1

Comprehensive survey of handoff management

challenges in wireless mesh networks and the evaluation

of MPLS-based solution for providing an efficient

transport

Anand Patil

1. Research Topic

A wireless mesh network is a self-organized backbone wireless network that can

dynamically maintain connectivity between its wireless nodes. The mesh here refers to an

interconnected group of mesh routers that help maintain connectivity throughout the mesh

network. A mobile terminal which is connected to a mesh network can maintain the network

connectivity as it moves from one Access Point to another within the mesh network. The

traditional wireless access methods such as Wi-Fi networks, WLAN, Bluetooth, and cellular

networks use single hop communication; for this reason they are connected to an Access Point

which connects directly to a wired backbone network. Expanding the coverage of traditional

wireless access methods is expensive because the wired backbone infrastructure which provides

the backhaul service requires new cables to be laid. In many situations laying new cables is not

feasible for practical and economical reasons. In contrast to single hop communication, the

wireless mesh networks (WMN) use multi-hop communication. In the wireless mesh networks

each Access Point (AP) is wirelessly connected to mesh routers. The mesh routers form the

wireless backbone for WMN networks by routing packets. In wireless mesh networks extending

the wireless coverage to larger areas is simple and cost-effective. This is the reason that service

providers such as Aruba Networks and Cisco are deploying next generation wireless mesh

2

networks because they are becoming the preferred way to deliver Video, Data and Voice in

outdoor environments. “A wireless mesh can deliver the same network capacity, reliability and

security that were once reserved for wired networks – but with the flexibility of wireless. With

today‟s state-of-the-art solutions, municipalities, public safety agencies, port authorities, and

industrial organizations can rely on mesh networks to provide essential connectivity to their

workers and constituents” (Aruba Networks, 2011). When the city of Austin, TX decided to offer

cheaper way of providing wireless access to its public on-the-go, the Austin city officials decided

to build a wireless mesh network in the city that would link the public places. The city of Austin

successfully installed wireless mesh solution with the help of Cisco‟s mesh infrastructure (Cisco,

1996-2006).

Handoff management which is part of the mobility solution for mesh networks is a

determining factor for successful deployments of wireless mesh network. WMN design poses

challenges in terms of the mobile device changing its attachment to the network across subnets

or IP domains. Wireless service providers such as Cisco and Aruba Networks are faced with the

problem of providing a consistent mobility management solution in a wireless mesh network.

Aruba Networks has adopted „High-speed outdoor roaming‟ which is a modified version of the

Mobile IP mobility solution. Aruba network‟s proprietary „MobileMatrix‟ technology for fast

roaming of mobile terminals across IP subnet is an example of a mobility solution (Aruba,

20011, pp-9). This paper will look at few of these vendor implementation of mobility

management solution. Two solutions for mobility management namely mobile IP and MPLS-

based modification of Micro-mobile IP are described later in the paper. This paper

analyzes handoff and routing needs of the WMN networks, examines the MPLS application

micro mobility solutions, thus evaluating the suitability of MPLS as an efficient transport

3

solution. The paper also looks at the current status of IETF and IEEE standards in terminal

mobility area.

2. Research Question

What are the mobility management challenges unique to the wireless mesh networks and

how have the standardization efforts in this area addressed the WMN deployment issues? Can

the MPLS routing features help achieve efficient handoffs for wireless mesh networks?

3. Background

A wireless mesh network is basically a collection of fixed wireless nodes, most of the

time consisting of regular wireless routers running adapted software. Its main goal is to provide

an inexpensive and easily deployable wireless backhaul that will connect distant LANs or

WLANs (Carrano, et al., 2011, p. 54). As opposed to wireless mesh network, the wired backhaul

refers to a wired backbone network capable of routing between nodes. Typically wired

backbones are an internet network composed of access routers, edge routers, and gateways

interconnected with each other using backbone wired network. The Mobile Terminals (MT)

accesses the network using end devices. These end devices are connected to part of networks

which are usually lower capacity network referred to as access networks. Access networks are

the network which MT are attached to. WLAN, for high-speed wireless data network, cellular

network, for voice and data connectivity, satellite communications for wireless access to

commercial applications, and Wi-Fi hotspot networks in public places for wireless network

connectivity are all examples of access networks (Akyildiz et al., 2004). A WMN has to be able

to integrate with these heterogeneous access networks with acceptable handoff latency.

4

The term mobility can refer to either nomadic(terminal) mobility, vehicular mobility,

service mobility or session mobility. Service mobility refers to the ability to maintain sessions

and obtain services without any interruption while user changes terminals and session mobility

allows the user to maintain a session while he/she changes terminals such as a laptop to a PDA

(Mohammad et al, 2009). Only the nomadic/terminal mobility is in the scope of this paper. The

nomadic mobility refers to maintaining the identity of the terminal as it moves from one location

to another. So as the mobile terminal moves from one location to another location, its peer

devices need to be able to continue communication with it using the terminal‟s known identity

irrespective of its location (Schiller & Voisard, 2004, p. 213). Nomadic mobility is also referred

to as terminal mobility. Mohammad et al. (2009) define terminal mobility as “The ability of a

terminal, while in motion, to access telecommunication services from different locations, and the

capability of the network to identify and locate that terminal either in the same or a different

administrative domain.”.

IP protocol is suitable for static networks. In wireless networks, the nodes are in constant

movement. In such environment IP protocol does not result in efficient operation and instead

cause lot of overhead traffic resulting in noticeable latency and unacceptable user experience.

Mobile IP (MIP) refers to protocol enhancements to standard IP protocol that allows transparent

routing of IP datagrams to the mobile nodes on the internet (Perkins, 2002). Some authors have

defined mobile IP as a routing protocol in itself. Authors Raab et al (2005) define mobile IP as

dynamic routing protocol where end devices signal their own routing updates and dynamic

tunnels eliminate the need for host route propagation. This means that instead of routing tables

updating in response to the terminal movement, the tunneling feature hides the address change in

the routing table. A handoff in the wireless mesh network occurs when a mobile terminal

5

changes it point of attachment to the wireless backbone network. A change of Access Point (AP)

while maintaining the connectivity is typically called a Handoff (Valko, 1999). Handoff latency

measures the delay between the point in time from when MT was connected to wireless mesh

backbone from a point of attachment and when it moves and is able to connect to the backbone

from another point of attachment. The delay is especially significant in multi-hop transmission

characteristics of WMNs due to multiple route discoveries, signaling message propagation when

multiple hops are involved (Xie & Wang, 2008). Handoff Management is the process by which a

mobile terminal keeps it connection active when it moves from one access point to another

(Akyildiz, Xie, & Mohanty, 2004). The handoff process in the WMN can happen between two

mobile nodes belonging to the same network domain, in which case the handoff is referred to as

micro-mobility, or between WMN nodes belonging to different network domains where it is

called Macro-mobility. Mobility Management enables telecommunication networks to locate

roaming terminals for call delivery and to maintain communication as the terminal is moving to a

new service area (Akyildiz et al., 1999). Mobility Management encompasses a set of tasks for

supervising the mobile user terminal (or mobile Station, MS), in wireless network. The tasks are

divided into registration and paging, admission control, power control and handoff (also called

handover) (Giannattasio et al., 2009). Recently the Multi-Protocol Label Switching has been

explored as an efficient tunneling protocol to be used in MIP. MPLS is a packet forwarding

solution where labels are assigned to packets. Routing is enabled by looking at labels. Label

lookup forwarding enables fast end-to-end routing without need for protocol specific lookups.

What this means is that packet header does not need to be parsed at each hop. If IP routing were

used, IP header has to be parsed to determine the next hop. MPLS label does away with parsing

6

for layer-3 or layer-2 specific protocols since it adds a label to the data before the layer-3 header

and after layer-2 frame.

4. Hypothesis

The wireless mesh networks are typically characterized by mobile users traversing across

homogeneous and heterogeneous access networks. The access network could be organized as

one whole layer-2 access network or sub divided as subnets. The layer-2 access mechanisms

could be different in heterogeneous access networks. Within the wireless networks, the access

methods used by mobile device to attach to the access network are different (CDMA, Wi-Fi,

802.11, etc.). As the user moves from one subnet to another or even across the IP domain,

maintaining the mobile user location information becomes an overhead. The handoff

management process needs to deal with location and addressability challenges of mobile nodes.

Since the protocol parameters can be different across each wireless access network, it is difficult

to achieve a smooth transition at each network boundary. The overhead caused by need to

maintain the location and addressability information for each node, results in higher latency in

wireless mesh networks compared to static wired network. MPLS inherently supports QoS

features and this coupled with its lower overhead in network stack lookups should result in

lowering handoff latency and reducing network resource allocation. Due to the lack of adequate

IEEE standards for addressing WMN mobility issues, the vendors such as Cisco and Aruba

Networks have adopted proprietary mobility solutions.

7

5. Scope and Assumptions

The scope of this paper is limited to a survey of handoff management issues inherent in the mesh

networks and application of MPLS to address those concerns. Current deployments of wireless

mesh networks by vendors such as Aruba Networks and Cisco is evaluated. The intended

audience is service providers who are planning to provide WMN solutions in enterprise,

municipalities, and emergency services. The IEEE standard 802.11s which is relevant to mesh

networks is covered. The IEEE 802.16 standard is out of the scope as well as the 802.21 standard

which deals with media independent handover between the cellular networks and the WLAN

network (De La Oliva et al. 2008). This paper is concerned with mobile terminal‟s terminal

mobility solution also referred to as nomadic mobility. The scope of the paper mainly focuses on

the intra-domain mobility issues. Thus cross-protocol handoffs which span across an

administrative domain are out of scope. Wireless Metropolitan Mesh Networks (WMAN) is the

deployment of mesh network over metropolitan area and not in the scope of this paper.

6. Importance

The past decade has seen tremendous growth in wireless LAN deployments such as in

enterprises, universities, and public wireless hotspots such as airports, restaurants, etc. However

providing mobility support for mobile devices which move from one wireless access structure to

another often necessitates fixed wired infrastructure as a backbone network to carry and route

traffic to the Internet (Nandiraju et al., 2007). This can be expensive for service providers. WMN

deployment can save CAPEX (capital expenses) for service providers because mesh nodes can

self-configure and acts like a router of backbone traffic without need for expensive T1 and T3

network pipes to be laid.

8

Signaling and traffic management are crucial aspects of any communications network.

Mobility Management plays an important role in signaling and traffic analysis as efficient traffic

management load analysis studies involve studying the mobility management (Markoulidakis,

Lyberopoulos, Tsirkas, & Sykas, 1997).

