Integrated Services Digital Network (ISDN) – Core Aspects of Frame Protocol for Use with Frame Relay Bearer Service
|Publication Date:||1 January 1991|
Scope and purpose
This standard provides a description of the protocol to support the data transfer phase of the Frame Relay bearer service as defined in ANSI T1.606, Frame relaying bearer service - Architectural framework and service description including Addendum 1. The protocol defined in this standard is a protocol operating in the lowest sublayer of the data link layer of the OSI reference model and is based on a subset of ANSI T1.602 (LAPD) called the "core aspects." These procedures are used by both demand and semipermanent virtual calls.
This protocol is intended to support multiple simultaneous frame relay bearer connections (i.e., connections at core sublayer) within a single bearer channel (i.e., a connection at the physical layer). This protocol provides for transparent transfer of user data and does not restrict the contents, format, or coding of the information, or interpret its structure. This standard is applicable to Frame Relay Bearer Service (FRBS). It is intended for use on any bearer channel, and when operating on the Dchannel, it is used concurrently with ANSI T1.602 procedures.
The core functions of the LAPD protocol that are used to provide this function are:
- frame delimiting, alignment, and transparency provided by the use of HDLC flags and zero bit insertion/extraction
- frame multiplexing/demulti
- inspection of the frame to ensure that it consists of an integer number of octets prior to zero bit insertion or following zero bit extraction;
- inspection of the frame to ensure that it is not too long or too short;
- detection of (but not recovery from) transmission errors; and
- congestion control functions.