A framework for the transmission of streaming media to mobile devices

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<ul><li><p>INTERNATIONAL JOURNAL OF NETWORK MANAGEMENTInt. J. Network Mgmt 2002; 12: 4159 (DOI: 10.1002/nem.420)</p><p>A framework for the transmission of streaming mediato mobile devices</p><p>By Kevin Curran and Gerard Parr</p><p>One interesting problem is the delay imposed upon mobile receivers whenswitching between wireless cells. We provide a solution to this in the formof an extension of Mobile IPs handoff algorithm. Our solution involvesthe exploitation of mobility prediction to predict a mobile terminalsfuture location based on its previous history (i.e. the last cell that it hasbeen in) and for the media stream to be already present and cached by nextcells base station ready for receiving by the mobile device. Copyright 2002 John Wiley &amp; Sons, Ltd.</p><p>Introduction</p><p>M obile IP specifies enhancements thatallow transparent routing of IP data-grams to mobile nodes in the Internet.In Mobile IP27 a Mobile Host always has a HomeAgent (e.g. the router of the subnetwork the hostusually is attached to). This Home Agent keepstrack of the current point of attachment of themobile host. Whenever the mobile host changesthe network it is connected to, it has to register anew care-of address (COA) with the Home Agent.This association of the Mobile Hosts home addressand the current care-of address is called bind-ing. The care-of address can either be the addressof a Foreign Agent (e.g. a wireless base stationnode) that has agreed to provide services for theMobile Host or the new IP address of the MobileHost itself. A care-of address can be acquiredeither through stateless or stateful address auto-configuration. Traffic to the Mobile Host is always</p><p>passed through the Home Agent, and then tun-nelled to the care-of address and in the case of aForeign Agent care-of address; forwarded to theMobile Host by the Foreign Agent. Out-going traf-fic from the Mobile Host does not need to gothrough the Home Agent but the host can directlycommunicate with Correspondent Hosts. By usinga Home Agent as an intermediary, CorrespondentHosts do not need to know the Mobile IP protocolor the current location of the Mobile Host. Theforwarding of packets to the current address of theMobile Host is transparent for other hosts.</p><p>The routing tables typically maintain the next-hop (outbound interface) information for eachdestination IP address, according to the numberof networks to which that IP address is connected.The network number is derived from the IP addressby masking off some of the low-order bits. Thus,the IP address typically carries with it informationthat specifies the IP nodes point of attachment.To maintain existing transport-layer connectionsas the mobile node moves from place to place,</p><p>Kevin Curran is a Lecturer at the University of Ulster, Magee College. His research is focused on the field of distributed computing especiallyemerging trends within wireless ad-hoc networks, distributed objects, dynamic protocol stacks, multimedia transport protocols and mobilesystems. He can be contacted at kj.curran@ulst.ac.uk.</p><p>Gerard Parr is a Professor of Telecommunications at the University of Ulster, Coleraine. His research interests include ISDN Systems andStandards, Queueing Systems, Asynchronous Transfer Mode Switch Fabric and Communications networks protocols.</p><p>Correspondence to: Kevin Curran, Telecommunications and Distributed Systems Research Group, Northern Ireland Knowledge Engineering</p><p>Laboratory, University of Ulster, Magee Campus, Northern Ireland, BT48 7JL, UK.E-mail: kj.curran@ulst.ac.uk</p><p>Copyright 2002 John Wiley &amp; Sons, Ltd.</p></li><li><p>42 K. CURRAN AND G. PARR</p><p>it must keep its IP address the same. In TCP(which accounts for the overwhelming majorityof Internet connections), connections are indexedby a quadruplet that contains the IP addressesand port numbers of both connection endpoints.Changing any of these four numbers will cause theconnection to be disrupted and lost. On the otherhand, correct delivery of packets to the mobilenodes current point of attachment depends onthe network number contained within the mobilenodes IP address, which changes at new pointsof attachment. To change the routing requires anew IP address associated with the new point ofattachment.</p><p>In Mobile IP the home agent redirects packetsfrom the home network to the care-of address byconstructing a new IP header that contains themobile nodes care-of address as the destinationIP address. This new header then shields orencapsulates the original packet, causing themobile nodes home address to have no effect onthe encapsulated packets routing until it arrivesat the care-of address. Such encapsulation is alsocalled tunneling, which suggests that the packetburrows through the Internet, bypassing the usualeffects of IP routing.