Humble beginnings
The humble beginnings of cable TV are a result of poor over-the-air reception in rural areas and small towns. This problem began to be addressed in the late 1940s with the cobbling together of a well-placed antenna from which the signals were run into town on a coaxial cable. The acronym CATV stood for Community Antenna Television, although it now is synonymous with cable television. The cable output was taken and distributed to the subscribers, who were thrilled to have access to NBC, ABC and CBS.

Today, of course, this basic structure has evolved considerably, though it took many years. The CATV system carried analog video television signals, in traditional amplitude-modulated, vestigial-sideband format, exclusively in the forward path from headend to the home. Over time, the only evolution that took place was the number of channels carried. Today, the evolution in a typical North American system has led to the analog video spectrum spanning typically 50 to 550 MHz in 6-MHz increments, with some exceptions in the FM band.

In the return path—from home to headend—signals traditionally existed in the form of simple modulation formats carrying status and polled information, allowing operators to communicate with their converter boxes. Minimal actual engineering of the channel was performed in the sense of optimizing its use at that time, because the CATV designer and operator was not thinking of the system as a two-way medium in any sophisticated way.

Digital video
By the 1990s, digitized video was a clear necessity for satellite TV but it was also being seen as advantageous to cable operators in terms of bandwidth efficiency and robustness. The former translates to multiple programs in each 6-MHz slot, which historically supported only one analog video signal. Thanks to advances in digital compression and quadrature amplitude modulation (QAM), the number of digital channels that can be supported in a single slot has increased to 10.

Today, digital video services are typically carried in the RF spectrum multiplex above 550 MHz. Subsets of QAM signals can also be allocated geographically in what is called narrowcast (as opposed to broadcast) systems. This involves sending groups of QAM channels to targeted areas where the channel set is a function of choices in the headend and demographics of the served neighborhoods. Wavelength-division multiplexing (WDM) is a technology well-suited to this service because of its ability to enhance the capacity of existing fiber while also providing a means to segment QAM traffic (wavelength selection). WDM is essentially an optical version of RF multiplexing or frequency-division multiplexing (FDM). For more on WDM and its cousin, dense WDM, go to page 24.

Data via Docsis
Docsis stands for data over cable service interface specification, and is the standard from which cable operators pounced on the highly valued market of serving high-speed Internet. Debuting in 1997, Docsis' growth has been ongoing, to the extent that cable modems have become a major technical and business success story for the cable industry.

Docsis is, at its core, an analog physical-layer system spanning the RF and linear optical domains specifically designed for the HFC plant. The system is designed to pass Internet Protocol packets and as a result is designed to interface using other common network interfaces and protocols at the edge of the network.

The two communicating pieces of the Docsis system—the cable modem and the cable modem termination system (CMTS)—are engineered in an effort to cover the worst-case scenario as well as to recognize the wide dynamic range of plant quality. As such, the devices are designed to operate under the conditions of what would be generally considered very poor forward and return paths. The cable modem has Ethernet and USB interfaces to a PC, either as an external modem or as a PC card, depending on vendor and model. On the HFC network side, the cable modem interfaces via the RF input to the home and splits off from the same cable that connects to the TV.

The CMTS puts a modulated RF QAM signal on the cable. One QAM signal supports many users, as does one return RF signal. In the return path, the CMTS is a multiport burst demodulator to the CATV network side, operating a very intricate, combination time-division multiple-access/frequency-division multiple-access protocol. For Docsis 1.0, the maximum burst rate is about 10 Mbits/ second, or 16-QAM modulation at about 2.5 Msymbols/s. The lowest-rate mode is 160 ksymbols/s using quadrature phase-shift keying (QPSK), giving 320 kbits/s.

Forward path
The forward path uses QAM and since it was critical that they coexist with existing analog video carriers, the QAM signals are also put on 6-MHz centers, and thus signal at a rate slightly greater than 5 Msymbols/s. Thus, for 64-QAM, a raw bit rate of about 30 Mbits/s is achieved, while for 256-QAM, about 40 Mbits/s is obtained. Of course, some of this is the overhead associated with forward error correction.

The M-QAM forward path takes advantage of the very high CNR needed to support high-quality analog video. CNR is the commonly used cable terminology for RF analog video signals. It is commonplace for analog video at the home to be designed for a CNR in the mid-40s, minimum—quite high compared with what typical digital communication links have available. As a point of reference, 64-QAM without error correction requires only 28 dB of signal-to-noise ratio (SNR), while 256-QAM requires just 34 dB. When error correction is considered, these numbers are generally 3 to 5 dB lower.

