design and validation of qos aware mobile internet access procedures for heterogeneous networks

Mobile Networks and Applications 8, 11–25, 2003 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.

Design and Validation of QoS Aware Mobile Internet AccessProcedures for Heterogeneous Networks

GIUSEPPE BIANCHIDipartimento di Ingegneria Elettrica, Universita’ di Palermo, Viale delle Scienze, Parco d’Orleans, 90128 Palermo, Italy

NICOLA BLEFARI-MELAZZID.I.E.I., Universita’ di Perugia Via G. Duranti, 93 06125 Perugia, Italy

PAULINE M.L. CHANDepartment of Cybernetics, University of Bradford, Richmond Road, Bradford, West Yorkshire, UK BD7 1DP

MATTHIAS HOLZBOCKGerman Aerospace Centre (DLR) Institute of Communications and Navigation, Oberpfaffenhofen, P.O. Box 11 16, D-82230 Wessling, Germany

Y. FUN HUDepartment of Cybernetics, Internet and Virtual Systems, University of Bradford, Richmond Road, Bradford, West Yorkshire, UK BD7 1DP

AXEL JAHNGerman Aerospace Centre (DLR) Institute of Communications and Navigation, Oberpfaffenhofen, P.O. Box 11 16, D-82230 Wessling, Germany

RAY E. SHERIFF ∗School of Engineering, Design and Techology, University of Bradford, Richmond Road, Bradford, West Yorkshire, UK BD7 1DP

Abstract. In this paper, the requirements for personal environments mobility are addressed from terminal and network perspectives. Practi-cal mobility and Quality of Service (QoS) aware solutions are proposed for a heterogeneous network, comprising of satellite and terrestrialaccess networks connected to an IP core network. The aim, in adopting a heterogeneous environment, is to provide global, seamless servicecoverage to a specific area, allowing access to services independently of location. An important assumption is that nomadic user terminalsattached to a particular segment should be able to exchange information with any other terminal connected to the network. This is to en-sure transparency of device technology. Different communication scenarios are investigated in support of IPv4 and IPv6 operating on userplatforms and over access segments. The heterogeneous network necessitates the need to perform handover between access segments toenable coverage extension and seamless connectivity. Handover procedures are analyzed, and an approach is presented that enables variousoperation and segment specific parameters to be taken into account when deciding upon the need to perform handover and in selecting theoptimum access segment. In order to ensure transparency of network technology, the need for end-to-end QoS support is discussed, bearingin mind the deployment of both IntServ and DiffServ enabled routers in the core network. Following this, a new admission control scheme,named Gauge&Gate Reservation with Independent Probing (GRIP), is proposed. The paper concludes with a description of a laboratorytestbed, which has been developed in order to verify the presented procedures, together with performance measurements of the handoverand the GRIP algorithms.

Keywords: Mobile IP, heterogeneous networks, admission control, QoS, Handover Management, Laboratory Demonstrator

1. Introduction

The next significant phase in the development of mobile com-munications will result in the convergence of mobile and In-ternet technologies, allowing nomadic user terminals to op-erate in an “always-on” mode [20]. The important part thatstandardization has played in the success of mobile communi-cations is well known. On the other hand, the development ofthe Internet has been less constrained by standards, as a resultthere are a number of different solutions currently supportedin the core network and on local platforms. Consequently, in

∗ Corresponding author.

order to facilitate the convergence between mobile and Inter-net technologies, it will be necessary to devise an adaptiveapproach to the problem, from both mobile access and Inter-net core network perspectives.

In the future mobile environment, it can be envisaged thatcomplementary networks will combine to provide a feder-ated heterogeneous service environment, which can be tai-lored according to the needs of the nomadic user. In the fol-lowing, a heterogeneous network is assumed, comprising offour access segments, each capable of supporting IP. Specif-ically, the network is constituted by GPRS, representing anevolved second-generation network; UMTS, the European

12 G. BIANCHI ET AL.

Figure 1. Communication scenario in a hybrid multi-segment network.

third-generation (3G) network; wireless local area network(W-LAN); and Mobile EuroSkyWay (M-ESW), a plannedgeostationary satellite network capable of delivering broad-band services [11]. This is shown in figure 1. These four ac-cess segments provide a realistic near-future service scenario,providing complementary services and coverage to the no-madic user. The access segments are connected to the Internetthrough edge routers (ER). In GPRS or UMTS, the ER func-tionality is provided by the Gateway GPRS Support Nodes(GGSNs), while in the case of the satellite network, ERs areconnected to local Gateways, which provide a fixed-link con-nection to the satellite. The core network is based on IP con-nectivity.

Rather than consider the needs of the individual nomadicuser terminal, in this paper, attention focuses on the morecomplicated scenario offered by Collective Terminals, thefunction of which is to serve several nomadic users with ac-cess segment connectivity. This can be achieved via localarea network (LAN) connections provided by a terminal in-terworking unit (T-IWU). The T-IWU also provides the inter-faces to the radio modules of each supported access segment.

Collective Terminals can be thought of as being highly mo-bile and are likely to cross the service coverage area of sev-eral networks during transit, thus, handover is necessary forseamless connectivity. By using the Mobile IP protocol suite,a nomadic user that is connected to the T-IWU can be reachedanywhere regardless of the current point of attachment [25].

The Collective Terminal provides the nomadic user withthe transparency of network technology. Not all nomadicusers connected to a Collective Terminal will be configuredfor Mobile IP. Thus, in addition to inter-segment handoverprocedures, the Collective Terminal is required to take overthe mobility support for its attached clients. In this respect,the Collective Terminal will behave like a mobile router ina mobile network [25], ensuring that transparency of devicetechnology is achievable.

