08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
The term "In Path Equipment" (IPE) is used for any telecommunication equipment within the (64 kBit/s) transmission path for the speech signal between two entities, which want to communicate via IS Messages, i.e. the IS Partners.
In modern telecommunication networks most of these IPEs are digitally transparent for the complete 64 kBit/s data stream all the time after call establishment until call release. These IPEs are optimal and need no consideration here.
Some IPEs are most of the time digitally transparent, but disturb the link every now and then. Examples are:
– switches, which interrupt the link during Handover;
– switches, which insert a kind of conference bridge for a short while during Handover;
– links, which do octet deletions or insertions (octet slips);
– DTMF generators, which insert DTMF tones sometimes for a short while; and more.
– Other IPEs are digitally transparent in one direction, but not in the other. Examples are:
– DTMF generators (again), which insert the DTMF tones only in one direction;
– Network Echo Cancellers (NEC), which let the signal pass unaltered towards the PSTN, but cancel the echo; and more.
Other IPEs are semi-transparent, i.e. let most or some of the bits pass, but not all. Examples are:
A/_Law – converters;
– /A_Law converters; and
– especially the tandem connection of A/_Law and /A_Law converters, or vice versa.
links, which insert inband signalling by bit stealing (T1 links); and more.
Other IPEs are not transparent at all to the digital bit stream, although the speech signal pass more or less unaltered. Examples:
– level shifters, which adjust the signal levels, e.g. between national networks;
– DCMEs (Digital Circuit Multiplication Equipment), which compress the bit stream by encoding/decoding the speech signal for cost efficient transmission; and more.
Many of these IPEs – for some time – will be not compliant with the IS Message principle described above. The IS Messages will not pass these non-compliant IPEs or not in both directions, or not always. Care must be taken to identify situations where IPEs are part-time-transparent or semi-transparent, when applying IS Messages. Other IPEs – at some point in time in the future – will be compliant to the IS Message principle. The rules they have to fulfil are described below.