SFP, SFP+ and DAC: Choosing the Right Uplink for Switch-to-Switch Links

The uplink between cabinets is where most surveillance networks are quietly under-built, and where a wrong module turns a commissioning day into a support ticket. The good news is that SFP selection is mostly bookkeeping: speed, reach, fibre type, wavelength and coding. Get those five right and the link comes up first time.

1. Match the cage, the speed and the port

The cage on the switch defines the fastest module it will accept, and a slower module in a faster cage is normally fine — the reverse is not.

• A 10G module in a 1G cage will not negotiate down; check the cage label, not the datasheet headline.
• Some switches accept 1G modules in SFP+ cages and some do not — this is a specification detail worth confirming.
• Port count matters as much as speed: two uplinks allow a ring or a failover pair.

2. Choose reach class by the fibre you actually have

Reach is set by the optics, and the optics must match the fibre in the ground. Multimode (OM2/OM3/OM4) inside a building; singlemode between buildings.

• Both ends must use the same class; SR against LR will not link.
• Wavelength must match pair-to-pair (850/850, 1310/1310).
• Duplex vs simplex (BiDi) matters: a BiDi pair uses two different wavelengths and must be bought as a set.
• Distance claims assume clean connectors — budget loss for patch panels and dirty ferrules.

3. DAC and AOC: cheaper, shorter, simpler

Inside one rack, a passive DAC (direct attach copper) is cheaper than two optics, needs no cleaning, and works up to about 5–7 m depending on gauge. Active optical cables (AOC) extend that to tens of metres with the same plug-and-play behaviour.

• Use DAC for rack-internal 10G links; it is the lowest-failure option.
• DAC is fixed-length — order the length you need, not the nearest one.
• Beyond about 7 m, use optics with fibre or AOC rather than stretching copper.

4. Coding compatibility is the usual "dead link" cause

Many switches check the module's coded vendor string before enabling the port. A perfectly good generic module can be refused by a switch that expects its own coding. Two ways out: buy modules coded for the switch brand, or use a switch that accepts generic MSA modules — and confirm this before the order, not during commissioning.

Our optical converters and the SFP cages on the L2/3 managed switches accept standard MSA modules; for a single long run without a switch, the RS1002K-1F is the simple point-to-point answer.

5. The pre-order checklist

• Cage type and speed at both ends.
• Fibre type and measured length — including patch leads and panels.
• Matching reach class and wavelength at both ends.
• Coding accepted by the switch, or generic-module support confirmed in writing.
• A spare pair of modules and one spare patch lead per site.

Tell us the switch model and the fibre run and we will confirm the module pair that will link first time. Get a quote.

FAQ

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