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<2> An interview with an AT&T Long Distance Operations Center Supervisor who
will be referred to as Mr. R.
<3> and some general ISDN articles from Southwestern Bell Newsletters.
ISDN Definition
===============
CCITT Definition : An end to end digital network that supports a wide range of
services accessed by a set of standard multipurpose user-network interfaces.
ISDN will allow for incredible new services that will drastically change the
telecommunications industry and everyone's lives. For example, one new service
ISDN will bring about is calling party identification. This will allow
businesses and individuals who subscribe to that service to know exactly what
number you are calling from before they even decide whether or not to answer
the phone.
In the case of dialups like MCI, the originating # will be stored in a computer
along with whatever code and number that person dialed which would greatly
hinder abuse of codes from a home phone.
This is just the tip of the iceberg as far as ISDN is concerned. This phile
will analyze and describe how The Integrated Services Digital Network will
operate when it is implemented.
Out of Band Signalling
======================
Essential to a network capable of providing such enhanced services as calling
party identification is out-of-band signalling. Until the late 70's, when AT&T
introduced the 4ESS toll switch and CCIS into the national network, switches
had communicated with each other over the same channels in which our voice or
data was transmitted (in-band). During this time, all signalling between
switches had to be limited to a type that could be accommodated in the 'voice'
channel thus making it impossible to offer any advanced services.
The development of the separate Common Channel Interoffice Signaling (CCIS)
network allowed for more freedom and flexibility and thus came about the AT&T
Calling Card service.
ISDN brings an interface from the network to the subscriber's equipment. This
is a completely digital interface subdivided into two types of channels :
The 'D' Channels are those used for sending signalling and control information
across the interface. The 'B' Channels are those used only for customer
information which can be in the form of voice, data, or video.
The 'D' Channel hence manages the information or 'B' Channels making the
signalling 'out-of-band' and not 'in-band' as it is now. This approach allows
for two distinct benefits :
<1> All the capacity in the information bearing channels is available for
customer use.
<2> The 'D' Signaling channel allows for distributed processing across the ISDN
Network.
ISDN Interfaces
---------------
The CCITT has defined two major interfaces that will be used in conjunction
with the 'D' and 'B' channels :
The Basic Rate Interface (BRI) consists of one D Channel and two B Channels.
This interface is used for locations where information transport is relatively
small like a residence.
The Primary Rate Interface (PRI) consists of one 'D' Channel and 23 'B'
Channels. It is used for large capacity vehicles such as PBX's.
Notice that there are 2 DIFFERENT 'B' Channels in the Basic Rate Interface.
This allows TWO different types of data to be sent over the same connection at
the same time. For example, you could be ULing files to a board on Channel 1
while talking to the SYSOP on Channel 2.
So, if both you and a board both have a BRI ISDN Interface, next time the SYSOP
says 'Go Voice', you simply pick up the handset, switch it to channel 2, and
start talking...
These multiple channels are also the foundation for the widespread use of Video
Phones. Just like you were sending data over channel 1, and talking voice on
channel 2, you can be sending video over channel 1 (allowing the party's to see
each other) and talking on channel 2.
ISDN Devices
============
AT&T Technologies, Advanced Micro Devices, and Intel are all in the process
of designing equipment that will be compatible with ISDN. So far, the two main
designs talked about through SWB and AT&T are :
<1> The Voice/Data Terminal This will look like any regular computer terminal
with the exception that it has a handset on the side of the terminal and a
couple of switches that will allow you to decide which channel is for DATA
and which channel is for VOICE. This will also (of course), allow two
customers with a V/D Terminal to be exchanging DATA over one channel while
talking voice over the other one.
<2> The Video Phone