Background of Ad hoc Wireless Networks

Student Presentations

Wireless Communication Technology and Research

Ad hoc Routing and Mobile IP and Mobility

Wireless Sensor and Mesh Networks

Mobile and Ad hoc Networks

Mobile IP and TCP for WirelessMobile IP and TCP for Wirelesshttp://web.uettaxila.edu.pk/CMS/SP2012/teAWNms/

Overview

Introduction to Multi-hop Ad hoc Networks Bluetooth Piconets and Scatternets and Mesh Mobile IP TCP for Wireless

Indirect TCP Snooping TCP Mobile TCP Transaction oriented TCP

Introduction (Ad hoc networks)

Ad Hoc Networks are collections of nodes connected together over a wireless medium.

These nodes can freely and dynamically self-organize into arbitrary and temporary, “ad-hoc” network topologies, allowing people and devices to seamlessly internetwork in areas with no pre-existing communication infrastructure, (e.g., disaster recovery environments).

Introduction (Single-hop Ad hoc Networks)

Ad hoc networking is not a new concept having been around for over twenty years, mainly exploited to design tactical networks.

Recently, emerging wireless networking technologies for consumer electronics are pushing ad hoc networking outside the military domain.

The simplest ad hoc network is a peer-to-peer network formed by a set of stations within the range of each other that dynamically configure themselves to set up a temporary single-hop ad hoc network.

Introduction (Bluetooth Piconets)

Bluetooth piconet is the most widespread example of single-hop ad hoc networks.

802.11 WLANs can also be implemented according to this paradigm, thus enabling laptops’ communications without the need of an access point.

Single-hop ad hoc networks just interconnect devices that are within the same transmission range.

This limitation can be overcome by exploiting the multi-hop ad hoc paradigm.

Introduction (MANETs)

In this new networking paradigm, the users' devices must cooperatively provide the functionalities that are usually provided by the network infrastructure.

Nearby nodes can communicate directly by exploiting a single-hop wireless technology (e.g., Bluetooth, 802.11, etc.), while devices that are not directly connected communicate by forwarding their traffic via a sequence of intermediate devices (Multi-hop).

Generally, these user devices are mobile, therefore these networks are often referred to as Mobile Ad hoc NETworks (MANETs).

Introduction (MANETs)

Being completely self organizing, MANETs are attractive for specialized scenarios like disaster recovery, vehicle-to-vehicle communications, and home networking.

Unfortunately, nowadays they have a very limited penetration as a network technology for mass-market deployment.

To turn mobile ad hoc networks into a commodity, we should move to a more realistic scenario in which multi-hop ad hoc networks are used as a flexible and “low cost” extension of Internet.

Introduction (Mesh Networks)

Indeed, a new class of networks is emerging from this view: The mesh networks

Unlike MANETs, where no infrastructure exists and every node is mobile, in a mesh network there is a set of nodes, the mesh routers, which are stationary and form a wireless multi-hop ad hoc backbone.

Introduction

Some of the routers are attached to the Internet, and provide connectivity to the whole mesh network.

Mesh routers are not users’ devices but they represent the infrastructure of a mesh.

Routing protocols running on mesh routers allow the backbone to be self configuring, self healing, and easy to set up.

Client nodes connect to the closest mesh router, and use the wireless ad hoc backbone to access the Internet.

Introduction (Opportunistic Networking)

Mesh networks are moving multi-hop ad hoc networks from emergency-disaster-relief and battlefield scenarios to the main networking market.

While mesh networks represent a short-term direction for the evolution of MANETs, opportunistic networking constitutes a long-term direction for the evolution of the ad hoc networking concept.

The bottom line of this paradigm is providing end-to-end communication support also to very dynamic ad hoc networks, in which users disconnection is a feature rather than an exception.

Introduction (Opportunistic Networking)

Nodes can be temporarily disconnected and/or the networks can be partitioned, and the mobility of nodes creates the communication opportunities.

The main idea is thus to opportunistically exploit, for data delivery, nodes’ mobility and contacts with other nodes/networks.

In opportunistic networks the communication is still multi-hop, with intermediate nodes acting as routers but, in this case, forwarding is not necessarily “on-the-fly”.

Introduction

Intermediate nodes store the messages when no forwarding opportunity exists (e.g., no other nodes are in the transmission range, or neighbours are not suitable for that communication), and exploit any contact opportunity with other mobile devices to forward the data toward the destination.

In this view, the existence of a simultaneous path between sender and receiver is not mandatory (as in traditional MANET) to communicate.

Introduction

This networking paradigm is well suited for a world of pervasive devices equipped with various wireless networking technologies (802.11 family, Bluetooth, ZigBee, etc.) which are frequently out of range from a global network but are in the range of other networked devices, and sometime cross areas where some type of connectivity is available (e.g. Wi-Fi hotspots).

