Channel Associated Signaling
`
`15
`
`Network Signaling
`As previously described, network signaling takes place between nodes in the core network.
`This is generally from the local switch, through the core network, and to the destination
`local switch-in other words, between the calling and the called party switch.
`
`Figure 1-3 shows where subscriber and network signaling occur in the PSTN.
`
`Figure 1-3 Subscriber and Network Signaling
`
`For obvious reasons, the signaling system employed on the local loop (between the sub(cid:173)
`scriber and the local switch) differs from that which is used in the core network. The subscriber
`must only generate a limited number of signals: on or off hook, called party digits, and pos(cid:173)
`sibly a few commands for supplementary services. In comparison, a modem core network
`must perform very complex signaling, such as those to support database driven services like
`Local Number Portability (LNP), credit or calling card validation, and cellular roaming.
`Therefore, subscriber signaling systems are simple compared to modem network signaling
`systems.
`
`Network signaling was previously implemented using Channel Associated Signaling ( CAS)
`techniques and systems. However, for the past two decades, it has been replaced with
`Common Channel Signaling (CCS) systems. Apart from a rare trace of Signaling System
`No. 6 (SS6) signaling, System No. 7 (SS7) is almost the exclusive CSS system; thus, CCS
`can almost be taken to refer exclusively to the use of SS7. The remaining sections of this
`chapter discuss CAS and CCS methods.
`
`Channel Associated Signaling
`The key feature that distinguishes Channel Associated Signaling (CAS) from CCS is the
`deterministic relationship between the call-control signals and the bearers (voice circuits)
`they control in CAS systems. In other words, a dedicated fixed signaling capacity is set
`aside for each and every trunk in a fixed, pre-determined way.
`
`
`
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`

`16 Chapter 1: The Evolution of Signaling
`
`Channel Associated Signaling (CAS) is often still used for international signaling; national
`systems in richer nations almost exclusively use Common Channel Signaling (CCS). CCS
`is replacing CAS on international interfaces.
`
`CAS can be implemented using the following related systems:
`
`• Bell Systems MF, R2, Rl, and C5 .
`• Single-frequency (SF) in-band and out-of-band signaling
`
`• Robbed bit signaling
`The following sections discuss these methods in context with the type of signal, either
`address or supervisory.
`
`Address Signals
`Multifrequency systems, such as the Bell System MF, R2, Rl, and C5, are all types of
`address signals used by CAS.
`
`Multifrequency
`The CAS system can be used on either analog Frequency Division Multiplexed (FDM) or
`digital Time Division Multiplexed (TDM) trunks. MF is used to signal the address digits
`between the switches.
`
`Multifrequency (MF) signaling can still be found in traces within the United States, and it
`is still often found on international interfaces. On international interfaces outside of North
`America, MF is still used via the CCITT System 5 (C5) implementation. C5 is quite similar
`to Bell MF and was developed jointly by Bell Laboratories and the British Post Office [102] .
`R2 is the MF system that was deployed outside North America and is still used in less
`developed nations. R2 was developed by CEPT (which later became ETSI; see Chapter 2)
`and was previously known as Multifrequency Compelled (MFC) signaling. The CCITT
`later defined an international version; see Chapter 2 for additional information regarding
`the international version [102].
`
`MF simultaneously sends two frequencies, from a choice of six, to convey an address signal.
`The switch indicates to the switch on the other end of a trunk that it wishes to transmit
`address digits by sending the KP (start pulsing) signal, and indicates the end of address
`digits by sending the ST (end pulsing) signal. The timing of MF signals is a nominal 60 ms,
`except for KP, which has a nominal duration of 100 ms. A nominal 60 ms should be between
`digits.
`
`
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`

`Channel Associated Signaling
`
`17
`
`Table 1-3 shows the tone combinations for Bell System MF, RI, and C5. R2 tone
`combinations are not shown.
