What is ISDN ?


    To begin, "ISDN" should not be thought of as just one more product.
is actually a family of services that are central to the fundamental
transportation of the entire world-wide telecommunications network.... t
United States and the world are going digital.

    A global economy requires a global telecommunications technology.

    Overseeing this world-wide evolution is the International Telegraph
Telephone Consultative Committee (CCITT), an agency of the United Nation
    The CCITT's membership consists of major governments, manufacturers
service providers from around the world , and it stablishes common
international ISDN standards.  In the past many different ISDN "flavors"
have been developed by competing manufacturers and countries.  It is the
CCITT's job to reconcile these differing flavors into one harmonious who

     These evolving universal standards will not only ensure that everyo
can effectively communicate with everyone else, but by the very fact of
their being universal they tend to protect ISDN providers' investment in
switches and infrastructure and their customers' investment in terminals
other equipment.

     Now the shift to digital transmission is certainly nothing new.  It
began in the early 1960's with the conversion (to T - carrier technology
of the inter-office trunk facilities - the backbone network that connect
telephone companies' central offices.   Since then, the other elements o
of the network have in turn also been included in this conversion proces
     The conversion process continues at a furious pace.  Toady in the
Ameritech region some of the last pieces of the puzzle are falling into
 place.  The last of the old 1ESS analog central offices are going digit

     It is interesting to not that a similar process has occured in the
world.  Today it is difficult to find an analog PBX being offered (let a
purchased).   In fact, virtually all major manufacturers also offer some
type of ISDN compatability in their lines.   This does not mean, of cour
that analog telephone sets are dead - many will continue to be sold (
especially in the lower end markets), but teir days are numbered in the
business world.

     Digital technology alone - as important as it is - is not the whole
 story, however.  To state that the network is digital is like saying th
 all automobiles have engines.  Ther are gasoline engines, but there are
 also diesel engines; there are big ones and small ones, foreign and
 domestic. And almost none of the parts are interchangeable.
     What ISDN is doing is building on that digital foundationa set of r
specifications detailing common features and services so the everyone ca
work from the same set of blueprints.  And the first building block of I
is the Basic Rate Interface (BRI).

     Here in Ameritech, we call this service ISDN Direct.


How does ISDN Work?

     At it's most basic level, ISDN provides a standard "pipe" called th
Basic Rate Interface.   BRI is transmitted over the normal 2-wire copper
cable facilities familar to telephone trnsmission all over the world.
     But unlike ordinary analog transmission which restricts this pipe to
conversation at a time, BRI combines, or multiplexes, three communicatio
channels into one pipe - all of which can be used simultaneously.

     The BRI consists of two 64 Kbs "B" (or Bearer) channels and one 16
"D" (Delta) channel.  This is the infamouse 2B + D many people have hear
about.   We can see that one on the more obvious achievements of ISDN
technology is that the capacity of the local copper loop is effectively
trippled.

SIGNALING

  Beyond the issue of capacity, ISDN signaling schemes not only provide
another break with the past, but also present opportunities for unique
telecommunications applications.

  Today's analog transmission uses a system called "in-band" signaling,
which means that all call processing (such as sending "off" or "on"
hooks," call set-up, and ringing) is done on the same path, or circuit,
that the conversation takes place.  This signaling "overhead" greatly
impairs network efficiency.

  ISDN, on the other hand, utilizes the D channel for all signaling
-"out of band".

   In simplified terms, what this means is that when a user lifts the
reciever off the hook and dials a number, the telephone itself sends a
short message, or packet, over the D-channel to the central office
saying, in effect, "I want to send a certain kind of message to this
address."  The central office first looks at this packet and determines
what kind of route the actual message will need to take (i.e. circuit
switched or packet switched).  It then hands off this signaling packet
to a seperate network used only for signaling, while reserving a future
path or route in the circuit-switched network for the upcoming message.

   This signaling network usually referred to as Signaling System 7
(SS7 or sometimes CCS) repeats the process - passing along the packet -
until the far-end telephone is reached.  Only when that telephone says
"OK, go ahead" is the transmission path actually established and the
call put through.

   When out-of-band signaling is examined we can see another key element
in the ISDN picture.  That signaling packet as it is passed from node to
node, finally reaching the terminal at the far end must contain all the
information for the call setup - and that includes the number of the
calling party.   Many of the most exciting applications being developed
to utilize ISDN make use of this information called Automatic Number
Identification (ANI) or sometimes Calling Line Identification (CLID).

