Bridge Those Network Gaps

Stuart Jamieson examines how preconfigured ATCA blades can span SS7 and signalling networks.

Financially bullish communications carriers are starting to migrate existing circuitswitched, time-division-multiplexing (TDM) networks to Internet Protocol (IP) packet networks and roll out new VoIP, data, and other multimedia services. However, cost containment remains a top priority; otherwise, carriers would need to replace TDM networks wholesale. What they’re doing is adopting hybrid solutions that meld the Public Switched Telephone Network (PSTN) and packet networks in order to exploit existing TDM infrastructure and applications while laying the groundwork for all-IP networks further down the road.

Open platforms like AdvancedTCA provide a good framework for quickly implementing high-availability telecom and datacom systems that bridge the PSTN and IP networks. They reduce time-to-market by making it easier for telecommunications equipment manufacturers (TEMs) to outsource generic functions like network access, control, and media processing, thereby letting them focus scarce engineering resources on value-added applications and services. They also cut cost by allowing TEMs to capitalise on increased competition and economies of scale.

ATCA provides a platform for building scalable, cost-effective, high-availability, high-density telecom systems. Its high-bandwidth (10Gb/s/link) switched fabric gives it the throughput and scalability needed to host advanced multimedia services and grow system capacity.

Thanks to its large form factor (8U) and high-power capability (200W per blade), ATCA can implement complex functions and accommodate a larger subscriber base in a smaller footprint. Redundancy and hot-swap capability minimise susceptibility to point failures and enable individual blades to be serviced and upgraded without disrupting overall service. And integrated systemmanagement facilities enhance availability by providing greater visibility into, and control over, blade-level operation.

ATCA blade bridges PSTN and IP

Signalling is a good example of a hybrid technology tailormade for open ATCA platforms. IP-based signalling is clearly the way of the future. In the interim, though, we’ve got SIGTRAN (Signalling TRANsport), which bridges SS7 (Signalling System Number 7) and IP (Internet Protocol) signalling networks. It enables service providers to deploy efficient IP signalling while still maintaining compatibility with existing SS7 networks.

SS7 is the principal signalling protocol used for wire-line and mobile communications in the Public Switched Telephone Network (PSTN), which facilitates call set up and tear down as well as routing through the PSTN. SS7 services are usually employed over a dedicated network of 56 or 64kb/s TDM lines, though carriers also use high-speed E1/T1 and even optical lines.

Until recently, the processing demand on legacy SS7 nodes was remarkably light. Because signalling only occurs at the start and end of a call, many voice circuits can share a signalling link. Moreover, SS7 is a fairly efficient protocol, which enables a single SS7 link (56 or 64kb/s) to service thousands of calls. As such, a metropolitan area may be served by a surprisingly small number of SS7 nodes implemented in relatively inexpensive rack-mounted systems. Each of those systems would contain a handful of T1 line cards and processing elements, along with a management interface.

SS7 remains a viable technology. But, increased use of phones, faxes, modems, and toll-free/managed rate numbers, together with new services such as Local Number Portability (LNP) and Short Message Service (SMS), have caused an exponential growth in signalling traffic and placed a tremendous strain on the system. So, too, is this the case concerning the increased usage of VoIP and mobile services—they use IP within the network core, but require SS7 for delivery to traditional wire line phones, modems, and faxes.

To deal with this increased traffic, service providers need to deploy signalling equipment that can handle a far greater number of calls within the same footprint. One approach is to upgrade the existing SS7 network with higher-speed links. A more efficient, scalable, and cost-effective method, however, is to use packet networks to carry the signalling traffic. IP networks not only carry 50% more signalling (and other) traffic for a given bandwidth, but also provide greater flexibility for re-routing and recovery within the network.

Anatomy of an ATCA signalling gateway

SIGTRAN gateways use a protocol called SCTP (Stream Control Transmission Protocol) to facilitate reliable transport of SS7 messages over IP networks. Sitting above the SCTP transport layer are user adaptation layers, like M2UA, that replace equivalent SS7 datalink layers such as MTP2.

The SIGTRAN gateway provides a transparent connection between the SS7 network’s MTP2 layer and through the equivalent SIGTRAN M2UA layer, distributed across the network and up to the MTP3 layer. This enables higher-level SS7 software layers (MTP3, ISUP, SCCP) to run unmodified over IPbased SIGTRAN networks and existing SS7 applications used across traditional TDM or IPbased transport infrastructure.

An ATCA SIGTRAN signalling gateway (SG) can be implemented as a standalone entity, or combined with a media gateway controller in the same chassis or shelf. The SG, in turn, may be incorporated into various systems that require signalling, including Signal Transfer Points, Service Control Points, Base Station Controllers, Home Location Registers, Softswitches, and Visitor Location Registers.

Figure 1 shows a typical platform for an SG and media gateway controller (MGC) implemented using a single ATCA chassis. The SG is implemented as a cluster of redundant, hot-swappable ATCA blades. It runs the SIGTRAN and SS7 stacks, taking TDM signalling information from the SS7 world and passing it on (and vice versa) to the IP world in a SIGTRAN format.

The MGC, which actually terminates the calls and forms the bridge between IP and SS7, is implemented as a minimum of a pair of separate blades. The MGC uses an ATM or Ethernet interface to access upstream IP network. Chassis management assures new blades are provisioned and made available to the application as they are added to the system.

Sample ATCA gateway

ATCA’s large-form-factor, highpower- handling capability and high-bandwidth switched fabric makes it a good platform for building high-density signalling gateways and integrating them with IP-enabled, ATCA-based network elements. Artesyn’s SpiderWare ATCA, for example, provides a SIGTRAN signalling gateway on a single ATCA blade (Fig. 2), supporting 32 to 128 SS7/SIGTRAN signalling channels with up to a 100% line utilisation (depending on packet size).

SpiderWare provides up to 32 E1/T1 spans for communications with the SS7 network, and multiple Gigabit Ethernet links for communications with the IP network. It also provides a STREAMS-based implementation for SS7’s lower-level MTP1 and MTP2 protocol layers and SIGTRAN’s SCTP and M2UA layers. Upper-layer protocols such as MTP3, M2PA, M3UA, ISUP, and SCCP are available, too.

This ATCA hardware platform has an MPC7448 processor running carrier-grade Linux to perform high-level protocol processing and host customer applications, and one to four PowerQUICC II processor mezzanine cards to perform low-level protocol processing. The blade’s ATCA fabric interface features high-speed control and data-plane connections. For control, two Gigabit Ethernet channels connect to the ATCA Base Fabric. And the data plane managed by a 24-port Ethernet switch uses eight Gigabit Ethernet ports to connect to the ATCA High Speed Fabric.

Open-architecture systems utilising preconfigured off-the-shelf blades like SpiderWare make it easy to outsource basic control, signalling, and media processing functionality, as well as build systems that integrate the PSTN with emerging IP networks. These systems not only take less time to develop, but cost less to manufacture, upgrade, and service.

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