Chapter 9: Ethernet Cards
Standard Ethernet Features
124 SmartBits System Reference
Example:
To configure the gap for 70% traffic load (frame size of 896 bits):
Frame Rate = [100,000,000/(896 + 0.00000096)] * .7 = 78125 frames per second.
Interframe Gap = [100,000,000 – (896 * 78125)]/78125 = 384 bit times.
where:
• 100% load in bit times per second = bits per second, based on the card speed =
100,000,000 (for 100 Mbps Ethernet).
• Frame size in bits = frame data size + CRC + preamble =
32-bit CRC + 64-bit preamble + 100 bytes of data) = 896 bits (Multiply frame
data size 100 by 8 to convert it from bytes to bits).
• Min Frame Gap. Minimum legal interframe gap based on the medium used =
0.00000096 bits per second.
• Desired Load. Percent of bandwidth that you want to use = 70%.
• Frame Rate. Desired frames per second = 100,000 frames per second.
• InterFrame Gap. Interframe gap (in bit times) for a desired load.
100,000,000 divided by (896 plus 0.00000096) gives the number of frames per second to
achieve 100% load (111607.1 frames per second). Then take 100% frame rate, and
multiply it by the percentage of bandwidth use desired (in this example 70% or 78125
frames per second).
Now that you have frames per second, calculate the correct interframe gap: [100,000,000 -
(896 * 78125 )] = 30,000,089.6. This is the total gap, or the amount of open space where
no frames are sent. Divide this amount by the number of frames per second to find the
interframe gap needed to achieve a 70% load at 100 Mbps. The result is a gap of 384 bits.
IP Header Checksum Generation/Validation
When IP-oriented streams are used, the transmitter automatically increments the
identification field in the IP protocol header. When IP frames are received, the receiving
card automatically verifies the IP header checksum. A counter indicates the number of
frames received with bad IP header checksums.
The transmitter also supports automatic IP header checksum generation. For normal IP
headers containing no options, the header checksum is automatically generated and
inserted regardless of VFD usage. For extended headers that contain options, the IP
header checksum is correctly generated as long as the options themselves do not contain
VFDs.
Jumbo Frames
A jumbo frame is an Ethernet frame with a length between 1519 and 9018 bytes with valid
CRC, or a length between 1523 and 9022 bytes with valid CRC and VLAN tag. If jumbo
frame reception is not enabled, jumbo frames are counted as oversize frames.
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