   This signaling network usually referred to as Signaling System 7
(SS7 or sometimes CCS) repeats the process - passing along the packet -
until the far-end telephone is reached.  Only when that telephone says
"OK, go ahead" is the transmission path actually established and the
call put through.

   When out-of-band signaling is examined we can see another key element
in the ISDN picture.  That signaling packet as it is passed from node to
node, finally reaching the terminal at the far end must contain all the
information for the call setup - and that includes the number of the
calling party.   Many of the most exciting applications being developed
to utilize ISDN make use of this information called Automatic Number
Identification (ANI) or sometimes Calling Line Identification (CLID).

   For example. an incoming ISDN call could be automatically routed to
the proper department or person (while simultaneously displaying all the
customer's records on his or her computer screen) - all before the call
is even answered.

THE B-CHANNELS

    With all signaling tasls being performed by the D-channel, this
leaves the two B-channels free for full 64 Kbs, "Clear Channel",
transmission.  Each may be used for circuit-switched voice or data
calls. ("Circuit-switched" means that the entire transmission path, or
circuit, remains "nailed up" end to end for the duration of the call.)

    It should be noted the full, clear-channel 64 Kbs transmissiom
between central offices requires seperate SS7 deployment between all the
srvving offices.   Until the new SS7 network is fully extended
throughout the nation some links may still be restricted to "only" 56
Kbs.  Incidentally, this situation is one of the primary causes for the
"ISDN islands" many people talk about.

    In addition to the circuit-switched network described above, the
B-channel can also be utilized for high-speed packet data transmission.
    The packet network is seperate from both the circuit-switched
network with which most people are familiar and the new SS7 signaling
network.    In packet transmission the message is broken down into small
discrete blocks of information called packets - similar to the signaling
packet described earlier.   These packets are then addressed and sent
out individually over the network much like mailing a letter.
     Packet data transmission is extremely efficient for "bursty" type
of transmission today.

     One interesting option for users is to combine two B-channels for a
total of 128 Kbs of available bandwidth.  Video conferencing
applications, for example, can effectively use this speed.

D-channel packet:

    Another benefit of ISDN derives from the fact that not all the 16
Kbs capacity of the D-channel is needed for signaling.  Up to 9.6 Kbs is
available for medium speed packet transmissionwhich provides still
another communications option to the user.   Actual throughput will
depend on the number of packet devices hooked up to the D-channel and
how bursty their transmission patterns are.

    To sum up then, it would be possible for one rather energetic ISDN
user to simultaneously talk to a colleague over one B-channel, exchange
data files over the other B-channel and check his electronic mail over
the D-channel.  And, instead if needing to use multiple telephone lines
and modems, everything is accomplished over one BRI ISDN line.
     So how does it all fit together?

   First of all, ISDN starts out on a special line card in the Bell
central office switch.  It is then carried over the normal(non-loaded)
2-wire telephone plant cable mentioned earlier using a special
transmission scheme called 2B + 1Q.  (By the way, the term "2B + 1Q"
should not be confused with "2B + D."   2B + D is a generic term that
describes the actual transmission scheme of the 2-wire loop from the
C.O. to the customer.)   Distances of up to 18,000 feet form the C.O.
can be accomodated. (Distances over 18 Kft are possible, but would
require "boosters" and thus cost more.)

   At the customer's premise the cable terminates in a devise called the
NT1 (Network Termination 1).  Among other things, this device
establishes the basic boundary point between telephone company
facilities and the customer's inside facilities. The NT1 also takes the
outside, 2-wire transmission mode and converts it to the inside
mode(and vice versa). This inside mode, or passive bus, can vary, but
it usually consists of 6 wires - 2 for transmitting, 2 for recieving
and 2 for local power.

   Customers need to be aware that local power is needed.   This will be
no surprise to many of our business customers as some of them already have 
commercial telephone systems.  But Residence and smaller users need to be
that in order to run ISDN the unti that makes this possible needs power.

   Up to eight devices can be connected to one BRI in the "passive bus"
arrangement.  "Passive bus" is just a term used to describe the way in
which all the devices can share the same 6-wire transmission medium and
all transmit and/or recieve at the same time.   This arrangement allows
each device can have its own individual directory number.

