TYPES OF HDLC FRAMES

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There are three types of HDLC frames :
·         Information transfer frame (I-Frame)
·         Supervisory frame (S-Frame)
·         Unnumbered frame (U-Frame)
Information Transfer Frame (I-FRAME)
I-Frame is used for transporting user data. It also carries acknowledgement of the received frames. The control field of the I-Frame is as shown in Fig.7. The first bit is 0 which identifies the frame as an I-Frame. The next three bits are the sequence number N(S) of the frame.
The fifth is Poll/Final (P/F) bit. Its use is explained later.

The last three bits are the sequence number N(R) of the acknowledgement (RR) which is piggy backed on the I-Frame. 

Supervisory frame (S-FRAME)

S-Frame does not have data field (Fig.3b) and is used to carry only acknowledgements, requests for retransmission, etc. It is identified by the first two bits of the control field (Fig. 8). These two bits are 10 in an S-Frame. The next two bits (SS) are used to indicate four supervisory states. Receive Ready (RR), Receive Not Ready (RNR), Reject (REJ) and Selective Reject (SREJ).
The fifth bit is Poll/Final bit. The last three bits are the sequence number associated with the supervisory states RR, RNR, REJ and SREJ indicated in the SS-Bits of the control field. Note that an acknowledgement (RR) can be sent either on a supervisory frame or piggybacked on an I-Frame as mentioned earlier. On the other hand RNR, REJ and SREJ are sent only through a supervisory frame.


Unnumbered Frame (U-Frame)
The U-Frames do not have data field as S-Frames. They are used for link establishment, termination, mode setting and other control functions. Controls field of an unnumbered frame is shown in Fig.9.
The first two bits of the control field are 11 which identify an unnumbered frame. The fifth bit is Poll/Final bit. The rest five bits are called modifier bits. They specify the control function. 



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