Switching & X.25Switch Datagrams

Packet Switching Each end-end data stream divided into packets• User A, B packets share network resources

• Each packet uses full link bandwidth • Resources used as needed, • There are two methods of packet switching: Datagram and virtual circuit.

Bandwidth division into “pieces”

Dedicated allocationResource reservation

Connection Oriented Packet Switch Network

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.25's 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 & OSI Model

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 carrier's facilities.

X.25 Networking Devices

• PSEs are switches that compose the bulk of the carrier's 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 (Cisco's 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.

Next Time ... Transmission Control Protocol