计算机网络英文版课件4.ppt

江西师大软件学院 Computer networkChapter 4The Medium Access Control Sublayer 郭 斌 E_mail: jxnuit@163.com Introduction In any broadcast network, the key issue is how to determine who gets to use the channel when there is competition for it. In the literature, broadcast channels are sometimes referred to as multiaccess channels or random access channels. The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer. 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 competition stations Topics The Channel Allocation Problem Multiple Access Protocols Ethernet Data Link Layer Switching The Channel Allocation Problem Static Channel Allocation in LANs and MANs Dynamic Channel Allocation in LANs and MANs 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. Problem: Long delay The line utilization is low Dynamic Channel Allocation All the dynamic channel allocation methods to be discussed are based on five key assumptions. Station Model. Single Channel Assumption. Collision Assumption. Time assumption (a) Continuous Time. (b) Slotted Time. Carrier sense assumption (a) Carrier Sense. (b) No Carrier Sense. Station Model The mo