Fiber Optics Characteristics
Fiber optic technology uses light pulses to transport digital information from one point to another. Specifically, fiber optics are thin filaments of glass through which light beams are transmitted. Advantages of fiber include:
- High information carrying capacity (bandwidth)
- Very low error rates
- Total insensitivity to electromagnetic interference
- Great immunity to eavesdropping
Alaska United includes 67 optical amplifiers (also called EDFAs, Erbium-doped Fiber Amplifiers). These undersea amplifiers are integrated into the cable and are powered by “constant-current” PFEs (power feed equipment) located at the three cable landing stations. These EDFAs amplify the optical signal without any conversion to an intermediate electrical signal.
Alaska United network system has been designed to support WDM (wavelength division multiplexing) whereby four separate wavelengths (colors) each can carry an OC-192 (optical carrier level 192 which is 9.95328 Mbps or 129,024 traditional voice circuits). Alaska United carries one OC-192 signal. Each fiber pair can be upgraded incrementally at OC-192 signal levels by adding shore-based electronics without changing the wet plant.
Alaska United has a minimum of four fiber strands in all cable cross-sections (i.e., 2-fiber ring in a common cable sheath). PSBUs (power-switched branching unit) are included in Alaska United – East and Alaska United – West providing a common point where three undersea cable segments are joined. The PSBU provides the ability to remotely switch the power configuration of the cable and thereby enhance system reliability. The SONET terminal equipment can interface standard asynchronous and synchronous circuit types including: DS-1, DS-3, OC-1, OC-3, OC-12, OC-48, OC192 and EC-1 (electrical carrier level 1). SEAFAST cable has 24 to 36 fibers in each cable.
SONET stands for Synchronous Optical NETwork and is a fiber optic transport standard developed by ANSI (American National Standard Institute). SONET has the key advantages of providing high-capacity fiber optic transport, defines a system of synchronous signal levels, includes a high-level of OAM&P (Operations, Administration, Maintenance, and Provisioning) capability, supports automatic protection switching, allows a high-degree of interoperability between different vendor platforms, etc.
Unlike most terrestrial SONET systems, subsea systems are often configured as a “collapsed ring.” A collapsed ring means that a 2-fiber transmission ring is contained in a single common cable sheath. The reason for this is the relatively high capital cost of subsea systems (i.e., it would nearly double the capital cost to lay two diversely routed cables). This is why the route selection and installation of a subsea system is so critical to prevent damage to the cable. Since Alaska United is a SONET system it includes automatic protection switching (APS) to protect against failure of any individual electronic or fiber optic component.
BER or Bit Error Rate
BER refers to the ratio of error bits to the total number of bits transmitted. Alaska United is an astoundingly small one bit error occurring every 10,000,000,000,000 bits sent (this is three to four orders of magnitude better than satellite or microwave).
Alaska United network has full “route diversity” between Anchorage, Seattle, Juneau, Ketchikan, and Warrenton. Fairbanks service is diversely routed with fiber along pipeline corridor and fiber along Parks Highway.
The primary causes of cable failures are external aggressions such as bottom fishing, ocean currents and geological events. However, 95 percent of submarine cable failures worldwide are attributable to fishing activities.
To guard against these factors, Alaska United was plow buried from shore to a depth of 4,900 feet or greater except in rocky areas where bottom conditions don’t permit burial or bottom contact fishing. Additionally, the cable route was selected to avoid as many external aggression factors as feasibly possible. Extensive research of fishing activities and practices was completed to avoid busy fishing areas. Cable armoring and burial is in areas where potential fishing is unavoidable. GCI ensures that the Alaska United cable route is identified as a “cable protection route” on nautical charts. GCI also actively communicates with the fishing industry and monitor fishing activities to decrease the possibility of damage to the cable.