Wireless LAN FAQ

Wireless LANs are finally venturing out of their vertical market niches and into mainstream business environments. But they pose some special deployment challenges when compared with their wired counterparts.

Here are six frequently asked questions IT managers should consider before installing a WLAN.

1. How do I plan capacity for a mobile network? Wireless experts suggest that you first identify which applications the network will support and how much bandwidth they will consume. Then calculate how many users need mobility and in what places within your organization they will require it.

Next, apply this information in designing a network of wireless access points (AP). APs are the infrastructure radios, usually ceiling-mounted, which connect to the wired network on one side and to users' wireless network adapters over the airwaves on the other. Wireless veterans stress the importance of site surveys and performance testing to optimize AP placement.

"This is because variations in firmware, antennas and physical layout can alter the performance and range of the very same [wireless] chip set," says Craig Mathias, an analyst at Farpoint Group in Ashland, Mass.

Because it's difficult to anticipate where walls and objects will interfere with wireless communication, a network staff member armed with a wireless laptop typically walks around, installs APs and tests coverage—often trying out multiple antenna options for maximum signal strength.

This exercise will reveal how many APs you need to meet capacity requirements and how to position adjacent APs to prevent coverage holes. There are some rules of thumb about how many users a single AP can support. Gartner Inc., for example, recommends about 20 knowledge workers per 802.11b (11Mbit/sec.) radio.

But these ratios vary. Warehouses often need lots of APs to ensure floorwide coverage. But warehouse applications don't consume much bandwidth, and utilization of these APs is often only 2%, says Rob Greenfield, chief scientist at SCLogic Inc., a reseller of WLANs in Beltsville, Md.

On the other hand, a crowded office space will require many more APs—not to achieve range, but to support more users and heftier applications.

"Shoot for under 40% to 50% AP utilization in office environments," Greenfield suggests. That will compensate for not knowing how many users will be in range at any given time.

2. How can I avoid interference? Having multiple devices that use the same frequency can result in interference with performance. United Parcel Service Inc. learned this when it combined short-range Bluetooth (1Mbit/sec.) wireless technology with 802.11b LANs in a warehouse pilot, explains Joan MacEachern, lead telecommunications analyst at the Atlanta-based worldwide package-delivery company. Both Bluetooth and 802.11b run in the 2.4-GHz frequency band.

To solve the problem, UPS and its technology vendor, Symbol Technologies Inc. in Holtsville, N.Y., synchronized the Bluetooth network with Symbol's 802.11b APs. "Time Division Multiple Access technology in Symbol APs has scheduled the two networks to transmit at different intervals to avoid overlaps," explains MacEachern.

And don't forget to alternate use of 802.11b channels in neighboring APs to further minimize interference, advises John Lawson, vice president of IT and CIO at Tulane University in New Orleans. The university has installed 1,200 APs from Andover, Mass.-based Enterasys Networks Inc. across multiple campuses.

To segregate traffic in neighboring devices, 802.11b offers three different channels. "I've tried installing APs without channel planning," says Lawson. "The result was unacceptable interference."

3. Is my WLAN really secure? This is a complicated question. Security advances collectively called Wi-Fi Protected Access (WPA), based on emerging IEEE standards, will ship in new products later this year. WPA overcomes the notorious static-encryption-key weaknesses in 802.11b's Wired Equivalent Privacy standard. In addition, comprehensive vendor-proprietary products and services have long been available to reinforce wireless privacy.

However, to optimize security in WLANs immediately, the best approach is to employ IPsec virtual private networks, which use Layer 3 encryption, even on a local enterprise campus, says Dave Passmore, an analyst at Burton Group in Sterling, Va.

"This requires IPsec client software on user laptops and a device that terminates IPsec tunnels inside the enterprise network firewall," he explains.

Passmore notes, however, that forthcoming WPA-compliant products should resolve the need for local IPsec use in many organizations.

4. Which technology should I choose from the alphabet soup of WLAN standards? Ken Dulaney, an analyst at Gartner in San Jose, has a simple answer: "802.11b is the only mature, worldwide standard that can be deployed today," he says. "For most enterprises, 802.11b bandwidth should suffice for most office tasks for at least the next four years."

UPS, for example, is "building to current requirements, which is 802.11b," both in its warehouses and at its IT headquarters, says Fred Hoit, UPS's radio frequency infrastructure manager. "We anticipate a five-year life cycle," Hoit says.

