Packet SwitchingEach end-end data stream divided into packets Bandwidth division into “pieces”• User A, B packets share network resources Dedicated allocation• Each packet uses full link bandwidth Resource reservation• Resources used as needed,• There are two methods of packet switching: Datagram and virtual circuit.
Why Do We Need X.25• In case of packet-switching networks, the attached stations must organize their data into packets for transmission.• x.25 is an ITU-T standard that specifies an interface between a host system and a packet-switching network.X.25 is specified on three levels :• Physical level• Link level• Packet level
X.25 Introduction• X.25 is a standard that defines the connection between a terminal and a PSN.• X.25 is an interface specification. It does not specify the characteristics of the PSN itself.• X.25 was designed to transmit and receive data between alphanumeric "dumb" terminals through analog telephone lines.• Later, X.25s capability was expanded to support a variety of networking protocols, including TCP/IP, Novell IPX, and AppleTalk.
X.25 Protocol Suite• The X.25 suite of protocols includes Packet Layer Protocol (PLP) Link Access Procedure, Balanced (LAPB) Physical-layer serial interfaces (e.g., X.21bis, EIA/TIA-232, EIA/TIA-449, EIA- 530, and G.703).• Both PLP and LAPB include mechanisms for error checking, flow control, and reliability.• By including these mechanisms at both Layer 2 and Layer 3, X.25 provides a high level of reliability.
X.25 Networking DevicesX.25 network devices fall into three general categories: Data terminal equipment (DTE). Data circuit-terminating equipment (DCE). Packet switching exchange (PSE).• DTE devices are end systems that communicate across the X.25 network. They are usually terminals, routers, or network hosts, and are located on the premises of individual subscribers.• DCE devices are communications devices such as modems and packet switches. They provide the interface between DTE devices and a PSE. X.25 DCEs are typically located in the carriers facilities.
X.25 Networking Devices• PSEs are switches that compose the bulk of the carriers network. They transfer data from one DTE device to another through the X.25 PSN.
Packet Assembler / Disassembler• The packet assembler/disassembler (PAD) is a device commonly found in X.25 networks. The PAD is located between a DTE device and a DCE device, and it performs three primary functions: Buffering. Packet assembly. Packet disassembly.• X.25 connection can be a permanent virtual circuit (PVC) or, more commonly, a switched virtual circuit (SVC).
PVC Vs. SVC• A PVC (Permanent) is similar to a leased line. PVCs use no call setup or call clear that is apparent to the subscriber. Any provisioned PVCs are always present, even when no data traffic is being transferred.• An SVC exists only for the duration of the session. Three phases are associated with X.25 SVCs: Call setup. Information transfer. Call clear.
Configuration of X.25• When you select X.25 as a WAN protocol, you must set appropriate interface parameters. The interface configuration tasks include: Define the X.25 encapsulation (DTE is the default (Ciscos encapsulation type)). Assign the X.121 address.(PDN – Public Data Numbers) Define map statements to associate X.121 addresses with higher- level protocol addresses. Other configuration tasks can be performed to control data throughput and to ensure compatibility with the X.25 network service provider.
Take Care ....• X.25 is a flow-controlled protocol. The default flow-control parameters must match on both sides of a link.• Mismatches because of inconsistent configurations can cause severe internetworking problems.