In the first two installments of this three-part series we presented a present-day view of the datacom market and the basics of data cabling, installation and testing. In the final article of this series we'll discuss the benefits of placing cables underground and review some installation techniques. As a wrap-up, we'll explain the basics of telephone networks, which are still the main cross-country conduit for data signals.
Underground data cable Several advantages exist to placing data cables underground versus locating them overhead (which is frequently a less-expensive option). A direct-burial cable is virtually free from storm damage and has lower maintenance costs than aerial cable. In addition, aerial installations often lack aesthetic appeal, and in some communities are prohibited.
High- density polyethylene jacketed cable is best-equipped for direct burial because it can withstand the compressive forces of the media in which it is buried. High- density polyethylene is non-porous and non-contaminating, and provides complete protection against normal moisture and alkaline conditions. Most direct-buried cables have an additional moisture barrier of water-blocking gel under the jacket. If water should penetrate the jacket, it can't travel down the cable and create further damage. The damage is localized and thus more readily repaired. Since the earth thermally insulates buried cable, its year-round temperature varies by only a few degrees.
The tools used to bury cable consist of trenchers, boring machines, backhoes and shovels.
Installation techniques The outer jacket is the cable's first line of defense. Any steps that can be taken to prevent damage to it will go a long way toward maintaining the integrity of the cable. These techniques cover many situations you'll encounter in the field.
Cable should be buried in sand or finely pulverized dirt, free of sharp stones, cinders or rubble. If the soil in the trench does not meet these requirements, tamp four to six inches of sand into the trench, lay the cable, and tamp another six to eleven inches of sand above it. A creosote- or pressure-treated board placed in the trench just above the top layer of sand, will provide some protection against subsequent damage that could be caused by digging or driving stakes. In particularly difficult installations, such as in rubble or coral, or where paving is to be installed over the cable, PVC may be used as a conduit. This pipe protects the cable and usually makes it possible to replace cable that has failed without digging up the area.
Examine the cable as it is being installed to be sure the jacket has not been damaged. The outer jacket can sustain damage while in storage or as it is being dragged over sharp edges on the pay-off equipment.
Lay the cable in the trench with some slack. A tightly stretched cable is likely to be damaged as the fill material is tamped.
Bury the cable below the frost line to avoid damage by the expansion and contraction of the earth during freezing and thawing.
The National Electrical Code (NEC) states specific requirements for cables to be buried underground. The NEC specifies 24 inches as the minimum burial depth for 0 V-600V nominal applications.
If an installation must meet all NEC requirements, consult with a local inspector during the planning phase of the project. Electrical inspectors frequently don't look at datacom cabling during inspections.
Underground raceways An alternative to burying cables directly in the earth is to place them in an underground raceway. A raceway offers superior protection and the ability to add conductors at a future date with a minimum of expense. Obviously, it is far more expensive to install a raceway than it is to direct bury a cable.
When installing underground conduits, it is important to install more raceway than you need at the time of installation. This allows you to add additional install more cables in the future at very little expense. Pulling more cables in a conduit that is already partially filled is not generally a viable plan. Another option is to install multiple inner ducts inside one large conduit. This allows you to have three or four separate mini-raceways inside one large raceway. This method often eliminates the need for pull rope and reduces the amount of lubricant required.
The U.S. telephone network It is important to understand the differences between analog and digital circuits. Analog signals vary continuously and represent particular values, such as the volume and pitch of a voice or the color and brightness of a section of an image. Digital signals have meaning only at discrete levels, that is, the signal is either on oroff, present or absent, 1 or 0.
Analog telephone lines are the legacy (old, traditional) systems of the telephone industry. Most residential telephone lines are still analog. Analog lines make-up what is known as a local loop-the connection between your home telephone jack and the telephone company's central office. Since a typical local loop is about 2.5 miles in length, the central office is most often an inconspicuous building in or near your neighborhood (Fig. 1).
At the central office, the analog signal is converted to digital so it can be switched across the telephone network. Aside from a few remote areas, the U.S. telephone network that interconnects central offices uses digital signaling.
Dedicated vs. switched lines Telephone lines can be dedicated circuits you lease or switched services you buy. If you order a T-1 line, you are renting a dedicated point-to-point circuit with a 1.544 Mbps capacity from the telephone company. These virtual circuits allow you (the customer) to specify the end points of the circuit, whereas the actual communication route is determined by traffic volume on the telephone company's network. Switched services, such as residential analog telephone service, are services purchased from the telephone company. You can select any destination on the telephone network and connect to it through the network of public switches. You generally pay for connect time or actual traffic volume, so unlike a dedicated line, the bill will be low if usage is low.
