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Public Safety Tech Topic #14 - Diversity, Redundancy, and Resiliency - in that Order

This topic returns to an earlier discussion in Tech Topic #7 (Core Concepts and Software Defined Radio for Public Safety) and to the core precepts for public safety communications. In this topic, another fundamental concept that contributes to, and is an auxiliary to, the core concepts of redundancy and resiliency is considered. The concept is route diversity and how diversity applies to the connectivity for Public Safety Answering Points (PSAPs).

Earlier this year, the National Communications System (NCS) published a draft of extended guidance regarding route diversity in the planning for Federal department and agency minimum communications. 1 Our current topic will expand on this guidance and apply the principles of route diversity to PSAPs. The impact and contribution that route diversity make on the core concepts of redundancy and resiliency will then be discussed; especially the applicability to PSAPs. From a PSAP perspective, route diversity could be applied to either the PSAP-to-public switched network connection or alternatively from the PSAP-to-dispatched elements in the field perspective. This topic deals specifically with route diversity principles between the PSAP and the public switched network. This connectivity is especially important because it is the link between the public as subscribers-at-large and the public safety community through the PSAP. It is also extremely important because public safety officials must make sure that alternative means are available for the PSAP to connect to the local switched network in the event of natural or man-made disaster.

Route diversity is generally defined as the communications routing between two points over more than one geographic or physical path with no common points. 2 For a PSAP, this means that the connectivity between the PSAP and the local central office (specifically to the 911 selective routers) should have alternative ways and/or means. It also means that there are no common points of connection along the way with the exception potentially at the end points (the PSTN selective router and the PSAP Private Branch Exchange or connection main frame). Strictly speaking, the alternative means may be achieved by completely distinct methods such as copper wireline, fiber optic cable, free space optical or other radio links, or even satellite link. The point is that in order to provide diversity, connections between the PSAP and the central offices should be by separate and distinct methods with no common points of connection along the way.

By providing separate and distinct routing methods, it is also implied that there are separate and distinct entry points for each transmission means as they terminate at the end points. In many instances, this means that the separate routes should enter/leave from distinct and separated entry points to the end point facilities. For example, typical route diverse wireline systems enter at separate wiring closets or entrance facilities that support either the central office or the PSAP.3 It should be noted that route diversity is NOT satisfied by two separate systems that follow a similar geographic path, for example parallel cable systems. Under such circumstances, both means are still vulnerable to outage as if they were a similar single path.

In addition, redundant paths that employ the same transmission means (e.g. two trunks that are on the same cable or two channels on the same radio system) also do not satisfy proper diversity requirements. This condition is particularly important when either economic considerations or simple geographic conditions dictate path routing. As discussed later in this note, the costs of diversity must be considered in the overall vulnerability assessment of the facility and the need for route diversity.

It may be difficult for a PSAP to ensure route diversity and end point separation at either the PSAP or at the central office points of termination. In the ideal situation, diversity should be gained by connection to separate central offices. This would allow the PSAP to switch or divert its routing in the case of lost connection via one of its central office (CO) connections. It is also desirable to employ separate entrance facilities at the PSAP for each connection to a different CO. Finally, it is important that diverse routing employs distinct termination equipment as well; after all, it would not be appropriate to terminate diverse routing on a common single point of failure at either end of the connections.

The alternate routing characteristics provided by diversity contribute directly to the fundamental public safety precepts of redundancy and resiliency. By providing an alternative means of connectivity through diversity routing, redundant means of connection between the PSAP and the local exchange are accomplished. This is a very important capability that all PSAPs should have. With guaranteed connectivity to the public network, the overall resiliency of the PSAP in providing public safety communications support is enhanced.

It should be noted at this point that diversity routing should apply to all forms of connection between PSAP facilities and the local networks. It applies to all communications; whether they are voice, data, or video connections. Although economic and geographic considerations will play a significant role in determining the extent that route diversity may be accomplished, consideration must be given in light of redundancy requirements and ultimate resiliency of the PSAP. (Recall an old Army communicator's adage: plan for 50% backup to all communications but take 100% with you!)

Routing diversity is a fundamental concept that is employed by common carriers for their networks at large as well as to PSAPs. In fact, route diversity and all of its manifestations are considered industry "Best Practices".4

In summary, route diversity applied to public safety PSAPs ensures that there are no single points of failure in the connection between a PSAP and local networks. There should be no coincident use of common carrier facilities at the local network connections, no common loop facilities or interconnection (physical or logical) along the pathway, and no coincident termination facilities at the PSAP. It should be noted that the circuit connections between the PSAP and the CO's should have priority restoration services (Telecommunications Service Priority - TSP ) applied to them consistent with any Service Level Agreements (SLAs) that may exist with the common carrier to insure that carriers take all necessary actions to restore services on these circuits soonest should there be an outage.


1 See NCSH 3-10-1: Guidance for Improving Router Diversity within Local Access Networks (Draft), March 18. 2008.

2 Id.

3 It should be noted that TIA/EIA Standard 568-B provides standardization guidance for the installation of commercial building wiring and cabling. See http://www.nag.ru/goodies/tia/TIA-EIA-568-B.1.pdf. Per Wikipedia at http://en.wikipedia.org/wiki/TIA-568B, "TIA/EIA-568-B also defines characteristics and cabling requirements for entrance facilities, equipment rooms and telecommunications rooms."

4 For an extended list of diversity 'Best Practices', enter "diversity" in the search criteria at the FCC's Network Reliability and Interoperability (NRIC) Best Practices web site at https://www.fcc.gov/nors/outage/bestpractice/BestPractice.cfm.


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