By Ajay Kumar Gupta, Director of Business Development,
Hughes Software Systems, USA
Urban growth in Latin America has outpaced the growth of
voice, data and video telecommunications services. These services
tend to be expensive, lack reliability and are hard to obtain.
One solution is the convergence of data, voice and video services
on to single high-bandwidth lines using ATM technology to
help maximise the traffic-handling capacity of existing networks.
This solution, nevertheless, presents a series of challenges
which the ITU, by providing effective standards, has helped
to minimise.
The steady growth of urban areas in Latin American countries
has exposed the serious lack of resources available for telephone
and Internet connectivity expansion within these regions.
Numerous businesses are finding it difficult to obtain the
bandwidth, phone lines and telephony services they need to
conduct business and manage their day-to-day activities. Additionally,
the services they can obtain are expensive and not necessarily
reliable.
Too Many Wires
In a traditional circuit-switched network, a path is dedicated
to a call for the duration of the call. While in use, this
path cannot be utilised by other calls, even during periods
of silence in the conversation. Although the presence of a
dedicated path guarantees reliable and immediate transmission
of voice, the utilisation of bandwidth is not efficient. For
each concurrent conversation occurring, another dedicated
line is required. Large businesses, or small businesses with
heavy call traffic, require dozens of expensive phone lines
that each require a physical connection to the telephony network.
The high costs associated with multiple lines and the difficulty
in securing these multiple connections can be prohibitive
even in large cities. Additionally, the often necessary requirement
of connecting multiple corporate offices to a single circuit
to share calls can be complex. A solution is needed that can
reduce telephony network costs, make better use of bandwidth,
and solve the complicated logistical issues of physical wires
for each phone line.
The ATM Solution
Asynchronous Transfer Mode (ATM) is a multi-service, high-speed,
scalable technology that has been designed to carry voice
and all types of data, such as facsimile and multimedia over
a single fibre line. Voice over ATM (VoATM) provides a platform
for the transport of plain old telephone service over a digital,
broadband access network. VoATM can support toll-quality voice,
a Quality of Service parameter associated with traditional
circuit-switched networks. In addition, ATM provides efficient
use of band-width, high network efficiency and, therefore,
lower operational costs.
The Wide Area Network (WAN) environment today is made up
of a variety of services fulfilling the requirements of a
multiplicity of applications. Each service provides unique
benefits for certain communication needs. However, ATM, being
a multimedia, multi-service technology, has the potential
to support all network applications more efficiently than
any other single internetworking protocol. This leads to improved
performance and lower communication costs for both the service
provider and the user. With the standards already evolved,
ATM’s capacity to transport voice and deliver high-quality
speech has been recognised. The advantages of ATM are:
Maximum network utilisation:
ATM provides virtual networking, making statistical multiplexing
of traffic over any network resource possible. Virtual networking
attempts to utilise as much of the network as possible, thus
conserving relatively limited WAN fibre. Network capacity
is shared among the numerous phone calls and data services
occurring on a network at any one time. Considering that typical
networks utilise only 35 per cent of the available bandwidth
and waste the rest during idle, quiet, times ATM networks
can take full advantage of network bandwidth by allocating
resources and bandwidth to a user only when required. This
allows the network to support more users, usually twice the
number of users that a typical Time Division Multiplexed network
can support.
High-quality phone service:
ATM supports extensive Quality of Service, which ensures
the reliable transmission of voice traffic across a network.
While typical Voice over IP (Internet Protocol) conversations
are filled with static and echoes, VoATM supports clear, full
duplex conversations that are undistinguishable from standard
phone quality.
Traffic management built in:
ATM switches have been designed with effective traffic management
capabilities. For example, call admission control, usage parameter
control and traffic shaping are supported.
Supports multiple services on a single line:
ATM supports the transfer of voice, data and video signals.
ATM can easily inter-network with the Public-Switched Telephone
Network. In addition, different ATM Adaptation Layers provide
support for different service class capabilities. No longer
do you need separate networks and separate services for your
voice and data needs. ATM brings true convergence to your
telephone and networks needs.
However, Not All Is Rosy
While ATM does provide low cost, single line solutions to
many of the challenges facing businesses and homeowners in
Latin American countries, the transport of voice across an
ATM network requires that a number of technical challenges
be successfully managed if the network is to provide an effective
voice transport mechanism. The convergence of data and voice
traffic on a single network yields significant economic benefits,
but, at the same time, it presents technical challenges.