In addition, analog video is also quite sensitive to nonlinearity in the form of intermodulation distortion. Because of the nature of analog video spectrum, which creates a spectral line at the carrier frequency, the distortion spectrum contains many discrete lines, and analog video signals are sensitive to this impairment. As a result, HFC systems are designed with very stringent linearity criteria for both the optics and the RF cascade. Thus, the channel from a QAM perspective provides a very high SNR and very high linearity, delivering the ability to implement such sophisticated modulation schemes.

There are a few impairment issues on the forward path that must be considered. At the top of the list are clipping of the fiber-optic transmitter (laser); microreflections associated with poor terminations in the plant, which are largely uncontrollable; and analog video distortions falling on top of the QAM channels. Also, with the addition of QAM, which has a noiselike spectrum characteristic, there are distortion components consisting of the analog video's carrier-wavelike tones mixing with QAM (as well as QAM x QAM), creating a broadband-noiselike component that shows up as a frequency-dependent degraded CNR called composite intermodulation noise, or CIN. CNR degradation due to CIN can be an issue for the analog video performance.

Return path
In North America, the spectrum allocated for the home-toward-headend return path is between 5 and 40 MHz, while in other parts of the world it varies, such as 5 to 65 MHz in Europe. In all cases, the return path is allocated spectrum below that of the forward path, although other architectures have been suggested. Prior to cable modems, the return services implemented were rudimentary systems designed for converter-box status, communication and polling, for example, for pay-per-view billing. Accompanying the simple service requirements were simply architected systems that relied on basic modulation types like low-rate binary PSK or frequency-shift keying; unsophisticated protocols like (in addition to polling) Aloha; and coarse power control.

Since 35 MHz was allotted to North America (Figure 2), and kilobit/second modulations employed, the conventional wisdom was that this was plenty of return bandwidth. Of course, it was impossible to foresee the evolution of the Internet or the streaming-media technologies that are capable of high-bandwidth consumption. In addition, the return path stretches over spectrum that tends to contain high levels of interference because of short-wave propagation effects, assigned over-the-air spectrum such as citizen's band radio at 27 MHz and the inability to completely control the cleanliness of the plant and homes connected to it.

Clearly, cable's 5- to 40-MHz spectrum presents a bandwidth constraint when considering that node-serving areas can be 2,000 homes per node. This number is dropping—either physically or logically—as network architectures evolve. Fortunately, for the path from the home to the headend, the traffic tends to be lighter than downstream to the home. Thus, some asymmetry in traffic is expected, although this has steadily decreased over time.

Because of the bandwidth limitations, bandwidth efficiency is a key return-path driver. However, interference issues due to plant and home conditions are still a major source of return-path problems. The spectrum from 5 to 15 MHz is particularly vulnerable to such degradation. In addition, while forward cable networks provide very high SNRs, there has never been such a need in the return path. Furthermore, noise accumulates in the return path, as this direction of the network is many-to-one instead of one-to-many. Noise from all sources combines along the way. Thus, both the theoretical SNR, as well as the practical SNR and signal-to-interference ratio, are considerably lower than in the forward path.

As a result of these problems, the return path in Docsis 1.0 is limited to 16-QAM as the most advanced modulation, with QPSK also available. QPSK is most widely implemented today. Data rates scale in octaves beginning with 160 ksymbols/s and ending with 2.56 Msymbols/s, adding an avenue of flexibility for both performance and traffic management. Finally, forward error correction is programmable, again to allow optimization against conditions.

The Docsis standard is not standing still. Docsis 1.1 added features in its media-access control to provide quality of service, in particular for the anticipated (insert current year plus one here) explosion of voice-over-IP services. A more sophisticated return-path physical layer is available with Docsis 2.0, which adds 64-QAM and doubles the maximum symbol rate to 5.12 Msymbols/s. The maximum return burst bit rate then becomes about 30 Mbits/s-equivalent to what is available in a Docsis 1.0 forward, and thus addressing the move toward traffic symmetry.

It is becoming ever more apparent that new bandwidth-intensive services have driven a need for nodes covering smaller serving areas, and with a bent toward being able to scale without major reinvestment as the need develops. Operators that have deployed plentiful fiber bundles are well-positioned to transform their networks smoothly. Other operators make use of technology choices that provide the necessary physical or logical segmentation to keep up with the need to dish up more bandwidth.

Next month, we will take off from this HFC 101 starting point and discuss the services driving alternatives and augmentations to straight HFC, along with next-generation options that HFC operators can debate or embrace.

Related Articles
1. "Ensuring Strong TCP Performance Over Docsis Modems"; www.CommsDesign.com/story/OEG20030409S0013
2. "Docsis 2.0 Places RF Demands on Upstream Signals"; www.CommsDesign.com/story/OEG2003/0116S0006

Rob Howald (rhowald@gi.com) is the director of systems engineering in the transmission network systems group of Motorola's Broadband Communications Sector in Horsham, Pa. He has a BSEE and an MSEE from Villanova University and a PhD from Drexel University.