</p><p>By using this architecture, a Mobile Host canroam between Foreign Agents and its HomeAgent. When the Mobile Host leaves the servicearea of its current Foreign Agent and registerswith a new Foreign Agent, the Home Agent hasto be informed about the change of address.This procedure is called handoff. During sucha handoff, it is possible that the Mobile Hostloses connectivity for a short period of time. Toprovide smooth handoffs and speed up the handoffprocess, the use of several care-of addresses ispossible where wireless service areas overlap.However, only one of those addresses can beregistered with the Home Agent (primary care-ofaddress).</p><p>The Mobile IP architecture is well suited forMobile Hosts that change their point of attachmentonly over relatively large time intervals. When fast-moving Mobile Hosts are forced to perform a largenumber of handoffs per time interval, registeringa care-of address with the Home Agent causes toomuch over-head and a too high delay, which inturn results in decreased protocol performance.Several approaches to solve this problem andto provide a more local, hierarchical form of</p><p>mobility management are discussed in references30 and 32.</p><p>Mobile IP Handoff</p><p>In Mobile IP all base stations advertise theirpresence by sending beacon messages at a pre-configurable time interval. Mobile Nodes store theaddresses of the base stations within range in alist. When no beacon message of a registered basestation is received for a certain amount of time, thelist entry times out and is removed. Mobile Nodesthat have to perform a handoff because they leftthe service range of their current Foreign Agentchose a base station from the list as their new For-eign Agent. If the list does not contain any entries,the Mobile Node sends an Agent Solicitation Mes-sage. Base stations that receive this message haveto send an advertisement, which then allows theMobile Node to register with them. The handoffis initiated with a Registration Request from theMobile Node. The base station then forwards therequest to the Home Agent of the Mobile Host. TheHome Agent updates the care-of-address (COA) ofthe Mobile Host and installs a so-called encapsula-tor to tunnel IP packets to the mobile host via thebase station. The Home Agent then sends a Regis-tration Reply Message to the base station and thebase station in turn informs the Mobile Node thatthe handoff was successful. From then on, the basestation acts as the Mobile Nodes Foreign Agent.</p><p>However, the handoff algorithm itself is keptvery simple. Whenever the Mobile Node receivesa beacon message from a Base Station, it sends aRegistration Request and from then on uses theBase Station as a Foreign Agent. This results ina dropout until the new connection is establishedalthough the Mobile Node could still communicatewith the rest of the network over its current ForeignAgent. The method also works only when theMobile Node hears a single Base Station. As soonas service areas of Base Stations overlap, the MobileNode constantly switches between Base Stationsand because of that often cannot establish anytransport connection at all. Since a handoff to anew Base Station generates a certain amount ofoverhead, the simple handoff algorithm producesan unnecessarily large amount of Mobile IP controlpackets.</p><p>Copyright 2002 John Wiley &amp; Sons, Ltd. Int. J. Network Mgmt 2002; 12:4159</p></li><li><p>TRANSMISSION OF STREAMING MEDIA TO MOBILE DEVICES 43</p><p>Ad Hoc Network RoutingAlgorithms</p><p>An ad hoc network is a collection of mobilenodes forming a temporary network withoutthe aid of any centralised administration orstandard support services regularly available onconventional networks. In this thesis, it is assumedthe mobile hosts use wireless RF transceivers astheir network interface, although many of thesame principles will apply to infra-red and wirebased networks. Some form of routing protocolis necessary in these ad hoc networks sincetwo hosts wishing to exchange packets may notbe able to communicate directly. One problemwith wireless network interfaces is they typicallyoperate at significantly slower bit rates than theirwire-based counterparts. Frequent flooding ofpackets throughout the network, a mechanismmany protocols require, can consume significantportions of the available network bandwidth. Adhoc routing protocols must minimise bandwidthoverhead at the same time as they enable properrouting to take place. Also, ad hoc networksmust deal with frequent changes in topology. Bytheir very nature, mobile nodes tend to wanderaround, changing their network location andlink status on a regular basis. Furthermore, newnodes may unexpectedly join the network orexisting nodes may leave or be turned off. Adhoc routing protocols must minimise the timerequired to converge after these topology changes.A low convergence time is more critical in adhoc networks because temporary routing loopscan result in packets being transmitted in circles,further consuming valuable bandwidth.</p><p>A n ad hoc network is a collection ofmobile nodes forming a temporarynetwork without the aid of any centralisedadministration or standard support servicesregularly available on conventionalnetworks.