Design of Soft Handover Protocol forMobile ATM Networks

Ahmed S. AliFaculty of Engineering,Helwan University

Cairo, EgyptAh_salah@softhome.net

Ihab A. ALiFaculty of Engineering,Helwan University

Cairo, Egyptihabaly@ieee.org

Ibrahim 1. IbrahimFaculty of Engineering,Helwan University

Cairo, EgyptIb-lsmail1sof1home.net

Abd Elrahman H. ElsawyFaculty of Engincerin,Helwan University

Cairo, EgyptAelsa%y~segypLcou

Abstract: Handover, or handoff control process is required todynamically support active connections during the migration of aMobile Terminal (MT) from one Access Point (AP) to another.This feature is critical for mobile multimedia services delivered toa Wireless ATM-WATM terminal, as well as for efficient supportof current Personal Communication Systems (PCS)/CellularSystem on an ATM ifrastructure. Handover requires networksignaling to maintaiu the communication link and may result incell loss due to cell misrouting and/or misordering. Several hardhandover schemes were proposed, these schemes have somepotential problems such as the ping-pong phenomenon, thepossibility of information loss and poor QoS. This paper presentsa soft handover solution for Mobile ATM networks to overcomethese limitations. Four soft handover schemes are presented basedon different configurations of the mobile ATM networks andaccordingly the way of achieving the matching of the ATM cellsfor both the upstream and downstream traffic.

Keywords: Wireless ATM Networks, Mobile Communications,Handover, Quality of Service.

1. INTRODUCTION

According to the tremendous growth of the wirelesscommunication systems, and since ATM is the expectedprotocol for future backbone networks, it is natural to extendthe ATM protocol to the wireless environrrent. The mostpromising long-term solution to integrate Broadband IntegratedServices Digital Network (B-ISDN) multimedia services andwireless networks is the so-termed Wireless ATM (WATM)networks. Therefore, ATM protocol must be augmented toprovide the functions of the wireless environment such as thehandover function, which is the most critical feature of wirelessnetworks.

The handover is the procedure by which a user's radio link itransferred from one radio port to another through the networkwithout an interruption of the user connection. Future PersonalCommunication Network (PCN) will employ a pico-cellmobile network structure, operating at very high frequencyranges. This has some severe implications for the handoverprocess in mobile ATM networks. First of all, mobileconnections will experience handover very frequently. Due tothe small cell size, inter handover interval is expected to be inthe order of seconds, or even less, rather than minutes, callingfor an optimized handover procedure with a minimum ofrouting decisions required and centralized call processing

involved during handover. Secon radicharacteristics and real time requiremnts of dy uitvemultimedia services will allow only for miniml hdoverlatency, i.e. a fast setup procedure for handover conetionswill be needed [61. This illustrates that e need for a fast,effective and efficient handover protocol is not only highlydesirable but essential for the success of mobile ATMnetworks.

In general, the handover process involves migrating multipleVirtual Circuits (VCs) associated with a terminal fon AP toanother, while maintainig routing oplity and QoS to thpossible extent. Also, during handover, cells may not only belost but also be out of sequence, this depends on the handovermechanisms. Therefore, designing an optimal handovermechanism that enables the network being protectod agInstcell loss, cell duplication, and loss of cell sequence with aguaranteed level of QoS is an 'mportant iue in mobile ATMthat needs further investigation. The optimal algrithm shouldalso have low delay and delay variation. It must alO ardooffbetween the network resources utilized by the call andsignaling load incurred on the network.

Several hard handover schemes were proposed in th literature[6], [14], [15], [16], and [17]. In [1] and [2] we presented aneffective lossless hard bandover scheme for both lntra andInter-handover. However, The hard handover schemes maysuffer from some problems such as the ping-pongphenomenon, the possibility of infornmion loss and poor QoSFurthermore, with hard handover, the communication link tothe new AP can be set up only after the forwarding of thebuffered cells in the old AP in order to avoid cell misroutingand nmsordering. Due to the mismatch between hightransmission rate over wired links and low transmission rate(ver wireless links, the number of the buffered cells may belarge, which corresponds to a long handover delay and mayresult in cell loss. Therefore, soft handover is intduced tosolve these problems. Because of the "nmak-before-break"switching function of radio links, soft handover are inherentlyseamless, and reduces/eliminates the undesirable "pingpong"phenomenon of back-and-forth handover between two adjacentcells in conventional hard handover. Also, soft handover offcrsless load on the network from handover signaling ad overheadas well as smoother user conununications without the "'clicks'typical of hard handover when transmissions are stopped