If you have access to a digital circuit, you don't need a modem to provide digital-to-analog conversion services between your terminal equipment (i.e. computers, fax machines, and digital telephone instruments) and the telephone system. Nonetheless, customer premises equipment must present the correct electrical termination to the local loop, transmit traffic properly, and support phone company diagnostic procedures.
A new family of telephone technologies called Digital Subscriber Line (DSL) looks promising, but may never see full implementation. There are several types of DSL services, such as ADSL, HDSL, GLite and others. They are collectively referred to as xDSL. All are similar in that they use electronic boxes on each end of a standard telephone line to achieve transmission speeds of 1.5 megabits per second or higher. For this technology to gain widespread acceptance the telephone companies (Telcos) must implement it. Unfortunately, it is not in their best interest to do so. The Telcos make far more money on T-1 lines. The predecessor of DSL - ISDN - was also left to "die on the vine" by the Telcos. The same fate probably awaits DSL.
Telephony Terms Following are some of the key terms that are used for defining links between data communications and telecommunications systems.
Asymmetric Digital Subscriber Line (ADSL). A method of carrying high-speed traffic over existing copper twisted-pair wires. Currently in the trial phase, ADSL offers three channels: a high-speed (between 1.5Mbps and 6.1Mbps) downlink from the carrier to the client, a full-duplex data channel at 576Kbps, and a plain old telephone service (POTS) channel. A key feature of ADSL is that POTS is available even if the extra ADSL services fail.
Automatic Number Identification (ANI). A system that identifies the telephone number of the calling party for the call recipient-known to most consumers as "Caller ID." When using a T-1 line, the ANI information also includes the geographic coordinates of the originating call's central office.
Digital Signal Level Zero (DS-0). This is one of the 64,000 bps circuits in a T-1 or E-1 line. It consists of 8-bit frames transmitted at 8000 frames per second. The usable bit rate is often only 56,000 bps. DS-1, or Digital Signal Level One, is often used as a synonym for T-1, but it more precisely refers to the signaling and framing specifications of a T-1 line. See T-1.
Dual-tone Multi-Frequency (DTMF). A description of the audio bleeps you hear when you dial a Touch-Tone telephone. Each row and each column of keys is assigned a separate frequency. Combining row and column frequencies produces each key's frequency. For example, the second column's assigned frequency is 1,336Hz, and the third row's is 852Hz; Pressing the number 7 generates both those tones. By decoding the two frequencies, the telephone company's central office, your PBX, or an interactive voice response system can detect which button was pressed.
Frame Relay. This refers to a shared-bandwidth wide area network based on a subset of High-Level Data-link Control (HDLC) called LAP-D (link access Procedure-D channel). Frame relay is designed to be carried over high-speed, high-accuracy links such as T-1 or the still emerging T-3; a 56Kbps line is the most common implementation. Individual frames can vary in size, but they are usually 4,096 bytes. Users reserve a specific data rate known as the Committed Information Rate (CIR) or attempt to burst data at higher rates. Extra frames are discarded if the carrier's network doesn't have sufficient capacity. High Bit-rate Digital Subscriber Line (HDSL). This circuit offers a full-duplex 784Kbps connection over two twisted pairs. HDSL can carry either a full T-1 connection over two twisted- pairs or a fractional T-1 connection over a single twisted pair of wires.
Integrated Services Digital Network (ISDN). A telephone system service that provides access to both the public-switched telephone network and to packet-switched services (such as X.25 and frame relay). ISDN offers two types of channels: B (bearer), which are 64Kbps voice channels, and D (delta), which are channels for setup, coordination, and control. Telephone companies offer ISDN in two main varieties: basic rate interface (BRI), which contains two B channels and an 8Kbps D channel, and primary rate interface (PRI), which has 23 B channels and a 64kbps D channel.
Interactive Voice Response (IVR). The basic voice-mail system that can decode DTMF signals. This is how your call can be routed without the aid of a real human being.
Private Branch Exchange (PBX). A telephone switch used within a business or other enterprise, as opposed to the switches used at a public telecommunication service provider's central office (CO). PBXs might offer basic telephone service, some level of computer-telephony integration (CTI), voice mail, or other features. X.25. This term refers to a standard for packet-based wide area networks. For both leased lines, such as T-1 lines, and public-switched connections, like ISDN B-channel links, you pay by the minute or month. However, an X.25 connection has the advantage of being measured and billed by the number of packets or bytes actually sent or received.