Computer network Learning Note _4

3-2_retransmissions

Presenter Arq, mainly used in the error is common and must be corrected situation, WiFi, TCP is used. The rule of ARQ is that Recerver receives the correct frame to be automatically postback Ack,sender if no ACK is received within the timeout, the frame is re-sent. Arq has two problems, one is how long the timeout is, and one is the frame repetition. Timeout fixed time can not be too long too short, resolving frame repetition is in frames and acks add sequence numbers. The problem with frame duplication is that if the sender sends a data receiver and then sends the ACK timeout back, then sender repeats the frame receiver again. If you just distinguish between the current frame and the next frame with the Stop-and-wait method on the line, with a sequence numbers can be distinguished by 0 and 1. Stop-and-wait has great limitations and is effective for LANs, with low efficiency for high BD (bandwidth delay product) and large bandwidth limitations. Like r=1mbps,d=50ms. Back and forth 2d=100ms,1s Transfer 10 frames, add 1 frame 10,000bits, then the maximum rate of 100kb/sec, and then improve R is useless. So there's a further way, sliding window, while marking W frames, one RTT (round trip time=2d) to transmit W frames.

3-3_multiplexing

The typical multiplexing is introduced. TDM (Time Division Multiplexing) and FDM (Frequency Division Multiplexing). More used in TELECOMMUNICATIONS,TV and radio stations (FDM), GSM (2G Cellular). These traffic continues, and network traffic is bursty and needs to be multiple access schemes.

3-4_randomized_multiple_access

This paper mainly discusses the development process of traditional Ethernet and traditional multiple access control. Traditional ethernet:10 Mbps over GKFX coaxial Cable,with baseband signals,multiple access with ' 1-persistent csma/cd with BEB '. CSMA (Carrier Sense Multiple access method), CD (collision detecting).

3-5_wireless_multiple_access

It is mainly about 802.11 phsical layer and link layer. Multiple Access uses csma/ca,rts/cts optional. WiFi and classic ethernet,the Multiple access problem are solved with randomization. AP (Access to the Internet).

3-6_contention-free_multiple_access

Random Multiple access is good under low load. Not very good under high load. Spoke of the turn-taking multiple Access Protocols,token ring method, as well as if Token lost processing method.

3-7_lan_switches

The box can be these:

　　

switch is used in modern Ethernet, which allows multiple users to use their bandwidth in parallel. Find the corresponding output port from the input frame by address. One difference between switch and hub is that switch is a pass-through, the hub is a pass-through, the hub is not selective. In addition, the backward learning algorithm of multiple switch is introduced, which is used in the no loops case.

Computer network Learning Note _4