Carrier extension impact on data throughput

In half-duplex mode operating at data rates above 100 Mb/s the slotTime used at lower rates is inadequate to accommodate network topologies of the desired physical size (200 m). Carrier extension appends non-data extension bits to frames that are less than minFrameSize + extendSize bits in length to give a transmission at least slotTime in size.

This approach results in reduced sustained throughput for small packets. Worst case, if traffic consists of only 64-Byte frames effective throughput is reduced to 120 Mb/s. At present, average Ehernet frame size is 200 - 500 Byte and Gigabit Ethernet would deliver only 300 to 400 Mb/s throughput.

None of the above is an issue for fiber-based duplex links.

Preamble

Destination

address

Source

address

Length

field

Data

payload

CRC

Postamble

64

48

48

16

32

8

Extension

minFrameSize

minFrameSize + extendSize

Duration of carrier event

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