`
`Table 1-3
`
`Tones Used to Create MF Signals
`
`Digit
`
`Frequencies
`
`700
`
`900
`
`1100
`
`1300
`
`1500
`
`1700
`
`1
`
`2
`
`3
`
`4
`
`5
`
`6
`
`7
`
`8
`
`9
`
`0
`
`KP
`ST
`11 (*)
`
`12 (*)
`
`KP2 (*)
`
`+
`+
`
`+
`
`+
`
`+
`
`+
`
`+
`
`+
`
`+
`
`+
`
`+
`+
`
`+
`
`+
`
`+
`
`+
`+
`+
`
`+
`
`+
`
`+
`+
`+
`+
`
`+
`
`+
`+
`+
`+
`+
`
`*=Used only on CCITI System 5 (C5) for international calling.
`
`As stated, many international trunks still use C5. Signal KP2 indicates that the number is
`an international number; by inference, KP indicates that the number is a national number.
`International operators also use codes 11 and 12. More details on C5 are available in
`ITU-T Q.152. Supervision signals for MF systems are performed on FDM trunks by the
`use of Single Frequency (SF), which we describe in the following section.
`
`For circuit supervision, both Bell System MF and RI use Single Frequency (SF) on FDM
`trunks and employ robbed bit signaling on TDM controlled trunks. C5 uses a different set
`of MF tones for supervisory signaling.
`
`
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`18 Chapter 1: The Evolution of Signaling
`
`Supervisory Signals
`Single frequency systems, robbed bit signaling, and digital signaling are all types of
`supervisory signals used by CAS.
`
`Single Frequency(SF)
`Single Frequency (SF) was used for supervisory signaling in analog CAS-based systems.
`North America used a frequency of 2600 Hz (1600 Hz was previously used), and Great
`Britain used 2280 Hz (as defined in British Telecom's SSAC15 signaling specification).
`When in an on-hook state, the tone is present; when in an off-hook state, the tone is
`dropped.
`
`NOTE
`
`Supervisory signals operate similarly to those used in access signaling; however, they
`signal the trunk state between two switches rather than the intention to place or terminate
`a call. Supervisory signals are also known as line signals.
`
`Table 1-4 details the tone transitions Bell System MF and RI use to indicate the supervision
`signals. CS uses a combination of both one and two in-band signaling tones, which are not
`presented here.
`Table 1-4 Bell System MF and RI Supervision Signaling
`
`Direction
`
`Signal Type
`
`Transition
`
`On-hook to off-hook
`
`Off-hook to on-hook
`
`On-hook to off-hook
`
`Off-hook to on-hook
`
`Forward
`
`Forward
`
`Backward
`
`Backward
`
`Backward
`
`Seizure
`
`Clear-forward
`
`Answer
`
`Clear-back
`
`Proceed-to-send (wink)
`
`Off-hook pulse, 120-290 ms
`
`As with the MF address signaling, SF is sent switch to switch. A trunk is initially on-hook
`at both ends. One of the switches sends a forward off-hook (seizure) to reserve a trunk.
`The receiving switch indicates that it is ready to receive address digits, (after connecting a
`digit received by the line by sending a wink signal. When the originating switch receives the
`wink signal, it transmits the digits of the called party number. When a call is answered,
`the called parties switch sends an off-hook signal (answer). During the conversation phase,
`both ends at each trunk are off-hook. If the calling a party clears the call, it sends a clear(cid:173)
`forward signal; likewise, when the called party hangs up, it sends a clear-backward signal.
`
`
`
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`Channel Associated Signaling
`
`19
`
`SF uses an in-band tone. In-band systems send the signaling information within the user's
`voice frequency range (300 Hz to 3400 Hz). A major problem with in-band supervisory sig(cid:173)
`naling, however, is its susceptibility to fraud. The hacker quarterly magazine "2600" was
`named for the infamous 2600 Hz tone, which could be used by the public to trick the phone
`system into giving out free calls. The subscriber could send supervisory tone sequences
`down his telephone's mouthpiece using a handheld tone generator. This enabled the sub(cid:173)
`scriber to instruct switches and, in doing so, illegally place free telephone calls.
`
`The other major problem with in-band signaling is its contention with user traffic (speech).
`Because they share the same frequency bandwidth, only signaling or user traffic can be
`present at any one time. Therefore, in-band signaling is restricted to setting up and clearing
`calls down only because signaling is not possible once a call is in progress.
