Active or Passive DAS
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The majority of DAS installations today involve active equipment. Active DAS simply means the components require a power source to operate. an active system utilizes fiber optic cables to connect with remote nodes. Passive DAS systems don’t need fiber optic cables in general and consist of simple BDAs (Bi-Directional Amplifier). An example of a passive component used in a DAS system would be a diplexer. It doesn’t require power. The signal just runs through it, much like water runs through a pipe. DAS installations consisting of only passive components are rare. This is because as DAS installations get bigger and more sophisticated, they need to be controlled and monitored remotely. This capability is made possible by active equipment at both remote and Head-End locations connected via fiber. DAS system integrators deploy SNMP-enabled equipment. SNMP allows the system to be accessed and controlled via a GUI (Graphic User Interface) using an IP network.
DAS equipment has built-in alarms which are triggered when a failure occurs and automatically send alert messages out to the proper field personnel. System administrators or network maintenance engineers monitor the RF power level of a particular RU (remote unit) of a DAS installation using a smart phone or portable device with an internet connection. This prevents expensive maintenance calls, saving money and labor.
The majority of DAS OEMs (vendors) build equipment that transport RF signals between a DAS Head-End and RUs via fiber optic cable. Thus equipment on both sides have fiber optic transceivers that perform Electric to Optical (E/O) and Optical to Electric (O/E) conversions.
Many smaller DAS installations can be passive. Think of a small office in need of enhanced cellular coverage. A system integrator will install a directional antenna pointing to particular carrier’s tower. The signal enters the building via low-loss coaxial cable. A Bi-Directional amplifier known as a BDA is installed to strengthen the signal. Integrators also use passive components such as splitters and directional couplers to distribute RF signals between a BDA and indoor antennas. These installations are relatively inexpensive but require a good degree of RF expertise to ensure proper in-building coverage. Especially when there are multiple carriers and cellular bands (ranges of frequencies) which need to be distributed. Adding public safety UHF (ultra-high frequency) and VHF (very-high frequency) bands to installations will increase the cost and complexity of the deployment even more.