Channel type of the data link layer

One-to-one: point-to-point channel

One-to-many: broadcast channel

Three key concepts of the data link layer

Encapsulate into frame transparent transmission error detection

(Memory points: take one word for each concept: seal, poor air permeability)

Encapsulated into a frame

Framing is the process of adding a head and tail to the front and back of a piece of data to form a frame.

After receiving the bitstream submitted by the physical layer, the beginning and end of the frame can be identified from the received bitstream based on the tags at the head and tail.

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Transparent transmission

The ability to transmit data over a link regardless of the combination of bits. As a result, the link layer is “blind” to anything that interferes with data transmission.

For example, when the combination of bits in the transmitted data happens to be exactly the same as a certain control message, appropriate measures must be taken so that the receiver will not mistake such data for a certain control message.

In this way, transparent transmission at the data link layer can be ensured.


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Byte fill method

When a flag byte appears in the data message to be transmitted, it is mistaken for a frame boundary. One way to solve this problem is for the data link layer at the sending end to insert another escape character (usually ESC) before the presence of the marker byte, and the data link layer at the receiving end to remove the inserted escape character before sending the data to the network layer.

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Error detection

The so-called error is the phenomenon of bit and frame errors in the bit

A dislocation is when a 1 becomes a 0 or a 0 becomes a 1

A frame error is a loss, repetition, or disorder in a sequence of bits

These errors are usually caused by noise.

Error detection

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Example: The data to be sent is 1101 0110 11, using CRC check, the generated polynomial is 10011, so the final data to be sent should be?

Ideas:

Computing redundancy codes:

1 plus 0

(2) Modular 2 division

Assuming that the generating polynomial G(x) is of order R, add r zeros.

10011 is expressed as a polynomial

X4+X1+X0

=X4+X1+1

Order of 4

TIPS: Polynomial N bits, order n-1.

The remainder of the data added by 0 divided by the polynomial is redundant code /FCS/

A sequence of bits of a CRC check code.

The final data sent: 1101011011 1110




PPP is an important protocol at the data link layer

Point-to-point Protocol (P2P) is the most widely used data link layer Protocol. Users use dial-up phones to access data

PPP (the protocol used by a user’s computer to communicate with an ISP) is commonly used on the Internet.

PPP protocol to achieve transparent transmission method:

• asynchronous transmission | character-oriented: byte filling method

• synchronous transmission | oriented bits: zero bits filling method

Zero bit filling: If the string of bits between two flag fields happens to have the same bit combination as flag field F (01111110), it will be mistaken as a frame boundary. To avoid this, HDLC uses zero-bit padding so that there are no six consecutive ones between two F fields in a frame. After sending 5 1s in a row, fill in 1 0

(Flag field F is 01111110, so “5110”)

Important protocol of data link layer: CSMA/CD protocol

Chinese name: Carrier Sense Multipoint access collision Detection

CS: Carrier sense/monitor, each station before and while transmitting data

Check to see if there are other computers on the bus sending data.

MA: Multi-point access: many computers are connected to a bus in multi-point access mode.

CD: collision detection (collision detection), “send and listen”, adapter send number

The data side detects the change of signal voltage on the channel, so as to judge oneself when sending data

If other stations are sending data.

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