 increases with distance and as user density decreases Bit rate also reduces with distance Therefore, per subscriber cost increases with distance and decreasing user density Physical Layer Connectivity The per bit cost benefits of fibre can only be realised as fiscal gain if the data rate can be matched efficiently to individual and group subscriber needs. The investment cost per subscriber increases with distance and as user density reduces. Bit rate decreases with distance for all the delivery options. The combination of these factors means that capitalisation and operational cost per subscriber increases with distance and decreasing user density. Inconveniently, individual income and household income also tends to be lower in for example rural and deep rural areas, which compounds this effect. Providing a dedicated wavelength division multiplexed multimode fibre to your elderly aunt in Rosyth whose nearest neighbour is ten kilometres distant is not an economically attractive proposition unless an existing copper run exists that can be easily upgraded. If no copper run exists then point-to- point wireless may be the only feasible option. In practice, the most economic mix ends up being a combination of several or all options. 2.6.3 Free-Space Optics Free-space optical hardware systems also known as free-space photonics or optical wireless systems have been in existence since the 1970s, initially for military applications but now increasingly promoted for use as alternatives to point-to-point microwave systems. The systems can support data rates of between 100 Mbps and 2.5 Gbps over distances of several kilometres but are sensitive to propagation impairments including fog, absorption, scat- tering, physical obstructions, building sway and scintillation. Longer-term use of wavelength division multiplexing techniques is claimed to be capable of increasing this to 10 Gbps. High-quality photodiode hardware is available for the 800-nanometre band and 1550 nanometres. A risk of eye damage limits the power in the lower