Switch
ISDN Digital Subscriber Line
PRA
TY2600/v4.1
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Signalling
Signalling is used within a network to set up, maintain and terminate a call.
Specific signals (pulses or audio tones) are used to indicate to the switches the required connection or service, although some of these signals are for interpretation by the subscriber.
An example follows and is illustrated in the diagram.
To initiate a call, a telephone subscriber lifts the handset off its rest (off-hook). This tells the local exchange to be ready to receive the number of the called subscriber. As soon as the exchange signals the dial tone back to the calling subscriber, they dial the wanted number. On older exchanges, this information is passed via a rotary dial by a series of makes-and-breaks of the subscriber’s connection, interrupting current flow, i.e. pulses. On more modern exchanges, voice-frequency (audio) tones are usually sent to the exchange as push buttons are pressed. These tones are usually called DTMF or Dual Tone Multiple Frequency. In due course the subscriber receives advice from the exchange about the status of the call, either by a ringing signal, an engaged or busy tone signal, an equipment-busy signal, or some other specialized tone.
Phone
Ready?
Off-Hook
Switch
Dial Tone
Dial Number Pulses or Tones
Call Subscriber (Ringing) Ringing Signal
Connected, or
not Connected Engaged or Busy Tone
Conversation
TY2600/v4.1 © Wray Castle Limited 2.31 Channel Associated Signalling (CAS)
Signalling systems may be categorized as being either Channel Associated Signalling (CAS) or Common Channel Signalling (CCS).
With CAS, every traffic channel has a dedicated signalling channel. Because of this, CAS tends to be inflexible and relatively slow. A CAS system is limited by the number of signalling states, depending on the number of Direct Current (DC) states, Alternating Current (AC) tones, or bits allocated. This slows the system down and therefore limits the variety of information that can be transferred.
CAS can be analogue or digital. With the analogue system it is possible to choose between DC and AC.
DC systems rely on the voltage and current around the signalling circuit, whereas AC systems rely on tones.
In general, CAS systems are inefficient in that they require live signalling to take place, either continuously or, in the case of digital CAS, at regular intervals. This means that signalling occurs even when there is no new information.
Types of CAS are:
DC – Loop Disconnect; E&M
AC – Switching System AC15 (SSAC15); R2 Multi-Frequency Compelled (R2MFC) Channel Associated Signalling (CAS)
Exchange PBX
A B C
A1 B1 C1 Each traffic channel has its
own signalling channel e.g. loop disconnect
Traffic Signalling e.g. loop disconnect e.g. R2MFC
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Common Channel Signalling (CCS)
CCS is an improvement on the CAS system due to its greater flexibility in signalling and its ability to evolve.
CCS systems are based on a secure data circuit that links together the call control processors in connected switches. The common channel carries data messages that convey signalling for the circuits between these switches, and may additionally carry network supervisory and management data. CCS only requires one signalling channel for many traffic channels, since it only signals when required (unlike CAS, which signals even if nothing is happening). The ratio of 1000:1 (plus 1 backup) is common, but within GSM networks the ratio is more like 200:1 (plus 1 backup). CCS is faster and more flexible and it allows more services.
The CCS channel is usually implemented as a 64 kbit/s circuit on an E1 or T1 bearer.
In CCS systems, the signals are generated only to convey new information and are, therefore, more efficient than CAS systems.
Examples of CCS include SS7 (National and International), Digital Access Signalling System No. 2 (DASS2) (UK specific) and Digital Subscriber Signalling System No. 1 (DSS1) (ITU-T).
SPC ST
– Stored Program Control – Signalling Terminal
Common signalling channel for all traffic channels typically implemented as a 64 kbit/s circuit
on an E1 bearer
TY2600/v4.1 © Wray Castle Limited 2.33 Access/Network Signalling
This figure illustrates how signalling within a network can be subdivided into systems providing customer access and systems for use within the networks themselves.
Access signalling is a signalling system that is extended out to the customer and works in conjunction with a variety of recognizable tones to keep the customer informed about the progress of the call. In general, it does not need to be as sophisticated as the network signalling system, but the greater the level of sophistication, the more flexible and complex the services offered can be.
The network signalling system is used solely within the network. It is not extended to the customer because it is too sophisticated for this purpose, and the network operators are understandably reluctant to extend the means to access network functions out to the customer.
In addition to access and network signalling, it is possible to provide the means for end users to communicate with each other using a specified signalling system.