The structure of the paper is as follows. In section 2 severalsolutions to the problem of IP mobility, taking into accountthe possibility that different wireless segments may support

IP version 4 or 6, are presented. In the context of PersonalEnvironments Mobility, this section addresses the need fordevice transparency. In section 3, the problem of handover isaddressed when operating in a multi-segment, heterogeneousenvironment. This is to ensure transparency of operation fromwhere networks and services are accessed. Several differenthandover schemes are analyzed, in particular giving attentionto the different triggers that may cause handover, such as seg-ment availability, fades of the wireless transmission channel,or quality of service (QoS) measures. In section 4, a new ad-mission control scheme to support QoS in the different accesssegments as well as in the core network is proposed. Thisaddresses the need for wireless and wireline network trans-parency. In section 5, a selection of results obtained using aMobility and QoS Testbed and Demonstrator are presented.

2. IP mobility in multi-segment hybrid networks

2.1. Definitions

From the perspective of the T-IWU, the term Mobile Networkimplies that nomadic user terminals are fixed with respect toeach other but collectively are mobile. The T-IWU will havea home location, meaning a location where it is usually at-tached while it is not moving, and where all the IP addressesof the network are topologically correct. In order to makematters simpler and without loss of generality, the T-IWU canbe thought of as a router to which all the nomadic user termi-nals of the Mobile Network are attached. This router can benamed the Mobile Router (MR), following the terminologyin [23].

On the network side, a Home Agent (HA) of the Inter-net Service Provider (ISP) enables T-IWU mobility. Nomadicuser terminals will not be aware of the T-IWU’s HA. Thosemobile terminals that have implemented Mobile IP will havea HA at their Home Network. Another important entity onthe network side is the network interworking unit (N-IWU).Its purpose is to support handover and QoS. In what follows,

DESIGN AND VALIDATION OF QOS AWARE MOBILE INTERNET ACCESS 13

Figure 2. Mobility support for mobile nodes using DHCPv4.

Figure 3. Mobility support for mobile nodes using Mobile IPv4.

nomadic user terminals are termed “Mobile Nodes” (MN), al-though this does not necessarily imply that they have imple-mented the Mobile IP protocol. Indeed, the majority of MNsis unlikely either to have installed the Mobile IP protocol or tohave installed specific QoS-supporting applications. Further-more, either IP version 4 (IPv4) or IP version 6 (IPv6) maybe supported by the access segments.

In the following paragraphs, in order to address the needfor transparency of device operation, mobility solutions forthree scenarios are considered, specifically:

(i) MNs using IPv4 without Mobile IP,

(ii) MNs using IPv4 with Mobile IP, and

(iii) IPv6 MNs.

2.2. Support of Mobile Nodes using IPv4

This scenario assumes that the MN does not use Mobile IPand that it is an IPv4 client. The protocol architecture is de-picted in figure 2. Segments supporting IPv4 are likely to as-sign only one IP address to the T-IWU. Thus, in this instance,the T-IWU cannot serve its attached MNs with co-located IP.

The T-IWU receives one IP address, IaddrSS, from the ac-cess segment. It then registers with its ISP-HA using this ad-dress and requests for a set of IP addresses, IaddrISP,1,...,n,belonging to the address space of the ISP-HA. The T-IWUcan now serve local MNs with these addresses. During boottime, an MN needs to acquire an IP address, IaddrISP, from

the T-IWU using DHCPv4 (Dynamic Host Configuration Pro-tocol version 4). Correspondingly, the T-IWU has to storeall assigned IP addresses in a table. Then, the T-IWU reg-isters the MN with its ISP-HA using IaddrSS as CoA andIaddrISP,1,...,n as home address. The MN is now globallyreachable with IaddrISP,1,...,n. The ISP-HA encapsulates allpackets directed to the MNs and forwards the packets throughan IP-IP tunnel with destination address, IaddrSS, and IP pro-tocol field set to 4. The T-IWU has to decapsulate all pack-ets from the tunnel and then forward them to the appropri-ate MN. If a segment is changing (that is its IaddrSS) due tointer-segment handover, the T-IWU must send a binding up-date to its ISP-HA for all attached MNs. Note that the MNswill not be aware of these binding updates; their IP addresses,IaddrISP,1,...,n, remain unchanged.

2.3. Support of Mobile Nodes using Mobile IPv4

This scenario assumes that the MN is using Mobile IPv4(MIPv4). The situation is depicted in figure 3. The proce-dures are the same as described in section 2.2. In addition, af-ter having received the CoA, IaddrISP,1,...,n, from the T-IWU,the MN registers with its own HA using its home address andIaddrISP,1,...,n as CoA. Thus, packets with a destination of thehome address of an MN are perceived from the MN-HA, tun-neled to the ISP-HA and then tunneled again to the T-IWU.The T-IWU decapsulates the segment tunnel and forwards thepackets to the MN, which decapsulates its packets from theIaddrISP,1,...,n tunnel.


Figure 4. Mobility support for mobile nodes using IPv6.

2.4. Support of Mobile Nodes using IPv6

This scenario assumes that an MN is using IPv6. During boottime, the MN requests an IPv6 address using DHCPv6. It maybe assumed that the T-IWU can receive a co-located CoA foreach MN from the segment when IPv6 is supported. Then, theremaining procedures follow the standard Mobile IP protocol.However, if the access segment does not support IPv6, thesituation is more complicated, as depicted in figure 4. Herethe T-IWU has to register with the ISP-HA with its IPv4 seg-ment address, IaddrISP,1,...,n, and request for IPv6 addressesbelonging to the IPv6 address space of its ISP-HA. MNs areserved with these addresses. An MN can now register with itsHA, which forwards all packets using IPv6 routing headersto the ISP-HA. An IPv6-in-IPv4 tunnel is then required fromISP-HA to T-IWU to encapsulate all IPv6 packets and tunnelthrough the IPv4 segment and back.

