Division of data link layer, protocols used in data link layer and detailed introduction

- Data link layer division ：

• The data link layer of LAN is divided into logical link layer LLC And media access control MAC Two sublayer .
  1、 Logical link control （Logical Link Control Or abbreviation LLC） It is the upper part of the data link layer in the LAN ,IEEE 802.2 Logical link control protocol is defined in . The user's data link service passes through LLC The sublayer provides a unified interface for the network layer . stay LLC Below the sublayer is MAC Sublayer .
  2、MAC(medium access control) Belong to LLC（Logical Link Control） Next sub layer . Two media access control methods widely used in LAN , Namely ：
  （1）、 Contention media access control , Also known as the media access control protocol with the machine , Such as CSMA/CD The way .
  （2）、 Deterministic media access control , Also known as ordered Access Control Protocol , Such as Token( token ） The way .

- The protocols used in the data link layer are ：

• The data link layer is mainly responsible for transmitting data ,⽤ The most is local ⽹ Etheric in ⽹ agreement , and ⼴ Domain ⽹ Medium PPP agreement ,HDLC agreement .

- Ethernet protocol ：

• Ethernet is the most widely used LAN technology . Because of its simplicity 、 The cost is low 、 High scalability 、 And IP Net can be well combined with other characteristics , The application of Ethernet technology is moving forward from the internal network of enterprises to the field of public telecommunication network . Ethernet access refers to the combination of Ethernet technology and generic cabling , As the access network of public telecommunication network , Provide users directly based on IP The transmission channel of various services . The essence of Ethernet technology is a two-layer media access control technology , It can be transmitted on five kinds of lines , It can also be combined with other access media , Form a variety of broadband access technologies . Ethernet and telephone copper cables VDSL Combination , formation EoVDSL technology ; Combined with passive optical networks , produce EPON technology ; In a wireless environment , Develop into WLAN technology . Ethernet technology because of its simplicity 、 Low cost 、 Advantages of easy expansion , It has become very popular in user desktop systems and enterprise internal networks , With the development of technology, innovation , Its application field is gradually moving towards access network 、 Metropolitan area network 、 Even Wan / Expand the backbone network , Formed on IP/Ethernet Seamless end-to-end connection . The protocol used by Ethernet is called Ethernet Protocol .

- PPP agreement （ Point to point protocol (Point to Point Protocol)）：

• PPP The protocol is a link layer protocol designed for simple links such as transmitting data packets between the same units . This link provides full duplex operation , And pass the packets in order . The design purpose is mainly to establish point-to-point connection and send data through dial-up or special line , Make it a variety of hosts 、 A common solution for simple connections between bridges and routers .PPP Has the following functions ：
  （1）PPP With dynamic allocation IP The ability to address , Allow negotiation at connection time IP Address ;
  （2）PPP Support multiple network protocols , such as TCP/IP、NetBEUI、NWLINK etc. ;
  （3）PPP With error detection capability , But they don't have the ability to correct mistakes , therefore PPP It's an unreliable transport protocol ;
  （4） No retransmission mechanism , The network cost is small , Fast ;
  （5）PPP With authentication ;
  （6） PPP It can be used on many types of physical media , Including serial cable 、 Telephone line 、 Mobile phones and fiber optics （ for example SDH）,PPP Also used in Internet Access .
• To configure PPP agreement ：

-  Basic configuration 
	1： Enable ppp
	RouterTest#config terminal
	Enter configuration commands,one per line. End with CNTL/Z.
	RouterTest(config)#interface serial 0/0
	RouterTest(config-if)#encapsulation ppp
	RouterTest(config-if)#
	2： Address configuration command 
	RouterTest(config-if)#ip address 10.1.1.1 255.255.255.0

