Guest Blog: Wireless LANs and distributed antenna systems

by Scott Thompson
April 9, 2010



Recently I attended a national tradeshow, in a very large exhibit hall, in the nation's capital. There were several thousand people in the hall. Within that hall there were popular, actively sought locales where peoople were dipping and turning, sometimes holding their heads sideways, looking upward, rotating, stooping, and thrusting upwards. Then, inevitably, looking in frustration at a device in their hands.

And those were the few people in the hall who actually had a cellular signal!

Most people have had the experience of poor cellular service inside buildings, often large public buildings. In addition to the user inconvenience caused by the poor connection, the carriers lose minutes, and the building's utility - whether as a mall, public hall, or other venue - is compromised. Everybody loses.

Fortunately there is a reasonable solution for an in-building wireless solution called a distributed antenna system, or DAS. DAS is a method for recreating the coverage of primarily out door cellular services, indoors. The purpose of the DAS is to overcome the severe attenuation caused by the walls and structure of a building. Even the newly auctioned 700-MHz band, with its preferred propagation characteristics, will be callenged to provide the high signal-to-noise ratio required for high-speed data services envisioned for the 4G network.

Clearly, as people become more dependent on mobile voice and data services, there is a growing expectation for network connectivity everywhere, indoors and outdoors. DAS and related in-building wireless systems provide a means to match this expectation, even in the most challenging indoor environmnents.

The DAS system may be as simple as an outdoor pickup antenna with bidirectional amplifiers and indoor antennas, or a much more sophisticated base-station with fiber-optic distribution to remote access RF converters and antennas. In any case, as the name implies, the antennas are distributed throughout the facility, and the equipment, whether a micro-base station, repeater, or other cellular equipment, is safely locked in the telecommunications room.

This, of course, is a different architecture than the standard 802.11 LAN architecture, which comprises distributed access points (with connected antennas), versus distributed antennas.

In the past, this difference in architecture has not been important. The wireless LAN is a private network, designed, built, and maintained by the premises network administrators, whereas DAS systems may have multiple stakeholders including building owners/operators, carriers, third-party integrators, and premises network administrators.

As DAS solutions emerge, one of their compelling features is to include private wireless LAN traffic over the DAS. The argument for doing so is, "Why build two wireless infrastructures?" The easy answer is that most vendors' 802.11 wireless networking products are designed for a distributed access point approach, and not a distributed antenna (DAS) approach. Generally speaking, when you use something in a way for which it was not intended, you don't get the results you want.

Cisco recently released a positioning statement indicating that "Cisco does not certify, endorse, or provide RF support for Wi-Fi deployments over ANY distributed antenna system."

Although not specifically precluding the use of Cisco wireless LAN products in a DAS, the statement recommends special consideration of signal coverage, client-to-AP density, client roaming, location-based services, and the impact on the 802.11n (MIMO) antennas used by 802.11n access points. Cisco's positioning statement goes on to recommend an appropriate design and deployment if a DAS approach is used, because the "DAS vendor and/or systems integrator is solely responsible for the support of the DAS products and for providing adequate RF coverage and supporting any RF-related issues."

Why would you operate the wireless LAN over the DAS in the first place? If the answer is "physical security" of the wireless LAN access points (and that's the only security at stake with modern APs), then you should plan to secure the APs in locking ceiling or wall enclosures. These enclosures provide a degree of physical security cmmensurate with the value of the AP. Sometimes the concern is not so much malicious theft or vandalism of the APs, but just accidental displacement, disconnection, or blockage from the desired location. Again, the locking ceiling or wall-mount enclosure is the answer.

If the answer is to reduce installation and labor cost by combining DAS and wireless LAN networks, then consider coordinated design and installation of conventional, but distinct, wireless LAN and DAS infrastructures, including shared infrastructure, pathways, and spaces where appropriate. This is an overlay design; it can include shared workspace telecommunications enclosures for access points, remote acces units, bidirectional amplifiers, converters, repeaters, and antennas.

Another emerging technology may truly converge public cellular service onto private wireless LANs, but this is a topic for a future blog post, and is not likely to be the best solution in every case.

In the meantime, the wireless LAn and DAS designer should consider vendor recommendations, risk, and cost-saving potential when deploying wireless LA over DAS, versus an overlay design comprising distinct wireless LAN and DAS components.

Scott Thompson is the engineering director and co-founder of Oberon Inc. Since 1997 he has designed, specified, and installed IEEE 802.3 and 802.11 standard-compliant networking solutions. We've invited Scott to blog for us periodically, and welcome your comments on this post.