In a conventional second generation mobile communication network, each geographical cell of the network is served by a Base Transceiver Station (BTS) which provides the network side of the air interface. A set of BTSs are physically connected to a Base Station Controller (BSC) which is in charge of the allocation of radio resources to subscribers. Each BSC is in turn physically connected to a Mobile Switching Center which is responsible for the routing of incoming and outgoing calls to the mobile network and is responsible, in particular, for handling the mobility of subscriber devices. The interface between the BSC and the MSC is termed the “A-interface”.
Traditionally, vertically integrated networks have been built to deliver single services such as telephony or data access. FIG. 1 illustrates this vertically integrated architecture in the case of a 2G network. However, in order to increase the “modularity” of network components and therefore reduce network construction and operating costs, a solution sometimes known as the Mobile Softswitch Solution (MSS) architecture has been introduced. This architecture is described in an Internet published whitepaper titled “Efficient Softswitching”, Ericsson A B, August 2006, which can be found at: http://www.ericsson.com/technology/whitepapers/8107_efficient_softswitching_a.pdf. Softswitching in combination with IP or ATM technology allows for a layered network architecture approach according to which service execution, control and connectivity can be horizontally integrated across multiple access networks. Softswitches separate the call control and switching functions into different nodes, consequently separating control and connectivity layers.
As a particular example, the conventional Mobile Switching Centre (MSC) is split in the layered softswitch architecture into an MSC Server (MSC-S) which handles the control plane signalling (acting as Media Gateway Controller (MGC)), and a Media Gateway (MGW) which handles user plane traffic. A Gateway Control Protocol (GCP), namely H.248, is used between the MSC-S and the MGW in order to allow the MSC to control user plane bearers.
In today's advanced MSS networks, server pooling is the standardised concept to achieve geographical redundancy in the networks. This avoids inter alia the failure of a single server from resulting in a failure of the network to provide services to subscribers. A generalised MSS server pooling architecture is illustrated in FIG. 2. In the case of an MSC-S pool, it will be appreciated that if one MSC-S fails, BSCs currently allocated to that MSC-S can be reallocated. In a pooled architecture, an MSC Pool Proxy can be introduced between the BSC and a pool of MSC-S in order to support BSCs that do not themselves support MSC-S pooling. This Pool Proxy acts as a single point of contact for BSCs to the MSC-S pool. In more general terms a pool proxy is used to connect network nodes to pool nodes, performing SS7 point code mapping on the A-interface. Such pool nodes can be pool servers. Whilst the MSC Pool Proxy can be a standalone node, it is preferable to integrate this functionality into the MGW in order to save on operating costs.
In the conventional 2G core network architecture, an MSC is physically connected to a BSC via a trunk cable. A Time Division Multiplex (TDM) protocol is used to multiplex user traffic onto the trunk. Each TDM time slot is identified by a Circuit Identification Code (CIC). The CIC is an identity employed at the call control level by the MSC. When TDM trunk handling in introduced into the layered network architecture, the trunk cables are connected to MGW, but the CICs are controlled in MSC-S. This is transported over the GCP interface by a termination identity, with the MSC-S performing a mapping between the CIC and the termination identity. The MGW in turn performs a mapping between termination identity and actual TDM time slots. When the MSS server pooling architecture is introduced into the 2G architecture, resulting in a given MGW being controlled by two or more MSC-Ss), the CICs associated with each trunk (connecting the MGW to a given BSC) are allocated amongst the MSC-Ss such that each MSC-S is statically allocated a sub-set of the CICs. An MSC-S uses the GCP ADD command to “seize” a particular CIC (TDM time slot).
Similarly, looking from the MGW towards a Public Switched Telephone Network (PSTN) exchange (termed a Point Of Interconnect or POI), the CICs representing TDM time slots on the trunk connecting the MGW to the PSTN exchange are statically allocated to the MSC-Ss. Of course, in any real implementation, the 2G core network will have multiple POIs including those to other Public Land Mobile Networks (PLMNs) and to emergency service centres. In order to reduce the number of MSC-Ss that are visible to the POIs, typically only two MSC-Ss are coupled to each POI via respective trunks. These MSC-Ss act as transit servers forwarding traffic to and from a chosen “operating” MSC-S. Thus, in the case of the interface towards a POI, there are always two MSC-Ss in the path.
Configuring the static allocation of CICs to MSC-Ss within a server pool is a challenging exercise. The addition of a new MSC-S within the pool will typically require reconfiguration of all other servers within the pool, and of the BSCs connected to the MSC-Ss. On the POI side, reconfiguration of transit MSC-Ss will be required (or in the absence of transit MSCs, the PSTN or other networks will require reconfiguration). Furthermore, in the event that a server within the server pool fails, the CICs allocated to that server become unavailable pending any reconfiguration. That is to say that the corresponding TDM time slots towards the BSCs or the POIs are unavailable, despite the fact that the remaining servers within the pool may have capacity to handle those slots, possibly leading to congestion on the trunks. This is particularly serious in the case of the POI interface, where the loss of a transit MSC-S will cause a dramatic reduction in capacity. A solution of course is to over-dimension the trunks, providing sufficient capacity in the event of the failure of an MSC-S. This is an expensive option.
Similar issues may arise in the case of 3G networks where TDM links will continue to be used between the MGW and the POI side (e.g. PSTN) despite the TDM links with the Radio Access Network being replaced with, for example, IP or ATM links.