EXAMPLE EXISTING SERVICES AND HOW THEY WORK

Arguably the most common service offered from an IN in the fixed network is basic number translation services. In fact it could be said that it was these services that created the desire for INs. The earliest launches of IN service in the US in the late 1980s, were by AT&T. AT&T used a centralised database connected to the telephone switches via an SS#7 network, which allowed the switches to request translations for 800 service numbers.

So how does such a service operate? Remember back to Chapter 1 on how the public switched telephone network is structured into a hierarchy of local, transit and international exchanges (switching layers). These layers are structured around the E.164 numbering plan with numbers having local, national and international significance. So when a new numbering range like the free phone services of 800 and local charge rate numbers such as 0345 (in the UK) are dialled, what is a local exchange supposed to do, they have no national or international significance with respect to the E.164 numbering plan.

The answer is that the routing tables in the circuit switches are config- ured to route the call to an additional layer in the network, the SSP layer. In some implementations this layer is combined with the trunk/transit layer and in others the individual local exchanges have been converted to SSP capable switches.

By way of an example, let us consider a layer of SSPs above the local exchanges combining the functionality of transit exchanges and SSPs. In this case the local exchange signals the called number up to the transit layer (via SS#7 Integrated Services User Part – ISUP), the combined tran- sit/SSP has a trigger point set for interruption of the basic call routing software (BCSM). This causes the SSP to initiate a ‘dialogue’ with the SCP by sending an INAP initial-DP (Detection Point) message containing (amongst other items) the calling party’s number, the called number (the 800 or 0345 number), a service key (to identify the service logic program to execute) and the event type in the BCSM that triggered the

INTELLIGENT NETWORK SERVICES 124

request. The call routing engine in the SSP then suspends the call proces- sing, waiting for a response from the SCP.

The SCP, using the service key, executes the SLP associated with that key. The most common number translation SLP applications are:

† time of day routing, based on the time of day a call can be routed to a number of destinations, for example a call centre application might want all calls after 10 p.m. to terminate on a recorded announcement;

† geographic origin, the call can be connected to a number of different numbers based on the E.164 national prefix of the originating caller’s number;

† proportional distribution of calls to different destinations, again for a customer with multiple call centres the calls can be evenly distributed amongst the different bureaux; multiple choice routing, for example for a find-me follow-me service the call may be directed to different numbers and based on the response (busy say) the call can be redir- ected until an answer is received or the caller can be diverted to voicemail;

† combinations of the above!

Once the service logic has determined the destination of the call, in the number translation scenario the SLP responds to the initial-DP message in one of two ways: Establish a Temporary Connection (ETC) or a simple CONnect (CON) procedure. Both ETC and CON contain the destination number to be connected to. The difference between the two methods of connection is that an ETC is still under the control of the SLP and is usually used to temporarily connect a caller to a recorded announcement or automated announcement service running on an Interactive Voice Response (IVR, called a Voice Response Unit (VRU) in the US) platform in the form of an intelligent peripheral or service node whilst the CONnect method completes the connection without further action.

Additional information can be requested from the SSP in the form of reports about for example charging. When the call completes, the SSP will send the SCP the report requested and the SLP invoked will return to an idle state.

What other services can make use of this kind of control, a whole host of them:

† Tele-voting, e.g. when a game show asks viewers to call in to register a vote for a contestant. The IN can automatically count calls to a specific number and the results can be obtained in real time.

† Virtual private networks. In a multinational company the connection of its private telephone network via the public telephone network can reduce the cost of routing calls by not having to lease expensive circuits between sites. IN can be used to add routing plans that adapt to time of day tariff changes for example.

† Wide area Centrex. Instead of a company connecting a private network together, the telephone company can create a network for them, including unique numbering plans. All of the intelligence and configuration is done on the telephone company’s switches and IN systems.

† Calling card services. Most telecoms network operators around the world (and a number of partner companies) offer phone card services that allow people to charge calls to a separate account irrespective of where the call is made from. This normally involves calling a free number, which connects the caller first to an IVR. The customer then keys in account and personal identification codes, followed by the number they wish to reach. The IVR system then connects the call. All of this is generally performed using an IN system.

† Mobile networks have also made extensive use of IN for pre-paid mobile services. Pre-paid phones are validated by a service running on an SCP. If the caller has credit, then calls are allowed. Rules are set to automatically divert calls to an automated service when a particu- lar threshold is reached on their credit.

In order for the SLP to perform its role in these services it utilises the services of a database in the form of the Service Data Point (SDP). The SDP contains the customer specific data that allows a specific SLP to be executed for many different customer instances. In real-world implemen- tations the SDP is sometimes embedded in the SCP, rather than being a separate physical entity.