Networking Connectivity
What is full duplex?

Full duplex is a point-to-point, collision-free Ethernet* connection over twisted pair or fiber cabling. It allows independent, simultaneous two-way transmissions in both directions.

Full duplex is primarily useful to extend the distance of an Ethernet where the limitation on length is due to propagation delay, not attenuation. For example, when using a single-mode fiber connection, the media is capable of supporting distances of up to 100 km. However, due to propagation delay, a maximum of 500 meters is allowed for Ethernet, since the round-trip delay time must be less than 5.12 (seconds. Full duplex removes the restriction on length due to propagation delay, since a collision cannot occur in full duplex.

Full duplex switching is a very simple way to improve server performance. Bandwidth is improved to the server since both transmission and reception can occur simultaneously. If server reads and writes are symmetric, 200 Mbps bandwidth can be realized. In actual usage, bandwidth improvements are more modest.

There is one caveat to having a full duplex server connection, however. Since all clients will be communicating with the server, there may be times during peak usage when there is more traffic destined for the server than the server port has bandwidth to accommodate. In this situation, it is critical that the client ports have flow control. If too many of the clients' packets are lost, the machines could lose their connection to the server. Flow control can manage the traffic destined for the server port and prevent users from experiencing unnecessary interruptions in their work.

Thus you must be very cautious in installing a full duplex connection. It is extremely important to keep in mind that flow control is not generally possible over a full duplex link. Therefore, if a station on a full duplex link tries to send more traffic onto another link across a bridge or switch than that link can handle, frames will be lost.

Note: The Express Switch does not support NWAY. You must force the duplex setting.

This applies to:

Intel® InBusiness™ Hubs and Switches
Xircom® NetStation 4 Port Switch (NSE4100)
Xircom® NetStation 8 Port Switch (NSE8100)

Solution ID: CS-014409
Last Modified: 13-Jul-2010
Date Created: 09-Aug-2004
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