USING MEDIA CONVERTER TECHNOLOGY FOR COPPER TO FIBRE CONNECTIVITY.

Depending upon the nature of their networks, IT and network administrators present differing opinions on the best new networking technology on the market. Switching? Routing? Fast or Gigabit Ethernet? ATM? However, one thing they can agree on is the fact that today's networks are being stretched by the continual demands placed upon them.

Network complexity and the growing number of nodes on a network are driving the trend to higher data rates and forcing longer distance requirements into the LAN. Additionally, the noise on the network caused by radiation, interference, and cross talk is causing data integrity and security to become critical issues.

All of these factors are driving implementations of fibre optic cabling. But few network managers have the luxury--in time or capital--of starting with a clean sheet of paper and designing a fibre optic network from scratch. Most of the time they have to deal with what's already there and incorporate both new and old. This is where media converters are playing an increasingly important role in networking.

Media converters allow the use of fibre where and when it is needed, effectively integrating new equipment into an existing cabling network.

In the Ethernet world, media converters connect one network medium to another, transparently. Put simply, media converters make one network media type "look" like another media type--without changing the nature of a company's network. For example, company A has a number of PCs and Unix workstations with Ethernet interfaces that work only with 10BaseT (twisted pair) copper wiring. However, company A wants to install a fibre-based or 10BaseFL LAN in the building. It cannot simply throw out computers that are still quite usable. Furthermore, buying new fibre optical Network Interface Cards (NICs) at $750 or more for each machine is not a cost-effective solution. A study by Frost and Sullivan predicts a steady growth rate (in units and revenues) for copper-to-fibre conversion hardware (Fig 1).

Small, cost-effective devices called media converters come to the rescue in these and similar situations involving local networks. Companies do not have to dismantle their infrastructures: media converters allow them to protect and leverage their existing infrastructure, while keeping pace with changing technologies. In a study by the University of Pennsylvania, media converters were transparent, showing no noticeable degradation in network performance. The study recorded delays in the range of 1-2 microseconds for each unit, which are not noticeable by users.

Media converters increase networking flexibility, while decreasing networking costs. These devices offer media connections that readily fit almost any environment and connect all segments of the network, regardless of the media mix: coax, twisted-pair, single-mode, or multi-mode fibre. Converters can be designed to work with any network type, including Ethernet, Fast Ethernet, Gigabit Ethernet, ATM, FDDI, and Token Ring.

Common Applications

A fibre media converter can reliably and inexpensively extend the distance between two 10Base2 devices or two 10BaseT devices up to 2,000 meters. This functionality is possible without the added expense of a repeater.

A converter can also be used to extend the distance between a network switch and a fileserver or between two switches (Fig 2). Using media converters in a 100BaseTX to 100BaseFX back-to-back configuration provides a single method of extending the distance between a full duplex switch and a fileserver up to 2,000 meters. In fact, media converters can function in either half-duplex or full-duplex mode. Full-duplex Ethernet over UTP runs at 20 or 200Mbit/sec, for example, while half-duplex Ethernet over UTP runs at either 10 or l00Mbit/sec. Full duplex Ethernet is especially valuable in linking two switches or connecting a switch to a file server. No adjustments are necessary when using either mode, as a media converter will automatically sense which mode is in operation.

Integrating Data And Telecommunications Across Existing Fibre

While single-mode fibre is most commonly used in telecommunications carrier networks, multi-mode fibre is predominately found in LANs. A special type of media converter called the Long Haul Media Converter (100BaseTX to 100BaseFX) can connect these two environments via single-mode fibre, effectively extending the network distance to 40km, past the 2km limitation of multi-mode fibre.

A media converter allows a great deal of flexibility in Ethernet cabling. Suppose a network administrator wants to realize the advantages of fibre cabling because of its greater maximum distance between nodes, its inherent safety, and its immunity to Radio Frequency and Electromagnetic Interference (RFI and EMI). The administrator wants to install fibre in an engineering lab on the manufacturing floor. However, the administrator doesn't want to rip out all the copper cabling running throughout the rest of the building because of the associated costs (dismantling copper, implementing fibre, and service downtime.) Media converters allow the network administrators to implement fibre where it is needed and still maintain an existing coax or twisted pair infrastructure. With this flexibility, administrators can expand and upgrade their networks incrementally.

As noted, media converters can be used to extend the distances between nodes. When two 10Base2 to 10BaseFL converters, or two 100BaseTX to 100BaseFX converters are used back to back, they provide a simple way to extend the distance between two devices up to 2,000 meters (6,600 feet) without using a repeater. The distance between a switch and a file server can also be extended. However, it is important to note that the 100BaseTX constrains the collision domain distance between two repeaters to 5 meters, and between the repeaters and terminal devices to 100 meters. So, using the 100BaseTX to 100BaseFX media converters will not extend the collision domain. But, in full duplex installations where collisions are not a consideration, longer distances of up to 2,000 meters are possible.

In addition, maximum effective distances depend on the signal attenuation of the cables used and the ambient RF noise in the environment. As noted above, fibre has significant advantages over copper media, not only in bandwidth but also in EMI and RFI immunity. Therefore, it is a good idea to keep twisted pair runs as short as possible and use fibre for extended distances.

Still, these considerations do not outweigh the benefits media converters can provide. The bottom line is that media converters deliver valuable networking flexibility, while decreasing cabling costs. Companies can preserve their investment in older equipment, while at the same time upgrading particular sections of the network for better performance, which is the goal of every network manager.

Stephen Stange is senior product manager at Transition Networks (Minneapolis, MN).