- PAP Configure the instance 
	Router(config)#hostname RouterA
	RouterA(config)#RouterB password 123456
	RouterA(config)#interface Async 0
	RouterA(config-if)#encapsulation ppp
	RouterA(config-if)#ip address 10.0.0.1 255.255.255.0
	RouterA(config-if)#dialer-map ip 10.0.0.2 name RouterB 6666666
	RouterA(config-if)#username RouterA password 654321
	Router(config)#hostname RouterB
	RouterB (config)#RouterA password 123456
	RouterB (config)#interface Async 0
	RouterB (config-if)#encapsulation ppp
	RouterB (config-if)#ip address 10.0.0.2 255.255.255.0
	RouterB (config-if)#dialer-map ip 10.0.0.1 name RouterA 6666666
	RouterB (config-if)#username RouterB password 654321

- CHAP Configure the instance 
	Router(config)#hostname RouterA
	RouterA(config)#RouterB password 123456
	RouterA(config)#interface Async 0
	RouterA(config-if)#dialer in-band
	RouterA(config-if)#encapsulation ppp
	RouterA(config-if)#ppp authentication chap
	RouterA(config-if)#ip address 10.0.0.1 255.255.255.0
	RouterA(config-if)#dialer-map ip 10.0.0.2 name RouterB 6666666
	RouterA(config-if)#username RouterA password 654321
	Router(config)#hostname RouterB
	RouterB (config)#RouterA password 123456
	RouterB (config)#interface Async 0
	RouterB(config-if)#dialer in-band
	RouterB (config-if)#encapsulation ppp
	RouterB (config-if)#ppp authentication chap
	RouterB (config-if)#ip address 10.0.0.2 255.255.255.0
	RouterB (config-if)#dialer-map ip 10.0.0.1 name RouterA 6666666
	RouterB (config-if)#username RouterB password 654321

- CHAP and PAP
	Router(config-if)#ppp authentication chap pap
	 To configure PPP Back to dial the 

-  Multiple links PPP
	 By using multiple links PPP, You can bundle multiple connections into a virtual connection .
	Router(config-if)#ppp multilink
	Router(config-if)#dialer load-threshold load [outbound | inbound | either]
	 command dialer load-threshol load Specify when more B The channel is added to MLP In link bundle . When all B After the total load of the channel exceeds the specified threshold , Dial up interface （BRI or PRI） Add the channel to the multi link bundle .
	 Again , If the total load is below the threshold , Will be removed B channel .
	 Parameters load Is the average load of the interface , Its value is 1（ No load ） To 255（ Full load ）.
	 Parameters outbound（ The default value is ） Specify that only outbound data flows are considered when calculating the load ; Parameters inbound Specify that only inbound data flows are considered ;either Specify when calculating the load , Choose the larger of the outbound load and inbound load .

- HDLC agreement ：

• HDLC（High-Level Data Link Control, Advanced data link control ）, It is an international standard of link layer protocol , It is used to realize resource sharing and information exchange among remote users .HDLC The protocol is used to ensure that the data transmitted to the next layer can be accurately received in the transmission process , That is, there is no loss in error release , And the sequence is correct .HDLC Another important function of the protocol is flow control , That is, once the receiving end receives the data , It can be transmitted immediately . In the field of communication ,HDLC The protocol is widely used , Its working mode can support half duplex 、 Full duplex transmission , Support point-to-point 、 Multipoint structure , Support exchange type 、 Non switched channel .
• HDLC The protocol uses a unified frame format , Easy to use ; Zero bit insertion method is adopted , Easy to implement in hardware , And supports arbitrary bit stream transmission , Realize the transparent transmission of information ; Full duplex communication , High throughput , When no reply frame is received , Information frames can be sent continuously , Improve the efficiency of data link transmission ; use CRC Frame check sequence , It can prevent frame leakage , Improve the reliability of information transmission .
  HDLC The agreement has four main features ：
  1、 For any kind of bitstream, it can be transmitted transparently .
  2、 High data link transmission efficiency .
  3、 All frames have a frame check sequence （FCS）, High transmission reliability .
  4、 Use a unified frame format to realize transmission .
• To configure HDLC agreement ：

	 To be added ...