April 1999
Pulling The Wires From VoIP
BY SAMUEL LEVENSON
The two fastest growing segments in the communications industry are wireless and
Internet-related products and services. Until recently, wireless that focused on the
Internet was a mechanism for data services such as e-mail and Web browsing. Now, voice
over IP (VoIP) is rapidly developing into a major commercial activity and IP access
services are being deployed in wireless networks. The obvious opportunity here is for
untethered access to Internet multimedia and data services through wireless networks. This
opportunity is not just for VoIP, but for multimedia as well.
3G AIR INTERFACES
Third generation (3G) wireless systems, which will be deployed starting in 2000, offer
higher speed packet data services. Bandwidth in the hundreds of Kbps range opens
opportunities for extensive end-to-end mobile multimedia IP services. Currently deployed
second generation (2G) cellular systems (such as GSM, CDMA and TDMA) are optimized to
carry voice quality almost equivalent to wireline quality using low bit rate encoding -
for example, 8 or 13 Kbps for CDMA. While extensive real-time multimedia IP services on 2G
interfaces may not be practical, these systems can benefit from the lower cost of IP-based
routing and transmission in their backbone networks, providing the infrastructure for more
extensive services enabled by 3G technologies.
3G interfaces are defined by the International Telecommunications Union (ITU) standard
for International Mobile Telephony (IMT) to have global commonality of design, high
quality, compatibility with other networks, and the ability to use small packet terminals
with worldwide roaming capabilities. The data rates available on these systems depend on
the environment in which they are deployed - in particular, the standards bodies have
envisioned three environments with varying data rates available (see table below).
Environment |
Available
Bandwidth |
Indoor Office |
2 Mbps |
Indoor or Outdoor Pedestrian |
144 Kbps |
Vehicular |
64 Kbps |
These bandwidths will allow for IP-based multimedia services over
wireless access. As subscribers move among these environments, available bandwidth for
such services will change. Applications in the users' mobile devices may need to adapt
accordingly - for example, switching to a lower bit rate vocoder when the user moves to a
vehicular environment. This may also be an opportunity for providers to offer services
that transform the data stream to subscribers based on their current environment. For
example, the service provider might transcode a VoIP stream to a lower bit rate as the
available bandwidth for a user decreases after leaving an indoor office environment.
The combination of Internet connectivity and mobility creates the possibility for other
new service offerings. Voice-enabled Internet access is particularly attractive in the
mobile environment, where "hands-free" operation may be required. Location
capabilities are also an integral part of 3G. While initially developed for emergency
services applications, location services could be combined with VoIP capabilities to
provide new revenue-generating service capabilities.
While 3G systems will enable direct voice and multimedia Internet connectivity over the
air interface, it is likely that most voice traffic will still use non-IP-based channels.
Internet telephony, in these cases, may start at a gateway between the wireless access
media and the cellular infrastructure.
2G DIGITAL SYSTEMS AND THE EVOLUTION TO 3G
The current 2G air interfaces are optimized to carry low bit rate voice. Typically, these
systems use channels of 8 or 13 Kbps for voice transmission, and data rates are typically
limited to 28.8 Kbps or less. While it may be inefficient to use IP directly over the air
in these systems, there are opportunities to take advantage of VoIP in the fixed
infrastructure supporting the wireless access portion of the network.
The voice traffic from the air interface is packetized within the cellular system for
either routing to PSTN gateways or directly over IP. As long distance and other
interconnect carriers offer direct IP access, this will become more efficient than today's
circuit switched cellular infrastructures.
There are a number of initiatives to extend the data rates of current 2G systems as an
evolutionary step to 3G systems. These so-called 2.5G systems include EDGE, GPRS, and
IS95B systems with data rates of 64 Kbps and higher. 2.5G could start to offer direct
over-the-air voice and multimedia services on IP. With the introduction of 3G systems, the
infrastructure developed for the 2G and 2.5G services can be extended to offer a complete
IP-based infrastructure, the final step of the migration.
FUTURE OPPORTUNITIES AND CHALLENGES
The second and third generations of air interfaces are highly optimized for voice. As
Internet traffic, including multimedia over IP, dominates communications, enhanced or new
air interface standards that are optimized for packet data traffic may need to be
considered. These standards would allow varying quality of service (QoS) attributes to
account for differing needs of real-time voice and streaming multimedia, Web browsing,
e-mail, and other types of applications.
As Internet telephony standards such as H.323, SIP, and MGCP develop, there may be a
need to include provisions for some of the unique aspects of the mobile environment. Two
examples are the spectrum limitations of wireless services, and mobility.
The need to efficiently use spectrum has driven the wireless world to develop and
deploy extremely efficient, high quality, low bit rate vocoders. As the wireline access
world becomes less bandwidth-restricted with the introduction of technologies such as
cable modems and ADSL, the need for extremely efficient vocoding may be less then in the
wireless world. Multiple transcodings among different speech encoding mechanisms will
degrade the quality and increase the cost of Internet telephony communications between the
wireline and wireless world. This should drive the industry to develop a set of vocoding
technologies that can be shared by the wireless and wireline environments.
Mobile Internet telephony will require unique capabilities in terms of security,
privacy, and billing, as compared to fixed wireline Internet telephony. The cellular world
has tackled many of these issues in internetworking standards such as GSM and IS41. Many
of the solutions developed in the wireless telephony arena may usefully be incorporated
into both wireless and wireline Internet telephony.
Samuel Levenson is a member of the technical staff for Motorola Corporation.
Motorola's Internet and networkingGroup (ING) delivers smart access solutions to connect
people and organizations in a world of converging communications. ING provides a full
complement of wireline and wireless access solutions, including corporate networking,
broadband communications, wireless content services, and platform software. For additional
information, visit ING's Web site at www.mot.com/networking.
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