The International Standards Organization (ISO) defines a layered communication model. At each layer, entities communicate with peers on the same layer using the services of the next lower layer. The lowest three layers of this model are:
Layer 1 (Physical) defines bit encoding, modulation schemes, plugs and connectors.
Layer 2 (Data Link) provides packet structure, flow control, and error detection.
Layer 3 (Network) provides accurate data transmission over a network or concatenation of networks.
The SyncLink adapter and API implement layers 1 (ISO3309) and 2 (ISO4335). Link layer standards are based on ISO4335, including the X.25 Link Access Protocol Balanced (LAPB), Q.921 ISDN Link Access Protocol for the D Channel (LAPD), and Link Access Protocol for Frame Mode Bearer services (LAPF). These standards are closely related with differences in address handling and the supported subset of ISO4335. Protocols, such as LAPD, that support multiple data link connections define protocol specific addressing schemes (HDLC address field) to multiplex connections over a single physical interface.
This document is not intended to be a detailed explanation of HDLC link layer procedures. Refer to ISO4335 for general procedures and to any protocol specific standard for more details. A useful document for identifying the options used by a particular protocol is ISO/IEC JTC1/SC6.
Layer 3 protocols use the services provided by the API. Examples of layer 3 protocols are the X.25 Packet Layer Protocol (PLP) or Digital Subscriber Signaling System #1 (Q.931).
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