Digital Telecommunications Standards

One key player in the supply of digital telecommunications services is quite familiar, the telephone company. Beyond traditional Plain Old Telephone Services (POTS, also used as an acronym for Plain Old Telephone Sets), telephone companies have developed a number of all digital communication services. Some of these have been around for a while, aimed at business users with heavy data needs. Several new all digital services are aimed directly at you as an individual consumer.

The range of digital services supplied by telephone companies is wide and spans a range of data rates.

Maximum Data Rates of Digital Telecommunications Standards

Certainly you will still talk on the telephone for ages to come (if your other family members give you a chance, of course) but the nature of the connection may finally change. Eventually, digital technology will take over your local telephone connection. In fact, in many parts of America and the rest of the world, you can already order a special digital line from your local telephone company and access all-digital switched systems. You get the equivalent of a telephone line, one that allows you to choose any conversation mate who's connected to the telephone network (with the capability of handling your digital data, of course) as easily as dialing a telephone.

At least three such services are currently, or will soon be, available in many locations. All are known by their initials: SDS 56, ISDN, and SMDS. Eventually you will probably plug your PC into one of them or one of their successors.

T1

The basic high speed service provided by the telephone company is called T1, and its roots go back to the first days of digital telephony in the early 1960s. The first systems developed by Bell Labs selected the now familiar 8 KHz rate to sample analog signals and translate them into 8-bit digital values. The result was a 64 Kbits/sec digital data stream. To multiplex these digital signals on a single connection, Bell's engineers combined 24 of these voice channels to create a data frame 193 bits long, the extra bit length to define the beginning of the frame. The result was a data stream with a bit rate of 1.544 Mbits/sec. Bell engineers called the resulting 24-line structure DS1. AT&T used this basic structure throughout its system to multiply the voice capacity of its telephone system, primarily trunk lines between exchanges.

As telephone demand and private business exchanges (PBXs) became popular with larger businesses, the telephone company began to offer T1 service directly to businesses. As digital applications grew, T1 became the standard digital business interconnect. Many web servers tie into the network with a T1 line.

A key feature of the DS1 format was that it was compatible with standard copper telephone lines, although requiring repeaters (booster amplifiers) about every mile. The signal itself is quite unlike normal analog telephone connections, however, and that creates a problem. Its signal transmission method is called AMI (Alternate Mark Inversion) which are in bipolar form. AMI represents a zero (or space) by the absence of a voltage; a one (or mark) is represented by a positive or negative pulse, depending on whether the preceding one was negative or positive. That is, marks are inverted on an alternating basis. The formatting code for T1 transmissions over twisted pair copper cable generates a signal with a bandwidth about equivalent to its data rate, 1.5 MHz. This high speed signal creates a great deal of interference, so much that two T1 lines cannot safely co-habit in the 50-pair cables used to route normal telephone services to homes.

Outside the United States, the equivalent of T1 services is called E1. Although based on the same technology as T1, E1 combines 30 voice channels with 64 Kbit/sec bandwidth to create a 2.048 Mbit/sec digital channel.

HDSL

The primary problem with T1 is the interference-causing modulation system it uses, one based on 1960s technology. Using the latest modulation techniques, the telecommunications industry developed a service with the same data rate as T1 or E1, but it requires a much narrower bandwidth, from 80 to 240 KHz. One basic trick to the bandwidth reduction technique is splitting the signal across multiple phone lines. For T1 data rate, the service uses two lines; for E1, three. Besides reducing interference, the lower data rate allows longer links without repeaters, as much as 12,000 feet.

Unfortunately, the "subscriber" in the name of the standard was not meant to correspond to you as an individual. It fits into the phone company scheme of things in the same place as T1-linking businesses and telephone company facilities.

SDSL

Two lines (or three) to carry one service is hardly the epitome of efficiency. By altering the modulation method, however, a single line can carry the same data as the two (or three) of HDSL. The commercial version of this service is termed Single-line Digital Subscriber Line or SDSL. It has an effective range of about 10,000 feet from the central office without repeaters.

The chief advantage of SDSL over other, higher performance services is that it is symmetrical. The data rate is the same in both directions. Web servers and wide-area network connection often require symmetrical operation, making SDSL the choice for them.

ADSL

Most of the advanced services aimed at consumer Internet use are asymmetrical. They have a higher downstream data rate from the server compared to their upstream rates, from you back to the server. Telecommunications companies are not ignorant of this situation and shortly after they developed HDSL they also created a higher speed but asymmetrical alternative, which they termed, logically enough, Asymmetrical Digital Subscriber Line or ADSL.

ADSL can move downstream data at speeds up to 9 Mbits/sec. Upstream, however, the maximum rate is about 640 kbits/sec. ADSL doesn't operate at a single rate as does T1 or SDSL. Its speed is limited by distance, longer distances imposing greater constraints. It can push data downstream at the T1 rate for, at most, about 18,000 feet from the central office. At half that distance, its downstream speed potential approaches 8.5 Mbits/sec.

ADSL Downstream Data Rates

The modulation system used by ADSL operates at frequencies above the baseband used by ordinary telephone service or ISDN. Consequently, an ADSL line can carry high speed digital signals and ordinary telephone signals simultaneously.

VDSL

The next step above ADSL is the Very high data-rate Digital Subscriber Line or VDSL. A proposal only, the service is designed to initially operate asymmetrically at speeds higher than ADSL but for shorter distances, potentially as high as 51.84 Mbits/sec downstream for distances shorter than about 1,000 feet, falling to one-quarter that at about four times the distance (12.86 Mbits/sec at 4,500 feet). Proposed upstream rates range from 1.6 Mbits/sec to 2.3 Mbits/sec. In the long term, developers hope to make the service symmetrical. VDSL is designed to work exclusively in an ATM network architecture. As with ADSL, VDSL can share a pair of wires with an ordinary telephone connection or even ISDN service.

SDS 56

Already available from some telephone operating companies in some exchanges, Switched Data Services 56 (sometimes shortened to Switched-56) gives you a single digital channel capable of a 56-kilobit-per-second data rate. Its signals are carried through conventional copper twisted pair wiring (the same old stuff that carries your telephone conversations). Telephone companies like PacBell view it as an interim service to bridge the gap between ISDN service areas. To link to your PC, you need special head-end equipment-the equivalent of a modem-for your PC. Of course, because the signal stays digital, there's no need for modulation or demodulation. The signal stays error free through its entire trip.

In some locales, SDS 56 is no more expensive than an ordinary business telephone line. Installation costs, however, can be substantially higher (PacBell, for example, charges $500 for installation) and some telephone companies may add extra monthly maintenance charges in addition to the normal dial-up costs.

To take advantage of SDS 56, you need to communicate with someone who already has SDS 56 services. Currently, SDS 56 is not an internationally agreed upon standard, so your access to the service and its subscribers will be less universal than a telephone-based modem link. The chief advantages of linking through SDS 56 are higher speed, greater reliability, and data integrity.

ISDN

The initials stand for Integrated Services Digital Network, although waggish types will tell you it means "I Still Don't Know" or "It Still Does Nothing." The latter seems most apt because ISDN has been discussed for years with little to show for all the verbiage. But ISDN is an internationally supported standard, one that promises eventually to replace your standard analog telephone connection.