Review of Chapter 3 of network planning (data link layer)

Zhenzi diary 2022-01-15 01:38:25

Catalog

Chapter III important contents  

Mind mapping

Detailed explanation of examination outline

One . Functions of data link layer

Two . Some basic concepts of link layer

1. link

2. The data link

3. network adapter

3、 ... and . Three basic problems of link layer

1. Package into frames

2. Transparent transmission

3. Error detection

Four . Point to point channels

1. PPP The agreement has three components :

2.PPP The frame format of :

3.PPP Working state of

5、 ... and . Broadcast channel (CSMA/CD)

1. Channel division

2.CSMA/CD agreement ( Carrier answer , Multipoint access , collision detection )

3. Electromagnetic wave in 1KM The propagation delay of the cable is about 0 5us

4.CSMA/CD The concrete analysis of

5. Truncate binary exponential backoff algorithm

6. Why is the minimum frame specified by Ethernet 512 The bit

7. Strengthen the collision

8.CSMA/CD Process summary

  6、 ... and . Two sublayers of the data link

7、 ... and . Adapter

  8、 ... and . Ethernet MAC layer

Nine . Extended Ethernet

1. Extending Ethernet at the physical layer

 2. Expand Ethernet at the data link layer

3. Characteristics of Ethernet switch

Ten . Virtual LAN


Chapter III important contents  

1. Functions of data link layer

2. Point to point channel of data link layer (PPP agreement ) And broadcast channel (CSMA/CD agreement )

3. Three basic problems of data link layer : Package into frames , Transparent transmission , Error detection

Mind mapping

no

Detailed explanation of examination outline

One . Functions of data link layer

Continue to serve the network layer based on the services provided by the physical layer , Its main function is to strengthen the function of transmitting the original bit stream in the physical layer , Change the physical connection that may be wrong in the physical layer to a logically error free data link , Make it an error free link to the network layer .

Two . Some basic concepts of link layer

1. link

A physical line from one node to another

2. The data link

The hardware and software implementing the protocol are added to the link

3. network adapter

Network adapter is also called network card or network interface card (NIC), English name NetworkInterfaceCard. It's a device that connects computers to the Internet . Generally speaking, the network card is to PC Machine and LAN Connected network adapter . network card (NIC) Plug in the computer motherboard slot , Be responsible for converting the data to be transmitted by the user into a format that can be recognized by other devices on the network , Transmission through network media . Its main technical parameters are bandwidth 、 Bus mode 、 Electrical interface mode etc. . Its basic functions are : Data conversion from parallel to serial , Assembly and disassembly of package , Network access control , Data cache and network signal . Right now it's mainly 8 Bit and 16 Bit network card .
 

3、 ... and . Three basic problems of link layer

1. Package into frames

stay IP Datagram plus header and tail , A very important role of the head and tail is frame definition , And contains various control information

Here are four methods of packaging into frames

1. Character counting

2. The beginning and end delimiter method of character filling

3. Zero bit filling

The zero bit filling method uses a special bit pattern :01111110 As a qualifier

4. The violation code is used as the beginning and end

In Manchester code , Front high rear low 1, Front low rear high 0. Others are high in front and high in back , The first low and the second low can be regarded as illegal coding  

2. Transparent transmission

For transmitted data , There is nothing to prevent the data transmission .( That is, the delimiter does not work )

3. Error detection

CRC Cyclic redundancy check

Four . Point to point channels

PPP The protocol is the user and ISP The protocol used for communication .( Byte oriented protocol in serial line communication , It can be realized on the synchronous line , It can also be implemented on asynchronous lines )

1. PPP The agreement has three components :

(1) Link control protocol (LCP)

  (2)    Network control protocol   (NCP)

  (3)    One will IP Datagram encapsulation method

2.PPP The frame format of :

1.0x7E Is a delimiter

2.A and C No definition is given yet

3. The agreement determines PPP Agreement is IP Datagram or LCP still NCP

4.FCS Check sequence for frame

3.PPP Working state of

 

  When users dial in ISP when , A physical connection is established

  User computer to ISP Send a series of LCP grouping , establish LCP Connect

  Establishment and completion LCP after , Identification

5、 ... and . Broadcast channel (CSMA/CD)

A LAN USES broadcast channels , Now Ethernet occupies a large market in LAN , So sometimes they can replace each other .

