07.033GPPTerminal Adaptation Functions (TAF) for services using synchronous bearer capabilitiesTS
This section deals with the specific requirements for non‑transparent X.25 access. Other cases, e.g. teletex, are dealt within other specifications.
Layer 2 Relay function is described in annex 1.
5.1 Rate Adaptation and protocol model
For the protocol model and rate adaptation function applied in this case see Models 4b and 4e of Figure 6/GSM 03.10).
For the cases where the method indicated in CCITT X.30 is used see Models 4a and 4d of Figure 6/GSM 03.10).
For the cases where the HDLC interframe flag stuffing shown in the recommendation CCITT X.31 is used see Models 4c and 4f of Figure 6/GSM 03.10).
5.2 Signalling Mapping
5.2.1 Interchange circuit signalling mapping
The interchange circuit signalling mapping is identical to the transparent case described in section 4.2.
5.2.2 Call establishment signalling mapping
The physical interfaces are mentioned in section 4.3.4 and the signalling mapping is described in section 8.
5.3 Flow Control
The passage of flow control information between L2Rs is described in annex 1.
5.3.1 Conditions requiring flow control towards the network
The L2R function will send immediately a "flow control active" indication in the following circumstances:
(i) If the receive buffer from the radio side reaches a preset threshold.
(ii) If local flow control is initiated by the TE2 (see section 5.3.3 a)). On receipt of this flow control indication transmission of data from the receive buffer towards the TE2 is halted.
On removal of the buffer congestion or local flow control the L2R will send a "flow control inactive" indication.
In addition, for the local flow control condition, transmission of data from the receive buffers will be restarted.
5.3.2 Conditional requiring flow control towards TE2
The L2R function will immediately activate local flow control (see section 5.3.3 b)) under the following circumstances:
(i) The transmit buffer reaches a pre‑set threshold.
(ii) The L2R receives a "flow control active" indication.
On removal of the buffer congestion or receipt of L2R/RLP "flow control inactive" the local flow control will be removed.
5.3.3 Local flow control
Only inband flow control is allowed:
a) from TE2:
RNR is sent to indicate flow control active. RR is sent to indicate flow control inactive. Where RR/RNR is utilized then the TAF will generate flow control active/inactive immediately.
b) From TAF: As from TE2.
Where this method is used, the L2R will pass the RNR/RR frames to the TE2.
5.4.1 TX buffers
Data received from the TE2 shall be buffered such that if the MT is unable to transfer the data over the radio path then data is not lost.
The buffer shall be capable of holding n1 bytes. When the buffer is half full, TE2 shall be flow controlled as per section 5.3.2. The value for n1 is up to the implementors.
5.4.2 RX buffers
Data for transfer to the TE2 shall be buffered such that if the TE2 is unable to accept data then data transferred from the MT is not lost.
The buffer size should be n2 bytes. The value for n2 is up to the implementors.
When the buffer becomes half full, the L2R will send a "flow control active" indication.