A.2 L2RBOP

07.033GPPTerminal Adaptation Functions (TAF) for services using synchronous bearer capabilitiesTS

The LAPB user information fields and interface status changes are transferred between L2Rs using the services of the radio link. The L2RBOP entity segments and reassembles the LAPB user information fields to fit into the service data units (SDUs) handled by the radio link. I.e. segments of LAPB user information fields and interface status changes are transferred between L2Rs in n octet Protocol Data Units (PDUs). This corresponds to the fixed length of the RLP frame information field. The octets within the L2RBOP‑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.

The L2RBOP also provides facilities for transferring LAPB connection control information between L2Rs. This LAPB connection control information allows concatenated LAPB connections to be established, reset and released.

The L2RBOP PDUs are coded as follows:

‑ Each octet contains a status octet, 1 – 8 bits of user information, control information or fill.

‑ Octet 0 shall always contain a status octet in case at least one status octet is transportet in the L2RBOP 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 are used to convey the interface conditions that are conveyed by the S and X bits in CCITT recommendations V.110 and X.30. In the case of V series interfaces the 3 status bits correspond to SA, SB and X bits specified in V.110. In the case of X series interfaces only 2 bits are used and these correspond to S and X bits specified in X.30. The V series SA, SB and X bits use bit positions 8, 7 and 6 respectively in the status octets. The X series S and X bits use bit positions 7 and 6 respectively, in this case bit position 8 is unused.

‑ LAPB user information is carried in L2RBOP‑PDU information octets such that the first LAPB user information bit, in any consecutive group of 8, received or transmitted corresponds to bit position 1 in the octet. The second to bit position 2, etc.

‑ Information octets are inserted into the L2RBOP‑PDU in order of arrival 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 the next status octet within the L2RBOP‑PDU. This indicates the number of information octets between status octets. Thus if two status octets are inserted into an L2RBOP‑PDU at offsets l and m the address field value for the status octet at offset 1 will be defined by m‑l‑1 (m>l+1). The low order bit of the address corresponds to bit 1 of the octet and the high order bit to bit 5.

‑ Status octets are inserted in the information 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 can not be sent after each other. The remaining codes are used to indicate:

‑ Last status change, remainder of L2RBOP‑PDU is empty. Address field value is 31.

‑ Last status change, remainder of L2RBOP‑PDU full of information octets. Address field value is 30.

‑ End of a LAPB user information field. Address field value is 29. This is used to delimit LAPB user information fields. In this case the 3 status bits do not have their usual meaning. They are used to indicate the number of information bits in the previous information octet. A binary number in the range 0 to 7 is contained in bit positions 8, 7 and 6, bit 6 is the low order bit. The values 1‑7 indicate the number of information bits used, value 0 indicates all bits used. If this octet is not on the last position in a L2RBOP‑PDU another status octet follows (e.g. an End of LAPB user information field in octet 0 is followed by a status octet in octet 1).

‑ Abort a LAPB user information field transfer. The address field value is 28. This is used to abort the transmission of a LAPB user information field after sending one or more segments in L2RBOP‑PDUs. If this octet is not on the last position in a L2RBOP‑PDU another status octet is following (e.g. an Abort a LAPB user information field transfer in octet 0 is followed by a status octet in octet 1).

– L2RBOP-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 information stream when no status change has occurred, e.g. to indicate that the remainder of an L2RBOP‑PDU is empty or to indicate end of a LAPB user information field, 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.)

‑ LAPB connection control information is transferred between L2Rs by use of a connection control PDU. Connection control PDUs consist of an L2RBOP PDU with the status octet in octet 0 containing address field value 0. The coding of the remainder of the L2RBOP connection control PDU is as follows:

‑ Octet 1 contains the connection number, always 0 for LAPB. Other values are reserved for future use.

‑ Octet 2 contains the connection control information. The connection control information values are 1 for Connect, 2 for Reset, 3 for Disconnect and 4 for loss of LAPB interframe fill. This octet is coded as a binary number with the low order bit corresponding to bit 1.

‑ The use of octets 3 to n‑1 is reserved.

‑ LAPB exchange identification frames (XID) are transferred between L2Rs by use of exchange identification PDUs. These PDUs consist of L2RBOP PDUs with the status octet in octet 0 containing address field values 0. The coding of the remainder of the PDU is as follows:

‑ Octet 1 contains the connection number, always 0 for LAPB. Other values are reserved for future use.

‑ Octet 2 contains the exchange identification indication. The values are 5 for an Exchange Identification Request and 6 for an Exchange Identification Acknowledge. The values 7 to 255 are reserved. This octet is coded as a binary number with the low order bit corresponding to bit 1.

‑ The octet 3 contains a normal status octet. The rest of the PDU and of the following PDUs, if any, is used to transfer the XID information and it is treated like normal user data information PDUs as far as the coding is concerned.