MSOs have an opportunity to seize another market from telcos.

Cable operators may have a golden opportunity in the business services market.

Old-style private branch exchanges (PBXs) in the enterprise equipment market are gradually being updated to IP PBXs, the dominant type of PBX being sold today. On the one hand, some new IP-based services are possible with these new systems. On the other hand, however, time-division multiplexing (TDM) trunking of those IP PBXs is still the predominant method of interconnection, limiting the advantages business customers can realize from IP voice in both capabilities and cost.

The solution is end-to-end VoIP, which frees IP PBXs and IP-enabled PBXs from the constraints of TDM trunking by peering between business customers and service providers. Cable companies are in a great position to exploit this opportunity.

Session Initiation Protocol (SIP) is the standard that IP PBX manufacturers and service providers have settled on to realize end-to-end VoIP. SIP was designed to be a flexible and extensible protocol, resulting in many ways to achieve the same interconnection tasks.

Flexibility and extensibility are desirable characteristics, but with SIP, they come with a cost. Different vendors not only interpret the specifications for the protocol differently, they implement features that are specific to only their product, complicating interoperability.

Issues such as security, quality of service (QoS), call admission control and monitoring that go beyond common signaling and signaling normalization must also be resolved to fully define a predictable interface model.

SIP Figure 1
Defining SIP trunking
Throughout the last century, phone calls have been made using TDM technology through public switched telephone networks (PSTNs). Over time, these PSTNs have evolved to be made up of digital switches, but they are often 15 to 30 years old, with a lot of copper wire connecting these switches to residential and business customers.

A trunk line is a circuit connecting telephone switching equipment, as distinguished from a local loop circuit, which extends from telephone switching equipment to individual telephones or information origination/termination equipment.

PBX trunk lines are the phone lines coming into the PBX from the telephony service provider. These lines differ from extension lines that usually connect the PBX to individual phone sets.

So a TDM PBX trunk line is defined as the physical line, call signaling and media that connect the PBX to a provider of telephony service. TDM PBX trunks use either analog or digital signaling carried over a single TDM connection or a bundle of TDM connections. A bundle of TDM connections with digital signaling is known as a “T1 PRI” in North America.

SIP trunking is a TDM PBX trunk replacement service delivered over an IP network using SIP signaling. This implies that a SIP trunk must meet all regulatory and other expectations associated with a TDM PBX trunk. One significant difference between a SIP trunk and a TDM PBX trunk is that SIP signaling is completely independent of the underlying transport technology and media.

Telephony services delivered over an IP infrastructure are substantially different from voice applications such as Skype that are delivered over an IP infrastructure. To reduce confusion from here on, we use the term telephony over IP (ToIP) for TDM replacement services to distinguish from services such as Skype, which are typically described as voice over IP.

SIP Figure 2
Market opportunity
The growth of IP-based customer premises equipment has helped businesses upgrade and converge network infrastructures and deploy new IP-based features and capabilities.

ToIP-based services (including hosted IP telephony and SIP trunking) and equipment offer a range of new features that are not available on TDM systems, including desktop integration for presence-based features.

ToIP also allows service providers the capability to offer new services, such as wireless/wireline integration, click-to-dial, telework/remote office applications and soft clients support. To extend these offerings most efficiently, service providers need to be able to connect directly to PBXs at customer sites.

IP PBXs are the predominant CPE call control device being sold on the market today. In a typical business network, the IP PBX is a server that is connected to the corporate LAN and provides call control to the IP phones, PCs, conferencing devices, wireless equipment and other communication endpoints. At the same time, it also usually interfaces with the public network through the PSTN. An IP telephony gateway establishes the PSTN connection, and this gateway can be integrated into the IP PBX or can be adjunct to the IP PBX. This gateway enables the conversion of the IP packets to TDM analog or digital signals – and the reverse.

However, this packet conversion to TDM introduces latency and the potential for echo that often degrades voice quality. In addition, advanced IP-based signaling information and features are often stripped from the transmission by the conversion, eroding the ability to deliver IP-based features. TDM routing of ToIP traffic is clearly inferior.

IP peering enables IP PBXs to connect directly with ToIP service providers, eliminating the need for gateways and TDM routing. Peering of ToIP services is a far better approach, enabling the full capabilities of packet-based communications.

While IP PBXs are the predominant new PBX systems in the market, roughly 75 percent of deployed PBX systems are legacy IP-enabled TDM systems that are still connected to the PSTN for public access. These systems can be connected to a ToIP service provider using a TDM gateway and for multi-location enterprises with a mixed network of TDM and IP PBXs.

TDM service providers in the United States are required to report various statistics to the FCC. The latest “Statistics of Communications Common Carriers” published in 2008 by the FCC contain the following data:

  • 4,521,928 PRI PBX trunk lines deployed
  • 41,630,524 multi-line business-switched access lines
  • 3,078,355 single-line business-switched access lines
  • Generate $31.7 billion in annual revenue for the reporting TDM service providers.

