Wireless MANs

Two Kinds of MANs

A wireless MAN, or WMAN, is a fixed wireless installation that interconnects buildings or locations. There are two basic types of WMANs: back haul and last mile.

Back haul is for enterprise networks, cellular-tower connection and Wi-Fi hotspots. It's an option for enterprises that can't afford to install or lease fiber to connect their facilities over a large campus or city. Back-haul WMANs also make sense when you can't justify a service provider's fiber 10-Mbps connection that requires six T1 leased lines at $50,000 a year. Fixed wireless is about half that price, without a monthly charge.

Although DSL or T1 work for back haul, a private broadband wireless system often provides 10 times faster transmissions. Capital and installation costs are about five times higher, but you get an ROI in just a few months. To interconnect a few sites, you can install one or more wireless PTP (point-to-point) links (see "Cheap and Available,"); for more sites, a private multipoint WMAN usually makes more sense. Alvarion, Proxim and others offer private WMANs.

Wireless back haul has the greatest short-term appeal, but last-mile solutions could establish wireless as an alternative to residential broadband DSL/cable modem. Some wireless ISPs, including TowerStream Corp., compete head-on with broadband, offering quick installation and lower cost, as well as wireless Internet service in areas without access. Last-mile WMANs are handy for temporary networks, too, such as large construction sites or areas where conventional network service is disrupted.Alternative Radio

You can deploy several PTP wireless connections with so-called highly directional antennas (which extend the range of your wireless base station), alternate radio channels and antenna polarization, where you orient your antennas to work together. It's not uncommon to see half a dozen or more remote sites connected to a single hub site using PTP products such as Proxim's Tsunami. But PMP (point-to-multipoint) setups, where a central point serves multiple remote sites, or spokes, make more sense when the density of links is half a dozen or higher. It all depends on the technology and your geography. Multiple PTP links in a single direction (sector) using 2.4-GHz 802.11 technology, for instance, are limited by the bandwidth required per link (22 MHz) and the total available unlicensed bandwidth (83.5 MHz). A PMP system, in contrast, typically uses a polling protocol to support higher-density applications.

Either way, you must decide whether to use licensed or unlicensed spectrum. Licensed gives you exclusive use of the spectrum in a certain geographical area; with unlicensed, you may share the airwaves. Some WMANs, including those based on the developing 802.16 standard, can operate in licensed or unlicensed spectrum.

Many organizations won't use unlicensed WMANs for fear of interference or security problems. But unlicensed systems, usually based on spread-spectrum and OFDM (orthogonal frequency division multiplexing) protocols, are designed to tolerate interference--directional antennas dramatically minimize this problem. Wireless is still vulnerable to jamming, however.

Meanwhile, today's PMP landscape is dotted by proprietary offerings like Proxim's Tsunami MP.11 and Alvarion's BreezeNet DS.11 Outdoor, which operate in the 2.4-GHz band and are loosely based on the 11-Mbps 802.11b, or Wi-Fi, standard. RoamAD uses a modified 802.11 specification to provide the necessary MAC (Media Access Control) layer needed for outdoor and long-range use.Unlike the IEEE's wireless LAN (802.11) and Wireless Personal Area Network (802.15) specifications, 802.16d was designed for geographically dispersed locations in licensed and unlicensed frequencies between 2 and 11 GHz. The higher frequencies offer more bandwidth, but their shorter wavelengths require line-of-sight, which is realistic for back haul of cellular or Wi-Fi links over open terrain, but not for PMP in a dense urban area or where an outdoor antenna isn't desirable.The as-yet unratified 802.16d standard, a combination of 802.16a, b and c, supports hundreds of concurrent connections and operates at distances of up to 30 miles (50 kilometers) and speeds as high as 75 Mbps. That's enough for at least 50 businesses to have sustained T1 access. Unlike the 802.11 standard, which uses CSMA/CA as a MAC protocol, the 802.16 MAC employs TDMA (time-division multiple access).