`
`Subscriber Line Signaling
`A regular subscriber line (that is analog) still uses in-band access signaling. For example,
`DTMF is used to signal the dialed digits and the frequencies used are within the voice band
`(see Table 1-1). You can prove that DTMF uses in-band signaling by using a device, such
`as a computer, to generate the tones for each digit (with correct pauses) . Simply play the
`tones from the computer speaker down the mouthpiece of a touch-tone telephone. This
`allows you to dial a number without using the telephone keypad. Because the signaling is
`sent down the mouthpiece, you can be certain that it traveled within the user's voice
`frequency range.
`
`FDM analog systems nearly always reserve up to 4000 Hz for each circuit, but only use
`300-3400 Hz for speech; therefore, signaling is sent above the 3400 Hz (and below 4000 Hz).
`This is known as out-of-band signaling and is used in R2 for supervisory signaling. Unlike
`with in-band signaling, no contention exists between user traffic and signaling. North America
`uses a frequency of 3700 Hz, and CCITT (international) uses 3825 Hz. Table 1-5 details the
`tone transitions that indicate the supervision signals used in R2 and Rl .
`Table 1-5 R2 Supervision Signaling
`
`Direction
`
`Forward
`
`Forward
`
`Backward
`
`Backward
`
`Backward
`
`Backward
`
`Signal Type
`
`Seizure
`
`Clear-forward
`
`Answer
`
`Clear-back
`
`Release-guard
`
`Blocking
`
`Transition
`
`Tone-on to tone-off
`
`Tone-off to tone-on
`
`Tone-on to tone-off
`
`Tone-off to tone-on
`
`450 ms tone-off pulse
`
`Tone-on to tone-off
`
`
`
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`

`20 Chapter 1: The Evolution of Signaling
`
`R2 does not use a proceed-to-send signal; instead, it includes a blocking signal to stop the
`circuit that is being seized while maintenance work is performed on the trunk. The release
`guard signal indicates that the trunk has been released after a clear-forward signaling,
`thereby indicating that the trunk can be used for another call.
`
`Digital
`
`Supervisory signaling can be performed for R2 on digital TDM trunks . On an El facility,
`timeslot 16 is set aside for supervisory signaling bits (TS16) . These bits are arranged in a
`multiframe structure so that specific bits in the multiframe's specific frames represent the
`signaling information for a given TDM audio channel. See Chapter 5, "The Public Switched
`Telephone Network (PSTN);' for explanation of facilities and timeslots .
`
`Limitations of CAS
`We discuss the general disadvantages of CAS for the purpose of reinforcing the concepts
`and principles we have introduced thus far. CAS has a number of limitations, including:
`
`• Susceptibility to fraud
`• Limited signaling states
`• Poor resource usage/allocation
`The following sections discuss these limitations in more detail.
`
`Susceptibility to Fraud
`CAS employing in-band supervisory signaling is extremely susceptible to fraud because
`the subscriber can generate these signals by simply using a tone generator down a handset
`mouthpiece. This type of device is known as a blue box; from the beginning of the 1970s,
`it could be purchased as a small, handheld keypad. Blue box software was available for the
`personal computer by the beginning of the 1980s.
`
`Limited Signaling Information
`CAS is limited by the amount of information that can be signaled using the voice channel.
`Because only a small portion of the voice band is used for signaling, often CAS cannot meet
`the requirements of today's modem networks, which require much higher bandwidth
`signaling.
`
`
`
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`

`Common Channel Signaling (CCS)
`
`21
`
`Inefficient Use of Resources
`CAS systems are inefficient because they require either continuous signaling or, in the case
`of digital CAS, at regular intervals even without new signals.
`
`In addition, there is contention between voice and signaling with in-band CAS . As a result,
`signaling is limited to call set-up and release phases only. This means that signaling cannot
`take place during the call connection phase, severely imposing technological limits on the
`system's complexity and usefulness.
`
`Common Channel Signaling (CCS)
`CCS refers to the situation in which the signaling capacity is provided in a common pool,
`with the capacity being used as and when necessary. The signaling channel can usually
`carry signaling information for thousands of traffic circuits.
`
`In North America, signaling can be placed on its own Tl carrier even though it only takes
`up one timeslot. This means that two physical networks, "speech" and "signaling;' can have
`different routings. (Please refer to Chapter 5 for a description of carriers and timeslots.)
