With the blueprint for Internet Protocol-based cable telephony completed following the release of CableLabs' PacketCable 1.2 specification, the cable industry is inching toward the deployment of full end-to-end IP voice service over its new networks.

But circuit-switched, or constant-bit-rate (CBR) telephony is the long-established architecture, and operators such as AT&T Broadband and Cox Communications continue to ring up subscribers based on their CBR arrangements. While everyone in the industry recognizes the utility and cost benefits of IP telephony, architectures for operators with Class 5 switches are emerging to bridge the IP and circuit-switched worlds.

Along the way, a new architecture, called Line Control Signaling (LCS), has been spawned and outlined by CableLabs. That approach, issued last summer, lays the foundation for hybrid IP/circuit switched telephony.

The actual steel in the bridge that makes the hybrid architecture hold up is the Telcordia GR-303 Architecture, a set of specifications for digital loop carrier systems. In an LCS model, GR-303 is the communications interface between what's termed the Internet Protocol Digital Terminal (IPDT) gateway and a Class 5 local digital switch, which links an IP-based cable network and the Public Switched Telephone Network, or PSTN. The IPDT, as outlined by CableLabs, provides media gateway controller, signaling gateway, media gateway and partial call management server functions.

As an extension of the PacketCable architecture, LCS is designed to let operators deploy an IP-based telephony service now in the access portion of a network–from cable modem to cable modem termination system (CMTS)– while allowing for a full IP migration when complete PacketCable architectures become available.

"I'm going to obsolete a Class 5 switch sooner than I obsolete circuit-switched carrier gear," says David Fellows, chief technology officer for AT&T Broadband. Responding to operators' needs to retain the Host Digital Terminals (HDTs) they deployed for circuit-switched telephony service, vendors have responded with platforms designed to ease the way into IP voice, without stranding investments in legacy gear. At the same time, vendors are honing end-to-end voice platforms in anticipation of CableLabs' PacketCable interoperability testing.

LCS boxes

TollBridge Technologies Inc. and Nuera Communications Inc. are two vendors that have developed IPDT gateways. TollBridge's TB 300 gateway decodes the Network Call Signaling (NCS) signals sent by Multimedia Terminal Adapters (MTAs) into signals that Class 5 switches understand, according to Kevin Woods, vice president of marketing and product management. In addition, the gateway injects IP into the access portion of a network, while providing the same level of service quality and features served up by Class 5 switches.

When an operator moves to pure IP voice, the TollBridge gateway software can shift from an IPDT implementation to a media gateway that supports the Trunking Gateway Control Protocol (TGCP), which connects to a softswitch.

Nuera ORCA RDT-21
Nuera’s ORCA RDT-21
Nuera's IPDT, the RDT-21, acts as the Call Management Server (CMS) or softswitch to control the MTAs, and as a media gateway and media gateway controller to provide a connection to the PSTN, says Joe Matibag, Nuera's director of product marketing. By changing out its IPDT software, the box becomes a media gateway for IP voice.

Straddling VoiP and circuit-switched

Two major circuit-switched cable telephony vendors, ADC Telecommunications and Arris, have devised strategies for operators that leverage Class 5 switches as well as those already moving ahead with IP.

"ADC intends to support Homeworx (ADC's CBR platform) as long as our customers continue to deploy it," says Jim Murphy, director of packet telephony product management for ADC. As operators with Class 5 switches expand telephony services into new areas, "we encourage them to use the LCS model," he adds. By adopting LCS, operators can move to a single local network structure based on DOCSIS, rather than deploying what is essentially two separate networks–one for high-speed data and one for circuit-switched voice.

ADC provides CMTSs and HDTs for telephony headend gear, home integrated services units at the customer premises, and provisioning/activation and network management software.

ADC has demonstrated interoperability with both the TollBridge and Nuera IPDTs, and is involved in a lab trial with the Cuda 12000 CMTS and Nuera RDT to support LCS-based VoIP.

"Our own recommended migration strategy for circuit-switched HDTs is to cap the investment in those NIUs (network interface units) and introduce new subscribers using the IPDT product," says Murphy. If an MSO wants to use a softswitch, an IPDT allows the HDT to be connected to an IP gateway and the softswitch, allowing the HDT to operate as if it was still connected to a Class 5 switch.

