Digital telephone systems

For the first 100 years of the telephone business, all telephone circuits were analog. They were designed around the characteristics and problems of analog current and were maximized for that use. Since computers have arrived, however, a great number of people want to send digital signals over telephone lines. Both recently and from now on, growth in communications will be overwhelmingly digital. Analog is on its way out.

It is important to understand the differences between analog and digital circuits. Analog signals vary continuously, and they 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 or off, present or absent, 1 or 0.

Analog telephone lines are the legacy (old, traditional) systems of the telephone industry. Nearly all residential telephone lines are analog. Fifty-year-old telephones will probably work on your local loop — the connection between your home telephone jack and the telephone company's central office. (The central office is usually not a large downtown building, since the average local loop is about 2.5 miles, so the “central office” is most often an inconspicuous building in or near your neighborhood.)

When you talk on the telephone, the microphone in the “receiver” (while you're talking, it's actually a transmitter) produces an analog signal that travels to the central office and is switched either to another local destination or to other switching offices that connect it to a remote destination. Dialing the telephone produces the in-band signals that tell the switching system where to route the call.

The telephone companies have learned a great deal about the electrical characteristics of human voice signals over the years, and they have determined that we will be reasonably satisfied with voice signals that do not transmit frequencies below 300Hz or above 3,100Hz. Note that high fidelity is usually considered to be a system that can reproduce frequencies between 20Hz and 20KHz without distortion. While voices are recognizable with the standard telephone frequency range, that range of frequencies is likely to be inadequate for other types of sounds. For instance, music sounds lousy over the telephone. Telephone companies allow an analog telephone channel a bandwidth of 4,000Hz to work with.

The local loop is sometimes referred to as “the last mile,” because residences, generally saddled with analog-only transmission facilities, are rarely capable of bandwidth greater than 4,000Hz.

Bandwidth determines the speed that you can transmit a signal. If you are just using your phone line to transmit and receive data, with an analog connection (called POTS — plain old telephone service), the best bandwidth you can normally get is between 28.8kbps and 56kbps. This service that allows you to connect to the interconnected fiber-optic backbones that make up the long-distance and Internet systems, is nicknamed the Cloud. This is the long distance network, which is 100% digital in the United States.

The national telephone systems are very rarely digital all the way to the curb.

When the telephone company reverses the process and digitizes an analog signal, it uses a 64Kbps channel. (This conversion is a worldwide standard.) One of these channels, called a DS0 (digital signal, level zero), is the basic building block for telephone processes. You can combine (the precise term is multiplex) 24 DS0s into a DS1. If you lease a T-1 line, you get a DS1 channel. With synchronization bits after each 192 bits (that is, 8,000 times a second), the DS1 capacity is 1.544Mbps (the product of 24 and 64,000, with another 8,000 sync-bits added).

The second important distinction to make about telephone lines is whether they are dedicated circuits you lease or switched services you buy. If you order a T-1 line or a low data-rate leased line such as dataphone digital service (DDS), you are renting a point-to-point circuit from the telephone company. You have dedicated use of such a circuit — with 1.544Mbps (T-1) or 56Kbps (low data rate) of capacity, respectively.

These services are sold as permanent virtual circuits, where the customer specifies the end points. 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 the circuit provided by the phone company is already a digital circuit, there is no need for a modem to provide digital-to-analog conversion services between the terminal equipment (equipment such as 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 line that supports ISDN service, for example, must be connected to a device called an NT1 (network termination 1). In addition to the line termination and diagnostic functions, the NT1 interface converts the two-wire local loop to the four-wire system used by digital terminal equipment. For digital leased lines, you'll need T-1 and DDS. For the digital services, the digital subscriber line from the phone company needs to be terminated by a channel service unit or CSU. The CSU terminates and conditions the line and responds to diagnostic commands.

Customer terminal equipment is designed to interface with a data service unit (DSU), which hands over properly formatted digital signals to the CSU. CSUs and DSUs are often combined into a single unit called (rather obviously) a CSU/DSU. The DSU may be built into a router or multiplexer. So even though end-to-end digital services don't require modems, a piece or two of interfacing hardware is always required for connectivity.

