Alcatel Submarine Networks (ASN) and Tyco Submarine Systems Ltd. (TSSL) have worked together to provide the Black Sea Fiber Optic Cable System BSFOCS which connects Varna (Bulgaria), Odessa (Ukraine) and Novorossiysk (Russia).

The BSFOCS architecture is based on well-established submarine cable, repeater, and transmission terminal technology, currently being deployed in systems such as SMW-3, Columbus-III, Americas-II and other current WDM systems.

This 1300-km system is designed with two fibre pairs operating at 2,5 Gb/s in an SDH ring mode. SDH technology in conjunction with high capacity availability enables the transfer of narrowband and broadband applications. The submersible portion includes one Full Fibre Add-Drop Branching Unit and is designed for the system to be upgradeable in the future via Wavelength Division Multiplexing (WDM) with up to four wavelengths.


The BSFOCS architecture uses two fibre pair cables throughout, with the fibre pairs interconnecting the three landings as shown below:

The station equipped in Bulgaria is housed in two separate buildings: St. Konstantin (located between 500 m and 1 km from the beach manhole) and Varna (located 15 km from St. Konstantin). These two stations are connected by an STM-16 Terrestrial Interconnect Link, which carries STM-16 optical signals between the SDH equipment located in Varna and, the SLTE SDH transmitter and receiver in the High Performance Optical Equipment (HPOE) located in St. Konstantin. The PFE is also located in St. Konstantin.

The BSFOCS cable system is implemented with submerged optical amplifier repeaters and two pairs of fibres, operating in the 1550-1560 nm wavelength range. The system offers the following features:

* High availability, based on “standard SDH” ring protection

* Automatic traffic restoration in the event of a broken fiber path

* Submarine Line Terminating Equipment (SLTEs) based on established WDM technology, initially equipped for single wavelength transmission; the SLTEs are reusable should the Purchasers choose to implement a capacity upgrade for the network via WDM

* Repeaters based on established optical amplifier technology

* Repeater supervision based on reliable passive optical High Loss Loop Back patented technique

* Branching Units (BUs) with power switching capability for maximum network resiliance

* Maintenance Controller equipment to monitor and manage the SLTEs, PFEs and submerged plant

* ADMs offering up to 16 x STM-1e or 16 x 140 Mbit/s

* 1:8 protection of the SDH tributaries.

2.1 System Design

The BSFOCS architecture is based on STM-16 (2.5 Gbit/s) Add/Drop Multiplexers (ADMs) configured as a self-healing ring. The submerged plant operates at STM-16 with Forward Error Correction (2.66 Gb/s). The diagram below shows the network configuration.

In the event of a problem between two ADMs, traffic can be diverted to go around the ring in the opposite direction, thus restoring traffic. This is a standard feature of the ADM and occurs almost instantly when required, providing very rapid restoration of traffic. It is an extremely effective way of protecting against failures, and thus giving a high availability.

2.2 Power Feed Protection

The normal power feeding configuration is show below:

The table below summarizes the approximate voltages required for each segment.











The voltages are based on a line current of 0.92 A, 16 V drop across each repeater and a maximum cable resistance of 1 O /km. From the table, one can see that the maximum voltage required (St. Konstantin-Novorossiysk) is 1330V.

The Low Voltage PFEs use redundant converters and are capable of producing up to 1600V, thus giving the capability for single-end feeding. This capability provides additional protection against failures or the effects of a shunt fault anywhere in S1, S2 or S3.

In the unlikely event of a complete failure at Odessa or a shunt fault in S2, BU could be reconfigured to allow the Novorossiysk PFE to power the S2 S3 section as shown below:

2.3 Network protection

SDH equipment for BSFOCS is mainly based on Alcatel 1664SM STM-16/STM-1 SDH Multiplexer, which allows to transmit signal at the STM-16, 2.5 Gbit/s level into the SLTEs (aggregate side) while accessing it at the STM-1, 155 Mbit/s or PDH, 140 Mbit/s level on the domestic network (tributary side). Within each 1664SM, all tributary signals and mapped into VC-4s and can be cross-connected with other VC-4s of the aggregate signals, thus providing routing flexibility.

The BSFOCS is made of a set of fibers pairs configured in rings. From the SDH point of view, one ring is made of 1664SMs (one per node, configured in Add-and-Drop Multiplexer mode, or ADM) connected to adjacent ones.

This ADM ring provides high system reliability, owing to protection mechanisms at the VC-4 level, whose principles and applications are depicted below.


Principles of Protection Mechanisms

VC-4 SDH ring protection mechanisms operates in path protection, or SNC-P-for Sub-Network Connection – Protection:

Path protection is based on a permanent broadcast of each VC-4 from originating node to recipient node, using the two physical routes of the ring. The recipient node selects the best one of both signals, based on SNC-P/I criteria, i.e. Signal Failure & Signal Degrade; traffic is recovered within 50ms. There is no need for communication between nodes, which makes this mechanism very simple and therefore intrinsically reliable.

As shown in the figure below, in case of failure, the selector on the receive side chooses the correct signal coming from the alternate route. This choice is single-ended and non-revertive, which means that a unidirectional failure may create diverse routing between both directions of a path; this can be overcome by manual intervention of the operator to force the switch of the other direction.

Regarding the network load, each path uses equal capacity on each span of the ring, owing to permanent broadcast. Therefore, the ring is globally equally loaded, and the limiting capacity factor is the total number of implemented paths, i.e. 16VC-4s in a two fiber STM-16 ring. In order to save bandwidth, it is possible to implement path protection partially, i.e. only for VC-4s that are considered as of high priority by the operator: protected paths are not lost in case of failure, while non-protected are; but this helps to implement more paths.




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