Incorporating Media Converters, Course 407

Applying Media Converters

We are going to study some media converter applications assuming we are using one of the more sophisticated devices that are capable of operating in different modes — depending upon configuration.

Forced 10 Mbps Mode

Most media converter applications require operation at only one speed. When inserting a media converter between a 10BASE-T port and a 10BASE-FL port, the media converter must operate at 10 Mbps and have 850 nm transceivers. For this situation, the media converter could be forced to operate at 10 Mbps. Auto-negotiation is disabled. This situation represents the simplest of applications.

Forced 100 Mbps Mode

This application is similar to the above, but this time we want to connect at 100 Mbps speed. On the copper side is a 100BASE-TX port with its speed set to 100 Mbps. On the fiber side is a 100BASE-FX switch port with its data rate set for 100 Mbps and its duplex set to either full- or half-duplex. Therefore, 1300 nm transceivers must be installed on the media converter. Auto-negotiation is disabled. (The switched fiber port does not generate auto-negotiation signals anyway.) Modern media converters usually support full-duplex transmissions as their inherent behavior. Therefore, it is not necessary to set the media converter to full-duplex. Half- or full-duplex communication is possible over a full-duplex link, so in this case the media converter is transparent for duplex issues. The Forced 100 Mbps Mode will work as long as the copper side device is fixed to 100 Mbps.

Non-Transparent Mode

Let's assume the above situation except that the copper port is set for auto-negotiation. If we left the media converter in the Forced 100 Mbps Mode, which does not auto-negotiate, the device on the copper side will not increase its speed to 100 Mbps from its default 10 Mbps speed. With one side at 10 Mbps and the other at 100 Mbps, communication is not possible. If we do not want to put restrictions on the copper device, we need to change the operation of the media converter itself. Ideally, we would like the media converter to facilitate auto-negotiation, but we cannot do that when the fiber side does not auto-negotiate — we do not know speed or duplex.

Actually, we do know speed on the fiber side. We could parallel detect the speed by observing the valid link condition which differs between 10 Mbps and 100 Mbps. Once we know the fiber speed, we could communicate this to the copper side. We could participate in the auto-negotiation process on the copper side as if we were the link partner. We still do not know the duplex and we need some manual intervention (usually through a switch setting on the media converter) to select duplex. Remember that the media converter inherently functions in full-duplex mode so duplex selection is only "advertised" to the copper side as part of the auto-negotiation process. Thus, nothing different happens within the media converter. In our example, we know we have a full-duplex port so we would set the media converter to "advertise" full-duplex. This mode of operation is called Non-Transparent since auto-negotiation is possible on the copper side but not on the fiber side.

When the media converter is operating in non-transparent mode, the media converter's copper output port is disabled, but the output of the fiber side is not. This allows the connected fiber device to "hear" the equivalent auto-negotiation signals sent by the copper device. Once the media converter recognizes that signals are being sensed on both the media converter's copper and fiber optic inputs, it enables its copper port output in order to participate in the auto-negotiation process. If the media converter receives FLP bursts on the copper side, it will send out its own FLP to the copper partner advertising the speed and duplex of the fiber side. In this way it is operating as a proxy for the fiber side. Once auto-negotiation is complete, communication begins at the required 100 Mbps data rate.

Transparent Mode

The transparent mode is used when both copper side and fiber side devices are capable of auto-negotiation. In this mode, the media converter functions exactly like the non-transparent mode except the media converter does not proxy the information from the fiber side. Instead, information from both link partners is transferred as if the media converter were not present. In other words, the media converter is transparent. In this mode, full support of 100BASE-SX is possible as long as 850 nm transceivers are used. However, transparent mode is not restricted to just 100BASE-SX.

For example, assume two back-to-back media converters operating in transparent mode but using 1300 nm transceivers. It is desired to extend the distance between two copper switch ports so we use media converters at each end with the fiber sides connected together. The two copper switch ports would auto-negotiate, and the media converters would simply pass the information. In this situation, something interesting could happen. What if the two copper switch ports negotiated 10 Mbps? Would communication over fiber using 1300 nm transceivers be possible? No matter what transceivers are used, as long as they are the same, communication is possible regardless of data rate. Although 1300 nm transceivers operating at 10 Mbps is not covered in the IEEE standards, it will still work. The resulting link is just termed proprietary.