Cable Modems and Hybrid Fiber-Coax Cable

Coaxial Cable

What is coaxial cable?

Typically used to connect a television to cable TV services, coaxial cable consists of a small copper tube or wire surrounded by an insulating material and another conductor with a larger diameter, usually in the form of a tube or copper braid. This cable is then encased in a rubberized protective material.

What is the hybrid fiber-coax network?

HFC networks combine both optical-fiber and coaxial cable lines. Optical fiber runs from the cable head end to neighborhoods of 500 to 2,000 subscribers. Coaxial cable runs from the optical-fiber feeders to each subscriber. Hybrid networks provide many of fiber's reliability and bandwidth benefits at a lower cost than a pure fiber network. As of late 1996, about 7 percent of cable systems had been upgraded to HFC.

The Internet and Cable

The Internet is the fastest growing sector of the massive computer industry. With its free, global transfer of data in a split second, the Internet has quickly become a communication tool as important as the telephone. The biggest stumbling block to the Internet is bandwidth. Bandwidth is the amount of data that can be transferred over a certain medium. Modems over phone lines, the standard way to connect to the Internet, are very slow. They can handle text data fast enough, but when you add pictures, sound and video they are insufficient.

There are alternatives to phone lines. Various digital line types are available, and they are fast. The problem is that they are too expensive for the average consumer to use. What is really needed is low cost, high speed, widely available Internet access. That is what cable modems promise to deliver. The biggest expense for introducing new types of Internet access is the massive cost required to lay large amounts of fiber optic and other types of digital cable. Cable modems nullify this cost by using the existing cable TV feed that nearly every house in North America has. The cable modems allow data to be sent at very high speeds on the existing line.

The largest stumbling block to overcome is that cable networks were designed for one way broadcasts, from the cable company to your home. Data transmissions need to be two way communications. The coaxial cable that cable TV is transmitted on can already support two way communications, but the problem is that these signals pick up all sorts of interference from devices like ham radios and cellular phones. To get around this problem the cable network itself has to be upgraded. Most cable providers are choosing to go the hybrid fiber-coax route.

Hybrid fiber-coax networks (HFC) are a mix of fiber optic and standard coaxial cables. Not only does a fiber optic network improve the reliability of two-way communications, it also allows more channels to be delivered to subscribers, around 33 more to be exact. The fiber optic network carries both TV and data to a laser receiver. One receiver usually services anywhere from 500 to 2000 subscribers. The information is sent from the receiver to each individual subscriber along coaxial cable. It is still expensive to replace large sections of coaxial cable with fiber optic, but not as expensive as replacing everything.

The cable company acts as an ISP; it leases a fast digital line, which is then connected to the HFC network. The data is then broadcast over the network on an unused channel. When a cable modem starts up it scans all of the channels for this channel, called the control channel. The cable that comes into your house has enough bandwidth to carry the TV signal and data at the same time. A splitter divides the line into two, one going to a TV and another to the cable modem. The modem converts the data channel into standard computer data, which is sent to a standard Ethernet card in the computer. Sending data back, the upstream, works the same way, except in reverse. All cable subscribers serviced by the same laser receiver share the upstream channel, so network performance depends on how many users are using it at one time.

From the Internet:

The cable company acts as its own ISP. It establishes a connection with the Net through a router, which receives data over a 1.5-mbps T1 line or 45-mbps T3 line leased from the local telephone company. The router links to the head-end controller via fast Ethernet, at rates of up to 100 mbps. Data travels over the Net in small chunks called IP packets. These must be converted into signals that can be transmitted, like television broadcasts, over coaxial cable.

To the cable company

The head end receives TV signals via satellite and local broadcast and converts them to signals that can be sent over coaxial cable to subscribers. The subscriber's television tuner--or cable box--converts the signal back to a video image. Coaxial cable service occupies the 40- to 550-MHz portion of the spectrum. This frequency is split into 6-MHz bands, where each band corresponds to one of approximately 60 television channels. To deliver digital data, the head-end controller modulates the IP packets--encodes them as a digital signal--and broadcasts the signal down the cable on an unused channel. Generally, the head-end controller also acts as a traffic cop, using a special control channel. The head-end controller tells each subscriber's cable modem when it can transmit, on which frequency band, and for how long. When the cable modem starts up, it scans all its assigned channels to locate the control channel, which can be identified by its unique header signal.

Hybrid fiber-coax

Cable TV's "tree and branch" architecture makes it hard to provide a signal of consistent quality along the cable. Every time the cable signal is split, it becomes weaker. Companies can use amplifiers to boost signal strength, but these often introduce noise that can cause errors. To address this problem, some cable companies are upgrading their systems to include fiber-optic lines as well as coaxial cable.

Because each television channel occupies a 6-MHz frequency band, a cable system with 400 MHz of downstream bandwidth can carry the equivalent of 60 analog TV channels. On the other hand, a hybrid fiber-coax system with 700 MHz of downstream bandwidth has the capacity for 110 channels.

Optical fiber is more reliable than coaxial cable and increases the number of available channels. Hybrid fiber-coax cable service occupies the 40- to 750-MHz portion of the spectrum; coaxial cable only covers the 40- to 550-MHz part of the spectrum. Optical fiber can also transmit signals over much longer distances before requiring amplification. But upgrading to fiber-optic lines is quite costly. This is why hybrid fiber-coax is used only by select cable operators. Many still use regular coaxial cable throughout the network.

Laser transmitter/receiver

To send the data over the hybrid fiber-coax network, laser transmitters convert 6-MHz signals sent from the head end into optical signals. At the edge of a neighborhood, a laser receiver reconverts the signals so they can again be transmitted over coaxial cable, which goes into each individual house.

