NDSL, Chang Gung University

Frame Relay• To provide a telecommunication service for cost-efficient

data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN).

• Frame Relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission of data) up to the end-points.

• This speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection– without having to pay for a full-time leased line, – while the service-provider figures out the route each

frame travels to its destination and can charge based on usage.

NDSL, Chang Gung University

• Frame Relay has its technical base in the older X.25 packet-switching technology, designed for transmitting data on analog voice lines.

• Unlike X.25, whose designers expected analog signals, Frame Relay offers a fast packet technology, which means that the protocol does not attempt to correct errors.

• When a Frame Relay network detects an error in a frame, it simply drops that frame.

• The end points have the responsibility for detecting and retransmitting dropped frames.

NDSL, Chang Gung University

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Introduction

Frame Relay (FR) is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model.

FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces.

Today, it is used over a variety of other network interfaces as well.

FR is an example of a packet-switched technology.

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What is Frame Relay? “A packet-switching protocol for connecting devices on a

Wide Area Network (WAN)” FR networks support data transfer rates at

T-1 (1.544 Mb/s) T-3 (45 Mb/s) speeds.

In fact, you can think of Frame Relay as a way of utilizing existing T-1 and T-3 lines owned by a service provider.

Most telephone companies now provide FR service for customers who want connections at 56 Kb/s to T-1 speeds. (In Europe, FR’s speeds vary from 64 Kb/s to 2 Mb/s.

Frame Relay is quite popular because it is relatively inexpensive. However, it is being replaced in some areas by faster technologies, such as ATM.

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Frame Relay Devices (cont.)

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Frame Relay Devices

Devices attached to a Frame Relay WAN fall into the following two general categories: Data terminal equipment (DTE)

DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer.

Example of DTE devices are terminals, personal computers, routers, and bridges.

Data circuit-terminating equipment (DCE) DCEs are carrier-owned internetworking devices. The purpose of DCE equipments is to provide clocking and

switching services in a network, which are the devices that actually transmit data through the WAN.

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Frame Relay Devices (cont.)

The connection between a DTE device and a DCE device consists of both a physical layer component (L1) and a link layer component (L2).

The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices.

One of the commonly used physical layer interface specifications is the recommended standard (RS)-232.

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Serial Point-to-Point Connection

Network connections at the CSU/DSUEIA/TIA-232 EIA/TIA-449 EIA-530V.35 X.21

End user device

Service Provider

DTE

DCE

Router connections

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Circuit-Switching

Long-haul telecom network designed for voice

Network resources dedicated to one callShortcomings when used for data:

Inefficient (high idle time)Constant data rate

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Packet-Switching

Data transmitted in short blocks, or packetsPacket length < 1000 octetsEach packet contains user data plus control

info (routing)Store and forward

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The Use of Packets

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Advantages with compared to Circuit-SwitchingGreater line efficiency (many packets can go

over shared link)Data rate conversionsNon-blocking under heavy traffic (but

increased delays). When traffic becomes heavy on a circuit-switching network, some calls are blocked.

Priorities can be used.

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Disadvantages relative to Circuit-SwitchingPackets incur additional delay with every

node they pass throughJitter: variation in packet delayData overhead in every packet for routing

information, etcProcessing overhead for every packet at

every node traversed

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Simple Switching Network

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Switching Technique

Large messages broken up into smaller packets Datagram

Each packet sent independently of the others No call setup More reliable (can route around failed nodes or

congestion) Virtual circuit

Fixed route established before any packets sent No need for routing decision for each packet at each

node

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Packet Switching: Datagram Approach

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Packet Switching: Virtual-Circuit Approach

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Virtual Circuits and Frame Relay Virtual Connections

Protocol Architecture

Control PlaneBetween subscriber and networkSeparate logical channel used

Similar to common channel signaling for circuit switching services

Data link layerLAPD (Q.921)Reliable data link controlError and flow controlBetween user (TE) and network (NT)Used for exchange of Q.933 control signal

messages

User PlaneEnd to end functionalityTransfer of info between endsLAPF (Link Access Procedure for Frame

Mode Bearer Services) Q.922Frame delimiting, alignment and transparencyFrame mux and demux using addressing fieldEnsure frame is integral number of octets (zero bit

insertion/extraction)Ensure frame is neither too long nor shortDetection of transmission errorsCongestion control functions

LAPF Core Formats

User Data Transfer

One frame typeUser dataNo control frame

No inband signalingNo sequence numbers

No flow nor error control

FRAME RELAY CALL CONTROLCall Control Alternatives In frame relay operation, a user is not

connected directly to another user, but rather to a frame handler in the network;

For X.25, a user is connected to a packet handler.

Switched access

Switched access

The local exchange does not provide the frame-handling capability.

Switched access must be provided from the user's terminal equipment (TE) to the frame handler

Connection will be a demand connection (set up at the time of the call) or a semi-permanent connection (always available).

Integrated access

Integrated access

The user is connected to a pure frame-relaying network or to a switched network

The local exchange provide the frame handling capability.

The user has direct logical access to the frame handler.

Access Connection

The connection between the subscriber and the frame handler is access connection.

Once this connection exists, it is possible to multiplex multiple logical connections as frame relay connections,

They may be either on-demand or semipermanent.

Frame Relay Connection

The subscriber must established an access connection to a frame handler

A frame relay connection, analogous to a packet-switching virtual circuit, must first be established between two users.

Each connection has unique data link connection identifier (DLCI).

Data Transfer

1. Establish a logical connection between two end points, and assign a unique DLCI to the connection.

2. Exchange information in data frames. Each frame includes a DLCI field to identify the connection.

3. Release the logical connection.

Data Transfer

1. Establish a logical connection between two end points, and assign a unique DLCI to the connection.

2. Exchange information in data frames. Each frame includes a DLCI field to identify the connection.

3. Release the logical connection.

Frame relay Connection

• The establishment and release of a logical connection will be carried out in a connection.

• Exchange of messages over a logical connection is dedicated to call control

• A frame with DLCI = 0 contains a call control message in the information field. At a minimum, four message types are needed:

Messages

• SETUP - request the establishment of a logical connection if it accepts the connection

• CONNECT - assign the DLCI by choosing an unused value and including this value in the SETUP message; otherwise, the DLCI value is assigned by the accepting side in the CONNECT message.

• RELEASE - to clear a logical connection• RELEASE COMPLETE - Not accept a

connection

Messages

Connection

• Exchanges involved for switched access to a frame handler

• The calling user must establish a circuit-switched connection to a frame handler

• Done with the usual SETUP, CONNECT and CONNECT ACK messages,

Data Transfer