1+1 Configuration
Understanding 1+1 Configurations
When telecommunication providers are routing aggregated data for an entire city (e.g., a core backbone link carrying 10 Gigabits per second), a hardware failure is completely unacceptable. To achieve "Five Nines" of reliability (99.999% uptime, equating to barely 5 minutes of downtime per year), engineers mandate a 1+1 Configuration.
Instead of mounting one radio to the tower, the engineer mounts two.
Types of 1+1 Redundancy
| Redundancy Type | The Architecture | The Engineering Benefit |
|---|---|---|
| 1+1 Hot Standby (HSB) | Two ODUs share a single antenna via an RF coupler. ODU 'A' transmits all the data. ODU 'B' is powered on but its transmitter is muted. Both ODUs receive the incoming signal. | Hardware Protection. If ODU 'A' suffers a power failure or its amplifier burns out, the IDU instantly unmutes ODU 'B', restoring the link in less than 50 milliseconds. |
| 1+1 Space Diversity (SD) | Requires two ODUs and two separate antennas mounted 20 feet apart on the exact same tower. | Atmospheric Protection. During the summer, temperature inversions cause microwave beams to bend and cancel each other out (Multipath Fading). The fade might destroy the signal at the top antenna, but the bottom antenna (20 feet lower) will see a perfect signal. The system instantly switches to the bottom antenna. |
| 1+1 Frequency Diversity (FD) | Two ODUs share one antenna, but they transmit on two completely different frequency bands (e.g., 11 GHz and 18 GHz). | Rain Fade Protection. A heavy rainstorm will utterly destroy an 18 GHz signal, but the 11 GHz signal will punch right through the storm. The link survives the weather event seamlessly. |
The Cost of Protection
While 1+1 configurations offer peace of mind, they double the Capital Expenditure (CapEx). You must buy two Indoor Units, two IF cables, and two Outdoor Units. Furthermore, if using Space Diversity, you must pay exorbitant tower leasing fees to the tower owner because you are occupying two physical antenna spots on the steel structure.
Key Equations
Both radios powered, one active
Asys = 1−(1−A1)×(1−A2)
Switchover criteria:
RSL degradation, BER > threshold, equipment alarm
Switch time: <50 ms (hitless)
Availability improvement:
99.99% per unit → 99.9999% system
Improvement: 1/(1−A) additional nines
Comparison
| Mode | Active | Standby | Switch | Application |
|---|---|---|---|---|
| HSB (hot standby) | 1 TX/RX | 1 warm | <50 ms | Standard backbone |
| SD (space diversity) | 1 TX, 2 RX | Diversity combine | <50 ms | Multipath fade |
| FD (freq diversity) | f1 | f2 | <50 ms | Rain fade |
| Hybrid (SD+FD) | Both | Both | <50 ms | Maximum A |
| 1+1 hitless | Both TX | Both RX | 0 ms | Critical link |
Frequently Asked Questions
What is the switching time for a 1+1 system?
Carrier-grade telecommunications standards (like SONET/SDH or Carrier Ethernet) mandate that a 1+1 protection switch must occur in under 50 milliseconds. This speed is fast enough that phone calls will not drop, and streaming video will not buffer during the hardware failure.
Does 1+1 double the data speed?
No. In a pure 1+1 protection setup, the second radio is strictly a backup. If the active radio transmits 1 Gbps, the total link capacity is 1 Gbps. If you want to use both radios to double the speed to 2 Gbps, you must change the configuration to a '2+0' setup, which sacrifices the hardware redundancy for raw bandwidth.
What is an RF Coupler?
In a 1+1 HSB setup, two radios must share one antenna. An RF Coupler (or Magic Tee) is a passive waveguide block that bolts between the two radios and the antenna flange. It splits the incoming received signal equally (50/50) to both radios so they can both listen simultaneously.