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

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Presentation on theme: "X.25 PROTOCOL Presented by : EFA, Daniel Jr. G. VERGARA, Jonvon."— Presentation transcript:

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

2 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


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

5 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).

6 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

7 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

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

9 7 Layers of OSI

10 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

11 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.

12 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

13 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.

14 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

15 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.

16 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 01111110 7E hex 01111110 7E hex

17 LAPB Frame Format Flag AddressControlDataFCS Flag: (8 bits) Indicates start and end of frame (01111110) 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)

18 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

19 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.

20 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

21 Thank you!

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