2/12/20021 IEEE Wireless Local Area Networks The future is wireless Presented by Tamer Khattab and George Wong Prepared for EECE571N - Advanced Networking

2/12/20022 WLAN Technology Overview Physical layer technologies Architecture Transmission Medium access control technologies

2/12/20023 Architecture Network connectivity life time Ad-hoc

2/12/20024 Architecture (Cont.) Network connectivity life time Infrastructure

2/12/20025 Architecture (Cont.) Connection type Point-to-point

2/12/20026 Architecture (Cont.) Connection type Broadcast

2/12/20027 Transmission Medium Radio frequency transmission Narrow-band transmission Spread spectrum transmission direct sequence frequency hopping Infrared transmission Laser diode sources Light emitting diode sources

2/12/20028 Wireless LAN Standards IEEE wireless LAN standards ETSI HIPERLAN wireless LAN standards

2/12/20029 Wireless LAN Standards IEEE wireless LAN standards

2/12/ Physical Layer IEEE Wireless Ethernet

2/12/ IEEE Physical Layer Medium type 2.4 GHz FHSS ( GHz) 2.4 GHz DSSS ( GHz) Diffused infrared DFIR ( nm) Rates: basic=1 Mbps, enhanced=2 Mbps

2/12/ FHSS Band MHz GFSK (Gaussian Frequency Shift Keying) Sub-channels of 1 MHz Only 79 channels of the 83 are used Slow hopping 3 main sets each with 26 different hopping sequences

2/12/ FHSS (Cont.) Sequences within same set collide at max. on 5 channels Min. hopping distance of 6 channels. No CDMA within same BSS Coexisting BSS in the same coverage area use different sequences from the same hopping set.

2/12/ FHSS (Cont.) Frequency Time Hopping distance >= 6 sub-channels (The distance in frequency between two consecutive hops) Sub-channel 1 MHz 400 ms

2/12/ FHSS (Cont.) MAC data could be at 1 Mb/s or 2 Mb/s Sync pattern 80 bit SFD 16 bit PLW 12 bit HEC 16 bit 4 bitPayload data (variable length) PSF PLCP preamblePLCP headerPLCP_PDU Physical layer header and preamble always at 1 Mb/s

2/12/ DSSS Band MHz DBPSK (Differential Binary Phase Shift Keying) Band divided into 11 overlapping channels each with bandwidth 11 MHz Coexisting BSS in the same coverage area use channels separated by at least 30 MHz. 11 bit Barker sequence is used for spreading No CDMA used within one BSS

2/12/ DSSS (Cont.) Frequency (MHz) Channel number 11 MHz

2/12/ DSSS (Cont.) Sync pattern 128 bit SFD 16 bit SG 8 bit HEC 16 bit SR 8 bit Payload data (variable length) PLCP preamble PLCP headerPLCP_PDU MAC data could be at 1 Mb/s or 2 Mb/s LN 16 bit Physical layer header and preamble always at 1 Mb/s

2/12/ Infra Red Wave length near visible light nm PPM (Pulse Position Modulation) Diffused transmission technique used Only used for indoor transmission

2/12/ IEEE a 5 GHz ( , , GHz) OFDM (Orthogonal Freq. Div. Multiplexing) 52 Subcarriers BPSK/QPSK/QAM Forward Error Correction (Convolutional) Rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps

2/12/ IEEE b 2.4 GHZ band DSSS (11-chip) Rates 5.5 and 11 Mbps M-arry modulation. Convolutional Codes Shorter Preamble

2/12/ Product Samples 2.4 GHz FHSS ISA 2.4 GHz FHSS PCMCIA 5 GHz DSSS PCMCIA

2/12/ MAC Layer IEEE Wireless Ethernet

2/12/ Overview of the Protocol Layers IEEE specifies a MAC layer that is designed to operate over wireless channel IEEE is in the same protocol layer as the IEEE IEEE Logical Link Control (LLC) MAC Layer Data Link Layer Network Layer IEEE Ethernet IEEE Wireless Ethernet Physical Layer IEEE Token Ring IEEE Token Bus

2/12/ IEEE – CSMA/CA CS – Carrier Sense Each transmitter listens to the physical link before transmitting MA – Multiple Access Many nodes are connected to the same physical link. CA – Collision Avoidance Methods used to avoid collision

2/12/ CSMA/CA Why not CSMA/CD? Difficult to detect collision in a radio environment Radio environment is not as well controlled as a wired broadcast medium, and transmissions from users in other LANs can interfere with the operation of CSMA/CD Radio LANs are subject to the hidden-station problem

2/12/ Hidden-Station Problem A knows the existence of B C knows the existence of B B knows the existence of A and C However, A does not know the existence of C B AC

2/12/ Hidden-Station Problem Since A and C are sufficiently distant from each other that they cannot hear each other’s transmission (Carrier Sense doesn’t work!) This condition will result in the transmissions from the two stations, A and C, proceeding and colliding at the intermediate station B (However, A and C cannot hear the collision!)

2/12/ Hidden-Station Problem CSMA/CA medium access control was developed to prevent this type of collision Data Frame A transmits data frame B A B C C transmits data frame and collides with A at B (a) (b) Data Frame C A

2/12/ Exposed Node Problem Suppose B is sending to A. C is aware of this communication because it hears B’s transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’ transmission This is not a problem since C’s transmisstion to D will not interfere with A’s ability to receive from B BCAD

2/12/ Collision Avoidance IEEE address these two problems, hidden-station and exposed node problems, with an algorithms called Multiple Access with Collision Avoidance Sender and receiver exchange control frames with each other before the sender actually transmit any data The sender transmits a Request to Send (RTS) frame to the receiver and the receiver then replies with a Clear to Send (CTS) frame

2/12/ Collision Avoidance RTS includes a field that indicates how long the sender wants to hold the medium CTS reserves channel for sender, notifying (possible hidden) station For Neighbors See CTS: keep quite See RTS but no CTS: ok to transmit Receiver sends ACK when it receives the frame Neighbors keep silent until seeing ACK

2/12/ Collision Avoidance BCAD

2/12/ UBC Wireless Network UBC is currently deploying the wireless network “It’s changed students’ lives” Christopher Macdonald, School of Architecture “It was like winning the lottery” Alan Steeves, Research Engineer

2/12/ Questions?