Chapter 11: Signaling Systems


This chapter covers signaling in circuit-switched networks. It explains how signals are used as an encoding method required to supervise the connection and to relay addressing signals.


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. Can be classified by the method of exchange signals:
·        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 2 types of 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 a separate network to carry the signals. Two examples:
·        D channel of ISDN (see Chapter 32 for elaboration)
·        Signaling System 7 (SS7)- separate common channel signaling network; this eliminates toll fraud.

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.

Differences Between In-Band and Out-Band Signaling

In-Band Signaling

·        Line busy or idle; Central Office (CO) monitors constantly the status
·        When you lift receiver, it signals CO intent to make a call
·        CO marks the line busy and sends back a dial tone
·        Caller dials the number and CO transmits the dual-tone multi-frequency (DTMF) or dial pulses
·        System translates digits and sends them to the terminating end thus producing a ring
·        The process of setting up the talking path determines whether the path is operative since audible signals are passed over the channel that will be used for talking.
·        Are limited to transferring call setup and supervision information, and are therefore incapable of supporting virtual voice networks.
·        Call setup consumes circuit time (takes several seconds to setup), which is important because it represents a direct expense in the access charges paid to the LECs.

Common Channel Signaling (Out of Band)

Replaces the In-Band Signaling and uses a separate data communications network to exchange signals and route the calls.
·        SS7 is the international common channel signaling system.

The Architecture of OUt of Band Signaling

·        Service switching points (SSPs): Software applications running in the central offices, and are linked via data circuits to
·        Signal transfer points (STPs): Network nodes that act as hubs for signaling messages.
·        STPs are linked to service control points (SCPs), which contain a database of network information that can be accessed by the network nodes.
·        Each stage of the call, ringing, connect, and disconnect, is signaled with a data message.
·        It is so fast, the circuit connection can wait until the called party answers, further increasing circuit utilization.
·        Important to the LECs’ market strategies; enables them to receive more revenue from CLASS.
·        Custom Local Area Signaling Services (CLASS): Uses a separate packet-switched network to pass call setup, charging, and supervision information.
·        Can access the carrier’s database to obtain account information, such as features and points served, on a virtual network.
·        Is required for local number portability - take your previous phone number to your new location.

Conveying Signals

Signaling on interoffice circuits uses two leads that are both used to communicate status to attached central office equipment:

Signaling equipment converts the binary state of line signals:

Direct Current Signaling Systems

Can be employed on metallic facilities (most subscriber loops and voice frequency interoffice trunks). Not Mandatory.
·        Loop Start= when the caller takes the telephone receiver off the hook, closing a direct current path between tip and ring and allowing loop current to flow; all subscriber loops that terminate in station sets use loop start signaling.
·        Glare=When a party picks up the phone to find that there is someone there without it ringing.
·        No problem for the callers, but a problem in the trunks, so use one-way signaling
·        Most PBXs have two-way trunks so they are subject to glare.
·        Cannot use loop-start two-way trunks because the only indication the PBX has of a call incoming from the central office is the ringing signals.

Trunk Signaling Systems

Single-Frequency Signaling (SF)

2600-Hz Single-Frequency is the most common analog trunk signaling system.
·        Voice frequency leads from a carrier channel connect directly to the SF set.
·        Contains circuitry to change the state of the E & M leads in response to the presence of absence of the SF tone and to turn the signaling tone on and off when the switching system or other central office equipment changes the status of its leads.
·        Talk-Off= hazard of SF; can occur when the user’s voice contains enough 2600-Hz energy to actuate the tone-detecting circuits in the SF set.
·        Voice filters minimizes this, but not for people with high pitched voices.

Addressing Signals

Are more reliable, faster than dial-pulse signals, but requires a sender to transmit pulses, uses less time to set up a call than dial-up pulsing, and can be passed through the switching system after the connection is established. They:

·        Travel over the voice path
·        Transmitted over trunks as DTMF, dial-pulse, or multi-frequency signals, or over a common channel as a data signal.

·        Dial pulse = requires from 1-10 pulses plus an inter-digital interval.
·        DTMF signals = requires a DTMF receiver in the central office to convert the tones to the addressing signals.

Signaling System 7

Improves call management; handles call setup and disconnect; handles end-to-end call supervision, and call timing and billing.
·        Enhances network management
·        Separates network control from hardware; a control system independent of the circuits and transmission equipment that make up the network.
·        Supports Integrated Service Digital network (ISDN)
·        Supports user database; in the form of virtual networks
·        Handles addressing and supervision
·        Supports 800 number portability; when dialed, a SS7 message to a central database returns with the identity of the IXC.
·        Supports local number portability.


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.


·        The signaling data link is the physical layer; provides physical links between network nodes.
·        The signaling link layer is a data link that has three functions: flow control, error correction, and delivering packets in the proper sequence.
·        Signaling network layer routes messages from source to destination and from the lower levels to the user part of the protocol; enables it to handle link failures and to route messages based on their logical address.

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.:
·        No signaling- some private lines require no signaling.
·        Ring down circuits- generator rings the bell of a distant station.
·        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.


Few published standards exist on the carrier single-frequency signaling system. Internationally, most carriers use the ITU nos. 6 and 7 standards for common channel signaling and ITU nos. 4 and 5 line-signaling systems.
·        These standards are of little concern to users because the IXCs administer them.

Evaluating Signaling Equipment

Most economical configuration in digital systems is the built-in equipment of the T carrier channel. Circuits either have:
·        End-to-End signaling= the easiest to test
·        Link-by-Link signaling= the signals are extracted at intermediate points and connected by a pulse link repeater, such as PBX equipment furnished by the user.

Signaling compatibility is rarely a problem and testing capability is important.
·        Circuit design is essential before selecting signaling equipment.
·        Where built-in signaling is not included as part of the channel unit, external signaling converters are required.

Signaling compatibility is an important consideration in acquiring signaling equipment.

The timing of signals, which is controlled by switching and transmission equipment connected to signaling sets, is a frequent cause of incompatibility, but this has little to do with the signaling equipment itself.

Key Points

Switched connections over the telecommunications network involve some analog signaling; even though most of the connections is digital, the subscriber loop remains analog and requires analog signals.

Today’s system works like this: If a station is busy, the signal stops at the end office, but if you have call waiting, the call in progress is interrupted. ISDN provides a separate out-of-bank signaling channel that allows the network to send alerting messages to the user, where they can display on the phone.

·        When local networks are converted to ISDN, analog signaling is eliminated.

·        With analog trunks the tone operates auxiliary single frequency (SF) signaling sets that show the line status by changing the status of direct current voltages on their signaling leads.

·        SF signaling have been replace in nearly all circuits because of toll fraud and wasted circuit time during call setup.

·        Signals pass among IXC, LEC, and users as data messages.