Telecommunication networks worldwide are undergoing dramatic changes to address impending business challenges. Specifically wireline and wireless markets are undergoing convergence, which can allow communications anytime, anywhere, from any device; the combining of applications in new ways to deliver new services; or the consolidation of separate networks onto a common service layer.

Mobility markets are also evolving to take advantage of the new IP multimedia subsystem (IMS)-based services that are on the immediate horizon. Mobility networks are evolving to support not only connectivity to IP networks allowing mobile access to IMS-based applications, but also extending an IP connection out to the mobile device to provide end-to-end session initiated protocol (SIP)-based services and applications between the mobile device and IMS client. This is evident by the ongoing activities of mobility standards organisations such as 3GPP (the UMTS standards organisation) and 3GPP2 (the cdma2000 standards organisation), which are defining standards-based IP architectures and interfaces for wireless networks. By evolving mobile networks to IP, new converged voice and data services can begin to appear, higher content voice and data services and integrated voice/data user devices can be enabled with VoIP.

The migration toward IP-based networks and services will require a core network infrastructure that is capable of distinguishing and carrying the different types of traffic by assigning appropriate class of service (CoS) parameters to ensure high-quality service delivery whether voice, data, audio or video. To that end, MPLS has emerged as the leading technology for providing the CoS required for today's telecoms networks.

Core IP/MPLS networks provide the necessary infrastructure to support the migration towards IMS-based services and applications (VoIP, converged voice/data, audio, video, etc.). Additionally, IP/MPLS networks will be used to facilitate the transport of packetised legacy voice service over IP networks reducing associated transport costs, as well as provide for IP soft-handoff connectivity within and between mobile switching centres (MSCs). The primary requirement of these core IP networks is that they must be carrier grade and able to serve the differing traffic characteristics of a multiservice transport network. The factors that make IP/MPLS well suited for carrier grade core networks are the following:

• Traffic engineering and optimisation - service providers can engineer MPLS LSPs to optimise network bandwidth. Directing traffic flow over specific core LSPs maximises throughput, reduces congestion, and better utilises the optical trunks between specific network switches.
• Service differentiation - IP/MPLS provides the ability to differentiate among services (voice, high-speed data, audio, video) in order to provide QoS as required by an application to ensure and enforce specific service level agreements (SLAs)/service guarantees.
• QoS for voice - Delay and packet loss have impact on voice quality as the packet core network introduces delays due to queuing, jitter and serialisation delays. IP/MPLS uses traffic engineering capabilities to mark voice packets as high priority and low latency so that voice packets are treated as real time. Additionally IP/MPLS ensures that voice service meets strict packet loss requirements to guarantee quality of speech.
• Reliability/recovery - IP/MPLS provides the necessary reliability and failure recovery mechanisms to provide a level of reliability expected in carrier grade networks.
• Encapsulation for most Layer 2 network protocols - The MPLS standards for signalling and data encapsulation allow Layer 2 services, such as ATM, frame relay, and Ethernet virtual LAN (VLAN) services, to run over MPLS.

Evolution of the core network to MPLS is not sufficient to meet the challenges operators face for efficiency in operation and the introduction of new services without evolving the Radio Access Network (RAN) to support multiple access technologies.

But when they evolve access networks to a more efficient packet architecture service providers face some unique challenges. Mobile network architects must address multiple issues in the backhaul network: the existing use of multiple protocols, including TDM, ATM, IP, frame relay, and inverse multiplexing over ATM (IMA); and the need to support multiple wireless services, including GSM, time division multiple access (TDMA), CDMA/CDMA2000, general packet radio service (GPRS), UMTS/W-CDMA, and fixed wireless (WiFi) while maintaining QoS.

Why integrate IP with a multiservice backhaul network?

When choosing their backhaul network architecture, service providers must consider:

• How to reduce costs while increasing available bandwidth to support blended lifestyle services
• How to simultaneously support both legacy and third-generation (3G) services and protocols while protecting QoS.

According to the Yankee Group, mobile network operators (MNOs) spend approximately 15% of their revenues on their backhaul network. Growing demand for blended lifestyle services, such as push-to-talk, mobile instant messaging, and videophones, will increase revenues, but those increases come at the cost of leasing additional bandwidth facilities to support the content-rich services. CDMA2000 1xEV-DO technology and the introduction of high-speed downlink packet access (HSDPA) deliver high-speed wireless data services to the mass market, while increasing the need for more backhaul capacity.

To reduce costs, some MNOs may shift from TDM-based DS-1 and E-1 interfaces to lower-cost high bandwidth packet interfaces. This shift introduces a new problem: how to architect a multi-protocol backhaul network that can service both the new Ethernet traffic and any existing TDM, ATM, IP and frame relay traffic from existing services. The widespread adoption of wireless LAN (WLAN) services as a complementary technology to 3G wireless services further complicates this multi-protocol picture by adding the requirement for 3G and WLAN access services to share a common backhaul network.

To control operating costs while scaling to support growth in multiservice traffic, MNOs need a backhaul network architecture that utilises the available transport bandwidth more efficiently, increases network capacity, leverages their existing investments in multi-protocol equipment, and provides the QoS to support a mix of service traffic and wireless applications.

Still, the need to support ever-larger traffic volumes and a wider variety of access technologies will ultimately lead MNOs to consider an IP-based backhaul network design. Implementing an IP backhaul network can reduce the capital and operating expenses for a mobile network with 50 mobile switching centres (MSCs) by $213 million over two years. These cost-savings stem in part from more efficient packet handling, which enables the IP backhaul network to better utilise the available bandwidth between the cell sites and the MSC. More efficient packet handling enables the network to support more calls per cell site - reducing the number of leased facilities required to support a given level of traffic and improving the network's ability to meet the high capacity demands of high-speed data and content-rich blended lifestyle services. Reducing the number of DS1s required to handle the current traffic load also simplifies backhaul facilities engineering and provisioning.

Conclusion
The IP/MPLS core network forms a solid foundation to offer blended lifestyle, high-speed data, WLAN or other dynamic high-bandwidth services that can drive future revenues.

It also enables the mobile operator to consolidate multiple networks resulting in OPEX savings. By migrating to a multiservice-optimised IP backhaul network, MNOs can ensure that their access networks scale to support multiple access technologies and multi-media services. At the same time, migrating to a more-efficient IP architecture in the RAN can reduce costs by more than 26%. To accomplish the migration while maintaining support for both existing revenue generating and future services, MNOs should build a backhaul network that is optimised for a combination of multiservice and IP traffic patterns. Implementing a multiservice access solution benefits both carriers who have already deployed multiservice devices in their backhaul network and those planning to deploy such devices in the near future.

Andy Williams is European president at Lucent Technologies.