7. Contestable

Even though there are several research papers which focus on the individual aspects of

the handoff management solutions, such as solutions at layer-2 MAC, network layer, micro-

mobility and macro-mobility solutions, a comprehensive survey paper on the handoff and

mobility challenges of WMN, which evaluates MPLS technology for solving mobility issues is

lacking. This paper brings together and provides a comprehensive survey of the handoff

management challenges enabling a better definition of the problem areas in WMN deployment. It

also explores the applicability of MPLS solution to micro mobility solution. The paper

contributes to the WMN deployment scenarios by analyzing the proprietary mobility solution

adapted by the service providers, especially Cisco and Aruba Networks.

Because the IEEE standardization attempts for WMN mobility are still in draft stage (De

La Olivia et al. 2008), this study has importance for service providers (such as Cisco and Aruba

Networks) who in absence of standards have implemented their own proprietary solution to

mobility problem in WMN. The IETF and IEEE work on mobility management solution is

discussed in the „Standards‟ section later in this paper. Even though the handoff process in

wireless networks has been studied, but real-world solution that is acceptable to service providers

has been difficult to achieve (Kretschmer & Ghinea, 2010).

9

8. Methodology

This paper identifies the micro-mobility requirements for mobility management in WMN

networks, identifies the current micro-mobility solutions, discusses their inherent issues, and

finally evaluates whether use of MPLS technology in micro-mobility solution can offer

significant advantages. In order to accomplish these goals the paper answers the following sub-

problems.

8.1 Sub Problem 1

The first sub-problem is to evaluate how the wireless mesh network architecture differs

from the traditional Wi-Fi wireless access architecture.

8.1.1 Data collection and research

Identify the issues specific to WMN mobility and the requirements of mobility

management for WMN deployments.

8.2 Sub Problem 2

Second sub-problem is to identify the handoff challenges for wireless mesh deployment

and to examine how the efforts by the standards committee have helped in standardizing the

WMN deployment issues.

8.2.1 Data collection and research

Identify the handoff mechanisms in the WMN networks. Explain the OSI layered

solutions to mobility management to understand the handoff challenges at link and network

10

layer. Identify and evaluate the standards committee efforts at standardizing the network and data

link layer mobility solutions.

8.3 Sub Problem 3

Does MPLS offer a better solution towards solving the mobility issues apparent in the

wireless mesh networks in term of reduced complexity and efficient route path selection?

8.3.1 Data collection and analysis of results

Evaluate how MPLS architecture can help towards efficient handoff in WMN. Can

MPLS tunnels provide reduced complexity compared to the IP-to-IP tunnels of the traditional

micro-mobility solutions in the WMN network?

9. Wireless mesh network Architecture

9.1 Traditional wireless infrastructure mode:

The traditional wireless access methods such as Wi-Fi networks, WLAN, Bluetooth, and

cellular networks use single hop communication; for this reason they have to be connected to an

access point which connects directly to a wired backbone network. In a traditional wireless

network each access point connects to a wired backbone network, which could be Ethernet LAN

or any other wired access methods that in turn is connected to the Internet backbone. The AP

serves to provide the mobile terminals with network access. When the mobile terminal moves

from one access network to other, the user has to re-establish the connection. The major

disadvantage of this configuration is that coverage can be increased only by adding new APs.

11

Since APs require a backhaul network, providing large wireless coverage areas becomes

expensive.

9.2 Wireless mesh networks:

In contrast to single hop communication the wireless mesh networks use multi-hop

communication. In the WMN networks each AP is wirelessly connected to mesh routers. The

mesh routers form the wireless backbone for WMN networks by routing packets between mesh

routers. A WMN network is composed of Mobile MT connected to AP, from which they get the

network access. The APs wirelessly connect up to the mesh routers. Two or more mesh routers

are managed by mobile gateway routers (MGRs) (Garroppo, Giordano, & Tavanti, 2009). The

mobile gateways routers (MGRs) are the WMN‟s connection point to the Internet backbone. The

service providers typically use a single IP domain in which all their equipment is located. This

single IP domain is divided in to subnets, with each MGR being part of a distinct subnet. Instead

of being connected to a wired backbone, the MRs wirelessly connects to mesh gateway routers.

Each of the MGR is an internet gateway (IG) and connects to the wired infrastructure. The entry

point in the wired infrastructure is via Access routers (AR). The mesh routers are interconnected

to each other and are capable of multi-hop communication. The mesh routers are the first point

of network access to the Access Points because they work at layer-3 and are equipped with

complete IP stack (Garroppo et al. 2009). Just like routers and gateways form the backbone for

the Internet, the MRs and MRGs form the backbone for WMN. This architecture enables mobile

nodes to send multi-hop messages to distance the Foreign Nodes (FN) or the Corresponding

Nodes (CNs).

12

Figure 1: Wireless mesh network diagram

Internet Backbone

Domain 1

MTMT

AP

APAP

MTMT

AP

APAPSubnet A

Subnet B

Domain 2

MR

Subnet C

MR

MR

Wireless Mesh BackboneAP

MR

MGR

MGR

MGR

ARARAR Access Routers

9.2.1 Basic Service Set and Enhanced Service Set:

As shown in the below figure, a simplest configuration for WMN is one which consists of Basic

Service Set. A BSS consists of an Access Point several MTs. A MT is confined to a single AP.

13

Figure 2: BSS and ESS movement of a MT

Access PointAccess Point

BSSBSS

ESS

BSS: Basic Service Set

ESS: Extended Service Set

MT 2MT 1 MT 3MT 4

AssociationAssociation

Wireless Mesh Client Network

SUBNET A SUBNET B

9.2.2 Homogeneous and Heterogeneous deployments

Because the WMN deployment is expected to cover a large area compared to traditional

WLAN deployment, the expectation is that there will be different access systems, service

providers, backbone networks which could become part of one single mesh network. The WMN

architectures can be divided in two categories, namely Heterogeneous WMN deployment and

Homogeneous WMN deployment. Homogeneous deployments are limited to a single service

provider‟s domain. The entire network is managed by a single provider as a single IP domain.

Single IP domain deployments contain homogeneous access network such as Wi-Fi, cellular

14

network, WLAN, and Bluetooth across the mesh network with similar protocols and interfaces.

As shown in the figure above, the domain 1 network has its access network divided into subnets,

the MT movement is within single domain but across subnets. In contrast a heterogeneous

deployment will typically span across IP domains. Each IP domain may be using different access

networks, assuming that the backbone is all IP; the signaling overhead and mobility management

can introduce delay and latency whenever mobile terminal moves from one IP domain to

another.

10. Mobility Management:

A Mobility management solution for a WMN is concerned with ensuring that as the MT moves

in the mesh network, the mesh router can keep track of MT‟s location information as well as

maintaining the user‟s active connections to the backhaul network. From MT‟s perspective the

network adjusts to accommodate MT‟s changing location, reachability, and active TCP

connections. Mobility can refer to either terminal mobility, service mobility, session mobility, or

vehicular mobility. These various types of mobility solutions were briefly discussed in the

section 3 titled „Background‟. In this paper the term Mobility refers to Terminal Mobility.

Mobility management is a broad term referring to various aspects of mobility of a mobile

terminal in the wireless mesh network. Depending on the architecture of the WMN, the entire

WMN may consist of one single IP domain consisting of separate subnets or may be composed

of different IP domains, each domain consisting of several subnets. The former architecture

consisting of a single IP domain is usually a homogeneous wireless access environment because

service provider uses a single access method to access all of its MTs. An architecture consisting

of multiple IP domains is a heterogeneous environment as different IP domain may implement

15

different access systems in them. One IP domain may use WLAN connection where as other

domain may use a cellular network to communicate between MTs. The mobility management of

MTs is needed in both the homogeneous and heterogeneous environments, but in heterogeneous

environments a layer-3 handoff solutions are needed, where as for homogeneous environments

both layer-2 and layer-3 are candidate solutions. Depending on the movement of the MT within

or out of its IP domain, the mobility solutions can be classified as intra-domain also called

Micro-Mobility and Inter-domain also called Macro-Mobility.

10.1 Macro and Micro Mobility of mobile Terminal:

When the mobile terminal changes it Point of Attachment (PoA) within an IP domain

from one subnet to another this kind of mobility is referred to as micro-mobility, also called

intra-system mobility. This kind of mobility is the result of network and access systems having

similar protocols and interfaces. In the above shown figure 2, the MT movement from Subnet A

to Subnet B is referred to as Micro-mobility (intra-system). If the MT were to move from one

domain to another, the movement is called macro-mobility (inter-system mobility) (Akyildiz et

al. 2004, p. 18). Macro-mobility is a result of the network spanning service providers, and the IP

domains each having a different protocol stacks and heterogeneous access networks.

10.1.1 Mobility Solution:

IETF introduced mobile IP as a solution for mobile terminals which needed to remain connected

to the internet when changing their point of attachment. The Mobile IP uses concept of Home

Agent, Foreign Agent and registration between the agents as a way to keep track of the changes

in the mobile terminal location and address. The Home agent and Foreign Agents are mesh

gateway routers (MGR) as shown in the figure 1. Because MGR (HA and FA in mobile IP) are

16

required to keep the location data for the mobile terminals in database, whenever there is a

handoff initiated by MT movement, the location change information needs to be registered and

propagated to mesh gateway router. This requirement for a MT to register every time it changes

the PoA even within the same MR attachment causes lot of overhead registration messages.

Under mobile IP solution, even when the MT moves from one subnet to other within the same

home domain, the MT is required to send the registration update to the Home Agent for that

domain (Akyildiz et al. 2004, p. 19). Due to inherently frequent handoffs in the wireless mesh

networks, such registrations introduce latency. Due to the latency introduced by the registration

and handoffs in mobile IP, the research community has adopted several variant of mobility

management solution. To reduce the latency due to mobile IP registration and overhead traffic,

the WMN architecture has divided the mobility architecture in two separate problem domains.

The mobility of MT when it moves within the IP domain is referred to as Micro-mobility and

when the mobility spans across IP domains, the mobility is referred to as Macro-mobility.

