ISO’s High-level Data Link Control (HDLC) uses the frame format described in our 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 and HDLC Link Access Procedure Balanced (LAPB). Usually when referring to HDLC people mean LAPB 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 received correctly. This means that data can continue to flow in situations where there may be long “turnaround” time lags without stopping to wait for an acknowledgment. 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.