Tài liệu tham khảo voice and telephony networking over ATM

Voice networking trends
and needs
The explosion of Internet traffic and the
rapid increase in the number of Internet
users has not gone unnoticed. What impact
will these developments have on the evolu-
tion of existing telecommunications net-
works?
The volume of data traffic is increasing at
a much faster pace than voice or telephony
traffic (Figure 1). In the recent past, the traf-
fic volume of telephony was much greater
than data, and data networking relied main-
ly on either using the telecommunications
network based on circuit switching, or using
separate networks. At some stage, there is a
crossover of traffic, such as when both types
of traffic occupy the same amount of capac-
ity in the network. Nevertheless, voice still
accounts for a greater degree of operator rev-
enue.
But wireline voice traffic is increasing
more slowly today, and there are several rea-
sons why. A growing segment of voice net-
working is handled in cellular networks.
Also, new possibilities exist for moving
everything that is data-on-voice, so that it
can be run entirely over data networks. Some
examples are e-mail—to which a file can be
attached, thus replacing a fax message—or
the demodulation of a fax message as it en-

ters the public network so that it can be sent
as data instead.
At some point, the volume of data traffic
will become so much greater than voice traf-
fic that it may prove more prudent to run
voice on a packetized data communications
medium instead of data-on-voice or over
separate networks. Some sources predict
that this changeover will occur sometime in
the next few years.
On the terminal or user side, the trend is
also toward using packetized voice. This is
already true of mobile phones, telephony
software clients running on personal com-
puters and local area network private branch
exchanges (LAN PBX or virtual PBX).
The prospect of providing voice transport
on bandwidths less than the common
64 kbit/s at lower transmission costs is en-
ticing to new operators in deregulated en-
vironments, as well as to established opera-
tors as a way of staying competitive. As tech-
nologies become available and viable for
voice coding, efficient software implemen-
tations that run on standard personal com-
puters or on digital signal processors
(DSP)—whose capacity is constantly in-
creasing—will be widely used. Yet another
trend is the way in which telecom networks
and services are evolving from a vertical to

a horizontal orientation (Figure 2).
The data and voice networking trends de-
scribed above are in line with the prospect
of a packet-based connectivity network.
Thanks to its flexibility and quality-of-
service (QoS) guarantees, asynchronous
transfer mode (ATM) has proven itself ca-
pable of delivering cost-effective switching
and transport for a connectivity network.
ATM has also been selected for providing
switching and transport in the third-
generation mobile access networks, which
strengthens the case for ATM as a part of the
connectivity network.
As things stand, voice seems to be mov-
ing from being circuit-switched to being
packet-switched, which heralds a new op-
tion for voice networking. Thus, established
operators will have to consider a change of
course from circuit-switched to packet-
switched networks; but the transition will
have to be run smoothly, without a negative
impact on services in terms of richness of
feature, quality of service, or reliability.
Moreover, by using ATM in their networks,
new operators will be able to provide voice
and telephony in a common network
together with data, video and Internet
services.
Operator and end-

user benefits
Moving voice networking to ATM benefits
the network operator; happily enough, it can
also benefit the end-user. In essence, the op-
erator’s benefits are wholly economical.
However, when it will become more eco-
nomical to run voice on an ATM network is
40
Ericsson Review No. 1, 1998
Voice and telephony networking over ATM
Jan Höller
Voice and telephony still represent the largest volume in today's telecom-
munications networks, both in terms of traffic and generated revenue. How-
ever, as new communications services are introduced—particularly services
generated by applications on the Internet—the telecommunications net-
work must evolve to meet the increasing demand. A connectivity network,
in common use, that can handle the emerging multitude of applications and
services as well as existing services, such as telephony, can reduce opera-
tional costs. ATM is a technology that provides the flexibility and strict
quality of service required by a network of this type.
The author describes how ATM may be used for introducing voice and telepho-
ny into a truly multiservice network without reducing the range or capacity of
existing services. Thus, operators can enjoy the full flexibility and capabilities
of voice and telephony over ATM, which offers seamless interoperability and
management, alongside existing telephony services and networks.
Box A
Abbreviations
AAL ATM adaptation layer
ADPCM Adaptive differential PCM
AM Application modularity

