Wireless Connections
Overview of Wireless VoIP
Wireless networking is becoming an easy and economcal alternative to running wires around your home or office. It also opens up new possibilities for connecting buildings which are up to several kilometres apart. There are currently three standards upon which wireless networking devices are built. The table summarises some of the features for each.
Standard Data Rate Frequency
802.11a 54 Mbps 5.8GHz
802.11b 11/22 Mbps 2.4GHz
802.11g 54 Mbps 2.4GHz
Comments
802.11a - Poor obstacle penetration at this frequency limits its usefulness. Larger number of channels and short range makes this standard suitable for high cell densities.
802.11b - The first system to appear at mass-market pricing. Suitable for both internal and inter-building applications.
802.11g - New standard promising the advantages of 802.11b at higher data rates
The following discussion applies mainly to the 802.11b standard.
Range
You will see specifications for different brands of wireless networking devices quoting wildly different ranges. Take these claims with a pinch of salt. Unless the manufacturers have got something very wrong, or are operating at illegal power output levels, then products from different sources will all behave similarly if they are built on the same standard.
There are two types of application in which wireless networking is used: internal and inter-building.
Indoors
Radio waves travel in straight lines and at 2.4GHz and above do not penetrate obstacles very well. Some surfaces reflect the signals quite well whilst others tend to absorb them. Water, which comprises most of you, is particularly good at absorbing the energy, so you will find that putting your hand over an antenna can reduce the signal substantially. (Your hand won't warm up very much because output power is limited to 66mW in Europe.)
As a general rule 802.11b devices will usually cover a house quite well but there are no guarantees. The signal passes better through wooden floors and ceilings than through brick walls, and has no chance at all through concrete or stone. The use of an access point in the loft connected to a directional antenna pointing down from the rafters has proved an effective way to get full coverage in a typical house. For a more restrictive range the built-in antennas often work very well.
Your choice of wireless network adapter may be significant. If your mini tower PC stands on the floor with it's back to a radiator, you can't a expect built-in adapter with integral aerial to work very well. Or if you tend to sit with your notebooks aerial sticking out of the left hand side and your access point is down the corridor to your right, then the computer itself will screen the signal. Try an adapter with an external aerial that can see over your keyboard.
Outdoors
With line of site, it is said that distances of around 20kms are achievable. It must be remembered that atmospheric/weather conditions can greatly affect the quality of operation.
You should ensure that you do not exceed any legal power density limits which apply in your region.
External antennas make good lightning conductors! Consider what equipment you are putting at risk if you choose not to invest in a lightning arrester.
Antenna Gain
Q. How does an antenna produce gain?
A. By focusing the available radio energy in one direction.
Q. OK, so how can an 'omni-directional' antenna have gain?
A. Because it radiates around itself in a disc pattern, stealing power from above and below.
The above Q&As should help you get a feel for the radiation pattern for different types of antennas. A 0dB antenna radiates equally over a complete sphere. Bearing in mind that 3dB represents a doubling of radiated power, you could imagine a 3dB directional antenna radiating its signal into one half of that sphere. A 3dB omni-directional antenna would have a radiation pattern in the shape of a sphere with a cone removed from the top and bottom.
Diversity Receivers
The wavelength of 2.4GHz radio waves is about 12 cm. Because the signals reflect readily off many surfaces, there will often be a pattern of patches around the room where reflected signals cancel out the direct signals leading to 'dead zones'. If your antenna is in such a dead zone you get no signal. However, for every dead zone there will be a 'double-power' zone. Wouldn't it be nice if you could have a second antenna, just a few centimetres away? There would be a good chance of this second antenna sitting in a better reception zone.
A system with two aerials in this arrangement is called a diversity receiver. Most wireless devices are diversity receivers with the notable exception of PCI cards. If you need to use a PCI card, an external antenna, which you can move around to get a good signal is a good idea.
Avaya IP Office Wireless
Leveraging the reliable technology from SpectraLink, a leader in wireless voice solutions for the workplace, the Avaya IP Wireless Solution offers an advanced voice over IP (VoIP) client for wireless networks.
This solution allows SME's to take advantage of the cost savings and simplified management of a converged voice and data infrastructure.
Both the 3616 and 3626 phones are optimized for Avaya IP telephony and emulate the wired 4606 IP Telephone. They work in conjunction with the Avaya Voice Priority Processor to ensure voice quality over Wireless LANs. They are field upgradeable through an integrated TFTP client, so handsets can be updated with new protocols, features, and capabilities as they become available.
Based on global standards for wireless LAN's, the Avaya IP Wireless Telephone Solution simplifies network infrastructure by enabling voice traffic to be carried along with data traffic over the same wireless network. Both the 3616 and 3626 telephones are available for direct sequence 802.11b Wi-Fi networks.
The SpectraLink Voice Priority (SVP) quality of service protocol is simple to implement and reduces packet queuing delays for voice traffic. SpectraLink Voice Priority-enabled access points are available from the leading providers of enterprise wireless networks such as Avaya.
Users can have a choice of an executive or rugged workplace telephone and all the productivity benefits of their desk telephone in this next generation of wireless telephone solutions.