Frames structure on HDLC procedure

1.5.1 Types of frames according to HDLC procedure

The HDLC (High Level Data Link Control) protocol is defined by ISO for use on both point-to-point and multipoint data links. It supports full duplex transparent-mode operation and is now extensively used in both multipoint and computer networks.

In HDLC protocol frames can be of three types: I, S, U.

I-frame is called information frame,

S-frame –supervisor frame,

U-frame –unnumbered frame.

The I-frame is used only for data (information) transmission from users or higher levels.

The S-frame provides transfer of the special support information on a condition of transferred I-frames. They are used for transfer of receipts on confirmation or request, readiness or unavailability to reception of the next I-frame.

For struggle against "losses" and "inserts", which are characteristic for automatic repeat request systems in I-frames and S-frames, numbers of corresponding frames are transferred. Therefore I-frames and S-frames are "numbered".

The U-frame also is necessary for support information transfer. But this information serves basically as management of a data link (channel). With the help of U-frame takes place a link initialization, a connection establishment/disconnection, change of an operating mode and other service functions. In this frame numbers of information frames are not transferred, whence and the name – «unnumbered frame».

1.5.2 I-frame structure

Each frame consists of fields. The I-frame has 6 fields, and S - and U-frames have 5 fields. The I-frame structure is shown in fig. 1.17.

Figure 1.17 – I-frame structure

Let's consider frames fields construction.

Frame beginning and end fields. For the frame beginning and end detection the principle of the start-stop cyclic synchronisation is used. The sequence of a kind 01111110 is applied as a starting combination. The similar sequence is used for a detection of the frame end. This sequence is called as «opening flag» or «closing flag» accordingly.

Address field. In this field the address (number) of corresponding station in the binary form is transferred. Each station has the unique address. In a frame containing commands, the address of remote station is transferred, and in a frame-answer the local address is transferred.

Address field expansion on 8 bits (1 byte) is supposed. The index of that is presence of zero in the first bit of the previous byte of an address field, excepting byte of a kind 00000000. Thus, the younger bit of the usual (not expanded) address should be equal 1.

Control field. It contains identifiers of a frame type and operations of HDLC protocol. The basic (8-bit) format of the control field is resulted in fig. 1.18. The order of bits begins with bits of younger categories.

	NR		P/F		NS
Identification of the frame type ↑.

Figure 1.18 – The basic format of the control field of I-frame

N_S – bits of a serial number of the given (sent) I-frame (modulo 8).

N_R – bits of a serial number of an expected (received) frame (modulo 8), i.e. correct reception of I-frames to number (N_R – 1) inclusive proves to be true.

P/F – bit of interrogation/termination. In a command frame this bit is interpreted as bit of "interrogation" P (poll). If on a sent frame it is necessary to receive the answer it is exposed P = 1; if the answer is not necessary, P = 0. In an answer frame this bit is interpreted as bit of "the interrogation termination» F (finish). If the frame with P = 1 earlier has been correctly accepted, in reciprocal frame F = 1, otherwise F = 0.

Except the basic (8-bit) format of the control field there is also an expanded format (16 bits). The term "expanded" means expansion of a range of serial numbers of sent and received frames to 127. For operations with expansion by a serial number the sizes of fields N_S and N_R increase from 3 bits (modulo 8) to 7 bits (modulo 128). Thus, the size of the control field increases from one byte to two bytes. The expanded format of the control field is resulted in fig. 1.19.

NR

P/F

NS

Identification of the frame type ↑.

Figure 1.19 – The expanded format of the control field of I-frame

Check field. In the check field the control sequence (CS), received as a result of coding by a cyclic code with generator polynomial P(x) = x¹⁶ + x¹² + x⁵ + 1 is located. As k information categories which will be protected by a correcting code, categories of fields use: address, control and information. Thus, content between opening and closing flags is a code combination of a cyclic code. For definition CS usual procedure of the allowed cyclic code combination construction by formula (1.4) is used

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.

Figure 1.20 – Structure of support 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 expanded format – in fig. 1.22.

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

P/F – bit of interrogation/termination.

Type of the frame
S-frame		NR		P/F	s	s
U-frame	u	u	u	P/F	u	u
	Identification of a frame type

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

Type of frame

S- frame

NR

P/F

x

x

x

x

s

s

U- frame

x

x

x

x

x

x

x

P/F

u

u

u

x

u

u

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

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

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

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

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.

Date: 2015-02-16; view: 850