计算机网络chapter3数据链路层.pptVIP

The Data Link Layer(数据链路层) Chapter 3 Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders Functions of the Data Link Layer (2) Relationship between packets and frames. 3.1 Data Link Layer Design Issues Services Provided to the Network Layer （为网络层提供的服务） Framing（帧） Error Control（差错控制） Flow Control（流量控制） Services Provided to Network Layer (a) Virtual communication. (b) Actual communication. 3.1.1 Services provided to the network layer Unacknowledged connectionless service（无确认的无连接服务） Acknowledged connectionless service （有确认的无连接服务） Acknowledged connection-oriented service （有确认的面向连接的服务） Services Provided to Network Layer (2) Placement of the data link protocol. 数据链路层的简单模型 数据链路层的简单模型( 续） 3.1.2 Framing Framing method(成帧的方法): 4 （1）character count（字符计数法 ） -[see fig 3-3] （2）Starting and ending character,with character stuffing （带字符填充的首尾界符法 ）-[see fig 3-4] Frame flags: DLE STX(Start Text), DLE ETX(End Text) DLE in data(between flags): DLE--DLE DLE, so that: DLE ETX-- DLE DLE ETX （3）Starting and ending flags,with bit stuffing（带位填充的首尾界符法 ）. --[see fig 3-5] （4）Physical layer coding violation （a 1 bit -high-low pair, a 0 bit -low-high pair） Framing (2) A character stream. (a) Without errors. (b) With one error. Framing (3) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing. Framing (4) Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing. 3.1 DATA LINK LAYER DESIGN ISSUES 3.1.3 Error Control（差错控制） Error detection Error correction Repeated data frame Lost frame Timer 3.1.4 Flow Control（流量控制） To solve the problem that a sender wants to transmit frames faster than the receiver can accept them. Feedback-based flow control Rate-based flow control 3.2 Error Detection and Correction