Lifting the veil on Full Duplex DOCSIS
Written by John Holobinko, Director Access Strategy, Cable Access Business Unit
Traditional cable access systems can only transmit data in one direction across any part of the spectrum. Compared to PONs, a cable access network is severely limited in the maximum symmetrical data speed it can support, usually under 200 Mbps.
What if spectrum was no longer dedicated to a single direction, but supported simultaneous data transmission in both directions simultaneously? The technology for doing this is being developed now, and Cisco is in the forefront of this breakthrough. FDX is a new technology that enables simultaneous downstream and upstream communications over the same cable RF spectrum. With FDX, D3.1 channels can be transmitted simultaneously in both directions without data throughput loss in either direction. FDX is being developed primarily for N+0 plant architectures, i.e. systems where there are only optical nodes and only passive coax with no amplifiers following.
The biggest challenge in bidirectional transmission on the same frequencies is how to recognize return path signals at the point where they are at their lowest RF level while forward path signals on the same frequencies are at their highest level. The point in the network where this occurs is at each optical node output/input port. In an N+0 network the output level of the highest forward path carriers can be over 60dBmV. In contrast, the received carriers from cable modems can be 10 dBmV. If you think of only the return path, this means that there is somewhat coherent “noise” level that is represented by the forward path signals, that is virtually 50 dB greater than the signal level!
Knowing the forward path signal and using digital subtraction, you can remove this unwanted forward path “noise” to recover the return path signal. If this were the only thing required, FDX would be easy. However, when forward path signals leave the node and reach the first coaxial cable tap they are reflected back towards the node. Assume the best taps provide 25 dB of return loss. If there is 2.5 dB of cable loss to the first tap, that means that the reflected signal is 60 dBmV – 2.5dB – 25.0dB – 2.5dB = 30 dBmV compared to a return path modem signal of 10 dBmV! We are used to measuring the signal to noise ratio as a positive number. In this case, the relative SNR is a negative 20 dB! Consider that there are many reflections at various frequencies. This makes recovery of the return path signals far more complicated.
FDX technology uses powerful digital signal processing algorithms to derive return path signal from forward path signals and reflections, and Cisco has led the CableLabs effort in contributing technology for the FDX standard. Based on cable modem output signal power limitations, the standard currently defines symmetrical operation up to 684 MHz. FDX supports traditional cable modems and FDX modems in the same network simultaneously. Therefore, to support the FDX standard requires a node based Remote PHY and requires that the node return path gain sections support 700 MHz bandwidth operation.
Cisco is currently developing an FDX capable node based on its 1.2 GHz GS7000 super high output node technology. Customers with existing GS7000 nodes will be able to upgrade these nodes to Remote PHY and FDX.