Two Years in Mississippi George Kizer AlcatelLucent NSMA Annual Conference May 19 amp 20 2015 Holiday Inn Rosslyn at Key Bridge Arlington Virginia Network Overview 142 Sites 488 ID: 477614
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A Study of Propagation in a Difficult EnvironmentTwo Years in MississippiGeorge Kizer , Alcatel-Lucent
NSMA Annual ConferenceMay 19 & 20, 2015Holiday Inn Rosslyn at Key BridgeArlington, Virginia Slide2
Network Overview
142 Sites 488 Links150 Paths 1952 T-Rs top of the state
near the gulfSlide3
Maximum Path Length = 32 miles
Minimum Path Length = 3 milesMedian Path Length = 18 milesAverage Path Length = 19 miles
Six paths were at 11 GHzAll others were at lower or upper 6
GHz
17% of the paths were non-diversity
83 % were space
diversity
Architecture = rings and spurs
Network Path VariedSlide4
System design was in accordance with standard Bell Labs criteria (below)
Path propagation performance of all 488 simplex paths was measured over one to two years (as network was created)Path PerformanceVigants, “Space Diversity Engineering,” Bell System Technical Journal, pp. 103-142, January 1975Vigants, “Microwave Radio Obstruction Fading,” and Schiavone, “Prediction of Positive Refractivity Gradients for Line-of-Sight Microwave Radio Paths,“ Bell System Technical Journal, pp. 785 – 822, July –Aug 1981Slide5
Overall the network measured path performance met customer specifications: 99.998% one way
Multipath FadingSlide6
You can drown in a river with average depth of one foot – or a network with average satisfactory performance
Multipath FadingSlide7
Path performance variations were primarily the following:
IP Network CharacteristicsPath Multipath FadingPath Obstruction FadingPath PerformanceSlide8
MW
network supported a two layer IP network: a Multiprotocol Label Switching (MPLS) network which supported another overlay proprietary LMR IP networkEach network used Open Shortest Path First (OSPF) for packet routingEach network used Bidirectional Forwarding Detection (BFD)
to monitor route reachabilityIP Network CharacteristicsSlide9
OSPF validity was tried to BFD
The lower level MPLS network went down after three consecutive 100 millisecond BFD probe failures(e.g., after a MW path outage greater than 0.3 secs)The higher level LMR network went down after three consecutive 300 millisecond BFD probe failures
(e.g., after a MW path outage greater than 0.9 secs)
IP Network CharacteristicsSlide10
MPLS OSPF was restored 15 seconds after BFD came up
The LMR OSPF was restored 45 seconds after BFD came upIP Network CharacteristicsSlide11
For router networks,
individual outage durations is not the only criteriaA single one second MW path outage can cause a 15 second outage in am MPLS protected network which can cause a 45 second outage in the customer LMR router protected networkOutage time extension in router protected network can be significant
The outage time experienced by the customer can be two to three orders of magnitude greater than the radio path outage time
For router networks,
number of outages
can be more significant than
individual outage duration
Individual Fading Events
IP Network CharacteristicsSlide12
Ring and Mesh networks can provide significantly enhanced network performance with compared to the performance of cascaded paths
In the State of Mississippi, the routers switched hundreds of times a week, but outages due to path propagation were much less frequentThe weakness of rings is when multiple paths are affected simultaneouslySpurs without route protection are much more vulnerable to outage time magnificationIP Network CharacteristicsSlide13
Spur performance was limited by cumulative outage duration but also by individual outage duration
Both durations were measured.Path Multipath PerformanceSlide14
Path Multipath PerformanceCumulative Outage Duration
observedestimatedSlide15
I
ndividual path performance varied widely:Worst Case Observed Outage / Predicted Outage (secs ratio) = 156Best Case Observed Outage / Predicted Outage (secs ratio) =
0Median (50%, typical) Observed / Predicted Ratio = 1.9
Average
Observed / Predicted
Ratio = 6.4
Ratio Not Exceeded for 33% of Paths =
1
Multipath FadingSlide16
Average Total Path Outage Duration
= 114 secondsMinimum Number of Path Outages = 0Maximum Number of Path Outages = 659Median Number of Path Outages =
27Average Number of Path Outages = 55
Individual Path Annual Performance Varied Widely
IP Network CharacteristicsSlide17
Path Multipath PerformanceCumulative Outage EventsSlide18
Path Multipath PerformanceIndividual Outage Duration was relatively stable
Above statistics are for 6 GHz paths; 11 GHz paths experienced no outages.Number of outage events Total outage secondsOne reason measure outage > predicted outage is that Vigant’s method predicts outage on an analog basis while we measure it on an error-second basisSlide19
Path Multipath PerformanceCumulative Outage Duration
Cumulative outage duration is directly related to Vigants C factorVigants, “Space Diversity Engineering,” Bell System Technical Journal, pp. 103-142, January 1975
Good: 0.5
Average = 1
Difficult = 2Slide20
Path Multipath PerformanceCumulative Outage Duration
Vigants (1975) C Factor = 2Measured C Factor = 7This is consistent with Vigants’ and Barnett’s latter suggestions for the Gulf coast.