ATM Asynchronous transfer mode
AVS AXE-served voice services
B-ISDN Broadband ISDN
CATV Community antenna TV
CBR Constant bit rate
CE Circuit emulation
CS-ACELP Conjugate-structure algebraic-
code-excited linear prediction
DCME Digital circuit multiplexing
equipment
DSP Digital signal processor
DSS1 Digital signaling system 1
IN Intelligent network
ISDN Integrated services digital net-
work
ISUP ISDN signaling user part
ITU International Telecommunica-
tion Union
IWF Interworking function
LAN Local area network
NNI Node network interface
PBX Private branch exchange
PCM Pulse code modulation
PRA Primary rate access
PSTN Public switched telephone net-
work
QoS Quality of service
RMP Resource module platform
SAM Service access multiplexor
SDH Synchronous digital hierarchy

STM Synchronous transfer mode
SVC Switched virtual connection
TDM Time division multiplexing
UNI User network interface
VAD Voice activity detection
VBR Variable bit rate
VCC Virtual channel connection
VPN Virtual private network
VPNA Virtual private networking over
ATM
VTNA Voice transit networking over
ATM
VTOA Voice and telephony over ATM
Ericsson Review No. 1, 1998
41
dependent on individual operator circum-
stances.
The benefits to end-users are basically
two-fold. They can be offered a flexible tar-
iff based on the desired quality of service.
Thus, a lower voice quality can be chosen as
a cheap “tourist class” service. On the other
hand, if users so request, a higher audio qual-
ity can be provided for certain applications
such as conference calls. Furthermore, end-
users can opt for integrated access to all ser-
vices, making life easier.
Equipment and operations cost savings
Thanks to the multiservice capabilities of
ATM, it is possible to provide a common

network for all services. This means that the
cost of node equipment is reduced, as it can
be shared by all applications. ATM allows
common access to residential users as well
as to business users, for whom voice is inte-
grated with other applications. The edge
and core switching equipment is also shared
among the different applications.
With a smaller amount and variety of
equipment in the network, the cost of oper-
ation and maintenance is lower. Compared
with a synchronous transfer mode (STM)
voice network, an ATM-based network,
with its higher switching speed, requires
less node-interface hardware.
Transmission cost savings
The obvious benefit that comes to mind
when we talk of voice over ATM is its po-
tential for lowering transmission costs. Sig-
nificant savings can be made on access to ser-
vices and other links where transmission is
expensive; for example, on international
links. Because ATM permits the transmis-
sion capacity used in the network backbone
to be reduced, operators who own their own
transmission networks can sell spare capac-
ity as leased lines to third parties.
Support for ways of saving on transmis-
sion cost is being explored and developed in
a number of standardization activities

(Box B). If ATM’s capability of supporting
on-demand bandwidth or resources is put to
use, then we need only reserve as much band-
width as is needed at any given instance. By
contrast, in a synchronous digital hierarchy
Volume
Crossover
Change-
over
Time
Data
Voice
Access, transport &
switching networks
Access, transport &
switching networks
Services
Today's solution Migratory solution Future solution
PSTN/ISDN
Data (FR etc)
Mobile
CATV
Connectivity networks
Service nodes
Service networks
CATV
Mobile
Data
PSTN/ISDN
Figure 1

