Leasing a T1 line from a local telephone company can consume 10% to 20% of a carrier's overhead. T1 lines are expensive, particularly along rural highways and in coverage fringe areas. Nevertheless, when cellular was young, almost all transmissions rode along T1 lines directly from point to point, from the site of origin to the MSC.

Today, however, an increase in PCS carriers in smaller markets, combined with the emergence of microcell sites and increased capacity required by data and digital services, have resulted in rapidly diminishing channel availability. Established carriers may find it difficult to add channels in order to respond to growing user demand, and new carriers may find it nearly impossible to find any channels at all.

To further complicate matters, many microcell sites are located in nooks and crannies that may not be easily accessible. And, each and everysite -- large or small -- demands routine upkeep and maintenance, so more technicians are needed to support the network infrastructure. Currently, most carriers rely on approximately one technician for every 10 sites, and this ratio is projected to exceed 1:25. Not only is this investment expensive in terms of salary, but also in training and indoctrinating qualified personnel.

In response to this crisis, new forms of circuit configuration and implementation have arisen. Among the solutions are variations of spectrum splitting, multiplexing on T1s and DS0 grooming. Daisy chaining is one variation that is a fairly simple way to link multiple cell sites along a single T1 by hopping from one cell site to another, all the way down the line to the MSC. In many cases, even more cell sites branch out from these primary links and connect by ports along the T1, which then connects to the rest of the daisy chain and eventually to the MSC. This design maximizes the T1 line's capacity, which must be leased at a basic cost.

DAISY CHAINING PROS & CONS Drop-and-insert, or daisy chaining, has many advantages. It is much less expensive for carriers because they pay for shorter T1 lines as they connect from port to port rather than directly to the MSC. It maximizes T1 bandwidth usage. And, it is useful where no T1 lines are available. Unfortunately, it is not a simple network to manage.

The daisy-chain network is only as strong as its weakest link. When a problem occurs somewhere along the chain, it is a challenge to determine which site has defaulted. The T1 operates much like the old-style Christmas tree lights. When one bulb along the string goes bad, each socket needs testing to find the problem. Although this scenario is frustrating, it pales in comparison with losing cellular or PCS service to hundreds of subscribers.

You cannot understate the importance of reliability. U.S. Cellular, for example, adheres to a 99.999% network-reliability standard, which means networks must function with a maximum of 300 seconds, or 5 minutes, of downtime a year. Therefore, despite the significant economic advantages (up to 75% per T1 line or circuit), many carriers have been reluctant to use daisy chaining.

RELIABLE DESIGNS The solution is to find a method of intelligent reporting back from each specific site at all times. With pro-active network management, the network controller knows at all times what alarms occur, how the circuit performs, and it has access to an entire database of management information that the network operations center (NOC) can read.

Vendors such as ADC, Lucent Technologies and Nortel currently are marketing either complete systems or add-on devices that can help monitor the individual cell sites. The cells are monitored as though they were all direct point-to-point. They relay back messages, alerting carriers of system malfunctions before they happen. The system alerts the NOC to signal degeneration before subscribers begin to notice any problem.

BellSouth Cellular is one carrier profiting from advanced fault-tolerance equipment. BellSouth previously used a less sophisticated type of daisy-chain topology for its wireless network. BellSouth daisy chained its cell sites with non-intelligent communications service units (CSUs), which did not provide any pro-active management capabilities. Because BellSouth could not determine fault information at each cell site, a truck run to each individual site was the only way to determine where a fault was.

The need for a pro-active intelligent CSU that supports BellSouth's daisy-chain topology led the company to install and implement ADC's CrossPATH II intelligent CSU. This new solution helps BellSouth lower its operating and support expenses while providing critical performance information for each cell site.

The CrossPATH II integrates three T1 ports and a V.35 interface onto a single T1 circuit. It provides loopback capabilities and bit-error-rate testing to each site in the chain without disrupting service at the other sites, and it allows management of all daisy-chained cell sites onto a single DS0. Through its remote diagnostics, CrossPATH II monitors the varying electrical levels at each cell site and, when high error rates occur, it sends a simple network-management protocol trap back to the MSC. Long before service degradation manifests itself to the end user as missed syllables or gaps intruding into a conversation, carriers are alerted to a service-affecting problem.

The CrossPATH II also can perform loopback testing of the V.35 data port, supporting CDPD traffic or fingerprinting verification. Kevin Hart, ADC marketing manager, mentioned some of the other ways in which ADC makes a continuing effort to provide carriers with convenient features. For instance, the CrossPATH II has performance light-emitting diodes and loopback buttons in both the front and back to accommodate how each carrier mounts its equipment.

"BellSouth had planned for fault-tolerant daisy chaining for a long time," said Scott Daniel, BellSouth Cellular senior manager of transport/interconnect. "Our internal name for our plans is 'Project Prometheus,' after the Greek god of forethought. We are concerned about future bandwidth requirements for updated services. We are building a system to evolve with the 3G customer requirements for supporting wireless data networks. Daisy chaining is just another cost-cutting tool in a carrier's bag. It's a business decision how to juggle capacity, circuit cost and maintenance. But, in order to do it effectively and fully support data as you daisy chain, you have to be able to tell where the problems are."

