Packet switching:
In communication, the two parties in communication use packets as a unit, and use the store-and-forward mechanism to achieve data exchange. This is called packet switching.
Packet switching is also called packet switching. It divides the user communication data into multiple smaller data segments of equal length, and adds necessary control information in front of each data segment as the header of the data segment, each with a header The data segment constitutes a packet. The header specifies the address to which the packet is sent. When the switch receives the packet, it will forward the packet to the destination according to the address information in the header. This process is packet switching. A communication network capable of packet switching is called a packet switching network.
The essence of packet switching is store and forward. It temporarily stores the received packets and queues them on the route in the destination direction. When it can send information, it sends the information to the corresponding route to complete the forwarding. The process of storing and forwarding is the process of packet switching.
The idea of packet switching comes from message exchange. Message exchange is also called store-and-forward exchange. The essence of their exchange process is store-and-forward. The difference is that the smallest information unit of packet exchange is a packet, while message exchange is a Messages. Since the transmission and exchange are performed in units of smaller packets, packet exchange is faster than message exchange. Message exchange is mainly used in public telegraph networks.
The essence of packet switching is to divide the data to be transmitted into many groups according to a certain length. In order to accurately transmit to the other party, each group is marked with a logo. Many different data packets are transmitted on the physical line in a dynamic sharing and multiplexing manner. It can make full use of resources. When the data packet is transmitted to the switch, it will be temporarily stored in the memory of the switch. Then, according to the current line's busyness, the switch will dynamically allocate the appropriate physical line and continue the transmission of the data packet until it reaches the destination . After arriving at the destination, the data packets are recombined to form a complete data.
The packet is composed of the packet header and the user data part after it.
The packet header contains the receiving address and control information, and its length is 3-10B. The length of the user data part is fixed, with an average of 128B and a maximum length of 256B.
There is a problem that needs to be explained: the packet length in the same packet network is fixed, but the packet length of different packet networks can be different.
Packet switching: After routing has determined the output port and the next node, switching technology must be used to transfer the packet from the input port to the output port to realize the transmission of bits through the network node.
Features of packet switching:
(1) The smallest unit of information transmission is grouping. The
grouping consists of a group header and user information. The grouping header contains routing and control information.
(2) Connection-oriented (logical connection) and connectionless working modes. The
virtual circuit adopts the connection-oriented working mode, and the datagram is the connectionless working mode.
(3) Statistical time division multiplexing (dynamic bandwidth allocation)
The basic principle of statistical time division multiplexing is to divide the time into time slices of unequal length. Time slices with different lengths are the time required to transmit packets of different lengths. For each communication There is no fixed allocation of time slices, but on demand. This means the length of time that this multiplexing line is used to transmit packets, which shows that statistical time division multiplexing dynamically allocates bandwidth.
(4) Information transmission is error-controlled
packet switching is an exchange method specially designed for data communication networks. The characteristics of data services are high reliability requirements, and real-time requirements are not as high as telephone communication. Therefore, in packet switching, data information Reliability, with CRC check, retransmission and other error control mechanisms to meet the needs of data service characteristics.
(5) Information transmission is not transparent.
Packet switching must process the transmitted data information, such as splitting and reorganizing information.
(6) Flow control based on the call delay system
In packet switching, when the data flow is large, the packets are queued for processing, unlike circuit switching, which is immediately lost. Therefore, its flow control is based on call delay.
Comparison of packet switching and circuit switching:
The end-to-end delay of packet switching is variable and unpredictable, so it is not suitable for real-time services, but it provides better bandwidth sharing, and is simpler than circuit switching, and has a lower implementation cost.
Packet switching can be dynamically shared and multiplexed on the same link for transmission, and the utilization rate of communication resources is high, which greatly improves the capacity and throughput of the channel
Overview of time delay in packet switching networks:
The packet starts from the source host and is transmitted through a series of routers to reach the destination. The packet suffers several different types of delays at each node. The most important of these delays is the node processing delay, queuing delay, and transmission. Delay and propagation delay. The total sum of these delays is the total node delay. The packet is at the transmission link node, and the packet can be transmitted on this link only when no other packets are being transmitted on the link and no other packets are in front of the queue
Processing delay: The time to check the packet header and determine where the packet is directed, including the time required to check for bit-level errors.
Queuing delay: When a packet is waiting for transmission on the link, it suffers from queuing delay.
Transmission delay: also known as store-and-forward delay. The actual transmission delay is usually on the order of milliseconds to microseconds.
Propagation delay: the time required from one router to another, and the propagation speed is equal to or the speed of light.