The volume of data traffic is increasing more
rapidly than the volume of voice traffic. At
some point in time, the data traffic volume
will completely dominate.
Figure 2
Networks are evolving away from a vertical
orientation toward a horizontal orientation. In
the future, several capabilities previously
dedicated to specific networks will be com-
mon. Specifically, the trend is toward sepa-
rated service networks and connectivity net-
works.
Box B
Standards for voice and telephony
over ATM
In the past two years, standardization bodies
have taken an increasing interest in voice and
telephony over ATM (VTOA). In a very short time,
the ITU-T drafted and approved the new speci-
fication, AAL type 2, which was developed with
voice over ATM as the target application.
Although it is not a standardization body, the
ATM Forum has great influence on standard-
ization work and is developing a number of
implementation agreements for VTOA. This
work is currently being carried out by the VTOA
working group of the ATM Forum.
ITU-T
I.363.1
B-ISDN ATM adaptation layer (AAL) specifica-

tion types 1 and 2, 1996
I.363.2
B-ISDN ATM adaptation layer specification type
2, 1997
I.Trunk
AAL2 SSCS for Trunking, Draft
ATM Forum
af-vtoa-0078.000
Circuit emulation service interoperability spec-
ification v2.0, 1997
af-vtoa-0085.000
Dynamic bandwidth circuit emulation service,
64 kbit/s trunking, 1997
af-vtoa-0089.000
ATM trunking using AAL1 for narrowband ser-
vices 1.0, 1997
btd-vtoa-lltaal2-00.02
ATM trunking using AAL2 for narrowband ser-
vices, Draft
(SDH) network, capacity is reserved on a
(semi-) provisioned basis dimensioned to
cater for the busy hour. ATM-switched vir-
tual connections (SVC), on the other hand,
are used to transport voice traffic as demand
requires. Bandwidth not used by voice may
then be used by other applications
(Figure 3).
In addition, the introduction of alterna-
tive schemes for voice coding with com-
pression and silence removal produces vari-

able bit-rate (VBR) voice traffic for which
the new AAL type 2 (Box C) was developed.
This results in still more substantial band-
width savings, since compression is used and
little or no bandwidth is consumed during
silent periods. By its nature, VBR also fa-
cilitates gains in dynamic multiplexing.
Service-related benefits
By preparing the way for a specific migra-
tion path, the voice network can evolve
smoothly toward an ATM-based network—
a smooth migration is much preferred to a
drastic replacement. By building on exist-
ing services in the public switched tele-
phone network (PSTN) and integrated ser-
vices digital networks (ISDN), but replac-
ing STM transport with ATM, we can en-
sure total and seamless interoperability with
the existing networks. In this way, parts of
the network may be moved to ATM, while
other parts remain in STM.
The consolidation of PSTN, ISDN and in-
telligent network (IN) services provides ser-
vice transparency; that is, the service offer-
ing is independent of transport technology.
It will not only be possible to offer seamless
service interoperability, but seamless service
management as well. Customers and ser-
vices can be centrally managed in the same
way regardless of whether they are connect-

ed to an STM or ATM network. Existing
narrowband services may be consolidated by
reusing the network equipment and soft-
ware in which investments have already
been made. This is described in the two fol-
lowing product applications, which were
designed to provide full support of voice and
telephony services in an ATM network.
42
Ericsson Review No. 1, 1998
Time in seconds/minutes
Capacity
Capacity used for
other applications
Bandwidth release
Bandwidth allocation
Allocated bandwidth
Actual telephony load
Maximum available capacity
Capacity used for voice
Box C
Circuit-switched vs packetized
voice over ATM
Preferably, circuit-switched 64 kbit/s
pulse-code modulated voice is transport-
ed over ATM using ATM adaptation layer
type 1 (AAL1). Besides being used for
circuit-emulation services, AAL1 supports
the transport of constant-bit-rate (CBR)
channels such as voice.

