Administrivia No lecture on Thurs. Last work will be out this week (not due, covers wireless) Extra office hours for next week and the week after. Channel 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. th generation cellular systems are shifting to explicit centralized scheduling of resources by the BS. CSMA/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 protocol Mobility Management Collision Avoidance: RTS-CTS exchange Mobility Management A RTS(A) AP reservation collision B RTS(B) General principles Cellular examples. RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) 6
What is mobility? Mobility: Vocabulary spectrum of mobility, from the perspective: no mobility high mobility : permanent of (e.g., 8.9.0/, Verizon) : entity that will perform mobility functions on behalf of, when is remote wireless user, using same access point user, connecting/ disconnecting from using DHCP. user, passing through multiple access point while maintaining ongoing connections (like cell phone) Permanent address: address in, can always be used to reach e.g., 8.9.0.86, (0) 6-807 7 0 Example (simple): mobility within same 80. subnet H remains in same IP subnet: IP address can remain same switch: which AP is associated with H? self-learning (Ch. ): switch will see frame from H and remember which switch port can be used to reach H BBS AP router hub or switch AP H BBS 8 Mobility: more vocabulary Permanent address: remains constant (e.g., 8.9.0.86) : wants to communicate with Care-of-address: address in. (e.g., 79,9..) : in which currently resides (e.g., 79.9./) : entity in that performs mobility functions on behalf of. Wide-Area Mobility Managment In the 80. example, mobility is handled at the link layer. For wide-area roaming, it must be handled at the layer. Issues: How to find the destination? How to route stuff to the destination? How to update and keep track of its location? Mobility: approaches Let routing handle it: routers advertise permanent address of -nodes-in-residence via usual routing table exchange. routing tables indicate where each located no changes to end-systems Let end-systems handle it: indirect routing: communication from to goes through, then forwarded to remote direct routing: gets foreign address of, sends directly to 9
Mobility: approaches Let routing handle it: routers advertise permanent address of -nodes-in-residence not via usual routing table exchange. scalable routing tables to indicate millions of where each located s no changes to end-systems let end-systems handle it: indirect routing: communication from to goes through, then forwarded to remote direct routing: gets foreign address of from, sends directly to Indirect Routing: comments Mobile uses two addresses: permanent address: used by (hence location is transparent to ) care-of-address: used by to forward datagrams to triangle routing: -- inefficient when, are in same 6 Mobility: registration Indirect Routing: moving between s End result: contacts : this is resident in my Foreign knows about Home knows location of contacts on entering suppose user moves to another registers with new new registers with update care-of-address for packets continue to be forwarded to (but with new care-of-address) mobility, changing foreign s transparent: on going connections can be maintained! 7 Mobility via Indirect Routing Mobility via Direct Routing addresses packets using address of intercepts packets, forwards to receives packets, forwards to replies directly to requests, receives foreign address of forwards to receives packets, forwards to replies directly to 8
Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to : must get care-of-address from what if changes? Components of cellular architecture recall: wired public telephone different cellular s, operated by different providers 9 Accommodating mobility with direct routing anchor : FA in first data always routed first to anchor FA when moves: new FA arranges to have data forwarded from old FA (chaining) anchor foreign new foreign foreign net at session start new foreign Handling mobility in cellular s : of cellular provider you subscribe to (e.g., Sprint, Verizon) location register (HLR): database in containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another ) : in which currently resides visitor location register (VLR): database with entry for each user currently in could be 0 Examples GSM: indirect routing to Mobile IP Cellular s Will focus on latter. consults HLR, gets roaming number of in user HLR Mobile VLR Mobile call routed to Public switched telephone sets up nd leg of call to in in completes call through base station to
GSM: handoff with common GSM: handoff between s old BS VLR Mobile old routing new routing Handoff goal: route call via new base station (without interruption) reasons for handoff: stronger signal to/from new BS (continuing connectivity, less battery drain) load balance: free up channel in current BS GSM doesn t mandate why to perform handoff (policy), only how (mechanism) handoff initiated by old BS Home anchor PSTN (b) after handoff anchor : first during cal call remains routed through anchor new s add on to end of chain as moves to new 8 GSM: handoff with common Soft vs Hard Handoff old BS VLR Mobile 7 8 6 0. Mobile measures signal strength from adjacent BS s based on beacons.. old BS informs of impending handoff, provides list of + s. sets up path (allocates resources) to. allocates radio channel for use by. signals, old BSS: ready. old BS tells : perform handoff to 6., signal to activate new channel 7. signals via to : handoff complete. reroutes call 8 -old-bs resources released 6 In GSM, a is connected to only one BSS at any one time. Handoff entails changing channels. In CDMA, all users occupy the entire bandwidth (. MHz). Two BSS s can be simultaneously listening to the and pass their decisions and their reliability to the. The decides based on the better decision at the moment. Soft handoff provides a form of macrodiversity. 9 GSM: handoff between s Home anchor PSTN anchor : first during cal call remains routed through anchor new s add on to end of chain as moves to new (a) before handoff 7