WHY SWITCHING????? : WHY SWITCHING????? When two or more computer owned by the same company or organization and located to each other need to communicate, it is often easier just to run a cable between them. LANs work this way. However ,when the distances are large or there are many computers or the cables have to pass through a public load or other public right of way, the cost of running private cables are usually prohibitive.. Furthermore in just about very country in the world , stringing public transmission lines across( or underneath) public property is also illegal. Consequently, network designers must rely on switching. 1 STRUCTURE OF TELEPHONE SYSTEM : STRUCTURE OF TELEPHONE SYSTEM Fully Interconnected N/W Centralized Switch N N N N 2 N N N N Two level Hierarchy : Two level Hierarchy 3 N N N N N N N N N SWITCHING : SWITCHING It is a technology that alleviate congestion in ethernet,LAN etc.by reducing traffic and increasing switches , and are designed to work with existing cable infrastructure for that they can be installed w minimal disruption of existing networks. Often they replace share hubs.
Basically there types of switching:-
Packet switching 4 CIRCUIT SWITCHING : CIRCUIT SWITCHING When a computer in a computer network wants to communicate with any computer ,by switching equipments seeks out a physical path all the way from senders computer to reciever computer.
An important property of a circuit switching is the need to set up an end to end path before any dat can be sent. Due to this –once the network is established it remaions until the communication goes on. i.e there is no danger of cangestion. That’s why this switching technique is used in connection oriented techniques like TCP/IP. 5 Circuit Switching : Circuit Switching 6 C1 S S S S S S C2 MESSAGE SWITCHING : MESSAGE SWITCHING In this form of switching, no physical path is established in advance between sender and receiver . Instead when the sender has a block of data to be sent it is stored in the first switching office(i.e. router) and then forwarded later one hop at a time. Each block is received in its entirety, respected for errors and then retransmitted. A network using this technique is called a store and forward network. 7 PACKET SWITCHING : PACKET SWITCHING With message switching, there is no limit at all on block size, which means that routers must have disks to buffer long blocks. It also means that a single block can tie up a router-router line for minutes, rendering message switching useless for interactive traffic . To get around these problems, packet switching was invented.
Packet Switching networks place a tight upper limit on block size, allowing packets to be buffered in router’s main memory instead of on disk. By making sure that no user can monopolize any transmission line very long, packet switching networks are well suited for handling interactive traffoic. 8 Slide 9: Advantages of Packet Switching :
The first packet of a multi-packet message can be forwarded before the second one has fully arrived.
Increased throughput 9 Slide 10: Packet Switching C1 S S S S S S C2 DIFFERENCE B/W CIRCUIT & PACKET SWITCHING : DIFFERENCE B/W CIRCUIT & PACKET SWITCHING Circuit switching requires that a circuit be set up end-to-end before communication begins. While packet switching does not require any advanced set up.
If a switch goes down, all of the circuits using it are terminated and no more traffic can be sent on any of them. So, circuit switching is less fault tolerant compared to that of packet switching.
Setting up a path in advance also opens up possibility of reserving bandwidth in advance. If bandwidth is reserved, packet will not have to wait, it can be sent out as long as it arrives. With packet switching , packets may have to wait for their turn to be forwarded. 11 Slide 12: If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of that circuit is wasted. Thus, packet switching is more efficient from a system wide perspective.
Packet switching uses store-and-forward transmission while in circuit switching, the bits just flow through the wire continuously. The store-and-forward technique adds delay.
Circuit switching is completely transparent. The sender & receiver can use any bit rate, format or framing method. The carrier does not know or care, with packet switching, the carrier determines these basic parameters. 12 Slide 13: 13 SWITCHING ELEMENTS : SWITCHING ELEMENTS Repeaters