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Preparing metro networks for 4G LTE backhaul

Wireless operators must ensure their metro network infrastructure is optimized to handle 4G wireless broadband LTE voice and data, because the backhaul capabilities determine how much data the cell can support.

Editor's note: This Expert Lesson, LTE: The preferred 4G solution for wireless operators, provides a comprehensive look at Long-Term Evolution (LTE). This article looks at how to optimize metro backhaul for LTE. The guide also looks at why many mobile operators are choosing LTE, how to deploy LTE-based services to take advantage of 4G capabilities, and how operators should analyze geographic and customer requirements for LTE deployment planning.

The transition to 4G wireless broadband using Long Term Evolution (LTE) technology has largely focused on how

Metro backhaul topology is always important, and with LTE, it is even more so.
Tom Nolle
PresidentCIMI Corp.
the radio network and handsets can be adapted to the new technology, or on the kind of new services and data capacity LTE can support. These are important issues, but connecting the cell sites through metro infrastructure to provide for 4G LTE backhaul is often neglected -- and this requirement can create additional project benefits or define new project risks, depending on how it is managed.

Wireless backhaul can be an expensive proposition. This is particularly true when the feeds to the cell sites carry circuit-switched voice rather than packet. As service providers evolve to a metro Ethernet, IP or Ethernet/IP hybrid infrastructure commitment for IPTV and consumer broadband delivery, the unit cost of bandwidth on these new networks is typically far lower than that of existing SONET/TDM infrastructure. Some operators quote a full 10x difference, in fact.

The cost of backhaul has been a great concern as operators migrate to advanced 3G data services because the backhaul needed per cell determines the effective maximum data load the cell can support, whatever the radio technology might offer. Circuit-over-packet emulation, particularly circuit-over-Ethernet, has been one mechanism for dealing with the backhaul needs of 3G networks. But 4G offers more approaches.

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Pure VoIP and IMS deployment are considered a likely voice services direction by service providers looking at their aging TDM plant. Femtocells also generate a new opportunity, as do unified communications and hosted voice services using fixed-mobile convergence (FMC). An LTE/IMS voice system would use gateways to interconnect with the existing 3G network and PSTN. This may be an issue for operators that don't plan a fast transition away from TDM voice for wireline or for those that are looking at a protracted evolution from 3G to 4G, with both networks in place during that period.

VoLGA overlay handles circuit-switched voice over 4G

For those with strong TDM commitments in the near term, Voice Over LTE Generic Access (VoLGA) is an additional LTE voice option represented in a new industry group, the Volga Forum. VoLGA allows 4G networks and users to access circuit-switched 3G services and is essentially a service overlay on packet infrastructure that handles circuit-mode traffic. The packet-mode 3G traffic is directly mapped to the packet infrastructures, and as such, it is an LTE evolution of the 3GPP Generic Access Network (GAN) capabilities.

Operators have multiple views of VoLGA's role. Some see it as a pure transitional approach, an aid in handling the customer shift between 3G and LTE. In this role, an eventual transition to IMS would gradually reduce the VoLGA commitment. In the meantime, there would be potentially considerable savings in service gateway costs, as well as a transition of the current service revenue models to LTE. Other operators view VoLGA as a possible long-term strategy for voice services, in the same way that UMA/GAN is viewed today as an alternative to IMS for 3G or 3G/WiFi/WiMAX hybrid phone support.

The debate about VoLGA's role is largely due to different views on LTE's early drivers. Where LTE is primarily justified as a data/content strategy, there may be a strong desire to support the current 3G voice and SMS services as-is for a long period of time, depending on how well voice services sustain pricing and margins over time. Where LTE is linked to a VoIP migration in wireline or FMC, a faster transition to IMS voice is likely, and so VoLGA might be a short-term benefit only.

Voice requirements for mobile backhaul are different for VoIP and VoLGA. In the latter case, circuit emulation is necessary, and the QoS of the metro infrastructure will have to be tightly controlled to insure voice quality. While VoIP also requires QoS, the service is more forgiving of some variability in packet delay and even loss, and thus may be better suited for applications where data traffic and revenue are expected to quickly swamp that of voice services.

Metro network topology takes central role for LTE

Metro backhaul topology is always important, and with LTE, it is even more so. In most cases, even mobile Internet traffic will be backhauled for authentication and access control, and so the topology of the network and how cell sites are aggregated for coordinated handling will be important. As IP traffic grows, it will be desirable to avoid concentrations of IP traffic competing for a small number of data network on-ramps. If this happens, performance will suffer, and service credibility and desirability may be impacted.

Metro infrastructure is far more important for 4G services than for 3G because of the expected growth in data traffic volumes and the mixture of best-effort and premium content and traffic that will surely exist. Unless the metro network is planned as carefully as the radio network, LTE services may not provide the revenue and profit benefits expected by those that deploy them.

About the author: Tom Nolle is president of CIMI Corporation, a strategic consulting firm specializing in telecommunications and data communications since 1982. He is the publisher of Netwatcher, a journal addressing advanced telecommunications strategy issues. Check out his SearchTelecom networking blog Uncommon Wisdom

This was last published in August 2009

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