1.5.3 Support frames structure

S- and U-frames have no information field, that’s why they consist of 5 fields. The S, U-frame structure is shown in fig. 1.20.

Opening flag	Address	Control	Control sequence	Closing flag
8 bits	8 (16) bits	8 (16) bits	16 bits	8 bits

Figure 1.20 – Structure of service frames

Opening and closing flags, address and check fields are described in item 1.5.2. Let's consider control field of support frames.

The control field contains identifiers of frame type and operations of HDLC protocol. The basic (8-bit) format of the control field is resulted in fig. 1.21, and the expand format – in fig. 1.22.

N_R – bits of a serial number of an expected frame (modulo 8).

P/F – bit of interrogation/termination of interrogation.

Type of the frame	Order of bits transfer of control field in the channel
	8	7	6	5	4	3	2	1
S-frame		NR		P/F	s	s	0	1
U-frame	u	u	u	P/F	u	u	1	1
							Identification of a frame type

Figure 1.21 – The basic format of the control field of support frames

Type of frame	Order of bits transfer of control field in the channel
	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1
S- frame				NR				P/F	x	x	x	x	s	s	0	1
U- frame	x	x	x	x	x	x	x	P/F	u	u	u	x	u	u	1	1

x – the bits which values are not defined (it is recommended х = 0).

Figure 1.22 – Expanded format of the control field of support frames

s – bits define S-frame type (it supervizor functions). Coding of types of S-frame is resulted in table 1.3.

Table 1.3 – Types of S-frames

Bit number	4	3	S-frame type (supervizory function)
Values of bits	0	0	RR - Receive Ready
	0	1	RNR - Receive Not Ready
	1	0	REJ - Reject
	1	1	SREJ - Selective Reject

u – bits define U-frame type. Now 18 types of U-frame are standardised only. Their coding is resulted in table 1.4.

Table 1.4 – Types of U-frames

Name	Command/ Response	C-Field Format
		8	7	6	5	4	3	2	1
Set normal response SNRM	C	1	0	0	P	0	0	1	1
Set normal response extended mode SNRME	C	1	1	0	P	1	1	1	1
Set asynchronous response SARM	C	0	0	0	P	1	1	1	1
Set asynchronous response extended mode SARME	C	0	1	0	P	1	1	1	1
Set asynchronous balanced mode SABM	C	0	0	1	P	1	1	1	1
Set asynchronous balanced extended mode SABME	C	0	1	1	P	1	1	1	1
Set initialization mode SIM	C	0	0	0	P	0	1	1	1
Disconnect DISC	C	0	1	0	P	0	0	1	1
Unnumbered Acknowledgment UA	R	0	1	1	F	0	0	1	1
Disconnect Mode DM	R	0	0	0	F	1	1	1	1
Request Disconnect RD	R	0	1	0	F	0	0	1	1
Request Initialization Mode RIM	R	0	0	0	F	0	1	1	1
Unnumbered Information UI	C/R	0	0	0	P/F	0	0	1	1
Unnumbered Poll UP	C	0	1	P	0	0	1	1
Reset RSET	C	1	0	0	P	1	1	1	1
Exchange Identification XID	C/R	1	0	1	P/F	1	1	1	1
Test TEST	C/R	1	1	1	P/F	0	0	1	1
Frame Reject FRMR	R	1	0	0	F	0	1	1	1
Configure for test CFGR	C/R	1	1	0	P/F	0	1	1	1
Beacon BCN	R	1	1	1	F	1	1	1	1

The frame, which consists of the expanded format of the control field, is called frame of the expanded format.

For transition from a mode of the basic format in expanded special U-frames are used. For example, with the help of U-frame SABME it is possible to pass in a mode of the expanded format from mode SABM.

1.5.4 Bit-staffing

It is obvious, that by frame transfer over communication channel its contents between two flags (opening and closing) should not have fragments of a kind 01111110; otherwise it will be identified by the receiver as the frame end. Therefore, for the purpose of creation of the "transparent" channel, contents of the composited frame before sending in the channel are exposed to special processing. If in sequence meets five units successively after them it is inserted 0. On reception, before frame decoding return operation is made: if after five successively following units is 0, it is excluded. This method is called as bit stuffing. For example, we will consider a fragment of a contained frame between flags:

After frame formatting in the transmitter

...010111110001111110...

After processing in the transmitter

...01011111 00011111 10...

In the receiver (before frame decoding)

...01011111 00011111 10...