6.0 SIGNALING 6.1 Supervisory Signaling 6.2 AC Signaling 6.3 Address Signaling 6.4 Compelled Signaling 6.5 Associated and Disassociated Channel Signaling

Signaling conveys the intelligence needed for one subscriber to interconnect with any other in that network. Signaling tells the switch that a subscriber desires service and then gives the local switch the data necessary to identify the required and hence to route the call properly. It also provides supervision of the call along its path. Signaling gives the subscriber certain status information such as dial tone, busy tone and ringing

There are several classifications of signaling General a. Subscriber signaling b. Interswitch signaling Functional a. Audible – visual (call progress and alerting) b. Supervisory c. Address signaling

Signaling Function Supervisory Address Control (forward) – Seize Hold Release Status (backward) Idle Busy Disconnect Station -Rotary dial -Push button -Digital Routing -Channel -Trunk Alerting -Ringing -Paging -Off-hook warning Progress -Dial tone -Busy tone -Ring back Signaling Function Supervisory Address Audible-visual

SUPERVISORY SIGNALING - Provides information on line or circuit condition and indicates whether a circuit is in use or idle. The assumption is that a telephone in the network can have one or two states; busy or idle, idle is represented by the “on-hook” condition. Supervisory information – status must be maintained end to end on every telephone call. It is necessary to know when a calling subscriber lifts telephone off hook, thereby requesting service.

It is important because we may start metering the call to establish charges and to know the called and calling subscribers return their telephones to the on hook condition. During the period of occupancy of a talk path end to end, the particular path is busy (is occupied) so that no other call attempt can seize it. Dialing of a subscriber line is merely interruption of the subscriber loop’s off hook condition, called “make and break”. “Make” is a current flow condition and “break” is no current condition.

Some supervisory signals: Request for service - off-hook Ready to receive address - dial tone Call alerting - ringing Call termination - on-hook Request for operator - hook-switch flash Called party station ringing - ring back Network/called station busy - busy tone

AC SIGNALING Direct current signaling, such as reverse battery signaling, has notable limits on distance because it cannot be applied directly to multiplex systems and is limited on metallic pairs due to the IR drop of the line involved. Many ways to extend these limits, but for a cost effectiveness standpoint. On trunks exceeding dc capabilities, some form of ac signaling will be used.

AC signaling divided into 3 categories, i. Low Frequency ii. In Band Signaling iii. Out Band Signaling

Low Frequency AC Signaling Systems Operating below limits of the conventional voice channel Signaling systems are one frequency systems, typically 50 Hz, 80 Hz, 135 Hz, 200 Hz Excessive distortion and band limitation to operate systems over carrier derived channel Low frequency signaling is limited to metallic pair transmission systems Needed to repeater depending on the type of circuit (open wire, aerial cable, buried cable) and gauge wire Distance limit of 80 – 100 km

In Band Signaling Using for an audio tone, or tones inside the conventional voice channel, to convey signaling information In Band Signaling divided into 3 categories i. One frequency or Signal frequency (SF) ii. Two frequency (2VF) iii. Multifrequency (MF)

In Band Signaling - Single frequency and two frequency signaling systems utilize the 2000 to 3000 Hz portion, where less speech energy is concentrated.

Out of Band Signaling Supervisory information is transmitted out of band (i.e above 3400Hz) Use “tone on when idle”, indicating the on hook condition, others use “tone off” Advantages is that either system, tone on or tone off, may be used when idle.

Out of Band Signaling - Talk down cannot occur because all supervisory information is passed out of band, away from the speech information portion of the channel Out of band frequency preferred by CCITT is 3825Hz

SF signaling (a) in-band (b) out-of-band.

Signaling Techniques In-channel signaling In-band Out-of-band SF Signaling (2600 Hz) MF Signaling DTMF Signaling DC Current (on-/off-hook) Dial pulses (10 pps) 20 Hz Ringing voltage

Address Signaling Directs and routes a telephone call to the called subscriber If there is more than one switch involved in the call setup, signaling is required between switches (interregister switching)

Address Signaling: DTMF Signaling(dual tone multi frequency) 1209 1366 1477 1633 1 2 3 A 697 4 5 6 B 770 7 8 9 C 852 941 * # D

Commonly used as an interregister mode of signaling employing the speech band for the transmission. There are various methods of using two voice frequencies to transmit signaling information, example CCITT No. 4 uses 2040Hz and 2400Hz to represent binary 0 and 1

Address Signaling – Multifrequency Signaling Used for interregister signaling, method utilizing five or six tone frequencies, two at a time. Multifrequency signaling works equally well over metallic and carried (FDM) systems.

