Broadband speeds

Recently a number of faster broadband alternatives have been released in Australia. These include ADSL 2/2+, Wireless (of various types) and faster speeds on ADSL 1 services. All of these services offer bigger “headline” download speed numbers. Unfortunately, these larger numbers do not necessarily translate into a better user experience. It is clear that for many customers the improvement offered by ADSL 2/2+ may be fairly small over the ADSL 1 service it replaces. Furthermore, a service with a much lower “headline” speed may vastly out perform one of these new services. So caveat emptor.

The remainder of this article addresses the technical limits on the performance of broadband connections

Technologies

The following graph compares the speeds of ADSL, ADSL2 and Canopy Wireless technologies at various distances from the exchange or basestation.

ADSL 1

For many years in Australia there has been an artificial restriction on the bit rates allocated to ADSL uploads and downloads. It is for this reason that ADSL was sold in the speeds:

• 256k/64k
• 512k/128k
• 1.5M/256k
• 512k/512k

The best explanation for this restriction that we have heard was that in an inexperienced market this allowed consumers to buy a known quantity. In reality, the lines provided speeds in excess of these limits on some sites and a little less on other sites. If it was significantly less then the service would not be provided.

When the restrictions are lifted if you are in a good position ie. close to an exchange with little noise on the line you will now be allowed to transfer up to 8Mb per second. What you actually get is still limited by congestion, over sell and the ratio of upload and download speeds.

ADSL 2/2+

ADSL 2/2+ is capable of significanty greater speeds than ADSL 1, but these speeds fall off rapidly as the distance from the exchange to your modem increases. This is particularly a problem as wires rarely go in a straight line, people across the road from each other can be on different exchanges or on the same exchange but with 100s of meters of line lengths difference. We know of at least one case where the wires go down one side of a cul-de-sac, around the turning circle and then up the other side!

One unfortunate characteristic of ADSL 2 is that although the gap between ADSL 1 and ADSL 2/2+ speeds diminishes as distance increases. Thus a change from ADSL 1 may not increase the speed much at all.

As there is little experience with ADSL 2/2+ in the Australian market and Australia uses a slightly smaller diameter of telephone line, the impact of noise and Australian cabling standards on theoretical distances are not well understood. Thus where we have used known reliable figures for ADSL 1 in the real world, we are relying on somewhat more rubbery figures for ADSL 2/2+ which may be somewhat overstated.

Wireless

There are a great number of wireless technologies used to deliver Internet. They have an advantage over ADSL in that any distance dependent performance is measured as the crow flies (straight lines from the base station) rather than as wire meanders (around obstacles and through conduits underground). All of these technologies are dependent on the level signal strength and noise in their transmission band. Some of these technologies are dependent on the number of subscribers on a particular base station. A knowledgeable wireless subscriber should be able to discuss issues arising from these considerations and how they are prepared to cope with changes in environment.

Other Technical Traps

Download Speed to Upload Speed Ratio

The majority of data shifted over the Internet uses TCP/IP. This protocol works by dividing your message up into small parts and then transmitting the parts to the receiver. To make sure each part gets to the other end the reciever sends back occasional messages saying which parts have arrived. These ACK messages limit the performance of the sender as an ACK message must be sent for each group of packets sent. Thus download speed depends on upload speed.

The theoretical limit is about 3% for big TCP transfers. Thus for every kb of upload you need at least 3b of download. This is in fact the best case and would only be achieved when moving very large files, such as music or movie files. In practice we use a 10% figure. If you are only doing web then 1/7 is closer to the mark. Thus to support an 8Mb download speed you need in theory 240kbps uplink, but in practice between 400kbps and 1.1Mbps uplink. Interleaving traffic will change these results somewhat but for a single connection they should give a rough guide.

Many of the faster offerings do not have an appropriate ration of upload speed and download speed to support full speed downloads for typical users.

End-to-end speed versus Line Speed

In practice most users really care about how fast they can fetch a file or get an e-mail. Delivering the internet is actually a collection of links in a chain. Improving the strength of a link in the chain only matters if it is the weakest link in the chain. Unfortunately, line speed doesn’t dominate all internet activities.

Factors such as congestion and over sell limit the capacity of Internet transmissions in many cases. In the case of congestion in the service providers’ network improving the speed of its customers ADSL connections could make things slower unless the supplier increases the size of their connection to the Internet. In some cases what happens is that access to services provided by the service provider become super fast (fetching e-mail, for example) but the customers see no other benefit.

Symmetric Services

Applications which produce data for use by other sites, such as remote desktop and webservers, require more upstream speed than downstream speed. Currently this demand is best met with symmetric technologies such as SHDSL or fibre (and some wireless offerings).

Conclusion

At Clarinet we are happy to discuss with our customers about what they need and what they can afford so that they end up with an appropriate solution for their requirements and means. We have been in the business of providing Internet for over a decade and never want our customers to feel cheated when upgrading. We encourage people to look beyond the big numbers in the headlines of adverts to work out what they are really getting and then to determine if they are getting value for money.