Is Your Wifi Network Ready for ADSL2+?

In lots of places 1 MBit/s ADSL connections are almost a thing of the past already and telcos are starting to offer fast ADSL2+ connections with speeds up to 24 MBit/s. These speeds are quite challenging for todays home networks for two reasons. First of all, the TCP parameters of PCs and notebooks in the network have to be tweaked. Otherwise, a single connection to a server on the Internet will level out at 5 MBit/s with the standard TCP window settings of Windows XP. More about TCP windows and how to adapt them can be found in this blog entry. Secondly, Wifi access points and Wifi adapters in PCs and notebooks used to connect to the ADSL line are also not always capable of transferring data at such high speeds.

If you still use either 802.11b access points or wireless network adapters in the PC/notebook, think about replacing this kit when getting a faster ADSL line. The maximum speed that can be reached in a 802.11b network is around 5 to 6 MBit/s. This is far too slow for an ADSL2+ connection.

Most people these days probably have an 802.11g wifi access point which in many cases has a built in ADSL or ADSL2+ DSL modem. But even here, with theoretical speeds of 54 MBit/s, the air gets "virtually" thin. Due to the way data is sent and received in a Wifi networks, actual data rates in 11g networks are much less than the advertised 54 MBit/s.

To see how well the combination of my wifi access points and devices in work together I used iperf to generate traffic on my wireless network. Iperf is a free program and can be found here. To simulate a download from the Internet one of the computers used in the tests was connected to the wifi access point via a 100 MBit/s Ethernet cable. Like when receiving data from the internet, packets are thus only sent once over the wireless link. Here are my results:

Netgear DG834GB access point and Intel Centrino notebook with 802.11g chipset: This is the fastest possible setup as both access point and client device are 11g compatible. No other devices were present in the network for the test. With packet bursting activated, the maximum transmission speed was 20.7 MBit/s. Not bad for a wireless link but not enough if you should be lucky enough to get the best possible ADSL2+ speed of 24 MBit/s.

Same setup as above with an additional Intel Centrino notebook with 802.11b chipset: As soon as 11g devices detect a device which still uses the older 11b standard, additional safeguards are automatically activated for peaceful coexistence of 11b and 11g devices in the same network. For details on how this co-existence works, take a look at the book on the left (sorry, shameless self advertisement). In this mode, the maximum data rate I got on the 802.11g notebook was 12 MBit/s. This is already far from the 24 MBit/s of a good ADSL2+ line. Also beware of neighbors using old 11b access points or devices on the same frequency band used by your network as the effect will be the same.

Shame on the Siemens SE515DSL

I repeated the same tests using my Siemens SE515DSL access point with the latest router firmware. No matter what I tried I was never able to get more than 12 MBit/s out of this router. Even switching to 11g only mode on the router and the notebook brought no improvements. Quite a frustrating experience.


802.11g reaches its limits quickly with new ADSL 2+ lines. Should you be one of the happy few to get the full 24 MBit/s out of your ADSL2+ line, you might want to think about other wireless network alternatives. Unfortunately, the long awaited 802.11n standard, which promises speeds of 100 MBit/s and more, is still not finalized. This blog entry describes the situation back in October 2006. Pre-N devices with Mimo (Multiple Input Multiple Output) technology are already on the market for quite some time now, but might not be to everybody’s taste due to a lack of compatibility between different products and bulky PC cards sticking out of notebooks.

The next generation of even faster Internet connections at home and office like for example VDSL with 50+ MBit/s per second and fiber are already on the horizon. Also, video streaming at home over wifi which requires high bandwidths is also getting more and more main-stream. Let’s hope 802.11n will be finished soon in order to leave notebook manufacturers, chipset vendors and the market enough time to put inter-operable devices into the homes and offices before wireless home networks become the bottleneck.