   There are a few restrictions.  One important one is that only up to
two circuit-switched devices (such as voice terminals) can be hooked up
to a single BRI - all other devices must be packet-switched devices.
(Upon reflection this arrangement makes sense since two circuit-switched
terminals would be able to access one of the two B-channels at the same
time while all the devices could share the "virtual" packet network.)

    Any terminal equipment designed to "talk ISDN" (called a "TE1" in
ISDN terminology) can be connected to the passive bus.   A complicating
factor here is the exsistance of those different, propietary "flavors"
of ISDN mentioned earlier.

    The bad news is that until this year it was impossible to mix and
match equipment designed for a particular manufacturer's central office
switch with those of another.   The good news is that equipment built to
the new National ISDN-1 (NI-1 or sometimes NISDN) standard that is now
in place will solve that problem - all ISDN CPE will be interchangeable.

    Of course other equipment not specifically designed to work with
ISDN (called a TE2) can still be hooked up to the circuit, but it must
work through a terminal adaptor ("TA") which translates the
communications protocols from non-ISDN to ISDN.

   A typical situation might involve a PC with normal RS-232
connections.  BRI and RS-232 are not compatable, but the user need not
throw out his existing PC in order to work with ISDN.   The user has a
choice:  For example, the PC could work through a stand-alone TA such as
the Nothern telecom M5000, or it might be connected through a circuit
card plugged into one of the computer's expansion slots like the Apple
ISDN NB card, or it might plug directly into a telephone with a TA built
in like the AT&T 7507.

BEYOND BASIC RATE INTERFACE:

  ISDN Centrex

     ISDN Centrex is offered as an enhancement of the standard Centrex
service with which we are familiar.  Working across the standard Basic
Rate Interface, customers have access not only to all the features of
ISDN Direct but also to all the Centrex voice features that have made
that product so successful.


  ISDN Prime

     Primary Rate Access (PRI) is for our customers, particularly PBX
users, who need larger access rates to the network.  Instead of BRI's
2-wire, 2B + D interface, PRI is presented to the customer over a 4-wire
interface with 23 B-channels and one D-channel.   All channels
(including the D-channel) work at 64 Kbs.

     If a customer needs more capacity than one PRI (roughly equivalent
to a T-1 circuit), Pri's can be aggregated with one D-channel
controlling multiple PRI's which increase circuit efficiency.

     Since ISDN is a switched service, a particular strength of ISDN
Prime is the ability to consolidate PBX trunks and data services.  This
dynamic call allocation feature often virtually eliminates the need for
expensive private lines and greatly reduces the number of trunks needed.


  Broadband ISDN

      Broad band ISDN (BISDN) is the ultimate goal of the digital
revolution taking place.  Transmitted over the fiber-optic based
Synchronous Optical Network (SONET), this optical technology has the
bandwidth capacity to handle any conceivable application well into the
next century.

      Like digital network evolution before it, the transition to a
SONET backbone will begin with interoffice trunks and then gradually
work it;s way out towards the customer.  Full deployment will take many
years.

      However, our customers' rapidly expanding needs for bandwidth
cannot wait for the complete SONET network to be in place.  So, the
first elements of BISDN such as Switched Multimegabit Digital Service
(SMDS) are comming online right now with transmission speeds up to 34
Mbs.

      Higher and higher speeds will become available as the network and
our customers' needs evolve.  Ultimately, switched speeds in the
billions of bits per second("gigbits") will be possible.



THE BIG PICTURE:


      We are all in the early stages of a fundamental transformation of
worl-wide telecommunications.  It is not an exaggeration to say that
this transformation is comparable in scope to changes that took place in
society with the introduction of the automobile...  or television.

(SMDS) are comming online right now with transmission speeds up to 34
Mbs.

      Higher and higher speeds will become available as the network and
our customers' needs evolve.  Ultimately, switched speeds in the
billions of bits per second("gigbits") will be possible.


THE BIG PICTURE:

      We are all in the early stages of a fundamental transformation of
worl-wide telecommunications.  It is not an exaggeration to say that
this transformation is comparable in scope to changes that took place in
society with the introduction of the automobile...  or television.

      And, like those befoer us who were also "in at the beginning" we
can only dimly perceive where this new road will take us.
  You may have noticed already in your area that new phone line are
being installed and these are the digital lines that the ISDN
architecture will be using.   ISDN is already being used here in the
United States right now.

   More manufacturers of telephone equipment have started to produce
telephone systems the is compatible with the ISDN structure (such as
Macrotel 80D).

   For More information on ISDN Networks please call your local phone
company.
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