G.hn Co-existence with other Home Wired Networks
PowerLine Coexistence
An ITU-T Contribution specifies a methodology for a coexistence protocol on power line wiring. The 
Contribution was proposed by all the major vendors of existing Powerline Communications (PLC) devices, including vendors of HD-PLC, HomePlug, and UPA/Opera technologies. ITU Q4/SG15 G.cx was formed to supplement the G.hn work to develop an ITU-T Recommendation based on the Inter-System Protocol (ISP) proposal.
The IEEE P1901 work group is also adopting the Inter-System Protocol (ISP) by drafting a standard for Powerline Communications (PLC) Access and In-Home communications.
G.hn transceivers implementing the G.cx protocol will coexist with P1901 devices. G.cx specifies the ISP protocol, which allows other technologies to coexist with G.hn devices. The ISP protocol is a low effort addition that should not require re-design of the underlying technology. The ISP protocol adds a TDMA structure to the medium, simply scheduling transmission time to G.hn and non-G.hn devices.
To coexist with technologies that do not support G.cx protocol, G.hn will use its frequency agility capabilities, such as tone masking, spectrum shifting, and PSD shaping. A G.hn transceiver is capable of adjusting its frequency spectrum and transmit power appropriately to coexist.
Coax Coexistence
Since a broad frequency spectrum is available on the coax to support many channels, coexistence between a G.hn network and an existing network on the same wired medium is easily supported. For the existing MoCA networks, the network transmits a beacon signal. This beacon signal can be used to allocate time and spectrum to avoid interference and collision with the MoCA and the G.hn transceivers. For coexistence with existing technologies which do not transmit a beacon, G.hn includes capabilities such as frequency agility, tone masking, and PSD shaping.
Summary
For G.hn to accepted, it needs to coexist with current power line and RF coax protocols. Coexistence is not the same as interoperability. G.hn devices need not inter-operate with current protocols but must coexist with them in the same wire line medium. G.cx addresses the coexistence for G.hn networks with older established wired networking devices with the Inter-System Protocol (ISP).
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