FRAME RELAY

FRAME RELAY

The most important technical innovation to come out of the standardization

Work on ISDN is frame relay. Although designed for ISDN, frame relay

Now enjoys widespread use in a variety of public and private networks that

Do not follow the ISDN standards.

Frame relay represents a significant advance over traditional packet switching

and X.25. We begin the lesson with an overview of the differences between these

two approaches. Next, the details of the frame relay scheme are examined. Then,

the key issue of congestion control in frame relay networks is discussed. For a discussion

of ISDN, see Lesson A.

10.1 BACKGROUND

The traditional approach to packet switching makes use of X.25, which not only

determines the user-network interface but also influences the internal design of the

network. Several key features of the X.25 approach are as follows:

Call control packets, used for setting up and clearing virtual circuits, are carried

on the same channel and the same virtual circuit as data packets. In

effect, inband signaling is used.

Multiplexing of virtual circuits takes place at layer 3.

e Both layer 2 and layer 3 include flow control and error control mechanisms.

The X.25 approach results in considerable overhead. Figure 10.la indicates

the flow of data link frames required for the transmission of a single data packet

from source end system to destination end system, and the return of an acknowledgment

packet. At each hop through the network, the data link control protocol

involves the exchange of a data frame and an acknowledgment frame. Furthermore,

at each intermediate node, state tables must be maintained for each virtual circuit

to deal with the call management and flow controllerror control aspects of the X.25

protocol.

All of this overhead may be justified when there is a significant probability of

error on any of the links in the network. This approach may not be the most appropriate

for modern digital communication facilities. Today's networks employ reliable

digital-transmission technology over high-quality, reliable transmission links,

many of which are optical fiber. In addition, with the use of optical fiber and digital

transmission, high data rates can be achieved. In this environment, the overhead of

X.25 is not only unnecessary, but degrades the effective utilization of the available

high data rates.

Frame relaying is designed to eliminate much of the overhead that X.25

imposes on end user systems and on the packet-switching network. The key differences

between frame relaying and a conventional X.25 packet-switching service are

e Call control signaling is carried on a separate logical connection from user

data. Thus, intermediate nodes need not maintain state tables or process messages

relating to call control on an individual per-connection basis.

Multiplexing and switching of logical connections take place at layer 2 instead

of layer 3, eliminating one entire layer of processing.

There is no hop-by-hop flow control and error control. End-to-end flow control

and error control, if they are employed at all, are the responsibility of a

higher layer.

Figure 10.lb indicates the operation of frame relay, in which a single-user data

frame is sent from source to destination, and an acknowledgment, generated at a

higher layer, is carried back in a frame.

Let us consider the advantages and disadvantages of this approach. The principal

potential disadvantage of frame relaying, compared to X.25, is that we have

lost the ability to do link-by-link flow and error control. (Although frame relay does

not provide end-to-end flow and error control, this is easily provided at a higher

layer.) In X.25, multiple virtual circuits are carried on a single physical link, and

LAPB is available at the link level for providing reliable transmission from the

source to the packet-switching network and from the packet-switching network to

the destination. In addition, at each hop through the network, the link control protocol

can be used for reliability. With the use of frame relaying, this hop-by-hop link

control is lost. However, with the increasing reliability of transmission and switching

facilities, this is not a major disadvantage.

The advantage of frame relaying is that we have streamlined the communications

process. The protocol functionality required at the user-network interface is

reduced, as is the internal network processing. As a result, lower delay and higher

throughput can be expected. Studies indicate an improvement in throughput using

frame relay, compared to X.25, of an order of magnitude or more [HARB92]. The

ITU-T Recommendation 1.233 indicates that frame relay is to be used at access

speeds up to 2 Mbps.

The ANSI standard T1.606 lists four examples of applications that would benefit

from the frame relay service used over a high-speed H channel:

1. Block-interactive data applications: An example of a block-interactive application

would be high-resolution graphics (e.g., high-resolution videotex,

CADICAM). The pertinent characteristics of this type of application are low

delays and high throughput.

2. File transfer: The file transfer application is intended to cater to large file

transfer requirements. Transit delay is not as critical for this application as it

is, for example, in the first application. High throughput might be necessary in

order to produce reasonable transfer times for large files.

3. Multiplexed low-bit rate: The multiplexed low-bit-rate application exploits the

multiplexing capability of the frame-relaying service in order to provide an

economical access arrangement for a large group of low-bit-rate applications.

An example of one such low-bit-rate application is given in (4) below. The

low-bit-rate sources may be multiplexed onto a channel by an NT function.

4. Character-interactive traffic: An example of a character-interactive traffic

application is text editing. The main characteristics of this type of application

are short frames, low delays, and low throughput.

Frame Relay Protocol Architecture

Figure 10.2 depicts the protocol architecture to support the frame-mode bearer service.

We need to consider two separate planes of operation: a control (C) plane,

which is involved in the establishment and termination of logical connections, and

a user (U) plane, which is responsible for the transfer of user data between subscribers.

Thus, C-plane protocols are between a subscriber and the network, while

U-plane protocols provide end-to-end functionality.

Control Plane

The control plane for frame-mode bearer services is similar to that for commonchannel

signaling in circuit-switching services, in that a separate logical channel is

used for control information. In the case of ISDN, control signaling is done over the

D channel, to control the establishment and termination of frame-mode virtual calls

on the D, B, and H channels (see Lesson A).

At the data link layer, LAPD (Q.921) is used to provide a reliable data link

control service, with error control and flow control, between user (TE) and network

(NT) over the D channel. This data link service is used for the exchange of Q.933

control-signaling messages.

