Cable operators are facing a new challenge on the voice over Internet protocol (VoIP) front: peering. VoIP peering is the relationship between IP networks for both the call setup and call completion that occurs each time we place a phone call from one cable operator’s network to another. That is, we need to know where to send a call being placed, and we need to use a direct facility to get there. The issue today is finding the other PacketCable line directly without traversing the public switched telephone network (PSTN). Another way to consider the issue is in the regulatory framework that brought us here. Telecommunications regulations tax the local phone lines just as the cable plant is taxed with franchise fees. This generates money for state government and municipal governments respectively. These levels of government expect to maintain certain fees and taxes. VoIP is eroding those balances. Government has been working hard to keep up with the technology. A game changer Up until a couple of years ago, the only way cable could offer voice was to circuit-switch it like the phone company does, thereby becoming subject to tariff, in turn paying both municipal franchise fees and state telecommunications fees. This was a good technology fit within the traditional PSTN model, one that government could understand. VoIP changed all of that. Further, VoIP was fought for as what it essentially is, IP being used for a voice application. Broadband, yes; primary line, well, that depends. Primary line for most subscribers depends on ability to dial 911 and get an ambulance to your door. Primary line for municipalities, state and federal agencies is also about public safety and also about law enforcement. Cable has implemented both policy and technology to address emergency services and Communications Assistance for Law Enforcement Act (CALEA) requirements. Using emergency services and law enforcement compliance as a measure, then cable has implemented both a broadband service and a primary line voice service. This is often considered facilities-based VoIP service having North American standard Number Planning (NANP) availability as opposed to classification as an information service. Use of NANP 10-digit dial number assignments means access to the PSTN by VoIP customers, and in turn PSTN customers can access VoIP lines. There are tariff factors that apply to the cable operator in the deployment of VoIP services with NANP numbers. In some cases, cable is paying into the universal service fund, and several states have also held VoIP regulatory reviews, in general deferring to Federal Communications Commission policy on the matter allowing services to proceed. States have been concerned with intrastate toll tariffs and the portions of VoIP service deemed interstate telecommunications revenues. Further tariff and revenue reconciliation issues surround when two cable operators decide to bypass an inter-exchange carrier (IXC) by directly peering VoIP rate centers to one another across private transport facilities using media gateways. Peering options Some of the VoIP peering technologies the cable industry has been investigating surround the use of direct peering media gateways, telephone number databases, number lookup protocols, line endpoint protocols and interconnection facilities. Media gateways have been a key functional element in the PacketCable network from the beginning. A media gateway has the role of termination of VoIP line (in the traditional cable sense, PacketCable media gateway control protocol, or MGCP, lines) to presentation of calls to the PSTN, typically via digital trunks to the incumbent carrier. When a line is terminated to another VoIP packetized domain, the media gateway is often referred to as a session border controller. Telephone number databases exist in the present PSTN within the Signaling System 7 (SS7) network. The SS7 carrier maintains a database of all numbers in its facilities, with 1,000 and 10,000 blocks of numbers assigned to each phone switch and softswitch facility. The identification of numbers to a switch is known as the carrier identification code (CIC). Finding a number in the PSTN uses SS7 today. The originating number dialing the seven or 10 digits of the destination number initiates SS7 messages via the switch and peripheral facilities present at both ends of the call. The initial operation is for the terminating softswitch or circuit switch to look up the destination number if it contains leading digits that are on the local switch. If, however, the number is not local to the switch terminating the calling party, the switch consults its signal source point (SSP) database service for the destination number CIC. When the switch knows the CIC, it notifies the correct outbound trunk peripheral known as the signal transfer point (STP) that a call from the originating number is in progress to the destination number. The STP has the role of connecting over the inter-carrier facilities to the foreign switch that owns the destination number. When the foreign switch acknowledges the STP that it does own the number and determines the line is on hook, the foreign STP will inform the originating STP and switch to present ringing to the calling party line. Ringing now occurs on the destination line, and a two-way speech path is established between each STP and both phone switches. All the associated signaling and message formats required to exchange call setup, call progress and call completion are within the SS7 protocol. Understanding the impacts of tariffs, revenue reconciliation, and how the phone network is processing calls today is crucial to understanding why cable as an industry is motivated to change the traditional, legacy, circuit-switching protocols and interfaces of the PSTN. Beyond gateways PacketCable networks today use media gateways as previously mentioned to terminate the VoIP line to the PSTN trunk. The use of media gateways in offering access to the PSTN limits the functionality of the service to that of legacy PSTN lines, even when the two calling parties are both using VoIP. A media gateway is constrained to the PSTN circuit-switching facility to which it trunks. This causes transcoding from higher quality VoIP codec and sampling used in the PacketCable domain to lower bit rate encoding for PSTN trunks. This also limits the service to legacy voice features. New multimedia, location, and enhanced voice, video, and data end user interactions are lost by traversing the legacy PSTN switching domain. In several situations, the PacketCable call management server (CMS) or softswitch may not have a local SSP facility. This means that the cable softswitch has no knowledge of foreign voice switches and the exchanges they service. Class 5 switching where the CMS is permitted to traverse the PSTN effectively as a large direct inward dial (DID) facility results in consultation of the incumbent carrier for lookup of each dialed number originated by locally served lines. Processing calls in this manner generates high transactional costs paid to the carrier for SS7 processing and call completion. The costs associated for looking up or “dipping” dialed numbers in the SS7 database facilities is a large factor driving cable to use alternate technologies not present in the legacy