Optical networks are high-capacity telecommunications networks based on optical technologies and components that provide routing, grooming, and restoration at the wavelength level as well as wavelength-based services. Fiber-optic systems are now the key solution to linking high-speed LAN and WAN networks within and among buildings. The number of high-speed networking applications using fiber as the physical-layer backbone continues to grow, such as:

• Fiber Distributed Data Interface (FDDI), running at 100Mbps
• Synchronous Optical Network (SONET), running at 155Mbps and 622Mbps
• ESCON, running at 200Mbps
• Gigabit Ethernet, running at 1,000Mbps and 10 Gigabit Ethernet running at 10,000Mbps
• Fibre Channel, running at 1,062Mbps and below
• High-Performance Parallel Interface (HIPPI), running at 1,200Mbps.

To help define network functionality at the physical level, networks are divided into several different physical or virtual layers. The first layer, the services layer, is where the services such as data traffic enter the telecommunications network. The next layer, such as SONET layer, provides restoration, performance monitoring, and provisioning that is transparent to the first layer. Emerging with the optical network is a third layer, the optical layer, which provides the same functionality as the SONET layer, while operating entirely in the optical domain.

The lowest level SONET signal is called the Synchronous Transport Signal Level 1 (STS-1), which has a data rate of 51.84 Mbit/s. Higher data rates are achieved by multiplexing N of these data streams up to a maximum of N=48. SONET uses single-mode fiber and different types of light source depending on the ranges involved. For local area networks (LANs) that use LED sources and multimode fiber the Ethernet standard (3 Mbit/s) is widely used, although when higher data rates are required the Fiber Distributed Data Interface (FDDI) standard is common. This is a 100 Mbit/s channel that through technological developments allows interfacing of a FDDI optical network with twisted-pair cable for final connection to transmit/receive users of the network. In these hybrid systems the electrical connection is typically only the last 100 m or so to the user from the fiber-based part of the system. The standard for interfacing FDDI networks via twisted-pair cables over these typically 100 m connections is called the Twisted-Pair Distributed Data Interface (TPDDI).

Key technologies in an optical network:

• Broadband WDM - The first incarnation of WDM was broadband WDM.
• Optical Amplifiers - The amplifier provided enormous cost savings over electrical regenerators, especially in long-haul networks.
• Dense Wavelength Division Multiplexing (DWDM) - Dense wavelength division multiplexing (DWDM) combines multiple signals on the same fiber, ranging up to 40 or 80 channels.
• Narrowband Lasers - Advanced lasers with narrow bandwidths provide the narrow wavelength source that is the individual channel in optical networks.
• Fiber Bragg Gratings - A fiber Bragg grating is a small section of fiber that has been modified to create periodic changes in the index of refraction.

Network management of optical network is critically important. NMS must manage the overall network architecture and monitor signal performance for each wavelength. With the addition of optical add/drop multiplexers and optical cross-connects, the end-to-end performance of wavelengths becomes more difficult. NMSs for the optical network must assist providers in troubleshooting the network by isolating questionable wavelengths and the possible location of degradation. As the number of wavelengths on each fiber approaches 40 or more, it is important to have an intelligent method to monitor all of them. Finally, it is critical for the NMS to manage and provide new services to customers quickly.

Fiber Optical Communication Network

Related Terms: FDDI, SONET, Gigabit Ethernet, DWDM, WDM, Optical Amplifier, Multiplexer