HDLC uses synchronous transmission. All transmissions are in the form of frames, and a single frame format suffices for all types of data and control exchanges.
Figure 5.16 shows the HDLC frame structure. The flag, address, and control fields that precede the information field constitute a header. The frame check sequence (FCS) and flag fields following the data field are referred to as a trailer.


The flag field of an HDLC frame is an eight-bit sequence with a bit pattern 01111110. The flag field marks the beginning and end of each frame and contains the special bit pattern. A station receiving this pattern known an HDLC frame is on its way. Since the frame size may vary, the station examines arriving bits and looks for this pattern to detect the frame's end.
This pattern presents a problem.       Since the protocol is bit-oriented, the data fields (and others as well) can consist of arbitrary bit patterns. If the flag pattern exists in another field, would not the station interpret it incorrectly at the end of the frame? We certainly do not want to constrain the data to disallow certain bit patterns from appearing.
This problem can be solved by bit stuffing. The sending station monitors the bits between the flags before they are sent. If it detects five consecutive ls, it inserts (stuffs) an extra 0 after the fifth 1. Figure 5.17 illustrates bit stuffing.

This breaks any potential flag pattern and prevents it from being sent. Since the data is no longer correct, the receiving station must correct it. Whenever five consecutive Is are followed by 0 it assumes the 0 was stuffed and removes it. Since the flag field is not subjected to bit stuffing by the sending station, it is the only place where the flag pattern can appear.
The address field is self-explanatory. It is used to identify the terminals. It has 8 bits for the standard format and 16 bits for the extended one. If a primary station sends the frame, the address field defines the identity of the secondary station where the frame is being sent. This is necessary in multipoint configurations where there are several secondary stations. It requires only 8 bits for address.
If secondary station is sending the frame, the address field contains the sender's identity. In this case, 16 bits are required for address, because address field' contains the identity of the senders also.
The control field is 8 (standard format) or 16 (extended format) bits long and is used to send status information or issue commands. Its contents depend on the frame type. There are three types of frames:
(1) Information frame (1-frame) (2) Supervisory frame (S-frame) (3) Unnumbered frame (U-frame)
Figure 5.18 illustrates the HDLC control fields. If the first bit of the control field is 0, the frame is an I-frame. If the first bit is a 1 and the second bit is 0, it is an S-frame. If both the first and the second bits are Is, it is a U-frame.

The control fields of all three types of frames contain a bit called the poll\final (P\F) bit.
An I-frame contains two 3-bit flow and error sequence, called N(S) and N(R), flanking the P\F bit. N(S) specifies the number of frames being sent. N(R) indicates the number of the frames expected in return in a two-way exchange; thus N(R) is the acknowledgement field. The control field of an S-frame contains an N(R) field but not an N(S) field. S-frames are used to return N(R), when the receiver does not have data of its own to send. S-frames do not transmit data and so do not require N(S) fields to identify them. The two bits preceding the P\F bit in an S-frame are used to carry coded flow and error control information. U-frames have neither N(S) nor N(R) fields, and are not designed for user data exchange or acknowledgement. Instead, U-frames have two codes fields, one of two bits and the other three, flanking the P\F bit. Their codes are used to identify the type of U-frame and its function.
The information field contains the user's data in an I-frame and network management information in a U-frame. Its length can vary from one network to another but is always fixed within each network. An S-frame has no information field. The frame check sequence (FCS) is used for CRC error detection. The field may be 16 (standard format) or 32 (extended format) bits long.

  Link Access Procedure Balanced (LAPB)
Link access procedure balanced (LAPB) is a simplified subset of HDLC used only for connecting a station to a network. It therefore provides only the basic control functions required for communication between a DTE and DCE (thus, it does not include poll and select characters). LAPB was issued by ITU-T as part of its X-25 packet-switching network interface standard. It provides only asynchronous balanced mode.
  Link Access Procedure, D-Channel (LAPD)
Link access procedure for D-channel (LAPD) is another simplified subset of HDLC used in Integrated Services-Digital Network (ISDN). LAPD was issued by ITV-T as a part of its set of recommendations on ISDN. LAPD provides data link control over the D-channel, which is a logical channel at the user ISDN interface.
  Link Access Procedure, Modems (LAPM)
Link access procedure for modems (LAPM) is a simplified subset of HDLC for modems. It is designed to asynchronous to synchronous conversion, error detection, and retransmission. It has been developed to apply HDLC features to modems.

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