How does distance affect optical module selection?

Distance is the primary driver of optical module choice. Matching module reach to actual link length affects cost, power, and reliability.

Reach Categories

• SR (Short Reach): 100-500m, MMF, 850nm
• LR (Long Reach): Up to 10km, SMF, 1310nm
• ER (Extended Reach): Up to 40km, SMF, 1550nm
• ZR (Zulu Reach): Up to 80km, SMF, 1550nm
• Extended ZR: Up to 120km with amplification

Selection Matrix by Distance

Don't Over-Provision

Using a 40km module on a 100m link is problematic:

• Wastes money (ER costs 5-10× more than SR)
• Receiver may saturate (needs attenuator)
• More expensive spares
• Higher power consumption

Don't Under-Provision

Marginal link budget causes issues:

• Higher error rates
• Failures with temperature swings
• Cannot compensate for connector aging
• Reduced service life

Match to Fiber Type

• MMF: SR modules only
• SMF: LR, ER, ZR modules
• Verify fiber grade (OM3/4 for 10G SR at longer distances)

Real Link Length Calculation

Actual optical path may be longer than straight-line distance:

• Cable routing (not straight)
• Patch panels add 5-10m each side
• Riser cabling in buildings
• Slack storage in trays
• Add 15-25% to physical distance

Future-Proofing

• Slightly over-specify for growth (LR instead of LX for 5km link)
• Consider higher-speed upgrades (10G optics on 1G speed today)
• Track fiber attenuation over years (may need better modules eventually)