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