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1、百度文库专用Information & Communication College,1,刘联海,WELCOME TO,COMPUTER NETWORKS,百度文库专用,2,Review, Specific Functions Services Framing Error Control Flow Control CRC Elementary Data Link Protocols Sliding Window Protocols HDLC,百度文库专用,3, So far: We have discussed the Data Link Layers functionality and som
2、e protocols related to point-to-point communication. A large class of networks is built on top of broadcast channels: a number of stations share the same “wire.” If one station sends, all the others get to hear it. Whats the problem: if youre sharing a channel, then two stations may decide to start
3、frame transmission at the same time = frame collision,Chapter 4. The Medium Access Control Sublayer,百度文库专用,4, Solution: Allocate the channel to one of the competing stations. Problem: Youll have to use that same channel to figure out the competition and the allocation. The protocols used to determin
4、e who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.,Chapter 4. The Medium Access Control Sublayer,百度文库专用,5,Main topics: MULTIPLE ACCESS PROTOCOLSALOHA CSMA/CD ETHERNET WIRELESS LANS DATA LINK LAYER SWITCHING,Chapter 4.
5、 The Medium Access Control Sublayer,百度文库专用,6,信道分配策略,随机访问 (争用)-负载较轻时,控制访问(负载较重时),静态分配(将频带或时间片等固定分给各站点)站点少且固定，数据量大时,频分多路复用同步时分多路复用波分多路复用码分多路复用,动态分配(异步时分多路复用)仅当有数据发时,才占信道发数据 。,轮转 (每个站轮流获得发送机会),预约 (各站先声明有数据要发送, 然后按预约顺序发送),百度文库专用,7,Three strategies for channel allocation:1. Exercise no control at all: si
6、mply let a station try to use the channel, and do something when a collision happens. Applied in contention（竞争） systems. 2. 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. 3. Let a
7、station place a reservation（预约） for the channel. Used in slotted systems. The problem is how to make a reservation.,4.1 The Channel Allocation Problem,百度文库专用,8,4.2.1 ALOHA4.2.2 Carrier Sense Multiple Access Protocols4.2.5 Wavelength Division Multiple Access Protocols4.2.6 Wireless LAN Protocols,4.2
8、Multiple Access Protocols,百度文库专用,9,4.2.1 ALOHA: Pure ALOHA,Principle: if you want to send a frame, just do it. If a collision occurs, finish your current transmission and retry later.,百度文库专用,10,纯ALOHA冲突重发,百度文库专用,11,Figure 4-2. Vulnerable period for the shaded frame.,4.2.1 ALOHA,百度文库专用,12,1, Let S（吞吐
9、量） be the average number of new frames submitted during a frame time Tframe (time needed to transmit a full frame). Poisson distributed（泊松分布）.2, Let G（载荷） be the number of old and new frame submissions during a frame time (GS). Also Poisson distributed. 在给定的一帧时间共生成k帧的概率如下： Pk frames submitted=GK*e-G
10、/k!3, Let P0(传输成功的概率) be the probability that frame does not suffer from collision = S=GP0.4, 生成0帧的概率为e-G，而两个帧时间长的间隔所生成的帧平均数是2G，所以：Probability that a frame will not be damaged is P0=e-2G5, S=Ge-2G, 当G0.5时，吞吐量最大，为S0.184，即信道利用率为18。,4.2.1 ALOHA: Pure ALOHA,百度文库专用,13,原理：由于采用时槽技术，所以冲突危险周期被减少了一半，所以在同一个时槽中
11、没有其他流量的概率是e-G，于是S=Ge-G，由此可以得出当G1时，其信道利用率最高，可达到36。,4.2.1 ALOHA: slotted ALOHA,百度文库专用,14,Pure ALOHA vs. Slotted ALOHA,Figure 4-3. Throughput versus offered traffic for ALOHA systems.,百度文库专用,15,Carrier Sense Multiple Access Protocols do better than ALOHA: you monitor the channel before and/or during tr
12、ansmission. Persistent CSMA Nonpersistent CSMA CSMA/CD,4.2.2 Carrier Sense Multiple Access Protocols,百度文库专用,16,1, 1-persistent（持续的CSMA）: Listen whether the channel is free before transmitting. If busy, wait until it becomes free and then immediately start your transmission. If a collision occurs, th
13、e station waits a random amount of time and starts all over again.2, Nonpersistent（非持续的CSMA）: Less greedy（至少没有贪婪） when the channel is busy, wait a random period of time before trying again. If you wait too long, the channel utilization drops.3, p-Persistent: Used with slotted systems. If you find th
14、e channel idle during the current slot, you transmit with probability p, and defer until next slot with probability 1-p. p1 is not really good, p0 makes you really polite. 对ALOHA的改进之处：1，保证了当检测到信道忙时，所有的站都不再传送数据；2，每个站检测到由冲突，它就放弃它当前的传送任务。,Persistent and Nonpersistent CSMA,百度文库专用,17,Improvement: sense t
15、he channel, but immediately stop transmission when you detect a collision. Ethernet works like this. 1. Listen to see whether the channel is free. Transmission is delayed until the channel is no longer used.2. During transmission, keep listening in order to detect a collision. If a collision occurs,
16、 transmission immediately stops.3. If a collision occured, wait a random period of time, and proceed with the first step again.,CSMA with Collision Detection,百度文库专用,18,Figure 4-5. CSMA/CD can be in one of three states: contention, transmission, or idle.,CSMA with Collision Detection,百度文库专用,19,Basic idea: Often, there are a number of base stations connected through guided media. A base station can communicate with a mobile computer. The mobile computers use radio/infrared signals for communication.Problem: There can be subtle interference.,
