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The Data Link Layer ( 数据链路层 )

The Data Link Layer ( 数据链路层 ). Chapter 3. 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）. 3.1.1 Services provided to the network layer.

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The Data Link Layer ( 数据链路层 )

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  1. The Data Link Layer(数据链路层) Chapter 3

  2. 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）

  3. 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）

  4. 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

  5. Functions of the Data Link Layer (2) Relationship between packets and frames.

  6. Services Provided to Network Layer (a) Virtual communication. (b) Actual communication.

  7. Services Provided to Network Layer (2) Placement of the data link protocol.

  8. H1 H2 应用层 应用层 R1 R2 R3 运输层 运输层 网络层 网络层 网络层 网络层 网络层 链路层 链路层 链路层 链路层 链路层 物理层 物理层 物理层 物理层 物理层 数据链路层的简单模型 主机H1向H2发送数据 路由器R1 路由器R3 主机H1 路由器R2 主机H2 局域网 广域网 电话网 局域网 从层次上来看数据的流动

  9. 数据链路层的简单模型( 续） 主机H1向H2发送数据 路由器R1 路由器R3 主机H1 路由器R2 主机H2 局域网 广域网 电话网 局域网 仅从数据链路层观察帧的流动 H1 H2 应用层 应用层 R1 R2 R3 运输层 运输层 网络层 网络层 网络层 网络层 网络层 链路层 链路层 链路层 链路层 链路层 物理层 物理层 物理层 物理层 物理层

  10. 3.1.2 Framing • Framing method(成帧的方法): 3 （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 ）. -（01111110）-[see fig 3-5] • Physical layer coding violation （1->高-低电平对，0->低-高电平对）

  11. Framing (2) A character stream. (a) Without errors. (b) With one error.

  12. Framing (3) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

  13. 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.

  14. 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

  15. 3.2 Error Detection and Correction • Error-Correcting Codes（纠错码） • codeword （码字） • Hamming distance (10001001,10110001=>00111000,3) • parity bit • 纠正d比特错 • 需距离为2d+1比特的编码 • Error-Detecting Codes（检错码） • polynomial code（cyclic redundancy code，CRC code） • 检测d比特错 • 需距离为d+1比特的编码

  16. 3.2.1 Error-Correcting Codes (*) Use of a Hamming code to correct burst errors.

  17. 3.2.2 Error-Detecting Codes • Calculation of the polynomial code checksum • 多项式编码 • （循环冗余码，CRC码） • 生成多项式 G(x) • 计算校验和的算法： • -[see page 197] • CRC-12 = x12+x11+x3+x2+x1+1 • CRC-16 = x16+x15+x2+1 • CRC-CCITT = x16+x12+x5+1 • 结论： • 具有r检测位的多项式能够检 • 测所有长度<=r的突发错误 • 长度大于r+1的错误逃脱的 • 概率为1/2r.

  18. 循环冗余检验的原理说明 1101010110←Q商 除数P →110101 101000110100000←2nM 被除数 110101 111011 110101 111010 110101 111110 110101 101100 110101 110010 110101 01110←R余数

  19. 3.3 Elementary Data Link Protocols • An Unrestricted Simplex Protocol（一种无限制的单工协议） • A Simplex Stop-and-Wait Protocol（一种单工的停-等协议） • A Simplex Protocol for a Noisy Channel（有噪声信道的单工协议） • 进程(process)、分组(packet)和帧(frame) • wait_for_event(&event), 3 events: • frame_arrival: Received correct frame • cksum_err: Received frame with checksum error • timeout: Timeout when waiting acknowledgement

  20. Protocol Definitions Continued  Some definitions needed in the protocols to follow. These are located in the file protocol.h.

  21. Protocol Definitions(ctd.) Some definitions needed in the protocols to follow. These are located in the file protocol.h.

  22. 3.3.1 Unrestricted Simplex Protocol(一种无限制的单工协议)

  23. 3.3.2 Simplex Stop-and-Wait Protocol(一种单工的停-等协议) • stop-and-wait,单向数据传输,发送方网络层一直有无限的数据要发送, 从不损坏或丢失帧. • 抛弃约束条件:接收方网络层一直可以接收,接收方缓冲空间无限大.

  24. 3.3.3 A Simplex Protocol for a Noisy Channel • 一个肯定确认的重传协议(PAR) • 计时器(timer)、序号(sequence)、PAR/ARQ、时间间隔(timeout) • network layer of sender has unlimited data to transmit. • Discard restricted condition:channel never damages or loses frame. PAR:Positive Acknowledgement with Retransmission ARQ:Automatic Repeat reQuest Continued 

  25. A Simplex Protocol for a Noisy Channel (ctd.) A positive acknowledgement with retransmission protocol.

  26. 3.4 Sliding Window Protocols（滑动窗口协议） • Bidirectional transmission, piggybacking（捎带）, better use of available bandwidth • slide window • sending window: a set of sequence numbers corresponding to frame it is permitted to send for sender • receiving window: corresponding to the set of frames which is permitted to accept for receiver

  27. Sliding Window Protocols (2) • A One-Bit Sliding Window Protocol • A Protocol Using Go Back N（后退n帧） • A Protocol Using Selective Repeat（选择性重传）

  28. Sliding Window Protocols (3) A sliding window of size 1, with a 3-bit sequence number. (a) Initially. (b) After the first frame has been sent. (c) After the first frame has been received. (d) After the first acknowledgement has been received.

