1+1 HSB
Understanding 1+1 Hot Standby (HSB)
When a telecommunications tower is located on a snowy mountain peak accessible only by helicopter, a simple hardware failure (like a blown capacitor in a radio transmitter) could take a week to fix. To prevent a week-long citywide internet outage, engineers deploy a 1+1 HSB microwave link.
The term Hot Standby means the backup equipment is fully powered on, fully booted up, and actively locked onto the network timing. It is sitting 'hot,' ready to take over the exact millisecond it is needed, unlike 'Cold Standby' equipment which sits in a box in a warehouse.
The Mechanics of the RF Coupler
To put two Outdoor Units (ODUs) on one antenna, engineers use a passive waveguide device called a Coupler (often an asymmetrical hybrid coupler).
- The Receive Path (Listening): When the microwave beam hits the antenna, the coupler physically splits the wave. It sends half the power to ODU A, and half the power to ODU B. Because both receivers are actively listening, the Indoor Unit (IDU) constantly compares the signal quality of both. If Receiver A starts generating bit errors, the IDU instantly grabs the flawless data stream from Receiver B.
- The Transmit Path (Speaking): You cannot have both radios transmitting at the same time, or their waves will collide in the coupler and destroy the phase. ODU A transmits at full power. ODU B is mathematically commanded to output absolute zero power (muted). If ODU A's amplifier dies, the IDU instantly mutes A and commands B to output full power.
The Asymmetrical Insertion Loss Penalty
Using a coupler introduces significant physics penalties. A standard symmetrical (50/50) coupler destroys exactly 3 dB of transmit power and 3 dB of receive power just to split the signal.
To maximize the link distance, engineers often use Asymmetrical Couplers (e.g., a 10 dB coupler). In this setup, the primary radio suffers only a tiny 1 dB loss passing through the coupler. The backup radio suffers a massive 10 dB loss. Because the backup radio is only used in absolute emergencies, the network operator accepts the fact that the link will have a severely degraded signal margin during a failover event, as long as the primary link operates flawlessly during normal conditions.
Key Equations
Detection: RSL < threshold or BER > 10−6
Switch: RF relay or IF switch
Revert: Manual or auto (with hysteresis)
Hitless switching:
Both paths active, receiver selects best
No traffic interruption (0 ms)
MTBF improvement:
MTBFsys = MTBF2/(2×MTTR)
Comparison
| Parameter | HSB | Hitless | FD 1+1 | Notes |
|---|---|---|---|---|
| Switch time | <50 ms | 0 ms | <50 ms | Hitless best |
| Traffic loss | Yes (brief) | None | Yes (brief) | Hitless best |
| Cost | Moderate | Higher | Higher | Extra radio |
| Spectrum | Same freq | Same freq | 2× freq | FD needs 2f |
| Protection type | Equipment | Equipment | Propagation+equip | FD covers rain |
Frequently Asked Questions
Can 1+1 HSB protect against Rain Fade?
No. Hot Standby is purely a hardware protection scheme. Because both ODUs are attached to the exact same antenna and transmit on the exact same frequency, if a torrential rainstorm blocks the microwave beam in the atmosphere, switching to the backup radio will accomplish nothing. The backup beam will be blocked by the rain just like the primary beam.
Why is switching time so critical?
In legacy TDM (Time Division Multiplexing) networks carrying thousands of live voice calls, a break in the data stream lasting longer than 50 milliseconds will cause the central office switches to drop the calls entirely. 1+1 HSB guarantees the hardware switch happens fast enough to prevent dropped calls.
What is hitless switching?
Hitless (or errorless) switching occurs on the Receive side of a 1+1 HSB link. Because both receivers are actively listening to the exact same signal, the IDU can seamlessly stitch their data streams together. If Receiver A starts failing, the IDU can switch to Receiver B without dropping a single bit of data. Transmit switching, however, is never hitless; it always causes a momentary micro-outage.