Pre-deployment Considerations
Multiple clients on the same AP radio or channel
The behavior of a wireless network where multiple clients are accessing the same AP or channel is very much like being connected to an ethernet hub. In this case, the wireless network is a shared resource where the activities of one client affect the realized performance or network availability of all other clients. If just one client is downloading files, or accessing audio or video via wireless, other clients on that same channel are penalized.
A single wireless client could achieve 5.5Mb/s (802.11b radio) or 24Mb/s (802.11a radio) throughput given a good, solid connection to an AP with no other wireless clients associated to it. In an environment with multiple clients and heavy resource use at any speed, all clients will start dropping speed if there are lots of collisions.
To maximize wireless network performance for multiple clients, hard set the AP to only allow higher modulation speeds and set the multicast rate higher. Typically, wireless clients that connect at lower speeds are often far away from the AP, have weak radios, or are hidden from other clients.
Multiple clients on different AP radios or channels
In an environment with multiple APs using different channels, it is possible to have interference caused between radios.
Turn down the power of the radios or separate the APs further from each other. Having the radios close together and turned up to full power can lead to a noisy wireless environment that is not suitable for the best wireless performance possible.
The best recommendation is to use more APs with lower configured power levels which will result in tighter wireless cells.
Interference
Interference could be from another station wanting to access the network or even a microwave oven.
If wireless clients are too far away from each other to "hear" each others communications with the AP, then interference can be caused by each client trying to communicate with the AP at the same time. This is called the hidden node problem.
Place your APs away from other devices that operate on the same frequency, such as cordless phones and microwave ovens.
RF Obstacles
Metallic objects, like file cabinets, in the path between the AP and a wireless client tend to reflect radio signals.
High density materials, like concrete or brick, tend to be much harder for a radio signal to penetrate.
Post-deployment Tweaking
Beacon Interval
The beacon interval is the amount of time between access point beacon transmissions.
Increasing the beacon interval reduces the number of beacons and the overhead associated with them. The downside to doing this is that associating and roaming clients may experience delays as they scan for available access points that beacon themselves less frequently.
The beacon interval can also be decreased, which increases the rate of beacons. This allows for smooth client associations and roaming, but more beacons means more overhead.
Additionally, decreasing the beacon interval while leaving the DTIM interval the same will cause clients using power save mode to awaken more often which reduces power save efficiency.
DTIM Interval
The delivery traffic information map (DTIM) timer determines how frequently the station must leave power save mode and determine if traffic is queued up for it.
Using a longer DTIM interval allows mobile stations to sleep for longer periods which maximizes battery life at the expense of timely broadcast and multicast frame delivery.
Using a shorter DTIM interval allows for more frequent broadcast and multicast frame delivery at the expense of more frequent power-up and power-down cycles thus reducing power save efficiency.
Depending on the applications used on the network, setting the DTIM interval correctly is important. For instance, wireless clients receiving multicast streams may require a shorter DTIM interval.
Fragmentation Threshold
In theory, many smaller frames are more likely to successfully reach their destination in a network with frequent collisions than one large one. If there are a relatively large number of collisions occurring, using fragmentation may improve throughput.
If very few collisions (less than 5 percent) are occurring, then using fragmentation may actually decrease throughput because the additional headers applied to each fragment would increase the overhead on the network.
The best way to find the optimal fragmentation threshold setting is to test the network using different fragmentation threshold values and watch for increased throughput and reduced collisions as you move towards the optimal value.
Long Preamble versus Short Preamble
The preamble is the beginning bits of a transmission used to sync up a transmitter and receiver prior to sending the actual data.
The original 802.11 specification (1 and 2Mbps operation), defined a long preamble that uses a 128 bit sync field.
With the later 802.11b standard (11Mbps operation), an optional short preamble using a 56 bit sync field was added.
This shorter sync field was intended to improve the efficiency of the wireless network for more time sensitive applications like streaming audio/video and VoIP applications; however, some legacy wireless cards may not support the short preamble.
Legacy stations that do not support short preamble will not be able to understand much of the communication in the BSS, and most importantly won’t be able to receive "Protection" frames. This could result in legacy 11b stations transmitting at the same time as 11g stations, which doesn’t help either one to properly get their data sent!
RTS Threshold
If there are a large number of collisions and the clients are relatively far apart and possibly out of range of each other, enabling RTS/CTS functionality on the client NICs may help.
After activating RTS/CTS, test to determine if the number of collisions is less and the resulting throughput is better as enabling RTS/CTS will introduce more overhead to the network. If throughput drops but collisions are reduced, discontinue the use of RTS/CTS. Improved throughput is what we are after.
If collisions are occurring between clients that are all within range of each other, the problem may be the result of high network utilization or possibly RF interference.
For APs, enabling RTS/CTS is a matter of setting specific packet size threshold (0-2346 bytes). If the packet that the AP is transmitting is larger than the threshold, it will use the RTS/CTS function. If the packet size is equal to or less than the threshold, the AP will not use RTS/CTS. Setting the threshold to 2346 bytes essentially disables RTS/CTS on the AP.
RTS/CTS need not be enabled on the AP necessarily. When an AP receives and RTS frame from a client, it will always respond with a CTS frame. Generally, an AP doesn’t suffer from the hidden node problem thus enabling RTS/CTS on the AP will likely increase network overhead and reduce throughput.
Focus on using RTS/CTS in the NICs to improve performance.
Wireless Client Performance Optimization
On the wireless client side, to ensure the best performance there are several things to do.
Disable power save mode on the client NIC. This is highly wireless card driver specific and is often difficult information to find.
Disable Microsoft Wireless Zero Configuration (WZC) probing. The WZC service determines if there are any changes in the wireless environment that would require switching the wireless connection. This occurs every 60 seconds. Do not disable WZC probing if the wireless client is expecting to seamlessly roam from AP to AP; roaming will suffer without WZC probing enabled.