Packet-Switching Networks-

                Packet switching network enable you to transmit data over an any-to-any connection Sometimes a packet-switched network is described as a mesh network.  When information is transmitted over the network, it is known what path the information will take between the sender and the recipient of t he data.

                The original data is broken into smaller packets.  Each packet is tagged with the destination address and sequence number.  As the packet traverses the network between the source and destination hosts, it travels on the best current path.  This way, if a network link goes down during the transmission of a stream of packets, not all the packets have to be re-sent.  Some of the packets will have found an alternate route when the link went down.

                The following are three common implementations of packet-switching networks:

·         X.25

·         Frame relay

·         Asynchronous transfer mode(ATM)

X.25 Networks-

                X.25 is developed in the 1970s to provide users with WAN capabilities over public data networks.  Phone companies developed it, and it attributes are international in nature,  It is administered by an agency of the United Nations called the International Telecommunications Union (ITU).

                In an X.25 network, a host calls another host tyo request a communications session.  If the call is accepted, the two systems. Can begin a full-duplex information transfer. Either host can terminate the session.

                A point –to-point connection takes place between data terminal equipment (DTE) at the client site and data circuit-terminating equipment (DCE) at the carries’        facilities.  The DTE is connected to the DCE through a translation device known as a packet assembler/disassemble (PAD). The DCE connects to packet switching exchanges (PSEs), more commonly known as switches.  The switches interconnect with each other until they reach the DCE of the destination host.  This DCE connects to the DTE connects to the DTE of the host complete the communications session.

                 An association known as a virtual circuit accomplishes the end-to-end communication between the two DTEs.  Virtual circuits enable communication between two defined end points to take place through any number of intermediate nodes.  These nodes do not have to be a dedicated portion of the network.  The circuit is not a physical data link; it is bandwidth that can be allocated on demand.  The following are the two types of virtual circuits:

·         Permanent virtual circuits (PVCs)-  PVCs are used for common data transfers known to occur on a regular basis.  Although the route is permanent, the client pays only for the time the line is in use.

·         Switched virtual circuits (SVCs)-  SVCs are used for data transfers that are sporadic in nature.  The connection uses a specified route across the network.  The route is maintained until the connection ceases.