They suggested (1992) the C factor should be 10 for this area.Loso,
Inserra
,
Brockel
, Barnett and
Vigants
, “U. S. Army Tactical LOS Radio Propagation Reliability,”
Proceedings of the IEEE Tactical Communications Conference
, pp. 109-117, 1992.Slide21
Path Multipath PerformanceCumulative path outage varied significantly from the average
R = measured cumulative individual path outage – average outage = cumulative path outage standard deviation
10 log () = 7 dB (measured)
10 log (
) =
10
dB
(reported by
Bellcore
*)*Achariyapaopan, “A Model of Geographic Variation of Multipath Fading Probability,”
Bellcore
National Radio engineer’s Conference Proceedings
, pp. TA1 – TA16, 1986.Slide22
Path Multipath PerformanceIndividual Outage Duration was relatively stable
95 % of the paths have cumulative outage durations of the following:8.2 x network average duration (measured 10 log () = 7)16.5 x network average duration (Bellcore 10 log () = 10)Since the typical outage duration one second, 95% of the number of outage events average the following:8.2 x average number of outages (measured 10 log () = 7)16.5 x network average number of outages(
Bellcore 10 log () = 10)
For router networks, the number of outage events is more significant than the actual outage durationSlide23
Path Obstruction FadingObstruction fading was relatively infrequent but could be quite troublesome when it occurredSlide24
Typical Multipath FadingSlide25
Classical Obstruction Fading
RSLs for Both Receivers(Space Diversity)
Threshold RSL
Nominal RSL
Transmit PowerSlide26
Slow Speed Obstruction Fading
RSLs for Both Receivers(Hot Standby Non-diversity)
RSLs for Both Receivers
(Hot Standby Non-diversity)Slide27
RSLs for Both Receivers (hot standby)
Moderate Speed Obstruction FadingSlide28
Receiver
OverloadReceiverOverloadReceiverOverload
RSLs for Both Receivers (space diversity)
High Speed Obstruction FadingSlide29
U
nexpected diversity effectObstruction FadingSlide30
Unexpected path diversity effect
Different Paths from the Same NodeSlide31
Path diversity effect
Path Pairs from Same NodeSlide32
Obstruction Fading Predicted vs Measured Outages
Currently airport refractivity measurements are taken at different times (typically at sun up and sun down) than when obstruction fading typically occurs (from about midnight to just before sun rise)Slide33
Obstruction Fading Lessons Learned
Obstruction fading comes in two forms:Relatively short duration amenable to space diversityRelatively long duration indifferent to space diversityObstruction fading very localized:Geographically close paths can be de-correlated
Obstruction Fading estimation unreliable:
Results are unpredictable
Models under estimate outages on some pathsSlide34
Radio Wave Propagation is a Function of Atmospheric Refractivity
n =
index of refraction 1.000319N = refractivity = (n – 1) 10
6
= dry component + wet component
dry component = [ 77.6 p ]
/
[ 273 +
T
]
wet component = [3.73 x 105 eS HR ] /
[ 273 +
T
]
2
p = atmospheric pressure in
millibars
= 1.33 (pressure in mm of mercury)
= 33.9 (pressure in inches of mercury)
T
= temperature in degrees Centigrade
H
R
= relative humidity (percent) / 100
N is a weak function of p
Refractivity increases as H
R
increases or T decreases
See G. Kizer,
Digital Microwave Communication
,
Chapter 12, pages 461 to 463.
