Once you’ve assessed your need for additional access points, the next question is where they should be placed. The two considerations are improving range and improving throughput.
You can improve your range by locating additional access points in dead spots in your location, places where signal strength is negligible. To improve throughput you’ll want to have access points overlap, but keep in mind that there will be contention among wireless signals.
If a laptop can detect two or more different signals, it will connect to the strongest access point to get the fastest throughput. When the signals are of approximately the same strength, the receiving device (your laptop, for example) can become confused and have connection difficulties.
Your NIC card is trying to lock onto the strongest signal, but the maximum signal is not coming constantly from the same device. With differences over 20db, most cards will lock on the strongest signal consistently. Another thing to keep in mind is that bandwidth is not dispersed equally among competing devices.
All things being equal (the same computer, the same wireless NIC card, and the same bandwidth demand) a computer that is closer to the access point and has a stronger signal will be able to demand and get more throughput than a system that is further away.
Unless you buy a wireless network adapter meant for the corporate or enterprise market, you are unlikely to be able to specify which access point you would prefer your system connects to. Some of the higher-end cards do offer this feature.
With some of the Cisco NICs, for example, you can specify your desired access point with a list of four prioritized MAC addresses. But few, if any, devices for the home, SOHO, and SMB market have this feature.
In some of the small network products, you can do MAC filtering at the access point. You can specify which clients can connect to an access point by adding their identities to a list. This isn’t a dynamic feature, but at least it gives you some control over the number of connections and bandwidth usage.
You can’t just do a throughput calculation and determine the number of access points required, although that is a useful thing to do for a first pass. Where you place the access points is going to affect how many access points you might need.
If you are locating access points in different rooms, floors, or buildings, their physical location will limit the amount of overlap and therefore the coverage you can provide for each part of your location.
You might need five access points based on your throughput calculation; however, if you have to cover six floors of a building and two of the floors have almost all of your clients, you might find yourself putting three access points on those two floors and an additional four access points, one each on the remaining floors.
Your life is easier when you are building a multi-zone wireless network that doesn’t require a roaming feature. Roaming is the ability to move from access point to access point without having to log on again.
Without roaming, you can use almost anyone’s access points in combination; however if you want roaming you need to make sure that the product you are buying supports that feature.
This is one reason for not mixing devices from different vendors. You’ll find this feature on higher quality “enterprise” or corporate access points such as those from Cisco and Proxim.
For other vendor’s products, check carefully if they support this feature and that their roaming is actually reliably implemented. Just as roaming can be an elusive feature in wireless access points for the home and small business market, you’ll also find that buying multiple access points to perform network load balancing is also problematic.
With load balancing, network requests are dynamically routed to the access point that has the least traffic. While error checking is part of the 802.11 protocol, load balancing is not. Don’t expect devices from different vendors to perform load balancing for you, and be wary about expecting this feature from lower cost devices.
Load balancing is not an easy feature to implement in wireless technology because throughput is a direct function of signal strength, and most wireless NIC cards don’t know the difference between a Cisco and a Crisco access point (a fried Cisco), or Proxim and an Approxim.
So it takes some system smarts at the access point to figure out how to balance connections from different systems so that the throughput is balanced out. Solutions to the problem of roaming and load balancing are therefore proprietary solutions.
Most load balancing routines use MAC address filtering to perform load balancing. Furthermore, because only channels 1, 6, and 11 don’t overlap for 2.4 GHz Wi-Fi access points, if you are using any more than three access points, you will have contention.
Each access point in a network must have the same SSID value, that of your network, or they don’t connect up. For three access points, you might assign channels 1, 6, and 11 to each. Once you get beyond three access points, you want to use your network map to start placing different channels next to each other.
Location’s impact on the overlap of access points is therefore important to determine. Access points are relatively inexpensive network devices. They aren’t much more expensive than Ethernet hubs or switches, so there’s a tendency to put more access points in a wireless LAN than is probably necessary.
We’re not big fans of this approach, however, because each access point needs to be managed and each access point represents another possible path for intruders to use to enter your network.
Therefore, we recommend that you “right size” the number of access points and use other methods to improve your WLAN range when throughput is not the issue. Siemens and Ashoka, among others, have wireless routers and access points that connect to your network via your power line using the HomePlug system.
The devices available are 802.11b devices, and realistically HomePlug can’t give you the kind of throughput that 802.11g and future 802.11 standards will. However, for a difficult-to-reach location, this can provide an easy-to-setup wireless connection.
