A.2 The L2RCOP

07.023GPPTerminal Adaptation Functions (TAF) for services using asynchronous bearer capabilitiesTS

Information is transferred between L2Rs in fixed length n octet Protocol Data Units (PDUs). This corresponds to the fixed length of the RLP frame information field. The octets within the L2RCOP‑PDU are numbered 0 to n‑1, octet 0 is transmitted first. The value of n depends on the negotiated RLP version and frame type (GSM 04.22). The bits within the octets are numbered 1 to 8, bit 1 is transmitted first.

The RLP version value 2 indicates RLP multi-link operation. The RLP version value 0 or 1 indicates RLP single-link operation.

‑ Each octet contains a status octet, an information octet or fill

Octet 0 contains either a status octet or a user information octet.

– Octet 0 shall always contain a status octet in case at least one status octet is transported in the L2RCOP PDU. In RLP-versions 0 and 1 a PDU always carries at least one status octet. In RLP version 2 a PDU carries status octet(s) only if actual status change(s) has taken place within the period represented by the PDU. Here the L2R status flag in the RLP version 2 header is set to 1 when status octet(s) is carried in the PDU.

– Status octets contain 3 status bits and 5 address bits. In cases where two status octets within the PDU are separated by more than 23 octets, the first status octet in octet m is followed by a pointer octet in octet m+1 forming a two-octet status field. The pointer octet contains one reserved bit and seven address bits indicating the number of characters between the status field and the second status octet.

‑ The 3 status bits correspond to SA, SB and X in CCITT Recommendation V.110. The SA, SB and X bits use bit positions 8,7,6 in the status octets. When a status bit changes the current state of all three bits shall be transmitted.

‑ Information octets are character octets or encoded character octets

‑ Character octets are coded in the following way:

‑ The first bit of the character received/transmitted corresponds to bit position 1 in the octet. The second bit to bit 2, …… and the seventh bit to bit 7. For order of transmission of IA5 characters see CCITT Recommendation V.4 [10].

‑ 7 bit characters are padded with a 0 in bit position 8. Received parity (if used) is inserted in bit position 8, if parity is not used bit 8 is set to 0.

‑ Any start/stop bits are removed by the L2R.

‑ Encoded character octets are provided by the compression function. They are encoded according to ITU‑T V.42bis.

‑ Information octets are inserted into L2RCOP‑PDUs in order of transmission in octets 1 to n‑1 for RLP single-link operation, in octets 1 to n‑1 for RLP multi-link operation with status octet transportation, and in octets 0 to n‑1 for multi-link operation with no status octet transportation.

‑ The address field in the status octets indicates the position of next status octet within the L2RCOP‑PDU. This indicates the number of characters between status octets. Thus if two status octets are inserted into L2RCOP‑PDU at offsets l and m the address value will be defined by m‑l‑1. Address bit 20 corresponds to bit 1 in the status octets. Address bit 21 to bit 2 etc.

‑ Status octets are inserted in the character stream whenever a status change needs to be transmitted.

‑ Only address values 1 to n‑2 (n‑2  23) in the address field of status octets are used for addressing purposes. The implication of not allowing address value 0 to be used for addressing is that two status octets cannot be sent after each other. The remaining codes are used to indicate:

‑ Last status change, remainder of L2RCOP‑PDU empty. Address field value 31

‑ Last status change, remainder of L2RCOP‑PDU full of characters. Address field value 30

‑ Destructive break signal, remainder of L2RCOP‑PDU empty. Address field value 29

‑ Destructive break acknowledge, remainder of L2RCOP‑PDU empty. Address field value 28

– L2RCOP-PDU contains at least two status octets which are separated by more than 23 characters; the address-field value in the first octet of the two-octet status field is 27 and the address bits in the pointer octet of the status field indicate the number of characters between the two-octet status field and the next status octet.

‑ Address field values from n‑1 to 26 are reserved. In case of a PDU more than 25 octets in length, address field values from 24 to 26 are reserved.

‑ When it is necessary to insert a status octet into the character stream when no status change has occurred, e.g. to indicate that the reminder of a L2RCOP‑PDU is empty or to indicate a break signal, the current status shall be repeated.

– In case when 64 data octets are carried by a 66-octet PDU, a status octet is carried in octet 0 and another status octet within the first 24 data octets. (The first status octet gives the address of the second status octet, which carries value 30 in its address field.)

Three examples of an L2RCOP PDU are shown in Figure 2.

8

7

6

5

4

3

2

1

0

SA

SB

x

0

0

0

1

1

1

1

1

0

0

0

1

1

1

IA5 "G" (odd parity)

2

1

1

0

1

0

0

1

1

IA5 "S" (odd parity)

3

1

1

0

0

1

1

0

1

IA5 "M" (odd parity)

4

SA

SB

x

1

1

1

1

1

(last status change, rest of PDU empty)

.

.

.

n‑1

Figure 2a Single-link RLP and multi-link RLP with status octet transfer in PDU.

8

7

6

5

4

3

2

1

0

1

1

0

1

0

0

1

1

IA5 "S" (odd parity)

1

1

1

0

0

0

1

1

1

IA5 "G" (odd parity)

2

1

1

0

1

0

0

1

1

IA5 "S" (odd parity)

3

1

1

0

0

1

1

0

1

IA5 "M" (odd parity)

4

.

.

.

n‑1

1

1

0

0

1

1

0

1

IA5 "M" (odd parity)

Figure 2b Multi-link RLP L2RCOP PDU with no status octet transfer

8

7

6

5

4

3

2

1

0

SA

SB

X

0

0

0

1

1

1

1

1

0

0

1

1

0

1

IA5 "M" (odd parity)

2

1

1

0

0

0

0

0

1

IA5 "A" (odd parity)

3

1

1

0

1

0

0

1

0

IA5 "R" (odd parity)

4

SA

SB

X

1

1

0

1

1

5

R

0

1

0

0

0

1

1

.

.

.

41

SA

SB

X

0

0

0

0

1

42

1

1

0

0

1

1

0

1

IA5 "K" (odd parity)

43

SA

SB

X

1

1

1

1

0

.

.

.

.

65

1

1

0

0

1

1

1

1

IA5 "O" (odd parity)

Figure 2c A 66-octet RLP L2RCOP PDU with status octets separated by more than 23 octets