How much capacity can be transported in a single fiber pair? In a recent trial on Verizon's real-world environment network in Dallas, it demonstrates the capability to transport 35.4Tbps over a 80km single fiber pair, using Cisco Network Convergence System NCS1004 equipment, operating with 600G coherent DWDM technology. The trial also demonstrates the following maximum capacities per fiber pair in different scenarios:

  • 25.6Tbps @ 400G over 800km with 75Ghz spacing
  • 32Tbps @ 500G over 400km with 75Ghz spacing
  • 35.4Tbps @ 600G over 80km with 81.25Ghz spacing

The following figure shows the network topology and connectivity for the trial in Verizon's network.

Real world environment trials on Verizon network
Real world environment trials on Verizon's network,      Source of Picture: Cisco


Cisco partnered with Verizon to in the trial to demonstrate the benefits of the granular control of its NCS 1004 in a real-world environment. Ten fibers in Verizon’s 80km Dallas loop were used with NCS 2000 SMR flex-grid ROADMs to build an 800km network. Channelized ASE noise loading was provided by NCS 2000 equipment.

According to Cisco, three scenarios were tested – 1) 400G over 10x80km i.e. 800km, 2) 500G over 5x80km i.e. 400km, 3) 600G over 80km. A single transponder carrier per channel was used in the test. For each scenario listed above, it was also tested to lower the baud-rate (raised bits per symbol) to trade off excess margin for more capacity.

For 400G over 800km, the test was started with the highest baud-rate possible at 71.7Gbd/s with a corresponding modulation of 3.88 bits/symbol. This gave a Q-margin of 2db and fit into 87.5Ghz. It was then traded off the excess margin on the link for additional capacity. The optimal point as per testing was 61.72Gbs/s with a corresponding modulation of 4.5 bits/symbol and Q-margin of 1db. This signal fit into 75Ghz and resulted in a fiber capacity of 25.6Tbps. This test was similarly repeated for 500G over 400km and 600G over 80km.

The below chart shows test results for how the system can trade excess margin for more capacity.

Transmission Capacity in a Single Fiber Pair vs Margin
Transport Capacity in a Single Fiber Pair vs Margin,     Source of Picture: Cisco


Achieving the capacity of 35.4Tbps over a 80km single fiber pair in a real world network environment,  it will significantly improve DCI connectivity among data centers in a metro network, with increasing demands for super high capacity.