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/fiber10Base5 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 Duan Computer Science, FSU

IEEE 802 LAN Protocol StackIS

O/O

SI

Ref

eren

ce M

odel

802.2Logical Link Control (LLC)

802.3CSMA/CD

802.4TokenBus

802.5TokenRing

802.11Wireless

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

A DCB A DCB

A->B, C->D A->B, D->C

Wireless communications

• Hidden terminal

A DCB

• Exposed terminal

A DCB

Zhenhai Duan Computer 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