10.1.1.1 Micro-Mobility:

Due to registration and tunneling overheads of mobile IP solution, the mobile IP has not

been used in micro-mobility domain instead the Micro-Mobility solutions have adopted modified

versions of the mobile-IP. When mobile terminal moves, mobile IP requires a new tunnel to be

setup. The tunnel between Home Agent and Foreign Agent is needed so that mobile terminal can

get the packets destined to it when it associates with the Foreign Agent‟s Access Point. The

delay is inherent in the round trip incurred by the mobile IP as registration request is sent to the

HA and the response sent back to the FA (Campbell et al. 2002). Registration and routing of the

packets via tunnel between HA and FA becomes more significant when the handoffs frequency

increases. Micro-mobility is characterized by frequent handoffs due to the mobile terminals

17

switching frequently between APs. Various micro-mobility solutions are available such as

Cellular IP, Hawaii, and Hierarchical mobile IP (Campbell et al. 2002). A common approach in

the micro-mobility solutions to reduce the signaling overhead associated with MT registration is

to localize the signaling overhead generated by the MT movement. For this purpose typically a

mobile Agent gateway is designated closer to the MT and signaling and registration need not

propagate all the way up to the Home Agent gateway (Xie & Wang, 2008, p. 37). Signaling

(location info) is derived implicitly or via ways transparent to a mobile terminal. The idea being

that MT should be freed from signaling overhead, after it has initiated a handoff process (Caceres

& Padmanabhan, 1996, p. 59). Instead of all the handoff signaling flowing to root node which is

the mesh gateway router in MIP, the micro-mobility solutions implement a sort of Hierarchical

router distribution. The Hierarchical router distribution helps with localizing the registration and

signaling traffic because registration updates as MT moves from one PoA to another is kept with

the nearest mobile router. Micro-mobility solutions such as Cellular IP, HAWAII, and HMIP

tend to keep the mobility changes visible in smaller and local area. This avoids the registration

messages between HA and FA from long trips, for example when HA and FA are far apart and

have to cross several network devices to reach one another. Cellular IP, HAWAII, HMIP are

essentially effect proprietary control messages for location management and routing within a

regional area of network (Yokota et al. 2002, p. 132).

10.1.1.2 Routing based and Tunnel based Micro-mobility:

The Micro-mobility solutions are either tunnel based or routing based (Chiussi et al. 2002). The

Cellular IP and Hawaii protocols fall under routing based solution where as Hierarchical MIP

falls under tunnel based approach. Routing based approaches leverage on the IP forwarding and

lookups to send the packets destined for mobile terminal to mobile terminal even when the

18

mobile terminal has changed its Point of Attachment (PoA). The tunnel based micro-mobile

solution use the tunnels similar to mobile IP, but the mesh routers are divided into hierarchical

router domain. The division of hierarchical routers helps to localize the tunnels and registration

overheads (Chiussi et al. 2002) and thus avoid the mesh gateway router updates every time a

handoff occurs. Whether the handoff is tunnel based or routing based, the Micro-mobility

solutions need location and handoff management techniques to enable efficient handoff. Every

mobility management solution has two parts, namely location management and handoff

management (Akyildiz, Xie, & Mohanty, 2004). The location management and handoff

management will be discussed in later sections.

10.1.1.3 Routing Based Handoffs

Cellular IP: Cellular IP overcomes the mobile IP limitation of having to propagate the

MT handoff registrations all the way to mesh gateway router. Instead of designating mesh

gateway router as root router (which is always part of every routing decision), the mesh router

closet to the Access Point assumes that functionality. This way when handoff occurs within the

MR domain, mesh gateway router is not even involved. Cellular IP uses the packets transmitted

by MT to mesh gateway router to determine the path information, thus reducing on the signaling

required to keep the location database updated. Instead of a Home Agent maintaining the

location database in a centralized manner, the Cellular IP requires that each mesh router keep a

node to IP address of the MT. Thus each MR knows which port to forward the packet to. This

hop-by-hop routing means that no single point of failure exists (Valko, 1999, p. 55). However to

keep the node-to-MT ipaddress mapping updated, a periodic beacon transmission is required to

be send by the MT. Since the MT may move between the APs, the mapping can become

outdated. For this reason cellular IP utilizes timers to reduce packet loss due to packets delivered

19

to old AP. Search and lookup times are improved by using caches. Cellular IP uses IP addresses

to identify the mobile terminals (Campbell, & Gomez 2000, p. 48).

Figure 3: Cellular IP architecture

Internet

AR AR

MR1

MR2

MGR

MR3

MT1 MT2

MT1 -- MR2MT2 – MR3

Host Specific Routing

Figure : Cellular IP Architecture

Mesh Gateway/Domain root Router

Mesh Router

Access Router

Host Specific routing Table

Home Domain

HAWAII: Like Cellular IP, Handoff Aware Wireless Access Internet Infrastructure

(HAWAII) allows the mobile terminal to retain its IP address as it moves in the access domain

from one mesh router to other. Thus the Home Agent (usually the mesh gateway router) is

unaware of the MT movement, avoiding expensive handoffs between HA and FA for each

address change of the MT (Ramjee et al. 2002). Instead of extracting signaling information such

20

as node reachability and port to next-hop address mapping from MT‟s normal data traffic the

Hawaii protocol requires the mobile terminal to send explicit signaling messages so that HA can

use these signaling messages to determine the IP routing path to MT. Location information (i.e.

mobile-specific routing entries) is created, updated, and modified by explicit messages sent by

mobile Host (Campbell et al. 2002, p. 73). HAWAII works on hierarchical network segregation

by diving access network in domains. Each MT has a home domain and foreign domain. Inside a

Home Domain, a domain root router is designated. Each MT send periodic infrequent signaling

message to root domain router. This establishes a „path setup‟ between Domain root router and

MT (Ramjee et al. 2002). By selecting only a few designated routers which participate in the

„path setup‟ updated messages, the signaling traffic overhead is minimized. Further HAWAII is

similar to Cellular IP in respect that the MT retains its IP address as it moves within the home

domain or within its foreign domain. But rather than using hop-by-hop tables like in Cellular IP

where each node maintains next-hop table with Host IP address and port mapping, the HAWAII

location management uses IP address forwarding technique, so as far as routing is concerned

HAWAII uses IP routing mechanism to reach mobile host.

10.1.1.4 Tunnel Based Handoffs

Hierarchical MIP: Hierarchical micro-mobility solutions do not use IP addresses or

hop-to-node mapping to reach from gateway to mobile terminal, instead they employ tree like

structure of Agents, and each Agent maintains destination MT‟s address to next Agent address

mapping. Thus the location database is distributed across several Agents. Once a MT registers

with it‟s HA, after that it maintains location database update only with its immediate next Agent

(Campbell, Gomez 2000). This way location database updates when MT moves is visible only to

the lowest Agent in the hierarchy of the Agent tree.

21

10.1.2 Location Management:

Reaching the roaming MT for delivering the packets destined to is a challenge that

location management tries to solve. In traditional wired infrastructure when an IP device moves

from one subnet to other the TCP connections are broken. TCP connections work on static

location addressing; IP addresses are assigned in a hierarchical manner. So when an IP device in

a wired network moves from one subnet to other, its new subnet will assign it new an IP address.

With the new IP address, the TCP connections need to be re-established. Whereas when a MT

moves from one subnet to another, the expectation is that the MT will still be reachable with the

same connection end points as if it has not moved from its original location from a network

perspective. Location Management involves keeping track of the MT while it moves from one

AP to another. This is necessary because packets destined for MT need to be routed to its current

location and not its home location. To make it possible to route MT A‟s packets to a location that

is MT A‟s current location, there is a need for maintaining a database with MT‟s current foreign

and home location information. The database needs to maintain the mapping and every routing

decision will necessitate a lookup for current location before packets destined to a MT can be

routed. Each of the three Micro-mobility solutions maintains the location database at different

mobile nodes in the mesh network. Also the composition of the location database varies

depending on whether mobility solution. In case of Cellular IP the location database contains the

mapping of destination MT IP address and interface port use to forward the packet. In case of

HAWAII, the location database consists of IP address to next mobile node address. In case of

Hierarchical MIP, specific set of agent nodes maintain database of destination mobile node

address and next Agents address for forwarding the packet. Location management involves

keeping the location data refreshed and current. This necessitates paging and beacon messages

22

between mobile gateway nodes and MT (Mohammad et al. 2009, p. 679). The location

management can be classified into (Xie & Wang, 2008):

10.1.2.1 Address Management:

Address management involves keeping track of the identity of the MT while it is

roaming.

10.1.2.2 Movement detection:

A MT has to know that it has entered a new location or area. This can involve either an

active periodic probe request message by the MT to the mesh routers or it can be a passive router

advertisement beacon message from mesh routers to the MTs. When this happens the MT knows

that it has entered a new area and thus registration has to begun. MTs can look at the subnet

mask to identify if it has moved under new AP and new MR or just done a layer-2 handoff.

10.1.2.3 Paging:

Paging involves determining the MR of the MT in question. This is needed because the

MT gets its network connection from its associated MR.

10.1.3 Handoff Management:

Handoff Management deals with keeping the connections active as the MT changes its

Point of Attachment. Handoff of the MT involves update of the location database because MT‟s

new location needs to be communicated to the mesh routers and mesh routers contain the

mapping data to reach the MT. The various solutions to handoff depend on which OSI stack

layer the handoff is being done. The handoff solutions fall in three categories and they are link

layer handoff, network layer handoff and cross layer handoff. There are tradeoffs in each

solution whether it is link layer, network layer or cross-layer, these handoffs which will be

23

discussed in subsequent sections. There are two types of MT movement in the wireless mesh

network. In first case the MT may move from attachment to a MR to another MR. In this case

the MGR attachment still remains the same. If the MT changes its MR attachment then it

performs Link-layer handoff, where as when the MT changes its MGR attachment it performs a

Network layer handoff (Xie & Wang, 2008, p. 39).

10.1.3.1 Design Issues for Handoff Management:

10.1.3.1.1 Handoff detection:

A MT needs to detect a handoff is necessary before it can initiate a handoff. Forced

Handoff occurs when the point of attachment of the MT or other mesh Hosts such as routers

changes in the mesh network. The MT initiates procedure to find new MR attachment. Unforced

Handoff occurs when MT finds a better path to the MGR and thus initiates a handoff to connect

to new MR which may or may not lead to new MGR attachment (Xie & Wang, 2008, p. 39).

10.1.3.1.2 Mesh gateway router selection:

If a handoff occurs and MT finds a new MGR point of attachment then a network-layer

handoff needs to be initiated by the MT. This leads to the third handoff issue namely, the QoS

maintenance.

10.1.3.1.3 QoS Maintenance:

If during the handoff a MT finds several MGRs to choose from then the MT needs to

consider the QoS maintenance issues. The MT needs to make sure that the minimum QoS that it

had with previous MGR is what it can get with the new MGR. The QoS guarantee involves

maintaining resource reservation along the path that the MT will reach the new MGR.

24

The MT‟s change in attachment to the mesh Network can result in link-layer or network-

layer handoff. The handoff can be broadly classified as Link-Layer handoff where the MT

changes its association or PoA from one mesh router to another and the Network-Layer handoff

where the MT changes its PoA with respect to the mesh gateway router, also called the Access

router. The MT gets its network layer address from the MGR/AR.

