What is a Branching Unit? | Submarine Cable Junction

Q: What is the difference between a fixed and switching branching unit?

A fixed BU permanently connects trunk to branches with passive couplers and no reconfiguration. A switching BU includes an electrically controlled optical switch that can redirect traffic between branches remotely, allowing rerouting around cable cuts without a cable ship. Switching BUs cost 30-50% more and require electrical power from the submarine cable conductor.

Q: How deep can branching units be deployed?

Modern BUs are rated for 8,000 meters depth at approximately 800 bar pressure. The housing uses titanium or beryllium copper with internal components potted in pressure-compensating gel. Installation beyond 1,500 meters requires ROV assistance.

Q: How does a branching unit handle power feed?

Submarine cables deliver 5,000 to 15,000 VDC to power optical repeaters. At the BU, power feed circuitry manages current distribution. Fixed BUs use passive splitting; switching BUs can redirect full current to the active branch.

Definition & Operating Principle

A branching unit (BU) is a subsea junction device connecting a main trunk submarine fiber optic cable to two or more branch cables routed to different coastal landing stations. Inside the pressure-rated housing, optical couplers or switches distribute WDM channels among branch legs while power feed circuitry manages DC current for downstream repeaters.

BU design emphasizes extreme reliability for 25-year subsea lifetime with no maintenance access. Housings use corrosion-resistant titanium or beryllium copper, optical joints are fusion-spliced and hermetically sealed, and electronics are potted in pressure-compensating compound to eliminate stress from hydrostatic pressure at depths up to 8,000 meters.

Key Specifications

Optical Insertion Loss:

IL = 0.5 – 2.0 dB (per branch)

Maximum Depth: 8,000 m (~800 bar) | Design Life: 25 years | Power Feed: 5,000-15,000 V_DC

BU Type Comparison

Parameter	Fixed BU	Switching BU	ROADM BU
Optical Path	Passive coupler	2×2 optical switch	WSS-based
Reconfigurable	No	Yes (full branch)	Yes (per wavelength)
Insertion Loss	0.5-1.0 dB	1.0-2.0 dB	3.0-6.0 dB
Power Consumption	0 W	5-15 W	30-80 W
Relative Cost	1×	1.3-1.5×	2-3×
Typical Use	Simple spur	Protected trunk	Mesh network

Practical Application

The SJC2 system uses switching branching units at mid-ocean locations to connect Singapore-Japan trunk segments with branches to Hong Kong, Taiwan, and South Korea. Each switching BU allows redirecting full fiber pair capacity between branches when a cable cut occurs, reducing restoration time from weeks to minutes via remote switch command. The BUs are deployed at 2,000-4,500 meters along continental shelf edges.

Frequently Asked Questions

What is the difference between a fixed and switching branching unit?

A fixed BU uses passive couplers with no reconfiguration. A switching BU includes an electrically controlled optical switch for remote traffic redirection, costing 30-50% more but enabling minutes-long restoration versus weeks for cable ship dispatch.

How deep can branching units be deployed?

Modern BUs are rated for 8,000 meters (~800 bar). Titanium housings with pressure-compensating potting protect internal components. ROV-assisted installation is required beyond 1,500 meters depth.

How does a branching unit handle power feed?

Submarine cables deliver 5,000-15,000 VDC for repeater power. Fixed BUs use passive current splitting; switching BUs can redirect full current to the active branch. Voltage clamping circuits protect against transients.

Branching Unit