3. Handover scenarios and architectural support

3.1. Introduction

A heterogeneous network comprising of terrestrial and satel-lite networks is envisaged to ensure that the nomadic user canoperate transparently of location. Although the problem re-lated to macro-mobility has already been addressed by Mo-bile IP, there is a need to control the mobility procedures re-lated to each access segment. In a particular target system,the T-IWU is assumed to be responsible for interacting withall segments to handle matters related to mobility manage-ment such as handover. For example, the T-IWU is responsi-ble for selecting the most suitable segment during handover,requesting for resources in the new segment and then releas-ing the resources of the old segment. This section addresseshow the procedures for each access segment are executed to-gether with Mobile IP procedures. The methodology adoptedin the system manages macro-mobility using Mobile IP, whilestill maintaining each access segment’s network procedures.This not only ensures minimum alterations to the access seg-ment, but also at the same time allows flexibility in the targetsystem to include as many access segments as possible.

Handover is the mechanism that allows the user to main-tain an on-going session at an equivalent QoS when movingfrom one segment to another. Since both the terrestrial andsatellite segments involve the use of various different mea-surements and criteria, different QoS classes have to be ne-gotiated for users during handover. The aim is, therefore, tomaintain the QoS requested by the user when a session is firstestablished. The most common reason for initiating a han-dover occurs when the Carrier to Interference (C/I) ratio andthe received signal strength fall below a specified threshold.In this section, a handover procedure, which uses a combina-tion of different handover parameters, is proposed to supporthandover between heterogeneous networks.

3.2. Handover strategies

The proposed handover algorithm is based on the concept offuzzy logic. This is a concept that was conceived for handlingpartial truth in order to model the uncertainty of natural lan-guage. The algorithm is separated into two distinct phases;handover initiation and handover decision.

Handover initiation depends on a number of control ele-ments; in particular, on the way a signal is averaged over timeand compared, the type of applications used and requirementsof the user. The main criteria, however, would depend on thevariations of the signal strength. Averaging a signal is usu-ally necessary to remove rapid fluctuations due to multipathfading [24]. This is to ensure that the handover rate is at an ac-ceptable level and prevents a user from being transferred backand forth from one segment to another. However, a questionthen arises as to the size and length of the averaging win-dow. In [21], it is suggested that for a system with a relativelysmooth path loss characteristic, the averaging interval shouldbe large enough to remove variations due to fading. On theother hand, for micro-cellular systems, a long averaging in-terval is not desirable due to sudden path loss that results dueto the corner effect.

For the target system, the algorithm for handover initiationinvolves averaging the signal received with different windowsize and length for each segment to ensure the accuracy of thehandover initiation procedure using a fuzzy logic controller.


In addition to averaging the signal, a fuzzy controller is alsoused to determine whether a handover is necessary. Both thecontrollers use the fuzzy expert system concept in which acollection of membership functions and rules are used to un-derstand and derive conclusions based on the data obtainedfrom the system. The rules of a fuzzy expert system are usu-ally expressed in the following form:

IF x is low and y is high THEN z is medium,where x and y are input variables and z is an output vari-able.

Several criteria will need to be taken into accountwhen considering handover initiation. These include signalstrength, bit error rate, network coverage and the user’s per-ceived QoS.

For handover decision, another fuzzy concept known asthe fuzzy multiple objective decision making algorithm is im-plemented. During handover decision, all available segmentsare evaluated to determine the most suitable segment for theuser. For handover decision, the criteria used include batterystatus, network latency, bandwidth, signal strength, chargingmodel and so on [12].

3.3. Handover procedures

In order to reduce complexities and modifications to the ac-cess networks, the intelligence of the system is shifted tothe terminal. This results in the use of Mobile ControlledHandover. Hence, during the handover initiation phase, theT-IWU collects information on the user profile, the QoS per-ceived by the user and the reports of the radio link availabil-ity. This information, together with information on segmentavailability, can be used to initiate handover when required.

Once handover is initiated, the T-IWU decides on the tar-get segment using the handover decision algorithm. This isaided by the user profile, which contains information such asthe minimum and maximum cost and the list of segments withthe highest and least priority. If the user does not specify anypreference, the default segment is the terrestrial segment, pro-vided this is available. Next, before the handover can finallybe executed, the T-IWU communicates with the N-IWU todetermine the status of the target segment to ensure that it hassufficient resources to support the QoS required by the user.

Several procedures are performed during handover execu-tion. First, the T-IWU must communicate with the accesssegments in order to request for connectivity. Once the ac-cess segment receives this message, it will attempt to set-up a connection for the user. In the GPRS and UMTS seg-ments, this procedure is known as the PDP Context Activa-tion. For the satellite segment, a normal satellite connectionis set-up. However, the satellite segment must have alreadybeen adapted to receive IP packets.

The signaling to set up a new session is performed on thenew segment, since forward handover is employed to reducethe probability of a sudden drop in the satellite link. At thesame time, data is still sent through the old segment. Thisapproach is known as signaling diversity and is adopted to re-duce the time needed for handover. Upon establishment of

IP connectivity in the access segment, the T-IWU obtains aCoA from the chosen segment. This is followed by the exe-cution of the standard MIPv6 procedures. At this stage, theMN will have two active IP addresses in the two segments.This is to reduce the probability of packet loss in a move toensure smooth handover from one segment to another. TheT-IWU is then responsible for issuing messages to the accesssegment to release the resources in the old segment. However,if the T-IWU is not able to send this message before the ses-sion is dropped, the responsibility passes to the N-IWU. Here,the N-IWU is required to check for any open connections andwhere necessary, it is required to inform the affected segmentto release its resources.

It should be noted that the T-IWU and N-IWU only triggerthe access segment to establish and release sessions. Essen-tially, all these procedures are segment specific proceduresand are not modified at all. This ensures that modification toeach access segment is minimal.

3.4. Impact of Mobile IP procedures on the access networkand handover

Fundamentally, the Mobile IP procedures are transparent tothe access network. The role of the access network is simplyto forward packets to the core network or the MN. Therefore,the operation of the access network should remain unaffectedby Mobile IP. However, the assignment of the CoA and therouting of packets from a Corresponding Node to the MN mayinadvertently affect the efficiency of the handover.

