Optical Networking Glossary

Optical Units Reference

Multipliers, Optical Power, Gain & Loss, Wavelength & Frequency, and Bandwidth & Optical Modulation Bandwidth

Optical Networks

Optical fibers connecting locations many miles apart, and carrying information in the form of on-off flashes of laser light

Optical Fiber

Very thin strands of pure silica glass through which laser light travels in an optical network

Optical Modulation

The switching on and off of laser light in order to represent information in an optical network

Wavelength Division Multiplexing (WDM)

Transmitting many different colors (wavelengths) of laser light down the same optical fiber at the same time, in order to increase the amount of information that can be transferred

Formatting for Transmission

The conversion of data into 1s and 0s, and the different ways in which these signals can be transmitted through optical fiber

Protocol Basics

Forms of communication among network devices to enable the exchange of information

Synchronous Digital Hierarchy (SDH) and Synchronous Optical NETwork (Sonet)

Physical-layer protocol that frames data for fast and reliable transmission over optical fiber

Internet Protocol (IP)

A network-layer protocol that puts Internet data into packets and helps them to be routed over interconnected local area networks (LANs)

Multiprotocol Label Switching (MPLS)

A protocol for the swift routing of data streams, bringing improved performance and quality-of-service options to IP traffic

Asynchronous Transfer Mode (ATM)

A data-link layer protocol allowing integration of voice and data with the ability to provide quality-of-service guarantees

Ethernet

A data-link layer protocol commonly used to transfer Internet Protocol (IP) packets over cables in local area networks (LANs)

Digital Wrappers and Forward Error Correction (FEC)

An encapsulation method for traffic of all protocols, allowing an optical network to be protocol transparent, its functions to be managed in an efficient manner, and its data to be transmitted more reliably

Laser Basics

Devices giving out intense light at one specific color

Distributed Feedback (DFB) Lasers

Lasers giving out a very sharply defined color of light

Tunable Lasers

Lasers that can be adjusted to emit one of several different wavelengths

Vertical Cavity Surface Emitting Lasers (VCSELs)

Lasers that emit light from their surface in contrast with regular 'edge emitters'

Optical Amplification

Boosting of an optical signal without any conversion of the light into an electrical signal

Erbium Doped-Fiber Amplifiers (EDFAs)

Optical amplifiers made of short lengths of optical fiber doped with the element 'erbium'. A laser excites erbium ions in the fiber, which can then give their energy to optical signals passing through

