Linking fiber and copper.

Media conversion tools bridge the gap between different technologies to help future-proof your network.

Steve Stange

When Sly and the Family Stone made a pop chart hit in 1968 out of 'Everyday People,' they couldn't have known how well the lyrics would come to describe the challenges of network integration 30 years later. 'There is a yellow one that won't accept the black one that won't accept the red one ...' is a fairly apt description of networking's complexities. IS personnel must meet today's needs on this year's budget while, at the same time, predicting the network's future migration and outlining a strategy which protects existing investment.

Solutions do exist for building and campus area networks that fall within budgets, support Fast Ethernet applications now and offer upgrade capabilities to Gigabit Ethernet. The seemingly incongruous solution is in a fiber backbone and alternate technology to the desktop. There are, however, multiple choices within fiber technology and challenges inherent in carrying the network through to the desktop. The optimal mixed-media solution is singlemode fiber for the backbone, offering nearly unlimited bandwidth and high reliability, and either copper or multimode fiber to the desktop, with the segments joined by media conversion tools that bridge the gap between disparate technologies.

In the relentless pursuit of bandwidth, fiber optic cabling is being deployed at an increasing rate for good reason. Cabling installed in many environments today cannot support the demands of Fast Ethernet, let alone ATM or Gigabit Ethernet. At Gigabit Ethernet speeds, multimode fiber is susceptible to modal dispersion, or 'noise,' over the wire, which causes blurring and shortening of the data pulses. When you send data at increasingly closer intervals, over multiple paths of light and in varying wavelengths down the fiber, reliability declines and devices at the end of the cable are often unable to read the distorted data.

Singlemode fiber is considerably narrower, its core diameter closer to that of the wavelength of light. Data is sent in a single path of light, eluding modal dispersion problems. Higher speeds are achieved by pushing data through the fiber using a powerful laser transmitter able to pulse quickly enough to achieve Fast Ethernet, ATM, and Gigabit Ethernet data rates. Singlemode fiber also extends distance limitations posed by multimode fiber - up to 20 km (and beyond in some cases) in full duplex vs. 2,000 meters - with greater reliability. For distances over two kilometers, singlemode fiber is not susceptible to the kinds of errors that befall multimode fiber in the same scenario. In the typical building or campus area network, backbone distances can easily exceed two kilometers.

On the down side, singlemode fiber's higher speeds and practically limitless capacity carry a price tag hard to justify for the horizontal network segment. Telecom pundits talk of data rates to the desktop reaching into the realm of FDDI, Gigabit Ethernet, and ATM, but the fact is singlemode horizontal fiber is not required for most organizations and generally is not necessary. For the vast majority of users, the bandwidth available through copper or multimode fiber is sufficient. The challenge becomes establishing connectivity between discrete network technologies and equipment.

Currently, some critical network equipment and very few desktop computers have a fiber network interface. In establishing the connection between distinct technologies or connecting a new device to its distribution frame, conversion technology is the peacemaker of the networking world. A media converter can link a singlemode fiber backbone to Ethernet, Fast Ethernet, token ring, FDDI/CDDI and ATM enterprise protocols.

Media converters, small devices that take in signals from one media type and send it out on another media type, are the vital tools to link the singlemode fiber portion of the network to either multimode fiber or copper. This integration provides smooth connectivity between a singlemode fiber backbone and almost any legacy equipment.

Consider the following scenario:

A device is connected to the distribution frame with twisted pair horizontal cabling. Two distribution frames between the device and the hub are connected by singlemode fiber optic cabling. Alternatives to enable the two devices to communicate include: Laying new cabling between the device and the hub. New network interface cards (NIC) may also need to be added, but many NICs are not available with fiber options and those that are can be expensive. Cost and installation challenges for cable installation and NIC card costs usually prohibit this solution; Replacing the switch/router chassis, modular devices, or stand-alone hubs to provide like interfaces between the new device and the hub. Few of today's stackable routing and switching products support fiber cabling in a cost-effective manner; Using media converters to provide an 'invisible' link between twisted pair horizontal cable and the fiber backbone without requiring new cable or replacement of expensive equipment. For example, in a Fast Ethernet environment, a 100BASE-TX to 100BASE-FX media converter connects a 100BASE-TX twisted pair device to a 100BASE-FX-compliant singlemode fiber port that has either a ST or SC fiber optic connector.

Fast Ethernet is good for now, but how can such a mixed network topology be cost-effectively scaled to Gigabit Ethernet in the future? Using a Gigabit Ethernet media converter enables converting one or more 1000BASE-SX ports on a Gigabit Ethernet switch to 1000BASE-LX for use in campus area networks or other applications requiring the distance advantages of singlemode fiber. It is only necessary to convert those ports required for backbone connection.

In a multimode application, it is often less expensive to purchase a switch with multimode ports and an external-multimode-to- singlemode-media converter than it is to buy a switch with singlemode installed internally. Additionally, some electronics are only compatible with multimode fiber. In such an instance, a singlemode to multimode media converter makes the connection.

With singlemode fiber in the backbone, multimode fiber or copper to the desktop, and media conversion tools providing connectivity between disparate technologies, the network manager can confidently plan for a future network incorporating Gigabit Ethernet and more.

Stange is product manager for Transition Networks, Eden Prairie, Minn.