Add-Drop (Link)
Understanding the Add-Drop Link
If a telecom carrier wants to provide 5G to 5 small farming towns located in a straight line along a massive highway, laying 100 miles of fiber-optic cable in the dirt is far too expensive. Building 5 completely separate microwave links from the main city is also a massive waste of radio spectrum. The brilliant solution is to build a single Add-Drop Link.
The Microwave Daisy-Chain
An Add-Drop Link is essentially a data train making stops along a track.
- The massive Central Hub in the city blasts a huge 10 Gbps microwave beam to Town #1.
- Town #1 receives the entire 10 Gbps beam. The router at the tower acts as an Add-Drop node. It "Drops" (extracts) 2 Gbps of data to feed the local smartphones in the town.
- The router then "Adds" the local users' upload data back into the stream, and immediately blasts the remaining 8 Gbps microwave beam down the highway to Town #2.
- Town #2 takes what it needs, and blasts the rest to Town #3.
The Massive Cost Savings
By daisy-chaining the towers together, the telecom carrier only has to pay the government for one set of microwave spectrum licenses. They create a massive, 100-mile long high-speed data pipe that perfectly services every small town along the route without requiring massive, redundant central hub antennas.
Key Equations
ILexpress = ILmux+ILdemux (pass-through)
ILadd/drop = ILfilter+ILswitch
Channel isolation:
ISO = 20log(Pdesired/Padjacent) dB
Target: > 25 dB
ROADM CDC (colorless/directionless/contentionless):
Any λ to any port to any direction
Comparison
| Architecture | Flexibility | IL | Cost | Application |
|---|---|---|---|---|
| Fixed OADM | None | 3–6 dB | Low | Static networks |
| ROADM 2-degree | Moderate | 6–10 dB | Medium | Ring networks |
| ROADM CDC | Full | 8–14 dB | High | Mesh networks |
| WSS-based | Wavelength flex | 5–8 dB | High | DWDM core |
| MCS-based | Port flex | 10–15 dB | Moderate | Access |
Frequently Asked Questions
What is the massive flaw of an Add-Drop Link?
The Single Point of Failure. Because it is a daisy-chain, if the microwave dish at Town #2 is struck by lightning and violently destroyed, the physical chain is broken. Town #3, Town #4, and Town #5 will instantly lose all internet access. To prevent this, elite carriers build 'Ring' topologies. The link loops all the way back to the main city. If Town #2 dies, the data simply reverses direction and travels backward around the ring to keep the other towns alive.
Does the data slow down at every jump?
Yes, latency is a factor. Every time the massive radio wave hits a tower, the router must catch the wave, demodulate the data, extract the local packets, repackage the wave, and blast it to the next tower. This digital processing takes a fraction of a millisecond. If the Add-Drop Link is 20 towers long, the tiny delays stack up, causing noticeable 'ping' issues for the users at the very end of the line.
Can an Add-Drop Link change capacity dynamically?
Yes. Modern links use advanced Ethernet switching. If Town #1 is hosting a massive music festival and suddenly needs 8 Gbps of capacity, the central network will dynamically shrink the data allocations for the other 4 towns, giving the vast majority of the pipe to Town #1 for the duration of the festival, maximizing the efficiency of the massive microwave trunk.