Typically this signalling is used between the PBXs and is referred to as private network signalling.
Network
* Supervisory tones are provided to keep the customer informed about the progress of the call e.g. DPNSS, Q.sig
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Basic Call Process and the ISDN User Part (ISUP)
ISDN networks support bearer circuits suitable for transporting both voice and non-voice traffic.
Basic call set-up involves call control signalling, where adjacent exchanges in a traffic route exchange information to set up, supervise or clear a traffic circuit. Call control signalling is provided by a call control protocol such as the ISDN User Part (ISUP).
SS7 uses signalling messages between exchanges in the network. As SS7 is a CCS system, the messages must identify exactly the PCM bearer and timeslot of the traffic circuit with which the message is associated. The Message Transfer Part (MTP) is responsible for the reliable, unduplicated and in-sequence transport of signalling messages between nodes in the network.
The example in the figure illustrates a call between two fixed-line phones. After picking up the phone and receiving the dial tone the customer enters the telephone number of the called phone. The originating exchange and intermediate exchange send Initial Address Messages (lAMs) during the first stages of a call to establish the traffic route through the switched network. The originating exchange allocates circuit 10 to the call and the intermediate node allocates circuit 20. When the call arrives the destination exchange triggers the appropriate access signalling to the called party.
The lAM must also carry enough information to facilitate onward routing, including the called party number and call type. Additionally the lAM may include data such as the calling address digits.
In addition to the establishment of 64 kbit/s bearers within a switched network, ISUP supports a range of teleservices including telephony and fax and a wide range of supplementary services defined for the ISDN, such as Calling Line Id (CLI), Call Forwarding (CF) and Call Barring (CB).
ISUP can be used in mixed digital/analogue network environments.
ISUP supports:
Bearer Services – 64 kbit/s: 3.1 kHz Audio Teleservices – Telephony, Fax
Supplementary Services – CLI, CF, CB
Intermediate
ISUP – ISDN User Part
PCM – Primary Channel Multiplex TS – Timeslot
IAM – Initial Address Message CIC – Circuit Identity Code Originating
TY2600/v4.1 © Wray Castle Limited 2.35
End-to-End Call Setup – Basic Procedure
The diagram illustrates a call being set up between two switches that terminate the calling and called parties. The two parties are assumed to be ISDN-connected, but could be connected to the network by analogue technology.
Overall, the call set-up procedure for both speech and non-speech connections is the same. En-bloc addressing (all address digits contained in a single message) and overlap addressing (address digits are transmitted over a number of messages) are supported.
In the call set-up procedure, routing analysis may take place at the originating exchange or may be requested from a remote database. The route chosen will depend on the connection type, and signalling method required, and the called party number.
If the subscriber has digital access, the Set-up message may contain Bearer Capability Information (BCI). The local exchange uses the BCI to determine the correct connection type and signalling requirements.
BCI can be transferred in the ‘User Service Information’ parameter of the Initial Address Message (IAM) for data calls.Connection types are:
Connection types are:
speech
3.1 kHz audio
alternative speech/64 kbit/s unrestricted
alternative 64 kbit/s unrestricted/speech Signalling Capabilities are:
ISDN-UP Preferred
ISDN-UP Required
ISDN-UP Not Required (any signalling system)
TY2600/v4.1
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Call Clear Down Diagram notes:
* Either party clears
Disconnect – traffic channel is cleared but not yet available for reuse Release – traffic channel is cleared and available for reuse
Release Complete – on receiving release complete the call reference is released
User AISDN
Local
Switch A Transit
Switch P Local
Switch B ISDN User B DSS1
DSS1 ISUP ISUP
Call Phase
Disconnect* Release
Release Complete
Release
Release Complete
Release Disconnect*
Release
Release Complete
Release Complete
TY2600/v4.1 © Wray Castle Limited 2.37 Services for Revenue Generation
In order to attract more custom, fixed-network and mobile-network operators are adding new services to basic phone provision.
These may take the form of Supplementary Services (SSs), which are services that have been defined by a standards body, such as call hold and call forwarding; or Value Added Services (VASs), operator–
or network-specific features that are open to implementation, such as voicemail.
These service types are implemented by means of a software function in the network.
Traditionally, SSs were implemented into network nodes. However, with the requirement for more sophisticated service types other mechanisms are required. The modern approach is to implement them at the optimum point using any one or a combination of three methods: node based, terminal based or IN based. Examples of each approach are shown in the diagram.
