CMPT771 Wireless Media 1 Wireless Basics CMPT 771 Internet Architecture and Protocols

CMPT771 Wireless Media 2 Wireless Basics Two types of “links”: r point-to-point m PPP for dial-up access m point-to-point link between Ethernet switch and host r broadcast (shared wire or medium) m traditional Ethernet m Satellite m wireless LAN m Cellular Or lecture room

CMPT771 Wireless Media 3 Multiple Access protocols r single shared broadcast channel r two or more simultaneous transmissions by nodes: interference m only one node can send successfully at a time multiple access protocol r algorithm that determines how nodes share channel, i.e., determine when node can transmit r communication about channel sharing must use channel itself! r what to look for in multiple access protocols:

CMPT771 Wireless Media 4 Ideal Mulitple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: m no special node to coordinate transmissions m no synchronization of clocks, slots 4. Simple

CMPT771 Wireless Media 5 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access r access to channel in "rounds" r each station gets fixed length slot (length = pkt trans time) in each round r example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle r Problem m unused slots go idle

CMPT771 Wireless Media 6 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access r channel spectrum divided into frequency bands r each station assigned fixed frequency band r example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle r Problem m unused transmission time in frequency bands go idle frequency bands time

CMPT771 Wireless Media 7 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) r Analogy m Public Key Encryption Only the one holing the key can decrypt r Motivation m Sender – Mix information encoded with “codes” of receivers m Receiver – Decode mixed information using its own code, and find that for itself

CMPT771 Wireless Media 8 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) r used mostly in wireless broadcast channels (cellular, satellite, etc) r unique “code” assigned to each user; i.e., code set partitioning r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data r encoded signal = (original data) X (chipping sequence) r decoding: inner-product of encoded signal and chipping sequence r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

CMPT771 Wireless Media 9 CDMA Encode/Decode

CMPT771 Wireless Media 10 CDMA: two-sender interference

CMPT771 Wireless Media 11 CDMA: two-sender interference Assume codes (1,-1) for recv1 (1,1) for recv2 Data bit 1 for recv 1: 1·(1,-1) Decoded using code for recv1: 1·(1,-1) ·(1,-1) =(1,-1) ·(1,-1)=2 Decoded using code for recv 2: 1·(1,-1) ·(1,1) =(1,-1) ·(1,1)=0

CMPT771 Wireless Media 12 Random Access Protocols r When node has packet to send m transmit at full channel data rate R. m no a priori coordination among nodes r two or more transmitting nodes -> “collision”, r random access MAC protocol specifies: m how to detect collisions m how to recover from collisions (e.g., via delayed retransmissions) r Examples of random access MAC protocols: m slotted ALOHA m ALOHA m CSMA, CSMA/CD (Ethernet), CSMA/CA ( WLAN or Wi-Fi)

CMPT771 Wireless Media 13 Slotted ALOHA Assumptions r all frames same size r time is divided into equal size slots, time to transmit 1 frame r nodes start to transmit frames only at beginning of slots r nodes are synchronized r if 2 or more nodes transmit in slot, all nodes detect collision Operation r when node obtains fresh frame, it transmits in next slot r no collision, node can send new frame in next slot r if collision, node retransmits frame in each subsequent slot with prob. p until success

CMPT771 Wireless Media 14 Slotted ALOHA Pros r single active node can continuously transmit at full rate of channel r highly decentralized: only slots in nodes need to be in sync r simple Cons r collisions, wasting slots r idle slots r nodes may be able to detect collision in less than time to transmit packet – but sill waste a slot

CMPT771 Wireless Media 15 Pure (unslotted) ALOHA r unslotted Aloha: simpler, no synchronization r when frame first arrives m transmit immediately r collision probability increases: m frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1]

CMPT771 Wireless Media 16 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: r If channel sensed idle: transmit entire frame r If channel sensed busy, defer transmission r No time slot – fully distributed r Human analogy: listen and don’t interrupt others r Seems perfect – no collision !

