03.503GPPTransmission planning aspects of the speech service in the GSM Public Land Mobile Network (PLMN) systemTS
The basic configuration for the interworking with the PSTN is shown in figure 1.
2.2 Model of the PLMN
A more detailed model of the PLMN used for the consideration of transmission planning issues for speech is shown in figure 2. This model represents the main functions required and does not necessarily imply any particular physical realization. Routeing of calls is given in GSM 03.04.
Any acoustic echo control is not specifically shown as it will be provided by analogue processing of digital processing or a combination of both techniques.
The main interfaces identified within the GSM specifications are shown in figure 1. For the purposes of the present document, the Air Interface and the Point of Interconnect (POI) are identified along with two other interfaces, Interface Z and a 13‑bit Uniform PCM Interface (UPCMI). These interfaces are needed to define the PLMN transmission characteristics and the overall system requirements.
The Air Interface is specified by GSM 05 series specifications and is required to achieve MS transportability. Analogue measurements can be made at this point by using the appropriate radio terminal equipment and speech transcoder. The losses and gains introduced by the test speech transcoder will need to be specified.
The POI with the PSTN will generally be at the 2 048 kbits/s level at an interface, in accordance with ITU‑T Recommendation G.703. At the point, which is considered to have a relative level of O dBr, the analogue signals will be represented by 8‑bit A‑law, according to ITU‑T Recommendation G.711. Analogue measurements may be made at this point using a standard send and receive side, as defined in ITU‑T Recommendations.
Interface Z might be used in the case of direct MSC to MSC connections. Interface Z is of the same nature as the POI.
The UPCMI is introduced for design purposes in order to separate the speech transcoder impairments from the basic audio impairments of the MS.
2.4 Configurations of connections
2.4.1 General configurations of connections
Figure 3 shows a variety of configurations of connections. There are a number of PSTN features which should be avoided from such connections. These include:
1) echo control devices in the international network. If present, and not disabled, these devices will be in tandem with PLMN echo cancellers and may introduce degradation;
2) satellite routeings. The delay inherent in the connections when added to the PLMN delay, may result in conversational difficulties. Double satellite links are likely to cause severe difficulties and special precautions should be taken to avoid this situation under call forwarding arrangements;
3) Digital Speech Interpolation systems (DSI). There is likely to be an adverse interaction between DSI and DTX;
4) ADPCM. The distortion introduced by ADPCM on routes where PSTN echo control is not provided is likely to reduce the echo cancellation provided by the PLMN electric echo canceller;
5) significant differences in clock rates on non‑synchronized digital network components. The resulting phase roll and slips are likely to degrade the performance of the PLMN echo canceller;
6) those analogue FDM routeings which exhibit phase roll. Any phase roll due to the absence of synchronization between the carrier frequencies on the two directions of transmission is likely to degrade the performance of the PLMN echo canceller;
7) tandem connections of sources of quantization distortion. The PLMN speech transcoder is estimated to be equivalent to 7 QDUs between uniform PCM interfaces (see ITU‑T Recommendation G.113).
It is recognized that on some connections it may not be feasible to avoid these features, but in many cases, especially if taken into account at the planning stage, this should be possible.
2.4.2 Reference configurations to illustrate delay and echo control issues
Three basic reference configuration types shown in figures 4 to 6 are defined to illustrate delay and echo control issues. Intermediate echo control devices as shown in the figures are disabled by appropriate signalling between the MSC and ISC or MSC and MSC.
Reference configurations A (see figure 4) represent national or international connections where there is no echo control device in the PSTN. These reference configurations include re‑routeing configurations where the overall delay of the transmission path has not been extended.
Reference configurations B (see figure 5) represent national or international connections where echo control is provided in the PSTN. These reference configurations include re‑routeing configurations where the overall delay of the transmission path has not been extended.
Reference configurations C (see figure 6) represent national or international connections where re‑routeing has lead to an increase in the overall delay of the transmission path beyond recommended limits.
2.5 4‑wire circuits in the PLMN
As shown in figure 2, the PLMN will usually contain transmission systems. Where present, they should provide 4‑wire circuits.
In the case of digital circuits which do not include any speech processing devices, the overall system requirements of the PLMN will not be affected by the presence of the link.
In the case of analogue links, the transmission characteristics (e.g. attenuation, attenuation distortion, noise) will affect the overall system requirements of the PLMN. ITU‑T Recommendations M.1020, M.1025, M.1030 and M.1040 describe several transmission characteristics for leased circuits. In cases where the analogue link introduces loss, provision will have to be made at the interface to restore the loss.