Packet switching - Informatics

Packet Switching

Channel switching technology does not provide efficient use of the data transmission medium due to the pulsating nature of the traffic. For example, the transmission of voice messages over the digital channel given to two subscribers is conducted at a constant rate of 64 kbit/s, regardless of whether they say or are silent, although additional information could be transmitted during each pause. The work of computer users on the Internet also alternates between periods of intense (when the web page is loaded) and weakened (when searching for necessary information) traffic. Quantitatively, its unevenness is estimated by the traffic pulsation coefficient of an individual network user, which is assumed to be equal to the ratio of the average data rate to the maximum possible value and can be 1/50-1/100. To improve the efficiency of the transfer of computer traffic (increasing the ripple factor), the packet-switched technology, , is the following: (9.11):

Illustration of the principle of packet messaging

Fig. 9.11. Illustration of the principle of packet-switched messages

• the transmitted message is broken into small packets (from 46 to 1500 bytes);

• Each packet is supplied with:

- the header with the address information required to deliver the packet to the destination node,

- the number of the packet that will be used by the destination node to assemble the message,

- the trailer (or trailer) containing the CRC check code for detecting errors;

• Packets are transmitted over the network as independent information blocks. The network switches receive packets from the end nodes and, based on the address information, transmit them to each other, and ultimately to the destination node.

Batch switches (unlike channel switches) contain buffer memory for temporary storage of packets, as well as a switching unit (Figure 9.12, a), which includes interface processors (for each port) and a central processor that coordinates their work. A packet switch requires buffering for:

• Deciding on the promotion of the package. Therefore, the received packet is sequentially bit by bit entered into the buffer of the input port to check the checksum in order to detect errors. If there is no indication, the switch starts processing the packet. At the destination address, it determines the next switch;

• matching the rates of incoming packets and their switching. If the switching unit does not manage to process the packets, then the input queues are organized;

• matching the data rates of external channels connected to the ports of the packet switch. In the case where the rate of arrival of packets of the channel connected to the input port exceeds the capacity of the channel connected to the output port, it is necessary to organize the output queue (see Figure 9.12, a), otherwise the packets will be lost.

The circuit in Fig. 9.12, b illustrates the organization of exchange in a packet switched network containing four 4-port packet switches. The data flow from each of the end nodes of the network (computers K) to the switches is unevenly distributed over time. However, switches 2,4 due to the presence of buffer memory in them allow to more evenly load the main channel connecting switches 1 and 3 of the upper level, i.e. get a lower ripple coefficient on the trunk channel than on the subscriber access channels.

Packet Switching Scheme (a) and Exchange Packet Exchange (b)

Fig. 9.12. Packet Switch Scheme ( a ) and organize packet exchange < strong> (b)

thematic pictures

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