Tiếng nói mạng xu hướng và nhu cầu Sự bùng nổ của Internet và sự gia tăng nhanh chóng về số lượng người sử dụng Internet đã không đi không được chú ý. Điều gì tác động sự phát triển này sẽ có trên sự tiến hóa của công trình viễn thông hiện có mạng lưới? Khối lượng của lưu lượng dữ liệu đang tăng lên với tốc độ nhanh hơn nhiều so với giọng nói hoặc lưu lượng điện thoại (Hình 1). Trong những năm gần đây, khối lượng buôn người fic của điện thoại là lớn hơn nhiều so với số liệu, dữ liệu và kết nối mạng dựa ly duy về cách sử dụng các mạng viễn thông dựa trên chuyển mạch, hoặc sử dụng các mạng riêng biệt. Tại một số giai đoạn, có một chéo của giao thông, chẳng hạn như khi cả hai loại giao thông chiếm cùng một lượng năng lực sản xuất trong mạng. Tuy nhiên, giọng nói vẫn chiếm một mức độ lớn của nhà điều hành enue rev. Nhưng lưu lượng thoại hữu tuyến đang tăng chậm hơn ngày hôm nay, và có rất nhiều những lí do tại sao. Một phân khúc phát triển của mạng lưới làm việc bằng giọng nói được xử lý trong các mạng di động. Ngoài ra, khả năng mới tồn tại để di chuyển tất cả mọi thứ đó là dữ liệu trên giọng nói, để nó có thể chạy hoàn toàn trên mạng dữ liệu. Một số ví dụ là emailto mà một tập tin có thể được đính kèm, do đó thay thế một thông điệp hoặc fax các giải điều chế của một tin nhắn fax vì nó khuyến ters các mạng công cộng để nó có thể được gửi như là dữ liệu thay thế.
Trang 1Voice 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 telecommunicaevolu-tions 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 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 bemov-ing circuit-switched to bemov-ing 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
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 netnet-work,
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
Trang 2dependent 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
Change-over Time
Data
Voice
Access, transport &
switching networks
Access, transport &
switching networks
Services
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
Trang 3(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 serser-vice 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
Time in seconds/minutes
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
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.
Trang 4Vir tual 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
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
Trang 5ed 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 sbear-ervices, 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
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
SAM SAM
(e.g ISUP)
Resource control (access view)
AXE
Defined routes
LE3 LE3 LE2 LE2
LE1 LE1 LE3 LE3
Defined routes
AXD 301
AXE
Defined routes
AXD 301
LE1LE1LE3 LE2 Exchange 1
Exchange 2
Exchange 3
LE12 LE13 LE23 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.
Trang 6ISUP 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 inin-terface (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 transportgrow-ing 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
ISUP signaling
ATM network
Figure 8 Separate signaling is used for call control and for establishing connectivity Thus, exist-ing signalexist-ing procedures can be retained.
References
1 ITU-T I.580, General arrangements for interworking between B-ISDN and 64 kbit/s-based ISDN