Timing Is Everything

Out-of-sync networks can suffer from cut-outs, jitter, blocks and drops.

“Does anybody really know what time it is … does anybody really care?” With apologies to Robert Lamm and '70s rockers Chicago, the answer from successful wireless carriers is an unequivocal yes. They are ardently focused on minimizing complaints about blocks, drops and cut-outs.

How do they do it? They have a few available options. Plesiochronous timing actually refers to tying each cell independently to an external clock source, usually a GPS pulse. Global positioning satellites and receivers require a super-accurate timing mechanism to perform their primary positioning function to expected accuracy. Cells and switches will piggyback onto this to tune themselves, guaranteeing seamless call control.

A hierarchical source-receiver system ties an entire network to a limited number of external sources, whose timing references are passed down sequentially through the systems. It's the timing equivalent of a PTA telephone tree. Fifteen years ago, according to Steve Clark, U.S. Cellular (www.uscc.com) vice president of network operations, atomic clocks served as communal master references for the entire North American network. There were two: the Naval Observatory, and a cesium clock in Hillsboro, MO, controlled and shared by AT&T (www.att.com). Although accurate at the source, they degraded ever so slightly as they were passed down through each network.

Mutual synchronization is a third common timing method. Basically, each cell sends and receives timing references from all other cells in the system, continuously. Each cell determines official network time-stamp by averaging the pulses it constantly receives from other cells.

Loss of timing can be visible to customers in a number of ways. Shutdown of a master timing source could bring down a digital network. Short-lived audio anomalies result from faulty timing contact between handset and base station in areas of fringe or obstructed coverage. Most commonly, timing problems between switches will manifest themselves to users as those brief, clean, silent cut-outs, which Clark called the “Max Headroom” effect.

You can hear a mistimed analog signal, however corrupted with cross-talk or static. A mistimed digital call is completely, if temporarily, cut out. This is jitter. Lengthier or more severe timing differentials can then cause blocks, lost handoffs or failed call setup.

Tied to GPS

In practice, CDMA networks use a form of plesiochronous timing. Each CDMA switch is tied to GPS reference.

“As long as that clock source is clean, within a certain range of drift, everything works OK,” Clark said. That synchronization, however, can be a house of cards if contact is lost with the primary reference. A CDMA network will run in freewheel mode for a period of time. Unchecked, timing will deteriorate to the point that call control no longer functions.

That's where a company such as Datum (www.datum.com) comes in. Datum, which specializes in providing timing and synchronization products to wireless carriers and other clients, perfected the rubidium oscillator, a miniaturized timing device that provides internal timing to wireless base stations. Datum also builds quartz, cesium and hydrogen-based devices, geared to aerospace and military applications.

The oscillators are designed to keep time in synch with an external reference (i.e. GPS), but also to be stable enough to maintain sync in “freewheel.” Datum markets the rubidium-based oscillator to vendors, such as Lucent (www.lucent.com) and Motorola (www.motorola.com), that will include it in GPS-linked cell sites. The unit will provide a longer holdover when keeping time on its own than will quartz systems, which Datum also produces. This slow degradation of a cell's timing accuracy in the absence of an external clock reference is often called drift or wander.

A quartz oscillator will keep time in freewheel mode for a few hours before drift becomes noticeable to users. Rubidium, according to Bob Buckley, Datum-Irvine marketing communications manager, provides a window of 48 hours or more before drift becomes problematic. Rubidium is more costly than quartz, but Datum is working to narrow that gap.

Datum has been marketing the Starloc-II quartz device and recently has introduced a Starloc-II Plus, which incorporates rubidium. Aerospace clients use Datum oscillators in GPS satellites, so the company has built a presence at both ends of the clock reference chain. It originally had expected the Starloc-II to have a limited run on the market, as GPS timing functions were transferred to handsets. The telecom slowdown has pushed back that process, though, which is one reason Datum decided to market a rubidium upgrade.

TDMA Techniques

TDMA timing is a little different, according to Eduardo Inzunza, Symmetricom vice president of (www.symmetricom.com) synchronization marketing. The TDMA air interface will use a combination of plesiochronous and mutual synchronization to subdivide and synchronize frames. TDMA will use timing references carried between cells, along with GPS references to ensure accurate timing. The averaged timing references will render an exceedingly stable signal.

Several separate aspects of network traffic must be synchronized to ensure clean transmission. Bit synchronization means that the transmitter and receiver must run at the same speed to avoid lost signals or delays. Frame synch aligns both ends of the transfer so that the beginning of each frame can be identified. Network clocks, or clock references, must be available at each end to avoid the loss, or necessitated retransmission of data. This is slippage.

So where do we go from here? Datum continually is pursuing miniaturization and improved durability to increase the range of applications and environments in which atomic-quality timing can be employed. Datum worked for years to match the accuracy of an atomic clock in a box the size of a cigarette pack, Buckley said. The mandate now is to keep shrinking. The ability to shrink devices to the point where they were usable on early spacecraft was key to the company's growth. Now, getting timing devices into handsets at low weight and power usage is a major push.

As for durability: “We got a letter in one time from Rockwell (www.rockwell.com) where they had to fire up the backup rubidium, and it came online and worked perfectly after being in orbit (unused) for seven years,” Buckley said. The Starloc-II Plus is rated to operate at temperatures far below freezing and humidity up to 90%.

Symmetricom also is focusing on improving remote monitoring and fault management. Software such as its TimePictra and TimeScan suites are designed to manage configuration, fault and security remotely down to the card and port level in browser-based, point-and-click GUI.

Remote management is an issue for U.S. Cellular's Clark. Although switches are engineered to alert network nerve centers to the presence of a problem, including loss of contact with its master timing reference, they're often only macroindicators.

For Inzunza, timing and sync also are growing in importance as demands on bandwidth grow to accommodate broadband data and video. New applications necessitate subdividing bandwidth into ever smaller and tighter tolerances. The responsibilities of base stations and network carriers are growing — but they do know, and care, what time it is.

Kintzel (kintzel@sunflower.com) is a freelance writer based in Lawrence, KS.

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