To your home

Cable's bandwidth is wide enough to let subscribers watch television and maintain a Net connection. A splitter in each subscriber's home sends one cable to the TV set and another cable to the computer's cable modem.

Cable box

Subscribers receive their cable TV service through a combination tuner/descrambler box. Although today's "cable-ready" TVs can receive cable transmissions without the box, they can't unscramble pay channels such as HBO.

Cable modem

The cable modem demodulates the incoming signal and translates it back into IP packets the computer can understand. The cable modem also sends data upstream to the Internet through the cable system, on a 5- to 40-MHz band. Since all subscribers share this relatively narrow band, the cable modem must be able to transmit data on any 2-MHz channels allocated on the upstream band. The head-end controller instructs the cable modem when to send data upstream and which channel to use. Until the industry adopts standards, the head-end controller and the cable modem in each system most likely must be made by the same manufacturer.

PC

The cable modem communicates with the subscriber's computer through 10BaseT Ethernet. The actual connection generally uses standard RJ-45 cable, just like the cable used in local area networks, but subscribers must have Ethernet interface cards installed.

Advantages

One large advantage with cable modems is that the network is always on. No more dialing up, no more time limits. When your computer is on, you are on the Internet. The other obvious advantage is the high speed. Cable modems can receive data at a peak speed of anywhere from 30 Mbps to 4 Mbps and send it at 10 Mbps to 512 Kbps, depending on the brand. The transmission will rarely ever reach these speeds, and if it does it will be between two subscribers to the same service.

Drawbacks

The first problem is that your PC can't keep up with the cable modem, which sends data to your computer via an Ethernet card. Although the cable modem may be receiving data at 30 mbps, your PC will only be able to read data at the rate your network interface can handle--typically a maximum of about 4 mbps.

Another problem is that the cable network is not always as fast as the cable modem. As with any regular ISP, the Internet is only as fast as the slowest server. The cable operator's server--called the head end--caches many popular Web sites so they can be delivered quickly over the cable network. If a site isn't cached, however, the head-end server has to go looking for it out on the poky Net, just as your ISP's server does. The head end at most cable operators connects to the Internet via a T1 line, which has a data rate of 1.5 mbps, significantly slower than your cable modem. Some systems have upgraded to a T3 line, which provides a rate of 45 mbps.

Even so, the more people who tap into the cable system, the slower the performance of the network. Make no mistake, downloading will still be a whole lot faster over cable than over a telephone wire, but it won't necessarily be smooth sailing all the time.

It's the upstream data flow--from your PC to the cable operator--that creates the real headaches. Cable networks were designed to send data, not receive it. Every time you click a hyperlink, you are sending data. And, of course, email and file uploads are upstream transmissions.

Because cable TV was originally designed to send signals downstream, the bandwidth allocated for upstream transmissions is a narrow, 5- to 40-MHz band that must be shared by 500 to 2,000 customers. Moreover, the upstream band is subject to all sorts of interfering noises that can garble data. Poorly shielded coaxial cable picks up noise from motors, CB radios, microwave ovens, and other appliances. Ham radios can wreak havoc on upstream data, as can VCRs and television receivers.

Because of these problems, the big cable networks use hybrid fiber-coax networks. The use of fiber increases the upstream bandwidth and reduces noise, making it easier for upstream data to reach its destination. But upgrading to hybrid fiber-coax is hugely expensive. Only 7 to 10 percent of existing cable systems are properly upgraded to handle two-way traffic.

Telcos

While large cable operators may have the funds to continue upgrading their systems to hybrid fiber-coax, smaller operators do not. So many of them resort to telco return; that is, you receive data though your cable system, but send data through your phone line. As a subscriber, you would still be receiving data more quickly, but you'd also be tying up your phone line. And even this solution isn't easy for many small cable operators, who either have to sink funds into banks of telephone switches or strike deals with local telephone exchanges. These obstacles mean that only a small number of operators can afford to get into the Net business--and, as we know, less competition generally results in higher prices for consumers.

ADSL

ADSL (asymmetrical digital subscriber lines) will provide the strongest competition for cable modems. ADSL uses filters to split the existing phone line into three separate frequency channels. A 0- to 4-KHz band carries the traditional analog telephone signal and makes sure your phone keeps working. A higher frequency allows upstream data transfers at rates up to 640 kbps. The rest of the band is used to pump data downstream--either from the Internet or as video-on-demand--at speeds of up to 8 mbps. The telcos have demonstrated working prototypes, and several trials have been launched, but ADSL remains largely untested. Another consideration: the special equipment needed at each subscriber's home adds a per-user cost that's significant compared to cable TV Net access.

Wireless Cable

Wireless cable TV is already a large and growing business, cutting into cable companies' markets by broadcasting multichannel TV programming to homes equipped with special antennas. Today, you can place a dish on your roof that can receive data in the 2- to 2.6-GHz range. The dish passes the information down through the coaxial cable to a special modem that converts the signal into data your computer understands. This process works for downloading; however, talking back to the cable provider still requires a standard analog telephone line.

Speed is Fast

Unlike traditional communication devices (modems and LAN connections) that send and receive data at the same speed, many newer devices receive data much faster than they can send it. Keep in mind, however, that while cable modems can receive data at speeds up to 30 mbps, your PC itself is limited by its Ethernet interface, which theoretically reads data at 10 mbps but is usually much slower. Also, the cable network may slow down considerably if lots of people are logged on or if the cable company's connection to the Internet is bogged down for some reason.

Page Developed by Shannon Nelson.   12/03/98