The need for speed.

Advances in media converter technology have focused on two major directions-Layer 1 media converters (10 Mbps, 100 Mbps and 1,000 Mbps) and Layer 2 media converters (10/100 Mbps--10/100Base-TX to 100Base-FX, as well as 10/100/1,000 Mbps-10/100/ 1000BASE-T to 1000BASE-SX or LX). Each offers different benefits and each is designed for different applications.

Straight 10-, 100- or 1,000-Mbps media converters operate in the Layer 1 of the open systems interconnection (OSI) model. OSI Layer 1 defines the most basic way for the two hardware devices to communicate at the mechanical and electrical or optical level.

Consequently, Layer 1 media converters offer a physical transport conversion, converting electrical to optical signals defined by Ethernet protocol. They do not look into the Ethernet packet, where it is going or what it contains. They do not buffer the packets and do not use store-and-forward technology. These media converters are capable of auto-negotiation and offer fast copper-to-fiber conversion regardless of packet size. Finally, the converters are compatible with the 10/100, 10/100/1,000 switches or router ports on either side of the link, as long as the speeds are matched.

10/100-Mbps or 10/100/1,000-Mbps media converters, operating in OSI Layer 2, can help determine a network topology, physical addressing, order of frames, error notification and flow control. Layer 2 converters are capable of looking at the Ethernet frame, and use store-and-forward technology, allowing for the buffering of the received packet in its entirety before forwarding it to the fiber port.

Layer 2 converters offer users a reliable distance extension in a half-duplex environment, as well as a universal media converter solution for any Ethernet topology. They introduce, however, an additional delay associated with the store-and-forward concept. The more store-and-forward devices on the link, the more latency is introduced to the link. The delay is generally negligible in gigabit networks, but becomes more of an issue in 10- or 100-Mbps networks.

The following scenarios explore several LAN deployments:

10/100 switch connected to another 10/100 switch over fiber. Using 100-Mbps media converters in this case lowers the latency, because the packets going through the converters are not buffered. Minimizing latency in a network is critical, even more so with new voice and video over IP applications. Since both ends of the link can be configured to full duplex, worrying about the collision domains is not necessary, and 10/100 media converters are not needed, unless network administrators see the possibility of downgrading the link speed to 10 Mbps. For those applications requiring the ability to pass jumbo packets, Layer 1 converters are the only option, as 10/ 100 or 10/100/1,000 solutions normally have a maximum packet size constraint of 1,536 bytes.

Fiber switch to 10/100 NIC in fiber-to-the-desktop application or voice-over-IP scenario, with the IP phone on the remote end. A straight 100-Mbps converter can be connected directly to the fiber switch using fiber, and the NIC card can be connected to the converters via a 100Base-TX port. The benefits of straight 100 converters are identical to the ones presented in the first scenario.

Fiber switch to a hub. Whenever a hub needs to be connected to a fiber on a Fast Ethernet or gigabit switch, a 10/100 or 10/100/1,000 media converter should be used. They bridge the speed gap and allow 10-Mbps copper to talk to a 100-Mbps or gigabit fiber port on the switch. They also offer an upgrade path for the hub side of the link. Furthermore, they separate collision domains and allow users to limit an impact of the 512-bit time rule to a small segment of their network, allowing them to extend a distance via a full duplex link.

Copper 10/100/1,000 switch to a "mix of products" over a fiber link. When connecting numerous devices to a 10/100/1,000 switch over fiber, selecting one converter type that fits all the applications may be advantageous. The Layer 2 10/100/1,000 media converters will allow integration of all legacy Ethernet devices over a fresh gigabit fiber link. This converter will also allow the user to gradually update the remote devices to gigabit at the pace dictated by the budget.

Less costly is use of a Layer 1 1,000-Mbps converter next to the 10/100/1,000 switch. This will allow slightly lower project cost and slightly improve latency.

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This article was provided by Sev Sadura, product manager at Transition Networks, Minneapolis, a wholly owned subsidiary of Communications Systems.