The working principle of traditional telephone, telegraph and IP telephone

The implementation technologies are circuit switching, message switching and packet switching. Brief introduction of three switching technologies 1. Circuit switching technology Network switching technology has experienced four development stages, namely, circuit switching technology, message switching technology, packet switching technology and ATM technology. Both public telephone network (PSTN) and mobile network (including GSM network and CDMA network) adopt circuit switching technology, and its basic feature is connection-oriented mode. Before the communication between the two parties, a communication circuit with fixed bandwidth needs to be allocated, and both parties will occupy the allocated resources until the communication ends, and relevant signaling protocols need to be used in the process of establishing and releasing the circuit. The advantages of this method are that it can provide users with enough bandwidth in the communication process, strong real-time, small delay and low cost of switching equipment, but it also brings the shortcomings of low network bandwidth utilization. Once the circuit is established, the allocated circuit is always occupied regardless of whether the two parties are in a talking state or not. 2. Message exchange technology Message exchange technology is similar to packet switching technology, and it also adopts the store-and-forward mechanism. However, in message exchange, messages are used as transmission units. Because the message length varies greatly, long messages may cause great delay, and it is difficult for each node to allocate buffers. In order to meet the needs of messages of various lengths and achieve high efficiency, nodes need to allocate buffers of different sizes, otherwise data transmission may fail. In practical application, message switching is mainly used for short message transmission and communication services with low real-time requirements, such as public telegraph network. Message exchange appeared earlier than packet exchange. Packet switching is based on message exchange, which divides messages into packets and balances transmission delay and transmission efficiency, so it has been widely used. 3. Packet switching technology Circuit switching technology is mainly suitable for transmitting voice-related services, but it has great limitations for data services. First of all, the expect the unexpected of data communication has a great difference between the peak bit rate and the average bit rate. If circuit switching technology is adopted, allocating circuit bandwidth according to peak bit rate will cause great waste of resources, while allocating bandwidth according to average bit rate will cause a lot of data loss. Secondly, compared with voice service, data service has no strict requirements on time delay, but it needs error-free transmission. Voice signals can be distorted to some extent, but the real-time performance must be high. Packet switching technology is a switching method proposed according to the characteristics of data communication services. Its basic feature is to divide the data to be transmitted into many small pieces of data according to a certain length, and add corresponding header fields for routing and checking data before the data as the basic unit of data transmission, that is, grouping. Using packet switching technology, there is no need to establish a connection before communication. Each node first receives the data packet sent by the previous node and stores it in the buffer, and then selects the appropriate link to send to the next node according to the address information in the packet header, so that the bandwidth can be dynamically allocated according to the user's requirements and the network capacity during communication. Packet switching has higher circuit utilization than circuit switching, but the delay is longer. Virtual circuit of service switching provided by packet switching refers to a temporary logical connection established between two users. Permanent virtual circuit-refers to the permanent logical connection established between two users. As soon as the user turns on the computer, a permanent virtual circuit will be automatically established. Packet switching network datagram network is a connectionless network, and virtual circuit network is a connection-oriented network, which maintains the status information of each connection. Characteristics and comparison of circuit, message and packet switching (1) Circuit switching: Because circuit switching needs to establish a physical path between the two communication parties before communication (which is made up of switching equipment and links between the two communication parties), it has the following advantages and disadvantages. Advantages: ① Because the communication line is dedicated to both users, the data is direct and the data transmission delay is very small. ② Once the physical path between two communication parties is established, the two parties can communicate at any time, with strong real-time performance. ③ When communicating, both parties transmit data according to the sending order, and there is no disorder problem. ④ Circuit switching is suitable for transmitting both analog signals and digital signals. ⑤ Circuit-switched switching equipment (switches, etc.). ) and control are relatively simple. Disadvantages: ① The average connection setup time of circuit switching is too long, which is not suitable for computer communication. (2) After the circuit-switched connection is established, the physical channel is monopolized by both communication parties. Even if the communication line is idle, it cannot be used by other users, and the channel utilization rate is low. (3) When the circuit is switched, the data is direct, so it is difficult for terminals of different types, specifications