Youtor
Packet Switching
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http://youtube.com/watch?v=fQLwqKOPb2I
Want to buy me a cup of coffee? Click here! • https://streamlabs.com/spyrosgeorgios... • As we have seen in the previous paragraph, • a basic feature of circuit switching networks is,that any call made, • binds specific network resources (switching circuits, lines, etc.) • who can not use any other station to communicate. • In telephone communications, • there is a large usage proportion of these resources, • since most of the time, either one or the other interlocutor speaks. • However, as circuit switches began to be used for data communications, • two things were perceived: • 1.In a regular communication of a computer user, • the way in which data is exchanged • is such that most of the time the line is not used, • so wasting valuable network resources. • Thus, it becomes clear that in data communications, the circuit switching technique can be made ineffective. • 2.A circuit switching network provides connections such as, • which allow the transmission of data at a constant rate. • Thus, each of the communicating devices must send and receive at the same rate. • This limits the ability of a network to finally interconnect different computers • and in general a variety of equipment. • Let us now see that the second switching technique, that of packet switching, • solves the above problems. • The messages to be transmitted are divided into small numbers of bytes. • The typical maximum packet length is 1000 bytes. • Each packet contains • part of the useful user data and more • a destination address and • a sequence number. • Each network node, also called a Packet Switching Node (PSN), uses the destination address of the packet to decide which node it will forward. • The packet serial numbers are used by the destination station, • to reconstruct the original message from the pieces it has received in the packets. • Let's look at the packet switching process with the help of the Figure: • Station A wants to send a message to station E. • The message is sliced into packets and packets are sent one at a time from station A to the switching node K1 with which it is connected. • When the node receives the entire packet, it examines the destination address and forward(switch) it to another switch node, let's say K2. • This process is repeated until the packet arrives at the destination station. • Each node decides, how to FORWARD the packet, • by looking at the destination address and traffic information on network nodes. • Note that each node moves the packet to a next node only after it receives the entire packet. • That is why this technique is called and packet switching with store and forward. • The packet switching technique has a considerable number of advantages over the • circuit switching: • There is better use of telecommunication lines. • This, because each line can be used to transmit packets from different connections, • as needed, so that there are no time intervals where the line remains untapped. • Packets are queued at the nodes and transmitted as soon as possible. • Instead, dedicated line capacity is allocated to the circuit switching in a connection, • which can not use any other connection. • Thus, if the connection remains silent for some time, the line capacity remains untapped. • In a packet switching network a data rate can be converted. • Indeed, two stations with different speeds can exchange packets, • since each one is connected to the corresponding node at the appropriate speed. • When the traffic in a circuit switching network increases too much, • calls may be blocked. • This means that the network rejects additional connection calls, • until the network load decreases (lines are released). • Instead, a packet switched network still accepts packets, • with an increased delivery delay at their destination. • A priority scheme can be implemented. Thus, if a node has a number of packets, • waiting to be transmitted, can transmit the high priority packets first. • These packets will obviously be less delayed to process than the lower priority.
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