The DOCSIS 3.1 standard came closer to the ability to deliver symmetrical 5 Gbps speeds, with consortium developer CableLabs announcing the addition of MAC Layer support to the standard’s Full Duplex (FDX) extension.
As described in a blog post by CableLabs Principal Architect of Network Technologies Karthik Sundaresan, FDX is an update and extension to DOCSIS 3.1 specifications that builds on the core Orthogonal Frequency Division Multiplexing (OFDM) technology. CableLabs released physical layer specifications for FDX back in October.
The new addition is focused on the Media Access Control (MAC) management messaging and operation needed to enable FDX between the Cable Modem Termination System (CMTS) and the cable modem. This includes FDX channel acquisition/initialization process by a cable mode, and new processes such as sounding, echo cancellation training and resource block assignment.
"An FDX CMTS will simultaneously receive and transmit in the same FDX spectrum, while FDX cable modems can either receive or transmit in the same FDX spectrum. Thus, communication is Full duplex from the perspective of the CMTS but frequency division duplex from the perspective of the cable modem,” Sundaresan said.
The FDX band is divided into sub-bands and the CMTS assigns which sub-bands each cable modem uses for upstream or downstream operation.
“A sounding method is used to identify groups of cable modems, called Interference Groups (IGs), that would interfere with each other if they were allowed to transmit and receive at the same time in a sub-band,” he explained. "IGs are grouped together into a small number of Transmission Groups (TGs). Cable modems in the same TG either transmit or receive on any given sub-band and time, as signaled by the CMTS in the RBA. Cable modems from different TGs have enough isolation to transmit and receive at the same time in the same sub-band.
“FDX uses a combination of interference cancellation and intelligent scheduling at the CMTS. On the CM, in order to prevent upstream transmissions from interfering with adjacent downstream channels in the FDX band, echo cancellation techniques are used,” Sundaresan added.
The Full Duplex additions are, of course, good news for cable operators including Comcast, Charter, Cox Communications and Mediacom, who have all made major investments in their hybrid fiber-coaxial networks.
Sundaresan said CableLabs is also developing Operations Support Systems (OSS) changes needed to support FDX. In addition, he said the consortium has initiated work on the changes needed to support FDX within the Remote PHY realm.