Digital Telephony: Sending High-Speed Data Over Phone Lines

Since new communication systems are overwhelmingly digital, analog is slowly on its way out. Therefore, local telephone companies may offer some or all of these digital services you can recommend to your customers. Until a few years ago, almost all telephone circuits were analog. In fact, developers designed them around the characteristics and problems of analog current-maximizing them for that use.

Since new communication systems are overwhelmingly digital, analog is slowly on its way out. Therefore, local telephone companies may offer some or all of these digital services you can recommend to your customers.

Until a few years ago, almost all telephone circuits were analog. In fact, developers designed them around the characteristics and problems of analog current-maximizing them for that use. Now, computers are commonplace, and you or your clients may want to send digital signals over existing telephone lines. So, how do you go about it? Let's look at the differences between analog and digital signals first.

A multitude of differences. It's important you understand the differences between analog and digital circuits.

Signals. Analog signals vary continuously. 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, on the other hand, have meaning only at discrete levels. In other words, a digital signal is either on or off; present or absent; 1 or 0; all or nothing.

Analog telephone lines are the legacy (old, traditional) systems of the telephone industry. Nearly all of our residential telephone lines are analog. Fifty-year-old telephones will probably work on your local loop. (This is the connection between your home telephone jack and the telephone company's central office, which is usually a relatively small, inconspicuous building in or near your neighborhood.)

When you talk on a telephone, the microphone in its "receiver" (although it actually operates as a transmitter when you're talking) produces an analog signal that travels to the central office. Here, the signal switches either to another local destination or other switching offices that connect it to a remote destination. Dialing the telephone produces the signals that tell the switching system where to route the call.

At the central office, the telephone company will probably digitize the analog signal to switch it across the telephone network. Aside from a few remote areas, the U.S. telephone network (which interconnects central offices) uses digital signaling. But the signals going in and out of a home or office are almost certainly analog. (See "Breaching the Digital Bottleneck," in the November 1998 issue, on page 64S, original article.)

Frequency. Almost all telephone companies transmit voice signals between 300 Hz and 3100 Hz. (Most consider high fidelity as the reproduction of frequencies between 20 Hz and 20 kHz without distortion. As you can see, voices are recognizable within the standard telephone frequency range. This range is likely to be inadequate for music.)

Telephone companies allow an analog telephone channel a bandwidth of roughly 4000 Hz (maximum) to work with.

Going digital. Your modem allows your computer to communicate with other computers by converting digital communications into analog format, so they can travel through the public phone network. But, there's a limit to the amount of information a common analog telephone line can hold. This limit is about 56 kbps, although very few telephone/modem combinations ever really operate at anything near that rate.

When the telephone company reverses the process and digitizes an analog signal, it uses a 64 kbps channel. (This is a worldwide standard.) One of these channels, called a DS0 (digital signal, level zero), is the basic building block for digital telephone processes. You can combine (the precise term is multiplex) 24 DS0s into a DS1. The commonly used T1 line is a DS1 channel. With synchronization bits after each 192 bits (that is 8000 times a second), the DS1 capacity is 1.544 Mbps (the product of 24 and 64,000, with another in 8000 sync-bits added).

Dedicated and switched lines. You can send data over phone lines in two ways:

  • Use the phone company's switched network for low-speed modem traffic, or

  • Use dedicated circuits that can function at high speeds more or less independently of the network switching system.

ISDN. This network (integrated services digital network) was supposed to be the widespread digital phone circuit. Basically, it's a system of digitizing phone networks that's been in the works for more than a decade. It allows you to transmit audio, video, and text data simultaneously across the world, through the use of end-to-end digital connectivity.

However, telephone companies haven't handled ISDN very well. This is understandable, considering the deep entrenchment of the phone companies and how slowly they adapt to new technologies. With ISDN being the first digital residential service, a lot of problems showed up.

ISDN allows you to operate multiple digital channels simultaneously through the same regular phone jack in your home or office. The change comes about when the telephone company upgrades its switches to handle digital calls. Therefore, you can use the same wiring, but you're transmitting a different type of signal across the line.

Previously, you had to have a separate phone line for each device you wished to use simultaneously. For example, one line each for the phone, fax, computer, and live videoconference. Transferring a file to someone while talking on the phone and seeing their live picture on a video screen would require several expensive phone lines.

