IES64702B2 - A method of operating a modem with rate renegotiation - Google Patents

Abstract

In order to mitigate loss of throughput due to unsuccessful fall-forward attempts in a modem implementing rate renegotiation, an initial fall-forward protection interval is set which is progressively increased, preferably geometrically, as successive fall forward attempts fail to increase the data rate.

Description

A METHOD OF OPERATING A MODEM WITH RATE RENEGOTIATION * This invention relates to a method of operating a modem with rate renegotiation.

ITU-T Recommendation V.34 includes a procedure for rate renegotiation during data mode to change to a new data rate, or to resynchronise the receiver without a full retrain, which takes considerably longer to complete.

However, the criteria for initiating a rate renegotiation are not specified in the Recommendation.

The primary use of rate renegotiation is to ensure that the data rate is always matched to the line conditions, to maximise data throughput. The technique for doing this is well known. The signal-to-noise ratio (SNR) is monitored in the receiver, for example by measuring the Euclidean distance between each received QAM point and the constellation point which was actually transmitted.

The supportable data rate can be calculated from the SNR. A rate renegotiation is initiated if this analysis indicates that either the SNR is good enough for a higher data rate to be supported, or that it has dropped sufficiently to make a lower data rate more efficient in terms of throughput. The actual thresholds depend on the modem receiver performance and the error correction protocol, if any, being used.

The range of available data rates is determined from capability information exchanged by the two modems * « during handshaking. A rate renegotiation should only be initiated if this information indicates that a more suitable data rate than the current one is available.

This prevents time being wasted attempting to 964702 renegotiate when the current data rate is the most suitable one available.

Consider a modem where the SNR is continuously monitored and a protection interval between rate renegotiations is implemented. If such a modem is used in a symmetric data rate environment, and the line conditions seen by the modems, or the modems.' receiver performance, differs sufficiently, the modems will have different maximum supportable receive bit rates. The restriction of symmetric data rates means that one modem is forced to operate below its maximum supportable rate.

The rate renegotiation control mechanism will therefore repeatedly initiate rate renegotiations to attempt to increase the data rate, within the known range of available rates. The other modem, however, will limit the data rate to its maximum supportable rate each time, and the result of the renegotiation will be a connection at the same rate as before. The result of this situation is a drop in throughput, caused by repeated unsuccessful rate renegotiations.

It is therefore an object of this invention to provide a method to mitigate loss of throughput due to unsuccessful fall-forward attempts in modems implementing rate renegotiation.

Accordingly, the present invention provides a method of operating a modem with data rate renegotiation, in which an initial fall-forward protection interval is set which is progressively increased as successive fall forward attempts fail to increase the data rate.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein: ·*· Figure 1 is a functional block diagram of a modem receiver which implements rate renegotiation, and *' Figure 2 is a flow diagram of an embodiment of a method according to the invention.

The modem receiver shown in figure 1 is implemented, using conventional technology, primarily by a microprocessor 10 controlled by software in ROM (so-called firmware), and thus the blocks shown therein relate to the functions performed by the modem rather than to discrete circuit blocks. However, the modem could be implemented entirely in hardware or a mixture of hardware and software.

The incoming analog signal is digitised by an analog to digital (A/D) converter 12 (which in this embodiment is implemented in a separate integrated circuit) and the digitised signal passed to the microprocessor 10 which subjects the signal to automatic gain control and filtering processes 14 and 16. The microprocessor also includes timing recovery software 18 which controls the sample timing of the A/D converter 12.

The filtered signal is fed to an equaliser 20 followed by carrier recovery software 17, whose output is a received point having two components. Decision software 22 then decides which constellation point is the closest match to the received point. The constellation point * indicates an encoded signal which is then decoded at 24 to produce the originally transmitted data. The signal . * at the output of the carrier recovery 17 and at the input to the decoder 24 are used to determine the signal-to-noise ratio in SNR software 26. Rate renegotiation software 28 attempts to negotiate a new rate when the SNR software 26 determines that a new data rate is appropriate.