Telecoms Network Additional
Features Supplementary
Services
Value-Added Services
Terminal Based
Node Based
Terminal Based
IN Based
e.g. last number redial
e.g. calling line ID e.g. prepay or
credit card calling
Software Function
TY2600/v4.1
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The IN Concept
INs provide a framework that allows for the cost-effective introduction, control and management of services. To achieve this, the IN concept aims to reduce implementation time and improve efficiency by separating service control from other network functions.
INs offer flexible network architectures, standard network interfaces and rapid service introduction.
An IN separates the switching and service functions within a network. This enables the rapid introduction of new services with minimum disruption to the existing network.
An IN is overlaid on the existing network. It is intended to be applicable to all telecommunication networks, both circuit-switched and packet-switched.
INs support a wide variety of services, including supplementary services, and utilize existing and future bearer services (e.g. as defined in the ISDN context).
Switching function Service function
Network
TY2600/v4.1 © Wray Castle Limited 2.39 IN Implementation Simplified
Any call requiring IN-based services is forwarded on to a Service Switching Point (SSP), where the required service can be controlled by separate service logic located in a centralized Service Control Point (SCP).
In effect, the intelligence has been separated from the switching. This centralized approach allows new services to be introduced without having to update every network node or reconfigure hardware. This, in turn, allows for more efficient implementation and reduced lead times.
The main elements of the IN architecture are described below.
The Service Control Point (SCP) performs the service control function. It contains the service logic to control all IN-based services.
The Service Switching Point (SSP) detects when an IN service is required and forwards the necessary information to the SCP for processing. In practice, SSPs are the exchanges within an IN that are connected to the SCP.
The SSP and SCP functions may be combined within one network node. This combined physical entity is known as a Service Switching and Control Point (SSCP).
Databases hold service parameters and subscriber data. They are accessed by the SCPs.
LE
TE (SSP)
TE (SSP)
TE (SSP) LE
LE
Service software
SCP
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An Example of an IN Service
An IN may use two types of signalling, circuit-related signalling and non-circuit-related signalling.
Circuit-related signalling is used to set up, maintain and clear down a telephony circuit, whereas non-circuit-related signalling is used between nodes to exchange call-related information.
This is shown in the call flow in the diagram above.
One of the first IN services to be implemented was the 0800 or Freephone service. This simply involves the translation of an 0800 number (which has no geographical significance) to another number representing a destination (with geographical significance).
1–2 Circuit-related signalling takes place between the calling party and the LE and then between the LE and the TE.
3 On detecting that 0800 has been dialled, the TE sends non-circuit-related signalling to the SCP, requesting a translation of the 0800 number.
4 The SCP performs the translation and then passes the new number back to the TE. Again, this is performed using non-circuit-related signalling.
5–6 The TE then routes to the LE, and the LE to the called party, in the normal manner using circuit-related signalling.
TY2600/v4.1 © Wray Castle Limited 2.41 IN-based Services
The implementation of INs has enabled the rapid introduction of new, sophisticated services. Some IN services are described below.
Freephone
The served user can be reached with a freephone number from all or part of the country (or internationally) and be charged for the call.
Virtual Private Network (VPN)
This permits a private network to be built using the resources of the public network. The subscriber’s lines may be connected onto different switches, but still allow call transfer, call hold, etc. (As an option, access may be allowed from any point on the network).
Credit Card Calling
This allows subscribers to place calls from any telephone or access point to any destination and have the costs automatically charged to the account specified. The caller dials the card number and a PIN followed by the called number.
Premium Rate
Part of a call cost is paid back to the called party (considered an added value service provider). The served user can be reached from all or part of the country or internationally via translation of a premium rate number.
Televoting
This enables subscribers to assist in surveys of public opinion by ringing a specific number according to their choice. Alternatively they may be asked to ring a unique number and, after prompting, to indicate their choice by use of voice or keypad.
Universal Personal Telecommunications (UPT)
Personal mobility is achieved by use of a unique Personal Telecommunications Number (PTN), which allows any type of service to be used across multiple networks at any network access. The PTN is translated to an appropriate destination number for routing.
Universal Access Number (UAN)
This service allows a subscriber with several terminating lines in any number of locations or zones to be reached via a unique directory number. The subscriber can specify which incoming calls are routed to which terminating lines based upon geographical area.
Freephone
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