Points of Failure in a DAS
Once the DAS is built and the customer starts thinking about the maintenance program. A common question that comes up is about the points of failure for a DAS. Knowing these points helps customers design effective DAS maintenance processes.
Let's start with a quick overview of a block diagram for a DAS.
Every DAS has a DAS head-end. A DAS head-end is a cluster of electronics that has the first contact with the RF signal from the carrier provided signal sources. DAS head-end takes the RF signal, converts it to an optical signal and then sends the optical signal to 'remote' electronics via fiber optic cable. These remotes convert the optical signal back to the RF signal and send it to the antennas to broadcast to the cell phones.
Theoretically, any faulty elements in a DAS can be a source of the failure. However, at a high-level following are the common points of failure:
Defects in DAS head-end electronics
Defects in Remotes
Damage in coaxial cable
Damaged fiber cable
Let's take a quick look at the main components of the signal sources. Keep in mind if the function signal source is disrupted it can also disrupt the DAS service.
In a typical set up we will have some sort of base stations (or small cell or DRFM) from the carriers (such as AT&T or Sprint). These base stations will be connected to the building electrical circuit for power. They will be connected to the carrier's core network through what is known as 'backhaul router'. Most base stations will be connected to GPS antenna so that their clock is synchronized with the core network (i.e., core network and base station both use GPS as the source of time). For a signal source, the following are the main points of failure and can cause major service interruptions when they don't function as expected. They are:
Base station electronics
Electrical circuit or source of power
Connection to internet (i.e., backhaul router and connection)
GPS signal
The connection between a DAS and signal source
The extent of disruption due to these failures:
Typically the significant disruptions are classified in 2 ways:
Critical: Critical failure may cause system-wide (for one DAS sector or all DAS sectors) service disruption either for all signals or just the signal of a particular carrier (for example, AT&T 1900 MHz).
Major: Major failure will cause service disruption in part of the total coverage area - but not across the whole coverage area
Carrier signal source failure (Critical):
If one of the Carrier Signal Source(s) electronics becomes non-operational it may cause a system-wide outage.
For example, let's say, we have a single sector DAS and a 1900 MHz Radio Head (i.e., signal source) of AT&T stops working. In that case, 1900 MHz service from AT&T will not work in the whole coverage area of that DAS. Does this mean that all the AT&T phones in the coverage area will stop working? Maybe not; it depends on how many signal sources of different frequencies were covering the area. If 1900 MHz radio stopped working, but 2100 MHz radio is still working, then user's phones may still work.
Power circuit failure feeding the carrier signal sources (Critical):
Following the same logic mentioned above if the cause of carrier signal source failing is power failure this may impact all of the DAS coverage areas. This is the reason most of the carriers will keep battery back-up at the site which will cover during the power outage for a certain duration. If the signal sources can be backed up by UPS or generator power, then the impact of power failure can be minimized.
When the signal source loses power it will produce alarm in DAS (because of low RF input into DAS head-end, but the Network Operating Center (NOC) of the carriers will also receive trouble tickets. Additionally, as the owner of the DAS venue, the customer will also know about power failure.
Backhaul router failure (Critical):
All the signal sources at the DAS head-end will be connected to the carriers' core network via a backhaul connection. If the calls from the users cannot go to the core network, calls will not be processed. This failure can be particularly sinister because it will not show up on any DAS alarm. So as the DAS administrator, you may wonder why the users are complaining so much but you are not seeing any DAS alarms. You may even see the signal bars on user phones. In a situation like this check with the carrier to make sure they are not dealing with any backhaul trouble ticket.
GPS connection failure (Critical):
GPS connection is critical for signal sources because the GPS network allows the signal source to stay in sync with the core network. However, if the GPS connection is disrupted, the impact may not be immediate. Usually, this will not become an issue for at least 24 hours - although this may differ on the type of signal source and its manufacturer.
Lost GPS connectivity will not produce any alarm in DAS, but the Network Operating Center (NOC) of the carriers will receive trouble tickets.
Disruption of DAS service if: connection between signal source and DAS head-end is disrupted, DAS head-end electronics fail or fiber connectivity to remotes are cut
The connection between signal sources and DAS (Critical):
Signal sources will be connected to typical DAS head-end via coaxial cable and run through a DAS Interface Tray. The function of this tray is to reduce the hi-powered signal coming out of the signal source (like base station) and break down the composite signal into downlink and uplink signals (if needed). Failure in this connection may have a critical impact like the way carrier signal source failure can cause. Who owns this problem? It depends on who provided the DAS Trays or the interconnecting cables, or whoever damaged the connections.
In this case, DAS will be in alarm because of low RF input into DAS head-end, but the Network Operating Center (NOC) of the carriers may not receive any alarm.
DAS Head-end Electronics (Critical):
Head-end has the name for a reason - it's basically the brain of the DAS and supplies lifeblood (i.e., signal) to the whole DAS. Depending on which electronics fail, it can take out the full coverage or parts of it out.
For example, if the interface card of a particular frequency for a carrier fails, it can disrupt coverage of that frequency for that carrier. Say, for example, if the 1900 MHz interface card dedicated to Sprint fails on a single sector DAS, it will disrupt coverage of Sprint 1900 throughout the DAS. If a fiber distribution box fails then it will disrupt service of however many remotes are connected to it.
DAS electronics failure will cause alarm in DAS, but not at NOC of the carriers.
Fiber connection failure (Critical / Major):
Critical impact if a backbone fiber is severed and it was transmitting the signal to multiple remotes, then all antennas connected to those remotes will be out of service. If a single fiber connection going to an individual remote fails, then it will disrupt the service of just the antennas connected to that remote.
Break in the fiber connection will usually produce alarm in the DAS.
Remote failure (Major):
If for some reason a remote electronics fails, it will disrupt the service of antennas connected to it. For example, if a remote is supplying the signal to 4 antennas and each antenna was covering on average 5,000 sq. ft. of area, then as the remote fails it may stop coverage over an estimated 20,000 sq. ft. of area.
Failure at remotes will produce a DAS alarm.
Coax or Antenna (Major):
If the coaxial cable connected to an antenna is damaged, the antenna may stop broadcasting. Similarly, if the antenna is damaged it will disrupt the signal.
DAS may or may not produce an alarm for damaged coax and antenna. It depends on the type of DAS.
Contrary to our expectation, unfortunately, we experience the failure due to coax and/or antenna most frequently - especially on big campuses where there is always some sort of construction is going on.
If you have just finished building a DAS, and in the process of putting together a Maintenance program, hopefully, this post will help.
