11Administrivia No lecture on Thurs. Last homework will be out this week (not due, covers wireless) Extra office hours for next week and the week after.2CSMA/CA: Recap Sensing in wireless medium is limited by hidden terminal problem. Collision detection is limited by half-duplex nature of radios (cannot talk and listen at the same time) Collision avoidance is the key! Random backoff after sensing busy slot RTS/CTS reservation protocol3Collision Avoidance: RTS-CTS exchangeAPA BtimeRTS(A)RTS(B)RTS(A)CTS(A)CTS(A)DATA (A)ACK(A)ACK(A)reservation collisiondefer4Channel Partitioning, Random Access and Scheduling Channel partitioning (GSM, CDMA, etc) is inflexible in accomdating bursty traffic. Random access allows “on-demand”allocation, but has significant overhead due to collision or RTS/CTS. 4thgeneration cellular systems are shifting to explicit centralized scheduling of resources by the BS.5Mobility Management6Mobility Management General principles Cellular network examples.27What is mobility? spectrum of mobility, from thenetworkperspective:no mobilityhigh mobilitymobile wireless user, using same access pointmobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)mobile user, connecting/ disconnecting from network using DHCP. 8hub or switchAP 2AP 1H1BBS 2BBS 1Example (simple): mobility within same 802.11 subnetrouter H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H19Wide-Area Mobility Managment In the 802.11 example, mobility is handled at the link layer. For wide-area roaming, it must be handled at the network layer. Issues: How to find the destination? How to route stuff to the destination? How to update and keep track of its location?10Mobility: Vocabularyhome network:permanent “home” of mobile(e.g., 128.119.40/24, Verizon)Permanent address:address in home network, can alwaysbe used to reach mobilee.g., 128.119.40.186,(510) 642-5807home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remotewide area networkcorrespondent11Mobility: more vocabularyCare-of-address:address in visited network.(e.g., 79,129.13.2) wide area networkvisited network:network in which mobile currently resides (e.g., 79.129.13/24)Permanent address:remains constant (e.g., 128.119.40.186)foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile12Mobility: approachesLet routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile located no changes to end-systemsLet end-systems handle it: indirect routing:communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing:correspondent gets foreign address of mobile, sends directly to mobile313Mobility: approachesLet routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile located no changes to end-systemslet end-systems handle it: indirect routing:communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing:correspondent gets foreign address of mobile from home agent, sends directly to mobilenot scalableto millions ofmobiles14Mobility: registrationEnd result: Foreign agent knows about mobile Home agent knows location of mobilewide area networkhome networkvisited network1mobile contacts foreign agent on entering visited network2foreign agent contacts home agent home: “this mobile is resident in my network”15Mobility via Indirect Routingwide area networkhomenetworkvisitednetwork3241correspondent addresses packets using home address of mobilehome agent intercepts packets, forwards to foreign agentforeign agent receives packets, forwards to mobilemobile replies directly to correspondent16Indirect Routing: comments Mobile uses two addresses: permanent address: used by correspondent (hence mobile location is transparentto correspondent) care-of-address: used by home agent to forward datagrams to mobile triangle routing: correspondent-home-network-mobile inefficient when correspondent, mobile are in same network17Indirect Routing: moving between networks suppose mobile user moves to another network registers with new foreign agent new foreign agent registers with home agent home agent update care-of-address for mobile packets continue to be forwarded to mobile (but with new care-of-address) mobility, changing foreign networks transparent: on going connections can be maintained!18Mobility via Direct Routingwide area networkhomenetworkvisitednetwork4241correspondent requests, receives foreign address of mobilecorrespondent forwards to foreign agentforeign agent receives packets, forwards to mobilemobile replies directly to correspondent3419Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to correspondent:correspondent must get care-of-address from home agent what if mobile changes visited network?20wide area network1foreign net visited at session startanchorforeignagent24new foreignagent35correspondentagentcorrespondentnew foreignnetworkAccommodating mobility with direct routing anchor foreign agent: FA in first visited network data always routed first to anchor FA when mobile moves: new FA arranges to have data forwarded from old FA (chaining)21Examples Mobile IP Cellular networksWill focus on latter.22Components of cellular network architecturecorrespondentMSCMSCMSCMSCMSCwired public telephonenetworkdifferent cellular networks,operated by different providersrecall:23Handling mobility in cellular networkshome network:network of cellular provider you subscribe to (e.g., Sprint, Verizon)home location register (HLR):database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)visited network:network in which mobile currently residesvisitor location register (VLR):database with entry for each user currently in
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