Broadband Cable-based LANs

Networks can be classified as baseband and broadband. Baseband LANs, such as Ethernet, ARCnet, and Token Ring, are much more common in the office environment. Broadband LANs are popular where multiple services, such as closed circuit TV, data, and voice, are needed. Broadband is also popular in factory environments.

Broadband LANs work in much the same way that cable television works. Broadband LANs transmit multiple radio frequency signals on the same cable, usually coaxial but sometimes fiber. This ability to send many types of communication simultaneously over the same cable, including voice, video, and data, distinguishes broadband LANs from the far more common baseband LANs. To accomplish this feat, broadband LANs use a technique called frequency division multiplexing.

Frequency division multiplexing is used to put several channels on the same cable simultaneously. To understand frequency division multiplexing, think of the cable as a highway. A highway has a width, which determines how many lanes are possible; the cable has bandwidth, which also determines its capacity. Highway width is measured in feet; LAN bandwidth is measured in Hertz or cycles per second. It is the difference between a higher and a lower frequency the greater the spread between the upper and lower frequency, the more information can be transported.

Each "lane" or channel on a cable uses a different set of frequencies. Just as cars travelling in different lanes of a highway do not collide, information occupying one frequency band or channel on a cable does not interfere with information on another band . Thus, frequency division multiplexing sends its different types of information at unique frequencies.

Frequency-division-multiplexed information is generally sent in analog, not digital, form. Digital signals are discreteone or zero. Analog is continuous, like a wave. To reconcile this difference, digital computer data must be converted into analog form for transmission over a broadband cable. The conversion device is called a broadband modem. Once the broadband modem converts the data to analog form, it puts the data on the correct channel.

A Tree Topology

Some common broadband LANs use a tree-and-branch topology. The root of the tree is the headend, or central retransmission facility. The trunk cable is attached to this root. Various branch cables are attached to the trunk cable. From there, user devices may be connected. Although most broadband networks use a single cable, some use a dual cable system, one for each direction to and from the headend. A dual cabling system has twice the bandwidth of a single cable system.

A headend is essential. All transmissions must pass through the headend. Broadband Cable LANs use a tree-and-branch topology. All transmissions travel up through the tree to the headend, where the signal's frequency is altered so it can travel to its destination. In this illustration, even though PC1 and PC2 are adjacent, their messages must travel through the headend because each device transmits on one frequency and receives on another. The headend is responsible for translating the device's transmit frequency to the receive frequency of another device. This frequency translation is called remodulation.

To illustrate how a broadband LAN works, let's follow a transmission, say a file transfer, from one PC to another PC. The PC sends the file to its broadband modem, where it is modulated into analog form. The modem sends data at one frequency, called the return frequency and receives at another called the forward frequency. These terms sound reversed because they are named from the headend's point of view.

Before getting to the headend, the signal probably passes through several splitters and couplers, where the signal loses strength. It also passes through amplifiers , where the signal gains strength. According to the principle of unity gain, the signal must arrive at the headend with the correct strength, usually at a level lower than the transmission strength.

At the headend, the packet is translated from the return frequency to the forward frequency, then sent back onto the cable to the receiving PC. The trip to the receiving PC is just like the trip to the headend, except in reverse. Once at the receiving PC, the receiving modem translates the file back to digital form so the PC can understand it.

Forward and return frequencies make a pair. The headend can handle more than one pair of frequencies, although it cannot mix and match them. One complex part of designing a broadband LAN is deciding which information will travel on which frequencies. In many designs, the bandwidth is divided into 6MHz slices, which is the same bandwidth a television channel uses. Each type of communication takes one or more slices. Thus, an Ungermann-Bass Net/One broadband network might run over two channels, a television security system runs over another channel, and voice communications take up other channels. In general, higher speed communication services, such as Ethernet and data services, take up larger chunks of bandwidth than slower services, such as security and voice communications.

Unity Gain

All components of a broadband system either amplify or weaken the signal strength. As radio frequency signals travel down a cable, they deteriorate or experience signal loss. Devices on the broadband network, such as splitters, couplers, power inserters, and equalizers, also cause loss. The amount of loss experienced by a particular signal depends on many factors, including the diameter of the cable, the components it encounters, the distance it has traveled, and its frequency.

Broadband LANs are equipped with devices called amplifiers to counteract this unavoidable loss. Amplifiers attach to the cable in certain places and regenerate signals to a level determined by the system design. Any distortion the signal has picked up is also amplified.

The goal in designing a broadband system is to reach unity gain. Unity gain means that the amount of loss caused by the components in the system is equal to the amount of gain caused by amplifiers in the system. Achieving unity gains is complicated since the amount of loss imposed by a component depends on the signal frequency. Thus, amplifiers and other components are placed strategically to keep signal conditions uniform as signals move from device to device.

Since different frequencies are affected differently by various components on the network, achieving and maintaining unity gain is an arduous task. Consequently, broadband LANsfar more so than baseband LANsrequire trained professionals for design and installation. Also, broadband systems are best suited for large installations with large staffs of networking professionals, although smaller installations that require multiple service types can also benefit from broadband.

Ups And Downs

Broadband shines in campus environments, especially if several different types of communication must travel to the same locations. Support for multiple services saves cable costs and provides the ability to add services as needed. For example, a company might start with a data network and later add voice, security, a building management system, environmental controls, process management, and closed-circuit television.

A centralized tree-and-branch wiring scheme makes more sense than stringing together multiple baseband LANs in different buildings . Another advantage is broadband's immunity to electrical noise. Broadband LANs typically use frequencies above most machine-generated electrical noise, which is in the low frequency range. Baseband LANs, except those using fiber, are not so fortunate.

The price for this flexibility is complexity. Designing, planning, and installing broadband LANs is extremely time consuming and difficult. Along with deciding where cables and devices go, a LAN planner must evaluate the radio frequency requirements. Hundreds, and possibly thousands, of calculations can be necessary for the design and installation of a broadband LAN cable plant.

Maintaining a broadband LAN is also more difficult than maintaining a baseband LAN. Over time, the radio frequency settings of the components will drift (literally going out of tune) and cause transmission problems. The components require periodic tuning, which is more tedious than difficult.

Most end-user companies are not prepared to install a broadband LAN unaided. Installations require the resources and expertise of a broadband design and installation company. Experienced network planners can guide an end-user company through the process of creating the network, deciding which services will be used, where different devices will be placed, and so on.

This tutorial, number 10, was originally published in the May 1989 issue of LAN Magazine/Network Magazine.