Building Voice Ready Wireless LANs
BY BRIAN JENKINS
A certain excitement is starting to build around the potential
intersection of three technologies: VoIP, wireless LANs, and a new
generation of cellular handsets. Individually, each has established a
proven market, but when converged, they represent an application that
offers substantial new possibilities for enterprise networks. The result
of this convergence is Voice over Wireless LAN (VoWLAN). Like many other
promising technology trends that preceded it, VoWLAN is currently in the
hype stage, with industry pundits offering varying predictions about when
widespread market adoption will occur. Still, most agree that VoWLAN
promises large enough cost savings and productivity gains to make it an
exciting future direction for networking.
Todayï¿½s VoWLAN deployments provide cordless phone or walkie-talkie
functionality in specific vertical markets such as health care and
manufacturing. But the real future of VoWLAN lies in newly announced
cellular phones that integrate Wi-Fi connectivity. With these phones,
users can maintain their ability to access the cellular network when
outside the corporate office, and then talk to each other over the WLAN
and use their corporate PBX when in the office.
There are two key benefits of VoWLAN. First, it consolidates the cellular
phone and landline phone into one device that is always at hand, allowing
employees to be reached immediately without having to be tracked down
through two or more landline and cellular phone numbers. Second, employees
can reduce their cellular minutes while theyï¿½re in the office, since
internal calls are carried over the WLAN and outside calls take advantage
of the economic benefits of the PBX. This results in both productivity
gains and cost benefits.
To realize the benefits of VoWLAN, however, network administrators must
properly architect their entire network, paying special attention to the
wireless LAN infrastructure. When implementing a WLAN of any scope, IT
administrators have several issues to consider, including security,
performance, and scalability. To properly prepare for VoWLAN, they also
need to understand voice traffic requirements to ensure high-quality calls
over their entire network. Whether theyï¿½re planning imminent VoWLAN
deployments or just watching the technology for another year or two, IT
administrators can ease deployment by ensuring that their WLAN
infrastructure can provide blanket wireless coverage, seamless mobile
connectivity, and guaranteed voice bandwidth.
Blanket Wireless Coverage
Like cellular users, VoWLAN users expect phone coverage throughout the
corporate office. Network administrators must consider blanket wireless
coverage as a requirement for their wireless LAN deployment. But providing
blanket wireless coverage can be expensive and difficult if the
infrastructure isnï¿½t properly designed.
With most access points (APs), it is a complicated matter to provide
blanket coverage due to the co-channel interference inherent in 802.11.
Unfortunately, APs interfere with each other -- APs too close together
cause interference, and APs too far apart cause coverage holes. This makes
it difficult to simultaneously provide coverage while avoiding
interference. To deal with co-channel interference, network administrators
must do RF site surveys to help identify potential areas of interference,
and then strategically place APs and manage RF channels. Administrators
who have deployed basic islands of wireless coverage (in conference rooms,
perhaps) will face a complex challenge in scaling their WLANs to provide
To build a WLAN that scales from limited coverage to blanket coverage,
network administrators should look for infrastructure products that
eliminate co-channel interference.
Although blanket wireless coverage is a necessary step, itï¿½s not enough
to ensure that the WLAN is capable of delivering high-quality voice calls.
Since VoWLAN users are mobile, they expect high-quality calls even when
they move throughout the corporate office. This requirement is difficult
to meet with todayï¿½s APs, since they donï¿½t meet the low-latency
mobility requirements for high-quality voice connections. As clients leave
the coverage area of one AP, they must scan for a new AP, associate with
that AP, and in most cases re-authenticate with the network. This AP-to-AP
handoff process usually takes between 200 and 500 microseconds. While
these delays are hardly noticeable with data applications, they are quite
noticeable for voice calls, since the human ear notices delays ranging
from 50 to 100 microseconds. (Note: the AP-to-AP handoff delays become an
even greater concern for users looking to implement the 802.11i security
specification, requiring network administrators to make concessions
between security and voice quality).
To build a wireless LAN capable of providing seamless mobility, network
administrators should look for WLAN infrastructure products that eliminate
AP-to-AP handoff issues. By eliminating handoffs, network administrators
can provide a high-quality experience throughout the duration of the voice
call, regardless of where the user goes. In addition, getting rid of
AP-to-AP hand-offs provides seamless support for the upcoming 802.11i
security protocol. Since users are not passed from AP to AP, they donï¿½t
have to deal with the additional delays associated with re-authenticating
via 802.11i. As a result, eliminating handoffs allows the network
administrator to implement both tight security and high-quality voice
without making concessions between the two.
Wireless Quality of Service
Once blanket coverage and seamless mobility are established, the network
administratorï¿½s next focus is guaranteeing the quality of voice traffic
between the VoWLAN client and the IP-PBX. The convergence of voice traffic
onto the data network requires bandwidth guarantees and prioritization to
ensure high-quality voice calls, and the network administrator must
address two infrastructure areas to meet these requirements: the wireless
network and the wireline network.
In the wireless LAN, there are two separate approaches to providing
traffic management across the airwaves. The first method is
straightforward: completely separate voice and data traffic by separating
the traffic onto different channels. With this method, network
administrators can guarantee quality of service because the voice traffic
never contends with the data traffic.
However, itï¿½s not always possible to use separate voice and data
channels. For instance, users who want to use their laptops or PDAs for
both voice and data require a mechanism to prioritize voice ahead of data
on the same channel. This mechanism is currently being defined in the
IEEEï¿½s new 802.11e specification. Until 802.11e is ratified, however,
network administrators should implement WLAN infrastructure products that
enable blanket wireless coverage on each of several RF channels with
separation of voice traffic, along with a clean upgrade path to 802.11e.
Wireline Quality of Service
The network administrator also needs to provide quality of service when
the wireless traffic hits the wired network. Fortunately, there are
well-established methods already in place for prioritizing and
guaranteeing traffic on wired Ethernet networks. With the standards-based
VLAN tagging and prioritization embedded in most Ethernet switches,
network administrators can queue voice traffic ahead of data traffic
throughout the wired LAN.
The early indications are positive for VoWLAN, and the promise of economic
benefits and productivity gains are quite compelling for enterprise
customers. Today, most network administrators are either considering WLANs
or have deployed basic islands of coverage. As they begin to consider
voice as an application for their existing enterprise networks,
administrators should understand the requirements for voice along with the
solutions that are available. Voice over WLAN can be challenging to
implement, but there are solutions that deliver wireless data connectivity
today while also providing the blanket coverage, scalability, and quality
of service that will be required for wireless voice.
Brian Jenkins is vice president of product marketing at AirFlow
Networks. AirFlow Networks makes secure wireless LANs that are designed to
be as easy to deploy and manage as wired Ethernet LANs. For more
information, visit the company Web site at www.airflownetworks.com.
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2004 Table Of Contents ]