Shulou(Shulou.com)06/01 Report--

1. The type of channel 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 physical line section of a point-to-point without any other switching nodes

Data link: communication protocol + software + hardware (generally use adapters, that is, network cards, to implement the software and hardware of these protocols, and general adapters include the functions of the link layer and the physical layer)

3. Three basic problems to be solved in the data link layer:

Encapsulated into a frame: the beginning and end of the frame are used to define the frame

Transparent transmission: insert the same characters at the beginning and end of the frame contained in the packet with escape characters (ESC) before sending.

Error detection: when the receiver uses cyclic redundancy detection (CRC) technology to calculate the remainder, it will receive it, otherwise it will be discarded.

(frame check series redundancy code FCS)

4. PPP protocol (P2P protocol, used in WAN):

1) three components:

Advanced data Link layer Protocol (HDLP): can be used for asynchronous or synchronous serial media

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

Network Control Protocol (NCP): allows the use of multi-layer network protocols on point-to-point connections.

2) Protocol frame format:

Flag field F = 0x7E

Address field An is set to 0xFF only. The address field doesn't actually work.

Control field C is usually set to 0x03.

Protocol field:

The information field of the 0x0021-PPP frame is the IP Datagram.

0xC021-the information field is PPP link control data.

0x8021-indicates that this is network control data.

0xC023-the information field is the security authentication PAP.

0xC025-the information field is LQR.

0xC223-the information field is the security authentication CHAP.

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

3) Zero bit filling method (when the ppp protocol is used in SONET/SDH links)

4) the reason why the PPP protocol does not use sequence number and confirmation mechanism is due to the following considerations:

When the probability of error in the data link layer is small, it is more reasonable to use a relatively simple PPP protocol.

In the Internet environment, the data put into the information field of PPP is IP Datagram. The reliable transmission of the data link layer does not guarantee the reliable transmission of the network layer.

The frame check sequence FCS field ensures error-free reception.

5) the process of dial-up Internet access:

Dial-up access ISP->

The router's modem confirms dialing 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 connected PC, which connects the PC to internet->

When the communication is complete, NCP releases the network layer connection and reclaims the originally assigned IP address->

LCP releases data link layer connections->

Release physical layer connection

5. Local area network topology:

Ethernet:

Carrier sense (CSMA/CD):

Multipoint access: multiple computers are connected to a bus by means of multipoint access.

Carrier snooping: each station should check whether other computers on the bus are sending data before sending data. If so, do not send data for the time being to avoid collisions.

Note: Ethernet using the CSMA/CD protocol cannot communicate in full duplex, but can only communicate alternately in two directions (half duplex communication).

6. MAC address:

Composition: 48-bit binary = 24-bit manufacturer ID + 24-bit intra-factory unique ID

MAC frame format:

The minimum interval of MAC frames is 9.6 subtleties, which is equivalent to the sending time of 96bit.

About the difference between PPP frames and MAC frames:

Ppp belongs to wide area network and MAC belongs to local area network. Ppp is a point-to-point protocol, logically connected to only one device, so there is no need for addressing.

7. Tools:

Network topology simulation software: Cisco Packet Tracer

Bag grabbing tool: Eethereal + WinPcap

Note: this series of articles is to view the notes of Mr. Han Ligang's computer network basic course.

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