In section 2, three different scenarios for the MNs weredescribed. In the first and second scenarios, packets destinedto an MN have to be routed a number of times before reach-ing it. This increases the likelihood of packet loss as the CoAmay have changed before the packet reaches the T-IWU. Al-though an MN is capable of having two active CoAs duringhandover, the old link could be lost due to severe signal degra-dation, thus, making the MN unreachable via the old segment.The problem would be more serious if the IP network is con-gested. This problem, however, could be reduced with the im-plementation of route optimization in the T-IWU and the useof efficient admission control in the core network to guaran-tee end-to-end QoS. This is discussed in the next section. Theproblem with latency is not so obvious in the third scenario, asroute optimization is one of the fundamental components ofMIPv6. However, some problems with latency are anticipatedif the access segment is unable to support IPv6. This problemwill be resolved once the standardization of IP-based wirelessdata access networks is finalized by 3GPP and 3GPP2 [22].

4. QoS support

4.1. State-of-the-art

In this section, a further requirement for Personal Environ-ments Mobility, specifically how wireless and wired networktechnology can be made transparent to the user, is considered.


For the mobile network under consideration to be compati-ble with the outside world, QoS architectures considered inthe field of IP-based fixed networks (i.e., Integrated Servicesand/or Differentiated Services) have to be adopted with theminimum possible modifications.

However, quoting [19],

“both the Integrated Services (IntServ) architecture andthe Differentiated Services (DiffServ) architecture havesome critical elements in terms of their current defini-tion, which appear to be acting as deterrents to widespreaddeployment. . . There appears to be no single comprehen-sive service environment that possesses both service accu-racy and scaling properties”.

In fact:

1. The IntServ/RSVP paradigm is devised to establish reser-vations at each router along a new connection path, andprovide “hard” QoS guarantees. In this sense, it inher-its its basic ideas from ATM and the complexity of thetraffic control scheme is comparable. The cost of RSVPsoft state maintenance and of processing and signalingoverhead in the routers is significant and thus there arescalability problems. In addition, RSVP is not back-ward compatible with existing routers, which are not ca-pable of keeping track of the offered flows; hence, itis not easily and smoothly compatible with existing in-frastructures. Finally, RSVP is not appealing in mobil-ity frameworks, due to both its stateful nature and itsend-to-end operation. In fact: (i) IP mobility is man-aged without the need for maintaining per flow states;bindings are simply in charge of translating IP addresses,and do not take into account whether a number of ses-sions originates or terminates on the same mobile node;(ii) with RSVP, user migration cannot be managed in alocalized way – as proposed by several micromobilityframeworks, such as Regional Registration, HierarchicalMobile IP, Cellular IP. Rather, user migration manage-ment requires RSVP signaling information to travel backand forth to the destination node, and involves highlycomplex procedures. These considerations lead us to pur-sue Internet-wise admission control and resource reser-vation approaches, which are localized and stateless bytheir nature, and thus can be seamlessly applied to mo-bility frameworks.

2. The success of the DiffServ framework does not uniquelyreside in the fact that it is an approach devised to over-come the scalability limits of IntServ. As in the legacyInternet, the DiffServ network is oblivious of individ-ual flows. Each router merely implements a suite ofscheduling and buffering mechanisms, to provide dif-ferent aggregate service assurances to different trafficclasses. Packets are marked accordingly with a differentvalue of the Differentiated Services Code Point (DSCP)field in the IP packet header. By leaving untouched thebasic Internet principles, DiffServ provides supplemen-tary tools to move the problem of Internet traffic control

up to the definition of suitable pricing/service level agree-ments (SLAs) between peers. However, DiffServ lacksa standardized admission control scheme, and does notintrinsically solve the problem of controlling congestionin the Internet. Upon overload in a given service class,all flows in that class suffer a potentially harsh degrada-tion of service. This problem is highlighted in [2], whichpoints out that

“further refinement of the QoS architecture is requiredto integrate DiffServ network services into an end-to-end service delivery model with the associated task ofresource reservation”.

It is, thus, suggested to define an

“admission control function which can determinewhether to admit a service differentiated flow alongthe nominated network path” [19].

The focus of the proposed solution for the QoS supportis then the definition of an admission control function ap-plicable in a DiffServ framework, but potentially able to op-erate also in an end-to-end path (e.g., comprising IntServ sub-networks). In other words, the approach can be applied bothin a full DiffServ network and in a hybrid network consist-ing of (access) IntServ subnetworks and (core) DiffServ sub-networks. In that case, it is necessary to foresee a mappingbetween RSVP requests and DiffServ operation [2].

4.2. GRIP: Gauge & Gate Reservation with IndependentProbing

GRIP is a fully distributed and scalable Admission Controlscheme, intended to operate over an enhanced DiffServ In-ternet, but, in principle, compatible with the legacy Internet.GRIP builds upon the idea that admission control can be man-aged by pure end-to-end operation, involving only the newflow ingress router (or source host) and egress router (or des-tination host). In this respect, GRIP is related to the family ofdistributed schemes [5–7,9,14,15] recently proposed in the lit-erature under the denomination (following [9]) Endpoint Ad-mission Control (EAC). In addition, GRIP inherits the ideaof combining EAC with measurement based admission con-trol, which was first proposed in [1], where the SRP (ScalableReservation Protocol) was outlined. In GRIP, some essentialideas of SRP are inherited, but in the light of the brand newparadigm of EAC.

GRIP can be envisaged as a mechanism composed of thefollowing three components: (i) GRIP source node proto-col, (ii) GRIP destination node protocol, (iii) GRIP InternalRouter Decision Criterion. The source and destination nodeprotocols can be considered to be running at the ingress andegress nodes (for obvious security reasons) of possibly differ-ent ISPs. However, from a logical point of view, the sourceand destination node protocols are more naturally envisagedas running on the users’ terminals.


Figure 5. The “ideal” GRIP router operation.

