Comp 361, Spring 20056:Basic Wireless 1 Chapter 6: Basic Wireless (last updated 02/05/05) r A quick intro to CDMA r Basic

Comp 361, Spring 20056:Basic Wireless 2 Channel Partitioning (CDMA) We previously saw two different ways to partition channels, TDMA and FDMA. They, respectively, partition the channels up into time slots and frequency bands. Now we will see CDMA, a new way to partition channels. It is frequently used in wireless applications.

Comp 361, Spring 20056:Basic Wireless 3 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) r unique “code” assigned to each user; i.e., code set partitioning r used mostly in wireless broadcast channels (cellular, satellite, etc) 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”)

Comp 361, Spring 20056:Basic Wireless 4 CDMA Encode/Decode

Comp 361, Spring 20056:Basic Wireless 5 CDMA: two-sender interference

Comp 361, Spring 20056:Basic Wireless 6 IEEE Wireless LAN r b 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 m widely deployed, using base stations r a m 5-6 GHz range m up to 54 Mbps r g m GHz range m up to 54 Mbps r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions

Comp 361, Spring 20056:Basic Wireless 7 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 r BSSs combined to form distribution system (DS)

Comp 361, Spring 20056:Basic Wireless 8 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

Comp 361, Spring 20056:Basic Wireless 9 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 shouldn’t cause a collision if you sense another transmission r Collision detection doesn’t work: hidden terminal problem

Comp 361, Spring 20056:Basic Wireless 10 IEEE MAC Protocol: CSMA/CA 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 (ACK is needed due to hidden terminal problem)

Comp 361, Spring 20056:Basic Wireless 11 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)

Comp 361, Spring 20056:Basic Wireless 12 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

Comp 361, Spring 20056:Basic Wireless 13 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

Comp 361, Spring 20056:Basic Wireless 14 RTS-CTS Exchange is Optional r RTS-CTS exchange introduces extra delays and consumes channel resources r Should only be used to reserve channel for long data frames r In practice, wireless station sets a threshold frame size; for frames below threshold size, RTS packet not sent. r Threshold is often higher than max frame size, so RTS is not used