10.2 Link Layer Handoff and Network Layer Handoff Issues:

10.2.1 Link Layer Handoff:

The link layer lives in second layer of the OSI protocol stack. This layer is responsible

for node to node movement and the addressing is based on MAC addresses instead of IP

addresses. Link layer handoff happens when a MT moves away from the radio range of one AP

to another. The MT and its attached AP form a Basic Service Set (BSS). At this point it leaves

one BSS and enters other BSS. During this link layer handoff the MT exchanges management

frames with the new AP to form a new BSS. Forming new association, exchanging credentials

leads to latency in handoff. When MT moves in the subnet such that its connection to the serving

MGR remains the same, the handoff is still link layer handoff. This is also known as access

handoff or intra-system handoff because the devices attached to MGR, such as MRs, are link

layer devices connecting to the MT by 802.11 link interface. The MT may move from on one AP

to other AP which maps to a different MR. This is still a link layer handoff, as long as the

serving MGR remain the same (Cisco, 2009). The roaming in the layer-2 network can be of two

types:

25

10.2.1.1 Layer-2 roaming in layer-2 network:

When a mobile terminal moves from one AP to another and both the APs are still

attached to the same MGR and in the same subnet, the roaming is called layer-2 roaming in

layer-2 network. This type of deployment allows the mobile client to roam from one AP to

another without needing a change in client IP address (Cisco, 2009).

Figure 4: Layer-2 roaming at Layer-2 OSI layer

Access PointAccess Point

BSSBSS

MT 2MT 1 MT 2MT 3

AssociationAssociation

Wireless Mesh Network

MGR

MGR

Mesh Routers

S U B N E T A

MGR

MR MR

Mesh Gwy Routers

26

10.2.1.2 Layer 3 roaming in Layer-2 network:

When the mobile client moves from being attached to one AP to another AP which is controlled

by a different MGR and in a different subnet, then this kind of mobility is called layer 3 roaming

in layer-2 network (Cisco, 2009). This type of mobility supports mobility from one type of

access network to a different type. For example Subnet A could be a WLAN 802.11 radio

network and Subnet B a cellular network. Thus heterogeneous access networks are supported by

layer 3 roaming. Figure below illustrates the layer-3 mobility:

Figure 5: Layer-3 roaming at Layer-2 OSI layer

Access Point

MT 2MT 1 MT 2MT 3

AssociationAssociation

Wireless Mesh Network

MGR MGR

MGRMGR

Mesh RoutersMR MR

Mesh Gway Routers

Subnet A Subnet B

27

10.2.2 Link layer handoff delays:

The link layer handoff follows the 802.11 handoff procedure. This procedure has several

steps (CodeAlias, n.d.)

10.2.2.1 Scanning delay:

The MT scans for suitable AP to connect to. This is needed when current AP‟s SNR is

low or other conditions where continued attachment will result in signal loss.

10.2.2.2 Association delay:

After selecting suitable AP, the MT has to associate with the new AP. The association,

also, allows the new AP to inform the link layer devices (bridges, switches) to update their L2

table so that packets in destination to the STA get forwarded to the new location.

10.2.2.3 Authentication delay:

The new association between MT and AP need to be authenticated. 802.1X enabled APs

only accept 802.1X frames from non-authenticated STAs. The 802.1X frames contain EAP messages that

are forwarded by the AP to a back-end RADIUS server over the RADIUS protocol. More detailed

description of the delays due to authentication can be found in CodeAlias (n.d.).

A link layer assisted handoff has been proposed by Yokata et al. (2002). This architecture

requires a layer-2 bridge to filter the MAC addresses before forwarding to the home domain.

When a MT finds a neighboring AP with which it associates, the neighboring AP broadcasts the

MT‟s MAC address in its local segment. This causes the MT‟s MAC address to be registered in

the MAC bridge (the MAC bridge filters the packets before handing over to the Home/Foreign

domain root router). With a 2 port bridge, the subsequent packets will now flow from HA to

MAC bridge and to Foreign Agent to new AP and thus to the MT. After the MAC bridge port

28

mapping timer expires, the flow is tunneled from HA to FA and to the new AP and the MT. With

the link layer approach using the MAC bridge, the need for HA registration does not need to

happen before the MT in new domain starts receiving the packets. The registration delays are

thus not a requirement before MT can start communicating after handoff. This avoids the

registration related delays evident in the mobile IP before communication can resume between

the MT and the Internet.

10.2.3 Network Layer Handoff:

10.2.3.1 Mobile IP

Mobile IP (RFC 3220) which is IETF‟s specification for network level mobility, incurs

lot of registration overheads, suffers from triangular routing problem, packet loss during handoff,

and does not support paging (used to locate the terminal‟s mesh router). In contrast to the link

layer handoff, the network layer handoff allows handoffs to occur between two domains or

subnets. When a MT moves from one subnet in a domain to another, often the Access router

needs to update its routing entries. This scope of the handoff is now not within a single domain

but across domains. To maintain the MT‟s IP continuity in such cases needs MT to execute a

network layer handoff. PMIPv6 (Proxy mobile IPv6) is one network layer handoff protocol

which a proxy agent helps the MT execute network layer handoff.

10.2.3.2 Proxy MIPv6

Proxy MIPv6 (RFC 5213) (Gundavelli, et al, 2008), the IETF proposed standard, is a

network-based local mobility protocol. It defines Mobile Access Gateway (MAG) and Local

Mobility Anchor (LMA). MAG is the access router which provides access to the Internet, where

as LMA is local home agent router (Lee & Min, 2009). The LMA provides the MT with its

29

anchor point connection to the network layer of the mesh network. The MAG assists the MT in

the handoff by detecting the movement of the MT from one subnet to another. When this

happens the MAG searches for the LMA in the new subnet and finds if the new LMA has the

binding Cache entry for the MT. The binding cache entry is mapping between the MT‟s Home

address and its Care of Address in the new subnet (RFC 5213, 2008). If not then MAG sends the

binding update to the new LMA in the moved to subnet. A tunnel is established between the

MAG in the home domain and the LMA in the visited domain. This enables the MT to be

addressed with its Home Address even though it has moved to a different subnet and has separate

Care of Address, called the Proxy CoA (RFC 5213, 2008), from the visited subnet.

The predecessor to the PMIPv6 namely the MIP required stack modification in the MT

because it needed the MT to be part of the handoffs. However the proxy MIPv6 which is network

based handoff takes that responsibility away from the MT and network layer instead handles the

handoff for the MT (Garroppo et al. 2009). There are many mobile devices which are not

necessarily mobile aware, so a implementation was needed which could free up the mobile

terminal from handoff mechanism. Thus the proxy MIP was born. In the PMIPv6

implementation, the entire handoff is performed for the MT by the network layer and initiated by

the MAG. Network layer involvement has known to cause handover latency and packet loss

before the MT in the new subnet can start receiving the packets destined to it (Lee & Min, 2009

p. 1085), for this reason modified version of PMIPv6 namely PFMIPv6 (Proxy Fast mobile IPv6)

was developed. In PFMIPv6, the handover procedure is done before the MT executes a handoff.

This saves on latency and packet loss is avoided. In the PMIPv6, the MAG detects the MT

movement and then initiates the network handoff by establishing tunnel with foreign LMA. In

PFMIPv6, the MAG depends upon the link layer trigger to initiate the tunnel before handover is

30

started. For example the MT link layer can detect the presence of a nearby AP which has

stronger signal and trigger signal to its home domain MAG that a handoff is necessary. The

home domain MAG sends MT identifiers such as MT id, MT-LMA id etc to the foreign LMA in

Handshake Initiate message. Thus the Foreign LMA has handover parameters that will be needed

for binding before the handover is executed (bi-directional tunnel between MAG and foreign

LMA establishment) (Lee & Min, 2009).

10.3 Mobility requirements for handoff:

When mobile clients move across the network, the following need to be incorporated for

a successful handoff:

10.3.1.1 Location Database:

Location database is information about the mobile terminals association with AP and

MAC address. The database is located in the MGR‟s client database. The entries stored for each

mobile terminal are client MAC and IP address, security context and associations, QoS, WLAN

and associated AP (Cisco, 2009, pp2-17). As the mobile terminal association with AP changes,

the MGR‟s client database is updated. This update is either the responsibility of MGR and

happens at the network level by communication between the MGR‟s or can be the responsibility

of the mobile terminal to update the client database.

10.3.1.2 Move Discovery:

There are two ways for move discovery. The mobile client‟s layer-2 service detects

disconnect at layer-2 level and issues a notification called Media Sense to the Windows OS

(Cisco, 2009, pp 12-4). This enables the mobile terminal to recognize a move and request new IP

from DHCP server. The other way is for mobile Client to receive a Foreign Agent advertisement.

31

Within a subnet the mesh router periodically sends a beacon message to all the devices

advertising its existence. This way any visiting mobile client can detect a change in association

with AP in the visiting subnet, request new IP and initiate a handoff.

10.3.1.3 Location discovery:

Once the move discovery is done and if it is a move to a different subnet, then either a

tunneling or routing update method is used for routing packets to the mobile client in the new

subnet. For tunneling method the mobile Client has to update the Host MGR about the MTs new

AP association and new Foreign MGR‟s address. The MAC being a flat assignment remains

same.

10.4 Challenges of Link Layer Handoff:

As we note above during the link layer handoff involves scanning, management frame

exchange, and authentication delays. There could be several APs served by a single mesh router,

all the network layer-2 elements and when the AP is involved in a handoff, the signaling

messages have to pass through all the layer-2 devices generating lot of network traffic and

overhead. Thus pure layer-2 handoff is not an efficient solution. Combination of layer-2 triggers

with layer-3 handoff establishment is a good working solution for handover.

32

11. Standardization efforts in wireless mesh network:

11.1 Current mobility standards and their status

11.1.1 IETF mobility standards

The Mobile IP (MIP), which is the RFC 3344 (Perkins, 2002), is the current IETF

standard for supporting mobility on the Internet. IETF introduced mobile IP as a solution for

mobile terminals which needed to remain connected to the internet when changing their point of

attachment. But the IETF Mobile IPv4 suffers from longer handover delays mainly due to AAA

(authentication, authorization and Accounting) signaling, IP address configuration, and packet

loss during handoff (Adibi, Naserian, & Erfani, 2005). To alleviate the Mobile IP related issues,

a hierarchical Mobile IP has been proposed by IETF as proposed standard in October 2008 called

the Hierarchical Mobile IPv6 Mobility Management (HMIPv6) in RFC 5380. Successor to the

Mobile IP was the Proxy MIPv6, which is the IETF proposed standard (RFC 5213). The PMIPv6

handles all the handoff related signaling in the network layer nodes, this way mobile terminal

does not have to get involved in the signaling and thus overhead is reduced. There are many

mobile devices which are not necessarily mobile aware, so a implementation was needed which

could free up the mobile terminal from handoff mechanism. Thus the proxy MIP was born. The

Proxy MIPv6 is discussed further in the section 10.2.3.2. Mobile IP with Regional Registration

(MIP-RR) was proposed in RFC 4857 to reduce the registration delays. MIP-RR uses

hierarchical levels for home and foreign agent, so that registration related signaling can be

handled locally and does not need to travel across to one single common home/foreign domain

agent for the entire domain. This reduces the signaling traffic at the upper level home agent.

