Signaling
Overview Signaling is the step by step process of completing a telephone call. For example, when you make a telephone call, you lift the handset off of the telephone cradle (known as going off the hook). This sends a signal to a switch located at the telephone company. f the phone company has enough available circuits to proceed with the call, a signal is sent back to you, with a dial tone, to proceed. A generic telephone system has two major components, the user or subscriber side and the trunk or switch side. Signaling occurs between a telephone and a switch (subscriber side signaling) or between two switches (trunk side signaling). The subscriber side consists of the telephone and the wires that connect it to a Private Branch xchange (PBX) or to a switch in a Central Office. This configuration is called the subscriber loop. telephone to ring, and a hybrid to convert from two wire to four wire. The simple act of lifting a handset off the cradle and dialing a telephone number involves a series of signals exchanged between the telephone and the switch or PBX. Signaling Types As indicated, signaling can occur between a telephone and a switch (subscriber loop signaling), or between switches (trunk signaling). While either type can be analog or digital, only analog signaling between a phone and a switch, and both analog and digital signaling between switches are examined in this document. Subscriber Loop Signaling Trunk Signaling PBX or Switch PBX or Switch The phone may be analog or digital, but for the purpose of this paper, only the analog interface is examined. The phone has various components that include a transmitter, a receiver, a switch hook (activated / deactivated by the button in the handset cradle), a dialer (either rotary or push button), a ringer, and a hybrid (two to four wire converter). Figure 1 Subscriber Loop Signaling and Trunk Signaling n general, there are four types of important signals. /onhook or line seizure signaling, which provides information about the status of the loops or trunks. Start dial supervision. Address signaling, which provides information to the switches about the call destination. The switch (or PBX) has various components which interface to an analog telephone including a battery to nformational signaling, which provides information operate the telephone, a current detector to detect onhook/offhook to subscribers about the call progress. transitions, a dial register to store dialed numbers, a tone generator for tones (dial tone and busy signals), a ring generator to cause an attached Signaling Page 2
Line Seizure Signaling Line seizure signaling provides information about the status of the loop or trunk. On the subscriber side, the loop can either be idle (onhook) or active (offhook). When directly connected to a telephone (or FAX), Hypercom provides support for loopstart signaling on the subscriber side. Loopstart is the most popular interface for subscriber side signaling. On Hook - dle Switch Opened When a subscriber lifts the handset off the cradle, the switch hook closes allowing a current supplied by the PBX or switch to flow through the loop. Two wires are used to implement the circuit. The wire carrying -48Vdc from the switch to the telephone is called RG. The wire returning to the switch from the phone is called TP. At the switch, the current detector monitoring the TP wire attempts to seize a trunk line and initiates a response with informational signaling (dial tone or fast busy) based on the trunk seizure success. When the handset is replaced, the hook switch opens and the current stops flowing, signaling that the circuit has now gone idle. RG TP -24 or -48Vdc Detector Loopstart signaling circuitry in this case is asymmetric (a -24 or -48Vdc battery located at the PBX/switch and a hook switch at the telephone). Telephone PBX or Switch One of the advantages of a loopstart circuit is no requirement for accurate ground references between the switch and the telephone. Off Hook - Active Switch Closed Telephone RG Subscriber Side Signaling - Loopstart TP Flow Figure 2 Subscriber Side - Loopstart Signaling -24 or -48Vdc Detector A disadvantage of loopstart is poor "glare" resolution. Glare occurs when both the local end and the remote end attempt to seize a trunk at the same time. Unfortunately, with loopstart, the telephone is not informed of an inbound seizure until ringing is detected. An additional concern caused by loopstart implementations is no far end disconnect supervision. When one end hangs up, the circuit is not terminated until the remote end also hangs up. Because of these limitations, loopstart is rarely used in switch to switch signaling. PBX or Switch Loopstart is used almost exclusively on subscriber loops (telephone to switch signaling) and is the simplest and most inexpensive form of signaling to implement. Signaling Page 3
