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[E309]Ethernet To Fiber Optic

by Colin Yao, Col

The SONET standards were developed in the mid-1980s to take advantage of low-cost transmission over optical fibers. It defines a hierarchy of data rates, formats for framing and multiplexing the payload data, as well as optical signal specifications(wavelength and dispersion), allowing multi-vendor interoperability.

SONET is sometimes described as "T-1 on steroids". Why is that? As we know, the digital hierarchy (DS-0, DS-1, DS-2, DS-3 and more) was created to provide cost-effective multiplexed transport for voice and data traffic from one location in a network to another.

SONET (Synchronous Optical Networking, used in the United States) and SDH (Synchronous Digital Hierarchy, used in Europe and other parts of the world) are two equivalent multiplexing protocols for transferring multiple digital bit streams using lasers or LEDs (light-emitting diodes) over the same optical fiber.

SONET and SDH were designed to replace PDH (Plesiochronous Digital Hierarchy) system to eliminate the synchronization problems that PDH had. SONET is synchronous, which means that each connection achieves a constant bit rate and delay. For example, SDH or SONET may be utilized to allow several Internet Service Providers to share the same optical fiber, without being affected by each others traffic load, and without being able to temporarily borrow free capacity from each other.

SONET and SDH are considered to be physical layer protocols since they offer permanent connections and do not involve packet mode communication. Only certain integer multiples of 64kbits/s are possible bit rates.

SONET is actually TDM(time division multiplexing) based and this makes it readily supports fixed-rate services such as telephony. Its synchronous nature is designed to accept traffic at fixed multiples of a basic rate (64kbit/s), without requiring variable stuff bits or complex rate adaptation.

The SONET data transmission format is based on a 125us frame consisting of 810 octets, of which 36 are overhead and 774 are payload data. The basic SONET signal, whose electrical and optical versions are referred to as STS-1 and OC-1, respectively, is thus a 51.84Mb/s data stream that readily accommodates TDM channels in multiples of 8 kb/s.

SONET can be used to encapsulate PDH and other earlier digital transmission standards. It can also be used directly to support either ATM (Asynchronous Transfer Mode) or Packet over SONET/SDH (POS) networking. So SONET/SDH is actually a generic all-purpose transport container for moving both voice and data traffic. They in themselves are not communications protocols.

SONET brings with it a subset of advantages that makes it stand above competitive technologies. These include mid-span meet, improved operations, administration, maintenance, and provisioning (OAM&P), support for multipoint circuit configurations, non-intrusive facility monitoring, and the ability to deploy a variety of new services.

Improved OAM&P is one of the greatest contributions that SONET brings to the networking field. Element and network monitoring, management, and maintenance have always been something of a catch-as-catch-can effort because of the complexity and diversity of elements in a typical service provider's network. SONET overhead includes error-checking ability, bytes for network survivability, and a diverse set of clearly defined management messages.

There are mandatory tests for every stage of the design, manufacture, and installation of the fiber optic components, link, cable plant, and network.

Most fiber optic test procedures have been thoroughly tested and codified as industry standards. These standards are part of EIA/TIA RS455 and are being adapted into IEC standards.

Most of these standards refer to tests of individual components under a variety of environmental conditions; therefore, only a few related to testing the performance of the installed cable plant.

With data communication networks, we are primarily concerned with three fiber optic test procedures: FOTP-95 for measurements of optical power, FOTP-171 for testing patch cables and OFSTP-14 for testing the loss of the installed cable plant.

Optical Fiber Testing

A great many tests must be performed on optical fibers. A fiber manufacturer must test a fiber to determine the characteristics by which the fiber will be specified. As a quality control measure during manufacture of fibers, the manufacturer must constantly test the fibers to ensure that they meet the specifications. Among the tests are the following: core diameter, cladding diameter, numerical aperture (NA), attenuation, refractive index profile and tensile strength.

Other tests performed on fibers or on fiber optic cables concern their mechanical and environment characteristics. Mechanical tests such as impact resistance, tensile loading, and crush resistance test the cable's ability to withstand physical and mechanical stresses. Environmental tests evaluate the changes in attenuation under extremes of temperature, repeated changes of temperature, and humidity.

Fiber Optic Cable Plant Testing

There are some testing concerns in a premises-cabling application based on the recommendations found in TIA/EIA-568A for generic cabling systems and TSB-67 on testing.

An important concept in testing is distinguishing between the link and the channel. The channel is the end-to-end system, including any patch cables at the equipment or work area. The link is the "behind-the-walls" cabling from the equipment-side patch panel to the work-area-side outlet.

Testing can be done on either a link level or a channel level. Installers of a building cabling system are usually concerned with the link. The link forms the basic infrastructure for the building cabling.

Fiber Optic Component Testing

Fiber optic data links are composed of three components: a transmitter, a receiver and the interconnection cable plant. These components must be compatible with the parameters of the intended application.

The loss budget must be adequate for the expected loss in the cable plant and the dynamic characteristics must meet the bandwidth requirements set by the network data transmission rate.

The loss budget is set by the output power of the transmitter and the sensitivity of the receiver. On a static basis, this is determined by the difference in optical power levels, but in reality the issue is a dynamic one, determined by the performance of the components at the data transmission rate of interest.

Continuity Testing

Simple continuity testing can be achieved by a flashlight: Does the light come through the fiber? Fancy flashlights ? called visual continuity testers ? are available specifically for fiber optic testing. A read light is easiest to see.

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