`Alternatively, the signaling might exist on a carrier with other user traffic, depending on the
`network operator.
`
`Outside of North America, the signaling is placed in its own timeslot on an El (that is,
`logically rather than physically separated) . The other timeslots on El are for user traffic(cid:173)
`apart from TSO, which is used for synchronization. El systems tend to use the TS16 timeslot
`for signaling; some core network equipment ignores TS16, expecting it to be used for
`signaling traffic because it has historically been the timeslot for digital CAS signaling.
`
`The only CCS systems that have been implemented to date are Signaling Systems No. 6 and
`No. 7 (SS6 and SS7) . The ITU for the international network originally standardized SS6,
`but they saw limited deployment. AT&T nationalized SS6 for the North American network
`and called it Common Channel Interoffice Signaling (CCIS) No. 6. SS6 saw a limited
`deployment after the mid-1970s because it had far less bandwidth and a much smaller
`packet size than SS7 . In addition, its evolutionary potential was severely limited because it
`was not a layered protocol architecture.
`
`CCS systems are packet-based, transferring over 200 bytes in a single SS7 packet, as opposed
`to a few bits allocated to act as indicators in digital CAS . The signaling information is trans(cid:173)
`ferred by means of messages , which is a block of information that is divided into fields that
`define a certain parameter or further sub-field. The signaling system's specifications (Recom(cid:173)
`mendations and Standards) define the structure of a message, including its fields and
`parameters.
`
`Because CCS is packet-based and there is not a rigid tie between the signaling and the
`circuits it controls, it can operate in two distinct ways. These two distinct ways are circuit(cid:173)
`related signaling and non-circuit-related signaling.
`
`
`
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`

`22 Chapter 1: The Evolution of Signaling
`
`Circuit-Related Signaling
`Circuit-related signaling refers to the original functionality of signaling, which is to estab(cid:173)
`lish, supervise, and release trunks. In other words, it is used to set up, manage, and clear
`down basic telephone service calls. Circuit-related signaling remains the most common
`mode of signaling. As it is with CAS, signaling capacity is not pre-allocated for each traffic
`circuit. Rather, it is allocated as it is required. Each signaling message is related to a traffic
`circuit. Because no dedicated relationship exists between the circuits and the signaling, it is
`necessary to identify the traffic circuit to which a particular signal message refers. This
`is achieved by including a circuit reference field in each signaling message.
`
`Non-Circuit-Related Signaling
`Non-circuit-related signaling refers to signaling that is not related to the establishment,
`supervision, and release of trunks. Due to the advent of supplementary services and the
`need for database communication in cellular networks and Intelligent Networks, for
`example, signaling is no longer exclusively for simply setting up, managing, and clearing
`down traffic circuits. Non-circuit-related signaling allows the transfer of information that is
`not related to a particular circuit, typically for the purpose of transmitting both the query
`and response to and from telecommunication databases. Non-circuit-related signaling
`provides a means for transferring data freely between network entities without the
`constraint of being related to the control of traffic circuits.
`
`Common Channel Signaling Modes
`A signaling mode refers to the relationship between the traffic and the signaling path.
`Because CCS does not employ a fixed, deterministic relationship between the traffic
`circuits and the signaling, there is a great deal of scope for the two to have differing
`relationships to each other. These differing relationships are known as signaling modes.
`
`There are three types of CCS signaling modes:
`
`• Associated
`• Quasi-associated
`• Non-associated
`SS7 runs in associated or quasi-associated mode, but not in non-associated mode. Associ(cid:173)
`ated and quasi-associated signaling modes ensure sequential delivery, while non-associated
`does not. SS7 does not run in non-associated mode because it does not have procedures for
`reordering out-of-sequence messages.
`
`Associated Signaling
`In associated mode, both the signaling and the corresponding user traffic take the same
`route through the network. Networks that employ only associated mode are easier to design
`
`
`
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`

`

`Common Channel Signaling (CCS)
`
`23
`
`and maintain; however, they are less economic, except in small-sized networks.Associated
`mode requires every network switch to have signaling links to every other interconnected
`switch (this is known as a fully meshed network design). Usually a minimum of two
`signaling links are employed for redundancy, even though the switched traffic between two
`interconnected switches might not justify such expensive provisioning. Associated signaling
`mode is the common means of implementation outside of North America. Figure 1-4
`illustrates the associated concept.
