The Data Communication Equipment

Published Date: 02 Nov 2017

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INWK 6111

TITLES:

X.25

FRAME RELAY

-SRIKANTH SK (B00620329)

X.25

INTRODUCTION:

X.25 was first introduced by International Telegraph and Telephone Consultative Committee (CCITT). Nowadays, it follows the standard of ITU –T (International Telecommunication Union).The X.25 was very popular between 1970 and 1980.It is a standard used to define the interface between the DTE and a packed-switching network. In circuit switching, it creates a dedicated link between the source and destination, so there is no need to specify the interface between the DTE and DCE whereas in packet-switching, the attached stations should transfer the data into packets for transmission. Therefore, the interface needs a protocol which has been defined X.25.

Data Terminal Equipment (DTE)

The equipment used in source or destination. For instance, routers, printers, servers, desktops, etc .

Data Communication Equipment (DCE)

Data Communication equipment is a device in which the data is put upon the local loop .It is also called as Data Circuit-terminating Equipment

X.25 PROTOCOL:

The operation of X.25 can be defined by three level functions which operate in the basic three layers such as physical, data-link and network layer.

The physical level function is handled by the physical layer

The link level function is handled by the Data-Link layer and

The packet level function is handled by the Network layer

The following diagram gives the detail information about the X.25 structure and working layer

C:\Users\keerthi\Desktop\x.25.png

Physical Level:

It states the physical link between the stations and the packet switching-node which is defined by using any one of the following protocols X.21, EIA -232, EIA - 441 and some other serial protocols.

Link Level:

It gives a reliable data transfer across the physical link by using the protocol LAPB (Link Access Procedure balanced protocol) which helps to manage and control the packet framing

LAPB FRAME

Flag Address control INFORMATION FCS Flag

Front Flag: It indicates the starting point of the frame.

Rear Flag: It indicates the end of the frame

Address Field: It is an 8 bit field which contains the command and response value of the DTE and DCE which is either 0x03 or 0x01.

Control Field: It contains the sequence numbers for flow control and it is also an 8bit field.

FCS: Frame Check Sequence is used for error control.

PACKET LEVEL:

It sets up the logical connection between the stations and that connection is called as virtual circuit. And also, this level is responsible for the call set up for data transfer between two DTE’s.

CALL SET-UP IN X.25:

http://www.doc.ic.ac.uk/%7Ekpt/Slides/X25/img007.GIF

Here, in the above diagram, first, a call request is send from the source DTE to the destination DTE to establish a connection .After that, if the call is accepted then the source sends data packets and receives the acknowledgement of each received packets. Once all the data transferred, the source sends a clear request to release the connection.

FRAMING OF DATA:

C:\Users\keerthi\Desktop\x2501.gif

Packet assembler/disassembler (PAD) is a device which is used to convert the user data into X.25 packets at the source terminal. And also, in the destination it again help to extract the data from the X.25 frame.

The framing of data in X.25 is achieved by the following steps,

The user data are sent to the lower layer which creates a X.25 packet by adding the control information as a header. In X.25 each node has more than one virtual circuit.so the header contains the information about the identity number of the particular virtual circuit to which the data is to be associated .And also, it contains the sequence numbers of P(s) for sequencing the outgoing packets and P(R) for acknowledging the received packet which are used for flow and error control.

After that, the X.25 packet is passed to the next lower layer which again adds control information in both the ends of the packet for LAPB protocol.

CONCLUSION:

The hop-to-hop error control and each hop maintains a separate state table which provides the most consistent communication by reducing the maximum errors.so still it used for credit card verification.

FRAME RELAY

INTRODUCTION:

Frame relay was introduced to overcome the drawbacks of X.25 which follows the standard of ISDN. Even though, the X.25 reduces the maximum number of errors, it takes more processing time. This is mainly because of the node-to-node error control .And also; it uses the same channel for the control information and the data, which causes some congestion in the channel. In frame relay, the node-to-node error control was eliminated by implementing end-to-end error control and it gives a separate channel for control information from source to destination. Also, frame really has completed the process of multiplexing in the layer 2 whereas in X.25, it needs another layer to complete the multiplexing. These functionalities cut down the processing time and augment data rate between two stations

FRAME RELAY PROTOCOL ARCHITECTURE:

http://2.bp.blogspot.com/-Do1xHsxjw2k/TZHH_GXQtZI/AAAAAAAAADI/OWcwUdDBcGw/s1600/frame+re.JPG

In this architecture, there are two main planes which play the major functions are

C-plane:

It supports to create the logical connection at the beginning of the communication and terminates it after the communication.

It also helps to define the protocol between the network and a hop.

It gives a separate logical path for control information which is defined by the protocol LAPD at the data-link layer. So a reliable data link service path is created between the user and the network which is used to transfer the Q.933 control signalling.

U-plane:

It helps for the transfer of data between two stations and the U-plane protocol defines an end-to-end control.

It provides the transfer of data between the two end users.

It is defined by using the protocol LAPF (Link Access Procedure for Frame mode bearer services) protocol and it is defined in Q.922.

LAPF Functions:

Ordering the frame and delimiting.

Multiplexing and demultiplexing of frames has been done by using the addressing field.

Checks the frame weather it is too long or too short.

It helps to detect the transmission errors.

It also controls the congestion in the communication.

LAPF Frame Format

Flag Address Information FCS Flag

The data in frame relay is transferred through the LAPF which is mostly same as LAPD. But it doesn’t have the control information field.

It is impossible to achieve all control operation on the connection, because the connection carries only the user information.

No sequence number is specified in this frame .So, it is difficult to control the flow of data and the errors.

Front Flag: It indicates the starting point of the frame

Rear Flag: It denotes the ending point of the frame.

Address Field: It varies between 2, 3 and 4 octets which contains the data link connection identifier (DLCI). The DLCI helps to find the particular logical connection which the data is to be associated.

FCS: Frame check sequence is used for error control which happens at the destination.

FRAME RELAY IN WAN:

In the following diagram, the DTE (Data Terminal Equipment) are connected to the WAN (Wide Area Network) and the DCE (Data Communication Equipment) are interconnected by trunk lines inside the WAN.

C:\Users\keerthi\Desktop\frame relay in wan.png

The frame relay defines the operation between the DTE and DCE and not between the DCE’s. So, the overhead is reduced and increases the efficiency. It also allows access to network and finds the transmission errors through standard cyclic redundancy check.

Frame assembler/dissembler (FRAD) is a device used to send the data packets as frames relay frames into the network at the source node and at the destination node it turns the frame relay frames as data packets.

CONCLUSION:

It is more efficient than X.25, because of the high data transfer with in the less amount of time. So, still it is used by some governments and telecommunications.

REFERNCES:

http://www.doc.ic.ac.uk/~kpt/Slides/X25/sld002.htm

http://en.wikipedia.org/wiki/X.25

William Stallings, "Data and Computer Communications, Eighth Edition."

CISCO, "CCNA exploration".