Spirent SmartBits System, Reference Manual

Page: 131 / 278

Chapter 9: Ethernet Cards
Standard Ethernet Features
SmartBits System Reference 123
The gap timer determines the amount of time elapsed between the last bit of the CRC of a
frame and the first bit of the preamble of the following frame. In a half-duplex
environment, the gap timer starts at the end of the successful frame transmission, and is
not necessarily the first transmission attempt. The resolution of the timer varies depending
on the port speed.
IFG Based on Frame Rate
Use this calculation to find the interframe gap for a given port when the frame rate and
frame size(s) is known. The unit used is bit times, varies depending on the speed of the
card. With a 100 Megabit card, there are 100 million bit times per second.
Subtract the total transmitted number of bits per second from the total number of available
bits per second. Then divide the remaining bits by the frames per second to obtain the
interframe gap.
Interframe Gap Based on Frame Rate and Frame Size
Interframe Gap = [100% load – (frame size * frame rate)]/frame rate
The following example calculates the interframe gap to achieve 100,000 frames a second
(with a frame size of 896bits):
Interframe Gap =
[100,000,000 – (896 * 100,000)]/100,000 = 104 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 pr 100 bytes of data) = 896 bits (Multiply frame
data size 100 by 8 to convert it from bytes to bits).
• Frame rate = desired frames per second = 100,000 frames per second.
IFG Based on Load
This equation is used to calculate the global interframe gap when the desired load (the
percentage of bandwidth used) is known. Use the load equation to first calculate the
frames per second based on the desired load. Then plug frames-per-second in the initial
rate calculation to find a correct gap.
The unit used is bit times, which varies depending on the speed of the card. With a 100
Megabit card, there are 100 million bit times per second.
Interframe Gap Based on Load%
Frame Rate = [100%load/(frame size + min frame gap)] * desired load
then
InterFrame Gap = [100% load – (frame size * frame rate)]/frame rate