The concept of packet switched networks

Packet switching network is a new type of switched network after circuit switching network and packet switching network, which is mainly used for data communication. Packet switching is a kind of storage and forwarding exchange, it divides the user's message into a certain length of packet to packet for storage and forwarding, therefore, it is more efficient than the circuit exchange, the delay is smaller than the message exchange, and has the ability of real-time communication. Packet switching utilizes statistical time division multiplexing principle to reuse a data link into multiple logical channels, which ultimately constitutes a message transmission path between the host and the called user, which is called virtual Circuit (V.C) to implement packet transmission of data.

Packet switching network has the following characteristics: (1) packet switching has the ability of multiple logical channels, so the circuit utilization of the trunk line is high; (2) The terminal interworking between different code types, rates and protocols of the packet switching network can be realized; (3) because of the ability of packet switching to detect and correct errors, the error rate of circuit transmission is very small. (4) The network management function of packet switching is strong.

The basic business of packet switching is Exchange virtual Circuit (SVC) and permanent virtual circuit (PVC). The Exchange virtual circuit as the telephone circuit, that is, two data terminals to communicate with the first use of the call program to establish the circuit (that is, virtual circuit), and then send data, communication after the end of the process with the removal of the virtual circuit. Permanent virtual circuit as a dedicated line, in the packet network between the two terminals in the application contract between the provision of permanent logical connection, no need to call the establishment and the removal procedures, in the data transmission phase, and the exchange of virtual circuit the same.

Packet switching data network is composed of packet switch, network Management center, remote concentrator, sub assembly and disassembly equipment and transmission equipment. (1) The packet switch realizes the interface protocol between the data terminal and the switch (x 25), signaling protocol between switches (such as x 75 or internal protocol), and packet storage and forwarding, the provision of packet network services support, with the Network Management center to complete routing, monitoring, billing, control and so on. According to the position of the packet switch in the network, it is divided into two kinds: the transfer switch and the local switch, (2) The Network Management Center (NMC) cooperates with the packet switch to ensure the network running smoothly. Its main functions are network management, user management, measurement management, billing management, operation and maintenance management, routing management, Collection of network statistics and necessary control functions are the core of the whole network management; (3) The main function of the sub-assembly and disassembly equipment (PAD) is to convert the non-grouping format of ordinary character terminals into component group format, The data streams of each terminal are grouped together, and the packets are interleaved and reused on the set channel, and the other side converts the received packet format to the opposite direction. (4) The function of the remote concentrator is similar to that of the packet switch, which usually contains pad function, it is only connected with a packet switch, no routing function, it is used in the area where the user is more concentrated, generally in the telecommunication department.