BellSouth Cellular manages its daisy chaining with components from TTC, which provides equipment to remotely mode test an alarm from a CSU, and Clear Communications, which provides, among other components, the alarm correlator that indicates whether the break is in the cell or the T1.

"This combined solution is like having a network technician standing at every cell site, with his test set plugged in and his phone at his ear, ready to alert us when a possible problem shows up," Daniel said.

Lucent Technologies' next-generation architecture, its Flexent product line, has CSU functionality built into the equipment, said Hank Menkes, Lucent Technologies wireless networks director of systems engineering.

"We are able to take corrective action should there be a T1 or DS0 functionality failure at the cell site," Menkes said. "We bridge out the faulty components so we maintain the functionality of the daisy chain."

However, because such technology does not protect carriers from faults within the links between cell sites, Flexent also offers loopback capabilities.

"The loopback capability of the last cell in the daisy chain back to the MSC is necessary if the fault or failure in the daisy chain is in the links between cells," Menkes said. "That may not be our equipment; that could be the local carrier's T1 facility."

Flexent supports TDMA and CDMA technologies.

Nortel Networks equipment performs daisy chaining by providing fractional T1 output in its base station that allows it to connect any carrier's fractional T1 input through that carrier's base station. The Nortel Networks equipment supports TDMA carriers such as AT&T Wireless, U.S. Cellular, Cellcom in Israel and BCP in Brazil. Sprint PCS, Telstra in Australia, AirTouch, and Bell Mobility in Canada are CDMA customers.

Mark Whitton, Nortel Networks director of product management for CDMA access products, pointed out that fundamental differences in the two technologies make daisy chaining somewhat different in each case. CDMA is a natural daisy chain because the voice is packetized until it gets to the MSC, which has the natural statistical multiplexing effect of combining packet streams together. If one CDMA site has high traffic and the other light, you fully use the backhaul to avoid blocking.

TDMA and GSM deliver voice in a circuit-switched stream with a finite amount of fixed bandwidth for each transmission, so some multiplexing efficiencies are lost.

"However," Whitton said, "that doesn't mean you don't get benefits from other systems, such as TDMA or GSM. In fact, GSM is used extensively on highway sites. In remote areas, such as Arizona and Mexico, it is used in conjunction with microwave and supports a lot of drop-and-inserts."

Typically, rural areas have fewer T1s and longer distances to cover. By implementing microwave, it is possible to transmit over a great distance at less cost. For instance, in a 25-mile T1, linking sites every 5 miles, and fault-tolerant infrastructure at each junction, no area of more than 5 miles is vulnerable at any one time. It costs each carrier less, because each T1 is shorter. Whitton added that one way to handle redundancy in daisy chaining without advanced remotely operated infrastructure is to create a ring in which the last link is connected directly to the MSC. But in most cases, since the last link is usually the farthest away from the metro-based MSC, that alternative is costly.

The software to daisy chain is a component of all of Nortel Networks access products when shipped.

"The ability to daisy chain effectively is inherent in the chip sets needed to terminate T1s for point-to-point connection, anyway," Whitton said. "We just do the additional software work that allows carriers to daisy chain, and it's extremely cost-effective. If it were expensive, we would make it an option rather than an integral part of the package."

Although some carriers said that microwave is the most common application for daisy chaining, most agree that there will always be a mix of fiber optics and microwave, depending on the service area. U.S. Cellular uses daisy chaining "almost exclusively" with its fiber-ring service with the local carriers handling T1 drop-and-inserts. Otherwise, U.S. Cellular prefers microwave, which gives it more control over system outages and maintenance, said Doug Blake, U.S. Cellular director of network operations.

"With microwave, you can have daisy-chain features in a lot of aspects down the line, such as redundant hot standby," he said. "Also, you can build in frequency and space diversity for backup in case a microwave dish blows off path."

On the other hand, WirelessNorth "never uses microwave unless (it has) to," according to John Novak, WirelessNorth switch manager. "Fiber is always the preferred method of carrying T1 traffic, unless you need a partial leg, or hop, to fill in somewhere," he said.

WirelessNorth now has reached a point where network monitoring is affordable and advisable. It intends to use a digital cross connect system (DCCS). A DCCS is used as a hub in the MSC for different cell sites. The individual pipes from the cell sites (DS0 and/or DS1) are bundled and sent via a larger pipe -- a DS3 or STS-1 -- toward the telephone switch, rather than traditional daisy chaining.

Regardless of whether you use daisy chaining with TDMA, CDMA or GSM networks, along a physical T1 or microwave, or in conjunction with a point-to-point configuration, opportunities to enjoy reliable fault-tolerant support are broadening. Now that such devices and networks are available and affordable, this technology may be implemented increasingly.