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Microwave Communications Divisionhttp://www.microwave.harris.com

R.U.Laine - Systems Engineering Document No.2217 March 2002Ver. 2.1

Outage Points, Availability, and Performance Objectives inDigital Microwave Radio-Relay Links

DefinitionsDigital microwave links are characterized by their

Availability, accommodating long-term outagescausing traffic-disconnects and loss of datathroughput, and Performance -short-term outage withno traffic disconnect nor loss of data throughput -during available periods, and quality, dribbling andburst errored-seconds (ES) with no outage.

The internationally defined outage or dynamic

thresholdof a digital microwave link is the Severely-Errored Second (SES) orsync loss point coincidingwith the

AIS - alarm indication signal, the all-onessignal in DS1/E1trunks,

AIS or Blue signal, the alternate ones and

zeros, in DS3/E3 trunks

LOF - loss-of-frame synchronization, with a >10 -

3 BER ES event in PDH (asynchronous) links, orthe

LOP - loss-of-pointer synchronization point at

about >2x10-5 BER in SONET/SDH links.

All other ESs are dribbling (RBER random orresidual) or burst ES events, not outages.

This terminology is compliant to all ITU-R, ITU-T,and North American (Bellcore, Industry Canada,etc.) digital trunk availability and performancedefinitions and objectives1.

The availability and performance of any digital trunkare easily verif ied with external BER test

instrumentation or the NMS (network managementsystem) internal to many radio-relay systems and allSONET/SDH multiplexing networks.

OutagesOnly the SES outage (sync loss) events definedabove typically occur during fade activity inoptimally configured and properly aligned medium-and high-capacity digital radio-relay links. Theseradios are configured with anticipatory (switching

before errors occur) algorithms that ensure errorlessdata switching between diversity receivers.

Such quality events as dribbling BERs at 10-6 RBER(residual BER) should never occur during dynamicfade activity in a properly engineered and aligneddigital microwave link.

Dribbling errors in radio-relay links, while commonin cable systems due to dispersion, amplifier

degradation, and NEXT, are indicative of equipmentproblems (transmitter PA non-linearity), dispersive(spectrum-distorting) fades caused by a long-delayed high-level multipath signal (unfavorablepath geometry with faulted antenna size selectionand/or alignment), high level interference, low fademargins, flawed diversity spacing, etc., requiringimmediate correction.

Since 10-6 RBER (or other) quality events are nevercaused by flat fade activity, non-outage qualityBERs should never be used as a non-standardthreshold for digital microwave link outagecalculations.

Static Threshold TestsThe 10-6 BER static threshold point is provided onmicrowave equipment data sheets for the purposeof ensuring proper equipment performance (andacceptable interference levels) during factory andfield static measurements conducted manually withpads or attenuators.

These in-service manual tests are always to thedigital radio receivers static 10-6, 10-8, etc. BERthreshold, never to its dynamic SES threshold(outage) point.

Static tests are conducted in the field during linkinstallation and commissioning. Static tests certifyradio link thermal and interference thresholds andfade margins without disrupting or disturbing traffic.In-service static tests, perhaps to the 10 -8 to 10-6

RBER static threshold point, are completed duringquiet daytime non-fading periods each year.

This provides assurance that the link hasexperienced neither equipment degradation (radios,antenna feeder systems) nor any increase in

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interference levels that degrade fade margins in thistest interval.Availability

All multiplexers, PABXs, channel banks, DACs,etc. connected to digital microwave T1/E1 trunkports have since 1975 a field delay setting (bystrapping, software command, etc.) of 2.5-10 sec

after outage before traffic-disconnecting CGA(carrier group alarm) or AIS execution.

This delay setting accommodates short-termoutages in digital microwave links with no disruptionnor disturbance to live traffic or data throughput. Inthis way, the normal multipath fade activity andswitching expected of radio links and systems goundetected by telephone company, cellular, PCS,electric utility, public safety trunking, data/IP, etc.subscribers.

Long-term outage events of more than the 2.5-10sec duration setting of this CGA/AIS delay willexecute trunk conditioning which disconnects livetraffic and initiates a period of trunk unavailability.

Such unavailability events are, of course, generallyunacceptable to most service providers and theirsubscribers. Traffic-disconnect events in radio linksmay be caused by power fading (e.g. rain outage inhigh-frequency microwave links and ducting inlonger low-clearance microwave links in difficultgeoclimatic regions), non-protected equipmentfailure, and infrastructure damage or degradation -power, antenna feeder system, etc.

Link availability is increased with equipmentprotection using hot-standby radios and modules,redundant antenna feeder systems (splittransmitters connected to separate diversityantennas), proper path clearances in duct-entrapment regions, and self-healing path-switchring arrangements (loop systems) providing routediversity.

Ring systems reduce radio equipment costs perhaps40%, and are considered essential to the acceptableoperation of some high-frequency microwave routessubject to rain outage and all copper and orfiberoptics cable systems.

And ring systems improve the performance of amicrowave system by automatically routing T1/E1trunks away from a fading path that may otherwisecontribute excessive SES outages.

Excessive Short Term Outages

Nearly all microwave systems experience short-termoutage due to multipath fade activity, and it is thisnumber of SES/yr (or SESR - SES ratio = numberof SES/time - internationally) that is predicted by thetransmission engineer for each microwave link tomeet the users performance objective. Sincerecognized models used to predict the numbers ofshort-term SES outages are based upon statistical

data for the regions geoclimatic (terrain, weather,etc.) conditions, a few optimally-configured links willexperience more than predicted.

This is normal, as these are outage predictionsmeeting performance objectives not requirements.In any system, many hops will experience lessoutage and some more, all dependent upon theterrain and climatic variabilitys associated with thatspecific link.

Excessive short-term outage occurrences may becaused by something as simple as an antennamisalignment or as serious as inadequate pathclearance over an area impacted by ABL(atmospheric boundary layer) formation. The impactof excessive short-term outage is dependent uponthe application.

Some types of mobile switching offices (cellularMSC/MTSO) are heavily impacted by continuousshort-term outages that even disconnect and isolatecell sites, while others will accommodate even long-term