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It’s All About Timing: Why Network Synchronization Matters
Martin Nuss, Vitesse Semiconductor
MAY 14, 2014 01:19 AM
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According to the International Telecommunication Union (ITU), mobile broadband subscriptions worldwide will reach 2.3 billion by the end of 2014. These subscribers no doubt contribute to the over six billion hours of YouTube videos that are watched in a single month. That’s nearly an hour for each person on Earth, according to YouTube.                                                                                   

Ask anyone, and you know that most people dislike when their calls drop, or when the dreaded ‘loading’ icon disrupts their live stream of the playoff game. Carriers realize this, and know that network speed and reliability are driving forces behind consumer satisfaction in today’s connected world. With the exabytes of data sent across today’s networks, however, speed and reliability can be difficult to maintain consistently.

What’s less commonly known is the role that network timing and synchronization play in this whole equation. 4G LTE networks, for example, rely on highly accurate timing and synchronization for smooth cell-to-cell transfers of the mass of voice, video and mobile data.

4G LTE, LTE-A – What’s The Difference?

But first, some background about what 4G LTE really means. LTE is a broad umbrella encompassing three different network types:

-        Frequency Division Duplexed LTE, or FDD LTE, uses paired spectrum – one for upstream traffic and the other for downstream. FDD LTE was used in some of the early LTE deployments and is still deployed today;

-        Time-Division Duplexed LTE, or TDD LTE (also sometimes called TD-LTE), is more spectrally efficient. Unlike FDD LTE, TDD LTE requires only a single spectrum band for both upstream and downstream traffic, flexibly allocating bandwidth by timeslot, and generating significant cost savings for carriers in spectrum licensing fees; and

-        LTE-Advanced, or LTE-A, is an upgrade to either of the two types outlined above, delivering greater bandwidth by pooling multiple frequency bands and allowing simultaneous data transmission from multiple base stations to a single handset.

These different ‘flavors’ of LTE need different types of synchronization, and wireless networks use what’s called frequency synchronization and time-of-day synchronization. FDD LTE only needs frequency synchronization. TDD LTE and LTE-A, on the other hand, require both. And therein lies the challenge.

Historically, wireless networks have used global positioning satellite (GPS) as the main timing source, since it can provide both frequency and time-of-day synchronization. But carriers now recognize its drawbacks, especially as networks rely more on small cells (femtocells and picocells) for increased coverage and capacity. Often, small cells installed at street level or indoors lack a direct line of sight to GPS satellites. Even if that weren’t the case, adding GPS technology to these units would make them too expensive to deploy on a mass scale. Add to that the growing concerns about GPS spoofing and jamming, plus the unwillingness of countries outside the U.S. to depend exclusively on the U.S. government-run GPS satellite system for their wireless networks, and clearly carriers need alternatives.

Fortunately, there is an alternative: IEEE 1588 Precision Time Protocol (1588 or PTP). Not only can it deliver the frequency and time-of-day synchronization needed in TDD LTE and LTE-A networks, but it’s more cost-effective than GPS as well. Especially as carriers rely more on heterogeneous networks, or HetNets, using 1588 as a GPS-alternative for network timing becomes more critical. By definition, HetNets are comprised of both fiber and microwave equipment, including the more widespread small cells mentioned above. Compounding this is the fact that most carriers use network equipment from several vendors, which may or may not offer 1588 support.

True, most wireless customers ultimately won’t care how they get their services, just that they work when and where they’re needed. But from a network infrastructure perspective, IEEE 1588 is here to stay. Carriers need to look for it and plan accordingly as they continue their network rollouts to support next-gen advanced wireless services. 

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