phone network. In several states, the serving tariff trunk classification from the cable phone network to the carrier phone network is one that must respect each local access and transport area (LATA) boundary. This LATA configuration preserves intrastate revenues; however, it creates the expense of multiple appearances to the legacy PSTN. A LATA is a service region of area codes also referred to as a rate center. For example, LATA 132 signifies NY Metro NY Area. Within LATA 132 exist all other carriers, including the incumbent Verizon, identified as operating company number (OCN) 9104. Verizon services one or more number plan areas (NPAs), which is the area code, or first three digits of a 10-digit phone number. Within each NPA exist one or many NXX exchanges, also known as the central office code or exchange servicing the number. This is the second part of the 10-digit dial number. As an example, Carmel NY LATA 132, phone numbers served by Verizon OCN 9104, are NPA 845, with NXX 225 and NXX 228. Thus phone numbers 1-845-225-NXX and 1-845-228-NXX are Verizon exchanges for Carmel, NY. Services from other providers will exist in other exchanges. If a cable operator wishes to service local exchange numbers and is not a local exchange carrier, the PacketCable CMS serving local phone numbers knows only the lines directly attached to it. The cable operator must hand call lookup and call completion to the incumbent carrier that owns the exchange because the PacketCable CMS will have no knowledge of foreign exchanges known to the SS7 network. If the cable operator has exchanges assigned to it, the processing of calls to other cable VoIP networks via PSTN carriers using SS7 transactions still incurs cost per call completed. Further, the cable VoIP network needs to support local number portability, maintaining an end-subscriber ability to retain a phone number regardless of phone service provider. The SS7 bypass New technologies are being investigated, even as the cable industry is driving new telecommunications policies. Cable operators know the full NPA-NXX of all PacketCable lines in their networks. There exists another telephony number database known as electronic numbering (ENUM), an Internet Engineering Task Force protocol used by domain name system (DNS) technology. (See Figure 1.) Each cable operator has PacketCable provisioning systems with locally significant databases containing the most current knowledge of PacketCable line to fully qualified domain name (FQDN). When a multimedia terminal adapter (MTA) is provisioned, its IP address is assigned in DNS to an FQDN such as “” A VoIP peering architecture may begin at the provisioning system capable of updating the DNS server with not only the FQDN of the MTA, but also its assigned phone number(s). PacketCable CMSs do not presently consult DNS or the ENUM protocol to process call setup operations between lines. Were a softswitch to look up a destination number via DNS for the association of dialed endpoint FQDN and dialed line packet gateway, the PSTN and SS7 protocol could be entirely bypassed. The use of DNS, ENUM and call processing is done within the session initiation protocol (SIP) and SIP proxy servers. (See Figure 2.) SIP determines the serving softswitch by consulting DNS for a service (SRV) record response to provide the foreign packet gateway or foreign proxy SIP service that owns a SIP line. A SIP endpoint address is essentially the FQDN of a SIP client application, identified via “sip:” prefix such as “” A SIP line, however, is often thought of in 10-digit dial terms for two reasons. First, we are all programmed to remember our phone numbers and not our SIP uniform resource identifier (URI) information. Second, the keypad of my telephone can’t dial “” This is where ENUM comes in. ENUM translates FQDN information into the inverse of a 10-digit phone number returning DNS resource records that belong to the number. Resources include typically both the SIP endpoint address and, where applicable, the SIP telephone number denoted in the resource record via the “tel:” prefix. ENUM can service requests either in private and Infrastructure form or in public form. The difference is simply authority of delegated resources. ENUM in DNS is the top-level domain. Thus, number assignments by country code require registries to provide policy and management. North America is country code 1. Country code 1 has delegation applications submitted to the International Telecommunication Union for trials of ENUM-based number processing. Should country code 1 become fully delegated, telephony exchanges of voice service will be assigned in much the same hierarchical manner as previously discussed. To enable an existing line in the 1.x PacketCable architecture to utilize DNS and ENUM to process calls will require a hybrid IP-based STP. A hybrid IP STP will present the best of both worlds to the cable industry between PacketCable 1.x and PacketCable 2.0. A hybrid IP STP would effectively groom outgoing trunks from a 1.x CMS, examine dialed digits, resolve the destination in a high-performance ENUM DNS resolver, and complete the call. Should the destination be unknown to the hybrid IP STP, the call would be processed directly to the PSTN. (See Figure 3.) If, however, the destination number has a resource record in DNS, either a foreign exchange proxy server or a packet gateway controller, the call would remain in IP format and traverse the IP data network facilities between operators. (See Figure 4.) Take a dip To enable cable VoIP peering, each CMS deployed would use DNS and an ENUM specific lookup based on phone number dialed by the end user. The response from ENUM to the CMS query could be either the session border controller of the dialed network or possibly the actual end user SIP FQDN. This is the essence of “dipping” for an outbound call today in the PSTN. In reality, the CMS becomes a proxy for all multimedia services enabling cable operators to preserve services across an all-IP infrastructure, inherently supporting any type of media, voice, video and other future applications. (Again, see Figure 4.) It is likely cable operators will decide to require all media events, VoIP or other traverse session border controllers to maintain surveillance, administration and billing practices. Thus, exposing the FQDN of an individual endpoint is less important than knowing the FQDN of the dialed endpoint session border controller. The success of the VoIP peering will be based both on the management of data from the clearinghouse, or registry, and the ability to manage subset replication and synchronization of data within the cable operator’s provisioning networks. Cable is at the forefront of new technologies that enable the industry to shed itself of the legacy PSTN network technologies. The use of a central number registry based upon DNS, ENUM, SIP and new provisioning systems signify the emergence of a set of tools at the disposal of the cable industry not only to locate phone lines in other cable networks, but also and ultimately to enable a cable network to exchange any multimedia conversation between subscribers both on and off its network. Chris Busch is VP broadband technology for Incognito Software. Reach him at

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