7.1 General

Point to Point and Broadcast connections carry the layer three messages between layer three peers via the data link layer.

Layer two provides services for layer three to allow it communicate with its peer.

In Acknowledged mode layer three employs the services of layer two by means of the following primitives:

DL-Data Request, Indicate and Confirm: This controls the transfer of I frames.

DL-Establish, Request, Indicate and Confirm: This activates and confirms the Establishment of a connection over layer two.

DL-Release, Request, Indicate and Confirm.: This activates the Release of a connection over layer two.

Each of these primitive categories provide confirmed services to Layer three in Acknowledged mode, i.e. layer three is informed of the outcome of the Request primitive.

Point to Point Connections can also be established in Unacknowledged mode and in this case Layer three is serviced by Layer two with the DL-Unit Data Request and Indicate primitives.

The DL-Unit Data primitive initiates the transfer of Layer three information over the layer two link by means of UI (Unnumbered Information frames).

It employs a non confirmed service to layer three, viz. layer three is not informed of the outcome of the DL-Data Request primitive.

(Note that these comments also apply to broadcast connections as these are always used for Unacknowledged operation).

7.2 Representation of communication between Layer Three peers via their associated Layer Two connections

Layer three peer to peer communication can be represented by schematics which show how the communication process is implemented by means of:

1. interlayer primitives, which are used as a means of communication between layers two and three and
2. how they activate the transmission of information and supervisory frames between layer two peers.

Layer two peer to peer communication may also be represented by State Diagrams. These offer considerable flexibility as a method of representing the various transitions which can occur as a result of the different commands and responses between layer two peers.

Consider for example a point to point connection in Acknowledged mode between two layer two peers. It is assumed that the originating layer two peer has had its TEI value assigned. This is termed the ‘TEI Assigned State’.

The intention is to establish a connection between the peers and initiate the transmission of numbered Information frames (I frames) carrying layer three information such as the Setup message and associated Information Elements.

This transfer of information in this manner is termed the ‘Multiple Frame Established State’.

Schematic representation of this process is shown in Figure 11.

Legend:

1. DL Establish Request: A layer three request to activate a data link service. This service will be a confirmed service, i.e. the originating Layer three entity which activates the service will be informed of the outcome.

SABME: An Unnumbered U Frame which initiates an Acknowledged Service if the terminating end can accept the call and culminates in the transmission of multiple I Frames containing layer three information.

2. DL-Establish Indication: A primitive passed by layer two to three to assess the capability of layer three to accept of reject the incoming call.

UA: In the event that the call can be accepted by layer three then it invokes the data link service and transmits the layer two frame, UA or Unnumbered Acknowledged, to the originating layer two entity. (If it cannot accept the call it will transmit the DM frame).

3. DL-Establish Confirm: A primitive passed by Layer two to layer three to indicate that the call has been accepted.

I Frame: Numbered Layer three Information frames which are acknowledged by the receiving side by Supervisory frames called S frames.

The I frames are transmitted by the originating TE’s layer two to its associated peer by means of the DL-Data Request/Indicate/Confirm primitives. These primitives, which may be represented in the same way as the DL-Establish primitive shown in Figure 11, are used to transfer Layer three numbered Information frames (I Frames) in Acknowledged mode.

These carry the Layer three information such as the Setup message. This message is communicated by the Layer three entity in the exchange to the ISUP of CCSS7 and is transmitted across the network to the destination exchange. This exchange reconverts the Setup message into the ISDN Layer three format. The conversion occurs at Layer three of the ET via the ISUP.

Point to Point Connection Using Unacknowledged Transfer Operation

Schematic Representation:

Legend:

1: This is the DL-Unit Data Request primitive which initiates the transfer of Layer three information between data link entities.

2: This is the DL-Unit Data Indicate primitive which indicates to layer three the start of the reception of layer three information in UI frames.

7.3 Representation by means of a State Transition

Figure 13. represents the transition between two Layer two states, viz. The ‘TEI Assigned State’ and the ‘Multi Frame Established State’.

The ‘TEI Assigned State’ is not in a position to transfer information with a point to point connection using Acknowledged mode. (It is in a position to establish a point to point connection and transfer information in Unacknowledged mode).

By means of the Establish Request primitive Layer 2 assumes the ‘Awaiting Establishment State’. This is a transitory state awaiting the outcome of the request to establish a connection at the incoming end when layer two presents layer three with the Establish Indication primitive.