In November, ADC introduced the Cuda Packet Telephony Module (PTM) for its Cuda 12000 IP access switch. The PTM is a single card or blade that fits into a Cuda 12000 and integrates a media gateway, media gateway controller and SS7 signaling gateway. It provides GR-303 or v5.2 support for access to a Class 5 switch.

By combining the PTM and Cuda 12000, operators can leave existing circuit-switched equipment, HDTs and proprietary customer premises equipment in place while deploying new services based on the DOCSIS 1.1 specification.

Once an operator moves to full IP and adds a softswitch, the unused capacity of the blade can be connected to the softswitch. Future iterations of the PTM will support multiple types of DS-1 connections–to softswitches, Class 5 switches, an SS7 signaling interface, etc.

In this way, Murphy explains, the potential exists for an MSO to operate and run three generations of product simultaneously: circuit-switched, LCS and IP.

"The key is tying all telephony networks together," he says.

ADC, together with Nortel Networks and Motorola Broadband Communications Sector, has been involved in building Callahan Associates' IP telephony platform in Germany, believed to be the largest cable IP voice endeavor to date. Also, Murphy points out that ADC is trying to work with as many cable modems and multimedia terminal adapters as possible.

Arris added a key component to its VoIP portfolio when it purchased Cadant Inc. last year, and obtained the start-up's DOCSIS 1.1-qualified C4 CMTS. Arris has won key DOCSIS 1.1 qualification for its Cornerstone 1500 CMTS, as well as DOCSIS 1.1 certification for its indoor Touchstone Telephony Modem. Arris also markets versions designed for the rugged outdoors.

The Cadant C4 is built with a 15:1 level of redundancy and with "cable access modules," each supporting one downstream and eight upstream channels. This reflects a shift in the ratio of upstream to downstream traffic. "Over the last two years, there's been a dramatic increase in the amount of traffic in the upstream," says Stan Brovont, vice president of broadband marketing for Arris. One of Arris' customers, in fact, has reported almost identical traffic going upstream from the subscriber's house and downstream into the network, says Brovont. This was happening independent of voice service, he adds.

As voice is a symmetrical service, the upstream path must be reliable, and provisioning for more upstream traffic is critical. With a telephony deployment hitting over 15 percent voice service penetration, "you immediately begin to need eight upstream [channels] per CMTS blade," says Brovont, noting that most operators prefer to allocate one CMTS blade per node, with some combining node traffic before it reaches the CMTS.

"We see our installed base of circuit-switched customers as an important market and we have an obligation to them to help them with Voice-over-IP," says Brovont. "We can help a customer preserve an investment in a Class 5 switch and the circuit-switched equipment in a subscriber's home. Our goal is they don't have to throw anything away moving to IP."

The critical components in the Arris circuit-switched architecture are the HDT and the voice ports, environmentally-hardened NIUs that convert voice transmissions into digital signals.

For its circuit-switched customers moving to IP voice in the access portion of the network, Arris supports the TollBridge IPDT and similar products from Nuera and other vendors.

Separately, for customers moving to full IP, "we will have a product that lives in the headend that looks like the HDT to voice ports, but connects via MGCP (Media Gateway Control Protocol) to the call server," says Brovont. In this configuration, "the call server thinks it's talking to an MTA, but it's talking to the voice port," he says.

Arris hasn't publicly announced specific details of this product yet.

Nortel: Dealmaker

One of the most aggressive companies in forming alliances has been Nortel Networks, which last month (April) announced a non-exclusive agreement with cable telephony veteran Arris. The combination is meant to leverage Arris' CMTSs and embedded MTAs together with Nortel's Communications Server 2000 softswitches, packet voice gateways, signaling gateways and application servers.

Nortel also swung a deal with Motorola Broadband in February, marrying (on a non-exclusive basis) Motorola's multimedia terminal adapter (MTA) and Broadband Services Router CMTSs with Nortel's softswitches, gateways and application servers.

Nortel is also serving as the lead integrator for Callahan's IP voice architecture.

Nortel is actually offering two types of telephony architectures for cable operators, according to Elaine Smiles, senior manager of cable VoIP marketing at the company. One is a hybrid model, with IP serving as the transport in the access portion of cable networks, and the MTA and CMTS serving as IP devices. A GR-303 gateway takes IP traffic from the access and converts it to the GR-303 spec, and sends it through a peripheral or aggregating device to a TDM (time-division multiplexing) switch.

