Some Wireless Design Specifics


Background.  While this is a difficult topic to get a handle on, and particularly difficult to find good sources, we will present at least some aspects and issues of wireless design.  This will be done largely in different segments.

One of the most basic issues has to do with how to assess whether a design is good.  Think about what you would want.

  • accessibility/coverage
  • good signal
  • lack of interference
  • mobility

These are probably the major attributes that users want.  Unfortunately, in the academic literature these are not often the goals of the design.  But it seems in reality these requisites should seem like common sense.  We will use these as our goals.

How Much Bandwidth Really?  I have constantly been concerned about representations of available bandwidth in all kinds of situations.  We all hear the cable and phone companies touting their broadband connections, yet users never seem to get anywhere near the download speeds they expect if they listen to the amount of total bandwidth these companies advertise.  Users almost always get channels which are much less than the bandwidth of the entire link.

This also seems to definitely be true for wireless connectivity.  But, I still have yet to see this clearly in writing.

Think of the 11Mbps that is advertised for 802.11b wireless standard.  First off you know that you need to use some of this capacity for sending and some for receiving.  Based on this some authors state that one should automatically half the bandwidth capacity of 11Mbps to 5.5Mbps.  In contrast think about the wire based switched connections for Fast Ethernet where you can essentially get 100Mbps for sending and another 100Mbps for receiving due to the switching and the wires in each direction.

But this situation becomes even more challenging because as you might expect wireless signals are channelized.  The number of channels that are configured based on the 802.11 standards are in the following table.  Though I have yet to fill in all of the entries.



Wireless Standard Theoretical Capacity IEEE Channels Non-Overlapping Width      
802.11b 11Mbps 14 3 22 MHz      
802.11a 54Mbps   8 20 MHz      
802.11g 54Mbps            


It is also the case that each "radio transceiver" on an AP is set to a particular channel/frequency.  As we will also see, it is important to make sure that different APs are configured for particular channel.  So this implies that the capacity of any given AP is really much less than the theoretical capacity.  I find these sorts of things disturbing in that it seems that the information we are given isn't as accurate as it should be.  It is often very hard to get realistic specifications!

Channels.  So now that we have possibly dismayed you about some of the differences between reality and advertising, we still need to deal with reality.  Most authors, at least those that I respect state things like the following.

User groups on separate access points within a wireless LAN must be supported on separate and distinct channels.  This is even more true for places where there is overlap between different APs.

We need some additional concepts.

adjacent channel spacing or active channel separation - this is the amount of space between contiguous or active channels

from a design perspective the quality and reliability of a wireless network is best when channels assigned to access points that overlap are selected from opposite ends of the spectrum

cochannel interference is an RF - radio frequency condition where channels within the same wireless spectrum interfere with one another

Cochannel interference can cause a client's service session to lock up.  It might also cause severe network failure or total network collapse.  Thus it seems important to operate APs with overlaps on different channels and the further apart the better.

Now that you know how CSMA/CA operates in wireless and you know about channels and some basic properties of intelligent implementation, what do you think about the bandwidth at any given AP?

Radio Transceivers/Antennas.  I need to spend some time discussing characteristics of radio transceivers/antennas.  The terms can be used interchangeably, though usually not with the exactitude they should.

It is usually the case that an AP is associated with a particular radio transceiver and radio channel.  The antennas are generally attached.  But there are plenty of models where a card is inserted and an antenna is wired in so that the antenna can be placed at least a few feet away from the AP.  The most typical devices I've seen for this allow for two such independent radio transceivers/antennas.  My impression is this is why they talk about APs having more than one radio transceiver, in order to distinguish their capability to operate on a different channel on each antenna.

It is also my understanding that why some 802.11g APs advertise they give 108Mbps is they make use of two active radio transceivers, each on their own channel.  One of these is focused on sending and the other on receiving.  This essentially doubles the capacity.  But it still is channelized!  One must also be even more careful to have appropriately distinct channel frequencies.  Though, this is much easier to do using the 802.11g specifications.

How many users should be configured on a supporting radio transceiver?  This is generally considered to depend on the distribution system capacity.  Since almost all wireless APs uplink to a wired Ethernet, we will discuss the two likeliest scenarios.

  • uplink to a 10Mbps wired Ethernet
    • most designers suggest a maximum of 20 users configured to use a radio transceiver
    • obviously this depends on the type of traffic and business of the wired network
  • uplink to a 100Mbps wired Ethernet (switched or not)
    • most designers suggest a maximum of 40 users configured to use a radio transceiver
      • this seems high to me considering specs on wired Ethernet and extra limitations of wireless
      • personally, I wouldn't want to make much use of  a system that was very busy in this situation
    • obviously this depends on the type of traffic and business of the wired network
    • obviously this depends on the size of the collision domains in the wired network and where switches are used to improve capacity

While there are more types of antenna than I am going to present we will consider three basic categories.

  • direct
  • area directed
  • omni-directional spread/broadcast

The first diagram below relates to the use of omni-directional antennas.



The next diagram represents antenna that have some spread/reach, but are more directed.



There are also antenna that are much more directionally oriented usually used for a variety of wireless bridges.  These antenna can end up looking like satellite dishes.

Which sort of antenna are chosen in a given situation depends on the situation and costs.  Remember, you always want to be concerned about signals bleeding into places where they shouldn't.  For example, this can be important to limit accessibility and/or to make sure signals don't intrude into other spaces.

One final note is that in some instances it can be important to install more than one AP to cover and area for different purposes.  For example, you might have a small coffee shop where you want people to have open access to your wireless.  But you also need to have some administrative capabilities via wireless that needs to be totally secure.  It is likely that the only reasonable way to do this is to make sue of two different devices.  These may well be two different APs, though there may be APs that allow two different radio transceivers with distinctly different settings for security and access.