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momentarily during handovers. On the other hand, softhandover poses the highest requirements on the radiotechnology, because it requires the mobile terminal to be able Ato communicate concurrently with two APs. Therefore, in the s5overlapping boundary region, it enables dynamic selection ofthe best radio path. Provided that the overlapping region issufficiently large and both APs can maintain a sufficientlystrong signal in this region, this ensures enhanced QoS for theconnection as well as handover reliability.One disadvantage of the soft handover is, the additionalnetwork resources that are used during a soft handover. Theseresources thus become unavailable for use elsewhere. Anotherdisadvantage is that soft hand is more complex to implemeit.It is clearly difficult to conclude that one type of handover isbetter in absolute terms. System designers have to determinewhether the advantages outweigh the disadvantages for theirparticular systems. Actually, Mobile ATM does not supportsoft handover, i.e., it does not permit an MT having more thanone radio connection at the same time. So far, no Mobile ATMhandover mechanisms have been proposed which apply softhandover techniques. Therefore, in this chapter we present asoft handover solution for the mobile ATM networks. Four softhandover schemes are presented based on differentconfigurations of the mobile ATM networks and accordinglythe way of achieving the matching of the ATM cells for boththe upstream and downstream traffic. All of the four schemesensure lossless and in-sequence delivery of user data. In eachof these schemes, we propose a certain modification in both theATM network and the MT. However, these modifications leadsto different methods of ATM cell matching for both theupstream and downstream traffic. One of the advantages of ourproposed schemes is that, they can be used for WATMnetworks to support soft handover and also be used for theother mobile communication systems (such as CDMA system)if it needs to use the ATM as a backbone network.

II. MOBILE ATM NETWORK ARCHITECTURE

Mobile ATM is mainly considered as an access to an ATMnetwork issue. The mobile ATM framework comprises MobileTerminals (MTs), base stations or Access Points (APs) andATM switches (ATM nodes). The AP acts as inteiface betweenthe MTs and the ATM backbone network. Each APimplements physical transport, multiple access, data linkcontrol and basic radio resource management. At the border ofthe ATM network, special Mobile ATM Switches (MAS)supporting end-system mobility are being introduced. Theyhave the necessary extensions to signaling and controlprotocols to provide means for connection handover andmobility management. ATM switches and APs belong to thefixed network segment, whereas MTs can directlycommunicate only with RAPs, which provide the interfacebetween the wired and wireless portions of the network. TheATM backbone network is responsible for providing theswitching and transmission functions, as well as interworkingfunctions with other systems. Fig I shows an example of aMobile ATM network configuration.

MAS3

Fig. 1. Mobile ATM network configuration

111. SOFT HANDOVER SCHEMES FOR MOBILE NETWORKS

The handover procedure in the network is performed to ensurethe continuity of a mobile connection and to minimizeinterference to the user in the coverage area of neighboringcells. Each handover includes actions at two levels: radio andnetwork. The radio level handover is the actual transfer of theradio connection between two APs; the network level handoveris to support the radio level handover by performing cellbuffering and rerouting. As mentioned in the previous section,soft handover is a process of establishing a link with a targetAP before breaking the link with a serving AP. Two basicfeatures must be taken into account during a soft handoverprocess: Matching of 'the two communication paths over thedifferent APs and dynamic path selection.In Mobile ATM system, soft handover macro diversity calls forATM cell level synchronization of two communication pathsbetween MT and MAS. Each path carries the same user datastream, but displays possibly different delay propertics.Therefore, to be able to gain from intelligently combining thletwo diversity paths, the cell streams have to be matched. Oncethe state of matching has been reached, it is possible todynamically select the best path, be it on a per-cell-streamsegment or even on a per cell basis. Matching and dynamicpath selection takes place in the MAS for the uplink diredionand in the MT for the downlink direction.In the following sections we provide four different scenariosfor matching the two communication paths over the differentAPs. Whereas this matching is used to protect cell sequenceand prevent cell duplication in the hard handover case.

