SpectraLink Deploying NetLink Wireless Telephones User Manual

Deploying NetLink Wireless Telephones

Best Practices

White Paper

Version 1.0

May 2004

White Paper

1.0 Introduction

Wi-Fi telephony is the convergence of wireless voice and data

applications using a common Wi-Fi wireless LAN. Wi-Fi telephony
bridges traditional telecommunications, data communications, and
mobile technologies. A Wi-Fi wireless telephone is a wireless LAN client
device, using the same network technology as wireless laptops and
PDAs, and sharing the same medium. A Wi-Fi wireless telephone is
also functionally equivalent to a wired telephone, requiring configuration
and management from the local enterprise telephone system. Lastly, a
Wi-Fi wireless telephone is a mobile communication device that requires
special considerations for continuous high-quality connections as the
user moves throughout the coverage area.

SpectraLink pioneered the use of Wi-Fi telephones in a wide variety of
applications and environments, making SpectraLink’s NetLink Wireless
Telephones the market-leading Wi-Fi telephone devices. This document
identifies issues and solutions based on SpectraLink’s extensive
experience with Wi-Fi telephony.

1.1 Comparing
Voice and Data

Voice and data applications have different attributes and network
requirements. The most significant difference is their tolerance for
network errors and delays. Whereas data applications are designed to
accept just about any amount of packet delays and retransmissions,
voice quality will suffer with just a few hundred milliseconds of delay or a
very small percentage of lost packets. Data applications are typically
bursty in terms of bandwidth utilization, while a telephone conversation
utilizes a consistent, and relatively small, amount of network bandwidth.

1.2 Implementation
Issues

Using a wireless LAN for voice is not complex, but there are some things
that need to be considered, particularly for enterprise applications. A
critical objective of deploying enterprise Wi-Fi telephony is to maintain
similar voice quality, reliability, and functionality as users expect from
their wired business telephones. The key issues in deploying Wi-Fi
telephony are coverage, capacity, quality of service (QoS), telephone
switch integration, and security. This white paper provides
recommendations for ensuring that a network environment is optimal for
SpectraLink’s NetLink Wireless Telephones.

2.0 Wireless LAN Layout Considerations

NetLink Wireless Telephones utilize a Wi-Fi network consisting of

wireless LAN access points (APs) distributed throughout an enterprise
environment. The required number of APs is driven by two factors:
coverage area and system capacity.

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2.1 Coverage

One of the most critical issues in deploying NetLink Wireless
Telephones is ensuring sufficient wireless coverage. Often enterprise
Wi-Fi networks are designed only for data applications and may not
provide adequate coverage for wireless telephone users. Quite often
these networks are designed to cover only areas where data terminals
will be used, and do not include coverage in other areas such as
stairwells, bathrooms, building entrances, or lobby areas where NetLink
handsets may be used.

The overall quality of coverage is also more important with telephony
applications. Coverage that is suitable for data applications may not be
adequate for Wi-Fi telephony. Most data communication protocols
provide a mechanism for retransmission of lost or corrupted packets.
Delays caused by retransmissions are not harmful, or even discernable,
for most data applications. However, the real-time nature of a full-duplex
telephone conversation requires that voice packets need to be received
correctly within tens of milliseconds of their transmission. There is little
time for retransmission; lost or corrupted packets must be discarded. In
areas of poor coverage, data application performance may be
acceptable due to retransmission protocols, but real-time voice quality
may not be acceptable.

Another factor to consider in determining the coverage area is the device
usage. Wireless telephone devices are used differently than wireless
data terminals. Telephone users tend to walk as they talk, while data
users are most often stationary. NetLink Wireless Telephones are
usually held next to the user’s head, introducing additional radio signal
attenuation. Data terminals are usually set on a surface or held out at
arms length so the user’s body has little affect. Because of these
factors, a wireless telephone may have less range than a data terminal
and the wireless LAN layout should account for a general reduction in
radio signal propagation.

2.1.1 Overlapping
Coverage

To provide comprehensive coverage for Wi-Fi telephony applications,
APs need to be positioned with sufficient overlapping coverage to
ensure that there are no coverage gaps, or dead spots, between them.
As NetLink Wireless Telephones move out of range of a particular AP,
they seek out another AP to hand-off to, or re-associate with, in order to
maintain their network connection. A properly designed Wi-Fi network
will provide seamless hand-offs between APs, ensuring excellent voice
quality throughout the facility.

The wireless LAN layout must factor in the transmission settings that will
be configured within the APs. The transmission of voice requires
relatively low data rates and a small amount of bandwidth compared to
other applications. The 802.11 standard includes data rate reduction
specifications so that as a user moves away from the access point, the
radio adapts and uses a less complex and slower mechanism to send
the data. The result is increased range (coverage) when operating at
reduced transmission rates. Access points should generally not be
configured to limit the transmission to only the higher rates if wireless

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voice is an application on the wireless LAN because the coverage area
of the AP will be greatly reduced. If a site requires configuring the APs
to only negotiate at the higher rates, the layout of the wireless LAN must
account for the reduced coverage and additional APs will be required to
ensure seamless overlapping coverage at the higher rates.

2.1.2 Wireless Bridges

2.2 Channel and
Power
Considerations

Wireless bridges are used to connect Ethernet LANs or extend the range
of existing wireless LANs. Such devices generally create bottlenecks for
network capacity and add delay to the overall network, which is not
tolerable in the boundaries of QoS requirements. SpectraLink does not
support a configuration that includes wireless bridges and does not
recommend using wireless bridges within any wireless voice network.

