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All access points are one to Extricom mobile clients
It is rare for someone in the Wifi industry to talk honestly about the snags as well as the advantages of the technology. They will boast of the range of the latest 11n links but omit to point out that doubling the range quadruples the area of contention (and that is not counting interference between networks in the vertical plane - ie on different floors).
They will tell you that the speed is 300Mbits/sec without pointing out that much of that goes on network overheads, and that the top speed anyway depends on bonding two channels which you are not supposed to do if there are other networks in range. In cities, of course, there are always networks in range so the only way you can get top speed is by playing road hog.
"People don't notice the problems because Wifi, particularly 11n, is so resilient. All they see is a drop in performance," said Phil Belanger, who worked on the 11n spec.
Belanger can talk about these things because his company, Extricom, offers a technology that addresses problems associated with Wifi networks that use multiple access points. There is apparently no standard way of setting these up - the Wifi specs do not cover it. But the usual way is to site access points to give minimal overlap in their coverage and ensure that neighbouring ones use a different channel.
There are some problems with this. The signal strength is erratic and tails off with distance, and 11n performance can be hit badly if the system has to cope with legacy 11a/b/c devices, which will inevitably be present on large public or campus sites. Also there can be an appreciable hiatus when a user crosses from one access-point area to another.
Extricom's approach is to connect all access points to a smart switch and present them to the roaming clients as a single device. The switch can decide, packet by packet, which access point to draw the data from; this can also help it decide which access point to use to send data. There is no channel contention because all access points use the same channel. To use the jargon: Extricom separates the MAC and PHY layers, putting the former into the switch.
There are no boundary problems within the network area because there are no boundaries. Moreover, using a system called Channel Blanketing, the overheads of coping with multiple protocols can be avoided by assigning a different channel to each of the different WiFi flavours: one for 11b/g at 2.4GHz, one for 11n at 2.4GHz, one for 11a at 5GHz and another at 11n at 5GHz (click in image to see larger version).
The latest development, announced at the Wireless and Mobile 09 show in London yesterday, is a cascading switch that allows you to double up two Extricom networks, each with 16 access points. This can be either to double the coverage area, or to provide resilience in places like hospitals where the network cannot be allowed to go down.
If Extricom has to go through all this trouble to avoid problems campus sites where access points can be carefully sited and managed, there must surely be questions about how 11n will scale up in crowded cities and blocks of flats where the siting and use of Wifi equipment is chaotic and people are being encouraged to throw HD video around.
But at least we are finally going to get a firm spec. Belanger tells me the one for 11n will be finalised this September, and that all current Draft 11n products should be compatible with it.
They will tell you that the speed is 300Mbits/sec without pointing out that much of that goes on network overheads, and that the top speed anyway depends on bonding two channels which you are not supposed to do if there are other networks in range. In cities, of course, there are always networks in range so the only way you can get top speed is by playing road hog.
"People don't notice the problems because Wifi, particularly 11n, is so resilient. All they see is a drop in performance," said Phil Belanger, who worked on the 11n spec.
Belanger can talk about these things because his company, Extricom, offers a technology that addresses problems associated with Wifi networks that use multiple access points. There is apparently no standard way of setting these up - the Wifi specs do not cover it. But the usual way is to site access points to give minimal overlap in their coverage and ensure that neighbouring ones use a different channel.
There are some problems with this. The signal strength is erratic and tails off with distance, and 11n performance can be hit badly if the system has to cope with legacy 11a/b/c devices, which will inevitably be present on large public or campus sites. Also there can be an appreciable hiatus when a user crosses from one access-point area to another.
Extricom's approach is to connect all access points to a smart switch and present them to the roaming clients as a single device. The switch can decide, packet by packet, which access point to draw the data from; this can also help it decide which access point to use to send data. There is no channel contention because all access points use the same channel. To use the jargon: Extricom separates the MAC and PHY layers, putting the former into the switch.
There are no boundary problems within the network area because there are no boundaries. Moreover, using a system called Channel Blanketing, the overheads of coping with multiple protocols can be avoided by assigning a different channel to each of the different WiFi flavours: one for 11b/g at 2.4GHz, one for 11n at 2.4GHz, one for 11a at 5GHz and another at 11n at 5GHz (click in image to see larger version).The latest development, announced at the Wireless and Mobile 09 show in London yesterday, is a cascading switch that allows you to double up two Extricom networks, each with 16 access points. This can be either to double the coverage area, or to provide resilience in places like hospitals where the network cannot be allowed to go down.
If Extricom has to go through all this trouble to avoid problems campus sites where access points can be carefully sited and managed, there must surely be questions about how 11n will scale up in crowded cities and blocks of flats where the siting and use of Wifi equipment is chaotic and people are being encouraged to throw HD video around.
But at least we are finally going to get a firm spec. Belanger tells me the one for 11n will be finalised this September, and that all current Draft 11n products should be compatible with it.
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