Elements of a wireless network. Elements of a wireless network. Some wireless link standards. Elements of a wireless network

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1 Elements of a wireless Elements of a wireless 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 - Elements of a wireless - ome 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 5 Data rate (Mbps) n 80.a,g G cellular enhanced Mobl G UMT/WCDM, CDM000 G G I-95, CDM, GM Indoor Outdoor 0-0m 50-00m Mid-range outdoor Long-range outdoor 00m Km 5Km 0 Km - Elements of a wireless - Elements of a wireless mode base station connects mobiles into wired handoff: mobile changes base station providing connection into wired 80. (WiMX) b -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 -

2 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 fast interference from other sources: 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 to a destination at slightly different times -7-8 Wireless Link Characteristics () NR: signal-to-noise ratio larger NR is good! 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) NR(d) QM5 (8 Mbps) QM ( Mbps) K ( Mbps) -9 Wireless characteristics dditional problems: 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 -0 IEEE 80. Wireless LN 80. LN architecture 80.b.-5 GHz unlicensed spectrum up to Mbps 80.a 5- 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 all use CM/C for multiple access all have base-station and ad-hoc versions - -

3 80.: Channels, association 80.: passive/active scanning 80.b:.GHz-.85GHz channels admin chooses frequency for 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 - 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 - 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 -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 - 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 -7-8

4 80. frame: addressing frame address address address duration 80. frame: addressing 0 - payload CRC seq address ddress : used only in ad hoc mode ddress : MC address of wireless host or to receive this frame Internet R router H R MC addr H MC addr ddress : MC address of router interface to which is attached dest. address source address 80. frame ddress : MC address of wireless host or transmitting this frame MC addr H MC addr R MC addr address address address 80. frame : mobility within same subnet 80. frame: more frame seq # (for RDT) duration of reserved transmission time (RT/CT) rotocol version Type ubtype To frame address address address duration seq address From More frag -0 payload CRC Retry 0 - ower More mgt 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 frame type (RT, CT, CK, ) - 80.: advanced capabilities Rate daptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, NR varies QM5 (8 Mbps) QM ( Mbps) K ( Mbps) operating point 80.: advanced capabilities 0- ower Management 0-0- ER NR(d) 0. 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 - 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 -

5 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) : 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 ~ miles ( city rather than coffee shop ) ~ Mbps point-to-point point-to-multipoint - 80.: 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 -7 5