3GPP TS

Release 1998383GPP TS 07.10 V7.2.0 (2002-03)DTEDT E.RxDFlowcontrolemulationRxD&FCS3FCS2FCS1Figure 14: XON/XOFF Flow Control on input5.5 Convergence LayersConvergence layers are defined to permit data which has implied structure to be conveyed through the multiplexerwithout losing the structure or other parameters which are associated with the data stream. Common uses ofconvergence layers are to carry the state of V.24 control signals through a DLC or to ensure that the boundaries of acoded voice frame are preserved.Convergence layers apply to data whether it is carried by error recovery mode or non-error recovery mode procedures.The use of particular convergence layers is implicitly linked to the DLCIs used but may be negotiated away from thesedefaults by the use of the multiplexer control channel.5.5.1 Type 1 - Unstructured Octet StreamData which consists of an unstructured sequence of octets, for example, 64 kbit/s uncoded voice or normalasynchronous data without V.24 control signals, is inserted directly into the I-field. In this case, it could be said that theconvergence layer is null.Type 1 is the default convergence layer for each DLC.5.5.2 Type 2 - Unstructured Octet Stream with flow control, break signalhandling and transmission of V.24 signal statesIf it is desired to convey virtual V.24 control signals associated with a data stream the first octet of each I-field containsa representation of the state of the signals in accordance with Figure 15. The use of the extension bit allows other octetsto be added to cater for other circumstances. At present, an optional second octet is defined for handling thetransmission of break signals.The mappings of the V.24 signals to the bits in the control signal octet for the receiver and sender are given in tables 23and 24, respectively.Bit No 1 2 3 4 5 6 7 8Signal EA FC RTC RTR reserved reserved IC DV0 0Description of the control signal byte:Figure 15: Format of control signal octet3GPP