Data link layer of computer network

1. The channel type used by the Data link layer:

Point-to-point channel (1 to 1)

Broadcast channels (1-to-many), such as: hubs, switches

2. Link and Data Link:

Link: A point-to-point physical line segment with no other interchange nodes in the middle

Data Link: Communication protocol + software + hardware (general adapter is the network card to implement the hardware and software of these protocols, the general adapter contains the link layer and the physical layer of the two layers of functionality)

3, the data link layer to solve the 3 basic problems:

Package Framing: Frame-to-tail for defining frames

Transparent transfer: Insert the same character as the frame header and tail in the packet with the escape character (ESC) before sending

Error detection: The receiving end uses cyclic redundancy detection (CRC) technology to calculate the no-number, only receive, or discard

(Frame Inspection series redundant code FCS)

4. PPP protocol (for the use of the Wide area network):

1) 3 Components:

Advanced Data Link Layer Protocol (HDLP): Can be used for asynchronous serial or synchronous serial media;

Link Control Protocol (LCP): Establish and maintain data link connections;

Network Control Protocol (NCP): Allows multi-tier network protocols to be used on point-to-point connections.

2) Protocol frame format:

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Flag field F = 0x7E

Address field A is set to 0xFF only. The Address field does not actually work.

Control field C is usually set to 0x03.

Protocol fields:

The Information field of the 0X0021-PPP frame is the IP datagram.

The 0xc021-information field is the PPP link control data.

0x8021-says this is the network control data.

The 0xc023-information field is security-certified PAP.

The 0xc025-information field is LQR.

The 0xc223-information field is security-authenticated chap.

Note: PPP is byte-oriented, and the length of all PPP frames is integer bytes.

3) 0-bit Fill method (PPP protocol when used on SONET/SDH link)

4) The PPP protocol does not use the serial number and confirmation mechanism for the following considerations:

When the probability of error occurs in the Data link layer, it is more reasonable to use the simpler PPP protocol.

In the Internet environment, the data that the PPP information field puts in is the IP datagram. The reliable transmission of the data link layer can not guarantee the transmission of the network layer is also reliable;

The FCS field of the frame inspection sequence guarantees error-free reception.

5) The process of dial-up Internet:

Dial-in Access ISP

The modem on the router confirms the dial-up and establishes a physical connection

The PC sends a sequence of LCP packets (encapsulated into multiple PPP frames) to the router

NCP assigns a temporary IP address to the newly accessed PC, which is a PC connected to the Internet

When the communication is complete, the NCP releases the network layer connection and reclaims the original assigned IP address.

LCP Release Data Link layer connection

Releasing physical layer connections

5. LAN topology structure:

Ethernet:

Carrier Monitoring (CSMA/CD):

Multi-point access: Multiple computers are connected to a bus in a multi-point access mode;

Carrier monitoring: Each station before sending the data to detect whether there are other computers on the bus to send data. If so, no data is sent to avoid collisions.

Note: Ethernet with the CSMA/CD protocol cannot perform full-duplex communication and only bidirectional alternating communication (half-duplex communication).

6. MAC Address:

Composition: 48-bit 2 binary = 24-bit manufacturer ID + 24-bit unique identity in the factory

Mac frame Format:

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The minimum interval for Mac frames is 9.6 subtle, which is equivalent to 96bit of send time.

About the difference between PPP frames and Mac frames:

PPP belongs to wide area network, Mac is the category of LAN. PPP is a point-to-point protocol that is logically connected to a single device and therefore does not require addressing.

7. Tools:

Network topology simulation software: Cisco Packet Tracer

Grab Bag Tool: Eethereal + WinPcap

Note: This series of articles is for watching Han Ligang teacher's computer network Basic course made notes.

This article is from the "Inner Peace, Fire Rebirth" blog, please be sure to keep this source http://itisforyou.blog.51cto.com/9778869/1618348

Data link layer of computer network