Further to our blog entry about Submarine Repeaters, here is some history of the technology used.
The first generation optical fibre cable systems, repeaters or ‘regenerators’, were placed along the cable periodically to boost the optical signal. Each regenerator was comprised of a receiver, to convert the optical signal to a digital electrical signal; a regenerator to work out whether each incoming pulse was a ‘1′ or a ‘0′ and to generate a new one; and a transmitter (laser) to convert the new digital pulse into an optical one for transmission over the next fibre section.
Although robust and reliable, the key disadvantage of this technology was capacity:
The ‘capacity’ of a submarine cable refers to the total amount of data bits it can transport per second. Increasing the number of fibre-pairs in the cable is one method of increasing the cable’s transmission capacity, but bearing in mind that each fibre requires dedicated amplifiers to boost the signal periodically, the number of fibre-pairs is generally limited by the number of amplifiers that can practically be accommodated within a repeater casing. For PPC-1, 2 fibre-pairs have been chosen.
The second method is to reduce the duration of each light pulse (‘bit’) such that more bits can be transmitted per second. The laser transmission rate PPC-1 is 10Gbit/s – that’s 10 billion laser on’s and off’s per second.
The third technique is to couple multiple lasers to each fibre, each transmitting a slightly different colour (wavelength) of light and each carrying separate data. By filtering the light received at the far end into the original colour components, it’s possible to recover the data transmitted on each wavelength. This technique is known as Wavelength Division Multiplexing (WDM) or Dense Wavelength Division Multiplexing (DWDM) where a large number of closely spaced wavelengths are used.
Each fibre on PPC-1 has been designed to support 96 wavelengths, each operating at 10Gbit/s. The total transmission capacity of PPC-1 is therefore:
2 fibre-pairs x 96 wavelengths x 10Gbit/s = 1920Gbit/s or 1.92Tbit/s.
To put this into perspective, a basic telephone conversation typically requires 64kbit/s to transmit. If the entire population of Australia made overseas calls at the same time, it would require only 1.3Tbit/s of capacity. This underlines the very significant impact that PPC-1 will have on Australia’s international telecommunications capacity.
Since the late 1980s, submarine cable systems have employed optical fibres as the primary transmission medium. Optical fibres are essentially flexible glass strands designed in such a way that light injected into the fibre travels along it, even around bends, without leaking out.
Similar to the early telegraph cables fibre, optical communications is a digital technology in that it carries ‘pulses’ (ons and offs) of light, in this case, which represent the ‘1′s and ‘0′s of the data being transported. The characteristics of the fibre enable the pulses to travel long distances before they need to be amplified – typically a pulse will lose half of its optical power over a distance of 15km of fibre.
Accordingly, over transoceanic distances it’s necessary to periodically amplify the light pulses travelling down the fibre. The spacing between amplifiers depends on many factors such as the pulse or ‘bit’ duration, the total length of the cable and the total design capacity of the fibre. For PPC-1 the amplifier spacing is approximately 93km.
Separate fibres are used for each direction of transmission, so for bi-directional communication, a pair of fibres (and their associated amplifiers) is required.
Last week while at Tyco Telecom’s Labs, we undertook interoperability testing between the Foundry MLX routers used in PIPE Networks’ domestic MPLS network, and the Tyco Telecom SLTE equipment used to terminate the submarine cable system.
This interop testing is designed to establish that the Foundry MLX routers operate correctly over the Tyco Telecom submarine equipment, enabling PIPE Networks to extend its domestic MPLS network internationally once PPC-1 is complete.
During the testing process we tested the performance of PIPE Networks’ standard MPLS products over a lab-based submarine system using STM-16 (2.5gbps), STM-64 (10gbps), 10G LAN PHY and 10G WAN PHY interfaces. Importantly, in the lab we were able to introduce realistic levels of latency to assess the impact of nearly 7000km of fibre network between the routers.
The actual performance testing was done using a Spirent Smartbits network analyser with two 10 gigabit interfaces, with a couple of the tests being repeated on an Agilent N2X tester to confirm the results.
