What are the six main functions of the data link layer?

The basic function of the physical layer is to use the transmission medium to provide a physical connection to the data link layer and realize the transparent transmission of the bit stream. The basic functions of the data link layer are:

Through various control protocols, error-free physical channels are transformed into error-free data links that can reliably transmit data frames. The basic function of the network layer is to select the most appropriate path for packets or packets through the communication subnet through the routing algorithm.

In computer networks, physical links are unreliable due to the presence of various interferences. Therefore, the main function of this layer is to make the faulty physical line become an error-free data link through error control and flow control methods on the basis of the bit stream provided by the physical layer, that is, to provide a reliable method of transmitting data through the physical medium. The physical addresses used in the data link layer, such as MAC addresses, only solve addressing problems within the network.

When communicating between different subnets, each subnet is assigned a unique address in order to identify and locate devices in the network. Since the physical technology used by each subnet may be different, this address should be a logical address (e.g., an IP address).

The primary function of the data link layer is to provide management of the creation, maintenance, and release of data link connections between two network entities.

The Data Link Layer is the second layer of the OSI reference model and sits between the physical layer and the network layer. In a broadcast multiplex access link (LAN), it can also be subdivided into a media access control (MAC) sub-layer and a logical link control (LLC) sub-layer due to the possibility of media contention, with the media access control (MAC) sub-layer dealing with contention and conflicts for media access.

The data link layer places a checkcode (parity, sum, crc) at the end of the frame to check the substance, transforms the potentially faulty physical connections provided by the physical layer into logically error-free data links, and encapsulates the original data of the physical layer.

The protocol of the data link layer

The synchronization protocol is a transmission unit in which many characters or many bits are organized into blocks of data - frames, which are synchronized at the beginning of the frame to maintain a fixed clock within the frame. In fact, the fixed clock is mixed in the data by the sender through some technology and sent out, for the receiver to separate the clock from the input data, the implementation is more complicated, this function is usually done by the demodulator.

Due to the use of frames as transmission units, the synchronization protocol can make more efficient use of channels, and it is also convenient to realize functions such as random combustion error control and flow control. Synchronization protocols can be divided into three types: character-oriented synchronization protocols, bit-oriented synchronization protocols, and byte-counting synchronization protocols.

The data link layer must have a series of corresponding functions, mainly including: how to combine data into data blocks, in the data link layer, this data block is called a frame, and the frame is the transmission unit of the data link layer; how to control the transmission of frames on a physical channel, including how to handle transmission errors, how to adjust the transmit rate to match the receiver; and the management of the establishment, maintenance and release of data link pathways between two network entities.

Physical link: It is composed of a transmission medium and a device. The original physical transmission line refers to the basic physical transmission medium and equipment without high-level error control.

Data link (logical line): On top of a physical line, the transmission of these data is controlled through some procedures or protocols to ensure the correctness of the transmitted data. The hardware and software that implement these procedures or protocols are added to the physical line, thus forming the transmission path of the data link from the point of sending the data to the point of receiving the data.

When multiplexing techniques are employed, there can be multiple data links on a single physical link.

(1) Link management.

2) Frame demarcation.

3) Flow control.

4) Error control.

5) Separate data and control information.

6) Transparent transmission.

7) Addressing.

1. Data link layer function.

Provides management of the creation, maintenance, and release of data link connections between two network entities. It forms a data link core data unit (frame: data frame or signal frame), and controls the frame demarcation, synchronization, and transmission and reception sequence.

Network flow control, error detection and error control in the transmission process.

Only data transmission from one end of the wire to the other is provided. The data link layer places a checkcode (parity, sum, crc) at the end of the frame to check the substance, transforms the potentially faulty physical connections provided by the physical layer into logically error-free data links, and encapsulates the original data of the physical layer.

2. The main functions of the network layer.

For the network layer, IP addresses are used to uniquely identify devices on the Internet, and the network layer relies on IP addresses to communicate with each other (similar to MAC addresses at the data link layer), see IPv4 and IPv6 for detailed addressing schemes.

The LAN divides the data link layer into two sublayers: the logical link control LLC sublayer and the media visibility access control MAC sublayer, allowing the LAN architecture to adapt to a variety of transmission media.

As a result, the physical and MAC sublayers of various types of LANs need to change depending on the medium used and the access method, and these differences are transparent to the LLC sublayer.

The LLC, as a sub-layer of the data link layer, uses the MAC sub-layer to provide services to it, and provides services to its upper network layer through interaction with the peer entity LLC sub-layer.

The MAC sublayer is the network entity used to implement media access control. The main functions of the MAC sublayer include encapsulation and unpacking of data frames, addressing and identification of frames, receiving and sending of frames, link management, error control of frames, and maintenance of MAC protocols.

Features of the data link layer protocol

A link is a physical link from a node to a neighboring node, while a data link adds some necessary hardware (such as network adapters) and software (such as protocol implementation) to the link.

The data link layer mainly uses two types: point-to-point channel and broadcast channel, and the protocol data unit transmitted by the data let the sail elimination link layer is frame. The three basic problems at the data link layer are: encapsulation into frames, transparent transmission, and error detection, cyclic redundancy check CRC is an error detection method, and frame check sequence FCS is a redundant code added to the data.

The main functions of the data link layer are: providing services for the network layer, realizing error control between the physical layer and the network layer, performing frame synchronization, realizing flow control, and realizing error control.

1. Provide services for the network layer.

The data link layer is responsible for encapsulating the bit stream provided by the physical layer and giving it to the network layer. At the same time, the data link layer is also responsible for identifying and correcting errors generated during data transmission to ensure the accuracy and reliability of data.

2. Realize error control between the physical layer and the network layer.

The data link layer ensures the reliability and integrity of data transmission through error control technology. At the data link layer, error control techniques include the use of cyclic redundancy check (CRC) codes provided by the physical layer to detect and correct errors during transmission, and techniques such as parity to detect and correct burst errors.

3. Synchronize the frames.

At the data link layer, frame synchronization techniques need to be implemented to ensure that the receiver can accurately identify the start and end of each frame. The Frame Synchronization Technique can be achieved by using the Synchronization Signal provided by the Physical Layer.

4. Achieve flow control.

The data link layer can avoid congestion in the network by implementing traffic control technology, so as to ensure the stability and throughput of the network. Flow control techniques can include measures such as limiting the rate of the sender and the buffer size of the receiver.

5. Realize error control.

The data link layer can improve the reliability and accuracy of data transmission by using error control technology to reduce errors in the data transmission process. Error control techniques can include the use of coding techniques to detect and correct errors, as well as the use of flow control techniques, among other things.

The most basic function of the data link layer is to provide a transparent and reliable basic data transmission service to the users of the collapsed layer.

Transparency means that there are no restrictions on the content, format, and encoding of the data transmitted on this layer, and there is no need to explain the meaning of the information structure; Reliable transmission eliminates the need for users to worry about lost information, distracted information, and incorrect sequencing.

These situations can occur in the physical wide return layer, where error correction codes must be used to detect and correct errors in the data link layer. The data link layer is an enhancement of the physical layer's ability to transmit the original bit stream, transforming the physical connection provided by the physical layer into a logically error-free data link, so that it behaves as an error-free line to the network layer. If you want to memorize the data link layer with as few words as you can, it's this:

Frame and Media Access Control".