WGBH-TV maintains a full-duplex 7Ghz inter-city microwave link between WGBH in Boston and its sister station WGBY-TV in Springfield, MA, 100 miles away. The link is used for program exchange and also handles network traffic like e-mail and Web access for WGBY. In 2001, these 30-year-old systems needed to be upgraded with the latest state-of-the-art equipment, capable of handling at least DS-3 rates. A digital upgrade was run on the existing microwave link because the link was already in full service with towers, dishes, RF plumbing, shelter space, etc. There would be a one-time-only cost to purchase and install the new equipment.
The Microwave Radio Communications (MRC) proposal met all the criteria set by WGBH and WGBY. The proposal specified variable-rate modems (VRM) at each end as the baseband interface and at each repeater site to re-clock the data. These modems met the bandwidth and interface requirements the station was looking for. The MRC 2RU modem chassis has four card slots for a variety of interfaces and uses quadrature amplitude modulation (QAM) of the IF carrier. The units installed in the system have two ASI interface modules and one T1 module, leaving one spare slot. One ASI interface is used to transport a multiplex of two MPEG program streams, each encoded at 12Mb/s. The T1 interface links the WGBH and WGBY networks.
Two Tandberg E5720 encoders and two TT1260 decoders are installed at both the WGBH and WGBY master controls, making it possible to send two program streams in each direction simultaneously. Each of the three repeater sites is configured with a modem pair for eastbound traffic and a pair for westbound traffic. A pair of modems is connected to the back. The ASI and T1 interfaces on the receive modem feed the inputs on the transmit modem to re-clock and regenerate the data, and to make it possible to breakout the ASI for troubleshooting. One challenge faced was the trade-off between bit rate and the robustness of the link. The station already had a handle on fade margins and performance on the old link. MRC helped develop a link budget and, based on the numbers, suggested 16 QAM as the level to ensure acceptable performance during worst-case conditions. The maximum bit rate on the link is 60Mb/s with the 16 QAM modulation spread over three IF channels in the variable rate modem.
The upgrade took three days of replacing radios and installing modems. After installing the new equipment at all of the repeater sites, the studio ends were turned on. With a few configuration modifications in the modems and encoders, staff were seeing pictures and network data was moving. Over the past nine months, the link has been reliable except for a modem ASI jitter issue early on, which MRC has resolved.
It is likely in the near future WGBH will be adding more services on this link, including both real-time and non-real-time video transfer.
Sal Blatti, systems eng.
Steve Harrington, dir. eng.
Michael Foti, systems eng. mgr.
Dave St. Onge, R-F systems mgr.
Ray Miller, chf. eng.
Variable rate modems
E5720 MPEG encoders
TT1260 MPEG decoders
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