Glossary term

Link Availability

Engineering definition of link availability covering outage time, service availability, rain fade, adaptive modulation, link margin and validation evidence.

Branch: Telecommunications Engineering
Glossary type: metric
Content: Glossary term
Updated: Jun 26, 2026
Revision: v1.0.0 · reviewed

Definition

metric

Link availability is the fraction of an observation period during which a communication link meets the stated service or performance threshold.

Link availability turns RF, optical or packet-link behavior into a service metric. A link may be considered unavailable when carrier is lost, when BER or packet loss exceeds a limit, when capacity falls below a committed rate, when latency violates a target or when a required protected path is not usable. The definition must state the boundary and threshold, not just whether the radio or optical receiver remains synchronized.

Link availability is the fraction of time during which a communication link meets the stated service threshold. The threshold may be carrier lock, received power, SNR, bit error rate, packet loss, committed capacity, latency or a protected-route condition. The key point is that availability is defined by the service rule, not by a vague impression that the link is “up”.

This distinction matters in microwave, satellite, wireless and optical systems. A radio can remain synchronized during heavy rain while adaptive modulation drops capacity below the committed service. An optical receiver can stay below overload but still fail an error-rate target. A packet backhaul can pass RF checks and still violate latency during congestion.

Basic Time Definition

For an observation period:

\displaystyle A_{link}=1-\frac{T_{out}}{T_{obs}}

where:

T_obs is the total observation time;
T_out is time when the link fails the stated service threshold.

As a percentage:

\displaystyle A_{link,\%}=100\left(1-\frac{T_{out}}{T_{obs}}\right)

The inverse is outage probability over the observation period:

P_{out}=1-A_{link}

Allowed Outage

For a target availability:

T_{out,max}=(1-A_{target})T_{obs}

This converts a percentage into an operational allowance. A monthly target of 99.99% over a 30-day month has:

T_{obs}=30(24)(60)=43200\ \text{min}

so:

T_{out,max}=(1-0.9999)(43200)=4.32\ \text{min}

If observed service outage is:

T_{out}=22\ \text{min}

then:

\displaystyle A_{link}=1-\frac{22}{43200}=0.999491

or:

A_{link}=99.9491\%

The link misses the 99.99% target by:

22-4.32=17.68\ \text{min}

Service Threshold

The outage rule must say what counts as unavailable. For a microwave backhaul, carrier availability may remain high while service availability fails because adaptive modulation falls below committed capacity. If the committed service is:

R_{commit}=120\ \text{Mbit/s}

and rain fallback mode provides:

R_{fallback}=50\ \text{Mbit/s}

then the link is service-unavailable during that fallback condition even if the radio remains locked:

R_{fallback}<R_{commit}

For a telemetry link, the threshold may be packet success probability or command latency. For an optical span, it may be received optical power, BER and protection switching time.

Partial Degradation

Availability reporting should define how partial service is counted. A link that carries emergency telemetry but not video may be available for one service and unavailable for another. A packet path that meets average throughput but violates latency for control traffic may pass a bulk-data target and fail a control-system target.

Logging resolution also changes the result. One-minute counters can hide repeated five-second interruptions, while event logs can overstate outage if they count every modulation change as a failure. The engineering rule should map raw events to a service decision before the availability percentage is calculated.

Relation To Link Margin

Link margin is an instantaneous or scenario-based surplus. Link availability is the time-based result after fading, interference, traffic, maintenance, equipment faults, power events and protection behavior occur across the observation period.

More margin usually improves availability, but the relationship is not one-to-one. A link with strong clear-weather margin can still fail availability if rain fade is severe, if an obstruction removes reserve, if interference is seasonal, if backup power fails, or if adaptive modulation cannot carry committed load.

Multi-Hop Paths

For independent serial hops:

A_{path}=A_1A_2\cdots A_n

Two 99.99% hops in series give:

A_{path}=0.9999^2=0.999800

or about:

A_{path}=99.9800\%

Adding a hop can reduce rain path length and improve RF margin, but it also adds equipment, power, site and maintenance exposure. The correct design depends on path-level service availability, not on one hop margin alone.

Validation Evidence

A defensible availability claim states the service boundary, observation period, outage threshold, measurement source, time resolution, exclusion rules, maintenance windows, capacity criterion, error-rate criterion, latency criterion, weather correlation, protection-switching behavior and whether partial degradation counts as outage.

Common mistakes include reporting carrier uptime when committed capacity is unavailable, mixing planned maintenance with service outage without a rule, using one clear-sky drive test to claim annual availability, ignoring short interruptions hidden by coarse logging, treating packet congestion as unrelated to RF fading, and comparing availability targets without stating the observation period.

REF

Disciplines