Introduction (Wireless Sensor Networks)

Among multi-hop ad hoc networks, Wireless Sensor Networks have a special role.

A sensor network consists of a large number of small sensor nodes, which are typically densely (and possibly randomly) deployed inside the area in which a phenomenon is being monitored.

Wireless multi-hop ad hoc networking techniques constitute the basis for sensor networks, too.

Introduction

However, the special constraints imposed by the unique characteristics of sensing devices, and by the application requirements, make the solutions designed for multi-hop wireless networks (generally) not suitable for sensor networks.

First of all, power management is a “pervasive” issue in the overall design of a sensor network.

Sensor networks utilize on-board batteries with limited energy that cannot be replenished in most application scenarios.

Introduction

Furthermore, sensor networks produce a shift in the networking paradigm from a node-centric to a data-centric view.

The aim of a sensor network is to collect information about events occurring in the sensor field rather than supporting the communications between users’ devices.

Multi-hop ad hoc network technologies have big potentialities for innovative applications of great impact on our everyday life.

Introduction (Research direction)

However, after almost a decade of research, ad hoc networking technologies are rarely used and have not yet affected our way of using wireless networks.

It is believed that this is due to a wrong approach in the research, which was dominated by simulation modeling and theoretical analyses with only few attempts to build network prototypes to understand how well MANETs work in reality.

Introduction (Research direction)

In the last few years, this stimulated a new community of researchers combining theoretical research on ad hoc networking with experiences/measurements obtained by implementing ad hoc network prototypes.

Bluetooth Piconets and Scatternets

Piconets and Scatternets

Piconet Basic unit of Bluetooth networking Master and one to seven slave devices Master determines channel and phase

Scatternet Device in one piconet may exist as master or slave in

another piconet Allows many devices to share same area Makes efficient use of bandwidth

Wireless Network Configurations

Network Topology

Piconet = set of Bluetooth nodes synchronized to a master node The piconet hopping sequence is derived from the master node

Scatternet = set of piconets Master-Slaves can switch roles A node can only be master of one piconet. Why? A node can only be one master in a piconet. Why? Slaves could belong to more piconets. (bridge nodes)

Piconet 1

Master

Master

Piconet 2

Scatternet

Slave

Frequency Hopping

Total bandwidth divided into 1MHz physical channels FH occurs by jumping from one channel to another in

pseudorandom sequence Hopping sequence shared with all devices on piconet Piconet access:

Bluetooth devices use time division duplex (TDD) Access technique is TDMA FH-TDD-TDMA

Scatternets

piconets

Each piconet has one master and up to 7 slaves

Master determines hopping sequence, slaves have to synchronize

Participation in a piconet = synchronization to hopping sequence

Communication between piconets = devices jumping back and forth between the piconets

Mobile IP

Motivation for Mobile IP

Routing based on IP destination address, network prefix (e.g. 129.13.42) determines

physical subnet change of physical subnet implies change of IP address to have a

topological correct address (standard IP) or needs special entries in the routing tables

Specific routes to end-systems? change of all routing table entries to forward packets to the right destination does not scale with the number of mobile hosts and frequent changes in the

location, security problems Changing the IP-address?

adjust the host IP address depending on the current location almost impossible to find a mobile system, DNS updates take too much

time TCP connections break

Mobile IP Requirements

Transparency mobile end-systems keep their IP address continuation of communication after interruption of link possible point of connection to the fixed network can be changed

Compatibility support of the same layer 2 protocols as IP no changes to current end-systems and routers required mobile end-systems can communicate with fixed systems

Security authentication of all registration messages

Efficiency and scalability only little additional messages to the mobile system required (connection

typically via a low bandwidth radio link) world-wide support of a large number of mobile systems in the whole

Internet

Terminology

Mobile Node (MN) system (node) that can change the point of connection

to the network without changing its IP address

Home Agent (HA) system in the home network of the MN, typically a router registers the location of the MN, tunnels IP datagrams to the COA

Foreign Agent (FA) system in the current foreign network of the MN, typically a router forwards the tunneled datagrams to the MN, typically also the default

router for the MN

Care-of Address (COA) address of the current tunnel end-point for the MN (at FA or MN) actual location of the MN from an IP point of view can be chosen, e.g., via DHCP

Correspondent Node (CN) communication partner

Example network

mobile end-systemInternet

router

router

router

end-system

FA

HA

MN

home network

foreign network

(physical home networkfor the MN)

(current physical network for the MN)

CN

Data transfer to the mobile

Internet

sender

FA

HA

MN

home network

foreignnetwork

receiver

1

2

3

1. Sender sends to the IP address of MN, HA intercepts packet (proxy ARP)2. HA tunnels packet to COA, where FA is, by encapsulation3. FA forwards the packet to the MN

