Post on 15-Dec-2015
Minimum Frame Size
• Why a minimum frame size is needed?• How long does it take for a station to notice a
collision?
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
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.
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