Address Signaling – Multifrequency Signaling 4 commonly used MF signaling system, i. SOCOTEL – used principally in France ii. Multifrequency Signaling in North America iii. CCITT No. 5 Signaling Code iv. The R-2 Code

COMPELLED SIGNALING - Signal element duration is important parameter, a call setup an initiating exchange sends 100 ms seizure signal. A fully compelled signaling system is one in which each signal continues to be sent until an acknowledgment is received The R-2 and SOCOTEL are examples of fully compelled signaling systems.3, figure below illustrates a fully compelled signaling sequence

Fully compelled signaling procedure.

COMPELLED SIGNALING Note the small overlap of signals, causing the acknowledging (reverse) signal to start after a fixed time on receipt of the forward signal. This is because of the minimum time required for recognition of the incoming signal

COMPELLED SIGNALING Fully compelled signaling is advantageous in that signaling receivers do not have to measure duration of each signal, thus making signaling equipment simpler and more. SOCOTEL is a European multifrequency signaling system used principally in France and Spain

Signaling/Switching Dependence Signaling and Switching are closely tied. Signaling allows switching to automate the network.

Call Processing - Local Call Detect off-hook condition Send dial-tone to calling station Collect dialed digits Translate digits to a called number Route call Prepare connection between stations Send ring voltage to called station / ring-back tone to calling station Detect off-hook by called station and cut-through the call Detect disconnect and terminate call

EFFECTS OF NUMBERING ON SIGNALING Numbering, the assignment and use of telephone numbers, affects signaling as well as switching. It is the number or the translated number, as we found out in Section 1.3.2, that routes the call. There is “uniform” numbering and “nonuniform” numbering. How does each affect signaling? Uniform numbering can simplify a signaling system. Most uniform systems in the nontoll or local-area case are based on seven digits, although some are based on six The last four digits identify the subscriber. The first three digits (or the first two in the case of a six-digit system) identify the exchange. Thus the local exchange or transit exchanges know when all digits are received

EFFECTS OF NUMBERING ON SIGNALING There are two advantages to this sort of scheme: 1. The switch can proceed with the call once all digits are received because it “knows” when the last digit (either the sixth or seventh) has been received. 2. “Knowing” the number of digits to expect provides inherent error control and makes “time out” simpler

Signaling Techniques Common Channel Signaling (CCS) Signaling Network Speech Path Dedicated data link between systems Trunk group associated Trunk group disassociated

Advantages of CCS One signaling path needed per trunk group Faster and simpler to transfer information between control processors No possibility of interference with speech path Signaling can’t be accessed by customer

Advantages of CCS Value-added services of a signaling control point Shared processing for small offices Allows centralized decision making (flow mgmt) Permits Advanced Intelligent Network (AIN) services

Disadvantages of CCS CCS links can be a single point of failure No inherent testing of speech path by call setup signaling CCS response time is critical

ASSOCIATED AND DISASSOCIATED CHANNEL SIGNALING Associated channel signaling, the signaling is carried right on its associated voice channel, whether in-band or out-of-band Figure below illustrates Two concepts: associated channel and separate channel signaling,

Conventional analog associated channel signaling (upper) versus separate channel signaling (which we call quasi-associated channel signaling) (lower). Note: Signaling on upper drawing accompanies voice paths; signaling on the lower drawing is conveyed on a separate circuit (or time slot). CCS = common channel signaling such as CCITT Signaling System No. 7.

Quasi-associated channel signaling, typical of E-1 channel 16 Quasi-associated channel signaling, typical of E-1 channel 16. As shown, the signaling travels on a separate channel but associated with its group of traffic channels for which it serves. If it were conventional analog signaling, it would be just one solid line, where the signaling is embedded with its associated traffic.

Trunk Group Associated CCS TR-303 Trunk Group DLC CO Switch TR-303 Control Link

Disassociated channel signaling is when signaling travels on a separate and distinct route than the traffic channels for which it serves. CCITT Signaling System No. 7 uses either this type of signaling or quasi-associated channel signaling. Figure below shows fully disassociated channel signaling.

Fully disassociated channel signaling Fully disassociated channel signaling. This signaling may be used with CCITT Signaling System No. 7

Trunk Group Disassociated CCS Signaling Switching Center Speech Paths CCS Data Link

Terminology in signaling often refers back to manual switchboards or, specifically, to the plug used with these boards and its corresponding jack as illustrated in Figure below. Thus we have tip (T), ring (R), and sleeve (S). Often only the tip and ring are used, and the sleeve is grounded and has no real electrical function.

Switchboard plug with corresponding jack (R, S, and T are ring, sleeve, and tip, respectively).