Telecommunications Switching Systems 6

7/29/2019 Telecommunications Switching Systems 6

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Signaling

Lecture 6

Signaling

SIGNALING can be defined as theexchange of information specificallyconcerned with the establishment andcontrol of connections and the transfer ofuser-to-user and management informationin a telecommunication network.


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SIGNALING is broken down in three

functional areas:

Supervisory

Address

Call progress: audible-visual

Supervisory Signaling

Supervisory signaling provides informationon line or circuit condition.

On- & Off-hook status of subscriberimplies to start & stop billing,establishment and release of voicechannel, status of line busy or idle


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Address Signaling

Address signaling directs and routes atelephone call to the called subscriber.

It originates as dialed digits or activated push-

buttons from a calling subscriber.

The local switch accepts these digits and, by

using the information contained in the digits,

directs the call to the called subscriber.

Address signaling between switches is calledinterregister signaling.

Call Progress: Audible-Visual

This type of signaling is categorized inforward direction and backward direction.

In the forward direction, it is utilized to alertthe called party that there is a telephonecall waiting.

This is often done by ringing a telephonesbell. A buzzer, chime, or light may also beused for alerting.


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Call Progress: Audible-Visual

The backward direction includes audible tones or voiceannouncements that will inform the calling subscriber thefollowing:

1. Ringback. This tells the calling subscriber that the distanttelephone is ringing.

2. Busyback. This tells the calling subscriber that the calledline is busy.

3. ATBAll Trunks Busy. There is congestion on therouting. Sometimes a recorded voice announcement isused here.

4. Loud Warble on Telephone InstrumentTimeout. This

occurs when a telephone instrument has been left off-hook unintentionally.

Another signaling breakdown is

Subscriber signaling

Inter-switch (inter-register) signaling

Intra-switch (register) signaling


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Signaling information can be transmitted bymeans such as

Duration of pulses (pulse duration bears aspecific meaning)

Combination of pulses

Frequency of signal

Combination of frequencies

Presence or absence of a signal

Binary code

For dc systems, the direction and/or level oftransmitted current

Pulse Dialing


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Signaling Tones


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Signaling

Signaling

In-channel Common Channel

Associated

Non-associated

DC

Low frequency

Voice frequency

PCM

Inband Outband


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Signaling


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Channel Associated Signaling

Common Channel Signaling


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Inband VF signaling

In-band signaling refers to signaling systemsusing an audio tone, or tones, inside the

conventional voice channel to convey signaling

information.

Single Frequency (SF) 2600 Hz

Two-Frequency (2VF) 2400 & 2600 Hz

These signaling systems used one or two tones

in the 2000- to 3000-Hz portion of the band,where less speech energy is concentrated.

SF signaling (inband)


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Drawback of Inband signaling

A major problem with in-band signaling is thepossibility of talk-down, which refers to the

premature activation or deactivation of

supervisory equipment by an inadvertent

sequence of voice tones through the normal use

of the channel.

Such tones could simulate the SF tone, forcing a

channel dropout (i.e., the supervisory equipment

would return the channel to the idle state).

Chances of simulating a 2VF tone set aremuch less likely.

It becomes apparent why someadministrations and telephone companieshave turned to the use of 2VF supervision,or out-of-band signaling.


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Outband Signaling

The major advantage of out-of-band signaling isthat continuous supervision is provided, whethertone on or tone off, during the entire telephoneconversation or may be used when idle.

Talk-down cannot occur because all supervisoryinformation is passed out of band, away from thespeech-information portion of the channel.

The preferred CCITT out-of-band frequency is3825 Hz, whereas 3700 Hz is commonly used inthe United States.

SF signaling (outband)


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Compared to in-band signaling, out-of-bandsignaling provides:

1.Faster call setup times (compared to in-bandsignaling using multi-frequency (MF) signaling

tones)

2.More efficient use of voice circuits

3.Support for Intelligent Network (IN) services,

which require signaling to network elements

without voice trunks (e.g., database systems)

4. Improved control over fraudulent network usage

Drawback(outband sign.)

In the long run, the signaling equipmentrequired may indeed make out-of-bandsignaling even more costly because of theextra supervisory signaling equipment andsignaling lead extensions required at eachend, and at each time that the carrier

(FDM) equipment demodulates to voice.