What Causes Obstruction FadingSlide35
The electromagnetic wave is reflected or refracted depending upon the angle of incidence
If refracted, the electromagnetic wave bends toward region of higher refractivity
Reflection
Refraction
Optical Wave PropagationSlide36
Radio Wave Propagation is a Function of Atmospheric Refractivity
Without wind or rain atmospheric refractivity a function of weather
When radio wave is inside a slab of high refractivity air, wave bends down (moves toward region of higher refractivity)
When radio wave encounters a nearby slab of significantly different refractivity air, wave is reflected if angle of incidence is less than Brewster’s angle (<< 10 degrees)
Radio Wave PropagationSlide37
Obstruction fading for a path inside a high refractivity layer
Layer acts like a large lake with slowly moving waves
Above a layer of different refractivity air, the radio wave is subjected to reflective (interference) fading
Within a layer of different refractivity air, the radio wave is subjected significant bending (earth bulge) or trapping (ducting)
Reflective fading for a path above a high refractivity layer
Obstruction Fading as a Function of HeightSlide38
October 24
th
Early Morning OutagesSlide39
October 27
th
Early Morning OutagesSlide40
October 24, 2013
Outages Occurred at Times of No Wind, Low Temperature and High Humidity
These conditions are conducive to formation of a dense atmospheric layer
Example of Jackson, Mississippi for day of October 24
th
(see
www.wunderground.com/history
)
See local airport for more exact weather.
Low Temperature
High Relative Humidity
No Wind
(Static Atmosphere)
Weather HistorySlide41
October 25, 2013
A typical day had no path propagation outages
Example of Jackson, Mississippi for day of October 25
th
(see
www.wunderground.com/history
)
See local airport for more exact weather.
Weather HistorySlide42
October 27, 2013
Outages Occurred at Times of No Wind, Low Temperature and High Humidity
These conditions are conducive to formation of a dense atmospheric layer
Example of Jackson, Mississippi for day of October 27
th
(see
www.wunderground.com/history
)
See local airport for more exact weather.
Low Temperature
High Relative Humidity
No Wind
(Static Atmosphere)
Weather HistorySlide43
For a graphical depiction of unusual air flow, go to
www.wunderground.com/history. Select Jackson, MS, Oct 27, View, and then select View Animated Radar Loop. Watch the cold moist Gulf air come inland in early morning and then blown out later that day.
Weather HistorySlide44
If reflective fading occurs during periods of unusual weather, we are dealing with “abnormal propagation” that cannot be predicted accurately.
This fading can significantly expand the fading season (summer for normal multipath, fall and winter for abnormal weather layering).Expect path to have history of outages significantly longer than one second.Obstruction FadingSlide45
Typical Remedies for Obstruction Fading:
Decrease the distance between sitesThe effect of distance reduction cannot be predicted accuratelyIncrease the antenna heightsThe placement of antennas cannot be predicted accurately without radiosonde measurements from nearby airports
Convert to space diversityImprovement typically moderate
Convert to adaptive
modulation
Improvement
can be dramatic
Turn linear routes into rings
Weather anomalies must be
localized
Powerful with tall antennas (moderate uncorrelated outages)Slide46
Linear router based paths will experience longer outages than the underlying microwave paths
Error extensive can be as much as two orders of magnitudeActual path performance may be significantly different than estimatesUse architecture (rings or meshes) to tame path performance.System performance can significantly exceed the performance of cascaded paths.Lessons LearnedSlide47
Remember
Availability estimates and clearance guidelines are no guarantee of path performance !> Use architecture to overcome propagation surprises <Slide48