7 Definition of SN‑PDU

04.653GPPGeneral Packet Radio Service (GPRS)Mobile Station (MS) - Serving GPRS Support Node (SGSN)Release 1999Subnetwork Dependent Convergence Protocol (SNDCP)TS

7.1 Format convention

7.1.1 Numbering convention

The convention used in the present document is illustrated in Figure 15. 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

Oct 1

2

N-1

N

Figure 15: Format convention

7.1.2 Order of transmission

SN‑PDUs are transferred between the SNDCP layer and LLC layer in units of octets, in ascending numerical octet order (i.e., octet 1, 2, , N‑1, N). The order of bit transmission is specific to the underlying protocols used across the Um interface and the Gb interface.

7.1.3 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 16 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

1st octet of field

24

23

22

2nd octet of field

21

20

Figure 16: Field mapping convention

Figure 17 illustrates an NSAPI field that spans from bit (1,8) to bit (2,1). NSAPI 15 is mapped to bit (1,8) and the other NSAPIs are mapped in decreasingly order until NSAPI 0 that is mapped to bit (2,1). A bit set to 0 means that the compression entity is not applicable to the corresponding NSAPI. A bit set to 1 means that the compression entity is applicable to the corresponding NSAPI.

Bit

8

7

6

5

4

3

2

1

1st octet of field

15

14

13

12

11

10

9

8

2nd octet of field

7

6

5

4

3

2

1

0

Figure 17: NSAPI mapping convention

7.2 SN‑PDU Formats

Each SN‑PDU shall contain an integral number of octets, and shall comprise a header part and a data part. An SN‑PDU shall contain data from a single N‑PDU only. Two different SN‑PDU formats are defined. The SN‑DATA PDU shall be used for acknowledged data transfer and SN‑UNITDATA PDU for unacknowledged data transfer.

Bit

8

7

6

5

4

3

2

1

Oct 1

X

F

T

M

NSAPI

2

DCOMP

PCOMP

3

N‑PDU number – acknowledged mode

Data segment

N

Figure 18: SN‑DATA PDU format

Bit

8

7

6

5

4

3

2

1

Oct 1

X

F

T

M

NSAPI

2

DCOMP

PCOMP

3

Segment number

N‑PDU number – unacknowledged mode

4

N‑PDU number – unacknowledged mode (continued)

Data segment

N

Figure 19: SN‑UNITDATA PDU format

More bit (M):

0 Last segment of N‑PDU.

1 Not the last segment of N‑PDU, more segments to follow.

SN‑PDU Type (T):

0 SN‑DATA PDU.

1 SN‑UNITDATA PDU.

First segment indicator bit (F):

0 This SN‑PDU is not the first segment of an N‑PDU.

The octet including DCOMP and PCOMP is not included in the SN‑DATA PDU or SN‑UNITDATA PDU format. Also the octet for N‑PDU number for acknowledged mode is not included in the SN‑DATA PDU format.

1 This SN‑PDU is the first segment of an N‑PDU. The octet for DCOMP and PCOMP is included in the SN‑DATA PDU or SN‑UNITDATA PDU format. Also the octet for N‑PDU number for acknowledged mode is included in the SN‑DATA PDU format.

Spare bit (X):

0 Shall be set to 0 by the transmitting SNDCP entity and ignored by the receiving SNDCP entity.

NSAPI:

0 Escape mechanism for future extensions.

1 Point-to-Multipoint Multicast (PTM-M) information.

2-4 Reserved for future use.

5-15 Dynamically allocated NSAPI value (see subclause 6.1).

SN‑PDU with an unallocated NSAPI value shall be ignored by the receiving SNDCP entity without error notification.

Data compression coding (DCOMP):

0 No compression.

1-14 Points to the data compression identifier negotiated dynamically (see subclause 6.6).

15 Reserved for future extensions.

SN‑PDU with an unallocated DCOMP value shall be ignored by the receiving SNDCP entity without error notification.

Protocol control information compression coding (PCOMP):

0 No compression.

1-14 Points to the protocol control information compression identifier negotiated dynamically (see subclause 6.5).

15 Reserved for future extensions.

SN‑PDU with an unallocated PCOMP value shall be ignored by the receiving SNDCP entity without error notification.

Segment number:

0-15 Sequence number for segments carrying an N‑PDU.

N‑PDU number – acknowledged mode:

0-255 N‑PDU number of the N‑PDU.

N‑PDU number – unacknowledged mode:

0-4095 N‑PDU number of the N‑PDU.