【数据通信与网络】Lecture10-MultipleAccessAndLAN

– Only decrement timer while medium is idle!
Yes
Busy?
No
No Decrement t No
Collision can only happen if two stations generate same timer value
t=0? Yes Send frame
Data communications and networking
(1) Random Access Prococal
MA—Multiple Access
Data communications and networking
• Aloha (随机多址通信)
– Packet radio protocol – Random-access method based on acknowledgements and backoffs
– Other stations remain quite during this period – Optional, as the overhead may be too high for small frames
Data communications and networking
1-persistent
– Send as soon as channel is idle
Non-persistent
– Wait a random period of time before sensing again
Data communications and networking
p-Persistent
10.2 IEEE802 LANs
10.3 Extending LANs: repeater, bridge, router
10.4 Summary
Data communications and networking
10.1 Multiple Access
Goal: To access a shared media in a controlled fashion.
Data communications and networking
CSMA/CD—— CSMA with Collision Detection
Exponential back-off
– Wait 2N ´ max_propagation_time after collision, where N is number of transmission attempts
Reduces the possibility of collisions
– Does not eliminate collisions – Propagation delay
• Takes time before all other stations can sense a transmission
Tpr
Tpr
Two (or more) stations sending at the same time Collisions must be detected by all stations Minimum packet size and mximum bus length
– 72 bytes (64 bytes at data link layer) and 500 meters for 10Base5 (―Thick Ethernet‖)
Data communications and networking
IEEE 802.11 CSMA/CA Wait Procedure
t = random(0, maxwait)
Sense carrier
When medium becomes idle, wait a random amount of time
Wireless Networks—Signal Strength Problem
• To detect collisions, a station needs to compare transmit and receive signals • The signal from station’s own transmitter is much stronger than the signals from other stations
Station 1
Station 2
Data communications and networking
Binary Exponential Back off Algorithm in CSMA/CD (二进制指数后退算法) How randomization Back off time is done when a collision occurs ?
Data Communications and Networking
Lecture 10. Multiple Access and Local Area Network
数据通信与计算机网络
Lecture 10. Multiple Access and Local Area Networks
10.1 Multiple Access
Data communications and networking
CSMA/CA With RTS/CTS
Request-to-send/clear-to-send (RTS/CTS) handshake
– RTS/CTS frames contain Duration field
• Period of time the medium is reserved for transfer
Data communications and networking
Wireless Networks—Hidden Terminal Problem
• Stations may not detect each others signals
Data communications and networking
Pure ALOHA (纯ALOHA) Slotted ALOHA (开槽ALOHA)
Data communications and networking
Pure ALOHA Basic idea: • let users transmit whenever they have data to be sent. • If the frame was destroyed, the sender just waits a random amount of time and sends it again Rate of channel utilization : 18 percent Slotted ALOHA Basic idea: Divide time into discrete intervals, each interval corresponding to one frame. Channel utilization : 37 percent
Solution: • After a collision, time is divided into discrete slots. slot’s length is twice of maximal propagation time in the network (2). Station waits for 0 or 1 slot. • • • After i collisions, a random number from 0 to 2i - 1 is chosen After ten collisions have been reached, the randomization interval is
Data communications and networking
Carrier Sense Multiple Access (CSMA)
(载波侦听多路访问)
Carrier sense
– Listen (sense) before talk(sending) (LBT) – Do not send unless the medium is idle
Send jam so other stations detect collision as well
Listen When Talk —— LWT :
– Abruptly stop transmitting as soon as the collision is detected
Data communications and networking
Difference: Slot ALOHA limits sending time at every time and Pure ALOHA does not.
Data communications and networking
Aloha Protocol
Data communications and networking
CSMA/CA—Carrier Sense Multiple Access with Collision Avoidance
– Control signals before transmission – Carrier sense:
• do not transmit immediately when medium gets idle (ppersistence) • Wait random time
Data communications and networking
CSMA/CA
冲突避免的载波侦听多路访问
Collision detection does not work well on wireless networks