Multiplexing is the process of combining numerous bit streams or signals so they all can be carried efficiently. All carrier-based transmission systems use frequency division multiplexing as a way of creating discrete channels, and preventing signals on different channels from interfering with each other. Frequency division multiplexing works by assigning channels to different blocks of a spectrum. A single TV channel might operate in the 6MHz block from 54MHz to 60 MHZ, while the next channel occupies the 60 to 66 MHz block. The third channel might occupy the 66MHz to 72 MHZ space.

Baseband digital transmission systems typically use time division principles to separate different messages. TDM interleaves data packets from multiple messages, separating them slightly in time.

Code division multiple access (CDMA) is another multiplexing method used by cellular and PCS networks. CDMA uses frequency division to create channels, and then a digital coding technique to stack time division signals within each channel.

Space division multiplexing is another carrier-based communications networks separate channels. Cellular and PCS networks are good examples of space division, where frequencies and channels are spatially separated from each other.

One of the most important technologies for breaking the bandwidth bottleneck is DSL: Digital Subscriber Line. There is actually not one single DSL technology, but several. The one most commonly used is Asymmetric Digital Subscriber Line (ADSL, sometimes called Asynchronous Digital Subscriber Line). ADSL is one of several similar technologies generically called “xDSL”. Another popular variant is HDSL — High bit-rate Digital Subscriber Loop.

All the xDSL technologies involve the installation of electronic boxes on the ends of relatively standard telephone lines, which allows for transmission speeds of as much as 1.5 megabits/sec for HDSL and 6 megabits/sec for ADSL. (Fig. 2, 3 and 4.)

ADSL is really an extension of HDSL. It's not only the fastest of the xDSL technologies, but it also handles packet-switched transmission technologies, such as ATM (Asynchronous Transfer Mode). Sending data in self-addressed packets, via routed rather than switched circuits, is a cheaper way to send all forms of data than via voice circuits. This is how information is transmitted over the Internet, and will eventually become the standard for all transmissions.

DSL's transmission rates of 1.5 Mbps to 6 Mbps are only a fraction of what would be available with optical fiber to the home or office (which would be at least hundreds of gigabits). But compared to regular telephone circuits, it's phenomenal. ADSL could transform the existing information network from one limited to voice, text and low-resolution graphics to a powerful, ubiquitous system capable of bringing multimedia (including full motion video) into millions of homes and offices.

DSL will play a crucial role over the next decade, as telephone companies enter new video and multimedia markets. The success of these new services depends on reaching as many subscribers as possible. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs and the Internet into homes and small businesses, DSL could make these markets viable for telephone companies and application suppliers alike.

Following are some key terms used for defining links between data communications and telecommunications:

ADSL — Asymmetric Digital Subscriber Line. 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 customer, a full-duplex data channel at 576Kbps and a telephone service (POTS) channel.

DS-0 A Digital Signal Level Zero. This is one of the 64,000bps circuits in a T-1 or E-1 line. It consists of 8-bit frames transmitted at 8,000 frames per second. 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.

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. Users reserve a specific data rate called the CIR, or committed information rate, but users can attempt to burst data at higher rates. Extra frames are discarded if the carrier's network doesn't have sufficient capacity.

HDSL — The High bit-rate Digital Subscriber Line. 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.

ISDN Integrated Services Digital Network. This telephone system service 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. Telcos offer ISDN in two 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. (See Fig. 1.)

PBX — Private Branch Exchange. 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. When you dial within your company, the PBX provides the dial tone; when you dial 8 or 9 for an outside line, the CO provides the dial tone.

T-1. T-1 is a North American standard for point-to-point digital circuits over two twisted pairs. A T-1 line carries 24 64,000bps channels (also known as DS-0) for a total usable bit rate of 1,536,000bps (if you include extra bits used to synchronize the frames, the actual bandwidth is 1,544,000bps). Customers may lease a fractional T-1, using only some of the 24 T-1 slots. A T-1C contains two T-1 lines; T-2 supports four T-1 circuits. A T-3 communications circuit supports 28 T-1 circuits, and a T-4 consists of 168 T-1 circuits. E-1 through E-5 are similar standards used in Europe and Japan, but they offer different numbers of channels. See DS-0.

X.25. X.25 refers to a standard for packet-based wide area networks. For both leased lines, such as T-1s, and public-switched connections, like ISDN's 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.