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International Standard

CCITT #7 or ITU-T 7

CCS standard CCS7

SS7 Signaling System #7

Quasi-associated signaling

This type of signaling employs a minimalnumber of nodes, thus minimizing delays.Quasi-associated signaling is the preferredsignaling mode for SS7.


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Common Channel Signaling SystemNo. 7 (CCS7 or SS7) is a global standardfor telecommunications defined by ITU-T.

The standard defines the procedures andprotocol by which network elements in thepublic switched telephone network (PSTN)exchange information over a digitalsignaling network to effect wireless

(cellular) and wireline call setup, routingand control.

The SS7 network and protocol are used for:

1. Basic call setup, management and teardown

2. Local number portability (LNP)

3. Toll-free and toll wireline services

4. Enhanced call features such as callforwarding, calling party name/number

display and three-way calling5. Efficient and secure worldwide

telecommunications


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SS7 messages are exchanged betweennetwork elements over 56 or 64 kilobit persecond (kbps) bidirectional channelscalled signaling links.

Signaling occurs out-of-band ondedicated channels rather than in-bandon voice channels.

Signaling Points

Each signaling point in the SS7 network isuniquely identified by a numeric point code.

Point codes are carried in signaling messages

exchanged between signaling points to identify

the source and destination of each message.

Each signaling point uses a routing table to

select the appropriate signaling path for eachmessage.


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There are three kinds of signaling pointsin the SS7 network:

1. SSP (Service Switching Point)

2. STP (Signal Transfer Point)

3. SCP (Service Control Point)

SS7 architecture


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SSP

SSPs are the telephone switches (endoffices and tandems) equipped with SS7-capable software and terminating signalinglinks.

SSPs are switches that originate,terminate or tandem calls.

An SSP sends signaling messages to

other SSPs to setup, manage and releasevoice circuits required to complete a call.

STP

STPs are the packet switches of the SS7network. They receive and route incomingsignaling messages toward the properdestination. They also perform specializedrouting functions.

An STP can also act as a "firewall" toscreen SS7 messages exchanged withother networks.


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SCP

SCPs are the databases that provideinformation necessary for advanced callprocessing capabilities.

SCPs and STPs are usually deployed in matedpair configurations in separate physical locationsto ensure network-wide service in the event ofan isolated failure.

Links between signaling points are alsoprovisioned in pairs. Traffic is shared across alllinks in the linkset. If one of the links fails, thesignaling traffic is rerouted over another link inthe linkset.

The SS7 protocol provides both error correction

and retransmission capabilities to allowcontinued service in the event of signaling pointor link failures.


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SS7 Signaling Link Types

A An "A" (access) link connects a signaling end point (e.g.,

an SCP or SSP) to an STP. Only messages originatingfrom or destined to the signaling end point aretransmitted on an "A" link.


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B

A "B" (bridge) link connects one STP to another.Typically, a quad (set of four links) of "B" linksinterconnect peer (or primary) STPs (e.g., the STPs fromone network to the STPs of another network).

C A "C" (cross) link connects STPs performing identical functions intoa mated pair.

A "C" link is used only when an STP has no other route available toa destination signaling point due to link failure(s).

Note that SCPs may also be deployed in pairs to improve reliability;unlike STPs however, mated SCPs are not interconnected bysignaling links.


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D A "D" (diagonal) link connects a secondary (e.g., local or

regional) STP pair to a primary (e.g., inter-networkgateway) STP pair in a quad-link configuration.

The distinction between a "B" link and a "D" link is ratherarbitrary. For this reason, such links may be referred toas "B/D" links.

E An "E" (extended) link connects an SSP to an alternate

STP. "E" links provide an alternate signaling path if an SSPs

"home" STP cannot be reached via an "A" link. "E" links are not usually provisioned unless the benefit

of a marginally higher degree of reliability justifies theadded expense.


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F An "F" (fully associated) link connects two

signaling end points (i.e., SSPs and SCPs). "F links are not usually used in networks with

STPs.

In networks without STPs, "F" links directlyconnect signaling points.

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Telecommunications Switching Systems 6