33

11.1.2 IEEE Standards for mobility management

The IEEE standards committee established the 802.11 Task Group named 802.11s TG for

the „s‟ amendment to the existing 802.11 WLAN standards to address the Wireless Local Area

Mesh Networking. The 802.11s is presently in the draft stage. The latest activity in the 802.11s

task group has been to forward the draft 802.11s document to the RevCom (Review Committee)

(IEEE, 2011).

The 802.16 working group is IEEE‟s effort to standardize the mesh architecture in Wide

Metropolitan area Networks (WMAN). The 3G cellular network‟s 3GPP program (Third

Generation Partnership Program) uses IETF defined Mobile IP for its mobility solution in

cellular network). The 3GPP networks handle the issue of mobility in two ways; the 3GPP2 uses

Mobile IP for IP mobility (i.e., terminal mobility) management and SIP for session mobility

management (Munasinghe & Jamalipour, 2008).

11.2 IEEE 802.11s standards Overview

A device which confirms to the 802.11 MAC and PHY specifications is identified as a

Station. A Station which can acts as a central device for other wireless LAN stations is called an

Access Point (AP) and this topology is called a Basic Service Set (BSS) (Hiertz et al. 2007). A

BSS forms a single hop network and all the devices on the basic service set depend on the central

AP to relay frames to each other. When several APs interconnect with each other, they form an

Extended Service Set (ESS). Stations can roam between APs in an ESS area. The IEEE 802.11s

takes this topology further and defines a mesh network. It defines a mesh Point (MP) as a mesh

network element which can relay frames between MPs using multi-hop communication. The

802.11s link management protocol is used to discover peer MPs which can then establish the link

34

layer connection with each others. The peer MP discover can involve passive scanning, which is

based on listening to the beacon frames from nearby MPs or active scanning by sending probe

request to nearby MPs. Once the neighborhood MPs are identified, they can form a mesh

network. The IEEE‟s 802.11 amendment „s‟ describes the necessary functions to form a wireless

mesh network. The 802.11s is an MAC layer approach to multi-hop communication between the

mesh Points, versus the earlier approaches to using network layer for multi-hop frame exchanges

(Carrano et al. 2011, p. 53). Typically 802.11s defines a wireless mesh network with point of

view of mesh routers being fixed in their location relative to each other. This contrasts with

mobile Ad hoc Networks (MANETS) in which there are no fixed routers. In a MANET the

routers are mobile and ad hoc without the need for infrastructure of mesh routers for routing

frames. The mesh routers in the WMN are responsible for exchanging the frames. Since the

mesh routers form the backbone wireless network, they need to constantly update their routing

tables. There are two route discovery methods for updating the routing table (Carrano et al. 2011,

p. 54):

11.2.1 Proactive Routing Updates:

In proactive approach of the route discovery, the mesh routers are constantly exchanging

the routing information regardless whether there is a need for data transmission between the

mobile terminals. The proactive approach tries to keep the routing tables updated. As evident this

approach results in the excessive traffic, and since the effort is to keep the routing tables updated

at all the time and since the nodes in wireless network are mobile, the tables can quickly get

outdated and thus greater need to exchanges routing entries frequently resulting in excessive

traffic overhead. Two examples of proactive routing are Optimized Link State Routing (OLSR)

and Destination-Sequenced Distant-Vector Routing (DSDV).

35

11.2.2 Reactive Routing Updates:

Reactive approach to route discovery takes an on-demand approach. When a mesh router

finds that there is a need for the routing tables to be refreshed, it initiates the path discovery

mechanism. Examples of reactive routing protocols are Dynamic Source Routing (DSR) and Ad

hoc On-Demand Distance Vector (AODV).

Recently however the focus of the routing protocols for WMN has shifted to layer-2

multi-hop routing. Layer-2 allows easy embedding in the network cards for the mobile devices

(Carrano et al. 2011). The 802.11s defines three entities that make up a wireless mesh network,

the mobile Station (MT), the mesh Station/router (MR), the mesh Access Point (AP), and the

mesh gateway router (MGR). The mesh gateway router is the portal for the mesh network to

connect to other networks. Each element in the mesh network needs an identity and this identity

is called the mesh ID. Mesh ID along with the path selection protocol in use in the mesh network

and the path selection metric constitutes the unique mesh Profile of each mesh element. All the

elements in the mesh cloud (formed by interconnected mesh routers) share the same mesh

Profile. mesh beacon frames are used by the MR to discover peer MRs. MRs form mesh peer

links with each other, each link identified by the MAC addresses of participating MRs and link

identifier.

11.2.3 Adaptive wireless routing

Recently there have been number of Adaptive routing protocols that have been suggested

fir wireless mesh networks. Traditional wireless network protocols such as OLSR and AODV

use static parameters which are preset and may not be suitable for all network conditions,

resulting in network degradation performance when used in environments that they were not

36

designed for (Azzuhri et al. 2010, p.145). The adaptive WMN routing protocols will dynamically

change certain parameters that are initially pre-set in the network. Adaptive AODV defines three

mobility levels low, normal and high. By checking the number of 1-hop neighbors it will use

appropriate mobility level (Azzuhri et al. 2010, p.145). So if it finds that there are a lot of 1 -hop

neighbors with which it has association it will reduce the HELLO message interval. Similarly in

adaptive routing version of the OLSR routing (OLSR is link state routing). Based on the link

condition the periodic link messages are either reduced or increased.

11.3 WMN Challenges for the Standards and Research bodies:

With the „s‟ amendment in the 802.11 specifications, IEEE redefined the MAC layer so

that multi-hop communication is enabled in the MAC layer for the mesh routers participating in

the mesh networks (Carrano et al. 2011). The challenges for WMN are:

11.3.1 Routing protocol:

Due to inherent stability issues of the wireless links, the wired routing protocols are not

suited for wireless mesh networks. For this reason ad hoc wireless routing protocols are used in

the WMN. These protocols broadcast frequent routing messages to gather routing metrics in

order to provide better QoS on the mesh (Hiertz et al. 2008). In wired links the layer-3 routing

protocols can access the neighboring links and determine the routing metrics, enabling them to

determine the routing paths and update the routing tables, but in the wireless mesh environments,

due to presence of radio links which IP layer cannot directly access, using IP protocols in

wireless mesh networks in a challenge. For this reason and since the MAC layer is closest to the

radio access physical layer, the standards organization foresee the integration of the routing and

the frame forwarding in to the MAC layer (Heirtz et al. 2008). Thus L2 routing which supports

37

MAC address based mesh path selection and forwarding using radio aware routing metrics is

what is proposed as part of the 802.11s standard. An essential component of the routing solution

is the use of metrics to determine the preferred route between source and destination (Faccin et

al. 2006). The airtime metrics used for radio links in the wireless mesh networks should ideally

be multi-dimensional, taking into account the link condition, bandwidth, QoS considerations,

power requirements etc… Thus the WMN networks call for routing protocols operating at layer-

2 MAC layer.

11.3.2 Link Management

Mesh points use passive scanning or active beacon transmission for finding the candidate

peer mesh point. Once the candidate mesh point is identified and mesh link established, the

airtime metrics is calculated (Heirtz et al. 2007).

11.3.3 Path selection

The airtime metrics of the peer mesh links calculated above is used during the path

selection. Path selection protocols at layer-2 are mentioned in the section___. The path selection

helps mesh point determine the path to the root or portal mesh point.

11.3.4 QoS:

In case of mesh Networks two main QoS issues are identified, namely for access network

traffic and backbone traffic (Faccin et al. 2006). For both of these traffic networks, since the

routing solutions for the WMN operate at layer-2 and layer-2 MAC layer is responsible for

forwarding frames from hop to hop, the challenge is to provide consistent QoS over end to end

link which covers multi-hop route path. Cross layer routing solutions have been proposed to help

layer-3 routing take advantage of the layer-2 proximity to the radio medium for providing QoS

38

over multi-hop links and at the same time improve up on the route optimization that comes with

layer-2 triggers.

12. MPLS for Mobility Management:

12.1 MPLS for routing:

In the conventional IP based routing, the packets are assigned to the stream based on the

longest prefix match of the destination IP address. This longest prefix match is done at every hop

at intermediate routers. At every router the network header is required to be parsed to compute

the longest prefix match. In the inter-domain routing where the routing tables are much bigger

than the inter-domain routing, this lookup and computation for longest match can add substantial

overhead to routing process. Multi-protocol label switching (MPLS) uses labels attached to the

packets for routing from hop to hop, the packets are assigned to stream based on the packet label,

unlike the IP address based routing where at each hop the packet may be assigned to a different

stream based on the longest prefix match computation. MPLS allows use of traffic engineering

and policy based routing. Because the routing is based on labels and not the destination address,

policy routing defines the stream to which the packets can be attached. The routers which assign

the labels to the packets are called the Label Edge routers (LER) and the routers which use the

labels for forwarding the packets are called the Label Switching routers (LSR). The path between

two LSR is called Label Switched Path (LSP). Packet with labels mapped to a common Forward

Equivalence class (FEC), will all flow through the same route.

39

12.2 Mobile nodes’ name and location identity with IP routing:

In the IP world, a node is identified and named by the IP addressed assigned to it. The IP

address identifies not only the identity but also the location binding of a network node. This

works fine in static wired networks where the routers are stationary. In wireless networks where

the mobile nodes are mobile, this kind of routing which is based only on IP address does not

work because as the mobile nodes move for example from a point of attachment in subnet A to

point to attachment in subnet B, the change in its location in IP domain necessitates an IP address

changes. The tying of the IP address with location information is needed in IP world because IP

addressing is hierarchical. Without the IP address change the other hosts on IP network would

not be able to communicate with mobile node. The IP address change of the mobile node causes

all the routing tables in the upstream network to change and this result in delays when handoff

needs to take place.

The mobile IP tries to solve this problem by keeping the moving mobile node‟s address constant

as it moves from one domain to other. This is achieved by using mapping between its home and

foreign addresses at the network anchor point. However bi-directional tunnels are necessary to be

setup between the mesh gateway router and the home anchor point. This tunneling introduces

delays because frequent mobile node movement causes registration messages to be sent to mesh

router gateway.