CMPT771 Wireless Media 17 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability

CMPT771 Wireless Media 18 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA m collisions detected within short time m colliding transmissions aborted, reducing channel wastage r collision detection: m easy in wired LANs: measure signal strengths, compare transmitted, received signals m difficult in wireless LANs: receiver shut off while transmitting r human analogy: the polite conversationalist

CMPT771 Wireless Media 19 CSMA/CD collision detection

CMPT771 Wireless Media 20 IEEE Wireless LAN r b (1999) m GHz unlicensed radio spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions r a (1999) m 5-6 GHz range m up to 54 Mbps r g (2003) m GHz range m up to 54 Mbps r n (2009) m 2.4/5GHz, MIMO m OFDM (instead of DSSS)

CMPT771 Wireless Media 21 Base station approach r Wireless host communicates with a base station m base station = access point (AP) r Basic Service Set (BSS) (a.k.a. “cell”) contains: m wireless hosts m access point (AP): base station

CMPT771 Wireless Media 22 Ad Hoc Network approach r No AP (i.e., base station) r wireless hosts communicate with each other m to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z r Applications: m “laptop” meeting in conference room, car m interconnection of “personal” devices m battlefield r IETF MANET (Mobile Ad hoc Networks) working group

CMPT771 Wireless Media 23 IEEE : multiple access r Collision if 2 or more nodes transmit at same time r CSMA makes sense: m get all the bandwidth if you’re the only one transmitting m try to avoid collision if you sense another transmission

CMPT771 Wireless Media 24 IEEE MAC Protocol: CSMA CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff CSMA receiver - if received OK return ACK after SIFS (make sure no collision)

CMPT771 Wireless Media 25 Enhancement: CSMA/CD ? r Collision detection doesn’t work: hidden terminal problem

CMPT771 Wireless Media 26 Collision avoidance mechanisms r Problem: m two nodes, hidden from each other, transmit complete frames to base station m wasted bandwidth for long duration ! r Solution: m small reservation packets m nodes track reservation interval with internal “network allocation vector” (NAV)

CMPT771 Wireless Media 27 Collision Avoidance: RTS-CTS exchange r sender transmits short RTS (request to send) packet: indicates duration of transmission r receiver replies with short CTS (clear to send) packet m notifying (possibly hidden) nodes r hidden nodes will not transmit for specified duration: NAV

CMPT771 Wireless Media 28 Collision Avoidance: RTS-CTS exchange r RTS and CTS short: m collisions less likely, of shorter duration m end result similar to collision detection r IEEE allows: m CSMA m CSMA/CA: reservations m polling from AP

CMPT771 Wireless Media 29 IEEE MIMO

Wireless Technology Evolution

CMPT771 Wireless Media 31 Progress: 1G/2G Wireless Cellular Networks r First generation - analog r Second generation cellular network (1992-) m Technology: TDMA, TDMA hybrid FDMA Systems: DAMPS(USA, IS-54), GSM m Technology: CDMA (Qualcomm) Systems: CDMA(IS-95) m Pros (vs 1G) Higher Frequency available, good Security, soft Capacity (with CDMA), higher Capacity m Cons (vs 3G) Speech Service, lower rate, no multimedia Service and no higher rate Service

CMPT771 Wireless Media 32 Progress: 3G Wireless Networks r Third generation 3G (1996-) m Objective truly anybody at any place to communicate with anyone at any time m Support Multimedia Service 144kb/s(Outdoor and higher velocity ), 384kb/s(from Outdoor to indoor, lower velocity), 2Mb/s(indoor) m Standards Japan: WCDMA (TDD/FDD) Europe: WCDMA (FDD model) and TD-CDMA(TDD). America: CDMA-2000 (TDD) China: TD-SCDMA(1998)

CMPT771 Wireless Media 33 Progress: 4G Wireless Networks r 4G Wireless Networks: Objective m Support interactive multimedia services: teleconferencing, wireless Internet, etc m Global mobility and service portability m Higher rate, lower cost r What's New in 4G m Entirely packet-switched networks m All network elements are digital m Higher bandwidths to provide multimedia services m Tight network security. r 3G vs 4G m Circuit and packet switchiong/Entirely packet switched networks m Combination of existing & evolved equipment/All digital elements m Data rate (up to 2Mbps)/Higher rate (up to 100Mbps) r Technology m MIMO-OFDM (Orthogonal Frequency division multiplexing) ?

WiMax r “Worldwide Interoperability for Microwave Access” m WiMAX is a standards-based Broadband Wireless Access m fast broadband connections over long distances r Standard based: Wireless MAN by IEEE m d: Fixed WiMAX (Jun. 2004) m e: Mobile WiMAX (Nov. 2005) m Industry to create interoperable complete ecosystem r WiMAX Forum (2001), Intel etc. involved m Claims there are over 455 WiMAX networks deployed in over 135 countries

LTE (Long Term Evolution) r LTE Air-interface m Downlink OFDM m Uplink SC-FDMA m Mutiple Antenna Scheme (MIMO)

WiMaX vs LTE (4G and beyond)

CMPT771 Wireless Media 37 More on Wireless Media r Power optimization m Transmission power m Circuit power r Joint source-channel coding m Channel ? r Roaming m Hand off m Vertical hand off r …