Virtual Circuit A Logical Connection

Published Date: 02 Nov 2017

Introduction

X.25 is a protocol to provide reliable data communications on public data networks. This protocol transfers the data through packet switching. In this method, the information is taken from different users and combined into discrete data packets. These data packets are further forwarded to the Packet Data Network (PDN). Using the routing information the data packet is routed through the network cloud.

X.25 packet switching uses virtual circuits. The data are converted into the packets which are transferred in a logical form over a circuit.

x.25 packet switching network devices are used in between the data terminal equipment (DTE), data circuit-terminating equipment (DCE), and packet-switching exchange (PSE). Data terminal equipment devices that communicate across the X.25 network are usually the end systems. These devices are usually terminals, computers, or network hosts. DCE devices are communications devices, such as modems and packet switches. They provide the interface between DTE devices and a PSE. PSEs are switches that carry the network. They transfer data from one DTE device to another through the X.25 PSN.

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Figure 1: DTEs, DCEs, and PSEs Make Up an X.25 Network

The above Figure 1 explains the DTEs, DCEs, and PSEs makes up an X.25 Network and the relationships among the three types of X.25 network devices.

PAD- Packet Assembler/Disassembler

The PAD has three functions which it performs between the DTE and the DCE device. The first is the buffering which is the storing the data until the processing of the device starts. The second and third are the packet assembly and the disassembly. It buffers the data that is transmitted and received to or from DTE device and also assembles the data into packets which further forwards it to DCE device. PAD also disassembles the packets that are received before it is forwarded to the DTE device. The communication period lasts as long as the DTE device terminates the connection itself. For every communication the connection should be established.

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Figure 2- PAD

X.25 Virtual circuit-a logical connection

The virtual circuit is the logical two way path between the DTE device and the x.25 network destination. There are many such virtual paths which is multiplexed into one circuit. At the remote end the paths are again demultiplexed towards the destination. The operation of the virtual circuit is that when the DTE device sends packets to DCE, it checks for the packet headers to use the appropriate virtual circuit and sends the packets to the nearest PSE. The PSE further transfers the traffic to the next node which can be a remote DCE device.

X.25 Protocol Mapping to OSI model layers

The lowest three layers of the OSI model are mapped to the x.25 protocol. The following are the protocols used: Packet-Layer Protocol (PLP), Link Access Procedure, Balanced (LAPB) and the physical layer interfaces (X.21bis).

PLP is the network layer protocol. It works on five modes: call setup, data transfer, idle, call clearing, and restarting. Call setup establishes switch virtual circuits in the X.25 network. PLP uses X.121 addressing scheme to establish virtual circuit.

The data transfer mode is used to transfer the data between the two terminal devices. PLP here handles the error control, flow control, segmentation, reassembly and bit padding.

The idle mode is assumed when the virtual circuit is established but the data transfer fails. Call clearing mode is used to terminate the session between the DTEs. Restarting mode is used to set the transmissions between the DTE and the DCE devices.

Link Access Procedure, Balanced is a data link layer protocol. It deals with the packet framing and the communication between the DTE and DCE device. This protocol based on bits and checks if the frames are error free. The LAPB frames are information frames, supervisory and unnumbered frames. The information frmae comprises of upper layer information. It has functions such as sequencing, flow control, and error detection. The supervisory frame carries control information. the function of this frame is requesting and suspending transmission, reporting on status and acknowledging. The unnumbered frame deals with the control information. this includes link setup and disconnection.

X.21bis is a physical layer protocol. X.21bis allows the activation and deactivation between DTE and DCE. It uses point to point connection, synchronous, full-duplex transmission.

PLP Packet encapsulation within LAPB and X.21bis Frame

Figure 3- PLP packet encapsulated in LAPB frame.

The above diagram explains that the PLP packet with the fields GFI, LCI, PTI, User data is enclosed within the LAPB frame.

LAPB Frame Format

The LAPB comprises of a header, data and trailer. The following are the fields in the frame.

Flag- It indicates the starting and the ending of the frame.

Address Field- The address field specifies source and destination address.

Control- The control bits are used for flow and error control.

FCS- It is used for error detection using cyclic redundancy control

Figure 4- LAPB Frame Format

Frame Relay

Introduction

Frame relay is a new wide area networking method. It is an evolution in X.25. It uses the packet switched technology that does not provide error check. This is because the modern digital and optical network transfer the data with low rate of errors. Therefore the frame relay leaves it to the end stations to look for the errors. Frame relay thus reduces the amount of overhead during transmission and allows fast packet switching. It is purchased as a dedicated lease line such as T1. A Frame Relay network consists of a number of DTE devices, such as routers, connected to remote ports on multiplexer equipment via traditional point-to-point services such as a T1.

Devices in Frame Relay

Data terminal equipment (DTE)

These devices are the terminating devices in a network and are used by the customers. Examples are PCs, routers and bridges.

Data circuit-terminating equipment (DCE)

The DCE equipment provides clocking and switching that transmit data through the wide area network.

The communication between DTE and DCE device takes place in both physical and data link layer. The physical layer interface used is RS-232. The data link layer components deals with the connection between the router and the switch.

Implementation of Frame Relay Network

A frame relay network utilises a T1 multiplexer. A frame relay network comprises of DTE devices such as routers connected to ports on multiplexer device through T1.

The above diagram explains the connection of different devices to different servers in wide area network. There are two methods to deploy frame relay:

Public carrier-Provided Networks

The frame relay switching device is owned by the central offices and the customers that subscribe for the network are charged for it. They are not allowed to administer and maintain the service. The the DCE device is owned by the service provider and the DTE device is customer owned.

Private Enterprise Networks

The private enterprise networks are owned by the private organisations around the world. Here the administration and the maintenance of the network are the organisation’s own responsibility.

Frame Formats

Standard frame relay frame

Flags- denotes the ending and starting of the frame.

Address-

DLCI – Data link connection identifier is 10 bit. The value specifies the connection between DTE device and the switch.

C/R - (1 bit) The C/R bit is used for the upper layers to determine commands and response

EA (1 bit) – this determines if this byte is last byte of address (0= more, 1 =last)

FECN (1 bit) – Forward Explicit Congestion Notification indicates congestion in the direction the frame is being transferred.

BECN (1 bit ) – Backward Explicit Congestion Notification indicates the congestion in the opposite direction the frame is being transferred.

DE (1 bit) – Discard Eligibility describes the frame as low priority.

Difference between X.25 and frame relay

The connection establishment and the release in X.25 uses the same virtual channel for data transmission which causes overhead. The frame relay uses different virtual channels to transmit data using LMI protocol.

X.25 uses packet switching on network layer of OSI Layer. The frame relay performs packet switching on data link layer of OSI Layer.

X.25 routers have to acknowledge each frame while in error detection. The frames have to be retransmitted and acknowledged which is a complicated process. The frame relay does not perform flow control between frame handlers.