12.3 How MPLS fits in the mobility solution:

12.3.1 MPLS as routing layer binding the layer3 and layer2:

Mobility solutions have been explored which try to separate mobile hosts‟ IP identity

from the mobile nodes‟ location identity. One such solution by Sethom, et al. (2004) implements

40

a virtual layer between the layer-2 and layer-3 to separate the mobile nodes‟ physical internet

identity from its location identity. This virtual layer uses MPLS routing mechanism for

independence for layer-2 and layer-3 routing. The MPLS helps achieve independence between

the layer-2 forwarding mechanism from the application identity (IP address) (Sethom et al.

2004). As noted earlier in the paper the layer-3 network handoff introduces latency because the

routing updates need to be made in the routing table for each handoff made by the mobile

terminal. The layer-2 handoff can reduce this handoff latency, but suffers from broadcast

messages and probe messages in entire layer-2 domain. The MPLS routing mechanism lies in

between the layer-3 and layer-2 routing solutions, on one hand it provides routing by using fast

label look-ups at each hop without needing label decoding function (unlike network headers that

are decoded at each Layer-3 devices), thus also providing layer-2 like fast MAC lookup

functionality.

12.3.2 Static binding between the mobile nodes identity and physical

location:

MPLS breaks the static binding between the mobile nodes identity and its physical

location in the internet addressing scheme. When a node physically moves from one location in

the internet addressing scheme to another, its IP address also has to change. MPLS breaks this

static link between the node identity and node location by using labels to do the routing function

while maintaining fixed IP address as its identity. The dynamic binding between labels and

terminal‟s physical points of attachment breaks the static association between the location and

identification in the current internet architecture, and enables transparent and efficient mobility.

Routing is then accomplished by labels instead of IP (Sethom et al. 2004, p. 66). The static

binding between the nodes identity and at the same time its use for routing packets destined for

41

the node creates limited scope for using the routing entry for anything other than routing

function. By decoupling the routing entity from the nodes identity, the MPLS labels can be used

for multiple functions at the same time. For example labels can identify the type of service that

packets should receive, the resources that the packet stream in the FEC should be allowed to use,

and the routing policies based on labels.

12.3.3 MPLS as hierarchical isolation between mobility anchor point

and the mesh routers:

As noted in the section 10.1.1 „micro-mobility‟, the micro-mobility solutions such as

Hierarchical MIP, Cellular IP, and HAWAII help in reducing the handoff delays by partitioning

the mobility domain into local mobility regions. This way the mobile terminal movement is

localized and home agent router is not involved for every handoff thus the registration delays

caused by the registration between the mobile terminal and the home agent router is avoided.

MPLS solutions for micro-mobility take similar approach in localizing the mobile agent

movement updates. The gateway home agent which is the anchor point for mobile terminals and

is the designated labeled switched router, the mesh router serving as an acting base station and

mobile terminal are the only nodes involved in the registration and handoff process.

12.3.4 MPLS with traffic engineering support in wireless mesh

networks:

MPLS brings with it the traffic engineering concepts which are critical to ensuring a user

acceptable QoS experience when moving from mobile terminal‟s home domain to foreign

domain. With WMN networks requiring carrying voice, data, and video there is a need for

reliable QoS mechanism which will not add signaling overheads to what can be sometimes be

42

unstable and bandwidth starved radio links between the mobile nodes in the mesh network. The

MPLS RSVP-TE can be used to implement an IntServ model. In the case of RSVP-TE the end

host-to-host connections are replaced by reservations between network elements. This reduces

the signaling overhead. By employing signaling protocols such as RSVP-TE a LSP can be setup

and managed dynamically by using dynamic or static routes. A LSP can be configured to provide

QoS guarantees and to follow automatic reconfiguration when a failure appears of network state

changes (Boringer et al. 2005).

12.3.5 MPLS tunnels in the Micro-Mobility solution:

As discussed in the section 10.2.3 under „Network layer handoff‟, when the mobile

terminal moves from one subnet to another, the first step that MT does it register with its Home

agent. The registration involves making the Home Agent aware of the MT‟s care of address

which could be the Co-located COA or Foreign Agent‟s CoA (RFC 3344). From that point on

when HA receives packets destined for the MT, it establishes tunnel between itself and the

visited Foreign agent. Even though the packets destined for MT‟s home IP address is

encapsulated in the IP header with CoA, the routers that are part of the tunnel have to re-examine

the network header for determining the next hop using longest prefix match. If this tunnel is

replaced with the Label Switch Path, then the label lookup will need to be done only at HA and

at FA and not at the routers in between these two edge routers. The HA and FA act as Label

Edge routers (LER) attaching and stripping the labels respectively. Once the label is stripped the

conventional IP address routing is used to reach the MT in the visited network. This saves 16

bytes per packet transmitted (Boringer et al. 2005). Thus the MPLS is an efficient light-weight

tunneling technology, using Label switched paths between the home and foreign agents an

overlay network is efficiently created and managed (Chiussi, Khotimsky, & Krishnan, 2002).

43

MPLS by providing LSP paths create an overlay network, this means that existing network nodes

by incorporating label switching capability turns the internet network into a more efficient label

based switching network riding on top of the existing IP network.

12.4 MPLS based handoff:

The handoff mechanism in MPLS based micro-mobility solution uses established LSP

paths between the Home agent and the foreign agent. When the mobile terminal moves from one

subnet to another, it sends the registration message to the new mesh router, in response to the

MR‟s agent advertisement message. The new mesh router registers with the gateway home agent

using registration request message. The LSP path is established between the gateway agent in the

domain and the foreign agent in the moved to subnet. The gateway updates its label database and

sends registration reply message to the new MR. The new MR sends the registration message to

old MR as well, so that old MR can send the mobile terminals packets to new MR.

12.5 MPLS compared to traditional micro-mobility solution for

registration and handoff delays:

Micro-mobility solution using MPLS defines a new mobility agent called Label Edge

Mobility Agent (LEMA). LEMA functions as any other Label Switched router (LER) in that it

maps the IP address of the mobile host/agent to a FEC. The FEC itself consists of next-hop and

the MPLS label. Thus FEC defines the LSP. In addition to a regular LER functionality, the

LEMA also accepts registration message and on receiving one, it maps the IP address in the

registration message to a FEC class. Thus the LEMA can on fly define new LSP when it receives

registration request message from the mobile host. LEMA is a mobility enhanced version of LSR

(Chiussi, Khotimsky, & Krishnan, 2002). The LEMA is any LSR which has the mobility

44

function added to it. When a mobile node moves from one AP to another in the LEMA domain,

on receiving the registration message from mobile node, the LEMA enabled node maps the new

CoA of the mobile node in the new subnet to a new FEC, thus defining new LSP path to the

mobile node dynamically. The overhead to create new LSP is much less than overhead of

creating routing table entry in the traditional micro-mobility solutions like HAWAII, and

Cellular IP.

12.5.1 LEMA network and attachments

As the mobile node in the MPLD network moves from one AP to another, the LSP paths

are setup dynamically based on which advertised LEMA path the mobile node chooses. In

contrast with other traditional micro-mobility solutions (such as Cellular IP, HAWAII) the

MPLS based solution allows the mobile terminal to chose a predetermined LSP path based on

which LEMA it chooses to register with. In the figure below the registration and path selection is

described (Chiussi, Khotimsky, & Krishnan, 2002).

45

Figure 6: LEMA registration and path selection

AR 1

LEMA 4

LEMA 7

LEMA 8

AR 2 AR 3

LEMA 6

LEMA 5

MT MT

LSP path advertisement LSP path advertisementLSP Path: (2, 4, 7, 8)

LSP path advertisementLSP Path: (3, 5, 7, 8); (3, 6, 8)LSP Path: (1, 4, 7, 8)

LEMA 1 1 2 3

4

5

6

7

8

Access Network 1 Access Network 2

When MT is attached to AR1, its LSP path is (1, 4, 7, and 8). When the MT moves and attaches

to AR2, based on the advertisement from AR2 about the available LSP paths, the MT can chose

path (2, 4, 7, 8). As can be seen the change is only at the first node of attachment from LEMA

node 1 to 2. When the MT completes its link-layer attachment to AR2 and parses the

advertisement (2, 4, 7, 8), it recognizes that the LEMA node match is at level 2 at node 4. Thus

in this case it needs to only change attachment at node 4. Thus the MT sends a registration

message to LEMA node 4, with change request to send the packets destined for MT to LSP path

46

(2, 4) instead of (1, 4), the rest of the LSP path (4, 7, 8) remains same for the MT whether it is

attached to AR1 or AR2. When the MT moves from coverage of AR2 to AR3, the AR3

advertises the LSP path (3, 5, 7, 8) and (3, 6, 8). When MT compares these new paths to the

current LSP path, it sees that change needs to happen at level 3 node. The MT has the option of

choosing any one of these LSP paths. The MT sends registration messages to the nodes which

changed in the handoff so that they insert the mapping of the MT‟s IP address to the new FEC

along the LSP path (3, 6, and 8).

12.6 Key Points in MPLS label routing compared to traditional

micro-mobility routing:

12.6.1 Simplified Registration:

The registration process is simplified, because when MT moves from one access point to

other, it just need to change the LSP path that it can chose based on path selection criteria and

this involves simple label swap and new FEC generation. The traditional route updates in

traditional micro-mobility solution is more time consuming, thus increased latency during

handoffs.

12.6.2 Flexible Path Selection:

The mobile terminal based on the advertisement received from the LEMA need to make a

path selection and then swap the Label for generating new FEC at the root LEMA for Path

change. In traditional approach the MT would have to propagate the mapping change all the way

to Home Agent. Though the Hierarchical approaches help in localizing the registration

propagation, the overhead is increased number of agents to keep track of local mobility. In

47

MPLS, tunnel redirection, which is a crucial ingredient of any mobility scheme, happens quickly,

at a change of a node in a single node in the network (Chiussi, Khotimsky, & Krishnan, 2002).

12.6.3 QoS Guarantee and Link Reliability:

LSP paths can setup RVSP-TE QoS guarantee mechanism which is lacking in the

traditional routing. Link reliability is much improved in MPLS networks. This is because when a

node experiences failure in IP routing network, since in the traditional IP addressing network the

packets travel nodes are all the link

12.6.4 Packet loss during handoff:

Traditional micro-mobility solutions experience packet loss when MT moves from one

AP to other. When the MT is in movement the packets that are received by the old AP may be

lost because it is not able to contact the MT. Some implementations of traditional micro-mobility

solutions use buffering at all of the nodes which avoids this problem. MPLS based nodes avoid

this problem due to implementation of the redirect message to the old node to send the packets to

new LEMA registered node to which the MT has moved to.

12.6.5 IP-to-IP Tunnel overhead is avoided:

The IP-to-IP tunnels are completely avoided with MPLS implementation. Instead of

tunnel establishment during handoff, the nodes setup LSP path and MT has ability to chose

which LSP path, instead of hierarchical IP address path setup in traditional micro-mobility

solutions based on network hierarchy.