4.3. GRIP source and destination node operation

The GRIP Source Node Protocol (SNP) is responsible for pro-viding a YES/NO admission control decision upon receivinga new connection set-up attempt. The simplest SNP opera-tion is the following. When an MN or source node requestsa connection with a destination node, the SNP starts a prob-ing phase, by injecting into the network, in principle, just oneprobing packet. Meanwhile, it activates a probing phase time-out, lasting for a reasonably short time (e.g., from tens tohundreds of ms). If no response is received from the desti-nation node before the timeout expiration, the SNP enforcesrejection of the connection set-up attempt. Otherwise, if afeedback packet is received, the connection is accepted, theprobing phase is terminated, and control is given back to theuser application which starts a Data Phase, simply consistingof the transmission of information packets (see figure 5(a)).

The simplest GRIP Destination Node Protocol (DNP) op-eration basically consists of monitoring the incoming packets,intercepting probing packets, reading their source address,and, for each incoming probing packet, simply relaying withthe transmission of a feedback packet, if the destination iswilling to accept the set-up request. In the basic operation de-scribed above, single probe and feedback packets are envis-aged, although redundancy (i.e., transmission of more probesand feedbacks) can be considered. Failed reception of probingpackets is used to discover, at the endpoints, that a congestioncondition occurs in the network. In such instances, the newadmission request would be rejected. This idea is extremelyclose to what the TCP congestion control technique does, buthere, it is used in the novel context of admission control.

The only mandatory requirement imposed on the GRIPmobile/source node and destination node operation is that theprobing packet (or packets) and the data packets are taggedwith different values of the DS codepoint field in the IP

packet header. The main reason for this requirement will beexplained in the following paragraphs. Besides, the packettagging, the simplest implementation, i.e., a single probingpacket and a single feedback packet, is compatible with theH.323 call set-up scheme using UDP, which encapsulates aH.225.0v2 call set-up PDU into a UDP packet. Note that theGRIP Internal Router Decision Criterion (DC) operation, de-scribed in what follows, is then perfectly compatible with ex-isting applications.

4.4. GRIP internal router decision criterion

It is assumed that probing and data packets can be distin-guished “on the fly” by internal network routers, since theyare tagged with different DS codepoint values in the IP packetheader. The “ideal” GRIP router operation is depicted in fig-ure 5(b). For convenience of presentation, it is assumed thatthe router handles only the GRIP controlled traffic. Other traf-fic classes (e.g., best-effort traffic) can be handled by meansof additional, separate (logical or physical) queues, eventuallywith lower priority.

At each router output port, GRIP implements two queues,one for data packets, i.e., belonging to flows that have alreadypassed an admission control test, and one for probing traffic.Packets are dispatched to the respective buffers according tothe probe/data DSCP tag. The router applies a forwardingpriority: probing packets are transmitted only when no datapackets are waiting in the buffer. This priority discipline isimplemented in most commercial routers. It ensures that theperformance of the accepted traffic is not affected by conges-tion occurring in the probing buffer.

Each ideal GRIP router implements an MBAC (Measure-ment Based Admission Control) module, which measuresthe AGGREGATE data traffic that it is handling. Based onthe running traffic measurements, the MBAC module imple-


ments a Decision Criterion (DC), which continuously drivesthe router to switch between two states: ACCEPT andREJECT. Several MBAC algorithms have been proposed inthe literature (see, e.g., [1,8,16, and references therein]) withthe capability to determine, only based on running measure-ments, whether, at a certain period of time, the consideredlink is able to accept new connections without QoS degra-dation. Based on its state, the DC controls the probing bufferserver. In particular, when it is in the ACCEPT state, the prob-ing queue accommodates probing packets, and serves themaccording to the described priority mechanism. Otherwise,when the DC switches to the REJECT state, the router dis-cards all the probing packets contained in the probing queue,and blocks all new arriving probing packets. In other words,the router acts as a gate for the probing flow, where the gate isopened or closed on the basis of the MBAC traffic estimates(hence the Gauge & Gate in the acronym GRIP).

4.5. GRIP rationale and performance

Note that the above DC need not be necessarily based on traf-fic measurements but could be independently defined by net-work operators. Two possible alternative solutions could be:

(i) implementing a separate probing queue with given ca-pacity and buffer space, to be tuned at runtime to suitablylimit accepted probing packets;

(ii) driving the GRIP gate by means of information generatedat lower layers, if the layers have QoS support capabili-ties, such as ATM or specific MAC layers.

In any case, the DC is completely independent of the ar-chitectural way of operation of GRIP. Each router is locallyin charge of deciding whether it can admit new flows, orit is congested. The notion of internal router congestion isnot standardized, and it is up to each specific router DC im-plementation to determine if and when, congestion arises.The internal router decision is summarized in the router state(ACCEPT versus REJECT). This state is not notified to theendpoints by means of explicit signaling information trans-mission. Instead, endpoints rely on probing packet losses(i.e., dropped by routers in the REJECT state) as an implicitsignaling pipe, of which the network remains unaware. Morespecifically, when the router is in the ACCEPT state, it ad-vertises that it can admit new connections. This informationis implicitly conveyed to the endpoints by allowing probingpackets to be served, and thus by leaving them traveling fur-ther toward their respective destinations. Conversely, whenthe router is in the REJECT state, no probing packets are for-warded. Dropping probing packets implies aborting all con-current set-up attempts of connections whose path crosses theconsidered router. Conversely, a connection is successfullyset up when all the routers crossed by a probing packet arefound in the ACCEPT state.

This driving principle provides a smooth migration pathconsisting of distributed admission control schemes of in-creasing complexity and effectiveness, which can indeed op-erate over a multi-provider and multi-vendor Internet.

4.6. Compatibility of GRIP with standard Per Hop Behaviors(PHBs)

Prior to addressing compatibility issues between the proposedsolution and IETF standards, the role of GRIP must be betterspecified.

If the application of GRIP is desired in a specific domain,then we are free to choose DS codepoints and to define PHBs.The PHB implementing GRIP should have two priority lev-els. The lower priority level should be such that the relevantdropping probability is equal to 100%, when the GATE is inthe REJECT state. A suitable DC drives the GATE. ApplyingGRIP to a specific domain simply means implementing thedescribed GRIP PHB described above.

