Network Technology Tutorials Notes (17)

<title>WAN and access Network technology</title> WAN and access Network Technology 1. Common WAN Technology--HDLC

The so-called character-oriented, that is, the data transmitted on the link must be the specified character set of characters. Furthermore, the error messages transmitted on the link must also consist of the control characters specified in the specified character set. This character-oriented link control rule plays an important role in the development of computer network.

With the development of communication technology, this character-oriented communication protocol gradually exposes its weaknesses. It is summarized as follows: The utilization of ① communication link is low, because it uses to stop waiting for the protocol, send and receive the two sides to carry on the work; ② all communication devices must use the same character code, ③ only to the data part of the error control, if the control part error can not be controlled error, so it is not high reliability ; The ④ is not easy to expand, and a new control character will be set for each function extension. Since there are some other shortcomings, it is necessary to design a new link control rule instead of the character-oriented rule.

Data Link Control protocol is divided into character-oriented protocol and bit-oriented protocol. The character-oriented protocol uses characters as the basic unit of transmission, with 10 special characters to control the transmission process. Bit-oriented protocol is a basic unit of transmission, and its transmission rate is high, and it is widely used in public data network.

HDLC is a widely used data link control protocol for bit-oriented

2. Common WAN technology--HDLC frame structure and type

The first field is identified with the End field 01111110, in order to avoid error recognition when the intermediate field appears 01111110, it will be filled with a 0 immediately after the occurrence of 11111, and there will be no error identification problem. When this frame is received, the Flag field 01111110 is found, the boundary is determined, the bit stream is scanned, and when 11111 is found, a subsequent 0 is deleted, so that the original bitstream can be restored. It ensures that no matter what bit combination is occurring, there is no judgment error in the frame boundary.

The Address field is also 8 bits, and the Address field is always written to the address of the station when the data is transferred in an unbalanced manner, and the address of the answering station is always written when the transmission is balanced.

The control field is the most replicated field, and many of the replication capabilities of HDLC are implemented by the control field, where the value of the first two bits divides the HDLC frames into three main classes: information frames, managed frames, and unnumbered frames. If the first bit of the control field is 0, it is the information frame. Manages a frame when the first second bit of the control field is 10. When the first to second bit of the control field is 11, then there is no numbered frame. Information frames are mainly used to host user data, management frames are mainly used for traffic and error control, no numbered frames are mainly used for link control.

Data link layer in Internet

There are two ways that users access the Internet: one is to use dial-up telephone lines to access the Internet, and the other is to connect using a dedicated line. Regardless of the method used, there is a data Link layer protocol at the time of the connection, and the most extensive on the internet is the slip and PPP protocol.
SLIP (Serial lines Internet Protocol, Serial Line Internetwork Protocol), an old industry standard for Windows remote access, is used primarily in UNIX remote access servers and is still used to connect some ISPs today. Because the SLIP protocol is for low-speed serial lines, it can be used for a dedicated line or for dial-up lines, and the modem has a transmission rate of 1200bps to 19200bps.
Today, point-to-Point Protocol (PPP) widely replaces SLIP because it has more features and more flexibility. Point-to-Point Protocol (PPP) provides a standard method for transmitting multiprotocol packets over a point-to-point connection. PPP was originally designed to provide an encapsulation protocol for IP traffic transfer between two peer nodes.
The most common PPP protocols are PPPOA and PPPoE. Currently users are most commonly used by PPPoE.

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