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Industry Imperatives
May 2004

VoIP And The Cable Guys


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.

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.

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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