The TCP/IP Guide  9  Networking Fundamentals       9  Network Performance Issues and Concepts

Theoretical and Real-World Throughput, and Factors Affecting Network Performance 1 2 Quality of Service (QoS)

Comparing Half-Duplex and Full-Duplex Operation

Of these three options, full-duplex is obviously the one that yields the highest performance. Full-duplex operation doubles the theoretical bandwidth of the connection. If a link normally runs at 1 Mbps but can work in full-duplex mode, it really has 2 Mbps of bandwidth (1 Mbps in each direction). Remember the key word “theoretical” however—you do not really get double the performance in real life, because communications usually do not involve sending lots of data in both directions at once. However, you certainly get better throughput than in a half-duplex mode.

In some cases, the mode of operation is a function of the technology and cannot be changed. In others, however, full-duplex mode is a matter of the correct hardware settings, and also whether the software supports full-duplex operation or not. Thus, getting higher performance in this area is sometimes simply a matter of ensuring proper configuration.

Full-duplex operation has been pretty much taken for granted in communications for years. The more interesting development has been the rise in significance of full-duplex operation for local area networking. Traditionally, LANs have always used half-duplex operation on a shared access medium. As the use of switches has increased, allowing dedicated bandwidth to each computer, full-duplex operation has become very popular. Full-duplex operation in Ethernet not only allows the simultaneous transmission of data in both directions, it also eliminates contention for the formerly shared access medium—no more collisions. The combination of these two effects improves performance, sometimes substantially.

Theoretical and Real-World Throughput, and Factors Affecting Network Performance 1 2 Quality of Service (QoS)