What is circuit switching, packet switching, and group switching?

1. Circuit Switching Technology

The network switching technology has gone through four stages: Circuit Switching Technology, packet switching technology, group switching technology and ATM technology. Both the Public Telephone Network (PSTN) and the mobile network (including the GSM network and CDMA network) Adopt the circuit switching technology. Its basic feature is that the connection-oriented approach is adopted before both parties can communicate with each other, A communication circuit with fixed bandwidth needs to be allocated to both parties, and the communication parties will occupy the allocated resources until the communication ends, related Signaling Protocols must be used during circuit establishment and release. The advantage of this method is that in the communication process, the user can be guaranteed to provide sufficient bandwidth, and the real-time performance is strong, the latency is small, and the switching device costs are low, however, the disadvantage is that the bandwidth utilization of the network is not high. Once the circuit is established, the allocated circuit is always occupied no matter whether the communication parties are in the call status.

2. Packet Exchange Technology

The packet exchange technology is similar to the group exchange technology. It also uses the storage and forwarding mechanism, but the Packet Exchange is based on the packet as the transmission unit. Due to the large difference in the message length, long packets may cause great latency, in addition, it is difficult to allocate a buffer for each node. To meet the needs of various length packets and achieve efficient performance, the node needs to allocate a buffer of different sizes, otherwise, data transmission may fail. In practical applications, Packet Exchange is mainly used for communication services with short message transmission and low real-time requirements, such as public telegraph networks. Packet Exchange compared to group exchange occurs earlier. group exchange divides messages into groups for Transmission Based on Packet Exchange, balancing transmission latency and transmission efficiency, thus it is widely used.

3. Group Exchange Technology

Circuit Switching technology is mainly applicable to speech-related services. This network switching method has great limitations for data services. First, data communication is very abrupt. The peak bit rate and the average bit rate differ greatly. If circuit switching technology is used, allocating circuit bandwidth at the peak bit rate will cause a great waste of resources, if the bandwidth is allocated at the average bit rate, a large amount of data will be lost. Second, compared with the voice service, the data service has no strict requirements on latency, but requires error-free transmission. However, the voice signal may be distorted to a certain extent, but must be highly real-time. The group exchange technology is a kind of exchange method based on the characteristics of data communication services. Its basic feature is to adopt the storage forwarding Method for connectionless data communication, divide the data to be transmitted into many small pieces of data according to a certain length, and add the corresponding header fields used for data selection and verification before the data, as the basic unit of data transmission, it is grouping. Using the group exchange technology, you do not need to establish a connection before communication. Each node first accepts the Group sent by the previous node and stores it in the buffer zone, then, select an appropriate link based on the address information in the group header and send it to the next node. In this way, you can dynamically allocate bandwidth based on your requirements and network capabilities during the communication process. Group switching is more efficient than circuit switching, but has a high latency.

Services provided by group exchange
Switching a virtual circuit refers to a temporary logical connection established between two users.
Permanent virtual circuit-a permanent logical connection established between two users. A permanent virtual circuit is automatically established when the user starts the machine.

Group Switching Network

A datagram network is a connectionless network.

A virtual circuit network is a connection-oriented network that provides connection maintenance information for each connection.

Features and comparison of route, packet, and group exchange

(1) circuit switching: Before communication, a physical channel exclusive to both parties must be established between the two parties (composed of the switching devices and links of both parties ), therefore, it has the following advantages and disadvantages.
Advantages:
① Because the communication line is dedicated to users of both parties, the data transmission latency is very small.
② Once a physical path is established between the communication parties, the two parties can communicate with each other at any time in real time.
③ When both parties communicate, data is transmitted in the sending order, and there is no out-of-order problem.
④ Circuit switching is applicable to both analog signal transmission and digital signal transmission.
⑤ The switching equipment (switches, etc.) and control of circuit switching are relatively simple.
Disadvantages:
① The average connection establishment time of circuit switching is too long for computer communication.
② After the circuit switching connection is established, the physical path is exclusive to both parties. Even if the communication line is idle, it cannot be used by other users. Therefore, the channel utilization is low.
③ When the circuit is switched, the data is directly transmitted. It is difficult for terminals of different types, specifications, and rates to communicate with each other and to control errors during communication.