On the other hand, if GRIP is considered as an Internet-wide solution, then, in principle, it would be necessary to de-fine and standardize the GRIP PHB. In the remainder of thissection, it will be shown that an already standardized PHB,the Assured Forwarding (AF) PHB [17], is semantically ca-pable of supporting GRIP.

The AF PHBs have been devised to provide different lev-els of forwarding assurances. Four AF PHB classes have beenstandardized, each composed of three drop levels. In whatfollows, the notation AFxj will be used to indicate packetmarks belonging to the AF class x, with drop level j (with1 � x � 4, 1 � j � 3). Conforming to [17], within aclass x, if i < j , the dropping probability of packets labeledAFxi is lower than that of packets labeled AFxj . The primaryaim of AF is to promote performance differentiation (in termsof packet drop), either among different traffic classes, e.g.,marked with different drop levels, as well as within the sametraffic class, e.g., marking traffic conforming to a policy spec-ification with a lower drop level than non-conforming traffic.However, low loss and low latency traffic support appears tobe out of the targets of the AF model. Nevertheless, it is ar-gued that the AF PHB definition contains all the necessarysemantic to support per-flow admission control.

In order to run GRIP over an AF PHB class, it is proposedto assign the data packets to the AF dropping level 1 of agiven class x, and the probing packets to the AF droppinglevel 2 of the same class x. The latter level has then the taskof notifying internal network congestion to the end points bymeans of packet dropping.

The DiffServ router output port operation supporting theAF PHB is depicted in figure 6. Packets routed to the rel-evant output are classified on the basis of their DSCP tagand dispatched to the relevant PHB handler. The AF PHBhandler for AFx class is composed of two mandatory sub-modules. A measurement module is devised to measure ag-gregate AFx1 traffic, i.e., traffic which has already passed theadmission control test, and performs smoothing and filteringfunctions on the run-time measurements. The measurementmodule does not interact with the forwarding of AFx1 pack-ets, i.e., these packets are forwarded to the FIFO (First In FirstOut) buffer placed at the output, regardless of the measure-ments taken. Rather, based on such measurements, this mod-ule triggers the AFx2 “gate”. In this sense, it keeps the gate


Figure 6. Router output port operation with AF PHB, used by GRIP.

open (ACCEPT state) when the module does not detect con-gestion, i.e., it estimates that the considered router output portis capable of admitting new flows without QoS degradation.Conversely, the measurement module keeps the gate closed(REJECT state), i.e., it enforces a 100% drop probability overAFx2 packets.

The GRIP operation is supported by using only the firsttwo AF drop levels of a given class x, namely AFx1 andAFx2. Although the AFx3 drop level of the same class x

appears in principle unnecessary, it is convenient to exploitthis additional drop preference level. A first possibility is todivide the probing packets into two classes, correspondingto two different data classes (the latter being handled in thesame data queue, AFx1). These two classes of probing pack-ets are transported with drop levels AFx2 and AFx3, respec-tively. This allows differentiating admission control betweendifferent kinds of traffic, thus permitting the establishment offairness or priority rules. In fact, sources with very differ-ent peak bit rates and/or call arrival rates could emit probingpackets with different DSCPs, i.e., AFx2 and AFx3. Thus,to avoid certain kinds of source “stealing” all the capacity,their requests could be accepted with different probabilities.A second possibility is to devote the AFx3 drop level to thetransport of traffic that:

(i) does not conform to the policy rules set by the operator(out of profile traffic);

(ii) has not passed the admission control test and agrees to betransported on a best-effort basis, while waiting for theeventual possibility of re-attempting the admission con-trol procedure.

Finally, it is stressed that compatibility of GRIP with theAF PHB must be intended in the sense that this PHB is ca-pable of supporting the proposed admission control scheme.However, it is still necessary to deploy suitable components,such as GRIP aware applications. It should also be under-lined that the suggested semantic mapping onto AFxi tags is

robust when considered over already deployed AF routers. Inother cases, assuming that a GRIP scheme is deployed only insome restricted domains and not in a whole DiffServ region,the relevant traffic, both data and probes, is supported as “nor-mal” AF PHB traffic in domains not explicitly implementingGRIP. Clearly, in such domains (without GRIP semantic), therelationship between AFx1 traffic measures and AFx2 dropprobability may be loose. Although this may translate in apossibly poor QoS support for AFx1 traffic in such domains,the operation of the EAC is not impaired by any means. Onthe contrary, the support of “legacy” (i.e., non GRIP) AF traf-fic in a domain where the AF PHB is used as a GRIP PHBrequires some suitable functions at the interface between thedomains. Further information can be found in [3].

5. Mobility and QoS testbed and demonstrator

5.1. Testbed and demonstrator architecture

A testbed has been set up to demonstrate the above men-tioned handover, mobility, and QoS protocols. The IP back-bone network is simulated by a “CoreBuilder” layer 3 switchin which the access segments are connected via ERs. AnISP LAN provides application servers such as video andaudio streamers as well as the ISP-HA functionality. AllERs, HA and T-IWU were set up using Linux worksta-tions.

In addition to this laboratory testbed, a pan-Europeandemonstration validated the concept in a wireless access net-work environment (see figure 7). This unique experiment in-volved ITALSAT Ka band transponders [10] with a groundearth station in Rome (Italy); a commercially operating GPRSnetwork in Vienna (Austria); and a W-LAN access point.A detailed description of the demonstration set-up can befound in [18].


Figure 7. T-IWU and segment terminals in the demonstration vehicle.