With the introduction of compressed
voice, the bit rate is lower, maintaining
typical values of, say, 8 kbit/s for CS-
ACELP. The new voice-coding schemes
can also be combined with voice activity
detection (VAD), which exploits the fact
that—on the average—60% of a normal
conversation is silence. During these
silent periods, the bandwidth used for
transmission may be significantly low-
ered. This effectively produces a variable-
bit-rate (VBR) source out of the speech
traffic. With its characteristic CBR at fixed
bandwidths, AAL1 is not suitable for this
type of traffic. However, a new AAL type 2
has been developed to provide
bandwidth-efficient transport of low-bit-
rate, real-time services such as VBR voice.
Moreover, AAL2 is capable of providing
dynamic multiplexing gain as well as
reduced delay characteristics. With AAL2,
voice and sources of different nominal bit
rates can be efficiently multiplexed into
the same ATM cell stream while main-
taining the strict quality-of-service require-
ments for voice services (Figure C1). Thus,
AAL2 provides an efficient means of trans-
porting packetized voice on ATM.
Figure 3
In an ATM network, bandwidth used for voice

can be allocated on demand. Momentarily
unused bandwidth can be used for other
applications. Thus it is possible to maximize
the use of resources.
Voice
Ch 155
Voice
Ch 1
AAL2
Common part
Voice
Ch 7
Voice
Ch 1
AAL2 user
ATM
Silence Ch 3
Silence Ch 77
Data
ATM cell ATM cell
Packet header
Cell header
Figure C1
AAL2 allows the multiplexing of voice chan-
nels—as well as data—with different charac-
teristics into the same ATM cell stream.
Ericsson Review No. 1, 1998
43
Virtual private networking
over ATM

The main goal of providing virtual private
networking over ATM (VPNA) is to enable
the operator to deliver voice and telephony
virtual-private-network services in a cost-
effective way, primarily to business cus-
tomers. Customers are connected to a multi-
service network that uses ATM as the com-
mon connectivity layer, as described earlier.
Existing PSTN/ISDN and intelligent-
network services—the latter being of pri-
mary interest to VPNA—provided by AXE
are consolidated and deployed on a switched
ATM network using the AXD 301. Voice
is transported entirely in ATM, end-to-end.
The typical broadband network into
which VPNA has been built is depicted in
Figure 4. The network is deployed to sup-
port the communication needs of any size
business. The services offered include exist-
ing services, such as telephony and data
communications, as well as new video and
multimedia services.
VPNA uses the service access multiplex-
or (SAM) as the integrated access equipment
for providing all services. The service inter-
faces supported by the SAM are primary rate
access (PRA) to PBXs, circuit emulation
(CE), and a range of data interfaces, such as
frame relay and native-ATM—for example,
for connecting to routers. The SAMs can be

located on customer premises or in a cam-
pus environment, thus serving one or more
customers.
The SAM is connected to a switched ATM
backbone network via a standardized ATM
user-network interface (UNI). The
AXD 301 shown in the figure could be an
integral part of the ATM network or it could
be connected to the network by a user-
network interface (UNI). The AXD 301
supports the telephony application with
AXE. The addition of the VPNA features
does not require any new functionality in
the ATM network other than support for
switched virtual connections according to
standards.
As mentioned above, the virtual private
network and intelligent network services,
which have successfully been implemented
in AXE, are used for voice calls that are
switched end-to-end in the ATM network.
This is facilitated by separating call control
from the bearer services by introducing two
new resource types into the resource mod-
ule platform (RMP) of AXE (Figure 5):
• the switch view—which represents the
ATM connectivity that is used for voice
and controlled by the AXD 301;
• the access view—which handles remote-
ly located primary-rate accesses that con-

nect PBXs to the service access multi-
plexers.
The switch view resource type is a virtual
switch in the AXD 301 that emulates a
switching fabric to be controlled by AXE.
The AXE-served voice services (AVS)—
which are an AXD 301 software applica-
tion—are capable of establishing, control-
ling and releasing ATM connections in the
network as the telephony call control in
AXE requires. To this end, the application
relies on the general ATM services provid-
ed by the AXD base system.
For AXE to provide the full set of ser-
vices, such as routing and billing, current
narrowband signaling procedures must be
retained. Thus, call-control signaling,
which uses protocols, such as digital sig-
naling system 1 (DSS1) and ISDN user part
(ISUP), is transparently transported
through the ATM network and terminat-
Centrex
SAM
PBX
Customer premises
SAM
RSS
PBX
Switched ATM network
PSTN/ISDN/PLMN

