VoIP And The Cable Guys
BY MICHAEL KHALILIAN & JOHN
FALZON Cable operators are moving aggressively to provide a
Triple Bundle Offering by adding voice to their high-speed data and video
services. By using VoIP to offer voice services on a converged network, they
can leverage their high-speed data (HSD) investment. Cox and Comcast have
successfully demonstrated the value of adding voice to their service bundle
through their circuit-switched offerings. Now, with the help of CableLabs
and the PacketCable architecture, cable operators are confident about
rolling out IP-based voice services with the quality their customers expect.
Cable operators are relying on the VoIP standards defined by PacketCable,
a CableLabs-led initiative aimed at developing interoperable interface
specifications for delivering advanced, real-time multimedia services over
two-way cable. The goal is to repeat their success in implementing HSD using
the CableLabs DOCSIS (Data Over Cable Service Interface Specification)
standard.
PacketCable has defined the following �softswitch� components as part of the
voice architecture. The major components are the Call Management Server
(CMS); the PSTN Gateway, which includes a Media Gateway Controller (MGC),
Media Gateway, and Signaling Gateway; the Record Keeping Server (RKS); and
the Media Server.
Call Management Server
The Call Management Server Call (CMS) provides centralized call-control
processing, exchanging call control messages with Multimedia Terminal
Adapters (MTAs) using PacketCable�s Network-Based Call Signaling (NCS)
specification. NCS is an extension of the IETF Media Gateway Control
Protocol (MGCP). MTAs, provided as a component of the user�s Cable Modem or
as a standalone box, provide standard telephony connections to residential
telephones. Once connections are set up, voice passes directly between
gateway endpoints in the form of Real Time Protocol (RTP) packet streams.
The CMS maintains the call state and responds to events represented by NCS
messages that occur during call processing. In addition to controlling call
processing to MTAs, the CMS controls call processing to Media Servers and
Voicemail Servers, provides event information to Wiretap Servers for tapped
subscriber lines, and sends event information to Record Keeping Servers (RKS)
for billing. The CMS also communicates to Signaling Gateways for ISUP calls
originated on or terminated to the PSTN and traditional SCP-based
applications (such as 8xx number translation, call routing) to send Local
Number Portability (LNP) and Customer Name (CNAM) requests using the SS7
TCAP protocol. The CMS can also communicate to other CMSs and to MGCs using
SIP via a SIP proxy.
PSTN Gateway
The PSTN Gateway is made of several components. The Media Gateway Controller
(MGC) provides centralized call-control processing, exchanging call control
messages with Media Gateways using the PacketCable Trunking Gateway Control
Protocol (TGCP) specification. TGCP is an extension of the MGCP protocol.
Signaling connection information (ISUP, TCAP) is communicated to the Public
Switched Telephone Network (PSTN) and SCPs through a Signaling Gateway.
Media connections with the PSTN are through ISUP trunks with a Media Gateway
providing the bearer connections. Connections with the PSTN are also
supported through Multi-Frequency (MF) trunks with MF-based Media Gateways
providing both the bearer and signaling connections to the PSTN switches.
Once connections are set up, voice passes directly between gateway endpoints
in the form of RTP packet streams. The MGC maintains call state and responds
to events represented by TGCP or ISUP messages that occur during call
processing. In addition the MGC provides event information to the Wiretap
Server for calls forwarding to the PSTN from tapped subscriber lines and
sends event information to a RKS for billing. The MGC can also communicate
to other MGCs and CMSs using SIP via a SIP proxy.
Record Keeping Server
The RKS is a repository for PacketCable Event Messages. It receives or
collects messages from various PacketCable components such as the CMS and
CMTS, collates the messages and passes them on to necessary back-office
support systems. It supports the collection of information necessary to
create PSTN-style Call Detail Records (CDRs) that may be used for billing,
settlements, traffic analysis, and other back office functions.
Media Server
PacketCable uses the terms �announcement server,� �audio server,� and �media
player� to describe a media processing element that supports playing
announcements, interactive voice response (IVR), and audio
recording/playback. Media servers also support all these capabilities. For
example, to play a network announcement, the media server receives NCS
signaling control messages from the CMS and sends announcements to MTAs and
Media Gateways using RTP packets. These communications include requests to
create a connection context, start sending a specified announcement file to
a specific address/port on the IP network, and terminate the connection
context and end the �play� operation. In addition to the functions listed
above, media servers can also support conferencing, speech recognition, and
text-to-speech, allowing cable operators to offer a broad range of VoIP
enhanced services.
Other Specifications/Standards
In addition to the major �softswitch� components already discussed, there
are some key specifications to support VoIP in a DOCSIS environment. The key
specifications are Dynamic Quality of Service (DQoS), Security, and Event
Messaging.
DQoS
The specification for Dynamic Quality of Service provides a QoS mechanism
that insures that packets are delivered in a guaranteed manner instead of
the traditional best effort manner in IP networks. It defines the
interaction between the CMS and other network components such as the MTA and
the CMTS. The use of DQoS insures that VoIP calls have the highest possible
quality.
Security
The security specification provides confidentiality for both media and
signaling packets across the network. It incorporates authentication,
encryption, and key management to ensure a secure environment.
Event Messages
The Event Message specification provides a mechanism for tracking network
resources requested and used by voice services. Event Messages are sent from
the various network elements to an RKS, which provides an interface to
billing and other back office systems. Event Messages are also used to pass
event information to the Wiretap Server.
With the help of CableLabs, the cable operators are poised to provide a
competitive service offering to the incumbent telephone companies. Several
major cable operators have launched their VoIP offerings and more are
expected to launch services in 2004 with significant deployments expected in
2005.
John Falzon is a Board Member of the International Packet
Communications Consortium (IPCC) and a V.P. of Telcordia. Michael Khalilian
is Chairman and President of the IPCC, an industry consortium of carriers
and solutions providers advancing packet-based communication technologies.
For more information, please visit the IPCC online at
www.packetcomm.org.
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