Glossary of Common Terms used and Models Employed in a Discussion OF LAPD |
General format of Layer 2 frames
The general form of the layer 2 frame is shown in Figure 2. below:
|
Figure 2 |
It comprises the following components:
Flag: Flags with the characteristic byte format, viz. 01111110 are located at either end of the frame and determine the beginning and end of a frame. A method called ‘bit stuffing’ is employed to ensure that these bit combinations cannot occur during the course of data transfer so that there can be no ambiguity with the characteristic flag format which signifies the start and finish of a layer two frame.
FCS: Frame Check Sequence performs an error check on the received frame by a calculation based on the premise that the total number of bits received in each frame should equal the number of bits transmitted.
Address Fields: Comprises two parts, viz. the Service Access Point Identifier (SAPI) and the Terminal Endpoint Identifier (TEI).
SAPI Part: This is used to identify the type of service supported by the call which is being established according to the following format:
 | SAPI=0: Call Control for circuit switched connections. SAPI=16: Packet Switching according to X25 protocol. SAPI=63: Operational, Administrative or Management Purposes. |
These derive from the Protocol Discriminator in Layer Three Messages which is used to identify the type of call to be established.
TEI Part: All TEs have a unique TEI value and these may be assigned automatically by the network on power up and assume values between 64 and 126.
They may, alternatively, be assigned manually by the customer and in this case they must be given values within the range 0 to 63.
Note that some TEs may have more than one TEI value to support a number of different call types.
A TEI and SAPI value uniquely identifies a specific logical link between layer two peers as shown in Figure 3.
|
Figure 3 |
Legend:
SAP: The Service Access Point is a virtual interface between layers 2 and 3.
The interconnecting lines between the SAPs in Figure 3. represent the logical links between the two peer entities. Each link is uniquely distinguished by a specific TEI value and an Identifier value associated with the SAP interface, i.e. the SAPI value.
Thus in the case shown in the figure there are two connections to TE1 (Terminal Equipment One). One of these supports the SAP value 0 and the other the SAP value 16, i.e. each link is uniquely identified.
Similarly there are two connections to TE2. One of these supports the SAP value 0 and the other the SAP value 16 and again each link is uniquely identified.
Figure 4. shows another scheme of logical connections to further demonstrate the unique method of identification using TEI and SAPI values.
|
Figure 4 |
In Figure 4. one set of connections with different TEI values, 75 and 82 are linked to an SAP with the value SAP=0. Another link with a TEI value of 82 is connected to an SAP with the value of 16. Thus one TE can accommodate a number of TEI values used for point to point data transfer.
Each of the logical links is uniquely identified.
In Figure 3., therefore, TE1 and TE2 can each be addressed by two types of incoming calls and in the case shown in Figure 4. TE1 can be addressed by three different incoming calls.
Each of the TEs should be equipped to deal with the different types of calls presented to it. These may include calls of the same service category with different TEI values as well as calls with different service categories with the same TEI values.
In each case a specific application associated with the TE is required to communicate via Layer three with each unique connection. In this way a TE may support a number of different applications.
Associated layer two peer entities are connected by means of the LAPD protocol. The protocol establishes logical links between the Layer two peers of the network entity and the SAP interface of those TEs which qualify to accept an incoming call.
The physical connection via the B channel is established by Layer three signalling procedures supported by Layer two frames. These layer three signalling procedures are implemented between peer entities which have set up logical links between their layer two entities. Layer three signalling information is presented to the user interfaces of the TEs at their Service Access Point situated between layers two and three.
Broadcast connections set the TEI value of the Layer two frame to 127 and will establish links with all SAPs with the same identifier value, i.e. those which support the same service, as shown in Figure 5.:
|
Figure 5 |
Frame types: There are three types of Layer two frames, viz. Numbered, Unnumbered and Supervisory frames. These are used in a variety of configurations to carry Layer three information between peers. They do so by means of the Information frames, I (Numbered frames), and UI (Unnumbered frames). They must implement the task of supporting Acknowledged and Unacknowledged service and by establishing point to point and broadcast connections as required by the application.
The schematic in Figure 6. has been included to provide an introduction to the way these frames are use in typical signalling configurations.
A Note on Command and Response Frames
The C/R bit in the layer two frame format will indicate the status of a frame, viz. is it a Command or Response frame.
Information frames are Command Frames, Supervisory frames may be Command frames in certain configurations and Response frames in others while some Unnumbered Frames are Command Frames and others are Response frames.
A Command frame is indicated if the C/R bit is 0 in the user to network direction and 1 in the network to user direction.
A Response frame is indicated if the C/R bit is 1 in the user to network direction and 0 in the network to user direction.
(The TEI and SAPI values together form the Data Link Connection Identifier, DLCI, which uniquely identifies the address of layer two entities. The address of the Command frame is determined by the address of its peer entity. The address of the Response frame is determined by its own DLCI value. Accordingly the DLCI values of Command and Response frames across the layer two link will be identical).
A Note on the P/F bit values
P and F stand for ‘Polling’ and ‘Final’, respectively.
Command frames have their P/F bit set to P=1 and response frames that respond to the polling command frame have their F bit set to 1.
The P/F bit is discussed in more detail below.
