PPP笔记

NRZ-L:Non-Return-Zero Line
AMI:Alternate Mark Inversion
HSSI:High-Speed Serial Interface

平时说的复用如TDM、FDM，是针对物理层来说的。

多种从ISO的HDLC派生链路访问协议：
x.25的LAPB
isdn的D信道LAPD
调制解调器的LAPM
帧中继的LAPF

HDLC Operations, and Frame Types

There are three fundamental types of HDLC frames.

    * Information frames, or I-frames, transport user data from the network layer. In addition they can also include flow and error

control information piggybacked on data.
    * Supervisory Frames, or S-frames, are used for flow and error control whenever piggybacking is impossible or inappropriate, such

as when a station does not have data to send. S-frames do not have information fields.
    * Unnumbered frames, or U-frames, are used for various miscellaneous purposes, including link management. Some U-frames

contain an information field, depending on the type.

show processes cup 检查cpu的负载

pap与chap组合时，步骤为：首先尝试第一个，如果第一个协商不成功，则尝试第二个，如果第一个验证方法失败，则不再进行第二个验

证。