In this case it is assumed that layer three at the terminating end is in a position to accept the call and, accordingly, transmits the UA frame to the originating end.

This initiates the completion of the call according to LAPD procedure and culminates in the transfer of the Establish Confirm primitive to layer three.

(This is an intrinsic part of the ‘confirmed service’ offered by layer two to layer three, associated with the Establish primitive).

These conditions initiate the transmission of numbered I frames which is an intrinsic part of the ‘Multi Frame Established State’.

These various states and inter state transitions are shown below in Figure 13.

With reference to Figure 11. consider the position of the TE at the terminating end.

On receipt of the SABME frame, layer two will inform Layer three (with the DL-Establish Indicate primitive) that Acknowledged operation is required.

If a connection can be established then layer two will respond to the incoming frame with a UA, Unnumbered Acknowledgement, frame and inform its layer three of the outcome with the DL-Establish Confirm primitive.

This is a summary of the peer initiated LAP establishment which will cause a state transition between the TEI Assigned State and the Multi Frame Established state as shown in Figure 14.

A combination of Figures 13. and 14. can be used to represent State transitions which take account of Point to point connections activated directly by peer initiated LAPD establishment procedures as well as via the transitory state, ‘Awaiting Establishment’, and this is shown in Figure 15. below:

Layer Three can start the release of a connection by invoking the services of the layer two link by means of the Release primitive, DL-Release Request. This may be represented schematically in a similar way to that shown for the Request primitive in Figure 11.

This initiates the transmission of the DISC layer two frame which generates the ‘Awaiting Release’ state. The LAPD release completed procedure is implemented in the following way.;

The DL-Release Indicate primitive is presented by layer two to layer three and the UA, Unnumbered Acknowledgement frame is sent in acknowledgement to the DISC frame.

The system then enters the ‘TEI Assigned State’.

It is represented by State Transitions in Figure 16 by combining the Release sequence with the State Transition Diagram shown in Figures 13. and 15. as shown in Figure 16 below.

In this way the State Diagrams can be conveniently built up to provide an overview of the way Layer three uses the services of Layer two to initiate peer to peer communications.

Finally note that LAPD initiated release can be activated by the reception of a DISC(onnect) command by a layer two entity in the Multi frame Established state. This will cause a transition to the TEI Assigned state by means of a ‘peer initiated LAP release’ as shown in Figure 17.

The advantages of building up State Transitions in the manner described becomes obvious.

However, it must be emphasised that the state diagram in Figure 17. is mainly for illustrative purposes to provide an overview of the state transitions.

This point is emphasised by the detailed description shown in Figure 18. of the LAPD initiated release procedure which is discussed below.

Legend:

Note 1: Discard I Queue

Note 2: F=P

Note 3: Stop T200, T203.

In Figure 18 the originating side which is in the Multiple Frame Established State is considered.

Note 1:
A DISC (Disconnect) frame is received from the terminating end. This is a Supervisory (S) command frame. The frame’s P, or Poll, bit is set to 1 and this means it is polling the originating end for a Response frame. If the originating end is in the Multi Frame Established State then a UA frame is transmitted to the terminating end as shown. It will set its F, or Final, bit to 1 in this UA frame in the manner normally employed by P/F bits working in the usual command/response mode employed by layer two frames.

Note 2:
The DISC frame has been initiated by the DL Release Request primitive generated by layer three at the terminating end. The reception of the DL Release Request primitive at the terminating end will cause the cessation of DL Data primitives and the I frames which it generates during the Multi Frame Established State.
Any such primitives and I frames which have been queued for transmission will be discarded.

The reception of the DISC frame over the layer two link at the originating end is indicated to Layer three by means of the DL Release Indicate primitive as shown in the figure.

Note 3:
T200: This is reset following the transmission of a layer two frame. In the case described for Figure 18, T200 will be reset upon reception of the UA response at the terminating end. If the UA frame is not received within the default value for T200, which is typically one second, then the DISC frame is resent by the terminating end. After a specified number of retransmissions of the DISC frame an error primitive shall be passed between the data link layer and its associated management layer and the Release Confirm shall be communicated to layer three.
T203: This is a timer to supervise the time interval between frames. In the case where its default value, which is typically 10 seconds, is exceeded then an error is indicated. Timer T203 is stopped when the data link layer entity leaves the Multi Frame Established State.