Data transmission

Abstract

A data transmission system (10) is provided for transmitting a data signal between a server (12, 14, 16, 18) and a communications terminal (20, 22) over a transmission link having a variable bandwidth. The data signal is a scalable compressed multimedia clip comprising a sequence of images. Each image has a base layer and a number of enhancement layers. To cope with variability in the bandwidth, the sequence is re-ordered so that the base layers have greater safety times than the enhancement layers. This allows all of the base layers to arrive in time to be played at the expense of the enhancement layers.

Description

FIELD OF THE INVENTION [0001] This invention relates to data transmission and is particularly, but not exclusively, related to transmission of data representative of picture sequences, such as video. It is particularly suited to transmission over links susceptible to errors and loss of data. BACKGROUND OF THE INVENTION [0002] During the past few years, the amount of multimedia content available through the Internet has increased considerably. Since data delivery rates to mobile terminals are becoming high enough to enable such terminals to be able to retrieve multimedia content, it is becoming desirable to enable mobile terminals to retrieve video and other multimedia content from the Internet. An example of a high-speed data delivery system is the upcoming GSM phase 2+. [0003] The term multimedia as used herein applies to both sound and pictures, to sound only and to pictures only. Sound may include speech and music. [0004] Network traffic through the Internet is based on a transpo...

AI Technical Summary

Benefits of technology

[0059] An advantage of the invention is that it can provide larger safety times than prior art arrangements with the same initial buffering. In addition, it provides some degree of protection against decreased transmission throughput. Therefore it helps in reducing the occurrence of pauses without sacrificing compression efficiency.

[0060] The invention also helps in transmitting over a unreliable network connection using transmission protocols which do not automatically retransmit but only retransmit those missing parts chosen by a higher level function, for example in response to feedback. In this case, it provides more time for retransmission of the vital parts of a data signal than prior art arrangements. Thus, vital parts of a stream are more likely to be transmitted successfully and the ultimate end user of the data signal is likely to perceive a better output quality.

[0061] Preferably the bandwidth of the transmission link available to the data signal is variable. This may be because the amount of data traffic using the link is variable, for example due to the need to re-transmit lost or corrupted data.

[0062] The invention provides a way of altering a data signal so that it can be transmitted over a transmission link having a bandwidth which would otherwise not be sufficient for it or for allowing certain of the bandwidth of its transmission link to be made available for other data, for example control information.

[0063] Preferably the effect of the invention is to increase the safety times of some data units relative to, or even at the expense of, the safety times of other data units.

[0064] Preferably the data signal is arranged in a hierarchy having a base layer and at least one enhancement layer. The term base layer means a most fundamental layer. The base layer may represent the least refined version of the data signal. In each unit, the (or each) enhancement layer, when combined with the base layer, refines the base layer to make it more like the data unit. Each enhancement layer may add more refinement to the base layer. Preferably the safety time of the base layer is increased relative to the safety times of the enhancement layers.

Problems solved by technology

Since no resources are permanently committed within the gateways to any particular connection, the gateways may occasionally have to discard datagrams because of lack of buffer space or other resources.

Thus, the delivery service offered by IP is a best effort service rather than a guaranteed service.

Otherwise, degradation of IP datagrams will in turn affect UDP datagrams.

However, the client must be able to recover from packet losses and possibly conceal lost content.

Even with reconstruction and concealment, the quality of a reconstructed clip suffers somewhat.

Firewalls, whether in a company or elsewhere, may forbid the usage of UDP because it is connectionless.

However, if the IP layer loses the outgoing segment or the return acknowledgement, the timer at the sending end will expire.

Now, if the sender waited for an acknowledgement for each packet before sending the next one, the overall transmission time would be relatively long and dependent on the round-trip delay between the sender and the receiver.

Typically the storage space taken up by raw (uncompressed) multimedia data is huge.

This can even cause pauses in the playback of a multimedia stream whilst lost or corrupted data are retransmitted.

Pauses in multimedia playback are annoying.

However, the server may not react to network congestion sufficiently quickly to avoid pausing in the client.

In addition, the server cannot control the retransmission mechanism of TCP (or other underlying protocols, such as IP).

If the maximum throughput of the channel is equal to the average bit rate of the multimedia clip, the client cannot recover from a drop in the amount of received bits after throughput has dropped.

Since this type of server has no control over the contents of the stream, no flow (bandwidth) control can be applied, and it cannot respond to network congestion.

Therefore, sudden pauses in the playback can occur.

Consequently there must be a relatively long initial buffering delay in the client before starting the playback to avoid such sudden pauses.

Fast-forwarding a multimedia stream from a standard WWW server is not possible.

However, asymmetric air interface connections are applicable only in non-transparent mode (see below).

The transmitted data are likely to contain bit inversion errors.

There are two sources of error, packet drops and corruption due to interference in the radio frequency path.

Consequently, a non-transparent connection is error-free but throughput and transmission delay vary.

Reducing the temporal redundancy reduces the amount of data required to represent a particular image sequence and thus compresses the data.

Simply reducing the redundancy of the sequence does not usually compress it enough.

Therefore, some video encoders try to reduce the quality of those parts of a video sequence which are subjectively the least important.

It should be noted that while B-pictures provide better compression performance than P-pictures, they are more complex to construct and require more memory.

Furthermore they introduce additional delays because bi-directional interpolation requires both reference pictures to have been received and additional calculations are required.

Generally, scalable multimedia coding suffers from a worse compression efficiency than non-scalable coding.

In some cases, when reference layer pictures are poorly predicted, over-coding of static parts of the picture can occur in the enhancement layer, causing an unnecessarily excessive bit rate.

Images