Things to Consider When Reviewing Wireless Lan Products

Because wireless LAN technology is so new and dynamic, Proxim thought it might be helpful to outline considerations for both the writer to address in wireless review "write-ups" and the reviewer to address in wireless comparison testing.

Before you get started ...

Remember to catalogue all of the various PCs used for clients and servers.

Site Survey Tool

The link quality is a measurement from 0-5 recording roughly how many packets sent out by the site survey tool are being received by the adapters within range. A reading of 5 means that most of the packets are being received while a reading of 0 means that only a few packets are received. The received signal strength parameter measures the radio signal strength between the various LAN adapters and the site survey machine. Although it might seem odd, the link quality and received signal strength parameters do not always track each other. For example, the received signal strength might be 50% but the link quality could still be 5.

Recommended Configurations for testing Wireless LAN Products

Here are a number of test configurations for wireless LAN products that can be used for product reviews and comparisons.

There are a number of ways to benchmark performance of a wireless LAN. Here are a few that are commonly used. Proxim does not recommend one method or the other except for the fact that the same test should be used in each configuration for comparison.

  1. File Transfer
  2. Printing
  3. Loading Applications over the Network
  4. Running Client/Server Applications

Single client to an Access Point

Measure throughput with the units at close range of 5-10ft. with line of sight between the antennas.

Measure the throughput moving the client to various points within the network using the locations on your building map

Measure the throughput at locations near the fringe of range

Multiple clients to a single Access Point

Measure throughput with one client at close range of 5-10ft. line of sight between the antennas. Add additional clients and measure the change in overall throughput and the throughput for each client

Repeat the same test with the clients spaced at various locations on your building map.

Multiple clients to multiple Access Points

Install multiple access points all within the same range. Space them roughly 10 ft. away and configure each to communicate on a different channel with the same domain number.

Have the client machines attach to the network. They will randomly attach to one of the multiple access points. Place the units at close range or within 5-10ft line of sight with the access points. Compare the throughput numbers to the previous test.

Repeat the same test with the clients spaced out at different locations on your building map.

Roaming

Install multiple access points throughout your building. Use the site survey tool to optimize access point placement and make sure you have overlapping wireless coverage.

Use one client and move throughout the building. The PC will beep when it roams from one cell to another. Note the performance at different locations on your building map. You may need to move your access points to optimize your wireless coverage.

Set up two access points out of range. See if you can still roam from one cell to another even though you're are disconnected from the network for a short period of time.

Cost of Wireless Coverage

It is critical that whenever wireless solutions are being ranked that price be a factor considered. This is important because both the price and the hardware requirements for existing solutions vary so widely. For example, a Proxim wireless LAN solution requires only two adapters costing between $595 and $695, while other solutions require hardware hubs, transceivers, bridges concentrators, and/or access points. Compare prices based on a complete wireless solution for a particular size building. For example, what would it cost to provide wireless access for 10 mobile PCs in a building that is 1000ft X 1000ft?

Power Management

Many mobile computing platforms are battery powered and require peripherals that use as little power as possible. In order to compare various wireless LAN options, it is important to measure the power consumption through various typical uses. For example, most users do not require constant access to the network so the test should run some kind of typical tasks such as printing every 15 minutes and sitting idle for the rest. Here are some example tests:

  1. Create a batch file that prints a file on a network printer every 15 minutes. For the rest of the time, the unit sits idle. Record the amount of time before the unit runs out of power.
  2. Logon to the network and let the unit go into suspend/resume mode. Monitor how long the unit still has power. Compare this test both with and without the wireless LAN adapter.
  3. Run a file transfer between the mobile computer and a network drive continually and record the amount of time before the unit runs out of power. This test will drain the battery quicker than any other test.
Proxims RangeLAN2/PCMCIA units support three different advanced power management features. All are important to conserving the battery life of the mobile computer.

  1. The first feature is known as doze mode. In this mode, the adapter powers down to a low level and will wake periodically to see if there are messages from the network. In this mode, the performance of the adapter is roughly the same as an adapter fully awake adapter but the power savings is substantial.
  2. The second feature is known as sleep mode. In this mode, the adapter powers down to a very low level and will only wake up if the mobile computer transmits information on the network. (In this mode, packets sent to the mobile computer from other wireless stations are not received)
  3. The final feature is know as power down mode. In this mode, the mobile computer shuts power off to the PCMCIA port. When the unit wakes, the RangeLAN2/PCMCIA product maintains the network connection as long as the server has not logged the node off the network.

Wireless Protocol Issues

Although you can measure the performance of wireless network in the same manner as a wired network, there a number of issues that are specific to wireless. These issues include the following:

  1. Near/Far Phenomenon
  2. Hidden Terminal
  3. Fringe Communication
  4. Overlapping Cells
  5. Varying throughput with range
  6. Microwave Interference

Near/Far Phenomenon

A phenomenon we refer to as "near/far" might be encountered during the review process. Each unit tested should be evaluated in this scenario.

Assume that two units are attempting to communicate with one wireless access point. Both units are within range of the wireless access point but unit A is closer than unit B. The radio signal received from unit A will be stronger than the signal from unit B. If both attempt to transmit at the same time, there is a chance that unit B will never communicate with the access point. Robust Radio protocols insure that all units can communicate regardless of the distance from the access point. In this example, both unit A and unit B should be able to communicate at all times with the access point.

Hidden Terminal

Another scenario you might encounter and should evaluate is the "hidden terminal" phenomenon. When multiple radios establish communications, messages are sent and received by all units to insure that only two units communicate at any one time. However in some environments, not all of the radios will be in range to receive these messages. For example, assume that three radio units are placed in a single row - unit A, unit B and unit C. Both unit A and unit C are in range of unit B, however unit A and unit C are not in range. If both unit A and unit C attempt to transmit to unit B at the same time, unit B will not be able to receive the transmission. Good radio protocols consider the possibility of "hidden terminal" issues allow both unit A and unit C to communicate with unit B even through unit A and unit C are not in range.

Fringe Communication

As you move to the fringe of the range of any wireless product, it should be robust enough to maintain communication as you move in and out of range.

Overlapping Cells

Wireless cells that overlap can create confusion for clients connected to a particular access point. The protocol must be able to resolve these conflicts even when the units are on the same channel.

Varying throughput with range

As the client moves farther and farther from the access point, the throughput will begin to degrade. Robust protocols allow communication to slowly drop off as the client reaches the fringe of coverage.

Microwave Interference

Many microwave ovens operate in the 2.4 GHz frequency range. The ovens operate at 600-1000W of power. Even though the units are shielded, a good amount of energy still can block the transmission of wireless LAN products. With robust protocols, wireless LAN products can still communicate even in the presence of microwave oven interference.

We hope that these points will prove useful to you in the review process.

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