Thus far the operation of the modem corresponds to the prior art and, where a fixed protection· interval is used, it suffers from a drop in throughput caused by repeated unsuccessful rate renegotiations, as described above. Accordingly, in the embodiment of the present invention the following method is used, which is implemented by the firmware in the modem. ft The modem first sets an initial fall-forward protection interval, that is, the minimum period of time between fall-forward attempts. The modem checks the SNR at fixed intervals and decides whether a rate renegotiation should be initiated or not. When the SNR and exchanged capability information indicate that a higher data rate is available and supportable, a rate renegotiation is initiated (provided the fall-forward protection interval has elapsed) in an attempt to fall forward. If the renegotiation results in a connection at the same data rate, because the other modem's receiver could not support a higher rate, the protection interval between fall forward rate renegotiations is increased. A geometric progression may be used, so that the time lost in unsuccessful fall-forward attempts rapidly becomes insignificant, and the effect on the throughput is minimised.

On the other hand, if the SNR drops, and a reduction in data rate is required, a rate renegotiation is initiated immediately, and the fall-forward protection interval is reset to its initial value. This means that the modem can react quickly to a degradation in line quality. Similarly, the protection interval is reset if a renegotiation results in an upward change in data rate. v The result of this process is that the negative effect of unsuccessful fall-forward attempts on throughput is t reduced, without any loss of flexibility in the case of changing line conditions.

The method according to the embodiment of the invention is shown in more detail in the form of a flow diagram in figure 2.

An initial fall-forward protection interval is set at step 30 and the SNR and supportable data rates checked at step 32. Assuming that neither a fallback or fallforward are indicated, as determined by steps 34 and 36, the process simply loops back from step 36 to step 32 to await the next SNR check. However, if a fallforward is indicated at step 36, a check is made at step 38 to see if the fall-forward protection interval has elapsed. If not, the process again loops back to step 32 to await the next SNR check. On the other hand, if the check at step 38 determines that the fall-forward protection interval has elapsed, then data rate renegotiation is initiated at step 40.

If it is determined at step 42 that the data rate has not changed as a result of the renegotiation, then the fall-forward protection interval is increased at step 44 and the process loops back to step 32 to await the next SNR check. Thereafter, each time the process passes directly down steps 32 to 44 of the flow diagram, * indicating successive unsuccessful attempts to increase the data rate, the fall-forward protection interval / progressively increases, preferably geometrically.

However, if at any time it is determined at step 42 that the data rate has increased as a result of the renegotiation, then the process loops back to step 30 and the fall-forward protection interval is reset at its initial value. Likewise, the fall-forward protection interval is reset at its initial value if step 34 indicates that fallback is required, after initiating a lower rate renegotiation at step 46.

Claims (5)

CLAIMS : V

1. A method of operating a modem with data rate £ renegotiation, in which an initial fall-forward protection interval is set which is progressively increased as successive fall forward attempts fail to increase the data rate.

2. . A method of operating a modem with data rate renegotiation, comprising the steps of: (a) setting an initial fall-forward protection interval, (b) checking the signal-to-noise ratio at intervals to determine if data rate renegotiation is indicated for the modem, (c) if fallforward is indicated as a result of a Check carried out at step (b) and the fall-forward protection interval has elapsed, initiating data rate renegotiation, (d) if no change in the data rate results from the renegotiation at step (c) , increasing the fall-forward protection interval, and (e) repeating steps (c) and (d) and this step (e) with the increased fall-forward protection interval.

3. A method as claimed in claim 2, wherein if the ft result of the renegotiation at step (c) results in an increase in the data rate, the fall-forward protection interval is reset to the initial value.

4. A method as claimed in claim 2 or 3, wherein if fallback is indicated as a result of a check carried out at step (b), data rate renegotiation is initiated and the fall-forward protection interval is reset to the initial value.

5. A method of operating a modem with data rate renegotiation substantially as hereinbefore described with reference to the accompanying drawings.