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 //                         DDSN Intelligent Network                      //
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 //    A full rundown of our LinkLine (0800) and LoCall (0345) Services   //
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 //                    Presented in full By Keltic Phr0st                 //
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"...the most sophisticated network of its type ouside North America."

Steve Webster, BT ; DDSN Development Team


In 1983, British Telecom identified a major market potential for automatic freephone and premium rate services. An Analogue Network, with extended register translation and call charging facilities overlayed on the PSTN was proposed as an interim solution. The analogue derived services network, consisting of eight fully-interconnected switching nodes, was brought into limited public service in April 1985 and full public service in July 1985.

The LinkLine 0800 service permits calling customers to make calls free of charge while callers to LinkLine 0345 service numbers are charged at the local call rate irrespective of distance. The balance of the call charge is billed to the called customer known as the Service Provider (SP).

In keeping with its buisness modernisation programmes, British Telecom awarded a contract to AT&T for the supply and installation of a digital derived services network (DDSN), comprising 5ESS-PRX digital switches to be implemented in two distinct phases:

Phase 1, which was completed in 1988, involved the supply of eight digital units, utilising CCITT No. 7 common-channel signalling, as replacements for their analogue units (Figure 1). In addition, two new digital units were provided in London.

Phase 2 makes provision for an advanced freephone service using an intelligent network architecture.


In a traditional telecommunications network, call control 'intelligence' resides in the call processing software in its switching nodes. One disadvantage of this approach for some services is that customer-specific data has to be replicated in each node. As features become more sophisticated, then system complexity increases. In the DDSN Intelligent Network, specialised customer feature and routing information is held centrally in a network database which can be accessed by all switching nodes using dedicated datalinks and common-channel signalling (Figure 2). These signalling datalinks are used to pass requests for call handling information to the database and return instructions to the originating switching node.

An Intelligent network centralised call management fucntion allows an economical implementation of advanced features, simplifies administration of complex services and assures optimum use of network-wide, rather than switch-based, resources.


Three network elements are concerned with call processing for service providers with advanced features:

The network architecture is illustrated in Figure 3, and the role of each of the elements will become apparent as the call processing aspects are explained.

Action Control Point

The Action Control Points (ACPs) are the 5ESS-PRX Switching Nodes, which serve as transit and terminating nodes for DDSN traffic. All ACPs are fully interconnected by digital line systems and CCITT #7 (BT) common channel signalling. The CCITT #7 (BT) signalling links are used exclusively for setting up speech paths both within the DDSN and between the DDSN and the PSTN.

A Second totally independent common channel signalling network, utilising a proprietary form of #7 signalling (C7 North American), is used for transporting non-circuit related signalling methods between the ACPs and the Network Control Points (NCPs). This network is used only for advanced feature calls. Two of the ACPs have been nominated as a signal transfer and end point (STEP) and funnel the signalling traffic from the remaining ACPs to the NCPs. ACPs load share the C7NA signalling messages across both STEPs in the ACP-to-NCP direction, and the NCPs load share the signalling messages across both STEPs in the reverse direction.

Network Control Point

The Network Control Point (NCP) constitutes the core of the intelligent network and holds the data defining the treatment for specific advanced feature calls. NCPs are always provided in mated pairs.

Each NCP consists of a duplex processor, duplicated hard discs for data storage, tape drives and interfaces to the other network elements through a Local Area Network. This network, called the Common Network Interface, consists of the signalling terminals for the C7NA links from the STEP nodes and two peripheral controllers which communicate with the duplex processor. The common network interface ring (Figure 4) is automatically reconfigured under fault conditions to isolate the faulty section.

Advanced freephone call handling data is duplicated both within and and between each NCP in the mated pair. Call routing queries from the ACPs are balanced between the two NCPs by designating specific dialled codes to each NCP, and the decision on which NCP to query is taken at the ACP where the call entered the DDSN network. Although data is held on both NCPs, the secondary NCP is only accessed if the primary is not available. Under these conditions, the remaining NCP is capable of handling 100% of the load. This architecture virtually guarantees 100% service availability.

Automatic network management controls initiated by the NCP maintain the integrity of the intelligent network under overload conditions by sending code gapping messages instructing the ACPs to throttle back on the number of queries being forwarded to the NCP and defining the treatment for failed calls.

Network Services Complex

The Network services complex (NSC) provides the capability to give callers standard or customised interactive spoken information pertaining to the number called, such as, call prompting, courtesy response and call queing announcements. During or after a call prompting announcement the caller may communicate with the NSC by keying-in appropriate digits on an MF keyphone or keypad. The NSC can collect up to 15 digits which it forwards, via its host ACP, to the NCP via a C7NA common channel signalling link.

Initially, two NSCs loaded with the same announcements have been provided in the DDSN intelligent network and are co-located with the NCPs. Each NSC can handle 60 simultaneous calls and provide up to 2000 different announcements which are stored on triplicated moving head discs. In the even of an NSC failure, calls requiring these features are routed to the remaining NSC.

The NSC architecture is given in Figure 5.


