What Does It Take to Meet 40G?

How can you measure whether your platform is truly 40G compliant, meaning that the chassis is able to deliver 40G to all slots, with reliable end-to-end connectivity?

There are various ways that we can talk about meeting 40G requirements.  In a lab environment we can measure things like insertion loss and return loss crosstalk.  At RadiSys, we use the Insertion Loss-to-Crosstalk Ratio (ICR) as overall measure of performance. Let’s break down these terms:

  • Insertion loss: Measures how much signal is lost, in decibels, between a transmitter and receiver over frequency (Hz).
  • Crosstalk: Measures the amount of interference, in decibels, that occurs when one signal in an active channel bleeds into a signal in another active channel.

Why is this ratio important? Well, imagine that you are at a dinner party and are trying to converse with a friend who is sitting across the table. The table is quite large, so some of your friend’s conversation does not carry all the way to you (insertion loss). You might still receive the gist of the conversation, particularly in a quiet room. But now imagine that a guest next to you is carrying on a very loud conversation of his own (crosstalk). So what you end up hearing is the diminished conversation from your friend across the table (attenuated signal), which is further compromised by the chatter next to you (crosstalk). That’s what the ICR measures: the difference between the attenuated signal and the crosstalk at the receiver.

The graph below shows the minimum ICR value (the red line) across the frequency. A chassis can be considered compliant when it meets or exceeds the minimum value at all points, as shown by chassis 1 (blue line).

Some chassis may appear to be compliant when you look at the overall average. For example, chassis 2 (purple line) may have an average value that is above the ICR minimum; however, it actually fluctuates above and below the minimum. These dips below the line can be costly and unpredictable. For example, this could lead to a higher bit error rate, interoperability issues, and communication failures.

The ICR specification is not easy to meet. In the next few blogs, we’ll talk about some ways to minimize insertion loss and crosstalk on your way to 40G.

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