tower installation between donor and service antenna is several meters), insertion into an attenuating environment (e.g. installing a metal mesh between donor and service antennas), and/or reduction of reflections (no near obstacles in front of the donor antenna such as trees or buildings). Isolation can be also improved by integrated feature called ICE (interference cancellation equipment) offered in some products (e.g., NodeG, RFWindow). Activation of this feature has a negative impact on internal delay (higher delay => approximately +5 μs up to standard rep. delay) and consequently a shorter radius from donor site. Amplification and filtering introduce a delay (typically between 5 and 15 μs), depending on the type of repeater and features used. Additional distance also adds propagation delay. Reasons for weak signal Rural areas In many rural areas the housing density is too low to make construction of a new base station commercially viable. Installing a home cellular repeater may remedy this. In flat rural areas the signal is unlikely to suffer from multipath interference. Building construction material Certain construction materials can attenuate cell phone signal strength. Older buildings, such as churches, often block cellular signals. Any building that has a significant thickness of concrete, or a large amount of metal used in its construction, will attenuate the signal. Concrete floors are often poured onto a metal pan, which completely blocks most radio signals. Some solid foam insulation and some fiberglass insulation used in roofs or exterior walls have foil backing, which can reduce transmittance. Energy efficient windows and metal window screens are also very effective at blocking radio signals. Some materials have peaks in their absorption spectra, which decrease signal strength. Building size Large buildings, such as warehouses, hospitals, and factories, often lack cellular reception. Low signal strength also tends to occur in underground areas (such as basements, and in shops and restaurants located towards the centre of shopping malls). In these cases, an external antenna is usually used. Multipath interference Even in urban areas (which usually have strong cellular signals throughout), there may be dead zones caused by destructive interference of waves. These usually have an area of a few blocks and will usually only affect one of the two frequency ranges used by cell phones. This happens because different wavelengths of the different frequencies interfere destructively at different points. Directional antennas can be helpful at overcoming this issue since they may be used to select