networks: The future?
are some global trends in the communication arena,
which are leading to the evolution of converged
networks and the challenges involved in building
generation networks must be capable of handling
voice, video, and data communications with proper
QoS in a packet-based transport/switching infrastructure
the future, when data will become the dominant component,
it will be more practical to carry voice over data
formation of the Bell Company in 1876 marked the
arrival of the Public Switched Telephone Network
(PSTN). From a network that supported
just voice calling, it has today evolved to offer
advanced services. It represents $250 billion in
network investment and hundreds of billions of dollars
in annual revenues.
The Internet originated in the 1960s as ARPANET
from a packet-switched research project sponsored
by Advanced Research Project Agency (ARPA).
IP networks and PSTN employ conceptually
The PSTN uses a switched circuit network architecture
(SCN) that is an end-to-end connection from the
calling party to the called party. This includes
two distinct layers, the circuit
layer and the switched transport
layer. With all signaling traffic
traveling on the Signaling System 7 (SS7) control
network, transport resources are freed up from signaling
traffic. PSTN consists of large, centralized, proprietary
Class-5 switches with remote switching modules (RSMs)
and digital loop carriers
(DLCs). PSTN offer very high reliability,
excellent QoS for voice communications and speech
delivery with exceptional clarity.
On the other hand, IP networks employ connection-less,
"best-effort" Packet Switched Network
(PSN) architecture, which offer very little or no
QoS guaranties. Though the PSDN user-base is substantially
smaller than PSTN, it is growing at an exponential
Telecom Reforms-The Catalyst
U.S. Telecommunications Act of 1996 prompted global
trends towards deregulation and liberalization of
telecommunications. This Act is now widely expected
to be the beginning of a new age of innovative and
affordable telecom services.
But consumers are yet to see any of this happen
in a substantial way. The primary reason for this
failure is that though the policies have changed,
there is little change in the underlying infrastructure
of the service providers and new service creation
continues to take a 12 to 18 month period.
Network providers at the transport layer see their
product as 70 percent network with a 30 percent
wrap of services, but customers view the same product
in the reverse proportion.
Network providers dealt with this rise in data by
carrying it on the analog
and TDM networks designed and optimized for voice
or by building separate, parallel networks for high
capacity data traffic. In the past, when voice traffic
dominated the network, this approach made sense.
In the future, when data will become the dominant
component, it will be more practical to carry voice
over data networks the "convergence networks".
Convergence Networks The Interim solution
protect multi-billion dollar investment in their
existing networks, operators will prefer a gradual
transition to an all-packet next generation network,
spread over a decade or more, instead of whole scale
replacement. Hence, some sort of "convergence
network" (next-generation data services layered
on top of the existing networks) to bridge the gap
between PSTN and emerging PSDN is necessary.
The converged network will allow co-existence of
PSDN, PSTN and also the calls that originate on
the IP-side of the network to terminate on the PSTN-side
and vice-versa. Voice over IP (VoIP) uses the Internet
Protocol (IP) to transmit voice as a packet over
an IP network. The voice signals are digitized,
compressed and converted into IP packets before
being sent across an IP network.
These new networks will be built on two basic building
blocks a core transport/switching layer and an access
edge. Initially, the core transport/switching infrastructure
will be based on IP/ATM core switches, which form
the backbone of the network with IP cores supporting
Internet-related traffic and ATM cores carrying
While IP and ATM will dominate the transport layer
and are well suited for Class-4/toll circuit switching,
Class-5 TDM will be required in the network for
some time. So circuit switching and packet switching
will coexist for some time, with IP, ATM, and TDM,
all playing complementary roles. Eventually, IP
and ATM networks will merge, with the ATM core becoming
transparent and the IP becoming the principal protocol.
However, there are major challenges in building
a reliable NGN that offers parity with PSTN in terms
of QoS, ease of dialing, and convenience features
such as call waiting, free phone. The first challenge
is to have reliable call-signaling capabilities
in packet switching. The second is to provision
and control QoS. The third is to build a converged
VoIP/PSTN as a transitional solution. The final
challenge is to migrate to NGN that employs a comprehensive
Packet Mode Architecture (PMA).
SoftSwitches The Heart of NGN
Traditionally, switch vendors used to supply proprietary
applications that ran on Class 4 and Class 5 hardware.
This meant long waits for the service providers
who wanted new applications to differentiate their
services from the competition. But this is all going
to change with the Open Application Programming
Interfaces (APIs) supported by Softswitch. These
open APIs will allow service providers and third-party
developers to develop
innovative applications at their own pace without
depending on switch vendors. Besides quick service
creation, other benefits offered by Softswitch include
service differentiation, interoperability across
heterogeneous networks, and the smooth transition
from circuit to converged networks and then to next
generation of packet networks.