IN
Internet
AXD301
AXE
AXD301
AXE
VPN
SAM
PBX
Campus
PBX
FR, DXI,
ATM
SS7
PSTN/ISDN
SS7
Figure 4
An ATM-based multiservice network that pro-
vides voice and telephony services to busi-
ness customers. Virtual private networking
over ATM consolidates the services of AXE,
but switches voice end-to-end in ATM.
XSS
Resource control
AVS
AXD base system
AXE
AXD 301
AM AM AM
RMP

Figure 5
A separation of call control from the bearer
services is facilitated by new resources in the
AXE RMP. The AXD 301 emulates the switch-
ing fabric to the RMP.
AM Application module
AVS AXE served voice services
RMP Resource module platform
XSS Existing source system
ed in AXE software (Figure 6). The
resource-control protocols required for the
access and switch views are terminated in
the access equipment and the AXD 301
application, respectively. In this way, AXE
and the AXD 301 work much like a tele-
phony server. Standard ATM signaling ca-
pabilities are used to establish and release
connectivity.
Maintaining ATM end-to-end means
only using one STM-ATM transition for
voice, which guarantees toll-voice quality,
especially when voice compression is used.
Moreover, in the same way that call con-
trol is completely separated from the bear-
er services, configuration and maintenance
of the ATM network are separated from the
administration of the telephony services,
such as route planning and billing func-
tions.
In addition to separating call control from

bearer services, the AXD 301 provides the
VPNA gateway to other networks, public
and private, such as to the PSTN, ISDN and
cellular networks.
Voice transit networking
over ATM
The product application known as voice
transit networking over ATM (VTNA) al-
lows large volumes of transit telephony traf-
fic to be transported over an ATM network.
As mentioned above, VTNA can serve as a
path over which parts of the PSTN/ISDN
can migrate—using ATM as the common
connectivity layer—to become a multi-
service network. A part of the migration
strategy might be to introduce ATM for
handling traffic expansion, or to provide an
alternative redundant transit plane. ATM
may also be used by new operators who will
be providing telephony services in a new en-
vironment.
In the existing STM-based networks,
trunk interfaces are dedicated to specific
routes, and trunking transport capacity is
provisioned according to busy-hour traffic
needs. With VTNA, it is possible for node
and network resources to be used more effi-
ciently. Because VTNA uses on-demand
ATM virtual-channel connections for trans-
porting groups of telephony trunks of op-

tional sizes, the ATM network bandwidth
is shared between the different routes, as are
other applications such as data communica-
tions. Using ATM also means that the
AXD 301 ATM interfaces are shared on-
demand between the different routes and ap-
plications as the traffic pattern requires
(Figure 7).
State-of-the-art voice compression is pos-
sible on links where transmission costs need
to be reduced; for example, in international
links where combined voice and data
digital-circuit-multiplexing equipment
(DCME) capabilities are required.
The VTNA application is based on the
principle of network interworking (as de-
fined in ITU-T Recommendation I.580),
where telephony services are provided
transparently by AXE using standard ISUP
(Figure 8). By using network interworking
in this first step, we ensure that service
transparency can be achieved—with max-
imized reuse of existing applications. The
44
Ericsson Review No. 1, 1998
XSS
Resource control (switch view)
AVS
AXD base system
AXE

AXD 301
AM AM
RMP
Call control
(e.g. DSS1)
Call control
PSTN/ISDN/mobile network
Switched ATM network
IWF
IWF
PBX PBX
Voice path
Voice path
SAMSAM
(e.g. ISUP)
Resource control (access view)
AXE
Defined routes
AXE
AXD 301
LE3
LE3
LE2
LE2
LE1
LE1
LE3
LE3
Defined routes
AXD 301

