07.033GPPTerminal Adaptation Functions (TAF) for services using synchronous bearer capabilitiesTS
The L2R function required to support LAPB non‑transparently consists conceptually of the three sub‑functions shown in figure 1, i.e. the LAPB entity, the BORE and the L2RBOP entity. These perform the following functions:
‑ LAPB entity ‑ This terminates the LAPB protocol from the terminal or the network. The service provided by the LAPB entity to the BORE is described in ISO DIS 8886.2 ‑ OSI Data link service definition.
‑ L2RBOP entity ‑ This uses the services provided by the radio link, see specification GSM 04.22 . The service provided by the LAPB entity to the BORE.
‑ BORE ‑ This concatenates the data link services provided by the use of the L2RBOP and LAPB.
The functions are described in more detail in the following sections.
A.3.1 Radio Link Connection Control
The L2RBOP entity uses the services of the radio link to establish, reset and release the connection to its peer L2RBOP entity. The radio link connection will be established and released as a result of indications from the signalling mechanisms when the supporting circuit switched connection is established.
After an RLP reset or RLP disconnect the L2RBOP entities shall assume that the remote LAPB connection is in disconnected state. No data can therefore be transported between the L2RBOP entities before an exchange of the connection control PDU "Connect" has taken place. All connection control PDUs transferred before the RLP reset are no longer valid and must not be acknowledged. All PDUs (except XID) received by the L2RBOP entities after an RLP reset or disconnect and before a new connection control PDU "Connect" has been received will be discarded by the L2RBOP entity.
A.3.2 Status transfer
The L2RBOP entity transfers interface status information between L2Rs via the status octets in the L2RBOP‑PDUs. The meaning of the bits is exactly the same as that defined in CCITT recommendation V.110 and X.30. Status changes are inserted in the L2RBOP‑PDU in the position corresponding to the position in the information stream at the DTE/DCE interface that the interface status change occurred. When the RLP is established or reset a L2RBOP‑PDU with the current status octet shall be sent.
A.3.3 LAPB connection control
The L2RBOP entity transfers LAPB connection control information between L2Rs via the L2RBOP connection control PDUs. This allows a LAPB connection to be established, reset and released when the remote LAPB connection is established, reset and released or vice versa. L2RBOP connection control PDUs containing connect or reset requests shall be acknowledged by a similarly coded L2RBOP connection control PDU in the reverse direction. Data transfer between L2Rs is not allowed until the connection control acknowledge PDU is received.
In the case of requests crossing they shall each be treated as acknowledgements of the other.
A.3.4 LAPB exchange identification
The L2RBOP entity transfers a LAPB exchange identification request/acknowledge between L2Rs via the L2RBOP exchange identification PDUs. This allows transfer of identification information prior to link establishment and/or during the link (especially with respect to ISO 8885/DADI). A L2RBOP exchange identification request PDU shall be answered by an associated exchange identification acknowledge PDU. In case of crossing of two requests each request shall be answered individually. A LAPB exchange identification request with identification information will be acknowledged by the LAPB entity from L2R only when the acknowledge from the remote LAPB connection is indicated by an exchange identification acknowledge PDU sent by the remote L2RBOP entity.
A.3.5 Data Transfer
The L2RBOP entity assembles and disassembles L2RBOP‑PDUs by segmenting and reassembling the LAPB user information fields.
A.3.6 Flow control
Flow control information is transferred between L2Rs in two ways, these are:
‑ back pressure caused by L2R buffer conditions
‑ use of the X‑bit in the status octet,
X = 1 flow control active
X = 0 flow control inactive
Annex B (informative):
Document change history
Status octet tranfer at RLP init
Specification version upgrade to Release 1998 version 7.0.0