The great data rate-reach-capacity tradeoff

Source: Gazettabyte

Optical transmission technology is starting to bump into fundamental limits, resulting in a three-way tradeoff between data rate, reach and channel bandwidth. So says Brandon Collings, JDS Uniphase's CTO for communications and commercial optical products. See the recent Q&A.

This tradeoff will impact the coming transmission speeds of 200, 400 Gigabit-per-second and 1 Terabit-per-second. For each increased data rate, either the channel bandwidth must increase or the reach must decrease or both, says Collings.

Thus a 200Gbps light path can be squeezed into a 50GHz channel in the C-band but its reach will not match that of 100Gbps over a 50GHz channel (Shown on the graph with a hashed line). A wider version of 200Gbps could match the reach to the 100Gbps, but that would probably need a 75GHz channel, says Collings.

For 400Gbps, the same situation arises suggesting two possible approaches: 400Gbps fitting in a 75GHz channel but with limited reach (for metro) or a 400Gbps signal placed within a 125GHz channel to match the reach of 100Gbps over a 50GHz channel.

Optical transmission technology is starting to bump into fundamental limits resulting in a three-way tradeoff between data rate, reach and channel bandwidth.

"Continue this argument for 1 Terabit as well," says Collings. Here the industry consensus suggests a 200GHz-wide channel will be needed.

Similarly, within this compromise, other options are available such as 400Gbps over a 50GHz channel. But this would have a very limited reach.

Collings does not dismiss the possibility of a technology development which would break this fundamental compromise, but at present this is the situation.

As a result there will likely be multiple formats hitting the market which align the reach needed with the minimised channel bandwidth, says Collings.