User Plane

For the actual transfer of information between end users, the user-plane protocol is

LAPF (Link Access Procedure for Frame-Mode Bearer Services), which is defined

in Q.922. Q.922 is an enhanced version of LAPD (Q.921). Only the core functions

of LAPF are used for frame relay:

Frame delimiting, alignment, and transparency

Frame multiplexing/demultiplexing using the address field

0 Inspection of the frame to ensure that it consists of an integral number of

octets prior to zero-bit insertion or following zero-bit extraction

Inspection of the frame to ensure that it is neither too long nor too short

Detection of transmission errors

Congestion control functions

The last function listed above is new to LAPF, and is discussed in a later section.

The remaining functions listed above are also functions of LAPD.

The core functions of LAPF in the user plane constitute a sublayer of the data

link layer; this provides the bare service of transferring data link frames from one

subscriber to another, with no flow control or error control. Above this, the user

may choose to select additional data link or network-layer end-to-end functions.

These are not part of the frame-relay service. Based on the core functions, a network

offers frame relaying as a connection-oriented link layer service with the following

properties:

Preservation of the order of frame transfer from one edge of the network to

the other

A small probability of frame loss

Comparison with X.25

As can be seen, this architecture reduces to the bare minimum the amount of work

accomplished by the network. User data is transmitted in frames with virtually no

processing by the intermediate network nodes, other than to check for errors and to

route based on connection number. A frame in error is simply discarded, leaving

error recovery to higher layers.

Figure 10.3 compares the protocol architecture of frame-mode bearer service

to that of X.25. The packet-handling functions of X.25 operate at layer 3 of the OSI

model. At layer 2, LAPB is used. Table 10.1 provides a functional comparison of

X.25 and frame relay, and Figure 10.4 illustrates that comparison. As can be seen,

the processing burden on the network for X.25 is considerably higher than for frame

relay.

FRAME RELAY CALL CONTROL

This section examines the various approaches for setting up frame relay connections

and then describes the protocol used for connection control.

Call Control Alternatives

The call control protocol for frame relay must deal with a number of alternatives.

First, let us consider two cases for the provision of frame handling services. For

frame relay operation, a user is not connected directly to another user, but rather to

a frame handler in the network; just as for X.25, a user is connected to a packet handler.

There are two cases (Figure 10.5):

Switched Access. The user is connected to a switched network, such as ISDN,

and the local exchange does not provide the frame-handling capability. In this

case, switched access must be provided from the user's terminal equipment

(TE) to the frame handler elsewhere in the network; this can either be a

demand connection (set up at the time of the call) or a semi-permanent connection

(always available). In either case, the frame relay service is provided

over a B or H channel.

Integrated Access. The user is connected to a pure frame-relaying network or

to a switched network in which the local exchange does provide the framehandling

capability. In this case, the user has direct logical access to the frame

handler.

All of the above considerations have to do with the connection between the

subscriber and the frame handler, which we refer to as the access connection. Once

this connection exists, it is possible to multiplex multiple logical connections,

referred to as frame relay connections, over this access connection. Such logical connections

may be either on-demand or semipermanent.

Frame Relay Connection

The discussion will perhaps be easier to follow if we first consider the management

of frame relay connections. So, let us assume that the subscriber has somehow

established an access connection to a frame handler that is part of a frame relay network.

Analogous to a packet-switching network, the user is now able to exchange

data frames with any other user attached to the network. For this purpose, a frame

relay connection, analogous to a packet-switching virtual circuit, must first be established

between two users.

As with X.25, frame relay supports multiple connections over a single link. In

the case of frame relay, these are called data link connections, and each has a

unique data link connection identifier (DLCI). Data transfer involves the following

stages:

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.

The establishment and release of a logical connection is accomplished by the

exchange of messages over a logical connection dedicated to call control, with

DLCI = 0. A frame with DLCI = 0 contains a call control message in the information

field. At a minimum, four message types are needed: SETUP, CONNECT,

RELEASE, and RELEASE COMPLETE.

Either side may request the establishment of a logical connection by sending

a SETUP message. The other side, upon receiving the SETUP message, must reply

with a CONNECT message if it accepts the connection; otherwise, it responds with

a RELEASE COMPLETE message. The side sending the SETUP message may

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.

Either side may request to clear a logical connection by sending a RELEASE

message. The other side, upon receipt of this message, must respond with a

RELEASE COMPLETE message.

Table 10.2 shows the complete set of call control messages for frame relay.

These messages are defined in ITU-T standard Q.933. They are a subset of a larger

collection of messages defined in Q.931 used for common-channel signaling

between a user and an ISDN.

Access Connection

Now consider the establishment of an access connection. If the connection is semipermanent,

then no call control protocol is required. If the connection is to be set

up on demand, then the user requests such a connection by means of a commonchannel

signaling protocol between the user and the network. In the case of ISDN,

and also many other digital networks, the protocol used is Q.931.

Figure 10.6 provides an example of the types of exchanges involved for

switched access to a frame handler, in this case over an ISDN. First, the calling user

must establish a circuit-switched connection to a frame handler that is one of the

nodes of the frame relay network; this is done with the usual SETUP, CONNECT,

and CONNECT ACK messages, exchanged at the local user-network interface

and at the interface between the network and a frame handler. The procedures

and parameters for this exchange are carried out on the D channel, and are defined

in Q.931. In the figure, it is assumed that the access connection is created for

a B channel.

Once the access connection is established, an exchange takes place directly

between the end user and the frame handling node for each frame mode connection

that is set up. Again, the SETUP, CONNECT, and CONNECT ACK messages are

used. In this case, the procedures and parameters for this exchange are defined in

Q.933, and the exchange is carried out on the same B channel that will be used for

the frame mode connection.