`
`Figure 1-4 Associated Mode
`
`Signaling
`
`Signaling
`
`Quasi-Associated Signaling
`In quasi-associated mode, signaling follows a different route than the switched traffic to
`which it refers, requiring the signaling to traverse at least one intermediate node. Quasi(cid:173)
`associated networks tend to make better use of the signaling links; however, it also tends to
`create a more complex network in which failures have more potential to be catastrophic.
`
`Quasi-associated signaling can be the most economical way of signaling for lightly loaded
`routes because it avoids the need for direct links. The signaling is routed through one or
`more intermediate nodes. Signaling packets arrive in sequence using quasi-associated
`signaling because the path is fixed for a given call ( or database transaction) at the start of a
`call (or transaction). Figure 1-5 shows the quasi-associated signaling mode, which is the
`common means of implementation within North America.
`
`Figure 1-5 Quasi-Associated Mode
`
`Voice
`Trunks
`
`Signaling
`i . - - - - - - - - - - .1 Transfer
`Point
`
`Signaling
`- - - - - - - Transfer
`Point
`
`
`
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`

`24 Chapter 1: The Evolution of Signaling
`
`Non-Associated Signaling
`Because the path is not fixed at a given point in time in non-associated mode, the signaling
`has many possible routes through the network for a given call or transaction. Therefore, the
`packets might arrive out of sequence because different routes might have been traversed.
`
`SS7 does not run in non-associated mode because no procedures exist for reordering out(cid:173)
`of-sequence messages. Associated and quasi-associated signaling modes assure sequential
`delivery, while non-associated signaling does not. Quasi-associated mode is a limited case
`of non-associated mode, in which the relative path is fixed .
`
`Summary
`CCS has evolved to address the limitations of the CAS signaling method. CCS has the
`following advantages over CAS:
`
`• Much faster call set-up time
`• Greater flexibility
`• Capacity to evolve
`• More cost effective than CAS
`• Greater call control
`Most CCS calls can be set up in half the time it takes to set up CAS calls. CCS achieves
`greater call control because no contention exists between signaling and user traffic as it
`does with in-band CAS. Because the subscriber cannot generate particular signals intended
`for inter-switch ( core network) signaling, CCS offers a greater degree of protection against
`fraud than analog CAS methods .
`
`CCS has the following disadvantages in comparison to CAS:
`
`• CCS links can be a single point of failure- a single link can control thousands of
`voice circuits, so if a link fails and no alternative routes are found, thousands of calls
`could be lost.
`• There is no inherent testing of speech path by call set-up signaling, so elaborate
`Continuity Test procedures are required.
`
`
`
`Page 54 of 156
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`

`

`CHAPTER 3
`
`The Role of SS7
`
`The purpose of this chapter is to introduce Signaling System No. 7 (SS7 /C7) and give the
`reader an indication of how it affects the lives of nearly two billion people globally. The
`chapter begins by providing a brief introduction to the major services that SS7 /C7 provides
`and explains how the protocol has been and will continue to be a key enabler of new tele(cid:173)
`communication services. It concludes with an explanation of why SS7 /C7 is a cornerstone
`of convergence.
`
`SS7 /C7 is the protocol suite that is employed globally, across telecommunications net(cid:173)
`works, to provide signaling; it is also a private, "behind the scenes;' packet-switched network,
`as well as a service platform. Being a signaling protocol, it provides the mechanisms to
`allow the telecommunication network elements to exchange control information.
`
`AT&T developed SS7 /C7 in 1975, and the International Telegraph and Telephone Consul(cid:173)
`tative Committee (CCITT) [109] adopted it in 1980 as a worldwide standard. For more
`information on the standards bodies, see Chapter 2, "Standards." Over the past quarter of a
`century, SS7 has undergone a number of revisions and has been continually enhanced to
`support services that are taken for granted on a daily basis.
`
`SS7 /C7 is the key enabler of the public switched telephone network (PSTN), the integrated
`services digital network (ISDN), intelligent networks (INs), and public land mobile
`networks (PLMN s) .
