Fiber-optic cabling uses light-emitting diodes (LEDs) and lasers to transmit data. With this transmission, light is used to represent binary 1’s and 0’s: if there is light on the wire, this represents a 1; if there is no light, this represents a 0. Fiber-optic cabling is typically used to provide very high speeds and to span connections across very large distances. For example,
speeds of 100Gbps and distances of over 10 kilometers are achievable through the use of fiber—copper cannot come close to these feats. However, fiberoptic cabling does have its disadvantages: it is expensive, difficult to troubleshoot, difficult to install, and less reliable than copper.
Two types of fiber are used for connections: multimode and single-mode. Multimode fiber has a fiber thickness of either 850 or 1300 nanometers (nm), and the light signal is typically provided by an LED. When transmitting a signal, the light source is bounced off of the inner cladding (shielding) surrounding the fiber. Multimode fiber can achieve speeds in the hundreds of Mbps range, and many signals can be generated per fiber. Single-mode fiber has a fiber thickness of 1300 or 1550 nm and uses a laser as the light source. Because lasers provide a higher output than LEDs, single-mode fiber can span over 10 kilometers and have speeds up to 100Gbps. With single-mode fiber, only one signal is used per fiber.
The last few years have seen many advances in the use and deployment of fiber. One major enhancement is wave division multiplexing (WDM) and dense WDM (DWDM). WDM allows more than two wavelengths (signals) on the same piece of fiber, increasing the number of connections. DWDM allows yet more wavelengths, which are more closely spaced together: more than 200 wavelengths can be multiplexed into a light stream on a single piece of fiber.
Obviously, one of the advantages of DWDM is that it provides flexibility and transparency of the protocols and traffic carried across the fiber. For example, one wavelength can be used for a point-to-point connection, another for an Ethernet connection, another for an IP connection, and yet another for an ATM connection.
Use of DWDM provides scalability and allows carriers to provision new connections without having to install new fiber lines, so they can add new connections in a very short period when you order them.
Let’s talk about some of the terms used in fiber and how they affect distance and speed. First, you have the cabling, which provides the protective outer coating as well as the inner cladding. The inner cladding is denser to allow the light source to bounce off of it. In the middle of the cable is the fiber itself, which is used to transmit the signal. The index of refraction (IOR) affects the speed of the light source: it’s the ratio of the speed of light in a vacuum to the speed of light in the fiber. In a vacuum, there are no variables that affect the transmission; however, anytime you send something across a medium like fiber or copper, the media itself will exhibit properties that will affect the transmission, causing possible delays. IOR is used to measure these differences: basically, IOR measures the density of the fiber. The more dense the fiber is, the slower the light travels through the fiber.
The loss factor is used to describe any signal loss in the fiber before the light source gets to the end of the fiber. Connector loss is a loss that occurs when a connector joins two pieces of fibers: a slight signal loss is expected. Also, the longer the fiber, the greater the likelihood that the signal strength will have decreased when it reaches the end of the cable. This is called attenuation. Two other terms, microbending and macrobending, describe signal degradation.
Microbending is when a wrinkle in the fiber, typically where the cable is slightly bent, causes a distortion in the light source. Macrobending is when there is leakage of the light source from the fiber, typically from a bend in the fiber cable. To overcome this problem over long distances, optical amplifiers can be used. They are similar to an Ethernet repeater. A good amplifier, such as an erbium-doped fiber amplifier
(EDFA), coverts a light source directly to another light source, providing for the best reproduction of the original signal. Other amplifiers convert light to an electrical signal and then back to light, which can cause a degradation in signal quality.
Two main standards are used to describe the transmission of signals across a fiber: SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy). SONET is defined by the Exchange Carriers Standards Association (ECSA) and American National Standards Institute (ANSI) and is typically used in North America. SDH is an international standard used throughout most of the world (with the exception of North America). Both of these standards define the physical layer framing used to transmit light sources, which also includes overhead
for the transmission. There are three types of overhead:
■ Section overhead (SOH) Overhead for the link between two devices, such as repeaters
■ Line overhead (LOH) Overhead for one or more sections connecting network devices, such as hubs
■ Path overhead (POH) Overhead for one or more lines connecting two devices that assemble and disassemble frames, such as carrier switches or a router’s fiber interface
Typically, either a ring or point-to-point topology is used to connect the devices. With carrier MAN networks, the most common implementation is through the use of rings. Autoprotection switching (APS) can be used to provide line redundancy: in case of failure on a primary line, a secondary line can automatically be utilized.
Table 2-1 contains an overview of the more common connection types for SONET and SDH. Please note that SONET uses STS and that SDH uses STM to describe the signal.
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