The above photo is an example of the optical repeaters to be used on PPC-1. The repeater will contain two fibre pairs on the trunk between Sydney and Guam. They’re spaced at regular intervals along the cable and their main task is to amplify the transmission signal as it propagates down the fibre. The repeaters are powered by a DC 1A current running down the copper conductor of the cable.
PPC-1 uses DC power feeding equipment (PFE) located in Sydney and Guam. The cable, as previously described, contains a central copper conductor which is insulated from the sea water by a layer of polyethylene. The PFEs are basically reliable and very stable high voltage DC power supplies. These devices power the submerged repeaters along the cable from Sydney to Guam. To complete the DC electrical circuit between Sydney and Guam, a sea earth return is used. This is shown in orange in the figure above.
The Ocean Ground Bed provides the connection from the PFE to the sea earth at a site normally adjacent to the landing point so that a low resistance to earth can be obtained. We typically would like to see resistances lower than 1-2 ohms for a “good” earth. The OGB itself is made up of a series of special long lasting electrodes bored 500-600 mm into the water table. In order to get the required resistance additional electrodes are installed in parallel until the required resistance is achieved.
A key piece of technology used on PPC-1 is the Optical Add Drop Multiplexing Branching Unit (OADM BU). The OADM BU allows PPC-1 to service thin route add/drops off one fibre pair without the added cost of back to back Submarine Line Terminal transponders at intermediate points (for example at Madang in the figure above).
A second advantage is that “express” traffic never needs to be routed over the spur cables so is, by definition, more secure (i.e. stays in deep water).
A third advantage is that for restoration purposes some protection/restoration traffic can be routed to the spurs as required and this can be done without interrupting the main express traffic from Sydney to Guam. The figure depicts a simple example of using an OADM BU to connect Madang.
The submarine cable will be supplied and installed by Tyco Telecommunications.
Also known as the wet segment, the submarine cable contains from 4 to 12 optical fibres which operate in pairs (2 – 6 pairs). The exact composition of PPC-1 will vary along the route to Guam due to changing conditions. Two examples of the types of cable are shown above. The first Lightweight (or LW) cable is installed on benign seabeds while Double Armour (DA) is installed over more aggressive seabed conditions.
PPC-1 will use the latest Gen 3 transmission technology from Tyco Telecom.
Fibre Optic Undersea Cable
The cables to be used on PPC-1 come from the SL17 (17mm) family. Tyco Telecom has manufactured many thousands of kilometres for numerous large cable systems.
The repeater amplifies the optical signal so that it can be transmitted over long distances. Tyco Telecom 980-nanometer undersea repeaters use state-of-the-art Erbium-doped optical amplifier technology to achieve high performance and high reliability to transmit multiple-wavelength signals on the fibre pairs over transoceanic distances. The current repeater design can accommodate one to eight optical amplifier pairs in two physical designs.
Undersea Branching Units (BU)
A Branching Unit acts like a fork in the road as it provides a means to divide a single cable into two cables. This functionality allows for the creation of “express routes” and “local routes”; the “express” route normally consists of a direct path, while the “local” route provides access to other cable stations. Both Power Switched and Passive BUs incorporate robust electrical and mechanical technology used since the beginnings of optical undersea systems.
Submarine Line Terminating Equipment (SLTE)
The SLTE create the high quality, multiplexed optical signal suitable for transoceanic transmission.
The main components of SLTE are:
- 10 Gb/s High Performance Optical Equipment (HPOE) – provides special grooming of the optical line signal to enable transmission over distances in excess of 12,000 km without regeneration.
- Wavelength Termination Equipment (WTE) – provides the Wavelength Division Multiplexing (WDM) and Wavelength Division Demultiplexing (WDD) functions.
- Terminal Line Amplifier (TLAs) – provides output & input amplification of the transmitted & received optical signal, respectively.
Line Monitoring Equipment
The optical line monitoring system provides in-service performance monitoring and out-of-service fault location for our undersea cable and repeaters.
Power Feed Equipment (PFE)
The PFE are used to power the undersea repeaters from shore. Two HV PFE’s one in Sydney, the other in Guam will provide up to 12,500 volts at at up to 1.6 Amps, sufficient for the single-end power feeding of the PPC-1 system.
Further information can be found at www.tycotelecom.com