Multiplex communications – Wide area network – Packet switching
Patent
1989-09-01
1992-08-04
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 24, 370 29, H04B 320, H04L 516
Patent
active
051365766
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to transmission systems in which signals are transmitted over a path which is wholly or partially common to both directions of transmission between two stations. Such is the case, for example, where a two-wire telephone channel is employed.
Often it happens that, for some unknown time period TP, Communication is required primarily in the direction from one station to the other, but during this period it must be possible for the latter station to transmit short messages back to the former. It is often desired e.g. for reporting errors in forward transmission, that such messages should be transmitted as quickly as possible. Commonly, too, it is desired to maximise the data rate (in a digital system) or available bandwidth (in an analogue system) in the forward direction.
Frequency division duplex systems such as the CCITT Group II facsimile transmission standard in which a small portion of spectrum is reserved for reverse signalling inherently involve a reduction of forward channel capacity and a slow return channel, as well as stringent filtering requirements, whilst echo cancelling duplex systems (e.g. CCITT V32) provide rapid return channel signalling but reduce the maximum forward signal to noise ratio (and hence data rate) due to uncancelled talker echo (particularly in the presence of phase roll).
According to one aspect of the present invention there is provided a method of transmission between first and second stations via a transmission link in at least part of which transmissions in two directions travel over a common path, transmissions by the second station occupying a bandwidth which is the same as or at least partially overlaps the bandwidth occupied by transmissions by the first station, in which the first station includes an echo canceller and in which simultaneous transmission by the stations occurs only during periods which immediately follow a period of transmission by the first station alone and are of short duration relative to the rate of change of the echo characteristics of the path. The bandwidth overlap referred to may be intentional, or may be the result of imperfect filtering in a frequency division duplex system.
This aspect of the invention is based on the realisation that, following a period of transmission by the first station alone, the station's echo canceller can be well adapted to the characteristics of the link and, initially, by the first station good quality reception of transmissions by the second station can therefore, be achieved. Because the first station's echo canceller cannot adapt so readily to variations in transmission path characteristics (indeed, in most cases its state would be "frozen" or switched to a slow adapting condition) this situation does not persist. If, however, the time for which the second station is permitted to transmit is sufficiently brief, this is not a problem. Thus, over a given path, a higher data rate/signal to noise ratio can be achieved than would be possible with full duplex.
Of course, assuming that the forward data rate (or required signal-to-noise ratio) is greater than that which would be possible using full duplex, it is to be expected that the forward transmission by the first station will be corrupted by the second station's transmission, since the latter, having been silent, will not have available a well-trained echo canceller. For many applications, however--for example for error reports by the second station, or where the second station desires to initiate a reversal of the primary transmission direction--this is unimportant since the corrupted transmission either can be repeated, or is unwanted.
Thus the invention can be of benefit even in cases where variation of the transmission path characteristics is not a problem and in the second aspect of the invention there is provided a method of transmission between first and second stations via a transmission link in at least part of which transmissions in two directions travel over a common path, transmissions by the second station occupying a ba
REFERENCES:
patent: 3404219 (1968-10-01), Couturier
patent: 4841521 (1989-06-01), Amada et al.
British Telecommunications public limited company
Chin Wellington
Olms Douglas W.
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