`
`Each time you place and release a telephone call that extends beyond the local exchange,
`SS7 /C7 signaling takes place to set up and reserve the dedicated network resources (trunk)
`for the call. At the end of the call, SS7 /C7 takes action to return the resources to the
`network for future allocation.
`
`TIP
`
`Calls placed between subscribers who are connected to the same switch do not require the
`use of SS7 /C7. These are known as intraoffice, intraexchange, or line-to-line calls.
`
`Each time a cellular phone is powered up, SS7 /C7-based transactions identify, authenticate,
`and register the subscriber. Before a cellular call can be made, further transactions check
`
`
`
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`

`44 Chapter 3: The Role of S87
`
`that the cellular phone is not stolen (network dependent option) and qualify permission to
`place the call (for example, the subscriber may be barred from International usage) . In addi(cid:173)
`tion, the SS7 /C7 network tracks the cellular subscriber to allow call delivery, as well as to
`allow a call that is already in progress to remain connected, even when the subscriber is
`mobile.
`
`Although the average person typically uses SS7 /C7 several times a day, it is largely unheard
`of by the general public because it is a "behind the scenes" private network - in stark contrast
`to IP. Another reason for its great transparency is its extreme reliability and resilience . For
`example, SS7/C7 equipment must make carrier grade quality standards-that is, 99.999
`percent availability. The three prime ways it achieves an industry renowned robustness is
`by having a protocol that ensures reliable message delivery, self-healing capabilities, and
`an over-engineered physical network.
`
`Typically, the links that comprise the network operate with a 20-40 percent loading and
`have full redundancy of network elements. SS7/C7 might well be the most robust and reli(cid:173)
`able network in existence.
`
`SS7 /C7 is possibly the most important element from a quality of service (QoS) perspective,
`as perceived by the subscriber.
`
`NOTE
`
`Here QoS refers to the quality of services as perceived by the subscriber. It should not be
`confused with QoS as it relates specifically to packet networks.
`
`QoS is quickly becoming a key in differentiating between service providers. Customers are
`changing service providers at an increasing pace for QoS reasons, such as poor coverage,
`delays, dropped calls, incorrect billing, and other service-related impairments and faults.
`SS7 /C7 impairments nearly always impact a subscriber's QoS directly. A complete loss of
`signaling means a complete network outage, be it a cellular or fixed-line network. Even a
`wrongly-provisioned screening rule at a SS7 /C7 node in a cellular network can prohibit
`subscribers from roaming internationally or sending text messages. A loss of one signaling
`link could potentially bring down thousands of calls. For this reason, the SS7/C7 network
`has been designed to be extremely robust and resilient.
`
`Impact of SS7 Network Failure
`The critical nature of the SS7 network and the potential impact of failures was demonstrated
`in January 1990 when a failure in the SS7 software of an AT&T switching node rippled
`through over 100 switching nodes. The failure caused a nine-hour outage, affecting an estimated
`60 ,000 people and costing in excess of 60 million dollars in lost revenue as estimated
`by AT&T.
`
`
`
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`

`Signaling System No. 7-Based Services
`
`45
`
`Signaling System No. 7-Based Services
`In addition to setting up and releasing calls, SS7 /C7 is the workhorse behind a number of
`telecommunication services, including:
`
`• Telephone-marketing numbers such as toll-free and freephone
`• Televoting (mass calling)
`
`• Single Directory Number
`• Enhanced 911 (E9 l 1 )-used in the United States
`• Supplementary services
`• Custom local area signaling services (CLASS)
`
`• Calling name (CNAM)
`• Line information database (LIDB)
`• Local number portability (LNP)
`
`• Cellular network mobility management and roaming
`- Short Message Service (SMS)
`- Enhanced Messaging Service (EMS)-Ringtone, logo, and cellular game
`delivery
`• Local exchange carrier (LEC) provisioned private virtual networks (PVN s)
`• Do-not-call enforcement
`The following sections describe these telecommunications services.
`
`Telephone-Marketing Numbers
`The most commonly used telephone-marketing numbers are toll-free calling numbers (800
`calling), known asfreephone (0800) in the United Kingdom. Because the call is free for the
`caller, these numbers can be used to win more business by increasing customer response.