Use CSMA/CD Ethernet protocol can only carry out half duplex communication

Local area networks can be divided into three categories according to their topology

  LAN uses a shared channel . So how to divide the channels ?

1. Channel division

① Static division channel : As mentioned in the previous chapter FDM,TDM,WDM,CDMA. But the price is too high

② Dynamic channel division : random access ( Can send messages at any time ), controlled access

2.CSMA/CD agreement ( Carrier answer , Multipoint access , collision detection )

Carrier answer : Use electronic technology to detect whether other computers on the bus are sending

Multipoint access : Many computers are connected to a bus

collision detection : That is, send while detect , When sending data, the current host also needs to detect whether there is other data on the channel

What's hard to think about here is , Why has a carrier answered ? And collision detection ?

Because carrier answering can only judge whether there is data being transmitted at present , Consider two cases where carrier reception cannot be detected .① The current host and the host on the bus send data at the same time ② The data is still transmitted on other hosts

3. Electromagnetic wave in 1KM The propagation delay of the cable is about 0 5us

4.CSMA/CD The concrete analysis of

The one-way end-to-end delay on the bus is recorded as “ set ”(t)

When cosai tends to 0 when , The end-to-end delay is 2t, It's called contention period , Contention period is also called collision window

5. Truncate binary exponential backoff algorithm

The specific backoff algorithm is as follows

(1) The agreement stipulates that the contention period for the basic backoff time is 2t, The specific requisition period is 51.2us. about 10Mbit/s Ethernet , It can be sent during the requisition period 512bit, It can also be said that the contention period is 512bit

(2) The retreat time is r Times the expropriation period ,r by 0 To 2 Of k Power -1        k=min( Number of retransmissions ,10)

(3) When the number of retransmissions reaches 16 When the time , Indicates that too much data is transmitted in the link , Discard the frame , And report to the upper level .

6. Why is the minimum frame specified by Ethernet 512 The bit

Because the expropriation period is 512 The bit , If it is less than 512 The bit , If there is a collision , It will cause the receiver to discard the frame , The sender does not resend the frame . In Ethernet , When the length of the frame received by the receiving end is less than 512 Bit time , They should be discarded

7. Strengthen the collision

When the station sending the data detects a collision , Except to stop sending current data . Do you want to continue sending 32 or 48 Bits of human interference data .

8.CSMA/CD Process summary

  6、 ... and . Two sublayers of the data link

In order to make the data link layer better adapt to the LAN standard , The data link layer is divided into two sub layers —— Logical link control layer (LLC) And media access control (MAC). Everything related to the transmission media is put in MAC Sublayer , and LLC It has nothing to do with the transmission media , No matter what transmission media and MAC All the LLC The layer is transparent . But with the evolution of the times , Ethernet adapters are generally only equipped with MAC Layer protocol .

7、 ... and . Adapter

The connection between the computer and the external LAN is through the adapter . The adapter is a network interface board inserted into the host , Network card for short . The adapter is equipped with processor and memory .

  8、 ... and . Ethernet MAC layer

PPP Data frames and MAC In addition to the difference of frame format , The main feature is PPP Data frames are used for point-to-point channels , and MAC Data frames are used for broadcast channels .

The adapter has filtering function , When the adapter receives each MAC Frame first detection MAC The destination address of the frame .

common MAC There are three formats for frames : Unicast frame , Broadcast frame , Multicast frame ( one-on-one , A pile of all , One to many )

MAC Frame format

  Type is used to represent what protocol is used in the upper layer

Nine . Extended Ethernet

1. Extending Ethernet at the physical layer

1. Transponder

2. Optical fiber and optical fiber modem

3 Using multiple hubs

 2. Expand Ethernet at the data link layer

① bridge

② Ethernet switch

3. Characteristics of Ethernet switch

All hosts connected to Ethernet switches are exclusive transmission media , Collision free transmission data .

Ethernet switch is established by self-learning algorithm

Bus Ethernet uses CSMA/CD agreement , Working in half duplex mode

Ethernet switches do not use a shared bus , There is no collision problem , So do not use CSMA/CD agreement

Ten . Virtual LAN

Virtual LAN is a logical group composed of some LAN segments independent of physical location . every last VLAN All frames have clear identifiers .

  Insert VLAN tag Of 802.1Q frame


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