Question

purpose of a signalling system

Answer 1

Signals travel between controllers either over the talking path or over separate date networks. Telecommunications networks have a variety of methods for setting up and taking down circuit connections, such as signaling. They are not carried through to the LECs and IXCs.

  Direct current (dc)- used in local loops and tie lines

Tone- local loops and tie lines

  Bit-robbed- hybrid in-band system, that is neither tone nor direct current signaling common channel.

there are two types in signaling:

In-band: signals are carried over the same circuit that carries formation during the session.

Has drawbacks, such as fraud; toll thieves are able to defeat automatic message accounting systems by using devices that emulate signaling tones.

out-of-band: uses separate network to carry the signals.

D channel of ISDN.

signals can be grouped into four functions:

Supervising - monitoring the status of a line or circuit to determine if it is busy, idle, or is
requesting service.

Alerting - indicates to the addressee the arrival of an incoming call; audible bells and tones or visual lights.

Call Progress Tones - informs the user of the status of the call setup process; eg.: busy signal.

Addressing - the process of transmitting route and destination signals over the network; such as dial pulses, tone pulses, or data pulses over loops, trunks, and signaling networks.

Line versus register signaling: Line signaling is concerned with conveying information on the state of the line or channel, such as on-hook, off-hook (answer supervisionand disconnect supervision, together referred to as supervision), ringing current (alerting), and recall. In the middle 20th century, supervision signals on long-distance trunks in North America were usually inband, for example at 2600 Hz, necessitating a notch filter to prevent interference. Late in the century, all supervisory signals were out of band. With the advent of digital trunks, supervision signals are carried by robbed bits or other bits in the E1-carrier dedicated to signaling.

Register signaling is concerned with conveying addressing information, such as the calling and/or called telephone number. In the early days of telephony, with operator handling calls, the addressing formation is by voice as "Operator, connect me to Mr. Smith please". In the first half of the 20th century, addressing formation is done by using a rotary dial, which rapidly breaks the line current into pulses, with the number of pulses conveying the address. Finally, starting in the second half of the century, address signaling is by DTMF.

Channel-associated versus common-channel signaling:

Channel-associated signaling (CAS) employs a signaling channel which is dedicated to a specific bearer channel.

Common-channel signaling (CCS) employs a signaling channel which conveys signaling information relating to multiple bearer channels. These bearer channels therefore have their signaling channel in common.

Compelled signaling:

Compelled signaling refers to signaling where receipt of each signal from an originating register needs to be explicitly acknowledged before the next signal is able to be sent.^[3]

Most forms of R2 register signaling are compelled (see R2 signaling), while R1 multi-frequency signaling is not.

The term is only relevant in the case of signaling systems that use discrete signals (e.g. a combination of tones to denote one digit), as opposed to signaling systems which are message-oriented (such as SS7 and ISDN Q.931) where each message is able to convey multiple items of formation (e.g. multiple digits of the called telephone number).

Subscriber versus trunk signaling:Subscriber signaling refers to the signaling between the telephone and the telephone exchange. Trunk signaling is the signaling between exchanges.

Protocol:Resembles the OSI model, but has 4 layers instead of 7.
Message Transfer Part (MTP)= first of three layers and is a data-gram service, which means it relays unacknowledged packets.
Signaling connection control part (SCCP)= the fourth layer and it is responsible for addressing requests to the appropriate application and for determining the status of the application.

Private Line Signaling:

Uses all in-band signaling and Selective Signaling, which is an in-band system for operation of certain private line switching systems. Special dedicated circuits require signaling arrangements that use the same techniques and equipment as the telephone network, but have no direct counterparts in switched systems, ex.:
1. No signaling- some private lines require no signaling.
2. Ring down circuits- generator rings the bell of a distant station.
3. Selective signaling- some private line networks use 4-wire selective signaling system to route calls without the use of switching systems.

Many private networks use tie trunks without-of-band signaling links.

Coin Telephone Signaling:Coin telephones are owned by the LECs and are generated in the telephone to enable the operator to distinguish between coin denominations. They are privatly owned are not connected to coin control. They use the dialing and ringing signals of ordinary telephones plus direct current signals that operate apparatus within the telephone to collect and return coins.