and rates to communicate with each other, and it is also difficult to control the errors in the communication process. (2) Message exchange: Message exchange takes the message as the data exchange unit, and the message carries information such as destination address and source address, and the transmission mode of store and forward is adopted at the exchange node, so it has the following advantages and disadvantages: ① Message exchange does not need to establish special communication lines for both communication parties in advance, and there is no delay in connection establishment, so users can send messages at any time. (2) Due to the transmission mode of store and forward, it has the following advantages: a. It is convenient to set up code detection and data retransmission facilities in message switching, and the switching node also has path selection. When one transmission path fails, it can re-select another path to transmit data, which improves the reliability of transmission; B it is easy to realize transcoding and rate matching in store and forward, and even the sender and receiver may not be available at the same time. This facilitates communication between computers of different types, specifications and speeds; C. Providing multi-target service, that is, a message can be sent to multiple destination addresses at the same time, which is difficult to realize in circuit switching; D it is allowed to establish the priority of data transmission, and the message with high priority is converted first. (3) The communication parties do not occupy a communication line permanently, but occupy this physical path at different times, thus greatly improving the utilization rate of the communication line. Disadvantages: ① The data has to go through the process of storage and forwarding after entering the switching node, resulting in forwarding delay (including receiving messages, checking correctness, queuing, sending time, etc. ), and the greater the network traffic, the greater the delay, so the real-time performance of message exchange is poor, which is not suitable for transmitting real-time or interactive business data. ② Message exchange is only applicable to digital signals. (3) Because there is no limit to the length of the message, each intermediate node should receive the whole message completely. When the output line is not idle, it may be necessary to store several complete messages for forwarding, which requires each node in the network to have a large buffer. In order to reduce the cost and the capacity of node buffer memory, messages waiting to be forwarded are sometimes stored on disk, which further increases the transmission delay. (3) Packet switching: Packet switching still adopts the transmission mode of store and forward, except that a long message is divided into several shorter packets first, and then these packets (carrying source and destination addresses and numbering information) are sent out one by one. Therefore, compared with message switching, packet switching has the following advantages and disadvantages: advantages: ① It accelerates the transmission of data in the network. Because the data packets are transmitted one by one, the storage operation of the latter data packet can be parallel to the forwarding operation of the previous data packet, which reduces the transmission time of the message. In addition, the buffer needed to transmit a packet is much smaller than the buffer needed to transmit a message, so the probability and waiting time of waiting for transmission due to insufficient buffer are inevitably much less. ② Simplify storage management. Because the length of the packet is fixed and the size of the corresponding buffer is fixed, the memory management in the switching node is usually simplified to the management of the buffer, which is relatively easy. (3) Reduce the error probability and the amount of retransmitted data. Because the packet is very short, its error probability will inevitably be reduced, and the amount of data retransmitted each time will be greatly reduced, which not only improves the reliability, but also reduces the transmission delay. (4) Because the packet is shorter, it is more suitable to adopt the priority strategy, which is convenient to transmit some urgent data in time, so packet switching is obviously more suitable for burst data communication between computers. Disadvantages: ① Although packet switching has a shorter transmission delay than message switching, there is still a store-and-forward delay, and its node switches must have stronger processing capacity. (2) Like message switching, packet switching adds source address, destination address, packet number and other information to each packet, which increases the transmitted information by about 5% ~ 10%, reduces the communication efficiency to some extent, increases the processing time, complicates the control and increases the delay. (3) When using datagram service in packet switching, there may be out-of-order, lost or repeated packets. When the packets arrive at the destination node, they need to be sorted according to the packet number, which increases the trouble. If the virtual circuit service is used, although there is no disorder problem, there are three processes: call establishment, data transmission and virtual circuit release. In short, if the amount of data to be transmitted is large and its transmission time is much longer than the call time, it is more suitable to use circuit switching; When the end-to-end path consists of many links, it is more suitable to transmit data through packet switching. From the point of improving the channel utilization of the whole network, message switching and packet switching are superior to circuit switching, in which packet switching has less delay than message switching, and is especially suitable for burst data communication between computers.