wireless hosts laptop, D, I phone run applications may be stationary (non-mobile) or mobile wireless does not always mean mobility base station typically connected to wired relay - responsible for sending packets between wired and wireless host(s) in its area e.g., cell towers, 80. access points 6-6- Characteristics of selected wireless link standards wireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various rates, transmission distance Data rate (Mbps) 00 5 5-80.n 80.a,g UMT/WCDM-HD, CDM000-xEVDO.8 UMT/WCDM, CDM000 G cellular enhanced G G I-95, CDM, GM Indoor Outdoor 0-0m 50-00m Mid-range outdoor Long-range outdoor 00m Km 5Km 0 Km 6-6- mode base station connects mobiles into wired handoff: mobile changes base station providing connection into wired 80.6 (WiMX) 80.5.056 80.a,g point-to-point 80.b 6-5 ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a : route among themselves 6-6
ER Wireless taxonomy Wireless Link Characteristics () single hop multiple hops Differences from wired link. (e.g., s) no host connects to base station (WiFi, WiMX, cellular) which connects to larger Internet no base station, no connection to larger Internet (luetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MNET, VNET decreased signal : radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless frequencies (e.g.,. GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times. make communication across (even a point to point) wireless link much more difficult 6-7 6-8 Wireless Link Characteristics () NR: signal-to-noise ratio larger NR easier to extract signal from noise (a good thing ) NR versus ER (it Error Rate) tradeoffs given physical layer: increase power -> increase NR- >decrease ER given NR: choose physical layer that meets ER requirement, giving highest throughput NR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 0-0 - 0-0 - 0-5 0-6 0-7 0 0 0 0 NR(d) QM56 (8 Mbps) QM6 ( Mbps) K ( Mbps) 6-9 Wireless characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): Hidden terminal problem, hear each other, C hear each other, C can not hear each other means, C unaware of their interference at C C s signal space C s signal ignal attenuation:, hear each other, C hear each other, C can not hear each other interfering at 6-0 IEEE 80. Wireless LN 80. LN architecture 80.b.-5 GHz unlicensed spectrum up to Mbps 80.a 5-6 GHz range up to 5 Mbps 80.g.-5 GHz range up to 5 Mbps 80.n: multiple antennae.-5 GHz range up to 00 Mbps Internet hub, switch or router wireless host communicates with base station base station = access point () asic ervice et () (aka cell ) in mode contains: wireless hosts access point (): base station ad hoc mode: hosts only all use CM/C for multiple access all have base-station and ad-hoc versions 6-6-
80.: Channels, association 80.: passive/active scanning 80.b:.GHz-.85GHz spectrum divided into channels at different frequencies admin chooses frequency for interference possible: channel can be same as that chosen by neighboring! host: must associate with an scans channels, listening for beacon frames containing s name (ID) and MC address selects to associate with may perform authentication will typically run DHC to get I address in s subnet 6- H assive canning: () beacon frames sent from s () association Request frame sent: H to selected () association Response frame sent: H to selected H ctive canning: () robe Request frame broadcast from H () robes response frame sent from s () ssociation Request frame sent: H to selected () ssociation Response frame sent: H to selected 6- IEEE 80.: multiple access avoid collisions: + nodes transmitting at same time 80.: CM - sense before transmitting don t collide with ongoing transmission by other node 80.: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CM/C(ollision)(voidance) C C s signal space C s signal 6-5 IEEE 80. MC rotocol: CM/C 80. sender if sense channel idle for DIF then transmit entire frame (no CD) if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no CK, increase random backoff interval, repeat 80. receiver - if frame received OK return CK after IF (CK needed due to hidden terminal problem) DIF sender CK receiver IF 6-6 voiding collisions (more) Collision voidance: RT-CT exchange idea: allow sender to reserve channel rather than random access of frames: avoid collisions of long frames sender first transmits small request-to-send (RT) packets to using CM RTs may still collide with each other (but they re short) broadcasts clear-to-send CT in response to RT CT heard by all nodes sender transmits frame other stations defer transmissions avoid frame collisions completely using small reservation packets! time DT () reservation collision defer 6-7 6-8
ER 80. frame: addressing 80. frame: addressing 6 6 6 6 0 - frame address address address address duration seq payload CRC control control H R router Internet ddress : MC address of wireless host or to receive this frame ddress : MC address of wireless host or transmitting this frame ddress : MC address of router interface to which is attached ddress : used only in ad hoc mode R MC addr H MC addr dest. address source address 80. frame MC addr H MC addr R MC addr address address address 80. frame 6-9 6-0 80. frame: more 80.: mobility within same subnet 6 6 6 6 0 - frame address address address address duration seq payload CRC control control rotocol version Type duration of reserved transmission time (RT/CT) ubtype To From frame type (RT, CT, CK, ) More frag frame seq # (for RDT) ower Retry mgt More WE Rsvd H remains in same I subnet: I address can remain same switch: which is associated with H? self-learning (Ch. 5): switch will see frame from H and remember which switch port can be used to reach H router hub or switch H 6-6- 80.: advanced capabilities 80.: advanced capabilities Rate daptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, NR varies QM56 (8 Mbps) QM6 ( Mbps) K ( Mbps) operating point 0-0 - 0-0 - 0-5 0-6 0-7 0 0 0 0 NR(d). NR decreases, ER increase as node moves away from base station. When ER becomes too high, switch to lower transmission rate but with lower ER ower Management node-to-: I am going to sleep until next beacon frame knows not to transmit frames to this node node wakes up before next beacon frame beacon frame: contains list of mobiles with to-mobile frames waiting to be sent node will stay awake if -to-mobile frames to be sent; otherwise sleep again until next beacon frame 6-6-
pream. 80.5: personal area less than 0 m diameter replacement for cables (mouse, keyboard, headphones) ad hoc: no master/slaves: slaves request permission to send (to master) master grants requests 80.5: evolved from luetooth specification.-.5 GHz radio band up to 7 kbps M M Master device lave device radius of coverage arked device (inactive) 6-5 80.6: WiMX like 80. & cellular: base station model transmissions to/from base station by hosts with omnidirectional antenna base station-to-base station backhaul with pointto-point antenna unlike 80.: range ~ 6 miles ( city rather than coffee shop ) ~ Mbps point-to-point point-to-multipoint 6-6 80.6: WiMX: downlink, uplink scheduling transmission frame down-link subframe: base station to node uplink subframe: node to base station - UL- M M burst burst downlink subframe burst n Initial request # # #k maint. conn. uplink subframe base station tells nodes who will get to receive ( map) and who will get to send (UL map), and when WiMX standard provide mechanism for scheduling, but not scheduling algorithm 6-7 5