Scheme A

Scheme A, presents a solution that is based on theenhancement of the ATM switches at the border of the ATMnetwork as well as the MT to handle the soft handover. Usingthe ability of the ATM switch to add and remove new paths,we can perform the soft handover mechanism by modifying the

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border ATM switch to contain a soft handover interface part asshown in fig. 2.

Fig. 2. The soft handover interface part of the ATM switch.

Connection identifier resolving stage: This stage checks for thesource and destination addresses to detect the cells which havethe same source and destination addresses and forward thesecells to the delay equalization stage.Delay equalization stage: This stage is used for canceling thedifference in delay for the various APs.Correlation stage: This stage is used to compare the cellscoming from the delay equalization stage and deliver thematched cells to the switched combining stage.Switched combining stage: This stage combines the matchedcells by using a switch-and-stay strategy.

Fig. 3. The soft handover interface part of the MT.

Also, we consider that the MT is a part of a B-ISDN system bytransmitting information in the form of ATM cells. The MTunit will need to be modified to support the soft handover, fig3 shows the soft handover interface part for the MT.The handover protocol of scheme A (and also the otherproposed schemes) is designed to perform both Intra-switchand Inter-switch handovers. In Intra-switch handover, the MTis moving to a destination AP that is connected to the sameATM switch to which the current AP is connected. The

complexity of the local handover procedure is limited as onlyone switch is involved and must switch the mcbile connectionfrom the port attached to the current AP to the port attached tothe new AP. In inter-switch handover, the MT moves to adestination AP that is.connected to a switch different from theoriginal switch. Consequently, the connection path over thewired network must be modified to reach the destinationswitch. In either case the connection interruption during thehandover may be significant and may produce a considerabletraffic disruption.Definitely, we can use any connection rerouting scheme in ourproposed algorithm but we choose the incrementalreestablishment (connection modification)of the connectionpath. This scheme requires only the establishment of a newpartial path which connects the MT to a portion of the originalconnection path, thus allowing VCs to be partly reused.The detailed analysis of the procedures for the proposed softhandover algorithm is described as follows:

Intra-switch soft handover.

Consider a handover situation where an MT is moving from thegeographical service area of a base station and entering theservice area of another base station belonging to the sameswitch. The two base stations are connected to the wirednetwork through different access points AP,, and AP2. Thehandover protocol starts when the radio link between the MTand AP,, becomes weak, radio hints show that the MT needs toinitiate a handover. During the normal operation, the MTmaintains a list of APs it can hear. The list is prioritized by thesignal strength of each AP. The MT requests a handbver bysending a Handover Request (HOR) message (1) to AP, asshown in fig. 4. The message is further forwarded to its localswitch MAS, (The message includes a list of the connectionsthat should be handed over and a list of possible new radiolinks). The MASI looks at its table containing all the APs underits control and attempts to establish new connections throughnew APs. The switch sends a Radio Resource Status Enquiry(RRSE) message (2) to all the APs on the list (Or some of themdepending on the status of the network) to find out which APscan support the connections. The APs check their resourcesituation and answer the enquiry with a Radio Resource StatusConfirm (RRSC) message (3). The switch selects the best AP(In our example AP2) which is able to provide QoS guaranteesfor all the connections of the call (e.g the radio bandwidthnecessary to continue the wireless connection through AP2).The Handover will be made and a Radio Rcsourcc Allocation(RRAL) message (4) will be sent to AP2. In this solution,resource allocation is done afler the switch has made thedecision. MASI can now inform the MT via a HandoverConfirm (HOC) message (5) about the right Radio Terminal(RT) to choose. MAS, sends a Start Soft Handover (SSHO)message (6) to AP, and temporarily stops data transmission.When AP, finishes delivering its buffered data to the MT, itreplies to MAS, by a Soft Handover Ready (SHOR) message(7). The rationale behind this postpone of the SHOR message

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Fig. 4 The sequence chart for Intra-switcli soft liandover

is to minimize the time diverisity between th( ~-aY paths.Immediately, MASI continues data transmission to AP, andAP2 (and all APs involved in the lhandover process) precededby Start Data Flow Downlink (SDFr,&,,,) message (8). Uponreceiving the SDFd0w,, the APs start to send the downstreamdata to the MT at the same time and the upstream data toMASI. The final point that must be addressed to complete thesoft handover procedure is how the synchronization betweenthe coming ATM cells from the different APs to the switch(upstream traffic) and the ATM cells coming from the differentAPs to the MT (downstreamn traffic).

