SONET runs over fiber and radios in Puerto Rico

Puerto Rico Telephone Co. has combined fiber with digital microwave radios to create a multimedia SONET network.

Stephen Hardy Editor in Chief

Is nothing sacred? First one discovers that, despite their name, Fibre Channel applications likely will not be predominantly fiber anytime in the near future (see Lightwave, January 1998, page 35). Now carriers such as Puerto Rico Telephone Co. (prtc--San Juan, PR) are proving that Synchronous Optical Network (sonet) traffic doesn`t have to be optical after all. The island`s principal phone company is improving its communications flexibility and reliability by constructing both fiber-optic rings and digital microwave radio links--both of which will carry sonet traffic.

Neither medium is new to Puerto Rico. prtc has been installing fiber in its network since the 1980s, principally in point-to-point applications, where the addition of redundant lines helped provide protection from cable cuts. However, the company decided in 1996 to further improve network reliability by constructing a series of fiber-optic sonet rings. At the same time, the existing microwave radio network was suffering from obsolete equipment that had become difficult to maintain. According to Roberto Correa, group director for network engineering at prtc, it seemed natural to extend the benefits of sonet technology to the company`s microwave radio infrastructure. Thus, the company awarded a $60 million contract to Alcatel Network Systems (Richardson, TX) to provide both fiber-optic and microwave radio transmission equipment based on sonet technology.

prtc is building four bidirectional line-switched fiber rings to serve as backbones for the transport of business and residential voice and data traffic. The company chose this architecture because its network is not a mesh type, but rather "homes" toward two access tandems located in the metropolitan San Juan area. The structure of the local switching elements also influenced the decision. The switching network consists of 29 "host" switching offices that serve 68 local calling areas through the host and an array of remote switching units. The company has connected the remotes to the host switch via a star configuration; the hosts, in turn, are connected to the access tandems through a ring configuration.

To date, two of the expected four sonet rings have been completed. The largest of these starts in San Juan, then runs to Arrecibo, Mayguez, and Ponce before returning to San Juan. The rings operate at OC-48 (2.5 Gbits/sec), using a combination of Alcatel`s 1648 SM sonet multiplexers for the rings and 1603/12 SM terminals for OC-3 (155-Mbit/sec) spurs off the backbone network, OC-3 point-to-point connections, and multinode OC-12 (622-Mbit/sec) bidirectional line-switched ring applications. Bidirectional line-switched rings are noted for their robust protection capabilities. In addition to the line-switched ring protection afforded by unidirectional path-switched rings, bidirectional line-switched rings contain a second protection algorithm: span switching. Thus, if there is a failure of an individual fiber or an individual circuit pack on the high-speed electronics, the system only makes a switch on the affected span in the ring, and the rest of the nodes are unaffected.

The network contains approximately 2290 mi of singlemode fiber-optic cable. Principal suppliers include Alcatel, Pirelli, Telway, Siecor, and Lucent Technologies, according to Correa. The network also makes extensive use of digital crossconnects provided by Tellabs. There will be 25 such crossconnects throughout the network when it is completed, Correa says.

Wireless sonet

Of course, the unique aspect of prtc`s network is the mixing of fiber-optic and digital microwave radio technology. The company will use 24 mdr-4000s radio systems for three point-to-point spurs off the fiber backbone. The links connect a central office in Humacao to a facility in El Yunque; the El Yunque location to another in San Patricio; and an office in Miradero to its counterpart in Monte del Estado.

"They provide connectivity in a sonet network where you can`t put fiber," says George Kizer, sonet radio product manager at Alcatel Network Systems. "They`re using the radios for large cross-sections of transmission using an unusual radio configuration called a multiline switch." Whereas most radio applications would call for a hot standby radio to provide redundancy for each radio used in the network, the multiline switch configuration uses one nonstandby radio to provide backup to three nonstandby radios. A multiline switch links the protection radio to its three counterparts. prtc is using the Alcatel DST-4000s Multiline Switch for its application.

The transmit/receive radios transport DS-3 (44.736-Mbit/sec), STS-1 (52-Mbit/sec), and OC-3 traffic. The radios are housed in racks; typically, two radios would be located in one 7-ft, 19-inch bay. The radios will be collocated with some of the digital crossconnect systems to provide connection with the fiber-optic backbone, says Correa.

The use of sonet technology extends network management and protection to the microwave radio links and provides a natural bridge between the fiber and wireless networks. sonet traffic moving from the fiber to the microwave radio network requires minimal grooming. "We put a few other housekeeping signals along with them, then bind them all together in a radio data framing format," says Kizer. "Since it meets the sonet standard, it has all the oam&p [operations, administration, maintenance, and provisioning] functionality and the overhead, interconnects, and that sort of thing you would expect sonet equipment to have. It just happens to go over the air instead of over fiber cable."

The radios typically operate in three bands. The most heavily used is what Kizer terms "the lower 6-GHz" band, which extends from 5.9 to 6.4 GHz. For short hops, the 11-GHz band, which covers 10.7 to 11.7 GHz, may be used. prtc also can broadcast in the 4-GHz region (3.7 to 4.2 GHz), but Kizer says this band is rarely used.

While the combination of fiber and microwave radios is not yet common, Kizer says such applications should appear with increasing frequency as carriers attempt to push the benefits of sonet farther away from densely populated--and fiber-rich--areas. In rural areas, or in areas where topology makes the laying of fiber cable difficult, sonet radios offer an alternative way to close sonet fiber loops, he says.

"More and more people are starting to do this in the access networks," Kizer explains. "And in that regard, the access network is pretty much sonet--radios are just becoming part of it. So I would say it is more and more prevalent as the access rings get outside of the deep urban area and start getting out where it`s getting harder and harder to close fiber rings. Radios have become more practical for that."

For example, Kizer reports that a hybrid fiber-and-radio sonet network is now under construction in Colorado Springs, CO. Here, the application called for a pair of rings, one operating at OC-48, the other at OC-3. The OC-48 ring is fiber, while the OC-3 ring--which intersects with the OC-48 loop--is composed entirely of microwave radios. The diversity in operating speed is typical, Kizer indicates; radios tend to run out of steam at OC-3 rates.

Finishing the job

Kizer believes that prtc plans to add optical interfaces to its radios in the future. Such interfaces would allow the company to close additional sonet loops with radios or run additional rings off of the microwave equipment. For its part, prtc appears to be concentrating for now on completing its current plans, which call for the four fiber rings to be completed next year.

The radios will be an active part of prtc`s network. "We are presently using those radios for different services, not as a backup," Correa explains. "We will probably continue like that, using them for alternative routing, maybe for some special applications. But most of the circuits will be for specific use."

Future traffic demands will determine how the two media will be mixed in prtc`s evolving infrastructure. But if applications like the one now taking shape in Puerto Rico are any indication, carriers will be thinking that sonet isn`t just for fiber anymore.

What`s next--wavelength-division multiplexing over copper? u