Synchronous Digital Hierarchy

Synchronous Digital Hierarchy The Synchronous Digital Hierarchy (SDH) defines the transmission infrastructure that will be used in international carrier networks outside North America for the next generation. Over time it will replace the existing Plesiochronous Digital Hierarchy (PDH) which is currently in place. Initially, SDH facilities will be used to transport signals from the existing digital hierarchy operating at rates of approximately 2 Mb/s, 34 Mb/s, and 140 Mb/s. The SDH is based on a hierarchy of transmission rates. The core rate is the Synchronous Transport Module Level 1 (STM-1) rate of 155.52 Mb/s. Currently, SDH also defines rates of STM-4, STM-16, STM-64, and STM-256, which operate at 4, 16, 64, and 256 times the STM-1 rate, respectively. Higher rates may be supported in the future. The North American counterpart of the SDH is called the Synchronous Optical Network (SONET). SONET supports the same family of transmission rates as are defined for SDH. In addition, in order to accommodate the North American PDH rate of DS-3 (45 Mb/s), SONET also defines a channel rate of 51.84 Mb/s, which is one-third of the STM-1 rate. SDH Transmission Functions Currently, SDH transmission functions are performed by three basic types of transmission equipment: terminal multiplexes (TMs), add/drop multiplexes (ADMs), and Digital Cross-connect Systems (DCSs). TMs aggregate lower-rate signals and multiplex them to an STM-N rate for transport to a network service node. Today, TMs would typically operate at the STM-1 rate. ADMs are used at intermediate points on a transmission span to drop off subrate signals for local distribution, as well as to pick up subrate channels which originate at that local switching office. ADMs are often used on fiber rings for subrate traffic distribution. Technologically, TMs and ADMs are very similar, with the major difference that the ADM supports two STM-level interfaces to provide both input and output functions while the TM supports only a single multiplexed interface. The SDH Digital Cross-connect System is used to switch/rearrange STM-level signals on an administrative basis. The STM-1 channel structure has three components: Virtual Container (VC-4), Section Overhead (SOH), and Path Overhead (POH). The Virtual Container provides the information-carrying capacity of the facility. The Virtual Container is treated by intermediate SDH equipment (e.g., an SDH DCS) as a single contiguous channel operating at 150.34 Mb/s. The SOH is a 5.18 Mb/s channel, which is used by the carrier to manage the SDH facility. Functions such as framing, error detection, protection switching, timing adjustments, and a variety of management features are defined for the SOH. The POH is a 576 Kb/s end-to-end management channel. It is used to pass management and operations information between the equipment which terminates the SDH span. The Virtual Container of the STM-1 is flexible in that it can carry multiple subrate signals from both the North American and international transmission hierarchies. STM-1 is also one of the rates defined by the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) for Broadband ISDN (B-ISDN). Accommodating Existing Digital Hierarchies The new transmission infrastructure defined by the SDH can only be acceptable if it can also accommodate the existing digital hierarchies defined throughout the world. To carry the existing digital signal hierarchy, SDH defines Tributary Unit Groups (TUGs). A TUG is a 6.912 Mb/s time division subchannel of the VC-4. Twenty-one TUGs can be carried in an STM-1.

Mappings are defined from each of the currently supported international and North American digital transmission rates into the appropriate TUG. For example, for the European 2.048 Mb/s rate, the TU-12 mapping has been defined. A TU-12 carries each of three 2.048 Mb/s signals plus overhead in a 2.304 Mb/s container. In this way an STM-1 is capable of carrying a total of sixty-three 2.048 Mb/s multiplexed streams. Different TUG types can be mixed in the same STM-1, but different TUs cannot be mixed in the same TUG. For signals operating at rates higher than STM-1 (e.g., STM-4), time division multiplexing (TDM) of the STM-1s is used. Each of the four STM-1s is treated as an independent time division bandwidth. Each of the channels has its own independent Section Overhead and Path Overhead. At network cross-connect points the STM-1s may be routed in different directions in the network. To create channels with individual payloads greater than 150 Mb/s, SDH introduces the concept of concatenation. In a concatenated interface, the individual STMs are joined to form a super-rate channel. For example, four STM-1s can be concatenated to form a single, contiguous time division channel of 622.06 Mb/s. This concatenated signal is called an STM-4c. Intermediate SDH transmission equipment (e.g., a DCS) will treat the STM-4c as a single channel with one SOH. The end equipment will use a single Path Overhead channel for end-to-end management of the entire bandwidth.

What is a Network? A network is simply a group of two or more Personal Computers linked together. What Types of Networks Exist?Many types of networks exist, but the most common types of networks are Local-Area Networks (LANs), and Wide-Area Networks (WANs). In a LAN, computers are connected together within a "local" area (for example, an office or home). In a WAN, computers are farther apart and are connected via telephone/communication lines, radio waves, or other means of connection.

A network is simply a group of two or more Personal Computers linked together.

What Types of Networks Exist?Many types of networks exist, but the most common types of networks are Local-Area Networks (LANs), and Wide-Area Networks (WANs). In a LAN, computers are connected together within a "local" area (for example, an office or home). In a WAN, computers are farther apart and are connected via telephone/communication lines, radio waves, or other means of connection.

How are Networks Categorized?Networks are usually classified using three properties: Topology, Protocol, and Architecture. Topology specifies the geometric arrangement of the network.