a-Upstream traffic (Forward link direction)When the switch receives the ATM cells from the diffet.-ntAPs, it first checks them for error. Then the connectionidentifier r;solving stage checks for the connection identifiers.Since, these cells have the same VPI/VCI, it forward these cellsto the delay equalization stage. The delay equalization stagecancels the difference in delay between cells and forwar-' 4emto the correlator. The correlator stage compares the -lls anddeliver them to tl - switclhed combining stage if ':aey are thesame. If they are different it sends the cell of die original APand buffer the cells of the other APs. For the next cell, the cellcomeing from thle original AP is firstly compared with thebuffered cellr, if matching exists, all matched cells are passedto the ne*. stage. Otherwise, the coming cells are comparedand this process will continue until the matclhing betwecn the

cells from the different APs is achieved. Afler achieving thematching, the switch selects the best cell unte all A?s aredropped except one which means that the handoveb iscompleted.

b- Downstreamn traffic (Reverse link directionsWlhen all APs involved in the lhandover process receives tleSDFd,,,, message, they start sending the same A2?M cells at thesame timne to the MT. As a result of the time delay diversityfrom the APs to the MT, the ATM cells arrives i tne MT atdifferent times. Therefore, the MT .xecute a rcorrelation processes until the syncnronization is &:4.evez..Before achiieving the cell synchronizatic l, the MT passes thecell of the original AP (if it exists) and L itfferes the othercells.The new received cell from the original AP is compared withthe cells received from the others starting with the bufferedcells. After achlieving thbt ..ynchronization the MT combines thereceived ATM cells untf! all APs are dropped (from thehandover process) except one

Schemle B

In this scheme, we assume that the ATM cells are prc3ablymodified for radio transmission. Fig 5 shows an exampleformat for suggested WATM data cell.While the details may vary from one implementation toanother, the main concept is to build on facilities currentl'provided by ATM keeping additional syntax to a minimum. Ir.our proposed algorithm, the WATM cell contain one, anadditional 1-byte cell sequence. Fig 5 shows the ATM cellpror,agat;"n scenario in WATM network.

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Fig. 5. WATM cell fomiat

Using this modification in the standard ATM cell, the problemof ATM cell synchronization will be significantly reduced forthe downstream traffic. Accordingly, the complexity of the softhandover interface part for the MT will also be reduced In thisalgorithm we replace the correlation stage by a cell sequenceidentification stage while all other stages remain the same. Thecell sequence identification stage is used to identify the cellnumber of the ATM cell coming from all paths and detects thematched cells and then it delivers them to the comlbiner.Therefore, using this method we can rapidly achieve the cellsynchronization which gives more guarantee that there will beno cell loss, no cell duplication and surly sequence delivery.In this scheme, the same sequence flow of messages is thesame as described in scheme A. We recall that the advantage ofscheme B is tat it has less complexity than scheme A.

Scheme C

As we seen from the two previous schemes, the greater numberof network entities involved in the case of Inter-switchhandover, and so it is considered the most complex case of thesoft handover. Therefore, in scheme C, we propose a soft

Fig. 6. The mobile ATM network model for scheme C

I-i L-dovei protocol that execute the Inter-switch handover ontwso steps greatly simplifying its operation. In order to describethe steps of the scheme we first consider the rrtworkconfiguration shown in fig. 6. In this configuLration, we use anoveilap links to connect each pair of APs at the edge of twoaCIjacenit swol h Ilisters (the switch cluster is the group of cellsserved by the sautie switch, MAS) with the MAS's servingtheopposing cell. This is illustrated in fig 6, where, adjacent cells1, 2 are the boundary of two neighboring cell clusters. Originallinks LI and L1, connect MASI and MAS2 to AP, and AP,respecti'ely, while I.,, and Lo2 interconnect AP? and AP, toMASI andc iN'AS- Irspectively.In this schetmice, w'e use the overlap links to perforn softhandover in the lnter-switch handover case through two steps.Firstly, we perform the handover procedure as Intra-switchhandover to get its advantages (less buffer requiremiits, lowhandover delay and fast achieving of the cell synchronization).Then we complete the new handover connection to the newswitch ovei the network.

