Subject: Tutorial on Telephone network architecture

From: Russ Nelson <[email-masked]> (by way of [email-masked] (Mitch Kapor))
Date: Wed, 11 Dec 1991 17:17:32 -0500

> [Tim Gorman <[email-masked]> provides the following useful
> tutorial on ISDN in the telephone network. -MK]

Tim left off the subscriber end, perhaps because this is more common
knowledge.  But I'll fill it in anyway...
    
Everyone knows that ISDN carries synchronous 2B+D, and that each B is
64Kbps packet or circuit switched, and the D is 16Kbps packet
switched.  Synchronous means that the data is constantly flowing at
the specified rate.  Most modems in use today use asynchronous
transmission, which means that each chunk of information has a
starting indication and an ending indication.

Packet switched means that the stream of bits is formatted
into packets, and each packet is transported as a unit.  Circuit
switched means that the stream of bits goes from one end to another
without being interpreted by the switching equipment.

There are two different flavors of 2B+D: U and T.  ANSI has defined
two different interfaces in the customer's premises, T (or S/T) and U.
The U interface connects directly to the switch over a 0 to 18,000 foot
loop, and uses one pair.  The T interface connects to the U interface
through a signal converter called an NT1.  It uses two pair (transmit and
receive) and runs at most 1,000 feet.

The U interface is electrically more sophisticated.  The transmission
rate is 80Kbaud and each pair of bits is encoded into four different
voltage levels.  This makes the data rate 160Kbps, plus it transmits
and receives in both directions at the same time.  It does this by
knowing what it's sending, and subtracting that from what it's
receiving.  Then it digitally filters the result.  This is necessary
because the local loop to your premises may contain multiple changes
in impedance.  That happens when your wire gauge changes, or you have
a tee connection.  These impedance changes muddy the digital signal.

There can be only one device on the U interface, most probably an NT1.
The T interface, on the other hand, can be shared between multiple
devices.  This is done because you can hang a telephone, a modem, and
a FAX machine off the same phone line.  When a FAX machine calls you,
your FAX machine answers.  When a modem calls you, your modem answers,
etc.

There are three concerns with ISDN:

  o Powering
  o Wiring
  o Extensions

Powering

The current telephone network (POTS -- Plain Old Telephone Service
[seriously]) is powered by the central office.  Ever notice that your
phone has a "REN"?  That stands for Ringer Equivalence Number.  It's
given in units of the standard telephone bell.  Your central office
can ring about five of these bells.  If the total of all your RENs is
over five, then your phone may not ring.

ISDN, on the other hand, uses no power from the central office.  That seems
like an advantage, but what happens when you lose local power.  The central
office still runs because it has its own batteries.  But you can't make
calls unless you have your ISDN sets on a UPS.

Wiring

POTS phones can and have been wired willy-nilly, in a star configuration,
or bus.  Because only two wires are necessary, sometimes only two wires
work.

ISDN, on the other hand, requires a single stretch of wire with terminators
on both ends, and it requires four wires.

Extensions

POTS sets can join into an existing phone call simply by going off hook.
Only one ISDN set can communicate on a B channel at a time.  That means
that only the originating or receiving set can communicate.  All the
other sets cannot transmit or receive.

--russ <[email-masked]> I'm proud to be a humble Quaker.
Peace is not the absence of war.  Peace is the presence of a system for
resolving conflicts before war becomes necessary.  War never creates peace.