The Role Of The Softswitch In The
BY SUDHIR GRAPTA
Tremendous growth in data services is forcing public carriers to take a
serious look at voice and data convergence. Packet-based networking
technologies such as ATM and IP are reasonably mature to address this task.
In addition, several of the standards organizations including ITU, IETF,
Multi-Service Switching Forum, ATM Forum, and International Softswitch
Consortium are promoting the cause of separation of call control from bearer
control. Therefore, with the availability of packet-based bearer
technologies and the decoupling of call control from bearer control, the
stage is being set for public carriers to move from the current "one
network -- one service" paradigm to a "one network -- multiple services"
Figure 1 (below) provides a comparison between the existing switch
architecture and the proposed next generation switch architecture.
The current switches are "monolithic" in nature, using vendor
proprietary protocols for communications between different subsystems. A
major advance brought forth by the next-generation network (NGN)
architecture is the "opening" of these protocols. IETF and ITU
have taken the lead in specifying standards for such protocols. For example,
MGCP and Megaco (H.248) define the interface between a softswitch and the
subtending media gateways. Similarly, JAIN/Parley group has defined
standards-based APIs between the softswitch and feature servers. These
feature servers are analogous to the Signal Control Points (SCPs) in the AIN
and provide repositories for the services and their associated databases.
With the distributed switch approach for the NGN, carriers shall be able to
procure "best of breed" network elements in a multi-vendor
environment and hopefully, at a lower cost.
Referring again to Figure 1, the NGN switching architecture is based on
three logically separate layers. These layers can change independently with
the technology evolution without affecting the overall functions of the
The transport layer is responsible for the transmission of the bearer
traffic. The transmission technology may be TDM, ATM, or IP based. However,
due to the bandwidth efficiency inherent in the packet networks,
next-generation networks will likely be based on packet technologies such as
ATM and IP.
The switching layer provides the control logic required for call
processing and routing of the bearer traffic. The "softswitch" as
a product resides in this layer.
The service layer provides the call logic and the databases for
supporting telephony services, e.g., 800 numbers, local number portability,
THE ROLE OF THE SOFTSWITCH
The softswitch provides the call processing intelligence. It communicates
with the subtending media gateways in the transport layer with
standards-based control protocols such as MGCP and Megaco (H.248). Using
these protocols it determines the two end points that need to participate in
a call (or a session). Once identified, it then commands the two end points
to establish a bearer path between themselves. The end points then use
appropriate mechanisms (ATM SVCs/PVCs and/or IP-based) for the establishment
of the bearer path. This separation of the two signaling mechanisms allows
the same softswitch and the associated service logic to be shared across
different types of bearer networks (TDM or ATM or IP). Another advantage of
this approach is that subscribers or media types (data, video) can be
added/dropped relatively easily to an existing call or session, thereby
facilitating "multimedia" communications.
Eventually the transport layer will become packet-based end to end.
However, the current PSTN is circuit-switch-based. Therefore, in order to
connect two end points that may reside in networks based on different
technologies, one needs a media gateway. A media gateway provides bearer
level interworking and may also provide signaling level interworking to
establish calls that traverse networks built with different media types such
as ATM and IP.
There are several different media gateways that are available from different
vendors. These media gateways have different names depending upon where they
are used in the network.
Residential Gateway: This media gateway is used in a residential/SOHO
environment. It may have two to four analog voice ports and an Ethernet LAN
port. The network side interface is typically ADSL-based. Using the LAN,
multiple PCs in a residence can simultaneously access the Internet or
corporate VPN. The voice ports provide regular RJ-11 interfaces for the
telephones. The unit provides interworking between the circuit-switched
voice to VoATM or VoIP on the network side.
Business Access Gateway: This media gateway is used in small to
medium sized businesses. It typically provides two to 16 analog voice ports
or a digital T1 interface for on-site connectivity to a Key system or a PBX.
The unit may also have one or more data interfaces -- an Ethernet interface
for LAN connectivity and a V.35 interface for an onsite router. The network
side interface is typically SDSL-based or a T1.
Voice Gateway Or Trunking Gateway: This media gateway is designed
for the central office environment. It provides a large number of voice
trunks -- typically a few thousand. It also provides interworking between an
ATM- or IP-based packet network on one side and the circuit-switched PSTN on
Distributed Class 5 Switch Application
A Class 5-type network based on a softswitch will interact with subtending
residential gateways, business access gateways, and voice gateways. Using
this network, a subscriber connected to a residential media gateway can make
a voice call to other residential or business subscribers connected to their
respective media gateways with end-to-end connectivity for the call over the
packet network. For calls for which one end is connected to the current
PSTN, the call will traverse the voice gateway with associated signaling
carried over the SS7 network.
Tandem Switch Replacement Application
Due to falling rates for long-distance telephony, the Inter-Exchange
Carriers (IXCs) are looking for ways to reduce costs for carrying voice
traffic. One obvious choice is to packetize the long-distance voice network.
This has several advantages. First, due to the inherent dynamic bandwidth
utilization capability of the packet network, coupled with advances in voice
compression technologies, carriers can carry a significantly large (two to
three times more, depending on the technologies used) number of voice calls
over their existing transmission networks. Secondly, the same transmission
network can also be utilized for carrying the data traffic. This reduces the
operations overhead for managing two different and separate networks.
The role that softswitches and media gateways play in next-generation
network architectures is very significant. Based on the intelligence
contained in the call agents, a variety of existing and new voice related
services, including those listed below, can be supported over a packet-based
- Local calling based on 7-digit routing
- Domestic long-distance calling based on 1+10 digit routing
- International calling per ITU's E.164 numbering plan
- Multi-line Hunting
- DNIS/ANI delivery
- Call waiting with Caller ID
- Cancel call waiting
- Call forwarding -- all, on busy, on no answer
- Three-party conference calling
- Account codes/Authorization codes
- Distinctive ringing
- Local number portability
- Pre-carrier selection (FG-D)
- Casual calling (10-10-XXX calls)
- Operator service
- 800 toll-free calls
Sudhir Gupta is director of strategic marketing at Accelerated
Networks. Accelerated Networks is a market leader in multiservice broadband
access (MSBA) products that enable telecommunications service providers to
bundle voice and data services over a single broadband access network. For
additional information, please visit www.acceleratednetworks.com.
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