Packet Switching Network

• One of the common communications method is packet switching, which divides messages into packets and sends each packet individually.

• All the data to be transmitted is first assembled in to message units called packets.

• One of the key features of a packet is that it contains the source and destination network address in addition to the data.

• They are then passed by the source computer to its local packet switching exchange (PSE).

• On receipt of each packet, the exchange first stores the packet and then inspects the destination address it contains.

• Each PSE contains a routing directory specifying the outgoing link’s transmission paths to be used for each network address.

• Each packet may go a different route; its header address tells it where to go and describes the sequence for reassembly at the destination computer.

• Each packet is sent with a ‘header address’. • This tells it where its final destination is, so it knows

where to go.• The header address also describes the sequence for

reassembly at the destination computer so that the packets are put back into the correct order.

• These packets are sent out from the computer and they travel around the network seeking out the most efficient route to travel.

• PACKET STORE and FORWARD• The PSE forward the packets on the appropriate link at

the maximum available bit rate. This mode is often referred to as packet store and forward.

• A number of packets arrive simultaneously at a PSE on different incoming links and each may require forwarding on the same outgoing link.

• If a number of long packets are waiting to be transmitted on the same link, other packets may experience unpredictable long delays.

• To prevent this and to ensure that the network has a fast transmit time, a maximum length is allowed for each packet.

• These packets are reassembled into a single message by the protocol at the destination. When received, packets are reassembled in the proper sequence to make up the message.

• One packet also contains details of how many packets should be arriving so that the recipient computer knows if one packet has failed to turn up.

• If a packet fails to arrive, the recipient computer sends a message back to the computer which originally sent the data, asking for the missing packet to be resent.

• X.25 protocol describes a method of encapsulating and handling packets with high reliability.

Circuit Switching Network

• A type of communications in which a dedicated channel (or circuit) is established for the duration of a transmission.

• Two network nodes establish a dedicated communications channel (circuit) before the nodes may communicate.

• The circuit remains connected for the duration of the communication session.

• The telephone system is the circuit-switching network , which links together wire segments to create a single unbroken line for each telephone call.

• A telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties.

• Traditional PSTN phone system uses circuit switching while VoIP uses packet switching.

• PSTN stands for Public Switched Telephone Network. It is the same thing as POTS (Plain Old Telephone System). It is simply the worldwide telephone network.

• Circuit-switching systems are ideal for communications that require data to be transmitted in real-time.

• It is ideal for the situation when data must be transmitted quickly and must arrive in the same order in which it's sent.

• This is the case with most real-time data, such as live audio and video.

• There are three phases in circuit switching:• Establish• Transfer• Disconnect• A circuit switched networks connection

provides a fixed data rate channel at which both subscribers must operate.

• Before any data can be transmitted, a connection must be set up or established through the network.