Adjacent APs need to use different radio channels to prevent
interference between them. The 802.11b standard utilized by NetLink
Wireless Telephones provides three non-interfering channels: channels
1, 6, and 11 for North America. Access points within range of each other
should always be set to non-interfering channels to maximize the
capacity and performance of a wireless LAN, as shown in the diagram
below:

Non-interfering, Overlapping 802.11b Channel Coverage

The transmission power of APs can also be increased or decreased to
provide more or less AP coverage area. Generally, the transmit power
setting should be the same for all APs in a facility. This minimizes the
chance of higher-power APs interfering with nearby lower-power APs,

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and provides consistent coverage.

2.2.1 Site Surveys

Performing a site survey can minimize the possibility of dead spots. The
AP equipment provider can usually perform a site survey. While many
tools exist that allow customers to perform their own assessment,
SpectraLink recommends that every site employ a professional site
survey to ensure optimum coverage and minimize interference.

Site surveys are a necessity for large or complex facilities. An extensive
site survey will ensure that the minimum number of APs are deployed,
but at the risk of having to significantly reconfigure the network if the
coverage area is remodeled. Wi-Fi infrastructure providers are making
significant developments to reduce the time, cost, and complexity of
determining AP locations. Because the cost of APs has significantly
dropped over the last few years, in some cases it is more cost effective
to install more APs with overlapping coverage rather than try to
maximize coverage for each AP with a rigorous site survey.

To verify coverage with an existing Wi-Fi network, NetLink Wireless
Telephones offer a site survey mode that can be used to test the AP’s
signal strength in the wireless LAN coverage area. This mode detects
the four strongest AP signals and displays the signal strength and the
AP channel assignments. This mode can be used to detect areas with
poor coverage or interfering channels. With the NetLink e340 and i640
Wireless Telephones, the entire coverage area should be checked to
ensure that at least one access point’s reading is stronger than –70 dBm
in all areas. Also, if the site survey mode indicates 2 APs using the
same channel, then at least one other AP must be indicated at 10 dBm
stronger than those APs to avoid channel conflicts. After a site survey is
completed, coverage issues can be resolved by adding and/or relocating
APs and overlap issues may be resolved by reassigning channels or by
relocating some access points. Another complete site survey should be
performed after any adjustments are made to ensure that the changes
are satisfactory and have not impacted other areas.

2.2.2 Access Point
Data Rates and
Power Output

All APs on the wireless network used by the NetLink Wireless
Telephones must be set to the same supported and basic data rates. If
this is not adhered to, the NetLink Wireless Telephones may not
associate to the closest AP if a more distant one supports a higher data
rate.

In addition, all APs must be set to operate at the same power output.
SpectraLink highly recommends a power output setting of 100 mW. If
this cannot be accommodated, SpectraLink recommends a 50 mW
setting and requires a minimum of 30 mW. With lower power output
settings, special attention must be made to AP placement to ensure
there are no frequency re-use issues. These problems may not be
evident when using the handset’s site survey tool as it is assumes 100
mW transmission power from the APs.

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2.3 Capacity

2.3.1 Access Point
Bandwidth
Considerations

The network capacity requirements also factor into the number of APs
required, although in most cases the coverage area is the primary factor.
Data traffic is very bursty and sporadic, but data applications can tolerate
network congestion with reduced throughput and slower response times.
On the other hand, voice traffic cannot tolerate unpredictable delays, but
at least the bandwidth requirements are constant and consistent for
every phone call. Also, telephone traffic can be predicted using
probabilistic usage models, allowing a network to be designed with high
confidence in meeting anticipated voice capacity requirements. Beyond
the normal IP telephony design guidelines, there are several additional
considerations that need to be addressed for Wi-Fi telephony with
NetLink Wireless Telephones.

There are several factors that determine the AP bandwidth utilization of
a telephone call. The first is the VoIP protocol used and its
characteristics. The type of codec utilized combined with the packet rate
will determine the size of the voice packets, along with any additional
overhead information required for the protocol. The payload information
makes up a little more than half of a typical voice packet, with 802.11
and IP protocol overhead filling the rest. The 802.11 protocols include
timing gaps for collision avoidance, which means bandwidth utilization is
more accurately quantified as a percentage rather than actual data
throughput. The percentage of bandwidth used increases for lower data
rates, but it is not a linear function because of the bandwidth consumed
by the timing gaps and overhead. For example, a call using standard 64
kb/s voice encoding (G.711) utilizes about 4.5% of the AP bandwidth at
11 Mb/s, and about 12% at 2 Mb/s. In this example, four simultaneous
calls on an AP would consume about 18% of the available bandwidth at
11 Mb/s or about 48% at 2 Mb/s.

The following table lists the theoretical percentage of available
bandwidth used per telephone call for each 802.11b data rate:

1 Mb/s 2 Mb/s 5.5 Mb/s 11 Mb/s

NetLink Telephony Gateway (24
kb/s), 20 ms sample rate

G.711 (64 kb/s), 30 ms sample rate 20.5% 11.7% 6.1% 4.5%

G.729 (8 kb/s), 30 ms sample rate 9.3% 6.1% 4.1% 3.5%

15.7% 10.0% 6.4% 5.4%

Theoretical Call Bandwidth Utilization of 802.11b Access Points

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