The Softswitch is software that resides on fault-tolerant
servers in a highly scalable, reliable and distributed
architecture environment and performs call control
functions such as protocol conversion, authorization,
accounting and administration operations. Because
of its call control functions, the Softswitch must
be able to communicate with both PSTN and IP networks
which means Softswitch will have to interpret several
protocols such as MGCP, H.323 and SIP.
Also creating a strong developer community for Softswitch
API is an equally important factor in its eventual
In spite of these issues, Softswitch is destined
to become the most crucial component of next generation
networks and is expected to give telecommunications
the same rate of growth the Internet experienced
in the 1990s.
QoS A Challenge
of Service (QoS) is important for the converging
networks to be successful. Besides simple assurance,
QoS should be clearly defined, quantified, easily
expressed and packaged simply enough for people
to work with, and pay for.
QoS is about providing an environment that preserves
the isochronicity of a signal i.e. the signal goes
in and comes out, undistorted, perceptually in "real-time."
In terms of IP voice and real-time media, this means
developing strategies to reduce and regulate end-to-end
delays such as codec and packetization delay, transmission
and buffering delay, and routing delay. Also of
concern are packet loss and jitter.
QoS encompasses all seven layers of the Open Systems
Interconnection (OSI) Model, as well as every network
element from end-to-end, operating systems (real-time
scheduling, threads), communications protocols,
data networks, scheduling and traffic management
The concept of M3 multi-service, multi-technology
and multi-vendor is driving and enabling Next Generation
Networks. But providing QoS assurances in this mixed
environment is exceedingly difficult. Each service
type has a different concept of what constitutes
high quality. Voice services can accept signal loss
to some extent, but very little
time delay or jitter in order to
ensure high call quality. On the other hand, data
services can accept
time delays to some extent, but not packet loss.
Though QoS and bandwidth are related, increasing
bandwidth to achieve QoS is not going to solve the
problem since it is likely that new applications
will fill the available bandwidth in no time. So
even with fiber-to-curb and purely optical networks,
certain issues may never be resolved.
The fundamental trade-off in quality of service
is between bandwidth and traffic engineering and
prioritization. As bandwidth becomes more constrained,
traffic engineering and prioritization become more
important. This trade-off in turn leads to two global
QoS strategies one linked to bandwidth (RSVP), and
the other prioritization (Diffserver).
Resource reservation (e.g. RSVP) is where network
resources are allocated according to an application's
QoS request, subject to bandwidth management policy.
Prioritization (e.g. DiffServ) is where network
traffic is classified and network resources are
allocated according to bandwidth management policy.
In the latter case, each application traffic flow
has an associated Class of Service (CoS), based
on which the network elements give preferential
treatment to the classes having more stringent requirements.
The type of QoS that is more suitable for an individual
flow depends on the kind of application and the
network topology. The most popular QoS protocols
are ReSerVation Protocol (RSVP), Differentiated
Services (DiffServ) and Multi Protocol Label Switching
ReSerVation Protocol (RSVP)
which has been used for videoconferencing for years,
provides the signaling to enable network resource
reservation generally on a per-flow basis. RSVP
emulates the closest thing to PSTN's circuit switching
on IP networks. The router must maintain the state
information for each flow to allocate resources
based on available capacity. RSVP lacks scalability
and good policy control mechanisms.
Differentiated Services (DiffServ)
DiffServ provides a coarse and simple way to categorize
and prioritize network traffic. Each packet is marked
to belong to a particular class of service and is
sent through the network. Each router in the path
inspects the packet header and determines "packet
scheduling". This eliminates the need for maintaining
individual flows on all routers. DiffServ system
supports "admission control" (refusing
customers when the capacity is not available), "packet
scheduling", "traffic classification",
and "policies" and "rules".
Multi Protocol Label Switching (MPLS)
MPLS provides bandwidth management via network routing
control, according to labels inside (encapsulating)
packet headers. MPLS routing establishes "fixed
bandwidth pipes" similar to ATM virtual circuits,
but with "coarser levels" of QoS compared
MPLS is now becoming a de facto standard and may
emerge as "the winner" in the QoS protocol
race. However, despite these differences, MPLS and
DiffServ are not mutually exclusive.
Eventually the networks of the next generation must
be capable of handling voice, video, and data communications
with proper QoS in a packet-based transport/switching
infrastructure. It must provide parity with PSTN
in terms of QoS, ease of usage, and convenience
features such as call waiting.
The next generation networks' revolution is not
about the convergence of voice and data, but about
packet-based broadband networks. Unlike, the B-ISDN,
information superhighway, and Internet II, NGN is
not going to fade away primarily due to the maturity
of the underlying technologies, cost savings offered
by migrating to packet network infrastructure and
revenue opportunities provided by explosive growth
of data in the core network.
V. V. Nageswara Rao,Engineering
Manager, Huawei Technologies, email@example.com