# Ethernet, Wireless LAN. Ethernet Frame Format (a) DIX Ethernet, (b) IEEE 802.3.

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Ethernet, Wireless LAN

Ethernet Frame Format (a) DIX Ethernet, (b) IEEE 802.3

Minimum Frame Size Why a minimum frame size is needed? How long does it take for a station to notice a collision?

Worst case

Minimum Frame Size So, if maximum delay is t, the minimum frame size is 2t*bit rate. t is about 50us. So the minimum frame size of 10M Ethernet is 512 bits. What if the speed goes up?

Ethernet Performance Suppose there are k stations. Let p be the probability that a station has a frame to send when the channel is idle. Assume it is independent across stations, and is independent for one station at different times. Find the average number of collisions before a frame is sent. First, the probability that one station got the chance to send is A=kp(1-p)^{k-1}. Second, maximized when p=1/k. So A is bounded by (1- 1/k)^{k-1}. Third, each contention is independent, so average number of collision is 1/A, which is e when k is large. Each contention is 2t, so channel efficiency is P/P+2et.

Ethernet Physical medium – thin cable/thick cable/twisted pair/fiber 10Base5 500 meters thick (cable) Ethernet 100 nodes/seg 10Base2 200 meters thin (cable) Ethernet 30 nodes/seg 10BaseT 100 meters twist pair 1024 nodes/seg 10BaseF 2000 meters fiber optics 1024 nodes/seg 10Base5/10Base2, cable connected to each machine 10BaseT -- connecting to a hub 10BaseF -- between building Connecting

Ethernet Fast Ethernet Keep everything in Ethernet, make the clock faster 100Mbps. Cable – 100Base-T4 100m category 3 UTP, 4 lines. – 100Base-Tx 100m category 5 twisted pair – 100Base-Fx 2000m Fiber optic

Zhenhai DuanComputer Science, FSU IEEE 802 LAN Protocol Stack ISO/OSI Reference Model 802.2 Logical Link Control (LLC) 802.3 CSMA/CD 802.4 Token Bus 802.5 Token Ring 802.11 Wireless Data Link Physical Application Presentation Session Transport Network

Wireless LAN Basic structure: – Stations plus an access point – Stations talk to the access point, then to outside – Access point talks to stations – Stations talk to stations Design goal: – A MAC protocol to determine who talks next

Wireless communications Signal decays according to a power law with the distance, at least to the power of -2 with distance Comparing to Ethernet, what is the difference (as far as MAC is concerned)? When a station is sending, not all stations can hear. No real 100% carrier sense. – In Ethernet, everybody can hear everybody

Wireless communications When a station is sending, he cannot hear other stations – cannot decide if there is a collision. No CD in wireless LAN. – In Ethernet, the sender can determine if there is collision and abort immediatelly.

Wireless communications Being able to sense the carrier does not mean that you can decode the data If received signal having power P means that you can decode the data, it may be true that at power P/2 you can realize that there is something going on

Wireless communication The received signal can be read if the signal to noise ratio is larger than a certain threshold. Whether there is a collision depends on the signal to noise ratio at the receiver. You may allow two transmissions at the same time without collision. – In Ethernet, two simultaneous transmission means collision ADCB ADCB A->B, C->D A->B, D->C

Zhenhai DuanComputer Science, FSU Medium Access Control (MAC) Layer Asynchronous Data Service – DCF (Distributed Coordination Function) Contention-Based Medium Access Control CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance For elastic applications like email, file transfer Time-Bounded Service – PCF (Point Coordination Function) Contention Free Medium Access Control Optional access method works like polling For time-sensitive voice/video applications

Goals How to design an efficient contention-based MAC protocol for wireless LAN? Goals – Collision avoidance to reduce wasted transmissions – Reasonable fairness – Cope with hidden terminals – Allow exposed terminals to talk

Problems What problems will occur if apply Ethernet MAC? No CD, does not know whether there is a collision No CD, channel waste could be large using 1- persistent Cannot hear all other people means the sender cannot be sure that he can reserve the whole channel.

Fixes No CD, use ACK. If there is no ACK, assume there is collision No CD, has to use non-persistent to reduce collision by AVOIDING COLLISION, CA Cannot hear other people, so devise some channel reservation technique

DCF When got a packet to send, sense the channel, if idle, send immediately (not completely non-persistent! Why?) When channel is busy, wait until idle, then backoff a random time. If still idle, send. (The non-persistent part. The CA feature). If busy before reaching zero, freeze it, and reactivate when idle again. After receives a packet, send ACK. If collision, use the exponential backoff.

DCF Do you want the ACK to have the same priority as data packets? How do you make sure that ACK has higher priority? Use time. You have to wait for a certain amount time before you can send. High priority packets wait shorter.

DCF The SIFS, DIFS. SIFS is for control packets. DIFS is for data packets. When a station wants to send, if it is a control packet, sense the channel for SIFS, then send. If it is a data packet, sense the channel for DIFS, then send.

DCF

Further improvement Further improvement by improving carrier sense The problem is other people cannot hear me sending, so they will send. So, how to make sure that they will know I am sending?

RTS/CTS RTS/CTS in the place for carrier sense – RTS – reserves channel for a bit of time, if sender hasn’t heard other CTSes – CTS – sender replies if it hasn’t heard any other RTSes – Both messages include time. Network Allocation Vector (NAV) – If no CTS, exponential backoff – “RTS-CTS-DATA”

RTS/CTS 802.11 standardized both CSMA/CA and RTS/CTS In practice, most operators disable RTS/CTS – Very high overhead! RTS/CTS packets sent at “base rate” (often 1Mbit) – Avoid collisions regardless of transmission rate – Most deployments are celluar (base stations), not ad hoc. Neighboring cells are often configured to use non-overlapping channels, so hidden terminals on downlink are rare Hidden terminal on uplink possible, but if clients mostly d/l, then uplink packets are small. THIS MAY CHANGE. And is likely not true in your neighborhood! – When CS range >> reception range, hidden terminal less important

PCF The AP acts as the master and sends out beacon signals for polling stations and stations can sign up for certain amount of bandwidth use Co-exists with DCF. How to make sure that beacon signals have higher priority? – PIFS

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