Scheine D

This schemiie is introduced to support the soft handover forcurrent PCN/cellular systems on an ATM infrastructure (i.e.,for non-ATM Nvireless access mobile networks which use theATM protocol as a backbone network). In general, the mobileATM network can be used to support both broadband WATMaccess as well as legacy wireless systemn in an integratedmanner. In a typical implementation, various radio cards can beplugged into generic ATM base stations which convertapplicable wiricless protocols to the common mobile ATMprotocol. I he standard ATM can be used to construct a genericinfi-astructure for existing PCS, cellular, and wireless datasystem in addition to end-to-end WATM services. In thisconfiguration, Al M base stations convert wireless accessprotocols such as Global System'i for Mobile communication

Fig 7. The structure of the proposed gateway

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(GSM), IS-136, Code Division Multiple Access (CDMA), orIEEE 802.11 to a common ATM format which supportsnetwork level mobility requirements in a generic way (WATMnet]. In fact, in order to co-operate the two technologies (thecurrent PCN/cellular systems and the ATM protocol) there is aneed for an interworking device that understands both systems,this device most likely as a gateway. In our proposedalgorithm, the gateway is placed at the edge of the ATM corenetwork (i.e., at the MAS) that is mainly responsible for end,to-end transmission. Such an ATM-based mobile infrastructureprovides important service integration and cost/performanceadvantages over existing mobile networks, while facilitatingsmooth migration to broad-band WATM services. Theproposed gateway in our algorithm will be responsible for bothinterworking between the existing PCN system and the ATMnetwork, and also for achieving the synchronization to supportthe soft handover. The structure of the proposed gateway isshown in fig. 7.

For the downstream traffic, the ATM cell / system packetconverter receives the ATM cells from the MAS andpacketizes them into system packets , while the systempacket/ATM cell converter do the inverse process for theupstream-traffic. Using these converters in both directions, theproblem of packet synchronization will be more simpler than inthe previous proposed algorithms. Accordingly, the complexityof the soft handover interface part for the MT will also bereduced. In this scheme we replace the correlation stage by apacket sequence identification stage while the all other stagesare remaining the same. The packet sequence identificationstage in each the gateway and the MT is used to identifies thepacket number of the packet coming from all paths and detectsthe matched packets and then it delivers them to the combiner.Therefore, using this method we can rapidly achieve the packetmatching which gives more guarantee that there will be nopacket loss, no packet duplication and surly sequence delivery.

IV. CONCLUSION

In this paper, we present a soft handover solutions for themobile ATM networks. Four soft handover schemes arepresented based on different configurations of the mobile ATMnetworks and accordingly the way of achieving the matching ofthe ATM cells for both the upstream and downstream traffic.All of the four schemes ensure lossless and in-sequencedelivery of user data. In each of these schemes, we propose acertain modification in both the ATM netwcrk and the MT tosupport the soft handover. In the first scheme, we proposed asolution that is based on the enhancement of the ATM switchesat the border of the ATM network as well as the MT to handlethe soft handover. The soft handover interface part fir both theswitch and the MT are presented. The handover signalingprocedures for the Intra-switch handover is also introduced. Inthe second scheme, we proposed a solution that is based onmodification of the ATM cell for radio transmission, where theproposed WATM cell contain only an additional 1-byte cellsequence field. This algorithm has the advantage that it has lesscomplexity than the previous algorithm. The third scheme

presents a simplified handover procedure for the Interswitchhandover which is considered the most complex case. Thesimplification is achieved by using overlap links to connecteach pair of APs at the edge of two adjacent switch clusterswith the MAS's serving the opposing cell. The advantage ofthis algorithm is that it needs less buffer requirements, presentslow handover delay and fast achieving of the cell matching.This is at the expense of the increased usage of networkresources. The last scheme presents a soft handover protocolfor current PCS/cellular systems on an ATM infastructure. Inthis scheme, we use a gateway placed at edge of the ATM corenetwork. The proposed gateway in our scheme is responsiblefor both interworking between the existing PCS system and theATM network, and also for achieving the cell matching tosupport the soft handover.

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