Circuit Switching is often compared with group switching. The main difference is that the communication line of group exchange is not dedicated to the information transmission between the source and destination. When data is required to be transmitted in a sequential manner at a constant rate, circuit switching is an ideal choice. Therefore, when real-time data is transmitted, such as audio and video, or when the quality of service (QoS) is high, a circuit switching network is usually used. Group exchange is more efficient in data transmission and can prevent delay and jitter in transmission processes (such as e-mail information and web pages.

(2) packet exchange: the Packet Exchange is a unit of data exchange. The packet carries information such as the destination address and source address, and uses the storage and forwarding transmission method at the exchange node, therefore, it has the following advantages and disadvantages:
Advantages:
① For message exchange, no special communication line needs to be established for both parties in advance, and no delay is established for the connection. You can send messages at any time.
② It has the following advantages because it adopts the storage and forwarding transmission mode:. in Packet Exchange, it is easy to set up code verification and data re-transmission facilities. In addition, the exchange Node also has a path selection, so that when a transmission path fails, you can reselect another path to transmit data, improves the reliability of transmission; B. it is easy to implement Code Conversion and rate matching in storage and forwarding, and even the sending and receiving sides can be available at different times. This facilitates communication between computers with different types, specifications, and speeds. provides multi-target services, that is, a message can be sent to multiple destination addresses at the same time, which is difficult to implement in circuit switching; D. data transmission priority can be set up to enable conversion of packets with higher priority.
③ The communication parties do not possess a fixed communication line, but partially occupy this physical path in different periods of time, thus greatly improving the utilization of the communication line.
Disadvantages:
① As data enters the exchange node, it must go through the process of storage and forwarding, resulting in forwarding latency (including receiving packets, verifying correctness, queuing, and sending time ), the larger the network traffic, the larger the latency. Therefore, the poor real-time Packet Exchange 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 on the packet length, each intermediate node must completely receive the entire packet. When the output line is not idle, several complete packets may need to be stored for waiting for forwarding, each node in the network must have a large buffer zone. In order to reduce costs and reduce the buffer storage capacity of nodes, packets waiting for forwarding are sometimes stored on disks, further increasing the transmission latency.

(3) group exchange: the group exchange still uses the storage and forwarding mode, but a long packet is first divided into several shorter groups, then, these groups (with the source, destination address, and serial number information) are sent one by one. Therefore, Packet Exchange has the following advantages and disadvantages compared with Packet Exchange:
Advantages:
① Accelerated data transmission over the network. Because groups are transmitted one by one, the storage operations of the last group can be parallel with the forwarding operations of the previous Group. This pipeline transmission method reduces the transmission time of packets. In addition, the buffer zone required for transmitting a group is much smaller than that required for transmitting a packet. In this way, the probability of waiting for sending due to insufficient buffer zone and the waiting time are also much less.
② Simplified storage management. Because the group length is fixed and the corresponding buffer size is also fixed, memory management in SWAp nodes is usually simplified to buffer management, which is relatively easy.
③ Reduces the error probability and resend the data volume. Because the grouping is short, the error probability is inevitably reduced, and the data size of each re-transmission is greatly reduced, which not only improves the reliability, but also reduces the transmission latency.
④ Because grouping is short, it is more suitable for adopting priority policies to facilitate timely transmission of some urgent data. Therefore, grouping exchange is more appropriate for burst data communication between computers.
Disadvantages:
① Although Packet Exchange has less transmission latency than Packet Exchange, there is still a storage and forwarding latency, and its node switches must have stronger processing capabilities.
② In the same way as Packet Exchange, each group must add information such as the source, destination address, and group number to increase the amount of information transmitted by about 5% ~ 10%, to a certain extent, reduces the communication efficiency, increases the processing time, makes the control complex, and increases the latency.
③ When the group exchange uses the datagram service, unordered, lost, or duplicate groups may occur. When the group reaches the destination node, sort the groups by number, which increases the trouble. If virtual circuit service is used, although there is no out-of-order problem, there are three processes: Call creation, data transmission, and virtual circuit release.
In short, if the data volume to be transferred is large and the transfer time is much higher than the call time, circuit switching is more appropriate. When the end-to-end path consists of many segments of links, it is more appropriate to transmit data through group exchange. In terms of improving the channel utilization of the entire network, packet switching and packet switching are superior to circuit switching. packet switching has lower latency than packet switching, and is especially suitable for burst data communication between computers.