X.25 PROTOCOL Presented by : EFA, Daniel Jr. G. VERGARA, Jonvon

A packet-switched wide area network (WAN) developed in 1976 by the CCITT to provide a common interface protocol between public data networks. Interface Protocol for Packet Switched Networks WAN Technology X.25 Protocol

X.25 NETWORK DTE DCE PAD PSE

X.25 PADDCEPSEDCEPAD X.25 Wide Area Network Conceptual View of X.25 DTE

X.25 Network Devices X.25 network devices fall into three general categories: Data terminal equipment (DTE). Data circuit-terminating equipment (DCE). Packet switching exchange (PSE).

Data terminal equipment (DTE) – End systems that communicate with one another across the X.25 data network and include terminals, PCs, and network hosts Data circuit-terminating equipment (DCE) – Communications devices such as modems and packet switches, that provide the interface between DTEs and PSE. Packet switching exchanges (PSE) – Constitute the majority of the network – Transfers data from one DTE to another through the X.25 network X.25 Network Devices

Packet Assembler/Disassembler (PAD) PADs provide buffering (data storage), packet assembly and packet disassembly. This operation includes adding an X.25 header. X.25 Network Devices

PAD in Action Terminal DTE Assembler/ Disassembler Buffer X.25 Packet Modem DCE

7 Layers of OSI

X.25 Protocol Layers Layer 3 Layer 2 Layer 1 OSI Network Layer OSI Data-link Layer OSI Physical Layer X.25 Packet Layer X.25 Frame Layer X.25 Physical Layer

X.25 Protocol Layers X.25 defines how packet mode terminals can be connected to a packet network. It also describes the procedures required to establish, maintain, and terminate a connection as well as a set of services that provide additional functions.

X.25 mapping to OSI Model Application Presentation Session Transport Network Data Link Physical PLP LAPB x.21 bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703 x.21 bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703 Other Services X.25 Protocol Suite

X.25 Physical Layer Protocol Called the X.21 digital interface. Designed to enable all-digital communications between DTEs and DCEs and to address the problems inherent in many of the preexisting EIA interface standards. It specifies how a DTE and DCE exchange signals to set up and clear calls.

X.25 Physical Layer Several well-known standards are used for X.25 networks – X.21bis – supports up to 2 Mbps 15-pin connector – RS-232 (EIA/TIA-232) – supports up to 19.2 Kbps 25-pin connector – RS-449 (EIA/TIA-449) – supports up to 64 Kbps 37-pin connector – V.35 – supports up to 2 Mbps 34-pin connector Uses serial communications in either asynchronous or synchronous modes

X.25 frame-layer protocol Layer 2 protocol intended to provide reliable data transfer between the DTE and DCE by transmitting data as a sequence of frames.

X.25 Frame Format Flag Field 8 bits Address Field 8 bits Frame check Sequence (CRC-16) Flag Field 8 bits Data field (variable length In 8-bit groupings) Control Field 8 bits F AC D CRC code F E hex E hex

LAPB Frame Format Flag AddressControlDataFCS Flag: (8 bits) Indicates start and end of frame ( ) Address: (8 bits) DTE address is maintained in higher layer so this field is used to identify command and responses between DTE and DCE. A value of 0x01 indicates a command from DTE and responses from DCE while a value of 0x03 indicates commands from DCE and responses from DTE. Control: (8 bits) Contains sequence numbers, commands and responses for controlling data flow Data: (varies is size) Contains upper layer data FCS: (16 bits) Frame Check Sequence used to determine if an error has occurred in transmission (variation of CRC)

X.25 frame-layer protocol functions Transfer data efficiently and in a timely manner Synchronize the link, ensuring that the receive is synchronized to the transmitter Provide error detection and recovery Identify and report procedural errors to a higher layer for recovery

X.25 Packet Layer Protocol (PLP) A layer 3 protocol Creates network data units called packets that contain user information as well as control information. Responsible for establishing a connection, transferring data over the connection, and then terminating the connection. Responsible for creating virtual circuits and negotiating network services between a DTE and DCE.

PLP Operates in Five Modes Call Setup – Used to setup virtual circuit for SVC Data Transfer – Used for transferring data with both SVC and PVC Idle – Used when SVC call has been established but no data is currently being transferred Call Clearing – Used to end communication between DTEs for a SVC Restarting – Used to synchronize DTE and DCE for all virtual circuits that exist between them

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