31-01-2013, 11:33 AM
Switching Techniques
Switching Techniques.ppt (Size: 113.5 KB / Downloads: 85)
In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels. There are three typical switching techniques available for digital traffic.
Circuit Switching
Message Switching
Packet Switching
Circuit Switching
Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.
Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection.
With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated.
Routing decisions must be made when the circuit is first established, but there are no decisions made after that time.
Circuit switching in a network operates almost the same way as the telephone system works.
A complete end-to-end path must exist before communication can take place.
The computer initiating the data transfer must ask for a connection to the destination.
Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer.
Message Switching
With message switching there is no need to establish a dedicated path between two stations.
When a station sends a message, the destination address is appended to the message.
The message is then transmitted through the network, in its entirety, from node to node.
Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.
This type of network is called a store-and-forward network.
A message-switching node is typically a general-purpose computer. The device needs sufficient secondary-storage capacity to store the incoming messages, which could be long. A time delay is introduced using this type of scheme due to store- and-forward time, plus the time required to find the next node in the transmission path.
Packet Switching
Packet switching can be seen as a solution that tries to combine the
advantages of message and circuit switching and to minimize the
disadvantages of both.
There are two methods of packet switching: Datagram
and virtual circuit.
In both packet switching methods, a message is broken into
small parts, called packets.
Each packet is tagged with appropriate source and destination
addresses.
Since packets have a strictly defined maximum length, they
can be stored in main memory instead of disk, therefore access
delay and cost are minimized.
Also the transmission speeds, between nodes, are optimized.
With current technology, packets are generally accepted onto
the network on a first-come, first-served basis. If the network
becomes overloaded, packets are delayed or discarded
(``dropped'').
Packet Switching: Datagram
Datagram packet switching is similar to message switching in
that each packet is a self-contained unit with complete
addressing information attached.
This fact allows packets to take a variety of possible paths
through the network.
So the packets, each with the same destination address, do not
follow the same route, and they may arrive out of sequence at
the exit point node (or the destination).
Reordering is done at the destination point based on the
sequence number of the packets.
It is possible for a packet to be destroyed if one of the nodes on
its way is crashed momentarily. Thus all its queued packets may
be lost.