Wireless access points

PREVIOUS: Wireless protocols learning guide intro

Installing wireless access points is a tricky manoeuvre at best when building a wireless network. Considerations such as deciding which wireless protocol works best in the enterprise, installing the appropriate APs and ensuring they don't interfere with each other or other objects are only a few issues that require some thought. This section of the Wireless Protocols Learning Guide offers expert advice on purchasing WAPs, placement and WAP configuration.

Wireless access points

When it comes to buying wireless access points (APs), now is a tricky time. Don't forget to keep an eye on the latest from IEEE. Draft 802.11n products are improving but are not yet mature.

Today's draft 802.11n products are doing their best to comply with the emerging IEEE standard, trying to get a jump on the market that will materialise when the standard is finalised. But, strictly speaking, there is no such thing as complying with a draft standard. Vendors that align products with a draft are gambling that significant problems will not emerge before the standard is fully baked. In particular, interoperability issues are likely to crop up. A benchmark test of early draft 802.11n products conducted by Farpoint Group in May 2006 found numerous interoperability problems.

If you can defer your upgrade until after the standard is set, you will be able to buy 802.11n-based products with a much higher degree of confidence. If you cannot wait to invest in new hardware, look for enterprise APs with field-replaceable components that provide a predictable upgrade path from 802.11g to 802.11n. Don't forget that you'll need a plan for upgrading clients, too, when Wi-Fi certified 802.11n products finally emerge.

To preserve battery life on small-footprint wireless devices, the 802.11 standard defines a Power Save mode. The AP does buffer frames received for a sleeping station. Stations that enter Power Save mode announce their intention to sleep before temporarily shutting down all radio functions, with the exception of a timing function. The timing function wakes the station briefly at regular intervals to listen for AP beacons. The AP lets all stations know whether unicast frames are pending transmission by sending beacons containing a Traffic Indication Map (TIM). When a sleeping station hears one of those beacons, it leaves Power Save mode, turns its radio back on and becomes ready to receive buffered frames. A slightly different mechanism is used to deliver buffered broadcast/multicast frames to all stations at specified intervals, known as the DTIM (Delivery Traffic Indication Message) interval.

There are very big differences between US$50 residential-grade APs and US$500 enterprise-class APs.

Entry-level home APs have a single 802.11b/g radio. Enterprise-class APs like the 1130 have dual radios that simultaneously support 802.11a and 802.11g. Enterprise-class APs are engineered for higher-density use, supporting more concurrent clients, with higher throughput. Enterprise-class APs are also more likely to support external antennas so that coverage can be optimised, as opposed to home AP "rubber ducky" dipoles.

From a performance standpoint, business-class APs offer more than just dual radios. Enterprise APs provide more extensive power controls that allow for high-density, perhaps even self-healing, WLANs. Their radios may have better receive sensitivity and range. They may provide hardware support for data encryption, increasing throughput when security is enabled. And a growing number of enterprise APs support Wi-Fi Multi-Media (WMM), which allows traffic to be prioritised for Quality of Service (QoS).

Wireless access point placement

Whether you're planning to deploy wireless or tweak your existing deployment, the conventional wisdom states that you need a "site survey." Most wireless vendors are now offering very sophisticated tools that practically do the survey for you, but if you're trying to locate the best placement for your wireless access points (WAPs) manually, the trick is to keep the following three key relationships in mind.

Wireless access point placement checklist:

Relation to users:

• There are two ways to approach this, depending on your requirements: Design to give the best signal to the most users or coverage area, or pick a minimum and ensure that every user or area gets that or better.
• Obviously, distance is key, since data rates go down as distance increases, so minimise distance based on the design objective above.
• Remember that signals are still shared, so the more users per WAP, the smaller each user's share of the bandwidth will be.
• For security, WAPs should be as far out of reach as possible -- preferably on or above the ceiling (depending on construction).

Relation to the facilities:

• Obviously, keep away from metal pillars, filling cabinets, duct work or concrete walls, or other sources of interference (such as machinery) where possible.
• Use the appropriate antennae for the area.
• The device will have to have electric and/or Ethernet cabling, so placement should minimise cable distance and expense if possible.

Relation to other WAPs:

• Some vendors recommend a 20% overlap in coverage for seamless roaming of voice devices. This is a good rule of thumb, but consult your documentation.
• Think about your channel usage when WAPs are within range of each other. This is particularly important in multi-tenant areas where you may not have administrative control of other radios.
• You can adjust the power to reduce or increase the signal to optimise the overlap instead of moving WAPs physically closer or farther apart.

Wireless access point configuration

An understanding of wireless LAN (WLAN) client and WLAN access point (AP) communication mechanisms is critical to anyone attempting to initially connect, configure or troubleshoot a WLAN connection.

Several features of the WLAN client require configuration in order to connect to a wireless AP and transmit data. These can include SSID, IP addressing, security, and channel configuration parameters. Basically, the WLAN client configuration will have to meet the configuration that has been set up on the AP. In most cases, an admin configuring a WLAN for office users or the user setting up a WLAN in a home office will be configuring the access point and can therefore match the client configuration settings and AP settings easily.

In order to connect to an AP, the client and AP must match in their configuration. The first thing that you want to ensure is that the client is configured properly. The following are the basic configuration parameters that must be met:

1. Service set identifier (SSID) setting: This is a unique identifier that distinguishes one WLAN from another. This used to be the only security enabled for a WLAN, in that only clients that were configured with the proper SSID could connect to the WLAN. In most cases the SSID is broadcast so that your WLAN card will identify it automatically.
2. Channel settings: APs have multiple channels on which they broadcast their signals. If there are multiple APs in the area, they will need to transmit on different channels to prevent interference. This means that you may use channel 1 at home but may need to use channel 6 or 11 at a Wi-Fi hot spot or hotel. Channels 1, 6 and 11 are for 802.11b WLAN networks. Again, in most cases this information is broadcast and the WLAN card will detect it automatically.
3. Security settings: Unlike the SSID and channel settings, this information has to be provided to you. The security settings are in place in order to prevent unauthorised access.
4. IP addressing: An IP address, subnet mask and default gateway are needed to transmit data over a WLAN connection. The three settings above relate to physically connecting the laptop/PC to the AP. The IP address is needed to communicate over that connection. Once connected, the AP has the ability to automatically assign this information to the WLAN client, but if the AP is not set up to do this, the client will need to be manually configured with this information. In addition, the information will have to be provided to you.

To find the best connection using encryption and authentication, keep these tips in mind when configuring WAPs.

• If you have enabled encryption, you must also enable encryption on all wireless clients in order to establish a wireless connection. Make sure that the encryption bit level is the same on the access point and the wireless client.
• Make sure that the SSID on the access point and the wireless clients are exactly the same. If they are not, your wireless connection will not be established.
• Try changing the channel on your router, access point and wireless adapter to a different channel to avoid interference. Keep your product away (at least 3-6 feet) from electrical devices that generate RF noise -- microwaves, monitors, electric motors, etc.
• When deploying several access points and wireless devices, make sure that access points in close proximity do not have overlapping channels. Nearby access points should be assigned channels that are at least four channels apart to prevent interference.

NEXT: Wireless network deployment and management