</p><p>For the purposes of this paper, an ad hoc rout-ing protocol is considered to fill the space betweentwo network extremes. At one extreme, the net-work topology is changing so rapidly the only</p><p>feasible routing mechanism is for every packetto be flooded throughout the network. At theother extreme, the network topology is sufficientlypermanent and static as to permit the use ofconventional routing mechanisms such as thoseemployed in the Internet. Ad hoc networks arenetworks which lack the support structure andpermanency of traditional networks, yet changesufficiently slowly as to permit the use of a rout-ing protocol to optimise transmission bandwidth.Some of the ad hoc routing algorithms in use atpresent include Destination-Sequenced Distance-Vector Routing (DSDV)26 which is an adaptationof a conventional routing protocol to ad hoc net-works. DSDV is based on the Routing InformationProtocol (RIP)14 used in parts of the Internet;Temporally-Ordered Routing Algorithm (TORA)20</p><p>is a distributed routing protocol based on a linkreversal algorithm. It is designed to discover routeson demand, provide multiple routes to a des-tination, establish routes quickly, and minimisecommunication overhead by localising the reac-tion to topological changes when possible; Ad HocOn-Demand Distance Vector (AODV)27 routing isessentially a combination of both DSR and DSDV.It borrows the basic on-demand mechanism ofroute discovery and route maintenance from DSR,plus the use of hop-by-hop routing, sequence num-bers, and periodic update packets from DSDV. Themain benefit of AODV over DSR is the source routedoes not need to be included with each packet.This results in a reduction of routing protocoloverhead. Unfortunately, AODV requires periodicupdates which, consume more bandwidth than issaved from not including source route informa-tion in the packets.7 Signal stability based adaptiverouting (SSA)11 is a variant of the AODV proto-col to take advantage of information available atthe link level. Both the signal quality of links andlink congestion are taken into consideration whenfinding routes. One important difference of SSAfrom AODV or DSR is that paths with strong sig-nal links are favoured over optimal paths. Whilethis may make routes longer, it is hoped discov-ered routes will survive longer; The cluster basedrouting protocol (CBRP)16 is a variation of the DSRprotocol. CBRP groups nodes located physicallyclose together into clusters. Unfortunately, CBRPdepends on nodes transmitting periodic hello pack-ets; a large part of the gains made by DSR arebecause DSR does not require periodic packets of</p><p>Copyright 2002 John Wiley &amp; Sons, Ltd. Int. J. Network Mgmt 2002; 12:4159</p></li><li><p>44 K. CURRAN AND G. PARR</p><p>any kind; Optimised Link State Routing (OLSR)15</p><p>is a link state routing protocol. OLSR is an adap-tation of conventional routing protocols to workin an ad hoc network on top of IMEP.10 The novelattribute of OLSR is its ability to track and usemulti-point relays. The idea of multi-point relaysis to minimise the flooding of broadcast messagesin the network by reducing/optimising duplicatere-transmissions in the same region. Each node inthe network selects a set of nodes in its neighbor-hood that will re-transmit its broadcast packets.This set of selected neighbour nodes is called themulti-point relays of that node. Each node selectsits multi-point relay set in a manner to cover allthe nodes that are two hops away from it. Theneighbours that are not in the multi-point relay setstill receive and process broadcast packets but donot re-transmit them.</p><p>Mobility Prediction in WirelessNetworks</p><p>Many recent papers deal with mobility predic-tion in cellular networks. The tracking schemeproposed in reference 17 uses a Gauss-Markovmodel to predict a mobiles future location for effi-cient paging. Based on the Gauss-Markov model, amobiles future location is predicted based on theinformation gathered from the last report of loca-tion and velocity. An extension of the ResourceReservation Protocol (RSVP) for cellular networksis proposed in reference 2. The proposed schemeuses mobility prediction to reserve bandwidth andit is based on the same framework presented inreference 1. In this scheme, each datum of mobilityhistory information consists of a tuple whose ele-ments include the identity of the mobile station, thelast location visited, and a timestamp indicating thetime at which the current cell was entered. Basedon this historical data, a prediction can be madeon the most likely location of the mobile station.This knowledge can then be used for intelligentpre-allocation of resources.</p><p>Another scheme that uses prediction to locate amobile in a cellular network is presented in ref-erence 1. Statistical search theory is used in thisapproach by maintaining a history of prior knownmobility patterns of users. Based on this priorinformation, a vector of probability mass functionsconcerning the likely location of a target station is</p><p>first computed. These probability mass functionsare then used as input to a search strategy thatspecifies the manner in which a mobile terminalis to be paged. In refere...</p></li></ul>

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