5.2. GRIP implementation

The GRIP algorithm has been implemented on the ERs.The set-up for the GRIP measurement is shown in figure 8.A number of sources for QoS traffic was generated in anMN attached to the T-IWU by means of a Poisson process.Furthermore, best effort traffic was generated by the users.IntServ was simulated by using dedicated channels in theaccess network, whereas DiffServ ran on the core network.GRIP was implemented as end-to-end admission control. To

demonstrate the function of GRIP/DiffServ, the core networkwas loaded additionally with background traffic generated bya commercial of the shelf (COTS) protocol tester. The ef-fect of the different traffic sources can be seen in the up-per part of figure 9. At the beginning of the experiment,only bulky and bursty background traffic in the core net-work plus best effort traffic were present. After 20 s, theQoS sources started with admission attempts by using GRIP.When the background traffic is low, almost all sources suc-ceed in call admission, QoS traffic dominates and can bemaintained with priority due to DiffServ operation. Addition-ally, the GRIP signaling traffic (i.e., probing packets) can beobserved. The lower part of figure 9 shows the number ofadmitted QoS sources as a function of time. The proposedsolution for the QoS support foresees that an operator canchoose target performance levels, the latter are mapped ontoa maximum number of acceptable connections and GRIP en-forces such values. In this experiment, the maximum num-ber of connections that can be accepted without a prede-fined QoS degradation is equal to 100. Figure 9 shows thatthe number of admitted sources never exceed the target (100sources). Additionally, a close agreement with theoretical in-vestigations of the GRIP algorithm [4] was observed. Themean number of accepted sources was equal to 97.5 (veryclose to the maximum). For simplicity, the sources emit con-stant bit rate traffic, but it was verified, by means of simula-tions, that the same conclusions apply also to variable bit ratesources.

Figure 8. Testbed for GRIP/DiffServ QoS traffic admission control.


Figure 9. GRIP measurement results.

5.3. Handover

Using the testbed, the concept for multi-segment mobility wasimplemented and proved to operate satisfactorily. Figure 10shows the graphical user interface of the handover managerrunning on the T-IWU. The attached MNs can be observed onthe left part of the screen with their IP home and CoA. On theright side of the screen, segment-specific data are displayed,such as the segment-specific IP address, IaddrSS, and the ad-dress of the GGSNs. The performance (in terms of delay andthroughput) of the segments, as reported from the N-IWU canalso be observed. Moreover, the connection of the terminalsto the different available segments can be monitored in thecenter of the screen. Handover was performed based on sev-eral trigger conditions:

(i) the segment availability as measured from the receivedpower of the radio modems,

(ii) segment performance,

(iii) segment pricing, and

(iv) a service profile chosen by the users.

An IP handover measurement is shown in figure 11. MP3traffic was streamed from the ISP MP3 streaming server toa client attached to the T-IWU. The traffic flow through thedifferent segments was observed using a COTS IP traffic an-alyzer. At the start of the experiment, the traffic was routedthrough the GPRS network. At t = 35 s, handover to theM-ESW satellite segment was initiated, and again back toGPRS at t = 55 s. It can be easily seen that all traffic


Figure 10. Handover manager of the T-IWU.

Figure 11. Measurement of IP traffic over two segments with handover.

is rerouted through the new segment after handover. Thereader should note that the visible crossover of the IP trafficat handover is only caused by the measurement window of 1s due to the traffic analyzer. There was no perceptible impacton the audio quality. Through IP tunneling, the applicationlayer could continue the ongoing session without realizingthe change of the access segment. The change of propaga-tion delay associated with terrestrial and satellite paths wascompensated by stream buffering of the application.

5.4. Demonstration results

Initial results of the field trials conducted in Vienna towardsthe end of 2001 are shown in figure 12. Here, in orderto investigate the influence of user preference, the alloca-

tion of two users with different user profiles (business –always-optimum segment based on throughput and value –always-cheapest segment) is shown in comparison to the seg-ment monitoring. Note: GPRS is always available duringthe recorded measurement period (100% time-share) and istherefore not illustrated in the figure. The availability of theM-ESW network is directly input (63.8% time-share), whilefor W-LAN (57.7% time-share) power measurements at theT-IWU are depicted. The W-LAN segment is assumed to bethe optimum and the best value, so both user types are tun-nelled through the W-LAN if available. When W-LAN is notavailable, the route taken by the traffic depends on the user’sprofile, that is the business user is routed via the M-ESW seg-ment, which offers the higher throughput rate and the valueuser via GPRS, which is the most economical.


Figure 12. Comparison of segment monitoring/availability and user alloca-tion.

Table 1IP round trip delays to the HA.

Delay [ms] Min. Max. Mean

W-LAN 62 161 69GPRS 2131 4100 3468ESW 640 904 747

The measured IP round trip delays to the HA are shownin table 1. It can be noted that the delay in the commerciallyoperated GPRS segment can exceed, by far, the delay of thesatellite segment.

6. Conclusions

This paper has presented a concept for mobility and QoS sup-port for Internet services in a multi-segment network compris-ing of terrestrial and satellite components. Several solutionshave been discussed to serve IPv4 and IPv6 MNs connectedto a mobile local LAN by means of binding updates to an ISP-HA. DHCP-only clients are also supported. Special handovertechniques have been devised to enable seamless connectionsduring an inter-segment handover and the concept of fuzzylogic has been suggested for the handover decision and initi-ation phases.

QoS is supported by a new admission control proto-col (GRIP). Here, it should be stressed that GRIP is not

a new reservation protocol for the Internet but rather anovel reservation paradigm that allows independent end-point software developers and core router producers to inter-operate without explicit protocol agreements. In a cer-tain sense, GRIP extends the principle at the basis ofTCP to the completely different and novel problem ofproviding explicit per-flow admission control over state-less Internet architectures. Finally, GRIP is envisagedas an Internet-wide solution. It appears comparativelystraightforward to devise interworking functions betweenGRIP and IntServ islands. Certainly, most of the con-sideration carried out in [2] applies equally to the pre-sented scheme, since interworking between IntServ andDiffServ is similar to interworking between IntServ andGRIP.

In order to verify the concepts proposed in this paper, alaboratory demonstrator has been assembled. Subsequently,trials have shown the viability of providing mobile Internetaccess over different access segments and with an acceptableQoS.

Acknowledgement

The background of this work is the IST project SUITED(Multi-Segment System For Broadband Ubiquitous AccessTo Internet Services And Demonstrator) [13], sponsored bythe European Union. The effort of the project’s partners isgratefully acknowledged. This paper, however, does not im-plicitly represent the opinion of the project partners.

