Wireless deployment at enterprise level
Deb Shinder
Published: 14 Sep 2006 15:40 BST
…for most organisations, so the ability to make telephone calls over wireless LANs requires high levels of availability and bandwidth capacity.
One of the biggest obstacles to implementing VoWLAN is bandwidth. An 11 Mbps 802.11b WLAN doesn't have much bandwidth to spare. But as wireless technologies increase in speed (54 Mbps for standard 802.11a and g, and with the 802.11n standards, expected to be completed in 2007, providing theoretical transfer speeds over 500 Mbps and estimated actual throughput of 100 — 180 Mbps), VoWLAN becomes a much more attractive proposition. You can read about 802.11n here.
One implementation of VoWLAN that may be coming into its own soon is UMA (Unlicensed Mobile Access). Mobile phones are being tested by some companies that allow GSM and GPRS mobile phones to operate over 802.11 Wi-Fi networks. With dual-mode telephone handsets, users can roam between the cellular network and unlicensed Wi-Fi networks — both public hot spots and private WLANs. You can read more about UMA here.
WLAN topology in the enterprise
It's important to remember that wireless networking components are actually radio transmitters and receivers that use specific frequencies and channels within those frequencies. For example, the standard for 802.11b defines 14 channels. 802.11b is said to operate at the 2.4GHz frequency, but within that frequency range you could have different WAPs with one operating on channel 1, at 2.412 GHz, and another on channel 2 at 2.417 GHz and so forth.
The FCC allows the use of channels 1 to 11 in the US, whereas in Europe you can use channels 1 to13. The problem is that there's overlap of signal, so you can't use channels 1 and 2 within the distance range of 802.11b because they would interfere with each other. Due to the overlap, only three of the 11 channels can be used at the same location (channels 1, 6 and 11). This means you have to plan carefully if you need more than three access points. Channels need to be assigned so that there's little or (ideally) no overlap of signal.
Thus large multiple AP WLANs are usually designed in a "cellular" topology, where the APs are located to create coverage in much the same way mobile-phone towers are distributed. The APs that use the same channel have to be far enough away from one another to avoid interference.
There are a number of disadvantages to this type of architecture, including latency when communications are "handed off" from one AP to another on a different channel when users are mobile, and collisions when there are large numbers of users on the WLAN who transmit at the same time.
There are alternatives to the cell-based topology, such as the "channel blanket" topology offered by Extricom, which uses UltraThin access points controlled by a central switch that directs all of the wireless traffic. You can find out more about their solution here.
Summary
Wireless networking at the enterprise level is different — but there are many products and services out there that make it easier to implement one or more WLANs in a large organisation, which can support high-bandwidth applications such as VoWLAN.
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