[ 5 pages — 2,924 words]


INTRODUCTION

Media converters are devices that allow for interconnecting two different cabling technologies. With Ethernet there are three types of cables — coaxial, twisted-pair and fiber optic. With modern Ethernet systems, the most common need is to connect a twisted-pair (copper) segment together with a fiber optic segment and there are media converters available to do the job. Fiber optics offer greater distances, galvanic isolation, and immunity to high levels of electromagnetic interference and lightning strikes. Product selection would seem to be easy — just make sure that connectors are compatible and select the lowest price. However, not all media converters operate the same and supplying a media converter requires some planning since an improperly installed media converter will degrade a system or simply will not function. This document reviews the issues of applying media converters to an Ethernet system.


What is an Ideal Media Converter?

The ideal media converter would provide a transparent conversion from one medium to another without introducing data latency. However, conversion electronics within devices introduce data latency. Physical layer devices such as repeating hubs introduce little data latency while switching hubs introduce much more due to the store-and-forwarding of frames. A media converter is a physical-layer device and therefore should introduce little data latency. Another trait of a media converter is transparency. Ideally, all data generated on the copper side should appear on the fiber side. The same should be said of data generated on the fiber side — it should faithfully appear on the copper side. This applies to special signals such as Link Integrity bursts and support for the Auto-Negotiation protocol. Media converters need to support these special functions as well. As a transparent device, the data rate on one side must be the same as the data rate on the other side. In other words, there can be no buffering of data within an ideal media converter.



A media converter allows for a simple conversion from one medium
to another such as twisted-pair to fiber.

A media converter does more than simply provide a compatible physical connection between two types of cabling technologies. The signaling on the two media could be different so active electronics, and an associated power source, are needed to make this signaling conversion. This also means that media converters are unique to the networking technology involved. Ethernet, ARCNET® and Controller Area Network media converters are all different and, therefore, incompatible with one another. We will address the unique requirements of Ethernet media converters that convert twisted-pair cabling to that of fiber optic cabling since that is the most popular need. Twisted-pair to fiber optic conversion can occur in a repeating hub or switching hub, but operation within a hub differs from a stand-alone media converter — as will be discussed later. We will start by reviewing the twisted-pair and physical-layer standards for Ethernet. We will focus our discussion on 10 Mbps and 100 Mbps, and twisted-pair and fiber optic cabling.


10BASE-T

This is the original twisted-pair standard for Ethernet with operation at 10 Mbps using two twisted-pairs. Two pairs allow for full-duplex operation when a switching hub is employed. With a repeating hub, only half-duplex is possible. Manchester encoding is used over the wires and cable length is limited to 100 m. Category 3 (CAT 3) or better cable must be used on 10 Mbps, Manchester-encoded systems. The 10BASE-T standard introduced the Link Integrity function to verify that a valid link exists. This is accomplished using a special signal called the Normal Link Pulse (NLP) that is sent in the absence of data. RJ-45 connectors are used along with a standardized pin-out.



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