Telos ZephyrExpress - Manual (Page 122)

ISDN 7-7
commercials remind us for voice calls, you can choose an ISDN long-distance company
on a per-call basis by prefixing the number with the “1010-xxx” carrier code.
You must dial the full number, including the 1 or 011 + country code, following the 1010-
xxx numbers.
i
ISDN TIP!
If you want to temporarily switch carriers, try prefixing the call with these numbers:
1010-222 for MCI 1010-288 for AT&T 1010-333 for Sprint
AT&T can be unfriendly to ISDN, and at times will block 1010 access for that service. You
may have to contact them before dialing their access code.
Some long-distance carriers — not the big three — can’t handle ISDN at all. For the lat-
est information check the section in the Appendix “ISDN, BRI, ZephyrExpress, and You”.
Some long-distance connections are limited to 56kbps/channel. This limitation is be-
coming more rare. There is no certain way to know in advance. In addition, some carri-
ers may work at 64Kbps and not 56Kbps. So, you may need to try both or temporarily
switch carriers.
In our experience, the only sure way to know the capacity of a given connection is to try it, first at 64kbps and then at 56kbps if the higher rate fails.
t
DEEP TECH NOTE!
This limitation arises from a quirk of the older telephone infrastructure. The channel
banks that have been widely employed in the long-distance network have a native
64kbps capability, but ÒrobÓ the low order PCM bit on every sixth frame in order to
convey supervision information (on-hook/off-hook and dial pulses).
As new telephone plant is built for clear 64kbps transmission and a technology called
ÒSignaling System 7Ó is deployed to allow the various elements of the phone network
to communicate without using the robbed bits, this limitation will disappear. The
European network has universal full 64kbps capability.
ZephyrExpress provides standard rate adaptation (officially known as ‘CCITT V.110’)
from 56 to 64 kbps and vice versa when required. Bitrate adaptation happens automati-
cally within the system depending upon the rate selected by the calling party. Since
ZephyrExpress can’t communicate at two rates simultaneously, it ignores rate adaptation
information on any second incoming call.
HOW TO ORDER ISDN
Dealing with The Phone Company
As is often the case when we broadcasters interface with the phone people, the lines of
communication on ISDN can get a little tangled. Face it: we are not the usual customer.
The first order of business is to find someone who knows what ISDN is. While your
usual account agent will be the normal entry point, you may be talking to a number of
phone people before you find one who understands your needs. Some of the regional