  29. 3.4.1 A One-Bit Sliding Window Protocol Continued 

  30. A One-Bit Sliding Window Protocol (ctd.)

  31. A One-Bit Sliding Window Protocol (2) Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case. The notation is (seq, ack, packet number). An asterisk(*) indicates where a network layer accepts a packet.

  32. 3.4.2 A Protocol Using Go Back N Pipelining and error recovery. Effect on an error when (a) Receiver’s window size is 1. (b) Receiver’s window size is large.

  33. Sliding Window Protocol Using Go Back N (2) • 接收窗口大小为W=1 • 抛弃所有后续的帧 • 最多允许有MAX_SEQ帧未被确认 • Stop-and-wait is not fitful for channel which has long round-trip delay, e.g. Satellite channel,270 ms • Bidirectional transmission, noisy channel, • Discard restricted condition: network layer of sender has unlimited data to transmit • size of receiving window = 1, receiver discards all subsequent frames after a damaged or lost data frame.

  34. Sliding Window Protocol Using Go Back N (3) 思考题 • MAX_SEQ+1 distinct sequence numbers(0,1,2,... MAX_SEQ),no more than MAX_SEQ unacknowledged frames,the sender window<=MAX_SEQ. Why? Consider the following scenario with MAX_SEQ=7(a sender window of size eight): • The sender sends frames 0 through 7. • A piggybacked acknowledgement for frame 7 eventually comes back to the sender. • The sender sends another eight frames,again with sequence numbers 0 through 7. • All eight frames belonging to the second batch get lost • All eight frames belonging to the second batch arrive successfully. • Another piggybacked acknowledgement for frame 7 come in, ambiguity will arise to sender: • All eight frames belonging to the second batch get lost,ack=7 • All eight frames belonging to the second batch arrive successfully,ack=7

  35. Sliding Window Protocol Using Go Back N (4) • A Protocol Using Go Back n( when the size of sender window=7,no ambiguity: • The sender sends frames 0 through 6 • A piggybacked acknowledgement for frame 6 eventually comes back to the sender • The sender sends another 7 frames:7,0,1,2,3,4,5, • Frame 7,0,1,2,3,4,5 belonging to the second batch get lost • Frame 7,0,1,2,3,4,5 belonging to the second batch arrive • Another piggybacked ack nowledgement comes in,no ambiguity • Frame 7,0,1,2,3,4,5belonging to the second batch get lost,ack=6 • Frame 7,0,1,2,3,4,5 belonging to the second batch arrive,ack=5 • the problem : • Since a sender may have to retransmit all the unacknowledged frames at a future time ,it must hang on to all transmitted frames until it knows for sure that they have been accepted by the receiver. • No acknowledged frames without data.

  36. Sliding Window Protocol Using Go Back N (5) Continued 

  37. Sliding Window Protocol Using Go Back N (6) Continued 

  38. Sliding Window Protocol Using Go Back N (7) Continued 

  39. Sliding Window Protocol Using Go Back N (8)

  40. Sliding Window Protocol Using Go Back N (9) Simulation of multiple timers in software.

  41. 3.4.3 A Sliding Window Protocol Using Selective Repeat • 允许接收后续的帧（接收窗口 >1） • 窗口大小为W<=(MAX_SEQ+1)/2 • 所需缓冲器的数量=W • 所需计时器的数量=W • 否定性确认帧-NAK 思考题

  42. A Sliding Window Protocol Using Selective Repeat (2) Continued 

  43. A Sliding Window Protocol Using Selective Repeat (3) Continued 

  44. A Sliding Window Protocol Using Selective Repeat (4) Continued 

  45. A Sliding Window Protocol Using Selective Repeat (5)

  46. A Sliding Window Protocol Using Selective Repeat (6) (a) Initial situation with a window size seven. (b) After seven frames sent and received, but not acknowledged. (c) Initial situation with a window size of four. (d) After four frames sent and received, but not acknowledged.

  47. * 3.5 Protocol Verification • Finite State Machined Models • Petri Net Models

  48. *3.5.1 Finite State Machined Models (a) State diagram for protocol 3. (b) Transmissions.

  49. *3.5.2 Petri Net Models A Petri net with two places and two transitions.

  50. *Petri Net Models (2) A Petri net model for protocol 3.

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