So now we’ve got a feel for the change, what do the new architecture and services look like? For starters the SSP as we know it will be no more. In Part 1 (Section 5.6), the technology of media gateway control (MEGACO) and SIP were outlined. The interim next-generation SSP is a decomposed MEGACO media gateway, signalling gateway and media gateway controller (aka call server and softswitch). The longer term outlook seeing the monolithic circuit switch SSP becoming a softswitch-based architec- ture.
Figure 10.2 shows how the SSP will be decomposed in the Megaco archi- tecture. The stored program controller containing the BCSM will be placed in a MGC, the peripheral connections for Time Division Multiplex (TDM) streams will be placed on media gateways and the SS7 signalling will terminate on signalling gateways. This architecture represents a step- ping-stone that allows early adopters of next-generation network technol- ogy to implement existing switch functionality, but take advantage of IP trunking of voice and data internally to their own networks. From the outside world’s perspective (other operators) the fabric of the early adop- ter’s network has not changed, it still looks like a TDM switch. What is the point of mimicking a TDM switch in this way? Simply put evolution. It will take time for the current TDM-based circuit switched network to evolve into the new multimedia network of the future. In the meantime each carrier around the world will be progressing at different speeds. Some
10.4 THE FUTURE OF IN
Figure 10.2 Decomposed SSP architecture
carriers will be jumping straight in with new kit, whilst others will have the financial and operational legacy of a circuit switched infrastructure that will require careful migration of customers and services, we cannot just turn the Public Switched Telephone Network (PSTN) off over night, as much as we might like to! The integrity and reliability everyone has come to expect from the current telephone network needs to be maintained, whilst its ability to cater for new enhanced services is improved.
The Megaco-SSP in Figure 10.2, as we have said, looks like a conven- tional intelligent network switching node (service switch point) as far as its functionality goes, if the MGC implements the same capability set and call model (BCSM) as theoldSSP.
In a Megaco solution with the MGC implementing the BCSM of the SSP, the current SCP will be able to control the call flow as before, by utilising a SS#7 protocol stack INAP interface connected to the signalling gateway.
The signalling gateway will package up the INAP messages and use Stream Control Transfer Protocol (SCTP, see Section 5.7) for transporting the messages to the MGC. It is also possible that the vendor of current IN platforms may choose to implement an SCTP/IP interface directly on their SCP, to communicate with the Megaco-SSP. This represents an evolu- tionary approach to protect investment in current IN services, whilst moving the core transport to a voice over IP (VoIP) solution. This type of solution is designed to allow existing TDM network operators to slowly (and safely) migrate to VoIP architectures. New softswitches and SIP proxy/application servers can be bolted on to this architecture as and when required, to either replace or augment the capabilities of the SCP.
The advantage with Megaco for an operator already committed to a TDM infrastructure is around the trunking of the resulting call. When a number of decomposed SSPs are connected to the same IP backbone, voice connections transit from one media gateway to another, directly. In the equivalent circuit switched environment the voice call may pass through a number of switching stages. This is where the economies of an IP-based core network are realised. Figure 10.3 shows this scenario.
The resulting overall network is shown Figure 10.4. In this network it can be seen that both circuit switched and next-generation components are able to deliver services in a reasonably seamless fashion. This is achieved through the connection of the IN SCPs through signalling gate- ways, to what the SCP believes to be a SSP implementing a BCSM.
In reality the SCP is communicating to a MGC, which implements the BCSM. The MGC also controls a number of VoIP gateways. That completes the picture of a decomposed SSP (as examined in Figure 10.2). We will see in Section 10.5 how a voice server can be added to this architecture to deliver enhanced interactive voice services.
One additional component that is shown in Figure 10.4 is the directory server, this component forms the repository of information on how gate- ways can look up configuration information automatically to find out the
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address of their MGC at power-up time. The media gateway control can also make use of the directory server to map, for example, message trans- fer part signalling point codes to media gateway voice circuit groups.
10.4 THE FUTURE OF IN
Figure 10.3 VoIP call from decomposed SSP to decomposed SSP media gate- ways
Figure 10.4 Interim network architecture
Eventually the media and signalling gateways will be removed, to make way for a fully IP-based interconnect between carriers and end devices. As the progress of the unbundling of the local loop gives greater low-cost bandwidth to premises via xDSL services, then the IP connection can be extended all the way to the end devices. This picture for the fixed network will be repeated in the mobile world as third-generation (3G) mobile networks get underway.