Trunk Side Signaling 2 or 4 Wires for Voice (2w &M / 4w &M) On the trunk side, there are a number of possible line seizure signaling interfaces, such as: Loopstart Ground Start &M Type &M Type &M Type &M Type V Supported by OT Supported by Supported by Supported by OT Supported by Supported by Loopstart Loopstart is a simple switch activated circuit that provides current when a circuit or trunk is requested. Because of the concerns about glare and far end disconnect supervision, loopstart is rarely used for switch to switch (PBX to ) signaling. &M Signaling &M is the most frequently used analog signaling scheme for switch to switch line seizure signaling. &M is the short familiar form of ar and Mouth. The name comes from telephone circuit diagrams where the receive line was known as ar (listen) and the transmit line was known as Mouth (talk). &M uses separate circuits (wires) for signaling and voice, unlike loopstart which uses the same wires for voice and signaling. The voice path can be carried over two wires (2 wire &M) or four wires (4 wire &M). There are additional wires used for signaling in &M. The voltages for signaling wires in &M can be either -48Vdc or -24Vdc. &M Type n &M type, the battery for both the and M leads is supplied by the PBX to the line or transmission equipment. n the idle, on hook position, the M lead is grounded at both ends. (OT: the voltage on the and M leads may not be identical). PBX Detect Mouth Lead ar Lead Figure 3 &M Type The onhook condition is represented by the M lead in the grounded position and the lead in the open (not grounded) position. is represented by the M lead attached to battery and the lead grounded. ven though &M Type is the most commonly used signaling interface in orth America, it has potential drawbacks. f two PBXs are improperly grounded, a difference can exist in the ground potential between the PBX and the signaling equipment. This can even occur when equipment is located on different floors of the same building. can flow down the M signaling lead, the remote PBX can detect current flow on the lead and can lead to a false seizure of the trunk. This is the most common installation problem with &M Type. Because of the difference in grounds, and the amount of current needed before current flow can be detected, faulty grounds can cause sporadic and unusual problems. Additionally, as an asymmetric signaling scheme (battery supplied by one end only), there is a potential for noise interference. &M Type V Detect Line quipment &M type V is the TU-T standard for &M signaling and is the most popular interface outside orth America. Both the PBX and the line equipment supply battery. While the interface does not provide isolation between power systems, there are minimal return currents in this symmetrical signaling scheme and therefore interference and false seizures are greatly reduced. Signaling Page 4
The onhook condition is represented by the and M leads being in the open (not grounded) position. is represented by both leads being grounded. Detect 2 or 4 Wires for Voice (2w &M / 4w &M) ar Lead Hypercom supports: Wink start mmediate start Delay start The following section details the trunk side signaling that takes place after an line seizure signal (offhook condition) is detected and prior to output of digits over the trunk circuit to the remote switch. SUBSCRBR SD Start Dial Signaling On the subscriber side, the switch/pbx presents a dial tone to the attached telephone (or FAX) to indicate that dialing may proceed. f the switch is unable to seize a line, a fast busy (reorder) is presented. (Dial tone and fast busy are defined in informational signaling.) TRUK SD PBX Mouth Lead Figure 4 &M Type V Detect Line quipment WK START Wink start operation is the most commonly used trunk signaling type. n wink start operation, the originating trunk circuit goes offhook. The remote switch detects the offhook signal, transmits an offhook pulse that is between 140 and 290ms in length, and returns to the idle (onhook) state. This wink on the lead is only sent after the switch has determined that it can seize a line. The originating trunk detects the wink back, waits for at least 210ms and outputs digits to the remote switch. When the call is answered, the terminating switch indicates this by an offhook condition. After the call completion and the current is in wink mode, the polarity on both &M leads reverts to the onhook condition. Because many phones (subscriber side) are attached to a single PBX and this PBX typically has fewer trunk side connections, there is contention for available trunk lines. ither the PBX or switch can contend for an available trunk line. The potential for glare and double ended seizures exists. To alleviate the potential for glare, the calling PBX may receive a double acknowledgment from the remote PBX. There are a number of protocols, known as the start protocols, designed to reduce the problems caused by glare. These protocols provide end-to-end seizure of a trunk line. These protocols are put into action after detecting an offhook condition. MMDAT START With immediate start operation, the originating trunk circuit goes offhook, and maintains this condition for at least 150ms. The originating trunk then blindly outputs digits (address signaling). Using the figure 6 implementation of immediate start, there is no protection against glare (where both ends seize the line then send digits simultaneously). DLAY DAL Delay dial is similar to wink start when the lead timing sequence is examined. The main differences are the offhook state is signaled immediately by the receiver, and the return to onhook state (equated as permission for the originator to proceed with address signaling) is Signaling Page 5