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[5] G. Bianchi, A. Capone and C. Petrioli, Throughput analysis of end-to-end measurement based admission control in IP, in: Proceedings of19th Annual Joint Conference of the IEEE Computer and Communica-tions Societies (INFOCOM 2000), Vol. 3, Tel Aviv, Israel (March 2000)pp. 1461–1470.

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(March 1996) 82–90.[25] J.D. Solomon, Mobile IP – The Internet Unplugged (Prentice Hall, Up-

per Saddle River, NJ, 1998).

Giuseppe Bianchi received the Laurea degree inelectronic engineering from Polytechnic of Milano,Italy, in 1990, and a specialization degree in infor-mation technology from Cefriel, Milano, in 1991.He spent 1992 as a Visiting Researcher at the Wash-ington University of St. Louis, MO, and 1997 as aVisiting Professor at Columbia University, NY. Hehas been an Assistant Professor at Polytechnic of Mi-lano from 1993 to 1998. He is currently an AssociateProfessor at the University of Palermo. His research

interests include wireless access protocols and network architectures, QoSsupport in both wireless and wired IP networks, and performance evaluation.E-mail: [email protected]

Nicola Blefari-Melazzi received his “Laurea” de-gree in electronic engineering in 1989, magna cumlaude, and earned the “Dottore di Ricerca” (PhD)in information and communication engineering in1994, both at the University of Roma La Sapienza,Italy. Since 1998 he is an Associate Professor at theUniversity of Perugia. In January 2002 he won anItalian national call for Full Professorship and waspromoted at the rank of Full Professor. Dr. Blefari-Melazzi has been involved in various consulting ac-

tivities and research projects, including standardization and performanceevaluation work. His research projects have been funded or co-funded bythe Italian Public Education Ministry, the Italian National Research Coun-cil, by industries and European organizations and programs. Dr. Blefari-Melazzi served as a referee, TPC member, session chair and guest-editor toIEEE conferences and journals. His research interests focus on modellingand control of broadband integrated networks, multimedia traffic modelling,architectures and protocols for wireless LANs, satellite networks, queuingsystems, mobile and personal communications, Quality of Service guaran-tees and real time services support in the Internet.E-mail: [email protected]

Pauline Chan received an electronic systems and in-formation engineering degree from Sheffield HallamUniversity, UK, in 1999, following the award of aJPA Scholarship from the Malaysian Government.She is currently in the final year of her Ph.D. at theUniversity of Bradford, UK, where she is research-ing into the design of satellite network procedures forheterogeneous packet-oriented environments. Sheparticipates in the EC’s IST SUITED project and waspreviously involved in the COST 253 Action.

E-mail: [email protected]

Matthias Holzbock received the Diplom-Ingenieur(Dipl.-Ing. Univ.) degree in electrical engineeringand information technology in 1996 from the Tech-nical University of Munich, Germany. From 1995 to1996 he was with the Optical Networks Group of theGerman Aerospace Research Establishment (DLR)as a diploma student and research scientist. SinceSeptember 1996 he is with the Institute of Commu-nications and Navigation of the German AerospaceCenter (DLR) as a research scientist and project

manager. Recently, he is also managing director of TriaGnoSys GmbH. Heauthored and co-authored more than 40 publications and several patent ap-plications.E-mail: [email protected]


Y. Fun Hu received a 1st Class B.Sc. degree in math-ematical sciences and a Ph.D. degree in informa-tion systems engineering, both from the Universityof Bradford, UK. In 1990, she joined the Universityof Leeds, UK, as a Research Fellow after workingtwo years in the satellite communications industry.In 1994, she was appointed as a lecturer at the Uni-versity of Bradford. She is now a senior lecturer inthe same University. Dr. Hu has been involved inseveral defense, European Space Agency and Euro-

pean Union funded projects, including SAINT, SINUS, SUMO, VANTAGE,SECOMS and SUITED. She is a co-author of the book Mobile Satellite Com-munications Networks (Wiley). Dr. Hu was a national delegate to the EUCOST 253 and COST 256 Actions. She is now a national delegate to theEU COST 272 Action and a member of the Executive Committee of the IEESatellite Systems & Applications Professional Network.E-mail: [email protected]

Axel Jahn received the Ph.D. and the Diplom-Ingenieur degree in electrical enginneering in 1999and 1990 from the Fern University at Hagen and Uni-versity Fridericana at Karlsruhe, Germany, respec-tively. Since September 1990 he is with the Insti-tute of Communications and Navigation of the Ger-man Aerospace Center (DLR) as a research scien-tist, project manager and group leader. Recently, heis also a managing director of TriaGnoSys GmbH.Dr. Jahn is a lecturer at the Carl-Cranz-Gesellschaft

for Mobile Satellite Communications since 1993 and in 1995/1996 he wasa lecturer at Technical University at Ilmenau. Dr. Jahn is a Senior Memberof the IEEE. He received the Best Paper Award of ITG conference “MobileCommunications” in 1993 and is listed in “Who is Who in Science”. Heauthored and co-authored more than 90 publications, with two scientific text-books and 13 scientific journal papers. He serves as a reviewer for manyjournals and magazines, and as a session chairman on several conferences.E-mail: [email protected]

Ray E. Sheriff graduated from the University ofLeeds, UK, in 1986. He then spent the next four andhalf years in the satellite communications industry.In 1991, he took up a Lectureship with the Universityof Bradford, UK, where he was awarded a Ph.D. in1995 and appointed Reader in Mobile Communica-tions in 2000. During his time at Bradford, Dr. Sher-iff has participated in a number of influential satellitecommunications projects, principally funded underthe EC’s R&D programmes. He is a co-author of the

book Mobile Satellite Communications Networks (Wiley). Dr. Sheriff is anExecutive Committee member of the IEE’s Satellite Systems & ApplicationsProfessional Network and a UK Delegate on the Management Committee ofthe COST 272 Action.E-mail: [email protected]

design and validation of qos aware mobile internet access procedures for heterogeneous networks

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