Must Cut Down Delays
Have you ever made an international phone call and had a
2–3 second delay or loud echo within your conversation? While
this still occurs, typical phone circuits now contain echo
cancellors to reduce the echo and delay that occurs during
a conversation. In a VoATM environment this problem must be
managed ‘up front’ for the service to be acceptable. There
are a number of issues that must be addressed to make ATM
delays acceptable to end-users. Because ATM networks transfer
voice and data in packets, there are delays created in encoding
the packets, buffering the data, and filling the packets with
the correct amount of data. A method is needed to reduce the
time ATM networks spends encoding data and creating the necessary
packets.
Silence suppression
The characteristics of voice can be employed to advantage
for optimal use of available bandwidth. A normal flow of conversation
consists of pauses between sentences and periods of silence
when there is no speech. Voice communication is typically
half-duplex—that is, one person is silent while the other
speaks. These characteristics can be used to save bandwidth
by halting the transmission of packets during such periods
of silence. This is known as silence suppression. ATM implantations
must take advantage of silence gaps in conversations to increase
network utilisation.
Synchronisation
When voice is transported across a network, it is important
to synchronise the data that is transmitted from the speaker
to the listener. This can be achieved by employing standard
mechanisms. In case of ATM, the easiest solution (owing to
the availability of the global timing standards) is to adopt
an externally synchronised model, where each node is synchronised
to some external clock source. ATM services must have these
synchronisation features built in to make them usable.
The Emergence of a Standard
The International Telecommunications Union/Telecommunication
Standardisation Sector (the ITU-T) has created a standard
for ATM data/voice usage known as AAL2.
AAL2 provides for the bandwidth-efficient transmission of
low-rate, short and variable packets for delay-sensitive applications.
AAL2 also provides bandwidth reduction support because of
voice compression, silence detection /suppression and idle
voice channel removal. AAL2 supports multiple-user channels
on a single ATM virtual circuit and also supports varying
traffic conditions for individual users or channels.
User Benefits of AAL2
A major benefit of ATM trunking using AAL2 for narrowband
services is bandwidth savings. This can be achieved by:
-
Compressing
voice. When voice is compressed, lesser bandwidth is allocated
per call.
-
Releasing
bandwidth when the voice application does not need it.
Bandwidth can be released when the talker is silent or
when the call is completed.
-
Routing
and switching narrowband calls on a per call basis. This
leads to further improvements in performance and efficiency.
The result is a usable, practical ATM solution that can bring
voice and data convergence on a single line to end-users around
the world.
The Future
Telecommunications companies around the world are beginning
to build solutions around the AAL2 protocol. Telephonic software
companies have now developed software stacks and tools that
are making it easier to build, support and implement ATM solutions
based on AAL2.
One such solution, called Loop Emulation Service (LES), drastically
reduces the developer’s time to market and provides rapid
application development for VoATM-based product development.
Besides being a modular, scalable, portable stack, a LES stack
can be customised for national specifications.
A reference model of LES, which uses AAL2, is depicted in
the Figure. The reference model consists of three main components:
-
AAL2-enabled
Customer Premises Equipment/Customer Premises Inter-working
function (CPE/CP-IWF). This gives voice packets priority
over data calls, ensures toll-quality voice delivery and
sends the packets over Digital Subscribers Link (DSL)
lines.
-
ATM-based
switched or permanent virtual circuits, which are used
to connect the customers premise with the central office.
-
Voice
gateway, which ‘unpacks’ voice packets and converts them
to a standard format for transmission to a voice terminal.
What it Means for your Business…
Owing to its low operational costs, multi-service capability
and higher network efficiency, ATM is emerging as a solution
for service providers and network operators, especially in
the WAN environment.
These solutions will especially help companies in areas of
the world such as Latin America that do not have the infrastructure
in place to lay additional phone lines and provide the necessary
services.
Mr. Ajay Kumar Gupta is the Vice-President and General
Manager of Hughes Software Systems Operations in the Americas.
Mr. Kumar has been working in the field of communications
for over 14 years and has been involved with both RF and Terrestrial
communications.
Previously, Mr. Kumar worked with Hughes Network Systems
(India). He was responsible for the Business Development of
the Wireless Local Loop Product line of Hughes in and around
India. He was also involved in setting up Hughes Ispat Limited,
the basic telephony operator in the state of Maharashtra.
Mr. Ajay Kumar Gupta has done his Masters in Systems
Engineering from the Indian Institute of Technology Kharagpur
and Bachelors in Electrical Engineering (Communications) from
Jawahar Lal Nehru University.
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Last updated : February 2, 2004
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