3.3 Delay

03.503GPPTransmission planning aspects of the speech service in the GSM Public Land Mobile Network (PLMN) systemTS

3.3.1 General

A significant propagation time between the two ends of a connection causes difficulties in conversation over the connection. This arises from two causes. Firstly, the signal is reflected back from the distant end causing an echo to the talker (this is considered in clause 3.4). Secondly, even if ideal echo control were achieved, the delay between a user talking and receiving a reply from the user at the distant end of the connection could cause conversational difficulty.

PLMNs will be connected to the PSTN at a point where present planning rules allow for a delay of less than 12 ms (see ITU‑T Recommendation G.114 clause 2.2 a). The delay within the PLMN will greatly exceed this. If unacceptable circuit delays are not to be experienced by users, action will have to be taken when planning routes or during call set‑up.

3.3.2 Sources of delay

3.3.2.1 Elements of the PLMN that cause delay

The delay of the PLMN is made up of the following elements:

1) speech transcoding delay;

2) radio channel coding delay;

3) PLMN network delay (i.e. fixed elements such as multiplexing, propagation, switching, echo control);

4) speech processing for echo cancelling and noise reduction in Handsfree Mode.

3.3.2.2 Elements of the PSTN that cause delay

ITU‑T recommendation G.114 identifies various elements present in some PSTN connections which cause delay. These include:

1) coaxial, radio and optical fibre terrestrial transmission systems;

2) geostationary satellites;

3) digital speech interpolators;

4) digital exchanges (see also ITU‑T recommendation Q.551);

5) echo cancellers.

3.3.3 Effects of delay

Some recent studies have suggested that under ideal conditions, i.e.:

1) effective control of all echoes without clipping by the use of good echo cancellers;

2) low background noise leading to an absence of perceptible noise contrast;

3) low distortion of transmitted signals;

4) ideal loudness ratings.

Users can tolerate a circuit delay well in excess of 400 ms (currently the maximum delay recommended in ITU‑T Recommendation G.114). Other studies indicate that the difficulty caused by circuit delay increases when impairments, such as imperfect echo control caused by echo suppressers, clipping and noise contrast, are present.

However, the mobile environment is very harsh, with high background noise levels and distortion from the speech transcoder. In particular, the use of acoustic echo suppression could give rise to severe speech clipping and noise contrast. Also the operation of the voice switching used with DTX will give impairments similar to those caused by echo suppression. All subjective tests performed with echo suppressers indicate that, because of the increased effect of clipping with increased delay, the difficulty experienced by users increases rapidly with delay. According to curve 2 of figure A.1 of ITU‑T Recommendation G.114, the percentage of users experiencing difficulties with echo suppressers reaches 20 % with a delay of 150 ms rising to 40 % with a delay of 300 ms. ITU‑T Recommendation G.114 annex A details the test conditions under which this curve was derived and it concludes that connections with more than 300 ms can only be used by very disciplined users who are aware of the problems involved in such a connection. However, recent work has indicated that delays of up to 500 ms can be used satisfactorily, provided that effective echo cancellation is incorporated in the link.

3.3.4 Allocation of delay to the PLMN

3.3.4.1 Allocation of delay to the PLMN when using a full rate system

Taking account of Recommendations on the separate factors described in clause 3.3.2.1, the maximum both‑way in the PLMN between the MRP/ERP and the Point of Interconnection (see figure 1) will be 180 ms. In the case that the transcoder is positioned outside the BTS, the maximum distance between the POI and the furthest border of the cell controlled by the BTS is limited by a one‑way propagation delay of 1,5 ms (approximately 300 km). If the transcoder is positioned at the BTS, the limit is 6,5 ms (approximately 1 300 km). These limits may be subject to increase resulting from savings made in the overall network.

3.3.4.2 Allocation of delay to the PLMN when using a half rate system

If it is assumed that the speech quality associated with the half rate system is the same as the full rate system (considering both the speech transcoder and the radio sub‑system), then in order to achieve the same overall transmission quality, the maximum delay within the PLMN should be maintained at 180 ms.

3.3.5 Delay of various network configurations

3.3.5.1 National and international connections with no echo control in the PSTN (reference configurations A)

Reference configurations A (see figure 4) contain no echo control in the PSTN because present planning rules require the use of echo control devices only when the PSTN delay between two fixed PSTN users exceeds 25 ms. This leads to a maximum PSTN delay of 22 ms from the point of interconnection to the PLMN (see clause 3.4.2).

3.3.5.2 National and international connections with echo control in the PSTN (reference configurations B)

Reference configurations B (see figure 5) contain echo control in the PSTN because present planning rules require their use when the PSTN delay between PSTN users exceeds 25 ms. However, action may have to be taken by administrations when planning routes or at call set‑up to limit the maximum delay.

Subclause 3.3.3 describes how the impairments from the harsh mobile environment when coupled with delay can give rise to difficulty. If very good cancellation of both electrical and acoustic echo can be achieved and there are no sources of speech clipping or noise contrast either in the PLMN or the PSTN part of the connection, the circuit delay should be kept below 400 ms. This means that every attempt should be made to avoid mobile to mobile calls via satellite (expected delay > 440 ms).

If acoustic echo suppression is used or DTX is enabled, or there is any other source of clipping or noise contrast present in the PSTN, the additional distortion introduced makes it desirable to avoid any satellite routeing whenever possible in order to keep the delay below 300 ms.

3.3.5.3 Connections where re‑routeing leads to a significant increase in transmission path length (reference configurations C)

A number of possible combinations of re‑routeing are described by reference configurations C (see figure 6), all of which increase the path length and hence the delay and some of which increase the number of impairments in the network.

These routeings are likely to cause severe degradation to the quality of the connection and may result in significant difficulty, particularly when the connection contains one or maybe more satellite links.

These connections should be avoided in network planning and, if this is not possible, then the facilities of Signalling System No. 7 should be used to control the routeing of the call at call set‑up to minimize the effects.

3.3.6 Delay related requirements on the MS

3.3.6.1 Full rate MS

In accordance with the outline of transmission delays in various GSM system elements contained in GSM 03.05, the round trip delay in the MS shall not exceed 143,9 ms as defined in annex D.

3.3.6.2 Half rate MS

The round trip delay in the MS shall not exceed 143,9 ms as defined in annex D.

3.3.6.3 Handsfree MS

The round trip delay in the MS, including the handsfree processing stages, shall not exceed 143,9 ms + 39 ms. The 143,9 ms is as defined in annex D, the 39 ms is to allow for additional processing for handsfree.

The method of measurement is defined in annex C, clause C.10.