4.1 Unacknowledged Service

For Unacknowledged service, interlayer (3 to 2) communication employs the DL-Unit Data Request and Indication primitives. DL-Unit Data Request initiates the transfer of Unnumbered Layer three information frames over the layer two link. The DL-Unit Data Indicate primitive informs the incoming end that Unacknowledged service shall be employed for layer three information transfer.

In this case the layer three information is transmitted in UI frames (Unnumbered Information frames) and is not acknowledged.

(However, acknowledgements may be implemented by the higher layer protocols).

4.2 Acknowledged Service

In this case the mode of service to be employed is indicated by the Establish Request primitive which initiates Acknowledged Service. Point to point connections only are catered for in this mode of operation.

Information transfer in Acknowledged mode is handled between layers 3 and 2 by the DL-Data primitive. This provides a confirmed service to layer three because it comprises the Request, Indicate and Confirm components, i.e. it is informed by the Confirm component if its Request has been allowed by its associated peer.