48

12.7 MPLS throughput compared to other micro-mobility during

handoff process

Xie et al. (2003) have studied the throughput that the MPLS based routing can achieve

when compared to the other micro-mobility solutions such as HAWAII and Cellular IP. In their

measurements, the MPLS based micro-mobility can support throughputs of 1.375 mbps

compared to throughput of 1.22 mbps when the mobile nodes speed is 15 meters per sec during

the handoff process. At higher speeds such as 40 meters per sec. the throughput for MPLS based

WMN falls gradually to 1.335 mbps where as for HAWAII based WMN the throughput falls

significantly to 0.8 mbps.

Figure 8: Throughput in mbps vs. the mobility handoff speed using MPLS and non-MPLS

micro-mobility solutions.

MPLS vs. Traditional Micro-mobility Handoff Performance

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

10 mps 20 mps 30 mps 40 mps

HAWAII (mbps)

MPLS (mbps)

49

13. Vendor implementation for mobility Solution

There have been several wireless mesh implementations in the commercial industry.

Aruba, Cisco, and other have implemented their own proprietary solutions for mesh networks.

13.1 Aruba Network’s wireless mesh network solution

Aruba Network‟s proprietary wireless mesh network implementation is called the

Airmesh multi-service mesh solution. It uses Adaptive wireless Routing (AWR) and

MobileMatrix in its mesh network deployment.

13.1.1 Adaptive Wireless Routing (AWR):

AWR is Aruba‟s proprietary routing solution for wireless networks. It works with layer-2

triggers for RF awareness and layer-3 for network routing intelligence to provide routing

solution that is optimized for wireless network. A routing protocol which is entirely layer-3

based can overwhelm the network, because it needs routing updates to be sent to all the layer -3

modes. In fast moving mesh network where nodes are highly mobile such routing update

overheads introduces significant latency. Aruba‟s AWR routing uses cross layer approach. The

layer-3 intelligence is combined with layer-2 information such as radio link strength and airtime

matrices discussed earlier in the paper to calculate. Such layer-2 parameters provide AWR

routing with intelligence to use wireless links based on conditions suitable for radio links. Using

this feature allows AWR routing to balance the overall traffic in case of radio interference

(Aruba Networks, 2011). For optimal video transmission, Aruba uses deep packet inspection,

MAC layer frame prioritization, and buffering techniques to achieve optimal video transmission.

Using the link layer metrics allows the network to use Air quality metrics which is specific to

50

wireless communications. Indicators for assessing current link quality include the link‟s data

rate, received signal-strength indicator (RSSI) and external interference (Aruba, 2010, p. 5). The

result is that AWR makes the routing decisions not just on network layer intelligence but adds

layer-2 metrics for better routing decisions in wireless networks.

13.1.2 Aruba Networks Handoff process enhancement:

Aruba Network uses MobileMatrix for mobile terminals moving from one subnet to

another. The MobileMatrix takes approach similar to mobile IP, but uses the AWR fast routing

convergence which helps alleviate the latency inherent in the mobile IP based handoff (Aruba,

2010, p. 10). When a handoff occurs the MobileMatrix roaming allows fast convergence for

routing table updates. The location data is maintained in the routing table with each node. For

mobile node position change, the routing tables are updated. The convergence happens faster

than traditional layer-3 convergence, due to proprietary AWR routing.

13.2 Cisco’s mesh network deployments:

Cisco uses three components for deploying the wireless mesh network:

1. Cisco 1500 Series mesh AP: This is the access point for the wireless clients.

2. Cisco Wireless LAN Controller (WLC): WLC is the central point for controlling APs.

3. Cisco Wireless Control System (WCS)

The Client mobile terminals connect to Cisco WMN with help of the Series 1500 mesh APs.

The mesh APs are of two types, the Roof-top APs RAP) and mesh APs (MAP). Both the APs use

dual radio channel for better bandwidth distribution between the client access channel (2.4 GHz)

and backhaul channel (5.8 GHz). The WLC manage the mesh APs in that it dictates the security

51

policy, the QoS on the links and mobility functions.WCS in-turn centralizes the management of

network management functions and dictates the RF prediction, network optimization functions

(Cisco, 2009). MAPs use Adaptive Wireless Path Protocol (AWPP) to determine best path

through other MAPs to WLC. There are two kinds of traffic on MAPs

Bridge traffic: This is the traffic between the devices connected MAP Ethernet ports.

Traffic between MAPs and WLC: This traffic carries WLAN traffic through the LWAPP

tunnels.

13.2.1 Cisco’s Light Weight Access Point Protocol (LWAPP):

LWAPP is Cisco‟s underlying protocol used in communication between the MAPs and

the Wireless LAN Controller (WLC) (Cisco, 2009, p.2-1). A MAP node establishes two types of

tunnels between itself and the WLC. Both of these tunnels are LWAPP tunnels. The first tunnel

is between MAP and WLC and carries the mobile client‟s layer-2 frame data, and the second

tunnel carries the LWAPP control signaling traffic. LWAPP supports both layer-2 mode of

operation where it transports layer-2 frame traffic between the MAP and WLC and layer-3 UDP

traffic. The Layer-3 USP traffic mode of operation is more preferred as it allows network layer

intelligence to be used in traffic distribution.

52

Figure 9: LWAPP tunnel setup

Wireless LAN Controller (WLC)

MAP/

RAP

MAP/

RAP

MAP/

RAP

MAP/

RAP

MTMT MT

LWAPP Tunnel Traffic

control messages

LWAPP Tunnel Traffic

W. Client Layer-2 traffic

13.2.2 Mobility Management in Cisco wireless mesh networks

Cisco defines a „mobility group‟ to group WLCs such that wireless clients can freely

roam between WLCs in the same „mobility group‟. If the client moves from attachment to a

WLC „A‟ to another AP which maps to WLC „B‟ and both WLC „A‟ and „B‟ both belong to the

same mobility group, then a layer-2 handoff is performed because the handoff involves same

subnet. The movement of wireless clients across mobility group, which is across subnets, is

defined as inter mobility group roaming.

53

Figure 10: Cisco mesh network mobility solution

WLC

WLC

WLC

WLC

Mobility Group ‘A’

WLC

WLC

WLC

WLC

Mobility Group ‘B’

MAP

13.2.2.1 Intra ‘Mobility Group’ roaming (layer-2 handoff):

To enable groups of WLCs to communicate with each other, Cisco defines a „mobility

group‟. A mobility group is a group of WLCs that together, act as a single virtual WLC by

sharing essential information such as Radio resources, and VLAN parameters (Cisco, 2009, p.2-

13). Any WLC in the mobility group can directly contact other WLAN in the same group using

pre-authenticated tunnels. This enables the wireless mesh clients to freely roam between the

54

WLC which are part of the same mobility group without wireless clients having to re-

authenticate. Cisco deployments use the mobility groups to facilitate seamless wireless mesh

client roaming between APs that are joined to different WLCs which are part of same mobility

group of WLCs. This way virtual WLAN domain is formed. The roaming within the „mobility

group‟ is movement within a subnet. Thus this involves layer-2 roaming. WLCs which

implement Cisco‟s LWAPP protocol use frame forwarding in layer-2 to exchange frames.

13.2.2.2 Inter Mobility Group roaming (layer-3 handoff):

If the wireless client is moving from one subnet to another a layer-3 handoff is initiated.

In this case an anchor/home WLC is defined and the anchor WLC becomes the gateway for the

mobile client. The layer-3 handoff can be asymmetric or symmetric. In asymmetric handoff the

path to wireless client is always via the anchor/home WLC and the path from the wireless client

is directly to the foreign WLC. In symmetric layer-3 handoff both the path to and from the

wireless client is via home/anchor WLC (Cisco, 2009, p.2-19).

55

Figure 11: Asymmetric layer-3 roaming

Anchor WLCA-WLC

Foreign WLCF-WLC

S U B N E T A S U B N E T B

Ethernet IP Tunnel

Who is MT’s WLC?

MTMT

MT’s details, home IP..

Mobility Group

MAP A MAP B

MAP C

56

14. Conclusion

The paper discussed the short coming of the micro-mobility management solutions as

well as the requirements of a mobility management system for the wireless mesh networks. The

IEFT and IEEE standards in the area of mobility management and their current status were

discussed. The paper found that the MPLS solution has several advantages in reducing the

complexity, satisfying the mobility management requirements, and simplifying the mobility

architecture. MPLS advantages were evaluated in terms of suitability for mobility management

solution. In conclusion, it is evident that the MPLS brings with it many features which help

reduce complexity of the handoff management and location management for wireless mesh

networks. This results in reduction in the handoff latency thus solving one of the critical issues

inherent in traditional wireless mesh network deployment. MPLS is particularly suited in the

mobile wireless architecture due to its dynamic path selection and ability to define end to end

QoS. Due to MPLS‟s inherent tunneling support, many of the disadvantages of IP-to-IP tunnels

in the micro-mobility solution are avoided. Since MPLS is a layer-2.5 solution it is much closer

to the layer-2 generated link metrics such as airtime metrics in deciding which nodes provide the

best backhaul path. Thus the conclusion of this paper is that MPLS with its inherent tunneling

technology and QoS support is best suited as routing technology for wireless mesh network

deployment.

