07.013GPPGeneral on Terminal Adaptation Functions (TAF) for Mobile Stations (MS)TS
In order to support non transparent bearer services a Layer 2 Relay (L2R) function is included in the mobile termination. The details of the particular L2R function for the different non transparent bearer services are contained in the appropriate GSM 07-series Specification. This section describes the general aspects of the L2R function.
The Layer 2 Relay (L2R) function provides for the reliable transportation of known, i.e. non transparent, user protocols across the radio interface of a GSM PLMN. The L2R functions are located in the Mobile Termination (MT) and the Interworking Function (IWF) associated with a Mobile Switching Centre (MSC). The L2R uses the services provided by the Radio Link Protocol (RLP) to transport the non transparent protocol information between the MS and the IWF.
6.1 Functions of the Layer 2 Relay
The complete protocol reference models for data and telematic services are described in GSM 03.10. The subset of those protocol reference models relating to the L2R function is reproduced in figure 2.
───────────── Transparent higher layer protocol ─────────────
│ L2R ├───── L2R Protocol ────┤ L2R │
─ NTP ──┤ ┌─┬───┤ ├───┬─┐ ├── NTP ──
│ │ │ ├─ Radio Link Protocol ─┤ │ │ │
├───┤ ├───┤ ├───┤ ├───┤
─ IFP ──┤ │ │ ├── IFP ──
NTP Non Transparent Protocol
IFP Interface Protocol
The Non Transparent Protocol (NTP) will normally be a layer 2 protocol for OSI conformant protocols or an equivalent in the case of non OSI protocols. The Interface Protocol (IFP) will normally be a layer 1 protocol for OSI conformant systems or equivalent for non OSI systems.
The L2R can be considered to consist of 3 sub-functions, see figure 3.
│ Relay Entity │
│ L2RP │
──────────┤ NTP Entity │
│ Entity ┌───┬────────────┤
│ │ │ │
│ │ │ RLP │
│ │ │ Entity │
The 3 sub-functions are:
– A Non Transparent Protocol Entity
– A L2R Protocol Entity
– A Relay Entity
The NTP-entity interfaces the L2R to the IFP-entity and provides an interface to the particular NTP.
The L2RP-entity interfaces the L2R to the RLP-entity and provides an interface to the appropriate L2R protocol.
The Relay-entity provides the mapping between the NTP-entity and the L2R-entity. If applicable, it contains the data compression function. The negotiable parameters are exchanged with the remote Relay-entity by means of the RLP XID frame.
It should be noted that the inter-layer interfaces within the MS and the IWF and within the L2R will not be specified by GSM, any description given is for explanatory purposes only and is not intended to indicate a method of implementation. Therefore, the specification of the L2R is in terms of the peer-peer protocols. Generally, the non transparent and interface protocols will be specified elsewhere, e.g. CCITT Recommendation X.25 Layer 2 and 1. Thus the main specification for the L2R will consist of the L2R peer‑peer protocols.
6.2 Radio Link Services Used
The L2R function uses services defined in GSM Specification 04.22 (Radio Link Protocol).
6.3 Flow Control – General Description
A flow control active condition can take place under a number of circumstances:
– End to end flow control (DTE to DTE matter);
– Backpressure (buffers filling);
– Receive not ready (RLP condition).
It is possible that there will be an interaction between flow control active and inactive conditions in each circumstance.
6.3.1 End to End Flow Control
A DTE may wish to send a flow control active condition to another DTE.
Provisions exists in the L2R entity to transfer a flow control active condition (sent by its associated DTE) to the other L2R entity as soon as possible. This mechanism in the L2R entities allows such a flow control condition to be put ahead of any queuing which exists in the L2R entities.
Such a mechanism avoids build up of data in buffers which can be undesirable.
The L2R entity, receiving a flow control active condition from its associated DTE, stops sending data to that associated DTE immediately.
6.3.2 Back Pressure
The L2R and RLP entities have buffers which may become full to a predetermined threshold for a number of reasons, e.g. severe radio fading, failure or slowness of DTE to react to end to end flow control, certain RNR conditions. When this predetermined threshold is reached, a flow control active condition is sent to the associated DTE which is then prevented from sending any data, subsequently, the flow control inactive condition is sent to the associated DTE when the L2R or RLP entities have indicated that there is sufficient free capacity in their buffers for data flow from the associated DTE to proceed.
The corresponding peer-layer procedure to assess the respective buffer conditions are a layer management matter and are not dealt with here. It is also considered an implementation matter to ensure that such procedure do not result in loss of data or considerable reduction in throughput.
6.3.3 Receive not Ready
When the RNR condition arises, an RLP indication is sent to the other RLP entity which in turn shall send a flow control active condition to its associated L2R entity. That L2R entity shall then send a flow control active condition to its associated DTE.
An RNR condition may result in the Execution of "back pressure" as mentioned under 6.3.2.