The jury might be still out on the future of 802.11a versus .11g, but our piece on How to choose between them should help you decide where to put your resources. To back that up, here's a technical comparison.
Life was simple when wireless LANs first appeared. 802.11b, 2.4GHz, 11 Mbit/s. That was it - take it or leave it. And we took it.
But going from a switched LAN of 100 Mbit/s to a shared portion of an 11 Mbit/s CSMA/CD network, which in itself is only capable of a maximum throughput of about 6 Mbit/s, was seen as a major limitation. So now we have 802.11a and 802.11g, both of which offer 54 Mbit/s speeds, in addition to 802.11b.
.11a appeared first and, because it uses a different frequency range, took longer for to get ETSI approval for use in Europe. Some kit that you can readily buy abroad still isn’t legal here. And what is more, often has to have the power ramped down for use. Although .11a is supposedly less susceptible to interference, this hasn't in fact proved to be of any great significance.
In some ways .11a was a stopgap till.11g gave us 54 Mbit/s in the 2.4 GHz frequency band. However, though .11g is backwardly compatible, all your .11g throughput will drop if you introduce any .11b clients into a .11g WLAN.
If you want to use .11g, you need to migrate everyone up, but you won't have to resite your access points (.11g has the same range as .11b, .11a does not) and you won't need a new site survey.
Don’t Forget .11b
While 802.11b’s dominant role in WLANs is fast receding, the technology is by no means dead. Look for .11b in low powered devices such as tablets, scanners and cameras. And phones - not just pure IP Telephony ones, either. Chip manufacturers are already producing .11b chips that support GSM. The technology to have a mobile phone that switches to .11b when you walk into your building has been available for a couple of years now. The issue is in the licensing, since you’d have to be a registered Telco with partnership deals with the GSM operator, but hopefully that is a problem that can be solved. In any case, the future’s still bright for .11b.
Speeds and Feeds
802.11b and .11g both operate in the 2.4GHz frequency range: .11a works in the 5GHz range. One of the features of this difference is that the range covered by a .11g access point can be up to twice that of a .11a one for similar speeds. Vendors’ figures quote distances of 90 feet for 54Mbps for a .11g WLAN, while the same tests gave 45 feet using a .11a access point. It should be noted that these numbers are for APs rated for use in the USA, so complying with FCC regulations (and therefore capable of transmitting at higher powers than European ETSI standards allow).
Remember that all flavours of 802.11 use shared Ethernet. While the wired network has pretty much moved to switched Ethernet, an access point is shared like a hub. So the speeds quoted aren’t what you’ll get in terms of usable data throughput. You’ll tend to find that an 11Mbps .11b WLAN will give you about 6Mbps, while the 54Mbps of 802.11g and .11a work out at about 22Mbps and 25Mbps respectively. And if you add .11b clients to a .11g network, this number drops dramatically. In this instance, the access points start to use a form of RTS/CTS flow control to allow .11b clients to operate without being aware of the .11g network. This extra overhead means that you might only get in the region of 8-10Mbps throughput for your co-existing .11g clients.
The speed you get will depend on signal strength, so obviously on distance from the AP. There are set speeds - the client will automatically downshift to the best possible. For .11b, this was 11, 5.5, 2 or 1Mbps. .11a and .11g clients will select between 54, 48, 36, 24, 18, 12, 9 or 6Mbps.
The other main operational difference between the specifications is in the number of simultaneous channels supported. 802.11b/g support three (you have more than that to choose from, but only three non-overlapping at a time), while .11a supports eight now and the specification is written to allow 12.
This has a pretty big impact on usable capacity and is probably the biggest thing in .11a’s favour. Assuming for the sake of argument the usable throughput of both .11a and .11g is 25Mbps, this gives a .11a AP an aggregate throughput of 200Mbps, compared to just 75Mbps with a .11g AP.
So to summarise, 802.11g has decent throughput, good range, but might be a bit limited in terms of capacity. 802.11a has the same throughput, more or less, the range is quite a bit less, but the capacity is significantly better. And 802.11b really doesn’t have anything going for it in terms of PC clients, but still has a future for other devices. .11a had a bit of an edge while we were waiting for the .11g standard to be approved over here, but now it is, is the increased capacity of .11a enough to keep it in the running?
Oh, and if that’s still a bit confusing, the IEEE 802.11 working group are looking at a new standard, 802.11n, which if ratified, would offer higher speeds and/or greater distances. You wouldn’t see any hardware for a couple of years but it just goes to show we haven’t reached the end of the line for WLANs yet.
What do you think? Join the discussion about 802.11a and 802.11g in our forum.
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