By using multiplexing (a method of combining separate data signals together on one channel such that they may be decoded again at the destination), it's possible to combine many different digital data sources and have the information routed to the proper destination. Since the line is digital, it's easier to keep noise and interference out while combining these signals.

Right now, ISDN is beginning to look like a dead horse. There are new technologies that do far more for the same price. (See "ADSL: The New Internet Link," in the December 1998 issue, on page 32A. You can do a search for this at If ISDN is available in your area, it still provides a much better channel than a regular phone line, and it may be worth using. Pricing still seems to be the least standardized part of ISDN, so the following common prices may not be applicable in your area:

  • Installation charges generally run from $200 to $400.

  • Monthly charges are often in the $15 to $45 range, plus usage charges.

Eventually (and if ISDN survives), you can expect pricing to be only slightly more than standard analog service.

T1 line. Presently, if you want anything even resembling real bandwidth from the telephone company, you'll have to get a dedicated T1 line. This does not give you a high-speed connection from your home or office to your choice of locations; it only gives you high-speed access from your home or office to one specific location. Usually, this location is a place where you can connect to an Internet backbone (main line) and send routed signals through the Internet.

Your best source for T1 lines is a local ISP (Internet Service Provider). You don't pay per-call for T1 lines; you only pay a flat (high) monthly fee.

When leasing a T1 line from either the telephone company or ISP (ISPs buy T1 lines in bulk from the phone companies, and resell them at lower prices than the phone companies charge), you can attach your network to one of the company's connectors. (In telephone company jargon, we call everything beyond the connector customer premises equipment, or CPE.)

However, you cannot attach your router to the telephone company connector by simply plugging in a cable. To make sure a network interfaces properly with a telephone line, you have to perform several functions. You'd use a channel service unit (CSU) and data service unit (DSU) to handle these functions.

The CSU. A CSU is the first device the external phone line encounters on the customer premises. In the early 1980s, the telephone company always owned the CSU and leased the device to the customer. But during the course of telecommunications deregulation, you can now buy and install your own CSU.

One of the principal functions of a CSU is to protect the carrier and its customers from any "events" (stray voltages, frequencies, etc.) your network might introduce onto the carrier's system. A CSU provides termination for the telephone line and performs line conditioning and equalization. It also supports loop back tests for the carrier, which means the CSU can reflect a diagnostic signal back to the telephone company without sending it through any customer premises equipment.

CSUs often have indicator lights or LEDs that identify lost local lines, lost telephone company connections, and loop back operation.

When the telephone company provided CSUs, the telephone line itself provided the power to them. Now, they generally need their own power supply, and perhaps a backup power supply at the user site.

The DSU. You connect a DSU (sometimes referred to as a digital service unit) between a CSU and customer equipment, such as routers, multiplexers, and terminal servers. DSUs are commonly equipped with RS-232 or V.35 interfaces. Their main function is to adapt the digital data stream produced by the customer equipment to the signaling standards of the telephone carrier equipment, and vice versa.

The digital signals produced by customer devices (with throughput less than 56 kbps) are asynchronous, which means each byte is distinguished by start and stop bits, and the time interval between bytes varies. However, most customer devices in the telecommunication system use synchronous signaling, in which senders and receivers coordinate with timing clocks to identify the boundaries between units of data.

In these cases, the DSU may be called upon to parcel out incoming asynchronous data at the stable rate the carrier line expects, and wrap start and stop bits around incoming synchronous data before passing it along to the user network.

Other digital services. Here are some other digital options to consider:

Switched 56. Unlike ISDN, most carriers already offer this service. It creates a virtual network over existing public phone lines with a 56 kbps data rate. This service is cheap, but slow. It's ideal for intermittent data swapping between wide area networks (WANs).

SMDS (Switched Multi-Megabit Data Services). Using a connectionless networking plan, each SMDS packet has its own address and does not require a virtual circuit. Proposed speeds are from 1.5 Mbps to 45 Mbps using a fixed-length packet of 53 KB. Many regional carriers are beginning to offer this service for local traffic.

ATM (Asynchronous Transfer Mode). Using the same 53 KB packets as SMDS, ATM uses virtual circuits to transfer data at speeds of 34 Mbps to multiple gigabits per second.

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