                               ==Phrack Inc.==

                   Volume Two, Issue Eleven, Phile #8 of 12


                         Telephone Signalling Methods
                         ----------------------------


                           Written by Doom Prophet


   This file explains the basic signalling methods in use by the telephone
system and is intended for general understanding. The text that follows is not
highly technical since this file is for basic understanding and aimed at less
experienced phreaks. Still, the more experienced readers may want to read it
as a review on the information.


    Analog--Analog signals are those that have continuously and smoothly
varying amplitude or frequency.  Speech signals are of this type when you
consider tone, pitch and volume levels that vary according to the person
speaking. When a person speaks into the transmitter on a telephone, the voice
signals are made up of acoustical energy, which are then converted into
electrical energy for transmission along a transmission medium.

    Analog carrier facilities may operate over different media, such as wire
lines, multi-wire cable, coaxial cable, or fiber optic cable. Copper wire is
the most commonly used for subscriber loops.


    A technique that allows for many signals to be sent along the same
transmission path is called Multiplexing. Analog signals use Frequency
Division Multiplexing or FDM.


    Digital--Instead of the voice signal being processed as an analog signal,
it is converted into a digital signal and handled with digital circuits
throughout the transmission process. When it arrives at the CO that serves the
called telephone, it is converted back to analog to reproduce the original
voice transmission.


    Pulse Code Modulation or PCM is when the binary signal is transmitted in
serial form. Binary coding represents bits or binary digits at 0 and 1 levels.
These levels have a definite time relationship with one another. Time Division
Multiplexing or TDM is the type of multiplexing, sometimes abbreviated as MUX,
done for digital transmission.


    Metallic--Metallic facilities carry only one Voice Frequency (VF) channel.
Typically, a metallic facility is used to connect business or residential
lines to a CO. Coaxial cable can be used to transmit both Analog and Digital
signals as well as Metallic signals.


    VF channels have a 4000 Hz bandwidth, from 0 to 4000 Hz. However, the
in-band range of the voice frequency is between 200 and 3400 Hz. Signals that
are out of this frequency range but still within the VF channel are out of
band signals. A supervisory equivalent to 2600 for out of band is 3700 Hz. The
amount of VF channels vary according to the transmission facilities that are
being used.


    CCIS (Common Channel Interoffice Signalling) is where control or
supervisory signals are sent on a separate data link between switching
offices. CCIS links operate at 4800 bps, or baud. Signal Transfer Points in
the switch send the supervisory information over the dedicated link. This
prevents supervisory tones from subscriber stations to register with the
telephone network as a change in trunk status.


    Reverse Battery Signalling- When the called end answers, the polarity and
condition of the Ring and Tip leads is reversed to indicate the status of the
connection. Conditions for a call being placed, but not yet answered, is
ground on the Tip and battery (the CO battery current is flowing through) on
the Ring.  When the called party answers, by the action of relays in the
switching equipment, current is reversed in the calling subscriber loop and
battery is placed on the Tip and ground on the Ring, which remains during the
talking.


    E and M- Leads connecting switching equipment to trunk circuits are termed
the E and M leads, for receive and transmit. The E lead reflects the far-end
or terminating end condition of the trunk. Ground on the E lead indicates that
a signal has been received from the other end. The E lead is open when the
trunk is idle. The M lead reflects the the near end condition of the trunk. It
is grounded when the trunk is idle, and goes to battery condition when the
called party goes off hook. Long interoffice and short haul toll trunks use
this signalling method.


    It should be noted that AC signalling is Alternating Current, and is used
on the intertoll network, and interoffice and short haul toll trunks. DC, or
direct current, is used on two wire or intraoffice connections, and local
interoffice trunks.

    Single Frequency (SF)- Single Frequency is an in-band 2600 Hz signalling
system. When a four wire trunk is idle, and is equipped for SF in band
signalling, a 2600 Hz tone is being transmitted in both directions. When the
trunk is seized at an originating position, the M lead is changed from ground
to battery state. This removes the 2600 Hz supervisory tone from the outgoing
trunk pair. The loss of the 2600 Hz will be detected at the far end by the SF
signalling unit, changing the far end E lead condition from open to ground,
causing switching equipment to function. When ground is restored to the M
lead, replacing 2600 on the near end trunk, the pulsing of address information
begins.


    Multi-Frequency (MF)- The MF pulsing method uses AC signals in the voice
frequency range, and transmits address information between COs by combinations
of only 2 of 5 frequencies. MF is used for the sending of address information,
as mentioned before. Other signalling methods are still required for trunk
control and supervision. There are six MFs comprising MF codes. These are 200
Hz apart in the 700-1700 range. Two frequencies are sent at once, thus
explaining the term 'Multi frequency.'


    MF pulsing is initiated by manual keysets and the TSPS switchboard, or by
MF outpulsing senders in ESS and Xbar. MF pulsing is very rapid and only
occurs when a connection is being established. KPs, or Key Pulses, are used as
a signal to start MF pulsing. STs, or STart tones are used as a signal to
indicate the end of MF pulsing.


    As an example of MF signalling, take a toll switchboard trunk connected to
a Xbar Central Office. The operator selects an idle trunk, and presses the KP
button on the keyset to signal the distant sender or register link equipment
to connect to a MF receiver. The S lamp on the keyset will light when the far
end is ready to receive MF pulses. After keypulsing the digits of the called
number, the operator presses the ST button, which indicates the end of pulsing
and disconnects the keyset from the operator's cord circuit and extinguishes
the KP and S lamps.


    At the terminating CO, the two MF tones of each digit are amplified and
limited in the MF receiver unit associated with the incoming sender and
register circuit. The frequencies are selected by channel filters in the MF
receiver and then detected. The DC voltage that results will operate the
proper channel relays to continue with the process of placing the call.



                            Written in July of 1986