57

15. References

1. Adibi, S., Naserian, M. & Erfani, S. (2005). IEEE Canadian Conference on electrical

and Computer Engineering. pp. 1090-1092. doi: 10.1109/CCECE.2005.1557166

2. Akyildiz, I.F., Xie J., & Mohanty, S. (2004). A Survey of Mobility Management in Next

Generation all IP-Based Wireless Systems. IEEE wireless communications, 11(4), 16-28.

doi: 10.1109/MWC.2004.1325888

3. Akyildiz, I.F., McNair, J., Ho, J.S.M., Uzunalioglu, H., & Wang. W. (1999). Mobility

management in next-generation wireless systems. Proceedings of the IEEE, 87(8), 1347-

1384. doi: 10.1109/5.775420

4. Aruba Networks (August 2010). The next step in the evolution of wireless mesh

networking. Sunnyvale, CA. Retrieved 13 Sep 2011, from

http://www.arubanetworks.com/pdf/technology/whitepapers/wp_Evolution-Wireless-

mesh-Networking.pdf

5. Aruba Networks (2011). Next-generation Wireless mesh networks: Combining a multi-

radio architecture with high-performance routing to optimize video surveillance and

other multimedia grade applications. Sunnyvale, CA. Retrieved 22 Oct 2011, from

http://www.arubanetworks.com/pdf/technology/whitepapers/WP_Wirelessmesh.pdf

6. Azzuhri, S., R., Portmann, M., & Tan, W., L. (2010). Adaptive Wireless mesh networks

Routing Protocols. IEEE Conference: 2010 7th International Conference on ubiquitous

intelligence & computing and 7th international conference on autonomic and trusted

computing (UIC/ATC), pp. 142-147. doi: 10.1109/UIC-ATC.2010.28

7. Boeringer, R., Saeed, A., Diab, A., Mitschele-Thiel, A., & Schneider, M. (April 2005). I-

MPLS: a Transparent Micro-Mobility-enabled MPLS framework. In 11th European

58

Wireless Conference 2005 - Next Generation Wireless and mobile Communications and

Services (European Wireless), pp. 9-13. Retrieved 27 Sep 2011, from

https://cuvpn.colorado.edu/stamp/,DanaInfo=ieeexplore.ieee.org+stamp.jsp?tp=&arnumb

er=5755378

8. Caceres, R., & Padmanabhan, V., N. (1996). Fast and Scalable Handoffs for Wireless

Internetworks. ACM MobiCom '96: Proceedings of the 2nd annual international

conference on mobile computing and networking. Retrieved 20 Oct 2011, from

https://cuvpn.colorado.edu/10.1145/240000/236405/,DanaInfo=delivery.acm.org+p56-

caceres.pdf?ip=198.11.24.2&acc=ACTIVE%20SERVICE&CFID=50138595&CFTOKE

N=86436961&__acm__=1319286271_f658beceb2d78550d5c94935fc3f753d

9. Campbell, A., T., & Gomez-Castellanos, J. (2000). IP Micro-Mobility Protocols. ACM

SIGMOBILE mobile Computing and Communications Review 4(4).

10. Campbell, A., T., Gomez, J., Kim, S., Wan, C., Turanyi, Z., R., & Valco, A., G. (2002).

Comparison of IP Micromobility Protocols. IEEE Wireless Communications 9(1), 72-82.

doi: 10.1109/MWC.2002.986462

11. Carrano, R., C., Magalhaes, L., C., S., Muchaluat Saade, D., C., & Albuquerque, C., V.,

N. (2011). IEEE 802.11s Multihop MAC: A Tutorial. IEEE Communications Surveys &

Tutorials 13(1).

12. Chiussi, F., M., Khotimsky, D., A., & Krishnan, S. (2002). Mobility Management in

Third-Generation All-IP Networks. IEEE Communications Magazine 40(9), 124-135.

doi: 10.1109/MCOM.2002.1031839

13. Cisco (1996-2006). Austin's Wireless mesh Provides Free Access and Test Environment.

Cisco Press. Retrieved 10 Oct 2011, from

59

http://www.cisco.com/en/US/prod/collateral/wireless/ps5679/ps6548/prod_case_study09

00aecd80563c29_ns621_Networking_Solutions_Case_Study.html

14. Cisco (2009). Enterprise Mobility 4.1 Design Guide. Retrieved 07 Oct, 2011, from

http://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/emob41dg/emob41dg-

wrapper.html

15. CodeAlias. (n.d.). Handoff delays in 802.11 wireless networks. Retrieved 15 Sep 2011,

from

http://www.codealias.info/technotes/performance_evaluation_of_wireless_security_systems_part

_3_-_factors_affecting_handoff_performance

16. De La Olivia, A., Banchs, A., Soto, I., Melia, T., & Vidal, A. (2008). An Overview of

IEEE 802.21: Media-Independent Handover Services. IEEE Wireless Communications,

15(4), 96-103. doi: 10.1109/MWC.2008.4599227

17. Faccin, S., M., Wijting, C., Kneckt, J., & Damle, A. (2006). Mesh WLAN Networks:

Concepts and System Design. IEEE Wireless Communications 13(2), 10-17. doi:

10.1109/MWC.2006.1632476

18. Garroppo, R., G., Giordano, S., & Tavanti, L. (2009). Network-based Micro-mobility in

Wireless mesh networks: is MPLS Convenient? IEEE Global telecommunications

Conference, ( GLOBECOM), 1-5. doi: 10.1109/GLOCOM.2009.5425578

19. Giannattasio, G., Erfanian, J., Wong, K., D., Wills, P., Nguyen, H., Croda, T., Rauscher,

K., Fernando, X., & Pavlidou, N. (2009). Wireless Access Technologies (pp. 11). In

Hanzo L. (Ed.), A Guide to Wireless Engineering Body of Knowledge (WEBOK).

Piscataway, Wiley-IEEE Press. Retrieved 14 Oct 2011, from

http://ieeexplore.ieee.org/servlet/opac?bknumber=5361026

60

20. Gundavelli S., Leung, K., Devarapalli, V., Chowdhury, K., & Patil, B. (2008). Proxy

Mobile IPv6. IETF Network working group. Retrieved 10 Nov 2011, from

http://tools.ietf.org/html/rfc5213

21. Hiertz, G., R., Max, S., Zhao, R., Denteneer, D., & Berlemann, L. (2007). Principles of IEEE

802.11s. Proceedings of 16th international conference on computer communications and

networks. doi: 10.1109/ICCCN.2007.4317949

22. IEEE (2011). IEEE P802.11 – Task Groups – Meeting Update. Retrieved 22 Sep 2011, from

http://www.ieee802.org/11/Reports/tgs_update.htm

23. Hiertz, G., R., Zang, Y., Max, S., Junge, T., Weiss, E., Wolz, B., Denteneer, D., Berlemann, L., &

Mangold, S. (2008). IEEE802.11s: WLAN mesh standardization and High Performance

Extensions. IEEE Network 22(3), 12-19. doi: 10.1109/MNET.2008.4519960

24. MeshDynamics (2010). MeshDynamics Wireless for the outdoor enterprise:MD4000 Brochure.

Santa Clara, CA: MeshDynamics. Retrieved 1 Nov 2011, from

http://www.meshdynamics.com/documents/MD4000_BROCHURE.pdf

25. Mohammad, R., H., Zuriati, A., Z., Nur, I., U., & Mohamed, O., (2009). Mobility support

across hybrid IP-based wireless environment: review of concepts, solutions, and related

issues. Annals of Telecommunications, 64(9), 677-691. doi: 10.1007/s12243-009-0107-0

26. Kretschmer, M., & Ghinea, G. (2010). An IEEE 802.21-based Approach for seamless

wireless mobile Integration Using QoS-aware Paths supporting Unidirectional Links.

IEEE GLOBECOM Workshop on Seamless Wireless Mobility (GC Wkshps), pp. 27-31.

doi: 10.1109/GLOCOMW.2010.5700326

27. Lee, H., & Min, S. (2009). A Network-Based Fast Handover Scheme over IEEE 802.16e Access

Networks. ISCIT 2009. 9th International Symposium on Communications and Information

technology, 2009. doi: 10.1109/ISCIT.2009.5341014

61

28. Perkins, C. (2002). IP Mobility support for IPV4. The Internet Society. RFC 3220.

Retrieved 12 Oct 2011, from http://www.rfc-editor.org/rfc/rfc3220.txt

29. Markoulidakis, J.G., Lyberopoulos, G.L., Tsirkas, D.F., & Sykas, E.D. (1997). Mobility

modeling in third-generation mobile telecommunications system. IEEE personal

communications, 4(4), 41-56. doi: 10.1109/98.612276

30. Munasinghe, K., S., & Jamalipour A. (2008). Architecture for Mobility Management in

Internetworked 3G cellular and WiMAX networks. Wireless Telecommunications

Symposium. pp. 291-297. doi: 10.1109/WTS.2008.4547578

31. Nandiraju, N., Nandiraju, D., Santhanam, L., He, B., Wang, J., & Agrawal, D.P. (2007).

Wireless mesh network: current challenges and future directions of Web-In-The-Sky.

IEEE wireless communications, 14(4), 79-89. doi: 10.1109/WMC.2007.4300987

32. Raab, S., Chandra, M., Leung, K., Baker, F. (2005). Understanding mobile IP. In mobile

IP Technology and Applications (chapter 2, pp. 9). Indianapolis, IN: Cisco Press.

Retrieved 28 Oct 2011, from

http://abc.safaribooksonline.com/book/networking/ip/158705132x

33. Ramjee R., Varadhan, K., Salgarelli, L., Thuel, S., R., Wang, S., & La Porta, T. (2002).

HAWAII: A Domain-based approach for Supporting Mobility in Wide-Area Wireless

Networks. IEEE/ACM Transactions on Networking 10(3), 396-410. doi:

10.1109/TNET.2002.1012370

34. Schiller, J., & Voisard, A. (2004). Location-Based Services. San Francisco, CA: Morgan

Kaufmann.

35. Sethom, K., Afifi, H., & Pujolle, G. (2004). Wireless MPLS: A new layer 2.5 Micro-

mobility scheme. MobiWac ’04: Proceedings of the second international workshop on

62

Mobility management & wireless access protocols. New York, NY: ACM. doi:

10.1145/1023783.1023796

36. Strix systems (2006). Wireless mesh networks for municipalities. Retrieved 5th Nov

2011, from http://www.strixsystems.com/dlctrl/files/mesh-Networks-for-

Municipalities.pdf

37. RFC 5213, 2008. Proxy mobile IPv6. IETF: Network working group. Retrieved 2nd

Nov

2011, from http://www.ietf.org/rfc/rfc5213.txt

38. Jiang, H., Zhuang, W., Shen, X., Abdrabou, A., & Wang, P. (2006). Differentiated

services for wireless mesh backbone. IEEE Communications Magazine, 44(7), 113-119.

doi: 10.1109/MCOM.2006.1668391

39. Valko, A., G. (1999). Cellular IP: A New Approach to Internet Host Mobility. ACM

SIGCOMM computer Communication Review 29(1), 52. Retrieved 8th

Oct 2011, from

https://cuvpn.colorado.edu/10.1145/510000/505758/,DanaInfo=delivery.acm.org+p50-

valko.pdf?ip=198.11.24.2&acc=ACTIVE%20SERVICE&CFID=49887834&CFTOKEN

=59826479&__acm__=1319141093_88055088a0ded060e5b8963a2653ea69

40. Xie, J., & Wang, X. (2008). A Survey of Mobility Management in Hybrid Wireless Mesh

networks. IEEE Network, 22(6), 34-40. doi: 10.1109/MNET.2008.4694172

41. Xie, K., Wong, V., W., S., & Victor, V., C., M. (2003). Support of Micro-mobility in

MPLS-based Wireless access networks. IEEE Wireless communications and Networking,

2003, WCNC, 2, 1242-1247. doi: 10.1109/WCNC.2003.1200551

42. Yokota, H., Idoue, A., Hasegawa, T., & Kato, T. (2002). Link Layer assisted mobile IP

Fast Handoff Method over wireless LAN Networks. MobiCom '02: Proceedings of the

8th annual international conference on mobile computing and networking.