CSMA/CA problems are magnified when you implement 802.11 in a wireless MAN. CSMA/CA doesn't fare well over long distances because it's contention-based and requires that client machines detect collisions. Using directional antennas aggravates the problem. Instead, 802.16 uses TDMA, which allocates time slots so that each node takes a turn in order rather than semirandomly. That means no collisions and better performance. And 802.16 has QoS (quality of service) built into the MAC layer, which lets WMAN providers offer time-sensitive services like VoIP and low-resolution video without additional protocol overhead. It's designed for hundreds of users and works best if a single cell is less than 100 nodes to avoid polling delays. QoS is being added to 802.11.

Unlike conventional 2.5G and 3G data services, latency is not a significant issue in WMANs because most systems can tolerate a 10-ms delay. Wireless MAN distances aren't large, so the delays caused by RF propagation are minimal. That makes optimizing radio-processing crucial. Proxim claims its multipoint products have delays of less than 1 ms. Emerging mesh-based wireless solutions, in which data travels through multiple intermediate router hops, tend to suffer from higher delay.

So should you install your own wireless MAN? Unless the antenna is integrated into the CPE or is designed to be suction-cupped to a window, it's best to have your vendor or other service provider install it for you. That's because most WMAN reliability and performance problems are caused by improperly installed antennas.

And even with WiMAX, it will be some time before true plug-and-play 802.16 exists because the protocol spec is so broad and the number of products so few thus far.Security Insecurity

Security is a big red flag for most network administrators planning a WMAN. But 802.16 comes with robust security in its MAC spec. Although there's no requirement to use encryption, most wireless vendors building 802.16 products likely will choose 3DES (128-bit) or RSA (1,024-bit), and hooks are also available for AES support. When a wireless link comes up, digital certificates are exchanged to authenticate the CPE, which prevents someone from inserting rogue wireless devices into the network. After an electronic handshake, the base station grants the CPE a security-association identity and an authorization key with the CPE's public key.

Each service the client device provides is also encrypted with its own security association and private key. This double-layer key exchange, as well as the expiration of the private keys, makes 802.16 more secure and robust than 802.11.

Meanwhile, the WiMAX consortium is banking on its WMAN technology spilling over to the enterprise. And 802.16 is the key to making standards-based WMANs a reality for businesses.

Frank Bulk is a technology associate with the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected]. Post a comment or question on this story.

The WiMAX consortium promotes the deployment of wireless broadband using the IEEE's 802.16 and 802.16a standards and sets interoperability rules. WiMAX members, which include Intel and Fujitsu, account for about 75 percent of sales in the 2- to 11-GHz broadband wireless access market. The consortium says it hopes the specs will encourage the manufacture of low-cost silicon and related components that will give WiMAX vendors an edge over microwave. But noticeably absent from WiMAX are Cisco Systems and Motorola.

Wireless isn't just for hobbyists anymore. There will be 4 million wireless broadband subscribers in the United States by 2008, according to Intel, a major developer of 802.16 technology. Shorter term, half of U.S. Internet access will be broadband by July 2004, up from about 40 percent in September of this year, according to Nielsen/NetRatings. Around 20 million households use broadband, Nielsen says. Today, the vast majority of these users run DSL or use cable modems. But users in remote areas who don't have DSL or cable options will need wireless, Nielsen says.

Indeed, Intel's role as a WiMAX chip provider brings credibility to this emerging market, and the company is pushing WiMAX silicon. Wi-LAN, in conjunction with Fujitsu, and the team of Wavesat and Atmel Corp. are also suppliers in this space. Intel is collaborating with Alvarion on the silicon, and Airspan Networks and Aperto Networks have committed to buying from them when the first 802.16a or d chips become available in the second half of 2004. AT&T, BT and Nextel, meanwhile, have announced they will perform 802.16 trials.

WiMAX proponents say they hope 802.16 WMANs will be simple enough for customers to install themselves. The DIY factor plus low CPE cost could give wireless MANs a shot at undercutting DSL and cable modem costs. But that means significant improvements in antenna technologies are needed--ideally, the antenna should be integrated into the CPE. So far, though, the range is limited to between 1 and 2 miles.0