5 Frame structure for peer-to-peer communication

3GPP44.006Data Link (DL) layer specificationMobile Station - Base Station System (MS - BSS) interfaceRelease 17TS

5.1 General

All data link layer peer‑to‑peer exchanges are in frames conforming to one of the formats shown in figure 1. Several format types are shown in the figure:

‑ Format A is used on DCCHs for frames where there is no information field.

‑ Formats B, Bter and B4 are used on DCCHs for frames containing an information field:

‑ format Bter is used on request of higher layers if and only if short L2 header type 1 is supported and a UI command is to be transmitted on SAPI 0;

– format B4 is used for UI frames transmitted by the network on SACCH;

‑ format B is applied in all other cases.

‑ Format Bbis is used only on BCCH, PCH, NCH, and AGCH.

‑ In addition there is a Format C for transmission of random access signals.

Format C frames are described in annex A. Format A, B, Bbis, Bter and B4 frames are described in the remainder of the present document.

Bit

8

7

6

5

4

3

2

1

Octet No

1

:

Address field

:

k

Control field

k+1

k+2

:

Length indicator field

:

n

n+1

:

Fill bits

:

N201+n

Format type A

Figure 1 (sheet 1 of 3): General frame formats

Bit

8

7

6

5

4

3

2

1

Octet No

1

:

Address field

:

k

Control field

k+1

k+2

:

Length indicator field

:

n

n+1

:

Information field

:

N

N+1

:

Fill bits

:

N201+n

Format type B

Bit

8

7

6

5

4

3

2

1

Octet No

1

:

:

:

Information field

:

:

:

N201

Format type Bbis

Figure 1 (sheet 2 of 3): General frame formats

Bit

8

7

6

5

4

3

2

1

Octet No

1

:

Address field

:

k

Control field

k+1

k+2

:

Information field

:

N201 + k+1

Format type B4

Bit

8

7

6

5

4

3

2

1

Octet No

short L2 header
type 1

1

2

:

Information field

:

:

:

N201

Format type Bter

Figure 1 (sheet 3 of 3): General frame formats

The parameter N201 is the maximum number of octets which are partially or entirely available for the information field of a frame. It depends on the type of channel and the format, see sub-clause 8.8.3.

5.2 Frame delimitation and fill bits

Frame delimitation is provided by the physical layer:

‑ in format type A, B and B4 frames at the beginning of the frame for determining the start of the first octet in the address field, in format type Bter frames for determining the start of the octet containing the short L2 header type 1 (which is also the first octet used for the information field), and in format type Bbis frames for determining the start of the first octet for the information field;

‑ at the end of the frame for determining the last bit of the frame.

The end of the useful part of the frame, i.e. the end of the length indicator field in type A frames and the end of the information field in type B frames, is determined by a length indicator contained in the length indicator field. The useful part of a Bbis frames takes all N201 octets of that frame. The useful part of a Bter frame takes all N201 octets of that frame except those bits of octet 1 which contain the short L2 header type 1. The useful part of a B4 frame takes all N201 octets of that frame except those octets which contain the address field and the control field.

If a frame contains a length indicator that has a value less than N201, the frame contains fill bits. Each fill bit shall be set to a random value when sent by the mobile station. Except for the first octet containing fill bits which shall be set to the binary value "00101011", each fill bit should be set to a random value when sent by the network. Otherwise, the network shall set all octets containing fill bits to the binary value "00101011".

5.3 Address field

The address field may consist of a variable number of octets. However, for applications on control channels the field consists of only one octet. The address field identifies the SAP for which a command frame is intended and the SAP transmitting a response frame. The format of the address field is defined in sub-clause 6.2.

5.4 Control field

The control field consists of one octet. The format of the control field is defined in sub-clause 6.4.

5.5 Length indicator field

The length indicator field may consist of a variable number of octets. However, for applications on control channels the field consists of only one octet. The format of the field is defined in sub-clause 6.6.

5.5a Short L2 header type 1

The short L2 header type 1 consists of two bits. Its contents are defined in sub-clause 6.4a.

5.6 Information field

The information field of a frame, when present, has the position in the frame defined in sub-clause 5.1.

The maximum number of octets in the information field (N201) is defined in sub-clause 8.8.3.

5.7 Transparency

Because of the frame delimitation technique used (see sub-clause 5.2), the frame can include any possible sequence of bits without the need for additional transparency mechanisms.

5.8 Format convention

5.8.1 Numbering convention

The basic convention used in this Technical Specification is illustrated in figure 2. The bits are grouped into octets.

The bits of an octet are shown horizontally and are numbered from 1 to 8. Multiple octets are shown vertically and are numbered from 1 to n.

Bit

8

7

6

5

4

3

2

1

Octet no

1

2

;

;

n

Figure 2: Format convention

5.8.2 Order of bit transmission

The order of bit transmission is defined in 3GPP TS 44.004.

5.8.3 Field mapping convention

When a field is contained within a single octet, the lowest bit number of the field represents the lowest order value.

When a field spans more than one octet, the order of bit values within each octet progressively decreases as the octet number increases. In that part of the field contained in a given octet the lowest bit number represents the lowest order value.

For example, a bit number can be identified as a couple (o, b) where o is the octet number and b is the relative bit number within the octet. Figure 3 illustrates a field that spans from bit (1, 3) to bit (2, 7). The high order bit of the field is mapped on bit (1, 3) and the low order bit is mapped on bit (2, 7).

Bit

8

7

6

5

4

3

2

1

4

3

2

1st octet field

2

2

2

1

0

2nd octet field

2

2

Figure 3: Field mapping convention