AXE
Defined routes
AXD 301
LE1
LE1
LE3
LE2
Exchange 1
Exchange 2
Exchange 3
LE
12
LE
13
LE
23
Route
ATM network
Allocated trunk
group VCCs
Load
Figure 6
AXE software controls the call services and
access and switch-view resources. The voice
path, however, can be switched end-to-end in
ATM. As depicted in this figure, the AXD 301
and AXE operate as a gateway to the public
network.
Figure 7
The amount of trunking capacity is allocated

depending on traffic load. The ATM network
bandwidth is shared between the routes as
well as between different applications.
Ericsson Review No. 1, 1998
45
ISUP signaling may be embedded in the
ATM virtual-channel connections for
transport to the far-end AXD 301 and AXE
or a separate standard SS7 network may be
used. The AXD 301 provides the dynam-
ic trunking capabilities as well as the nec-
essary interworking with ATM. In this
context, AXE is a full-fledged standard
AXE.
A new type of device, which represents
the dynamic ATM trunk group and controls
the resources of the AXD 301, has been in-
troduced into AXE. When AXE requires
more trunk-group capacity, this is commu-
nicated to the AXD 301 via a duplicated in-
terprocessor bus between the two systems.
The AXD 301 then uses standard ATM sig-
naling procedures, over a user-network in-
terface or node-network interface (NNI), to
establish the necessary ATM virtual chan-
nel connections through the ATM network
to the appropriate destination. By mapping
trunk groups—that is, several voice chan-
nels—on individual ATM virtual-channel
connections, voice delay due to ATM cell

packetization can be kept at a minimum,
and the signaling load presented to the ATM
network can be kept low.
Future developments
Thanks to the flexibility of ATM, future
voice and telephony-like services will be
able to support sub-rate voice, or any voice
rate at all. The newly standardized AAL
type 2 protocol is perfectly suited to achieve
this objective. Although the development
of AAL2, in which Ericsson was instrumen-
tal, was primarily meant to facilitate the use
of mobile applications, AAL2 may also be
used in wireline networks where VBR voice
transport is one of the considerations. The
use of compressed voice with adaptive dif-
ferential pulse code modulation (ADPCM)
and conjugate-structure algebraic-code-
excited linear prediction (CS-ACELP) is of
particular interest, in that it can provide
voice transport down to 8 kbit/s with voice
of near-toll quality.
Because the product applications pre-
sented here support the separation of call
control from the ATM bearer services,
switched voice over ATM is feasible using
any underlying physical infrastructure. One
area of interest is in a community antenna
TV (CATV) network, where ATM is being
supported.

Conclusion
As the amount of data traffic exceeds the vol-
ume of voice traffic, it may prove more vi-
able to run voice on data than as separate
networks or data-on-voice. There is a grow-
ing trend toward transporting voice in pack-
etized format.
ATM technology is very well suited for
voice traffic as it provides on-demand,
flexible-bandwidth connections on a large
scale. This, in combination with the guar-
anteed quality of service that ATM brings
to real-time services, makes ATM an ideal
choice for circuit as well as packetized voice
transport.
Building a multiservice network with
ATM switching for voice, video and data
services provides the operator with a num-
ber of cost advantages in the service pro-
duction chain.
By combining the versatility and flexi-
bility of the AXE system with the ATM
switching capabilities of the AXD 301 sys-
tem, it is possible to consolidate existing
PSTN, ISDN and IN services, while at the
same time providing voice switching over
ATM. This technique is utilized in both the
virtual-private networking over ATM and
voice-transit networking over ATM prod-
uct applications, which support the smooth

and seamless migration to a multiservice
network of existing networks as well as in-
teroperability between new ATM-based
networks and existing networks.
XSS
Resource control
AVS
AXD base system
AXE
AXD 301
RMP
UNI/NNI
Bus
STM ATM
ISUP signaling
ATM network
Figure 8
Separate signaling is used for call control
and for establishing connectivity. Thus, exist-
ing signaling procedures can be retained.
References
1 ITU-T I.580, General arrangements for
interworking between B-ISDN and 64
kbit/s-based ISDN