`Telephone-marketing numbers also provide premium rate lines in which the subscriber is
`charged at a premium in exchange for desired content. Examples of such services include
`adult services and accurate road reports.
`
`Another popular telephone-marketing number is local call, with which a call is charged as a
`local call even though the distance might be national. In recent years in the United Kingdom,
`marketing numbers that scarcely alter the call cost have been a popular means of masking
`geographical location. These numbers allow for a separation between the actual number
`and the advertised number.
`
`
`
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`46 Chapter 3: The Role of S87
`
`Televoting
`Televoting is a mass calling service that provides an easy method of surveying the public
`on any imaginable subject. The host (for example, a deejay at a radio station) presents
`specific questions and the caller uses a telephone keypad to select a choice; the caller's
`action adds to the vote for that particular choice. The conversation phase is usually limited
`to a simple, automated "thank you for ... " phrase. Televoting can also be used in many other
`areas, such as responding to fundraising pleas and telephone-based competitions. A single
`night of televoting might result in 15 million calls [ 110]. Televoting services represent some
`of the most demanding-as well as lucrative-call scenarios in today's telephone networks.
`Revenue generation in this area is likely to grow as customers shift more toward an "inter(cid:173)
`active" experience, on par with convergence.
`
`Single Directory Number
`Another service that uses SS7 /C7 and has been deployed in recent years is the single direc(cid:173)
`tory number, which allows a company with multiple offices or store locations to have a
`single directory number. After analyzing the calling party's number, the switch directs the
`call to a local branch or store.
`
`Enhanced 911
`E911, which is being deployed across some states in the United States, utilizes SS7 to transmit
`the number of the calling party, look up the corresponding address of the subscriber in a
`database, and transmit the information to the emergency dispatch operator to enable a faster
`response to emergencies. E911 might also provide other significant location information,
`such as the location of the nearest fire hydrant, and potentially the caller's key medical details.
`The Federal Communications Commission (FCC) also has a cellular 911 program in progress;
`in addition to providing the caller's telephone number, this program sends the geographical
`location of the antenna to which the caller is connected. Enhancement proposals are already
`underway to obtain more precise location information.
`
`Supplementary Services
`Supplementary services provide the subscribers with more than plain old telephony service
`(POTS), without requiring them to change their telephone handsets or access technology.
`Well-known supplementary services include three-way calling, calling number display
`( CND), call-waiting, and call forwarding. Note that the exact names of these services might
`differ, depending on the country and the operator.
`
`
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`Signaling System No. 7-Based Services
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`47
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`Recently, supplementary services have been helpful in increasing operators' revenues since
`revenues against call minutes have been on the decline. Usually the subscriber must pay a
`fixed monthly or quarterly fee for a supplementary service.
`
`Custom Local Area Signaling Services (CLASS)
`Custom local area signaling services (CLASS) are an extension of supplementary services
`that employ the use of SS7 signaling between exchanges within a local geographical area.
`Information provided over SS7 links, such as the calling party number or the state of a
`subscriber line, enable more advanced services to be offered by service providers. A few
`examples of CLASS services include:
`
`• Call block-Stops pre-specified calling party numbers from calling.
`• Distinctive ringing- Provides a distinct ringing signal when an incoming call
`originates from a number on a predefined list. This feature is particularly beneficial
`to households with teenagers.
`• Priority ringing- Provides a distinct ring when a call originates from a pre-specified
`numbers. If the called subscriber is busy and has call waiting, the subscriber receives
`a special tone indicating that a number on the priority list is calling.
`• Call completion to busy subscriber (CCBS)-If a subscriber who has CCBS calls
`a party who is engaged in another call, the subscriber can activate CCBS with a single
`key or sequence. When activated, CCBS causes the calling party's phone to ring when
`the called party becomes available; when the calling party answers, the called party's
`phone automatically rings again. This feature saves the calling party from
`continuously attempting to place a call to a party is still unavailable.
`Note that the exact names of these services might differ, depending on the country and the
`operator. In addition, the term "CLASS" is not used outside of North America.
`
`Calling Name (CNAM)
`Calling name (CNAM) is an increasingly popular database-driven service that is only
`available in the United States at this time. With this service, the called party receives the
`name of the person calling in addition to their number. The c