VoDSL

The four basic elements that make up the VoDSL
architecture are the IAD (Integrated Access Device)
the DSLAM (Digital Subscriber Line Access Multi-plexer),
the data switch and, the voice gateway.
The IAD will reside at the subscriber’s location and
most frequently consists of two to sixteen analog POTS
(Plain Old Telephone Services) ports. The POTS lines
can connect any analog telephones, fax machines,
modems, KTS (Key Telephone Systems), or PBX
systems. The IAD will also have a data connection of
some type such as Ethernet, USB, ATM25, or V.35.

The DSLAM usually resides at the ILEC’s central
office. The DSLAM’s main purpose is to terminate
DSL lines from the customer premises and act as a
backhaul facility for data and voice traffic from the
subscriber. The role of the DSLAM in the VoDSL
architecture is vital because it provides the perfect
physical pathway for voice and data—enabling them to
coexist on a single copper pair.

The data switch uses an ATM, Frame Relay, or IP
protocol that receives traffic from each DSLAM and
redirects it to an appropriate termination point. This
switch is an efficient way to link data traffic to the ISP
infrastructures and direct voice traffic to the final
component of the VoDSL architecture—the voice
gateway.

There are multiple choices of VoDSL products today
and each uses a different protocol such as ATM, Frame
Relay, or IP for connection to the network. Jetstream is
a company that offers ATM and Frame Relay VoDSL
products. Coppercom and Accelerated Networks offer
only ATM products. Tollbridge and Lucent Technolo-gies
offer IP VoDSL products. Sprint ION will go with
ATM as its packetized voice solution (Fred Dawson,
"FocusOn: A Work in Progress," Inter@ctive Week,
July 1999) and Covad has announced plans to provide
VoDSL to consumers and businesses also utilizing
ATM transport technology.
VoDSL with Frame Relay is required by some
DSLAMs, such as Lucent’s DSLTNT and Copper
Mountain’s Copper Edge which can only use Frame
Relay on the network between the IAD and the
DSLAM. While standard development is underway,
most Frame Relay networks don’t currently have built-in
QoS (Quality of Service) that is needed to prioritize
the voice and data traffic out to the network. As a
result, a proprietary priority mechanism needs to be
implemented in the DSLAM to support the traffic.
This mechanism is needed because the one VC (Virtual
Circuit) that carries voice and the other VC that carries
data will compete to gain access to the network. Since
voice is time-sensitive and any delays can jeopardize
voice quality, it must be given the highest priority.
Both Lucent and Tollbridge’s IP-based VoDSL products
are flexible because they are independent of the
DSLAM subscriber technologies. However, it is still
necessary to maintain priority of the voice and data
traffic.

The best way to insure the integrity of voice traffic is to
utilize the QoS that ATM protocols provide. ATM has
a built-in mechanism to handle time-sensitive data,
such as voice, and ATM allows voice to transmit out of
the DSLAM uninterrupted. In this protocol the voice

VC is given priority over the data VC. The data VC is
typically set up as UBR (Unspecified Bit Rate), while
the voice VC is configured as VBR (Variable Bit Rate)
in real time.

Most of the solutions mentioned require ATM connec-tion
from the DSLAM to the Voice Gateway, typically
using a DS3 or OC3 uplink for connection to the ATM
network.

Voice Quality

The Service Providers have their own guidelines for
offering voice and data services and the key to the
success of VoDSL is that it has to be a transparent
element. Subscribers will not tolerate any voice service
that is not as good as what they are receiving today.

The industry has accepted two modulation techniques
for toll-quality voice: 64Kbps PCM (Pulse Code
Modulation) and 32Kbps ADPCM (Adaptive Differen-tia
lPulse Code Modulation). All the Companies and
products previously mentioned support these modula-tion
techniques.

VoDSL Solutions

Next-generation DSL access concentrators will offer
high performance and high capacity traffic aggregation
and scalability. Service providers need them to support
both frame relay and ATM protocols for communica-tion
with customer premise equipment. Furthermore,
they must offer diverse QoS levels for subscribers and
easily accommodate the VoDSL infrastructure.
Interoperability with the likes of Jetstream,
Coppercom, Tollbridge, and Lucent’s PathStar will also
be essential feature next-generation DSL access
concentrators.

The PathStar integrates the voice gateway and the
Class 5 switch, significantly reducing the complexity of
deploying voice services. This also lowers the cost per
subscriber versus the combined voice gateway and
Class 5 switch, enabling independent telcos and CLECs
to more easily and cost-efficiently provide alternative
voice service. As CLECs become better equipped to
provide VoDSL solutions at lower operating costs, they
will be able to compete with the larger carriers and
offer end users high-quality voice and data with equal
or higher levels of guaranteed service.