TELECOM Digest OnLine - Sorted: Re: How a Telephone Works

Re: How a Telephone Works

William Warren (
Tue, 13 Sep 2005 16:03:51 -0400 wrote:

> Though I am in the telecommunications field ( software side) I am a
> bit confused about how everything works, though I have a high level
> overview. So I am stating my undestanding, so that someone can
> correct or fill up the gaps.

> 1. Each home suscriber has a twisted copper pair that runs from his
> telephone to a cable containg thousands (why no multiplexing here and
> send it through a single wire??) thousands of such pairs; to the local
> excahnge or the central office.

There's no multiplexing because it means putting active equipment at
the end of the wire, and that means the company has to build a weather
protection enclosure, connect power, maintain batteries, and pay for
easement(s), maintenance, etc. It's more cost-effective to have the
pair go back to the CO., at least for most single-family homes.

Apartment buildings, especially large ones, are more likely to be
multiplexed, since the ILEC doesn't pay for the space needed and it's
less expensive to run fiber than copper for the same number of lines.

> 2. In the central office there is a hardware equipment (LTG ??) which
> has a lot of ports, to one of which the copper pair that runs from the
> suscribers telephone is plugged in.

> (I hope I am correct here.)

Yes, you are correct. Most companies use the term "Central Office" to
refer to both the telephone exchange equipment and the building which
houses it.

> 3. The central office is connected to the tandem office via trunks
> which I hope are a thick co-axial cable or optical fiber through which
> multiplexed traffic from various CO flows.

Almost all fiber; coaxial cables were retired long ago and are now
used only for TV transmission, and even then only in locations where
the coax is "retired in place" (as far as trunk usage goes) and
there's no demand for digital transmission.

> Also there is a seperate cable for SS7 siganlling, connecting various
> CO to TO .

There are separate _circuits_ for SS7, which share the same
transmission layer as inter-office trunks, but are always routed to
two geographically diverse Signal Transfer Point locations via routes
that have nothing in common.

> There is also a switch at the tandem office.

Yes, it's a tandem switch. The definition _used to_ be along
"two-wire" vs. "four-wire" switches, but since all digital paths are
four-wire, the distinction is less clear now. It's very common for
"local" exchages to do double-duty as small tandem offices, e.g., for
E-911 switching to a PSAP, and the only real difference between
"local" and "tandem" switching is the circuit packs used at the edges,
since all digital exchanges have to have "four-wire" (i.e., separate
paths for transmit and receive) internal switching anyway.

> 4. Now if a suscriber dials a number, the DTMF tones are resceived at
> the CO which has a directory (databse ???) look up. It finds that this
> number is at antother exchange and sends a SS7 signal to that . From
> there how is the trunk reserved ????

In the Bell System, SS7 is an overlay on the old MF signalling method,
so each CO handles trunk reservations the same way for both signalling
methods. The exchange keeps an internal list of which trunks are in
use, and assigns a vacant trunk to each inter-office call as needed.
Intermediate tandems assign trunks in turn, in a daisy chain fashion,
until the call is completed or there are no trunks available.

There is no "database" of trunks; i.e. they are _not_ preassigned at a
central "brain" before the call setup is attempted. Each office
maintains a local list, and makes its own decision about which trunk
to use for the next hop, with the assignments taking place in sequence
from office to office.

> 5. Also how is the incoming call from a modem and telephone
> distinguished at the CO. Or does the modem also dial DTMF signals???

The modem uses either DTMF or dial-pulse, depending on how it has been
programmed, and it dials the call in the same way a subscriber would.
The CO is unaware that a modem is being used, either for data or fax.

> I hope someone can answer my questions.

I hope I have.


(Filter noise from my address for direct replies)

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