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第 6 讲 数据链路层( 2 ). 知识回顾. 数据链路层的功能 帧定界 差错检测 循环冗余检码. CRC 工作原理. 举例 CRC. 本讲内容及教学目标. 掌握 PPP 协议在数据链路层的工作原理 了解面向比特的链路层协议 HDLC 的帧 理解停止等待协议 理想化的数据传输 具有最简单流量控制的数据链路层协议 实用的停止等待协议 理解滑动窗口协议,实现流量控制 了解传输层类似的流量控制功能. 数据链路层协议分类. 点到点线路 广播线路 局域网使用 Internet 中主要的数据链路层协议 SLIP ( Serial Line IP ) - PowerPoint PPT Presentation
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6 2
CRC
CRC
PPP HDLC
InternetSLIP Serial Line IP ;PPP Point-to-Point Protocol -SLIPPPPISPInternet
PPP PPP (Point-to-Point Protocol) PPP PPP;PPPISP
ISP PPP
IP ISPPPP
PPP
Maximum Transmission UnitMTU IPMTU1500IP1500IP64KBIPIP
PPP
PPP 1992 PPP 1993 1994 PPP [RFC 1661] PPP IP LCP (Link Control Protocol) NCP (Network Control Protocol)
PPP () F = 0x7E 0x 7E 01111110 A 0xFF C 0x03PPP PPP
PPP PPP 2 0x0021 PPP IP 0xC021, PPP 0x8021 IP 121112 1500 PPP 7EFF03FACFCSF7E
PPP PPP PPP 0x7E 2 (0x7D, 0x5E) 0x7D , 2 (0x7D, 0x5D) ASCII 0x20 0x7D PPP SONET/SDH PPP
0 1 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 00 1 0 0 1 1 1 1 1 1 0 0 0 1 0 1 00 1 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 0 F 8 5 1 0 5 1 0 F 0 0
PPP PPP PPP IP FCS
PPP ISP PC LCP PPP PPP NCP PC IP PC NCP IP LCP
LCP LCP LCP NCP LCP NCP LCP LCP
HDLC HDLC1974IBM SDLC (Synchronous Data Link Control) ISO SDLC HDLC (High-level Data Link Control)ISO 3309CCITT HDLC LAP (Link Access Procedure)HDLC LAP LAPBB(Balanced) LAPB ()
HDLC F (Flag) 6 1 0 8 bit 888168 Info F F A C FCSFCS
HDLC01111110511010
HDLC F 6 1 5 1 0 F 5 1 0
A BAP2AP1
cont. 1 2 2
A B
()
cont.
ABDATADATADATADATA B B B BABDATA BDATA B
ABACKACK(a)
A(timeout timer) tout B A
1 B B B A ACK B A ACK
0 1
N(S) 0 1
ARQ ARQ ARQ
(slide)--1n0~2n-1sending windowreceiving window
WWWll+WWj
ll+WiW
30~73012345670123456701234567012345670123456701234567012345670123456701234567012345670123456701234567
-10~11010101010101010101010101
piggybacking
go-back-n11112n-1(n)
selective repeat112n-11/2
PPPPPP
P105,3-09,3-11,3-13,3-14,3-19ARQARQ