TheMediumAccessSublayer.ppt

CHAPTER 4 THE MEDIUM ACCESS CONTROL SUBLAYER(介质访问子层) The channel allocation problem Multiple access protocols 802.3: Ethernet 802.11: Wireless LANs 802.15: Bluetooth 802.16: Broadband wireless Data link layer switching THE CHANNEL ALLOCATION PROBLEM Two types of network: Those using point-to-point connection. Those using broadcast channels. In any broadcast network, the key issue is how to determine who gets to use the channel when there is competition for it. Consider a telephone conference call. 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). The MAC sublayer is the bottom part of the data link layer. The Channel Allocation Problem: Introduction How to allocate a single broadcast channel among competing users (信道分配问题) Static channel allocation in LANs and MANs TDM FDM Dynamic channel allocation in LANs and MANs ALOHA CSMA Collision free protocols Limited-contention protocols … The Channel Allocation Problem: Static channel allocation in LANs and MANs The traditional way of allocating a single channel among multiple competing users is Frequency Division Multiplexing (FDM, 频分多路复用). When there is only a small and fixed number of users, each of which has a heavy (buffered) load of traffic, FDM is a simple and efficient allocation. When the number of senders is large and continuously varying or the traffic is busty, FDM has some problems. The Channel Allocation Problem: Static channel allocation in LANs and MANs Mathematical analysis Similar for other static channel allocation methods such as TDM The Channel Allocation Problem: Dynamic channel allocation in LANs and MANs Five key assumptions for the channel allocation problem Station Model (站模型). The model consists of N independent stations, each with a program or user that generates frames for transmi