ITU J.52
An elementary problem during a terrestrial transmission connected to a line is the availability of data rates higher than 64 kbps. This can be explained by the fact that a point-to-point transmission with 256 kbps requires four ISDN B-channels in total. Those can be switched through different routes by the network operator (in Germany: Deutsche Telekom), for example during a transmission from Frankfurt to Munich three B-channels almost direct and 1 B-channel evtl. via Dusseldorf and Cologne (see Picture 14). Time delay between the single B-channels, which must be compensated in order to provide a transparent channel of certain data rates, are resulting from that. Now, there are basically two different possibilities to provide a transparent channel with high capacity: the first one is to measure the channels and to adjust once to the analysed time delay, the second one is to check almost uninterruptedly the time delays and to adapt if need be.
For the first method the time delay between the single B-channels will be directly documented after the connection has been set up. It has the advantage that afterwards there won't be any data capacity required anymore for a new documentation, there are according to that n x 64 kbps available. This leads however during a changing of the connection via the network operator to a break down of the transmission.
The second method uses constantly a small part of the data rate in order to almost continuously check which time delay exists. The advantage is that the transmission can be continued also at changing of the connection switch.
The recommendation J.52 that have been developed within the framework of ITU plans for the transmission of ISO MPEG 11172-3 or 11318-3 signals following points:
Compensation of the time delay: the B-channels that belong to an application can be switched through different ways in ISDN and can so show a delay that has to be compensated.
Organization of the B-channels: the receiver must know which B-channels belong to a virtual channel.
