X.25 Overview

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Layers Interface

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Layer7:

The application Layer interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.

Layer6:

The Presentation layer establishes a context between application layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them.

Layer5:

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.

Layer4:

The Transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented.

Layer3:

The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors.

Layer2:

The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system.

Layer1:

The Physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.

X.25 Overview

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The Mnemonic

In Network Stack

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THE MNEMONIC TRADITIONALLY SUGGESTED BY IT TRAINERS IS:

ALL PEOPLE SEEM TO NEED DATA PROCESSING

X.25 Overview

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What Is Protocol ?

In Network Stack

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An agreed-upon format for transmitting data between two devices. The protocol determines the following:

the type of error checking to be used

data compression method, if any

how the sending device will indicate that it has finished sending a message

how the receiving device will indicate that it has received a message

X.25 Overview

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Layers Protocols

In Network Stack

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LayerTCP/IP suite OSI suite

# Name

7 ApplicationNNTP, SIP, SSI, DNS, FTP, Gopher, HTTP, NFS, NTP, SMPP, SMTP, SNMP, Telnet,

FTAM, X.400, X.500, DAP

6 Presentation MIME, XDR, SSL, TLS (Not a separate layer) ISO 8823, X.226

5 SessionSockets. Session establishment in TCP. SIP. (Not a separate layer with standardized API.)

ISO 8327, X.225

4 Transport TCP, UDP, IPSec, PPTP, L2TP TP0, TP1, TP2, TP3, TP4

3 Network IP, ARP, ICMP, DHCP, RIP, OSPF, BGP, IGMP, IS-IS X.25 (PLP), CLNP

2 Data Link PPP, SLIP X.25 (LAPB), Token Bus

1 PhysicalX.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703)

• You can find this table with more details in http://en.wikipedia.org/wiki/OSI_model

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Introduction

In X.25 Overview

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X.25 is an ITU-T standard protocol suite for connection to packet switched wide area

networks using leased lines, the phone or ISDN system as the networking hardware.

X.25 is designed to operate effectively regardless of the type of systems connected to

the network. It is typically used in the packet-switched networks (PSNs) of common

carriers, such as the telephone companies. Subscribers are charged based on their

use of the network. The development of the X.25 standard was initiated by the

common carriers in the 1970s. At that time, there was a need for WAN protocols

capable of providing connectivity across public data networks (PDNs). X.25 is now

administered as an international standard by the ITU-T.

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X.25 Devices

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X.25 network devices fall into three general categories:

1. data terminal equipment (DTE)-Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers.

2. data circuit-terminating equipment (DCE) - DCE devices are communications devices, such as modems and packet switches, that provide the

interface between DTE devices and a PSE, and are generally located in the carrier’s facilities.

3. packet-switching exchange (PSE)-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.

06

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X.25 Devices

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X.25 Overview

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PAD (Packet Assemble r /D i sassemble r )

In X.25 Overview

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The PAD is located between a DTE device and a DCE device, and it performs three

primary functions:

buffering (storing data until a device is ready to process it), packet assembly, and

packet disassembly. The PAD buffers data sent to or from the DTE device. It also

assembles outgoing data into packets and forwards them to the DCE device. (This

includes adding an X.25 header.) Finally, the PAD disassembles incoming packets

before forwarding the data to the DTE. (This includes removing the X.25 header.)

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X.25 Session Establishment

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X.25 sessions are established when one DTE device contacts another to request a

communication session. The DTE device that receives the request can either

accept or refuse the connection. If the request is accepted, the two systems

begin full-duplex information transfer. Either DTE device can terminate the

connection. After the session is terminated, any further communication requires

the establishment of a new session.

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X.25 Virtual Circuits

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A virtual circuit is a logical connection created to ensure reliable communication

between two network devices. A virtual circuit denotes the existence of a logical,

bidirectional path from one DTE device to another across an X.25 network.

Physically, the connection can pass through any number of intermediate nodes,

such as DCE devices and PSEs. Multiple virtual circuits (logical connections) can

be multiplexed onto a single physical circuit (a physical connection). Virtual

circuits are demultiplexed at the remote end, and data is sent to the appropriate

destinations.

