==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.