CN

Data transfer from the mobile

Internet

receiver

FA

HA

MN

home network

foreignnetwork

sender

1

1. Sender sends to the IP address of the receiver as usual, FA works as default router

CN

TCP for Wireless Networks

Motivation

Transport protocols typically designed for Fixed end-systems Fixed, wired networks

TCP congestion control Packet loss in fixed networks typically due to (temporary) overload

situations Routers discard packets as soon as the buffers are full TCP recognizes congestion only indirectly via missing

acknowledgements Retransmissions unwise, they would only contribute to the congestion

and make it even worse Slow-start algorithm as reaction

TCP Slow Start

Sender calculates a congestion window for a receiver Start with a congestion window size equal to one segment Exponential increase of the congestion window up to the congestion

threshold, then linear increase Missing acknowledgement causes the reduction of the congestion

threshold to one half of the current congestion window Congestion window starts again with one segment

TCP Fast Retransmit/Recovery

TCP sends an acknowledgement only after receiving a packet If a sender receives several acknowledgements for the same packet, this is

due to a gap in received packets at the receiver However, the receiver got all packets up to the gap and is actually

receiving packets Therefore, packet loss is not due to congestion, continue with current

congestion window (do not use slow-start)

Influences of mobility on TCP

TCP assumes congestion if packets are dropped typically wrong in wireless networks, here we often have packet loss

due to transmission errors furthermore, mobility itself can cause packet loss, if e.g. a mobile node

roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible

The performance of an unchanged TCP degrades severely however, TCP cannot be changed fundamentally due to the large base

of installation in the fixed network, TCP for mobility has to remain compatible

the basic TCP mechanisms keep the whole Internet together

Indirect TCP (1) Indirect TCP or I-TCP segments the connection

no changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol

optimized TCP protocol for mobile hosts splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections,

no real end-to-end connection any longer hosts in the fixed part of the net do not notice the characteristics of the wireless part

mobile host access point (foreign agent) wired Internet

“wireless” TCP standard TCP

I-TCP socket and state migration

mobile host

access point1

Internet

access point2

socket migrationand state transfer

Indirect TCP (2) Advantages

no changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work

transmission errors on the wireless link do not propagate into the fixed network

simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host

therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known

Disadvantages loss of end-to-end semantics, an acknowledgement to a sender does

not any longer mean that a receiver really got a packet, foreign agents might crash

higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent

Snooping TCP (1) Transparent extension of TCP within the foreign agent buffering of packets sent to the mobile host lost packets on the wireless link (both directions!) will be retransmitted immediately

by the mobile host or foreign agent, respectively (so called “local” retransmission) the foreign agent therefore “snoops” the packet flow and recognizes

acknowledgements in both directions, it also filters ACKs changes of TCP only within the foreign agent (+min. MH change)

„wired“ Internet

buffering of data

end-to-end TCP connection

local retransmission correspondenthostforeign

agent

mobilehost

snooping of ACKs

Snooping TCP (2)

Data transfer to the mobile host FA buffers data until it receives ACK of the MH, FA detects packet loss via

duplicated ACKs or time-out fast retransmission possible, transparent for the fixed network

Data transfer from the mobile host FA detects packet loss on the wireless link via sequence numbers, FA answers

directly with a NACK to the MH MH can now retransmit data with only a very short delay

Advantages: Maintain end-to-end semantics No change to correspondent node No major state transfer during handover

Problems Snooping TCP does not isolate the wireless link well May need change to MH to handle NACKs Snooping might be useless depending on encryption schemes

Mobile TCP Special handling of lengthy and/or frequent disconnections M-TCP splits as I-TCP does

unmodified TCP fixed network to supervisory host (SH) optimized TCP SH to MH

Supervisory host no caching, no retransmission monitors all packets, if disconnection detected

set sender window size to 0 sender automatically goes into persistent mode

old or new SH reopen the window Advantages

maintains semantics, supports disconnection, no buffer forwarding Disadvantages

loss on wireless link propagated into fixed network adapted TCP on wireless link

Mobile TCP

Transaction oriented TCP

TCP phases connection setup, data transmission, connection release using 3-way-handshake needs 3 packets for setup and 3 for release,

respectively thus, even short messages need a minimum of 7 packets!

Transaction oriented TCP RFC1644, T-TCP, describes a TCP version to avoid this overhead connection setup, data transfer and connection release can be combined thus, only 2 or 3 packets are needed

Advantage efficiency

Disadvantage requires changed TCP mobility no longer transparent

References

Multi-hop Ad hoc Networks from Theory to Reality

Editors: Marco Conti (Inst for Informatics and Telematics, Pisa Italy) ; Jon Crowcroft and Andrea Passarella (Univ. of Cambridge)

https://www.novapublishers.com/catalog/product_info.php?products_id=5556

Assignment #9

Write note on the topics highlighted in Yellow.

Q&A

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