The DDSN Intelligent Network will permit a range of new features to be offered as Advanced LinkLine to LinkLine service providers. These include (Advanced LinkLine feature name is in brackets) :


The true power of intelligent network call processing is not solely its list of advanced features, but combinations of the feature set which can be defined to meet a service provider's own unique telecommunications needs and, consequently, buisness requirements. An example of a simple call routing plan is shown in Figure 6. The data defining the call treatment(s) for a service provider are held in the NCP database in a service provider record.


Service Administration for Advanced LinkLine features is handled by the network subscriber transaction, administration and recording system (NETSTAR), which has on-line access to the NCPs. NETSTAR provides user friendly access to the NCP advanced feature database to modify, create or delete service provider call routing plans via dedicated or dialup/dialback links to VDUs. An NCP can have only one active call routing plan for any service provider number, but additional plans may be prepared and held in NETSTAR for transmission to, and activation at, the NCP when required. NETSTAR holds security backup copies of all call routing plans and NCP operating parameters.


Derivation of the Calling Subscriber Geography (CSG)

All 0800 and 0345 calls are routed via a DMSU to a DDSN action control point (ACP) (Figure 7). During Call set-up, the ACP requests additional set-up information to be sent via the C7BT Link. This cause the calling line identity (CLI) to be forwarded from the first exchange in the call path with C7BT signalling.

If a call is originated from a local exchange with C7BT signalling, a full calling line identity (FCLI) is returned to the ACP. The FCLI includes the caller's national number group (NNG) code, or all figure numbering (AFN) code in the case of a director area.

If the call is originated from an analogue local exchange (ALE), then a partial calling line identity (PCLI) is derived by the first digital exchange in the call path. This will normally be a DMSU, but in cases where an ALE is parented on a digital concentrator centre exchange (DCCE), the DCCE generates the PCLI. A PCLI must comprise sufficient information to uniquely identify the digital entry point to the PSTN used by that ALE. This information includes the region, area and unit identity portions of the network nodal identity plus the telephony process number and route numbers used by the call processing software of the digital exchange.

Whe a PCLI or FCLI is received by a DDSN action control point, the call processing software searches through a set of look-up tables for a comparison with the CLI sent. This search will result in the calling subscriber geography (CSG) being identified.

Figures 8 and 9 illustrate the CLI and CSG derivation process.

Global Title Translation

Call processing for service providers with basic features is handled within the DDSN switching nodes. To differentiate between calls to SPs with advanced and basic features, the ACP checks for the existence of a translation for the number dialled. If a translation exists, the call is routed to the specified network termination. If the translation does not exist, call handling instructions are returned from the NCP database in response to a query message from the originating ACP. A number of query messages are neccesary for some types of call; the initial query is therefore termed QRY1. The process is illustrated in Figures 10 and 11.

The QRY1 message includes:

  1. 10 digit dialled number, which excludes any leading 0 but includes a trailing 0 as padding if only 9 digits long.
  2. Calling subscriber geography (CSG).
  3. The ACP which originated the query. This is used to reference a table in the NCP which defines the capabilities of the ACP,; for example, whether it has an NSC.
  4. The destination of the query.

The route message includes a network code of up to 10 digits which is used by the ACP to route the call to its destination. This is normally a service provider (SP) line, but can be an NSC announcement.

A final treatment command is sent to the ACP when the NCP cannot route a call normally. The final treatment command results in either a tone or an announcement being returned to the caller.

Calls Requiring an NSC

As not all ACPs are hosts to an NSC, a call which requires an NSC at some point during the call treatment must be setup in two parts. After the QRY1 Message, the call is routed to an ACP/HOST, using C7BT in the normal manner, where a voice trunk to the NSC is allocated. This action is termed a 'service assist' if the NSC is required as intermediate step in the call treatment (SelectLink) or a 'hand-off' if the NSC is required to play an announcement as the final routing conclusion (CourtesyLink). During a service assist or a hand-off, the ACP/HOST then queries the NCP a second time (QRY2) with details of the NCP and call number used for the QRY1 message. The call treatment now continues with a list of commands being sent from the NCP to the NSC. This could be to play an announcement and collect digits from the caller. NCP/NSC communication takes place via the C7NA links with any digits collected being returned to the NCP to determine the final disposition of the call.


In response to a query message from the originating ACP, the NCP returns a billing command instructing the ACP what details to record; the ACP acknowledges receipt of the instructions to the NCP. On answer, the terminating exchange sends a message to the originating ACP giving either 'answer / no charge' or 'answer / charge' depending on which LinkLine (0800/0345) is defined. On Call termination, the ACP records the details of the call in an automatic message accounting (AMA) record.

The originating ACP normally controls the call and is responsible for generating an automatic message accounting record. These records are periodically polled by an on-line data collector which validates them before passing them to an off-line charge raising system which calculates call charges in preparation for the production of the service provider's bill. Where a 'hand-off' has occurred, the ACP/HOST takes over control of the call for supervisory and logging purposes.


The Multi-Function Operations System (MFOS) is central to the operations and maintennance fucntions for the DDSN intelligent network. These functions include:

Connection between the multi-function operations system processors, the network elements and the users is achieved using a virtual circuit switch for flexibility.

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