X.25 , Frame Relay and ATM

Presented by

Y.Raju

309175710118

x.25 is ITU-T protocol standard for WAN communications.

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x.25 network device fall into three general categories

1.DTE

2.DCE

3.PSE

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DTE are usually terminals DCE devices are communication devices such as modems and packet switches and PSE as generally located in carrier’s facilities

X.25 Network

x.25 defines the procedure for the exchange of data between user device and packet network node

It is interface between DTE and DCE for terminals operating the packet mode on public Data network.

x.25 standard specifically calls for three layers of functionality

1.physical layer

2.link layer

3.packet layer

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The X.25 Protocol

The X.25 Protocol Layers

Layer Name

Description

• Physical layer deals with physical interface between computer and the link that attaches that station to the packet switching node.

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• Link layer provides for the reliable transfer of data across the physical link by transmitting the data as a sequence frames.x.25 packet is carried within the LAPB frame as the information field(i).

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• x.25 allows both SARM and SABM to support LAP and LAPB

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X.25 Protocol

X.25 interface diagram

Packet layer provides an external virtual circuit service.

Relation between user data and x.25 protocol control information

User data are passed down to x.25 level 3, which appends control information as a header, creating a packet. X.25 protocol uses this control information for its operation

LAPB Frame Format

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.

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Control: (8 bits) Contains sequence numbers, commands and responses for controlling data flow.

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Data: (varies is size) Contains upper layer data.

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FCS: (16 bits) Frame Check Sequence used to determine if an error has occurred in transmission (variation of CRC).

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• X.25 connection can be a permanent virtual circuit (PVC), a switched virtual circuit (SVC).

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• A PVC is similar to a leased line. Both the network provider and the attached X.25 subscriber must provision the VC. PVCs use no call setup or call clear that is apparent to the subscriber. Any provisioned PVCs are always present, even when no data traffic is being transferred.

Frame Relay

• Frame Relay was originally designed for use on Integrated Services Digital Network (ISDN).
• Usually considered a replacement for X.25 using more advanced digital and fiber optic connections.Does not perform error correction at intermediate nodes making it faster than X.25.

When an error is detected (FCS) the frame is discarded and correction is left up to higher layer protocols

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• Original standard proposed in 1984 but widespread acceptance did not occur until the late 1980’s
• Service Description Standard

Overall service description and specifications, Connection Management

• Core Aspects

Frame Format, Field Functions, Congestion Control

• Signaling

Establishing and Releasing switched connections and status of permanent connections

Frame Relay Devices

• Data Terminal Equipment (DTE)
• Terminals, Personal Computers, routers, and bridges typically at the customer location
• Data Circuit-terminating Equipment (DCE)
• Typically packet switches owned by the carrier that transmit data through the WAN

Sample Frame Relay Network

Frame Relay Assembler/Disassembler (FRAD)

• To handle frames from other protocols a FRAD is used to provide conversion to Frame Relay packets
• A FRAD can either be a separate device or part of a router/switch

Frame Relay mapping to OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

LAPF

Other Services

Frame Relay Protocol

Any Standard

Frame Relay Physical Layer

• No specific protocol is defined
• Any protocol recognized by ANSI can be implemented

Frame Relay Data Link Layer

• Link Access Protocol for Frame Modes Services (LAPF) is the protocol defined for Frame Relay Layer 2 services

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• LAPF is a version of HDLC
• Does not provide flow or error control
• Uses Address field for DLCI (addressing) as well as for congestion control

LAPF Frame Format

DLCI: (10 bits) Data Link Connection Identifier is used to identify the Virtual Circuit number

C/R: (1 bit) Provided for up layers to determine commands and responses

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

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C/R

EA

DLCI

DLCI

EA

DE

BECN

FECN

Extended Addresses

• To increase the number of virtual circuits the DLCI can be expanded from 10 bits to 16 bits and 23 bits
• The EA field is set to 0 to indicate that additional address bytes are present. The last address byte will have a 1 in the EA field

FECN: (1 bit) Forward Explicit Congestion Notification indicates congestion in the direction the frame is traveling

BECN: (1 bit) Backward Explicit Congestion Notification indicates congestion in the opposite direction the frame is traveling

DE: (1 bit) Discard Eligibility indicates that a frame is low priority when set

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Meaning of ATM: Asynchronous Transfer Mode is a high-speed network technology that supports the transportation of voice, data, and video signals over a single stream.

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ATM combines both circuit and packet switching methods into one flexible technology that makes for simple network processing functions.

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Asynchronous Transfer Mode

Multiple logical connections over single physical interface.

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Flow on each logical connection is in fixed sized packets called cells.

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Minimal error and flow control.

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Data rates (physical layer) 25.6Mbps to 622.08Mbps

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Key issues in ATM

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Businesses and institutions who:

*connect LANs with fiber optic facilities to support specific applications .

*often send high volumes of data between several of their locations.

*have linked sites using applications such CAD/CAM or image processing

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ATM is designed to support

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virtual channel and virtual path?

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*A virtual channel is a logical connection similar to virtual circuit

*In ATM,virtual channels which have the same end points can be grouped into virtual paths.

*All the circuits in virtual paths are switched together .this offers increased efficiency, architectural simplicity, and the ability to offer enhanced network services.

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ATM Logical Connections

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*ATM makes use of a 53 byte fixed length cell

While the frame in frame relay is much longer,

and may vary in length

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*Error checking is only done on the header in

ATM rather than on the whole cell or frame

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*Virtual channel of ATM that follow the same

route through the network are bundled into

Paths. A similar mechanism is not in frame relay.

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How does ATM Differ from Frame Relay?

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1.Universal switching standard.

2.Full support of multimedia.

3.Single network access.

4.Reduction in network delay.

5.True bandwidth-on-demand.

6.Optimization of network resources.

7.Technical long life.

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Advantages of ATM

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1.Cost, although it will decrease with

time.

2.New customer premises hardware and

software are required.

competition from other technologies

100 Mbps FDDI,100 Mbps Ethernet

and fast Ethernet.

3.Presently the applications that can

benefit from ATM such as multimedia

are rare .

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Disadvantages of ATM

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