GPRS/3G Data Compression

This note describes how one GPRS/3G network provider (Vodafone) compresses images that you download from websites. They do this as standard (via a web proxy server which parses the data for images and re-compresses them) and there appears to be no way to disable it. Many people won't notice this if they are merely browsing websites but in some applications it could make GPRS/3G unusable. Fortunately there is a way around it, using a 3rd party data compression service, but you have to pay for it...

HTML data is hugely inefficient and compressing it would bring data volume savings of perhaps 10:1, and the client application (e.g. web browser) would not see any difference. Unfortunately this requires software at both ends of the link. As a default (no software installed at the client) Vodafone appear to implement image compression only; this works but obviously only by degrading image quality, and it cannot be disabled or controlled in any way. The following images show the effect of the default Vodafone GPRS/3G implementation:

JPEG - original
JPEG - after download via Vodafone GPRS

GIF - original
GIF - after download via Vodafone GPRS

It is unfortunate that Vodafone compress even already-compressed image types like GIF which are often used for images that need to be preserved.

Fortunately there is a way around this, which is to modify the data stream so the Vodafone server cannot recognise the images. There are a number of subscriber services that offer data compression. To implement compression which is transparent to the client, software is required at the client PC. One example is Onspeed which costs £25/year. I've used it for a few months and it works generally well. Because they have control over both ends of the link, they can compress the entire data stream. Their driver offers user-selectable image quality levels, including an "original quality" setting, as shown below.

JPEG - after download via Vodafone GPRS but using Onspeed "original" quality
JPEG - after download via Vodafone GPRS but using Onspeed "lowest" quality

GIF - after download via Vodafone GPRS but using Onspeed "original" quality
GIF - after download via Vodafone GPRS but using Onspeed "lowest" quality

Other ways to avoid this problem would be to set up one's own compressing web proxy, or a compressing+encrypting VPN, etc.

Vodafone's software appears to come from an outfit called Bytemobile which refuses to answer communications, citing contractual obligations. Vodafone's support don't seem to know anything it either but that is only to be expected from that company! The only evidence of it is a folder on a CD which comes with the Vodafone-branded GPRS PCMCIA card, plus a lot of googled press releases saying that Vodafone (and others) have signed up for the Bytemobile technology. On the CD is what looks like a driver which just might implement the client side of a lossless compression system. As there is no real info on it, and no evidence that the image compression is adjustable, I haven't tried it. It is free but is almost certainly not going to work with GPRS/3G providers other than Vodafone, whereas Onspeed works with everything and once one has paid for the login+password one can (technically, anyway) install it on anything.

One drawback of Onspeed is that their server evidently needs to receive quite a bit of data before it decides how it should compress it, so the data flow is very intermittent. This is OK with GPRS/3G where one pays so much per megabyte that one is grateful for anything! But one should disable the Onspeed client program if running on WIFI or some other fast network which is not charged per MB. Another drawback of Onspeed is dreadful "customer service" which is virtually incapable of sorting out any issue, and I did get some problems with them requesting an account prepayment about two years too early.

Page last updated 3rd September 2008

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