Bit-Oriented Protocols
In character-oriented protocols, bits are grouped into predefined patterns forming characters. By comparison, bit-oriented protocols can pack more information into shorter frames and avoid the transparency problems associated with character-oriented protocols. Given the advantages of bit-oriented protocols and the lack of any pre­existing coding system (like ASCII) to tie them to, it is no wonder that over the last two decades, many different bit-oriented protocols have been developed such as Synchronous Data Link Control (SDLC) and High-level Data Link Control (HDLC).
In 1975, IBM pioneered the development of bit-oriented protocols with synchronous data link control and lobbied the ISO to make SDLC the standard American National Standards Institute (ANSI) modified SDLC and it became ADCCF (Advanced Data Communication control Procedure) and subsequently ISO modified ADCCP to HDLC (High-level Data link Control). All of these protocols are based on the same stuffing for data transparency.
The ITU-T was one of the first organizations to embrace HDLC. Since 1981, ITU-T has developed a series of protocols called link access protocols (LAPs) such as:
• Link access procedures, balanced (LAPB)
• Link access procedures, D-channel (LAPD)
 • Link access procedures, modem (LAPM)
All link access protocols are based on HDLC. All bit-oriented protocols are related to the HDLC bit-oriented protocol published by ISO. HDLC supports both half-duplex and full-duplex modes in point-to-point and multipoint configurations. We now examine HDLC a bit more closely.

HIGH-LEVEL DATA LINK CONTROL (HDLC)
The most important data link control protocol is HDLC. Not only is HDLC widely used, but it is the basis for many other important data link control protocols, which use the same or similar formats and the same mechanisms as employed in `HDLC. HDLC defines three types of stations:
Primary station (control station): Primary station is responsible for controlling the operation of the link. It means, the station manages data flow by issuing commands to other stations and acting on their responses.
Secondary station (guest station): This station operates under the control of the primary station. It means, the secondary station responds to commands issued by a primary station. Furthermore, it can respond to just one primary station at a time.
Combined station:   This station combines the features of primary and secondary. A combined station may issue both commands and responses.
HDLC defines two link configurations. Unbalanced configuration: Consists of one primary and one or more secondary stations and supports both full-duplex and half-duplex transmissions.
Balanced configuration:    Consists of two combined stations and supports both full­duplex and half-duplex transmission.
HDLC defines three data transfer modes:
Normal response mode (NRM): This is used with an unbalanced configuration. The primary station instructs to the secondary station and then the secondary station can transmit the data. in which the primary station communicates with a single secondary station. in which the primary station can communicate with several secondary stations. Of course, it must manage and keep separate, the different sessions it maintains with each of the secondary stations.

Asynchronous response mode (ARM): ARM is used with an unbalanced configuration. ARM involves communication between primary station and one or more secondary stations like normal response mode. In this case, secondary station is more independent. Specifically, it can send data or control information to the primary station without explicit instructions or permission to do so.
Asynchronous balanced mode (ABM): It is used with a balanced configuration. ABM is used in configuration connecting combined stations. In this case, either combined station may initiate transmission with the station without permission. Figure illustrates in which a combined station communicates with another combined station. Table 5.2 shows the relationship between the 3 data transfer modes.

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