Data Link Layer

Channel types used by the data link layer
The data link layer belongs to the bottom of the computer network, the channel type used by the data link layer mainly has the following two kinds:
? The data link layer of the point-to-point channel, which uses a one-to-one point-to-point approach to communication between the user and the ISP.
? Broadcast channel: This approach uses a one-to-many broadcast communication method, using a dedicated channel sharing protocol

Problems in Data Link layer research
Although LAN is a network, but the main problem in the LAN is how to forward the packet from one host to another, without going through the router, belonging to the data link layer of the scope.
Mostly point-to-Point protocol PPP, and broadcast CSMA/CD protocol

The relationship between the data link layer and the network layer

The data link is in the next layer of the network layer, so the network layer also uses the services provided by the data link layer, the Data link layer to the network layer of data, plus frame head frame end , encapsulated into a frame , and then the frame to the physical layer for transmission, Just like the relationship between the data link layer and the physical layer, the network layer and the data link layer have the same relationship, the task of the data link layer is to shield out the differences between different communication channels , so that the network layer can not feel these differences, the network layer does not know whether the data link uses a point-to-point channel or a broadcast channel, The network layer only know, the data to the data link layer, the Data link layer can help it to send packets out, of course, the data chain layer also read the content of the network layer of data, only know is 0 or 1, and these 0 and 1 What does it mean? It does not know, what source IP address, destination IP address Ah, the data link layer can not understand, it just the network layer of the group, as the data portion of the frame, and then sent out.

The coupling degree between the data link layer and the network layer is also very low, and the thought of network layering is the development of computer network.

Data links and Frames
The similarities and differences between links and data links:
Link: A link is a physical line from a node to an adjacent node, which can be wired or wireless.
Data Link: the hardware and software of the real communication protocol are added to the link, which constitutes the data link.
These protocols can be implemented using network adapters, including hardware and software, and the general adapters include the data link layer and physical capabilities.
Protocol Data Unit-----frames for the data link layer
The data link layer sends the data frame of the network layer to the link, and the data in the received frame is taken out and handed to the network layer, and the Protocol data unit used by the network layer is the IP datagram (datagram, packet or packet).
The main steps of the data link layer of the point-to-point channel in communication:
? Node A's data link layer adds headers and tails to the IP datagram that the network layer has delivered
? The data link layer of Node A sends frames to the data link layer of Node B through the physical layer
? The data link layer of Node B receives this frame for error checking, and if there is no error, the IP datagram in the extraction frame is given to the network layer, and if there is error, discard the frame.

Three issues to be addressed by the data link layer

There are a number of protocols for the data link layer, but there are three issues that are common:
? Encapsulation into Frames
? Transparent transmission
? Error detection

Encapsulating into frames
The data link layer passes the network layer to the IP datagram, adds the header and the tail, constitutes a frame, the receiving end can recognize the beginning and end of a frame according to the frame head and the frame tail, and the header and tail play the role of frame delimitation. Each Data Link layer protocol specifies the maximum length MTU of the data portion of the frames that can be transmitted.

Transparent transmission
Transparent transmission means that no matter what combination of bits of data can be transmitted through the data link layer, such as: Before the "SOH" and "EOT" before the frame delimiter insert an escape character "Esc", called a byte fill, the receiving side when receiving, delete "ESC."

Error detection
CRC cyclic redundancy testing technique is widely used in the data link layer, and the frame test sequence FCS is generated using CRC technology, and FCS is added before sending. CRC is a method of error detection, the FCS is added after the data redundancy code, in the error-checking method, the CRC can be used, or other techniques can be used.
Process:
? Using the CRC operation to get the FCS sequence
? After adding a sequence to the data section
? The receiver takes the CRC test in the frame unit
Mistakes are discarded, error-free receive
Reliable transmission: Reliable transmission refers to the sender of what the receiving end to receive, but the data link layer can only make an extremely special error to receive the frame: the error is discarded, no error is received.
Transmission errors can be divided into:
? bit error
? frame lost, duplicated, out of sequence
The data link layer can realize the transmission without bit error

The OSI point of view is that the data Link layer provides reliable transport services, and in addition to CRC testing techniques, it is necessary to increase frame numbering, acknowledgement, and retransmission mechanisms. But now the quality of communication improved, data link layer error probability is small, now the Internet uses a differentiated approach:
Communication quality of the cable transmission link, the data link layer does not provide reliable transport services, the network layer provides
The wireless transmission link with poor communication quality, the data link layer provides reliable transmission service,
The purpose of this is to improve communication efficiency.

Data Link Layer