ISDN and 56k dialup are not the same
 

ISDN dialup greatly improves your Internet access by improving the type of telephone service you use. It requires that you have ISDN digital telephone service from your telephone company.

56k dialup speeds up your Internet access without  changing your telephone service. You just use a new and faster kind of modem with your normal telephone service.

For both ISDN and 56k the Internet service provider you use must have digital telephone service. Your provider must use ISDN digital telephone service to offer ISDN dialup.

To offer 56k, your provider can use lower grade digital telephone service but will normally use ISDN where it is available. In that case, 56k dialup and ISDN will share a common access number, but they are very different services.

The telephone network has two parts:

• Trunks. These carry calls from town to town and have been digital for years.
• Subscriber lines. These carry calls from telephone company facilities to homes and offices and are normally analog.

Making the trunks digital made the telephone network a data network. The data usually represents telephone calls, but it can be other things too, which makes possible bank networks, other corporate networks, and the Internet, among other things.

ISDN (Integrated Services Digital Network) makes the subscriber lines part of this digital network. ISDN is telephone service that's digital data networking from end to end.

As phone service, ISDN is a way to call an Internet Service Provider offering ISDN Internet service. We offer it almost everywhere in our service area:

• NetHeaven Dialup Services and Prices
• NetHeaven Access Numbers

Unlike the speed ratings of conventional modems, when ISDN is labelled 64k, it means exactly 64k, every call, all the time, in both the upload and download directions. An ISDN call to an Internet provider is like being wired to the provider with a digital wire.

An ISDN phone "line" has two phone numbers and two "B channels." They can be used to make two simultaneous calls that can be combined into a single 128k connection [1].

To use ISDN for Internet access, you need an ISDN phone line from Bell Atlantic, an ISDN access account from us, and an ISDN terminal adapter (commonly but inaccurately called an ISDN modem). Some routers and computers have built in ISDN terminal adapters.

ISDN can be used to make "voice" calls (after all, it's telephone service) or "data" calls. The type of call is specified by the caller's equipment when the call is placed.

These labels refer to the normal use of the calls, but ISDN "voice" calls can be used for data. In this part of the country they cost less than data calls. There's a 12.5% bandwidth loss though.

ISDN "data" calls must be transported on special trunks, and Bell Atlantic charges a per-minute surcharge on these "data" calls for use of the special trunks. However, ISDN "voice" calls can be transported on the same trunks as non-ISDN telephone calls, and there's no data surcharge.

On voice trunks, up to 1 out of every 8 bits is used by the phone company for telephone signaling. The overall data rate is 64k, but it's partitioned into 56k for the user and 8k for the phone company.  "Data Over Voice" really means "data over voice trunks", and since these trunks only have 56k available for the user, DOV runs at 56k.

DOV, also known as Data Over Speech Bearer Service (DOSBS), is true ISDN and is exactly 56k and bi-directional.  In contrast, V.90, K56Flex, and X2 "56k" dialup are "up to" 56k in one direction.  They are not ISDN, but DOV is real ISDN, just at 56k per channel instead of ISDN data calls' 64k per channel.

Most ISDN terminal adapters ("modems") can be configured for DOV, and two 56k DOV calls can be combined into one 112k connection.

Telephone regulations consider using ISDN "voice" calls to send data as being analogous to using normal voice calls to send data with modems [2].

Conventional analog phone lines can run for miles out of town. ISDN can only be carried on such lines for a little over 3 miles.

ISDN has been around for some time in places where there are many potential subscribers within 3 miles of telephone company switching offices , but where lakes or cows or mountains are favored neighbors ISDN often is unavailable.

More than half of our Glens Falls area ISDN prospects have been unable to get ISDN phone service, and the Glens Falls area is hardly the most sparsely populated portion of our service area.

ISDN can be made available in more distant places by transporting it with trunk lines to a closer facility. If there is a Bell Atlantic facility of the right kind near enough to you, BA will do this.

For many areas, the "closer facility" is the switching office for the local telephone exchange. Most BA exchanges are not equipped for "native" ISDN, so BA trunks it in from some other exchange. They call this "virtual" ISDN and charge an extra $10/mo for it. The data call surcharge is larger also.

• Dan Kegel's ISDN page

[2] In fact, you can use ISDN "voice" service for ordinary modem calls. Some ISDN "modems" even have regular modems built in. However, using ISDN for modem calls is like using race cars to carry cargo.

[3] It's more precise to say that in that past some bits used to be needed for signaling. They generally are no longer used, but a lot of older telephone equipment still has the 56k/8k bandwidth partitioning built in.