delayed until the receive digit register is found. f the remote switch is not ready to receive digits, improper call completion can result. The offhook condition must be present for a minimum of 140ms. The offhook to onhook transition must not occur for at least 210ms after the offhook condition. M - Lead - Lead Wait At least 150 ms Line Seizure Forwarded (Address Signalling) Called umber Answers Address Signaling Address signaling is used by the caller to control and direct the switching system. The network obtains and transfers that information by address signaling. M - Lead Line Seizure Figure 6 mmediate Start Protocol Forwarded (Address Signalling) The most commonly used methods of address signaling are dial pulse and Dual Tone MultiFrequency (DTMF) signaling. Dial pulse signaling is most closely associated with rotary dial phones while DTMF signaling is most often supported on a push button telephone. Hypercom only supports DTMF signaling. - Lead M - Lead - Lead Wink Line Seizure Forwarded (Address Signalling) Receive Register Found Called umber Answers Figure 5 Wink Start Protocol Called umber Answers DTMF (Dual Tone Multiple Frequency) signaling consists of two tones (dual tone), combinations of which are transmitted depending upon the signaling digit. The keypad is composed of 12 keys. For each row of keys, there are individual low frequency tones, while each column is represented by a different high frequency tone. By pressing key, the switch receives a digit represented by both a low and high frequency tone. For example, the number "5" is represented by a 770 and 1336Hz tone. The following matrix describes the tone pairs used for the keypad digits. Figure 7 Delay Start Protocol Signaling Page 6
Col. 1: 1209Hz Col. 2: 1336Hz Col. 3: 1477Hz Row 1: 697Hz 1 2 3 Row 2: 770Hz 4 5 6 Row 3: 852Hz 7 8 9 Row 4: 941Hz * 0 # Figure 8 DTMF Frequencies ote: The keypad has been standardized but the tone tolerances may vary. n the U.S., the tone generators should be within a 1.5% tolerance, while the switch ignores any tone that exceeds a tolerance of 3.5%. Within the orth American phone system, telephone companies often support the use of the octothorpe (#) to end the interdigit time-out period. This allows calls to be processed faster if the dialing digits are immediately followed by the "#" stop-dial signal. A RS-470 specifies that DTMF generators utilize the following levels: Per Frequency: +6dBm to - 4dBm Per Frequency Pair: +2dBm For automatic dialing, the minimum digit cycle time is specified as 100ms. The duration of the DTMF signal must be at least 50ms, and the minimum interdigit interval is specified as 45ms. The maximum interdigit interval is 3 seconds. DTMF receivers should register digits with ontime as short as 40ms. While there is no requirement to bridge a gap in the DTMF tone, the DTMF receiver must recognize 40ms of no tone as an interdigit interval. t should be noted that some receivers may recognize 20ms of no tone as an interdigit interval. Hypercom has a built-in DTMF detector/generator that complies with A RS-470 for transmit and receive functionality. nformational Signaling nformational signaling is simply the collection tones and recorded messages used by the network to inform the user of the progress of the call. The informational signals users are most familiar with are busy signal, fast busy, dial tone, and ring back. These signals are composed of precise tones generated at specific cadences. The audible tones most commonly used in orth America are as follows: Dial tone is a composed of two tones (dual tone) and is defined as 350 + 440Hz continuous. t is an indication that the switch/pbx is ready to receive address digits. Ring back tone is defined as a dual tone 440 + 480Hz with a pattern of 2 second on and 4 seconds off. The ring back tone allows the call initiator to know that the remote telephone is in the process of ringing. Busy signal is also a dual tone signal represented by 480 + 620Hz. t has a defined pattern of 0.5 seconds on and 0.5 seconds off. The busy signal is used as an indication that call could not be completed because the phone at the remote end is busy. Fast busy signal is an indication that a trunk line was not available for the call. t is represented by the frequency pair of 480 + 620Hz and has a pattern of 0.2 seconds on and 0.3 seconds off. Ringing is another form of informational signaling. To ring the attached phone, the outputs a 86Vac 20Hz down subscriber loop to drive a bell in the phone. The cadence of the ringing is 2 second on and 4 seconds off. Digital Signaling Digital links have become very popular for a number of reasons, including high numbers of user per physical ports (a T-1 circuit can carry 24 voice calls simultaneously) and better quality voice call (no line loss problems). There are two methods to transfer signaling information over digital lines, Channel Associated Signaling and Common Channel Signaling. Signaling Page 7
