Switching : Switching is process to forward packets coming in from one port to a port leading towards the destination. When data comes on a port it is called ingress, and when data leaves a port or goes out it is called egress. A communication system may include number of switches and nodes. At broad level, switching can be divided into two major categories:
Connection-less: The data is forwarded on behalf of forwarding tables. No previous handshaking is required and acknowledgements are optional.
Connection Oriented: Before switching data to be forwarded to destination, there is a need to pre-establish circuit along the path between both endpoints. Data is then forwarded on that circuit. After the transfer is completed, circuits can be kept for future use or can be turned down immediately.
Circuit Switching: A circuit switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. Each circuit that is dedicated cannot be used by other callers until the circuit is released and a new connection is set up. Even if no actual communication is taking place in a dedicated circuits then, that channel still remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle. Circuit switching is used for ordinary telephone calls. It allows communications equipment and circuits, to be shared among users. Each user has sole access to a circuit during network use.
Circuit switching can be relatively inefficient because capacity is wasted on connections which are set up but are not in continuous use. On the other hand, the connection is immediately available and capacity is guarantee until the call is disconnected;
Communication using circuit switching involves three phases discussed below:
a. Connection establishment: Before any signal can be transmitted, an end to end circuit must be established.
b. Data transfer: Information can now be transmitted from source through the network to the destination using the dedicated path established.
c. Termination: After some period of data transfer, the connection is terminated
Call setup time with conventional equipment is typically on the order of 5 to 25 seconds after completion of dialing. Trade-offs between Circuit switching and other types of switching depend strongly on switching times.
Message Switching: Message switching was the precursor of packet switching, where messages were routed in their entirety and one hop at a time. It was first introduced by Leonard Klein-rock in 1961. Message switching system is nowadays mostly implemented over packet-switched or circuit-switched data networks.
Hop-by-hoop Telex forwarding are examples of message switching systems. E-mail is another example of a Message switching system. When this form of switching is used, no physical path is established in advance in between sender and receiver. Instead, when the sender has a block of data to be sent, it is stored in the first switching office then forwarded later at one hop at a time.
Each block is received in its entity form, inspected for errors and then forwarded or re-transmitted. It is a form of store-and-forward network. Data is transmitted into the network and store in a switch. The network transfers the data from switch to switch when it is convenient to do so, as such the delays can happen. The source and destination terminal need not be compatible, since conversions are done by the message switching networks.
A delay for putting the message on the communications link is also incurred at each node enrooted. Message lengths are slightly longer than they are in circuit switching, after establishment of the header includes information identifying must be included with each message. The header includes information identifying the destination as well as other types of information. Most message switched networks do not use dedicated point-to-point links.
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Packet Switching: In packet switching network unlike CS network, it is not required to establish the connection initially. The connection/channel is available to use by many users. But when capacity or number of users increases then it will lead to congestion in the network. Packet switched networks are mainly used for data and voice applications requiring non-real time scenarios.
if user-A wants to send data/information to user-C and if user-B wants to send data to user-D, it is simultaneously possible. Here information is padded with header which contains addresses of source and destination. This header is sniffed by intermediate switching nodes to determine their route and destination.
In packet switching, station breaks long message into packets. Packets are sent one at a time to the network. Packets are handled in two ways, viz. data-gram and virtual circuit.
In data-gram, each packet is treated independently. Packets can take up any practical route. Packets may arrive out of order and may go missing.
In virtual circuit, planned route is established before any packets are transmitted. The handshake is established using call request and call accept messages. Here each packet contains virtual circuit identifier instead of the destination address. In this type, routing decisions for each packet are not needed.
Packet switching enhances line efficiency as packets from multiple applications can be multiplexed over the carrier. The internet uses packet switching technique. Packet switching enables the user to differentiate data streams based on priorities. Packets are stored and forwarded according to their priority to provide quality of service.
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