ISO's High-level Data Link Control (HDLC) uses the frame format described in the
section: Asynchronous and Synchronous Communications. This standard
corresponds to Layer 2 (the Data Link Layer) of the ISO 7-layered architecture. It is
responsible for the error-free movement of data between network nodes.
The job of the HDLC layer is to ensure that data passed up to the next layer has been
received exactly as transmitted (i.e., error free, without loss and in the
correct order). Another important job is flow control, which ensures
that data is transmitted only as fast as the receiver can receive it.
There are two distinct HDLC implementations: HDLC NRM
(see (SDLC) and HDLC Link Access Procedure
Balanced (LAPB). Usually when referring to HDLC people mean LABP or some variation.
LAPB is a bit-oriented synchronous protocol that provides complete data transparency
in a full-duplex point-to-point operation. It supports a
peer-to-peer link in that neither end of the link plays the role of
the permanent master station. HDLC NRM (also known as SDLC), on
the other hand, has a permanent primary station with one or more secondary stations.
HDLC LAPB is a very efficient protocol. A minimum of overhead is required to
ensure flow control, error detection and recovery. If data is flowing in both directions
(full duplex), the data frames themselves carry all the information required to ensure
data integrity.
The concept of a frame window is used to send multiple frames before receiving confirmation
that the first frame has been correctly received. This means that data can continue to flow
in situations where there may be long "turn-around" time lags without stopping to
wait for an acknowledgement. This kind of situation occurs, for instance, in satellite
communication. The window sizes vary, but are typically 7 frames for most terrestrial
lines and up to 128 frames for satellite links.
There are three categories of frames:
- Information frames transport data across the link and may encapsulate the higher layers of the OSI architecture.
- Supervisory frames perform the flow control and error recovery functions.
- Unnumbered frames provide the link initialization and termination.
The maximum frame size depends on the number of CRC bytes at the end of the frame.
It can be shown that for frame sizes of up to 4096 bytes, 2 CRC bytes gives a very high
probability that no erroneous frame will be passed as good. For larger frame sizes, up
to 16kbytes, a four byte CRC is used.
HDLC LAPB is the Link Layer (Level 2 of the OSI model) that is usually used
by X.25, the ITU (previously called the CCITT) standard for
Packet Switched Networks.
Our units offer the following HDLC support:
- LAPB to ISO 7776 standards.
- Frame size to 4099 bytes.
- 16 bit and 32 bit CRC.
- Modulo 8 and modulo 128.
- Settable counters and timers.
- Raw, protocol free HDLC streaming mode supported.
- HDLC NRM supported: Primary and Secondary.
- NRZ, NRZI, Manchester, FM0 or FM1 encoding.
