25-08-2017, 09:32 PM
Switching Techniques
Switching Techniques[.ppt (Size: 119.5 KB / Downloads: 284)
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.
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.
Packet Switching
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 size
The size of the packet can vary from 180 bits, the size for the Datakit® virtual circuit switch designed by Bell Labs for communications and business applications; to 1,024 or 2,048 bits for the 1PSS® switch, also designed by Bell Labs for public data networking; to 53 bytes for ATM switching, such as Lucent Technologies' packet switches.
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.
Packet Switching:Virtual Circuit
In virtual circuit, the route between stations does not mean that
this is a dedicated path, as in circuit switching.
A packet is still buffered at each node and queued for output over
a line.
The difference between virtual circuit and datagram approaches:
With virtual circuit, the node does not need to make a routing
decision for each packet.
It is made only once for all packets using that virtual circuit.