1901 devices. HomePlug 1.0 devices do not interoperate with HomePlug AV devices. Although it is technically possible to achieve such backward compatibility, doing so is not economically feasible because of the high cost of circuitry that would have to support different forward error correction (FEC) techniques and feature sets. HomePlug devices will not interoperate with devices that employ other powerline technologies, such as Universal Powerline Association (UPA), HD-PLC, or G.hn. In the case of G.hn, it was deemed prohibitively expensive to implement both HomePlug's turbo coding forward error correction and G.hn's low density parity check (LDPC). However, IEEE 1901 allows co-existence within the same deployment of both HomePlug AV and HD-PLC via its Inter-System Protocol (ISP). G.hn also supports the ISP. HomePlug devices are not compatible with certain power strips, surge protectors, and uninterruptible power supplies incorporating filters, which block the high-frequency signal. In such cases, the installer must plug devices directly into building electrical receptacles. If a spare power point is not available, a double adapter can be used in many cases with the incompatible device on one side and the HomePlug device on the other. EMI concerns One of the concerns with all powerline systems, when compared to dedicated data wiring, is that the route of the wiring is not known in advance, and is generally already optimized for power transmission. This means that there will be situations where the system will radiate a significant fraction of the energy, in the form of radio frequency interference, or be vulnerable to the ingress of external signals. Given that the shortwave band is used both by low-power long-range telemetry and high-power broadcast signals, this is a potentially serious drawback. To attempt to minimize the effects of incoming interference and frequency-dependent path losses, the HomePlug standard requires each node to maintain 'tone maps' updates during operation, so the equipment 'learns' to avoid certain troublesome frequencies and to put more data onto those frequencies that exhibit a low loss. However, while this mitigates against ingress, if there is sensitive receiving equipment nearby then there is no easy way to tell the HomePlug apparatus to 'turn down' the radiated interference. In comparison to the received signals in a radio communication equipment, the signal levels in a powerline system are quite high. Typically the power density is −50dBm per Hz, as each carrier occupies a channel of 24 kHz, each carrier is injected at