Pure IP: The other architecture

In the hybrid model, Smiles explains that some of the biggest challenges involve managing the trunk lines coming into and going out of the Class 5 switch matrix, and adjusting for different calling patterns. With five ports to manage in and out of the switch (generally three incoming and two outgoing trunks and peripherals), and accounting for possibly high bandwidth requirements on trunks to a long distance carrier as subscribers sign up with that carrier, relying on a Class 5 switch can be complex.

The advantage of IP voice, says Smiles, is the ability to place more services on the network "rather than micro-managing each path."

With a TDM-based switch, the line peripherals, trunk peripherals and switching matrix have to stay together physically and an operator has to have a switch in each geographic location. For operators with relatively low penetrations of telephony subscribers, this can create costly connectivity issues.

In an IP environment, a communications server (or softswitch) and trunking gateway (or packet voice gateway) can be placed separately on the network, Smiles points out. The trunking gateway doesn't have to be co-located with a softswitch (or communications server, in Nortel's parlance). Only the signaling portion of the call needs to come back to the communications server, while the bearer path–the actual voice portion of the call–can go through the trunking gateway. This allows the operator greater flexibility in placing softswitches and gateways in regional and local portions of the network.

Nortel's Communication Server 2000, says Smiles, "brings a full set of TDM functions into the voice-over-IP world" and represents an evolution of the traditional Class 5 switch, with an added gateway controller. Of particular interest to those operators that have a digital central office switch, the CS 2000 "can continue to run TDM peripherals," says Smiles. In this way, the CS 2000 can connect to another TDM switch or an IP network for operators that are running both circuit-switched and IP voice service. Nortel's Communications Server 2000-Compact is a software-only version of the Communications Server 2000 and runs on Motorola hardware and the Linux operating system. The Compact version can take advantage of advances in server processing power, Smiles says.

In addition to its relationships with Arris and Motorola, Nortel has worked with ADC's CMTS in the Callahan deployment, although no formal alliance has been forged. Nortel has also supplied its softswitch technology for an IP voice trial with Charter Communications.

IP straight up

Focusing strictly on IP voice are several vendors, including Unisphere Networks Inc. and Clarent Corp. In November, Unisphere rolled out its IS Voice platform, which is comprised of the company's SRX-3000 softswitch, ERX edge router and SMX-2100 media gateway.

"IS Voice links the access network QoS (Quality of Service) to the [operator's] interdomain network," says Gary Greenberg, Unisphere's senior systems architect. The scheme creates an "IP circuit" by leveraging call admission control features of the gateway and queuing features of the router to validate available routes. The IP circuits are based upon Multi Protocol Label Switching (MPLS) paths. "For each flow or IP circuit, there's an LSP (label switched path)," says Greenberg.

Clarent's architecture is comprised of a distributed softswitch; a call rating, routing and authentication center; two types of media gateway controllers; and an SS7 signaling server. The company is in trials with two cable operators, one of which is generating revenue and supporting over 700 VoIP subscribers.


Primary PacketCable components

Multimedia terminal adapter (MTA)

A client device comprised of interfaces to the subscriber's telephone and to network-side signaling call control elements. Provides codecs and all signaling and encapsulation functions for media transport and call signaling, and supports the Network Call Signaling (NCS) protocol.

Cable modem termination system (CMTS)

Among the many functions of the CMTS are assigning the required QoS to the cable modem based upon policy configuration; allocating upstream bandwidth based on cable modem requests and network QoS policies; and forwarding downstream packets to the DOCSIS network while forwarding upstream packets based on assigned QoS.

Call management server (CMS)

Provides call control and signaling services for the MTA, CMTS and media gateway controller, signaling gateway and media gateway. Includes the call agent, which provides signaling services to the MTA via the NCS protocol.

Provisioning server

Performs initialization, authentication and registration functions for MTA provisioning.

Media gateway

Terminates the voice bearer paths and transcodes media between the IP and the PSTN (public switched telephone network) networks.

Signaling gateway

Provides a signaling interconnect between the PSTN SS7 signaling network and the operator's IP network.

Media gateway controller

Maintains the call state and controls the behavior of the signaling gateway and media gateway.

Record-keeping server (RKS)

A "data repository" for billing information. Call agents, PSTN gateways, provisioning servers and MTAs send billing data to the RKS.

Key distribution center (KDC)

The primary point of security and key distribution in a PacketCable 1.2 network, based on the Kerberos trusted third party authentication and authorization protocol.