Group exchange technology (1)

The demand for broadband services such as videophone, visual text, image communication, and multimedia will also greatly promote asynchronous transmission technology (ATM) and Synchronous Digital series (SDH) and Broadband User Access Network Technology is constantly improving and widely used.

The group exchange technology has developed to a certain extent in computer technology. In addition to direct communication by phone, people can communicate with computers through computers and terminals, an exchange technology came into being when the quality of transmission lines is low and the network technology is still relatively simple.

Packet Switching is also called packet switching. It divides the data transmitted by users into a certain length, and each part is called a group. Add a group header before each group to specify the address to which the group is sent. Then, the switch forwards the group to the destination based on the address flag of each group, this process is called group exchange. A communication network for group exchange is called a group exchange network.

Comparison between group exchange and other exchanges

From the development history of exchange technology, data exchange has gone through the development process of circuit switching, packet switching, group switching and integrated business digital exchange.

Circuit Switching

When a computer terminal communicates with each other, one party initiates a call to exclusively occupy one physical line. When the switch completes connection, the other party receives the signal from the initiator, and both parties can communicate with each other. This circuit is used by both parties throughout the communication process. It features high real-time performance, low latency, and low switching device costs. However, it also results in disadvantages such as low line utilization, long circuit connection time, low communication efficiency, and inability to communicate with different types of end users. Circuit Switching is suitable for communication between users with large amounts of information and long packets.

Packet Exchange

Stores user messages in the memory of the switch. When the required output circuit is idle, the message is sent to the receiving switch or terminal, which transmits data in the network in the "Storage-forwarding" mode. The advantage of Packet Exchange is the high utilization rate of the relay circuit. It can be transmitted by multiple users on one line at the same time, enabling interconnection between terminals with different rates and procedures. However, its shortcomings are also obvious. Messages are stored and forwarded in units. The network transmission latency is high, and a large amount of switch memory and external storage is occupied. This does not meet the requirements of users with high real-time performance. Packet Exchange is suitable for communications between network users with short transmission packets and low real-time requirements, such as public telegraph networks.

Group exchange

In essence, group exchange is developed based on "Storage-forwarding. It has the advantages of circuit switching and packet switching. On the group exchange line, dynamic multiplexing technology is used to transmit data that is divided into many small pieces by a certain length-grouping. After each group ID, dynamic multiplexing technology is used on a physical line to transmit multiple data groups at the same time. Store the data originating from the user in the storage of the switch, and then forward the data in the network. At the receiving end, remove the grouping header and re-assemble each data field into a complete packet in order. Compared with circuit switching, group switching features higher circuit utilization and lower transmission latency and better interaction.

Asynchronous transmission mode (ATM)

Integrated Business digital network is a network that integrates voice, data, text and fax, videophone, and other services. It is suitable for different bandwidth requirements and diverse business requirements. Asynchronous Transfer Mode (Asynchronous) is an exchange technology used for Broadband Integrated Service Digital Networks. ATM is developed based on group exchange. It uses a fixed-length group and idle cells to fill the channel, so that the channel is waited for a long time. Because optical fiber communication provides a transmission channel with low error rate, traffic control and error control can be moved to the user terminal. The network is only responsible for information exchange and transmission, thus reducing transmission latency. Therefore, ATM is suitable for high-speed data exchange services.

Features of group exchange

In the group exchange mode, because the data can be temporarily stored and exchanged in the group mode, after the switch is processed, it is easy to implement communication between terminals with different rates and procedures. The main features of group exchange are:

High Line utilization rate: The group switch achieves channel multiplexing in the form of virtual circuits to achieve resource sharing. Multiple logical channels can be provided on one physical line, greatly improving the line utilization rate. This significantly reduces transmission costs.

Different types of terminals can communicate with each other: standard interfaces are provided to users through the X.25 protocol. data is stored and forwarded in groups to enable terminals at different rates, devices of different protocols can communicate with each other after the protocol conversion function provided by the network.

High Information Transmission reliability: when each group is transmitted in the network, the error verification and re-transmission functions are used between node switches. Therefore, the error rate transmitted in the network is greatly reduced. In addition, when a fault occurs in the network, the routing mechanism in the network will automatically enable the Group to select a new route to avoid the fault point and avoid communication interruption.

Group Multi-Channel Communication: because each group contains control information, the group Terminal can communicate with multiple user terminals at the same time and send the same information to different users.

Billing has nothing to do with the transmission distance: The network billing time is long, the amount of information is charged, and has nothing to do with the transmission distance, especially suitable for users who are not real-time, but have little traffic.

Network Structure of group exchange

A group exchange network is generally composed of a group switch, a network management center, a remote concentrator, a group assembly and removal device, a group Terminal, a non-group Terminal, a transmission line, and other basic devices.

Group switch function: provides basic network services: Switching virtual circuits and permanent virtual circuits, and other supplementary services, such as closed and user groups, and network user identification.

Route Selection and Traffic Control for end-to-end computer communication. Provides various communication procedures, data forwarding, maintenance, fault diagnosis, billing, and network statistics.

Network Management Center (NMC)

Network configuration management and user management, and collection and statistics of daily operation data.

Route Selection management, network monitoring, fault alarms, and network status display.

Complete billing management based on the billing information provided by the switch.

Remote concentrator (RCU)

Allows the access of group terminals and non-group terminals. With the procedure Conversion Function, each terminal can be integrated into the middle and high-speed lines of the group switch for multiplexing.

Group device (PAD)

Remove the character information from the asynchronous terminal (non-grouping terminal) and assemble it into groups, and send it to the group exchange network. The receiver restores the group information as a character and sends it to the user terminal. With the development of grouping technology, the functions of RSU and PAD are no different.

Group Terminal/non-group Terminal (PT/Treaty)

A group terminal is a data communication terminal device that has an X.25 protocol interface and can directly access the group exchange data network. It can be connected to the network through a physical line, and multiple virtual circuits can be established, and conversations can be made with multiple online users at the same time. For terminals that execute non-X.25 protocols and terminals without regulations, they are called non-group terminals. Non-group terminals need to be installed and removed by group before they can be connected to the switch port. Through the group exchange network, group terminals and non-group terminals can communicate with each other.