May 2004
Wireless VoIP - Other Voices
BY RICHARD WATSON
In earlier articles, the discussion has
focused on wireless VoIP that was telephony orientated, with full duplex
capability and enhanced telephony features. In the world of �voice,�
however, this is not the only value-added feature that can utilize audio
transmissions across a network fabric. As VoIP and WLAN technologies mature,
there are more variants of audio applications emerging, offering different
application functionality than standard telephony services. Since these
applications rely on being able to send a voice packet stream over the
network, they still depend upon high-quality transmission of voice from a
mobile terminal to some application entity within the network, but must meet
a more diverse application requirement. This article looks at some of these
�other voices� and how they are technically addressed and bring greater
value to various segments of the enterprise market.
PUSH-TO-TALK (WALKIE-TALKIE)
In many enterprises, associates make use of �Walkie-Talkies� � dedicated
VHF-based devices used to communicate with multiple team members
simultaneously. This mode is half-duplex where only one person within a
group can speak while all other members are in a listen mode. When the
initiator finishes speaking, another member of the group may initiate a
response where all can listen to the new speaker. Because operation of this
device requires pressing a button while speaking, it is also known as
�Push-to-Talk.�
As traditionally implemented in commercial
products, the Walkie-Talkies have been devices that support this single
function. Depending upon the scope of coverage and number of service
channels required, these devices can be quite expensive. With the deployment
of WiFi within an enterprise, the question arises: �can this function be
supported over WiFi?� The answer is �YES.�
There are new products emerging in the WiFi
market that support features labeled as �Walkie-Talkie� or �Intercom� that
replicate the traditional half-duplex functionality and even go beyond this
functionality. The architecture for these products is based on support of
audio packets being transmitted via multicast addressing. This architecture
is straightforward and aligned with basic network design fundamentals;
however, there are some wireless hurdles to be addressed.
Using a MAC layer multicast to carry
real-time audio data poses a problem in a standard WiFi (802.11) product. As
originally architected, a WiFi Access Point must transmit any multicast or
broadcast frames immediately if there are no mobile clients in Power Save
Polling (PSP) mode. However, in an operational wireless LAN, there is a high
probability that at least one client will be in PSP mode and the frames will
be queued and transmitted on a Delivery Traffic Indication Map (DTIM) frame
(a frame that announces to all associated devices that there is data queued)
interval. On the DTIM interval expiration, all queued multicast/broadcast
frames are then transmitted, which may sound okay, but the default DTIM
interval for some infrastructure products can be as much as one second. This
means that any inbound multicast frames will be queued and transmitted on a
one-second interval, which results in choppy, totally unacceptable voice
quality. Now that vendors are supporting multicast-based applications, how
can good voice quality be achieved? In the current marketplace, there are
two approaches to creating an environment where such applications will work
well.
The simplest solution involves a minor
configuration change of the wireless network to shorten the DTIM interval
setting. In fact, many of the commercially available products recommend this
approach, which can reduce the DTIM interval to less than 100 milliseconds (msec).
While this produces the desired effect in improving the voice quality with
multicast applications, there is a downside. The downside is that all
associated devices must respond to the rapid DTIM signaling, causing a
severe impact on the observed battery life of each device. With this
approach, network managers are left with a quandary: optimize the voice
application or device battery life?
A more elegant solution involves enhancing
the functionality of the Access Points to provide special handling for
multicast traffic. In this case, the Access Point must be aware of
application requirements based on packet addressing values. Some currently
available WiFi products allow administrators to specify a multicast MAC
address to identify the �Walkie-Talkie� application audio frame. Once
identified, the AP will apply special processing to these frames causing
them to bypass queuing and simply scheduling it for immediate transmission.
