3.1 Introduction on Layer Two frames

Information or I Frames

I frames carry the Layer three messages. A characteristic feature of these frames is that they include information on the number of the frame being sent, N(S), and the number of the next frame expected to be received, N(R).

Supervisory Frames

These S frames supervise the layer two connection. They are used to control the transfer of I frames and ensure that frames are received in the right sequence. They include the N(R) sequencing control parameter which indicates the number of the next frame to be received, so that the originating end knows what is the number of the next frame, viz. the N(S) value, expected at the terminating end.

Supervisory frames may be used in Command or Response mode.

The signalling schematic in Figure 6. provides examples of how they may be used.

They include the following frames which are discussed below:

RR (Ready Receive), RNR (Receive Not Ready), REJ (Reject).

Unnumbered Frames

(i) UI frames are used to transmit layer three information in Unacknowledged mode. The number of the frame being sent or the next expected frame to be received is not recorded in UI frames.

(ii) Other important Unnumbered frames include the SABME frame which is the first frame used for the setting up the Multi Frame Established State which caters for the transmission of I frames, the UA (Unnumbered Acknowledgement) frame which responds to the SABME frame if the incoming call can be accepted and the DISC frame which is used for terminating the Multi Frame Established State.

These are discussed in more detail below.

Note that in the following discussion on LAPD frames that reference should be made to Figure 6 as required.

3.2 Detailed Descriptions on the Frames

(i) Information Frames: 

These have been described in a variety of configurations in the schematic. In addition, a configuration, which demonstrates how the I frames use both numbered values, viz. N(R) and N(S) in a full duplex mode of operation is shown in Figure 7. below. Both of these values can only be effectively exploited in interworking configurations with frames transmitted in the opposite direction which also have N(R) and N(S) values.

The only frame type to have these values is the I frame. Thus the originating frame will set N(R) to the value of N(S) in the next frame which it expects to receive.

The intention here is also to demonstrate the role of P/F bits in this type of configuration.

The P bit for Command frames is set to 1 when the this frame is polling for a response frame and in this case the response bit will be set to 1.

However, the case considered in the Figure 7. below deals with the full duplex exchange of I bits. In this situation the P bit of the outgoing Command frame is set to 0 and the F bit of the received I frame is set to 0.

The N(R) and N(S) values are set to those shown in the figure to ensure appropriate sequencing of the I frame transmission and reception.

This type of full duplex I frame transfer may be required to support certain Layer three signalling configurations.

(ii) Unnumbered Frames

SABME: . This is a Command Frame. It is the first frame sent by the originating end in Acknowledged mode to initiate a layer two connection in response to a DL-Establish Request primitive (see the note on primitives below) from layer 3 and establish the Multi Frame Established State.

(This is the state where I frames are sent to the terminating end).

If the terminating end can establish a connection it will respond with UA, the Unnumbered Acknowledgement Frame. If it cannot establish a connection it will respond with the DM frame which will initiate the release of the connection.

DISC: This is a Command frame and is used to terminate the Multi frame Established State.

FRMR: This is a Response frame which is sent in the event of error detection during frame transmission.

(iii) The Supervisory or S Frames are listed below

RR: The layer two entity indicates it is ready to receive an I frame.
In command mode it may clear a busy condition that was indicated by the earlier transmission of an RNR frame.

RNR: Receive Not Ready tells the transmitting end to stop sending I frames.
Because its buffer is full.

REJ: The reception of this frame indicates the rejection of an incoming frame due to an error condition. The Reject frame tells the originating end to start retransmitting I frames with a N(S) value equal to the value of the last frame correctly received and incremented by one.

3.3 A Note on Primitives-implementation of interlayer communication

(i) The Service Access Point

The SAP or Service Access Point is a conceptual interface between the ISDN layers. It is the interface which is employed by the higher layer to request services from the lower layer and which is used by the lower layer to provide services to the higher layer. In this way layer three invokes the services of layer two and layer two provides services to layer three. These services are communicated between the layers by means of primitives. The specific role of key primitives will be referred to in succeeding sections below.

(ii) Generic Parameters of a Primitive

These comprise the following components as shown in Figure 8.: Request (1), Indicate (2), Respond (3) and Confirm (4) as set out in Figure 8. between two peer entities.

Primitives associated with Acknowledged Service:

1. DL-Data Request, Indicate and Confirm.
2. DL-Establish Request, Indicate and Confirm.
3. DL-Release Request, Indicate and Confirm.

Primitives associated with Unacknowledged Service:

1. DL-Unit Data Request and Indicate.