X.25PROTOCOL

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

DTE

DTE

DTE

DCEDCE

DCE

DCE

PAD

PSE PSE

PSEPSE

X.25

PAD DCE PSE DCE PAD

X.25 Wide Area Network

Conceptual View of X.25

DTE 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

TerminalDTE

ModemDCE

PAD

Buffer

Data

Assembly/Disassembly

Data

X.25 Packet

PSE

PSE

PAD in Action

TerminalDTE

Assembler/Disassembler

Buffer

X.25 Packet

ModemDCE

7 Layersof

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

PhysicalPhysical

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 LayerSeveral 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

FlagField8 bits

AddressField8 bits

Frame checkSequence(CRC-16)

FlagField8 bits

Data field(variable length

In 8-bit groupings)

ControlField8 bits

F A C D CRC code F

01111110

7E hex

01111110

7E hex

LAPB Frame Format

Flag FlagAddress Control Data FCS

Flag:Flag: (8 bits) Indicates start and end of frame (01111110) (8 bits) Indicates start and end of frame (01111110)

Address:Address: (8 bits) DTE address is maintained in higher layer so this field is used to (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 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 command from DTE and responses from DCE while a value of 0x03 indicates commands from DCE and responses from DTE.commands from DCE and responses from DTE.

Control:Control: (8 bits) Contains sequence numbers, commands and responses for (8 bits) Contains sequence numbers, commands and responses for controlling data flowcontrolling data flow

Data:Data: (varies is size) Contains upper layer data (varies is size) Contains upper layer data

FCS:FCS: (16 bits) Frame Check Sequence used to determine if an error has occurred in (16 bits) Frame Check Sequence used to determine if an error has occurred in transmission (variation of CRC)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 recoveryIdentify and report procedural errors to a

higher layer for recovery

X.25 Packet Layer Protocol (PLP)

A layer 3 protocolCreates 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

Thank you!