Opinions are mixed about 802.11a, the successor to 802.11b that runs in the 5-GHz frequency band with much faster speeds (54Mbit/sec.) but a shorter coverage range. On the plus side, you avoid interference when running 11a and 11b in parallel, because the two LANs use separate frequencies. But Greenfield says it's worth waiting a year for 802.11g-compliant products to gain cross-compatibility with 802.11b.

The shorter transmission range of 80211a "means you'll need many more APs for the same coverage, which will be more expensive," he says. Like 802.11a, 802.11g supports aggregate 54Mbit/sec. speeds but runs in the 2.4-GHz band and was designed for backward compatibility with 802.11b.

Finally, emerging dual- and multimode products "will eventually make upgradability a nonissue," says Mathias. Several enterprise-class vendors offer APs that support 802.11a and a slot for either an 802.11b or 802.11g radio.

Meanwhile, trimode 802.11 b/a/g chip sets recently began shipping, so client adapters and APs that support all WLANs could be available within a year.

5. What's the best way to manage my radio infrastructure? A wave of new WLAN "switch" architectures from established companies such as Symbol and Proxim Inc., as well as from a bevy of start-ups, aim to ease AP configuration and management in large installations. The idea is to centralize control of a dumb AP infrastructure in a "smart" collapsed-backbone device.

This approach purports to lower the cost of each AP—which adds up in large shops that might require hundreds or thousands of them—and to enable greater scalability and lower operations costs.

But that doesn't mean that if you want a smart AP, you can't get centralized management, too—unless this option blows your budget. For example, Cisco Systems Inc. offers its Wireless LAN Solutions Engine, a data center appliance and CiscoWorks management module that centrally configures and manages up to 500 APs. But Cisco Aironet 1100 and 1200 APs are far from dumb; they run the full complement of Cisco Internetworking Operating System routing software—enabling, for example, wireless virtual LAN (VLAN) support.

Symbol also supports wireless VLANs, but it builds them into its Mobius Axon wireless switch, not into the AP. VLAN profile information is enforced before incoming traffic hits the wired network or outbound traffic is distributed to wireless users.

6. Can I integrate WLANs with other networks? Network nirvana is when users can roam among disparate wired and wireless networks without having to reconfigure settings or reauthenticate. Multimode products are emerging that combine several versions of 802.11, as well as connectivity to 2.5- and 3-generation (2.5/3G) mobile WANs from the licensed carriers. For example, Texas Instruments Inc. in March announced chip sets for PDAs that combine 802.11b, GSM/GPRS and Bluetooth capabilities. NetMotion Wireless Inc. in Seattle has client software for roaming among any type of packet-based wired or wireless LAN or WAN. The software simply detects the most robust network connection available and transparently connects the user to it, keeping VPN sessions intact.

Such internetwork roaming will grow increasingly handy as WLAN-based public services, called Wi-Fi hot spots, continue to sprout up in airports, hotels, malls and other public locations. The licensed 2.5/3G carriers have begun offering 802.11b-based hot-spot services to complement their ubiquitous, but slower, 2.5/3G packet data services (see chart above).

Today's 2.5/3G services provide 20K to 60Kbit/sec. throughput per user, depending on the technology used. So multimegabit-speed Wi-Fi networks, where available, offer a better-performing option.

Wexler is a freelance writer in Campbell, Calif. Contact her at joanie@jwexler.com.

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MOBILE DATA SERVICE OPTIONS
Carrier*/Mobile WAN technology 2.5/3G WAN service coverage (U.S.) Wi-Fi services? Number of Wi-Fi hot spots
AT&T Wireless GSM/GPRS 5,000 cities and towns; GPRS roaming agreements in 25 countries Yes 600 in North America
Cingular Wireless GSM/GPRS 67 cities and towns on self-run network; roaming agreements with AT&T Wireless and T-Mobile in the U.S. expand footprint No N/A
Sprint PCS CDMA/1XRTT 4,000 cities and towns No N/A
T-Mobile GSM/GPRS 8,000 cities and towns; GPRS services available in 15 countries Yes 2,300 in North America; 200 in Europe
Verizon Wireless CDMA/1XRTT 900 cities and towns No N/A

* Includes North America's largest mobile WAN wireless licensees. Nextel Communications Inc. isn't listed because its network, based on Motorola Inc.'s iDEN technology, isn't a standards-based 2.5/3G network and runs at just 15Kbit/sec.

Source: Carrier-supplied information

Copyright © 2003 IDG Communications, Inc.

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