For almost a century, Americans have taken for granted the nearly unfailing service provided by the public telephone network, often referred to as plain old telephone service (POTS). If cable to is to emerge as a legitimate alternative, there are many technical issues that must be addressed. Perhaps the most fundamental of these is the evolution of the nation's cable infrastructure from a one-way, broadcast medium to a two-way, personal-communications medium.

Half-Duplex vs. Full-Duplex Cable Infrastructure
Cable was first introduced in the United States in the late 1950s. For the next 30 years, nearly every mile of buried cable was half duplex. This means that it was capable of broadband transmission in the downstream direction, i.e., from the head end to the subscriber but not in the upstream direction. Communication from the subscriber back to the head end was possible only via a telephone line.

This makes half-duplex lines cumbersome even for premium TV services, such as pay-per-view, that require upstream communication. It makes half-duplex lines extremely inconvenient for Internet service due to the fact that outbound e-mail messages and hypertext transport protocol (HTTP) requests have to be sent via the phone. And it renders half-duplex lines completely useless for voice because such service requires packets to be sent upstream and downstream continuously.

In recent years, cable operators have been investing heavily in upgrading their buried cable from half to full duplex as a necessary first step to capitalize on the demand for integrated data and voice services. While upstream transmissions still are not as fast as downstream (typically 1.5 to 3 Mbps downstream and 500 Kbps to 2.5 Mbps upstream), full-duplex lines offer sufficient throughput to support cable-based IP telephony. As cable operators compete for subscribers with xDSL providers, the speed with which cable operators replace older lines with full-duplex lines will be critical to their ultimate success.

Telephony Service across a Broadcast Media
Unlike POTS, which was developed from the outset as a point-to-point communication technology, cable networks were designed originally to broadcast one signal to many recipients. There was no concept of dedicated circuits and there was no need to parcel out bandwidth to individual subscribers. To enable cable-based IP telephony, modifications must be made to the way bandwidth is allocated and packets are delivered. This must be done without using the bulk of the cable spectrum because most of the bandwidth will continue to be used for TV broadcasts.

Direct Connect
Callers must be able to send and receive only their own voice packets, and these packets must be given priority over data packets to ensure that callers experience smooth, uninterrupted conversations. The first step in this process was addressed by the Data over Cable Service Interface Specification (DOCSIS). DOCSIS established universal ground rules for the transmission of packets across cable networks, ensuring that packets won't be routed incorrectly.

DOCSIS was later enhanced (in version 1.1) with quality of service (QoS) and security features that are necessary for voice communication. DOCSIS 1.1 also enables the prioritization of packet traffic. This allows cable operators to give certain packets (i.e. voice) the right of way and allows other traffic to be sent with a "best-effort" priority as determined by bandwidth availability. However, even this second-generation DOCSIS standard was not intended to address all of the technical issues associated with cable-based voice service.

To fill in the gaps left by DOCSIS, CableLabs®¹ created the packet cable specification known as the network-based call signaling (NCS) protocol for signaling voice calls over cable networks. NCS leverages the existing media gateway control protocol (MGCP) and the protocol is sometimes referred to as MGCP NCS. NCS uses network-based call agents to negotiate cable-based IP telephony calls. Call agents, which will be discussed later in this tutorial, ensure that voice packets traverse the network and are audible only at the two conversation end points.

Security
While POTS is considered an extremely secure service, cable-based IP telephony is not. Much like cellular telephony, cable-based conversations are susceptible to illegal wire-tapping and inadvertent "chat" conditions. To address this untenable situation, DOCSIS and NCS support multiple security services.

NCS currently supports the IPSec authentication specification. Adequate protection of telephony connections can be achieved if the telephony gateway accepts only packets that have been authenticated by IPSec. As described in the CableLabs on-line magazine Specs Technology, DOCSIS supports an encapsulation protocol for encrypting packet data across the cable network. The encapsulation protocol defines the frame format for carrying encrypted packet data, the set of supported data-encryption and authentication algorithms, and rules for applying the cryptographic algorithms to packet data.

CableLabs further reports that DOCSIS currently employs the cipher block chaining (CBC) mode of the U.S. data encryption standard (DES) to encrypt packet data. The protocols are extensible, can support multiple encryption algorithms, and will, in all likelihood, be extended to support the new advanced encryption standard (AES) once it is in place.

Power Consumption
As most people know, traditional telephones draw all the power they need from POTS lines. Because the public phone system has evolved to such a reliable state, and it is essentially immune from the effects of power outages, it is exceptionally rare that service is lost. Electrical utilities in most areas do not offer this degree of unfailing reliability. Therefore, head-end and customer-premises cable equipment that relies solely on the local electric company for power puts users at risk of losing phone service should a power outage occur.

To address this issue, "lifeline service" requirements are being implemented across the country that require IP phones, such as those that connect to cable lines, to provide at least four hours of battery backup. To meet this requirement, equipment manufacturers must develop phones that can be powered by as little as three watts. A key to achieving this is a telephony chipset that minimizes idle processing cycles and offers sufficient onboard memory to handle all signal processing.

The ideal cable-based IP–telephony system is typically built with a reduced instruction set computing (RISC) microprocessor to handle the signaling functions and digit collection. The necessary telephony peripherals, such as a local-area network (LAN) controller and universal serial bus (USB), are on a single chip to conserve power, and dedicated hardware should be used for the cable-communications protocol. Several megabytes of high-speed random access memory (RAM) are needed for the signal processing, and the same amount of nonvolatile memory is needed to store the telephony application. The nonvolatile memory should be electrically reprogrammable, such as a FLASH memory, to enable online software updates.

A high-performance, low-power digital signal processor (DSP) is needed to support the analog functionality, e.g. codecs, noise reduction, and echo cancellation. A programmable DSP can greatly reduce application-development time for solution providers.

Billing
Telephony billing is an extremely complex process. Most cable TV customers receive the same bill each month. Aside from pay-per-view requests, there is no need to meter or monitor customer usage. Telephone billing is quite different. A typical bill includes recurring monthly service fees, international and long-distance charges that vary based on time and day, and premium services, such as *69 and directory assistance, that are billed on a per-use basis.

To enable timely, accurate billing, call agents or network interfaces (NI) must collect all relevant usage data. The NI is the cable equivalent to the phone box that is outside every home. In the absence of an NI, cable-based IP telephony can also be delivered using voice-enabled cable modems inside a customer's home. If the call agents collect the billing data, the NI or cable modem does not have to be involved. Otherwise, the software inside the NI or cable modem must provide application programming interfaces (APIs) so that the billing system can access the relevant data. Depending on each cable operator's implementation, the data may be contained in standard management information base (MIB) files or in unique files set up specifically for telephony metering.

¹CableLabs is a research and development consortium of cable operators from across North America and South America. The organization's guiding concept is interoperability as cable networks evolve from regional to national in scope and the industry branches into Internet and telephony services.