计算机网络英文版4

Station Model
The model consists of N independent stations (e.g., computers, telephones, or personal communicators) Each station has a program or user that generates frames for transmission. The probability of a frame being generated in an interval of length Δt is λΔt, where λ is a constant (the arrival rate of new frames). Once a frame has been generated, the station is blocked and does nothing until the frame has been successfully transmitted.
《Computer Network v4》 cs.sjtu 2004-11-24
Dynamic Channel Allocation
All the dynamic channel allocation methods to be discussed are based on five key assumptions.
《Computer Network v4》 cs.sjtu 2004-11-24
Multiple Access Methods
Three strategies for channel allocation: Exercise no control at all: simply let a station try to use the channel, and do something when a collision happens. Applied in contention systems. Employ a round-robin technique: each station in turn is allowed to use the channel. Applied in token-based systems – the station that has the token may use the channel. Let a station place a reservation for the channel. Used in slotted systems. The problem is how to make a reservation.
《Computer Network v4》 cs.sjtu 2004-11-24
Carrier Sense Assumption
Carrier Sense. Stations can tell if the channel is in use before trying to use it. If the channel is sensed as busy, no station will attempt to use it until it goes idle. No Carrier Sense. Stations cannot sense the channel before trying to use it. They just go ahFra Baidu bibliotekad and transmit. Only later can they determine whether the transmission was successful.
《Computer Network v4》 cs.sjtu 2004-11-24
Single Channel Assumption
A single channel is available for all communication. All stations can transmit on it and all can receive from it. As far as the hardware is concerned, all stations are equivalent, although protocol software may assign priorities to them.
Chapter 4
The Medium Access Control Sublayer
Li-ping Shen 申丽萍 Email: shen-lp@cs.sjtu.edu.cn
The Medium Access Control Sublayer
Broadcast channels (multiaccess channels or random access channels ): a number of stations that share the same “wire.” If one station sends, all the others get to hear it. Problem: if you’re sharing a channel, then two or more stations may decide to start frame transmission at the same time, it will lead to frame collision, which means rubbish on the wire. Medium Access Control: Allocate the broadcast channel to one of the competing stations
《Computer Network v4》 cs.sjtu 2004-11-24
Topics
The Channel Allocation Problem Multiple Access Protocols Ethernet Data Link Layer Switching
《Computer Network v4》 cs.sjtu 2004-11-24
Station Model. Single Channel Assumption. Collision Assumption. (a) Continuous Time. (b) Slotted Time. (a) Carrier Sense. (b) No Carrier Sense.
《Computer Network v4》 cs.sjtu 2004-11-24
《Computer Network v4》 cs.sjtu 2004-11-24
Collision Assumption
If two frames are transmitted simultaneously, they will result in a collision. All stations can detect collisions. A collided frame must be transmitted again later. There are no errors other than those generated by collisions.
In pure ALOHA, frames are transmitted at completely arbitrary times. Collisions
《Computer Network v4》 cs.sjtu 2004-11-24
Throughput of Pure ALOHA
Let N be the average number of new frames submitted during a frame time Tframe (time needed to transmit a full frame). Poisson distributed. Let G be the average number of old and new frame submissions during a frame time (G >=N), Also Poisson distributed: Let P0 be the probability that frame does not suffer from collision => the throughput S=G*P0 A frame will not suffer a collision if no other frames are sent within one frame time before and after its start (totally two frame times long) Probability that a frame will not be damaged is
Static Channel Allocation
FDM /TDM with each user allocated equal-sized frequency/time portion is simple and efficient when the number of users is small and constant, and each user has a heavy traffic. It is inefficient if number of user is variable or when the traffic is bursty. The mean delay using FDM is N times worse than if all the frames were arranged orderly in a big central queue. interpret from queuing theory calculation
《Computer Network v4》 cs.sjtu 2004-11-24
Pure ALOHA
Principle: let users transmit whenever they have data to be sent. There will be collisions, of course. If a collision occurs, finish your current transmission and retry after a random amount of time. Contention Systems: multiple users share a common channel in a way that can lead to conflicts
The Channel Allocation Problem
Static Channel Allocation in LANs and MANs Dynamic Channel Allocation in LANs and MANs
《Computer Network v4》 cs.sjtu 2004-11-24
《Computer Network v4》 cs.sjtu 2004-11-24
Time Assumption
Continuous Time. Frame transmission can begin at any instant. There is no master clock dividing time into discrete intervals. Slotted Time. Time is divided into discrete intervals (slots). Frame transmissions always begin at the start of a slot. A slot may contain 0, 1, or more frames, corresponding to an idle slot, a successful transmission, or a collision, respectively.
《Computer Network v4》 cs.sjtu 2004-11-24
Multiple Access Protocols
ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Limited-Contention Protocols