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X.25 Virtual Circuits

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X.25 Overview

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X.25 Virtual Circuits

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Two types of X.25 virtual circuits exist:

1. Switched virtual circuits (SVCs)-SVCs are temporary connections used for sporadic data transfers. They require that two DTE

devices establish, maintain, and terminate a session each time the devices need to

communicate.

2. Permanent virtual circuits(PVCs)-PVCs are permanently established connections used for frequent and consistent data transfers.

PVCs do not require that sessions be established and terminated. Therefore, DTEs can begin

transferring data whenever necessary because the session is always active.

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X.25 Protocol Suite

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X.25 Overview

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Packet Layer Protocol

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PLP manages packet exchanges between DTE devices across virtual circuits. PLPs

also can run over Logical Link Control 2 (LLC2) implementations on LANs and over

Integrated Services Digital Network (ISDN) interfaces running Link Access

Procedure on the D channel (LAPD).

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Packet Layer Protocol

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The PLP operates in five distinct modes:1. Call Setup

-Call setup mode is used to establish SVCs between DTE devices. A PLP uses the X.121 addressing scheme to set up the virtual circuit. The call setup mode is executed on a per-virtual-circuit basis, which means that one virtual circuit can be in call setup mode while another is in data transfer mode. This mode is used only with SVCs, not with PVCs.

2. Data Transfer-Data transfer mode is used for transferring data between two DTE devices across a virtual circuit. In this mode, PLP handles segmentation and reassembly, bit padding, and error and flow control. This mode is executed on a per-virtual-circuit basis and is used with both PVCs and SVCs.

3. Idle-Idle mode is used when a virtual circuit is established but data transfer is not occurring. It is executed on a per-virtual-circuit basis and is used only with SVCs.

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Packet Layer Protocol

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4. Call Clearing-Call clearing mode is used to end communication sessions between DTE devices and to terminate

SVCs. This mode is executed on a per-virtual-circuit basis and is used only with SVCs.

5. Restarting-Restarting mode is used to synchronize transmission between a DTE device and a locally connected DCE device. This mode is not executed on a per-virtual-circuit basis. It affects all the DTE device’s established virtual circuits.

Four types of PLP packet fields exist:

• General Format Identifier (GFI)-Identifies packet parameters, such as whether the packet carries user data or control information, what kind of windowing is being used, and whether delivery confirmation is required.

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Packet Layer Protocol

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• Logical Channel Identifier (LCI)-Identifies the virtual circuit across the local DTE/DCE interface.

• Packet Type Identifier (PTI)-Identifies the packet as one of 17 different PLP packet types.

• User Data-Contains encapsulated upper-layer information. This field is present only in datapackets. Otherwise, additional fields containing control information are added.

X.25 Overview

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LAPB (L ink Access Procedure Ba lanced)

In X.25 Overview

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LAPB is a data link layer protocol that manages communication and packet framing between DTE and DCE devices. LAPB is a bit-oriented protocol that ensures that frames are correctly ordered and error-free.Three types of LAPB frames exist: 1. Information(I-frame)

-carries upper-layer information and some control information. I-frame functions include sequencing, flow control, and error detection and recovery. I-frames carry send- and receive-sequence numbers.

2. Supervisory(S-frame)-carries control information. S-frame functions include requesting and suspending transmissions, reporting on status, and acknowledging the receipt of I-frames. S-frames carry only receive-sequence numbers.

3. Unnumbered (U-frame)-The unnumbered frame (U frame) carries control information. U-frame functions include link setup and disconnection, as well as error reporting. U frames carry no sequence numbers.

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X.21bis Protocol

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X.25 Overview

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X.121 Address Format

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The X.121 Address field includes the International Data Number (IDN), which consists of two fields:

1. Data Network Identification Code (DNIC):DNIC is an optional field that identifies the exact PSN in which the destination DTE

device is located.This field is sometimes omitted in calls within the same PSN. The DNIC has two subfields:

Country: subfield specifies the country in which the destination PSN is located.

PSN: specifies the exact PSN in which the destination DTE device is located.

2. National Terminal Number (NTN): identifies the exact DTE device in the PSN for which a packet is destined. This field varies in length.

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X.121 Address Format

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PRESENTED BY:

MEISAM MOHAMMADKHANI