The FCC also collects data from service providers that are competitive with the TDM operators. In 2010, the Wireline Competition Bureau’s Industry Analysis and Technology Division at the FCC reported that 91.9 percent of residential ToIP was delivered by cable operators, and that only 2 percent of business-switched access lines were ToIP.

The explosion in data traffic from smartphones and tablets has brought these legacy networks to their knees. The existing TDM wireline service providers are focused on maintaining and adding capacity to their wireless networks. Cannibalizing their TDM-switched access line “cash cow” by delivering ToIP services is not high on their priority list.

Clearly, the cable operators have the core network infrastructure built to deliver not only ToIP as a service, but to deliver the underlying IP network connectivity, as well. As most cable operators do not deliver legacy TDM trunking services or TDM Centrex services, cannibalization is not an issue. Business voice services represent a greenfield opportunity for cable operators. By some calculations, as much as $31.7 billion in annual switched access revenue is available to operators that deploy SIP trunking or Hosted IP Telephony (HIPT).

Technical issues, resolutions
The SIP standard was written to ensure interoperability when integrating SIP equipment from different sources. However, inconsistencies continuously arise as different vendors interpret the SIP specifications differently.

To remedy this, the signaling normalization function is assigned to an enterprise edge device called an Enterprise SIP Gateway (ESG). The details and different methods of handling authentication for the SIP trunks are also performed by the ESG device. The ESG device performs SIP registration on behalf of each individual phone without any changes to the other elements.

SIP Figure 3
SIP security has not been a large public issue to date, due to its small number of deployments. On the other hand, the PSTN presents many opportunities for an attacker to gain access to ToIP subsystems and alter or deteriorate their operation. The attacker can discover appropriate parameters (if an underlying security mechanism is not utilized) needed to launch an active or passive attack.

To mitigate these attacks, an ESG device should implement the following security measures:

  • Authentication
  • Encryption
  • Signaling messages parser that acts as a firewall to reject malformed, invalid or unwanted messaging and to rate limit valid messaging that is used for various continuity checks
  • A SIP and media firewall that only opens ports to authorized SIP signaling, and corresponding media is implemented on the ESG.

Another main concern about ToIP and SIP trunking is with regard to the QoS and reliability. The bottleneck on the Internet is nearly always the connection to the customer premises.

There are primary methods that service providers use to deliver adequate QoS for real-time communication. One method is for the service provider to control the link from their point of presence (PoP) to a piece of equipment on the customer premises. The other method is for the service provider to over-provision the link. Both methods will be able to offer excellent quality.

In order to ensure call quality, prioritization must be available for both outbound and inbound traffic, and it needs to be dynamic so that bandwidth can be reallocated, depending on call volume. At maximum call volume, data traffic can be delayed, but not starved to the point that TCP retransmission becomes a factor.

Operationally, to monitor the health of the network, to troubleshoot any voice quality problems and to monitor security, several things need to be done. To monitor security, the SIP firewall must produce several firewall logs, including all permitted inbound access requests, all permitted outbound access requests, all dropped or denied access requests from private and public networks, etc.

This monitoring capability not only gives the operator the same monitoring capabilities as a TDM circuit, but it also gives the ability to play test recordings that originate from the customer premises and can be used to troubleshoot network impairments that negatively affect call quality.

Cable operators are in the unique position of having deployed core networks that support and provide 91 percent of fixed-line residential ToIP service in the United States. Replacing TDM business-switched access lines with ToIP business lines that use either SIP trunking or HIPT represents tens of billions of dollars in new revenue for the cable operators with minimal investment.

Because IP PBXs are dominant PBX technology deployed in the enterprise equipment market, and because almost all PBXs are IP-enabled – and because a large number of TDM trunks are still in service – this represents an opportunity for the cable operator to enter a new market and for the enterprise customer to realize the true advantages of an IP PBX.

An ESG is a specification for a CPE device that connects an existing voice core to an IP PBX or a TDM PBX with an IP gateway that enables the operator to deliver SIP trunking, as well as HIPT.

The ESG provides the demarcation point between the operator’s network and the enterprise customer’s network and delivers the following functions:

  • Connects – Session border controller (SBC) function in the ESG performs SIP trunking normalization between the core and the customer’s PBX using a defined rule set, as well as provides IPv4-to-IPv6 interworking and vice versa.
  • Protects – The SBC acts as a SIP NAT, SIP firewall, media firewall and media relay.
  • Optimizes – The SBC performs traffic shaping, call admission control and packet marking to provide prioritization for real-time traffic.
  • Manages – Through the telemetry and SETA functions, the ESG manages and monitors networking and call statistics.

By deploying ESGs, cable operators can increase the utilization of their packet voice cores through delivery of SIP trunking and HIPT services.