Channel Associated Signaling (CAS) is also known as Robbed Bit Signaling. n this method, certain bits within the voice circuits are robbed and used to transfer signaling information. n the Common Channel Signaling method, a single channel is dedicated to signaling information, which allows all bits within a user s payload to remain untouched. This is very important for data calls, but has little impact on the quality experienced by a voice user. Hypercom supports Robbed Bit/Channel Associated Signaling only. n the previous section four types of signaling are described: line seizure signaling, start dial supervision, address signaling, and informational signaling. The digital signal of interest is the line seizure signaling. ach type of line seizure signaling supported by Hypercom (loopstart and &M) has its own set of signaling states. ach of these states is represented by a specific digital pattern. For example, &M has only two states, idle and busy, but loopstart must convey multiple states, including loop open, loop closed, ring present, and ring removed. Because of the multiple states, loopstart signaling requires two signaling bits. n a D4 SF, every 6th and 12th frames have the Least Significant Bit (LSB) from every 8-bit word overwritten to indicate the hook state. To differentiate between the signal bits, the signal bits in the 6th frame are referred to as the A-bit while the signal bits in the 12th the frame are the B-bits. n SF, there are also signal bits in frames 18 and 24. These are labeled as the C and D-bits respectively. When a PBX places an outbound call, it picks-up the line by asserting the A-bit. Upon the A-bit transition, the line equipment raises its A-bit for a short period of time, then drops it. This is called the wink. When the A-bit drops from the line equipment, the PBX dials the desired numbers and perform call progress. When the remote end has answered the line, the line equipment asserts its A-bit to indicate call connection. When the call has ended, both ends drop their A-bits. DS-1 &M Signaling &M is a two-state signaling scheme (onhook/offhook) commonly used on switch to switch connections. As only one bit is required, the signaling state is transmitted by A-bit signaling. The B-bit (or B, C, D-bits in the case of SF) follows the same state as the A-bit. Tx Rx M-Lead A B A B -Lead Onhook 0 0 0 * Onhook 1 1 1 * Rx Tx -Lead A B A B M-Lead Onhook 0 * 0 0 Onhook 1 * 1 1 Figure 9 Two-State Signaling Scheme DS-1 Foreign xchange Signaling Ground start and loopstart use signaling states used within Foreign xchange Station (FXS) and Foreign xchange Office (FXO) modules. This signaling supports two-wire FX (Foreign xchange) trunk and OPX (Off-Premise xtensions) applications. Foreign xchange Office (FXO) modules provide analog circuit connection to a central office, while Foreign xchange Station (FXS) modules provide analog circuit interconnection to a station (telephone or PBX trunk). These signaling methods require four-state signaling, so both the A and B-bits are used to represent these signaling states. (n the case of SF, the C-bit follows the state of the A-bit and the D-bit follows the state of the B-bit). Signaling Page 8
FXO Transmit States Tx Rx A B A B FXS Receive States o Ringing 0 1 0 * o Ringing Ringing 1 1 1 * Ringing Rx Tx FXO Receive States A B A B FXS Transmit States Loop Open 0 * 0 0 Loop Open Loop Closed 1 * 1 1 Loop Closed Figure 10 Four-State Signaling Scheme For -1 lines, channel 16 is used for CAS signaling. MultiFrame 0 Time slot 16 is set to 0000 XYXX. This is known as the MultiFrame Alignment Signal (MFAS), the signaling reference point for the multiplexers. n Multiframe 1, the 16 time slot is used to convey the signaling states for the first channels, channels 1 and 16. The first nibble (four bits) is used for ABCD signaling bits of channel 1, while the second nibble is used for the signaling bits from channel 16. Multiframe 2 time slot 16 transports the signaling bits for channel 2 and 17. The first nibble is for channel 2, while the second nibble is for channel 17. The pattern repeats itself, 3 and 18, 4 and 19, up through channels 15 and 30. The pattern then repeats itself, MFAS, channel 1 and 16, channel 2 and 17 through channel 15 and 30. otice that since 0000 is the alignment signal, a signaling pattern of all zeros is illegal. Signaling Sequences Through an etwork There are three possible paths for voice connectivity through an network. ither a telephone handset or PBX/Key system is connected to the device. The signaling flow for these three combinations are represented in the diagrams below. These signal flows are valid with optioning through release 1 voice. Handset - Handset Call Setup through two s FR Call Request Ring Connection stablished Figure 11 Handset Call Setup Through Two s Signaling Page 9
PBX - Handset Call Setup through two s FR P B X Call Request Ring Connection stablished Figure 12 PBX- Handset Call Setup Through Two s PBX- PBX Call Setup through two s P B X FR P B X Loop, immediate or wink start Response to above Call Request Ring Connection stablished Figure 13 PBX - PBX Call Setup Through Two s Signaling Page 10
2851 West Kathleen Road Phoenix, AZ 85023 (800) 577-5501 (within the U.S.A.) (602) 504-5000 FAX (602 504-5166 -mail: info@hypercom.com Web Site: http://www.hypercom.com Signaling Printed in the U.S.A. July, 1997 Document umber 940100-001 Version 1.1 Copyright 1997, Hypercom Corp. All rights reserved. The ntegrated nterprise etwork is a trademark and Hypercom and the Hypercom logo are registered trademarks of Hypercom Corp. All other products or services mentioned in this document are trademarks, service marks, registered trademarks of their respective owners.