Other non-specified multicast and broadcast frames will be handled in the
standard manner based on DTIM interval transmission. The assumption is that
the voice-active clients will not be in PSP mode and will receive the frame
while any PSP clients will remain in that state undisturbed. This approach
is the best compromise to support good voice quality for the multicast
applications, while also optimizing battery life for associated client
devices. However, without a standards-based specification for this feature,
the implementation is proprietary. Thus, it is important to inspect the
vendor support feature set if you desire to implement this kind of
application.
ALTERNATE VOICE APPLICATIONS
For certain vertical markets there are industry specific applications that
need to take advantage of the underlying telephony (PBX) functionality.
These applications meet industry-specific data functionality, but also place
a requirement on being able to support some kind of voice connectivity.
One example of such an application is
customer relationship management (CRM) where a user may wish to talk to a
customer and desires to simply select a name within an application window to
initiate a phone call. This reduces the operation to a single task rather
than requiring the selection of a �softphone� application window and
explicitly entering the customer�s phone number. This creates a real
productivity advantage.
Another example of such industry specific
application is the Nurse Call application. This application has a more
complex configuration requirement, but still involves the use of a
full-duplex phone call to fulfill the functional requirements. With a Nurse
Call application, a hospital patient signals for assistance by pressing a
button at the bedside. This alert is processed by the central Nurse Call
system, which then signals all the active ward nurses, via text messaging
and asks if they are available to respond. Depending upon the nurse�s
availability, they respond to the text page and the system automatically
sets up a phone call between the nurse and the requesting patient. With such
an application, the nurse�s interaction with the system is minimized, while
maximizing response times to the patient need. Again, a significant
productivity advantage is created in an industry where there is an extreme
shortage of nursing staff.
The underlying technology requirement for
these applications remains reliable wireless VoIP support. As various market
segments adopt VoIP and WLAN technologies, there will be additional kinds of
�alternate� voice applications.
VOICE RECOGNITION
A more familiar telephony feature is voice recognition, most frequently
encountered with Interactive Voice Response (IVR) systems, often encountered
when calling into large corporate support centers, banks or a doctor�s
office. This technology is not traditional telephony, but still requires the
generation of a reliable audio packet stream. Implemented as a �phone call�
between an active user and the voice recognition engine, a voice �session�
must be established where the user is prompted by pre-recorded auditory
instructions and the user responds by speaking the appropriate response.
This response is then processed by the speech recognition engine, which then
acts upon the information provided to fulfill the desired operation. Simple
voice recognition can be used to navigate menu options for service or
support systems (e.g., �press �1� or say �one��). More complex uses of this
technology can be used for phone directory lookup or for user authentication
via voice print matching.
OTHER AUDIO APPLICATIONS
Other applications are possible where a user dialog is recorded by archiving
the RTP packet stream, which can be useful in a number of industries. For
example, call centers typically record conversations �for training
purposes.� Doctors traditionally make patient notes verbally and later
transcribe them to hardcopy. These applications can be accomplished through
the use of an Ethernet VoIP device, but a mobile WiFi appliance enhances the
usability and user productivity for these applications.
The potential of developing wireless devices with voice capability has yet
to be exhausted. As new �voice enabled� WiFi devices come on the market,
other applications will be developed to meet expanding needs and provide
productivity to a variety of user communities.
SUMMARY
As the WiFi market continues to expand, more �voice enabled� devices will
enter the market to fill an ever-expanding set of application needs. Basic
wireless telephony addresses a large segment in the general market, but
industry specific applications are emerging that rely on the same basic
primitive set of services � reliable, secure audio stream transport.
Management of this broad mix of telephony resources may become a real
challenge, especially when all the applications attempt to use these
services at once. In this case, configuration management, application
design, and user training will need to be addressed to help control the user
experience. All in all, this evolving technology is bringing a richer set of
features and functions to the end-user community.
Richard Watson is director of telephony product marketing for
Symbol Technologies� Wireless Systems
Division in San Jose, CA. Prior to taking on the marketing role for Symbol�s
NetVision family of WiFi Telephony products, he managed the software
engineering team for three years and was responsible for developing Symbol�s
WiFi Telephony products.
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