STEM newsletter

Migrating separate voice and data services to an NGN platform

30 April 2005

The majority of incumbent operators worldwide carry voice traffic on traditional circuit-switched networks. Data services such as ATM or frame relay are typically handled by separate network architectures where each service is switched separately.

Meanwhile VoIP is rapidly emerging as a service in its own right, with many users enjoying cheap and reasonable quality long-distance calls over best-efforts Internet. VoIP is also the inevitable future transport for voice in the core network. Operators believe they can achieve major capital and operational cost savings by migrating their existing voice and data services to a common next generation network (NGN) platform, where individual service interfaces are delivered at the edge of the network through so-called multi-service access gateways.

The best strategy will vary according to the design and age of the existing network. Our latest reference model explores the cost implications of different scenarios for this transition and offers a scaleable methodology for modelling these diverse network architectures.

Please register to download a copy of this reference model

Network topology

The network consists of five trunk exchanges (A to E) which are connected via an SDH backbone with six links. A set of local exchanges is connected to each trunk exchange via local exchange rings. Voice customers are connected to the local exchanges via remote concentrators and remote concentrator rings, while data customers are connected directly at the local exchange. The five trunk exchanges in the network are modelled individually. The local exchanges and other equipment linked to these trunk exchanges are averaged over those trunk exchanges.

Services and demand

Only a small selection of types of services are included in the reference model: voice, 64kbit/s ATM and 2048kbit/s ATM. For each of the possible routes between the trunk exchanges, a set of services is defined, and can be costed separately. A route is defined simply in terms of its two endpoints, for example node A to node C. The intermediate path taken is captured in a core traffic matrix. For the five trunk exchanges shown, there are 15 possible routes, and therefore 15 sets of services.

For the voice and ATM services, demand is specified for each of the 15 routes, without being specific about individual local exchange sites. So, for example, traffic for route A–E is from any local exchange on trunk switch A to any local exchange on trunk switch E, whereas traffic for route A–A is from any local exchange on trunk switch A to any local exchange on the same trunk switch.

The traffic generated by the 15 services (routes) is mapped onto the trunk exchanges using an access matrix and a core matrix. These matrices are used to calculate the traffic on each trunk exchange interface by multiplying the traffic carried for each service by route by the multiplier for the exchange and summing over all services. The five exchanges are modelled separately, based on a common template definition.

Migration scenarios

There are two steps to the migration from the traditional network to NGN. The first step is to add the IP network equipment and run it alongside the traditional infrastructure while customers are being migrated. The second step, once all customers are migrated, is to remove the legacy network equipment.

A media gateway is installed at a remote concentrator site. This gateway converts TDM circuits to IP and multiplexes them onto a gigabit Ethernet network (see Figure below). The GigE network is connected to the access router at the local exchange. The media gateway equipment includes new line cards (which are voice/DSL-capable) and the GigE interface. A soft switch (or gateway controller) is deployed at each trunk exchange site, which establishes call sessions and identifies the destination IP addresses of the call recipients for media packets.

Remote concentrator – NGN

An IP access router is deployed at each local exchange site. The remote concentrators are connected to this access router via a GigE ring. Each router is connected to other access routers on a local exchange ring and to the backbone routers via further GigE rings. ATM access circuits are migrated from the traditional ATM switch to the access router via a multi-service media gateway.

Local exchange during migration

Three scenarios are modelled: a proactive migration, where equipment in the traditional networks is removed before the end of its life and replaced with IP equipment; a migrate-as-required migration, where equipment is replaced only when it reaches the end of its life; and no migration, which is used as a base case to examine the effects of not migrating to IP at all.

Remote TDM concentrators gradually replaced by IP media gateways in as-required scenario

The purpose of the migration is primarily to reduce capital and ongoing costs, and therefore the key model results are the different operating costs, capex and depreciation for the various networks and scenarios considered.

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