Most residents of Massachusetts and its neighboring states refer to the Boston Central Artery/Third Harbor Tunnel project as "The Big Dig." Often touted as America's largest construction project, it involves moving the city's main elevated highway underground.
A critical piece of this huge project is a communications network for Third Harbor Tunnel security that operates 24 hours a day to ensure traffic safety and efficiency. To maximize security network reliability, several precautions were taken. These precautions included specifying a primary and backup fiber optic cable with a self-healing ring topology and a fiber optic multiplexer that supports a "multiple master" self-healing ring topology.
TC2800 4/8/12-Channel Multi-Drop Fiber Optic Multiplexer
This "multiple-master" multiplexer requirement was fulfilled by the Model TC2800 from TC Communications of Irvine, California. It provided maximum reliability through its "multiple master" configuration. In this particular multiple-master configuration the TC2800 can maintain system integrity with up to six device or linkage faults. (See diagram on this page).
According to Kevin Driscoll of the Boston-based Norfolk Electric, a primary contractor for the project, the TC2800 multiplexer was chosen for the job because "it is significantly more reliable than the more common 'single' master topology."
Maximum Reliability
A multiple-master multiplexer is more reliable than a single
master multiplexer because, in extreme situations where both
rings are broken in two or more places, master units simultaneously
poll the slave units, but through different half rings.
The self-healing ring topology consists of two data paths running
in opposite directions. Under normal operation only the primary
data path is used. Where there is a fault, the data path will
turn around before it reaches the fault or failed unit and use the
secondary data path to complete the link.
Three TC2800 Multiple-Master multiplexers are linked to three
main computers over separate data paths. One circuit is "live,"
one is a "hot " backup and the other is an emergency backup.
The Master multiplexer units poll five Slave units connected to
the security network control panels.
In this application, the multi-drop multiplexers form a sub-network
that links the access control panels/intrusion detection
system to the security computer and two main network computers.
The system interfaces with card readers, keypad control units, automatic door latches and various alarms located near controlled entrances to buildings.
Three TC2800 Master multiplexers are linked to three main
computers over separate data paths. One circuit is "live," one is a
"hot" backup and the other is an emergency backup. The Master
multiplexers poll five Slave units connected to the security network
control panels.
The end result is that motorists traveling through the Third
Harbor Tunnel have maximum protection with more reliable data
paths.
In many traffic control applications, the TC2800 can be used
as a low cost alternative to SONET or T1 Rings. High speed
SONET networks are often overkill and it easily provides the
desired network ring fault protection.
For example, the TC2800 can create a self-healing ring network
for linking traffic intersections with multiple RS-232 drops
for traffic lights, cross walks, camera PTZ, etc. A similar application
uses the TC2800 to link PTZ, radar detectors, ram meters,
weather stations, etc. along a stretch of highway.
The TC Communications Model TC2800 is transparent to all
protocols and supports data rates up to 38.4 Kbps. Each unit provides
two pairs of fiber interfaces and 4, 8 or 12 independent
channels. Optical signals are regenerated at each drop. A hardened
temperature version is available for extreme environments.