These early generation BU’s are in stark contrast to BU’s being planned for today’s regional systems and conceptualized for the next generation of their evolution. This paper addresses the various stages of BU evolution and provides a snap-shot of what the future may hold for these devices.
Branching Units Get Smart The Early Passive Branching Unit
By Ray Chrisner
For a small regional system comprised of 3 cable end-points (cable stations or communications platforms), in the absence of a Branching Unit, station-to-station connectivity is achieved by single cable landings at two stations and a double cable landing at the one in the middle. By utilizing a BU, all three stations can be connected by single cable landings and the quantity of cable deployed will be minimized (see Figure 1).
In the age of regenerative repeatered underwater systems, Branching Units (BU’s) typically were passive devices used to reduce the quantity of cable in regional systems (i.e., UNISUR). The passive nature of these BU’s did not permit any power of fiber switching. As such, when cable cuts or faults did occur, capacity re-routing or power reconfiguration was not possible.
Station C (-)
Station A
(+)
Fiber Pairs 1 & 2
(-)
Station B
Passive BU’s were used in undersea fiberoptic systems through the evolution of the technology from regenerative repeaters into the introduction of repeaters with optical amplifiers. Next Generation Power Switched Branching Units
Prior to the introduction of multiple wavelength These early generation BU’s are in stark (WDM) transmission, purchasers of long-distance contrast to BU’s being planned for today’s transcontinental systems wanted the flexibility Figure 1 – Branching Unit Connecting 3 Stations regional systems and conceptualized for the to provide BU connectivity via 1 or 2 fiber pairs next generation of their evolution. This paper but be able to dynamically bypass branches with Branching 3 sides, one designated as the trunk and two as branches. Theormain addresses the various stages of BUThe evolution andUnit has faults bypass stations no longer wanting to be B where the PFE voltage polarity is negative (-). power path in Figure 1 is from Station A to Station B with PFE powering being positive Station provides a snap-shot of what the future may hold part ofinhe system. These requirements led to the Powering from Station C is negative PFE polarity A and negative in Station B. For the BU, the side facing the station with positive Power Feed for these devices. development of power switched branching units to a ground at the BU. Equipment (PFE) voltage polarity (+) is called the trunk. The main power path continues to (PSBU’s). polarity (-). ofPowering The Early Passive Branching UnitStation B where the PFE Withvoltage a passive BU,isinnegative the event a shunt from faultStation C is negative In a two fiber pair system, PSBU’s provided PFE polarity to a ground at the BU. along the main power path (A-B), PFE voltage is For a small regional system comprised of 3 cable connectivity as depicted in Figure 1 but also could automatically readjusted, power to the repeaters end-points (cable stations or communications drop fiber With a passive BU, inisthe event of a shunt fault along the main power path (A-B), PFE both voltage is pairs to the branch station (Station maintained, and transmission continues. In the platforms), in the absence of a Branching Unit, readjusted, C) as shown in Figure 2. automatically power to the repeaters is maintained, and transmission continues. In event of a cable break along the main power path, station-to-station connectivity is achieved by of a cable break the event along theismain transmission is but lost in the leg with the transmission lost power in the path, leg with the break, single cable landings at two stationsbreak, and abut double PSBU has the other two remain connected. the event of a shunt or cable break in the advantage over passive BU’s the stations other two stations remain In connected. In the fault The cable landing at the one in the middle. By utilizing in that if there the leg that is powered into of theaground at theor BU (Station C toinBU Figure event shunt fault cable break theinleg that1), transmission is were a transmission affecting fault 1 a BU, all three stations can be connected by between Station C and the BU, the PSBU could is powered into the ground at the BU (Station C to lost in that leg . single cable landings and the quantity of cable switch all fiber pairs as a straight through path BU in Figure 1), transmission is lost in that leg . deployed will be minimized (see Figure 1). from Station A to Station B and bypass the faulted Passive BU’s are excellent devices for undersea applications. Because of the absence of active branch leg. Once the repair was made, the PSBU 9 Passive BU’s excellent devices for undersea components they have very low FIT are rates (failures in 10 hours). could be reconfigured to restore the original The Branching Unit has 3 sides, one designated applications. Because the absence active Passive BU’s were used in undersea fiberopticofsystems throughofthe evolution of the technology connectivity of the system. as the trunk and two as branches. The main components they have very low FIT rates (failures from regenerative into the introduction of repeaters with optical amplifiers. power path in Figure 1 is from Station A to Station repeaters in 109 hours). The control of the PSBU was via a sequence of B with PFE powering being positive in Station A and negative in Station B. For the BU, the powering steps performed via PFE’s at Stations side facing the station with positive Power Feed A, B, and C. The steps controlled a network 1 shunt fault is between theIfBU thefault firstisrepeater C,first transmission may be the and shunt betweentoward the BUStation and the Equipment (PFE) voltage polarity (+) Ifisthe called the of relays inside the BU which made the fiber maintained – in all other cases, transmission is C, very likely to bemay lost.be maintained repeater toward Station transmission trunk. The main power path continues to Station Power Conductor CCCCCoCCConducto
– in all other cases, transmission is very likely to be lost.
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