1. An Introduction to LAPD

LAPD (Link Access Protocol D Channel) is the protocol employed for D channel signalling between layer two, or digital link peers (adjacent layer two entities). It provides services to layer three by carrying messages between layer three peers. It does this in conjunction with the Physical Layer, Layer One.

It communicates with Layer three by means of ‘primitives’. This is the term applied to the exchange and control of information between adjacent layers. Information is exchanged in this way between Layers one, two and three. Layer two primitives provide information services to layer three while layer one primitives provide services to layer two.

Thus when an application informs Layer three to establish a connection, this information is communicated to layer two by means of the DL-Establish primitive (in the case of the Acknowledged mode of operation).

Note from the Schematic in Figure 1. that different types of primitives are used by the two modes of operation, viz. acknowledged and unacknowledged mode.

Connections may be established between Layer two entities by means of Acknowledged or Unacknowledged Service.

Generally the Acknowledged service is employed but unacknowledged service may be employed when the rapid transfer of information is required. This mode of operation must be employed for broadcast connections and these connections must be employed when the terminating end has a multipoint configuration, ie. a number of TEs terminated at the user interface.

In Acknowledged mode the Layer three Information (I) messages (such as the Setup message including its associated Information Elements) are acknowledged by the terminating end so that information using this mode is transmitted error free. In this case the transmission of I frames is acknowledged by Supervisory or S frames which control and supervise the error free transmission of information. The transfer of information by layer two under these conditions represents a State which is referred to as the Multi Frame Established State.

In Unacknowledged mode layer three Information is transmitted in UI (Unnumbered Information) frames and no account is taken of errors during the transmission phase although error control may be implemented at higher layers such as by the application itself.

Acknowledged mode of operation employs point to point connections only. This means that the Layer two entity at the terminating exchange is required to establish a connection with the layer two entity associated with just one ISDN Terminal Equipment (TE) connected at the user interface.

Unacknowledged mode of operation employs broadcast connections to facilitate the connection of a layer two entity with the layer two entity associated with one TE at the terminating end of a multipoint configuration. A multipoint configuration may employ a number of TEs which could connect to the calling TE.

Point to point connections may, in principle, also employ the unacknowledged mode of service. (Note when one TE is the only termination connected at the user interface, but can accept different types of calls such as video, voice, fax etc., these calls are established by means of point to point connections).

The schematic in Figure 1. above shows the key components involved in the implementation of the LAPD protocol and the following discussion will describe how these various components are interrelated.

These relationships are discussed under the headings listed below: