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ISDN has become widely used for standard voice connections in many European countries, but has failed to catch on in the USA for this purpose. Could be the cost of phone sets, ordering troubles, or the use of confusing SPID numbers was the cause. Nevertheless, ISDN is near universally available and works perfectly well to get a pure digital connection from the telephone network to our digital studio gear.
The advantages are clear and compelling:
Superior audio quality All kinds of interference from power lines and other noise sources plague analog lines; while digital ones are immune. One side of Telco A-to-D and D-to-A conversion is eliminated. Since Telco analog converters are usually not so good, this helps a lot with signal-to-noise and distortion.
Near perfect hybrid null The studio side of the connection is now “4-wire,” with independent send and receive paths. We still need an adaptive hybrid because the caller’s analog connection has mixed send and receive audio and these need to be accurately separated. But a careful digital hybrid design combined with the digital connection on the studio side can reduce leakage to near perfectly absent.
Better call setup and control The main advantage is eliminating the common dial-tone blast problem, where a host punches up a call only to be greeted with rather unfriendly dial tone. This happened because the caller hung-up and the studio phone system didn’t know it. The Telco CO took too long to signal the status change, so there was an ambiguous period when the two ends were not in sync. With ISDN, this never happens because the state of the call is signaled instantly.
So far, we have been talking about ISDN Basic Rate Interface, with its two 64 kbit/s channels. But that is certainly not the end of the digital telephone service story. In the 1960’s the Bell System was running out of copper capacity, probably owing to the effects of the baby boom on population and the rapid growth of the American suburbs. To solve the problem, engineers invented a way to carry 24 phone calls on a wire pair. These were named T1 lines signifying the lowest level scheme for digital call transmission. Here we have 24 channels x 8 bits x 8 kHz = 1.536 Mbit/s total capacity. T1s are still widely used in North America, despite their age and the use of primitive signaling methods derived from analog pulse dialing. For example, dialed telephone digits are conveyed by wiggling a bit as if it were being controlled by a rotary dial. “Wink start” and “hook flash E&M” begin and terminate calls. Not exactly computer age, but functional. The Telcos save a lot of copper: only two pairs are needed for all of the channels, one for each direction. This compares to the 24 pairs that would be required for analog.
In Europe, E1 lines are used. These are identical to T1s, but with 32 channels, rather than 24.
Primary Rate ISDN modernizes T1 and E1 lines. One of the channels is taken over for call-control data, so you have either 23 or 32 channels remaining for voice. This 64 kbit/s data channel allows more sophisticated information to be transmitted, rather than the simple dial pulses.
T1s are widely available and used in the USA, and costs are falling dramatically since the FCC opened up competition in 1996. Larger cities have a dozen or more companies competing to provide them. The usual application is a direct connection to a long distance carrier, bypassing the incumbent Telco and saving the around 2.5 cents/minute that the Telco would collect as its share for routing and connecting the call locally. There may also be another T1 for the local service. The PBX uses least cost routing to send the long distance calls directly to the T1 bound for the ling distance carrier and the local calls to the T1 going to the local Telco CO. Of course, it is possible to have both local and long distance on a single T1. You save the recurring base expense for one of the T1s, but calls will usually cost more per minute.
Carriers and customers in the USA are slowly migrating from T1 to Primary Rate ISDN. Since the underlying transport is T1, changing means installing a new card in the CO and PBX to support the more advanced signaling. But the wire pair and terminating equipment can often be re-used. ISDN PRI is readily available in most of Europe and is widely and routinely used to connect PBXs to the telephone network. ISDN PRI is better than T1/E1. For example, Calling Party Number (caller ID) is not generally carried on a 1-800 long distance T1 trunk, while this is routinely available on an ISDN PRI.
When you buy your T1 or ISDN from someone other than your incumbent Telco, you are still likely to see their familiar logo on the trucks in your driveway come installation day. That is because they will probably still provide the raw wire pairs and some terminating equipment. The company with whom you signed the contract will provide the switching and billing. But this is not always the case; sometimes a vendor may have its own cable and terminal, this being more probable if you are in a big office building in a major city center. And other arrangements are possible, owing to the variety of companies and services competing for your business. Pricing varies considerably as well, so it pays to shop and compare.
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