Different form factors, different rates

SFP+ is a small form-factor pluggable module for 10G links, fitting SFP+ slots on switches, routers, and servers, and carrying one 10G lane over fiber or copper. QSFP28 is a quad small form-factor pluggable module for 100G links, fitting QSFP28 slots, and carrying four 25G lanes over fiber, for a total of 100G, or one 100G lane, depending on the module's design. The two are not interchangeable: a QSFP28 module is larger than an SFP+ module, with more pins, and it cannot fit an SFP+ slot, while an SFP+ module in a QSFP28 slot, if it fits at all, cannot carry 100G, because it has only one 10G lane.

A buyer who needs 10G should order SFP+, and a buyer who needs 100G should order QSFP28, and a buyer who orders the wrong form factor has ordered a module that cannot serve, however right its rate appears on paper. Check the equipment's slot type before ordering, from its documentation, because the slot type is a fact of the equipment, and because a module ordered for the wrong slot cannot be adapted without an adapter that adds loss and failure points.

When 10G is enough

10G remains the workhorse rate for enterprise networks, campus links, and server connections where 10G suits the traffic, and where 100G would waste capacity and money. A network whose switches are 10G, whose servers are 10G, and whose traffic fits 10G needs SFP+, not QSFP28, because the equipment itself limits the rate, and because a faster module cannot make slower equipment faster. The 10G family on this site is the largest, with 176 sitemap locations, reflecting 10G's continued importance, and a buyer who needs 10G will find the SFP+ family well stocked.

10G also suits links where the distance is short, the traffic is moderate, and the budget is tight, because SFP+ modules cost less than QSFP28 modules, and because a network built on 10G can be upgraded later, when traffic grows, without replacing the fiber, which is the expensive part. A buyer who orders 10G today, with fiber that supports 100G tomorrow, has kept the upgrade path open, while a buyer who orders 100G today, for 10G equipment, has paid for capacity that cannot be used.

When 100G is needed

100G is needed where a link must carry very high traffic: between data centre switches, between data centre sites, or between a data centre and its users, where 10G links are saturated, and where aggregating more 10G links costs more than a single 100G link. A data centre whose switches are 100G, whose servers are 25G, and whose traffic grows year on year needs QSFP28, because 10G links would saturate, and because the cost of many 10G links, in ports, in power, and in management, exceeds the cost of fewer 100G links.

100G also suits links where the traffic is bursty, where peaks matter more than averages, and where a 10G link would saturate during peaks, causing loss and delay, while a 100G link would absorb the peaks without saturating. A buyer who measures peaks, not just averages, will often find that 100G is needed where 10G averages suggest it is not, because averages hide the peaks that cause the failures.

The fan-out: one QSFP28 to four SFP28

A QSFP28 port can fan out to four 25G SFP28 ports, through a breakout cable or a breakout module, so one 100G switch port can serve four 25G server connections, and a data centre can connect four servers to one switch port, saving switch ports, saving power, and saving management. The 100G AOC 10m cable on this site fans out from one QSFP28 end to four SFP28 ends, for short rack-to-rack links, and it illustrates the fan-out in a single product: one end, four ends, 100G total, 25G each.

A buyer who needs fan-out should confirm the breakout form, the cable length, and the equipment at both ends, because a breakout cable that mismatches at either end cannot be adapted without replacing it, and because a 10m cable cannot serve a 15m link. Fan-out suits data centres where servers are 25G and switches are 100G, and where the fan-out saves ports, and it does not suit networks where the servers are 10G, because a 25G server connection cannot serve a 10G server port.

Reach and fiber: the same rules apply

The reach, fiber type, and connector rules apply equally to SFP+ and QSFP28, because a 100G module's reach differs by fiber type just as a 10G module's does, and a 100G module with the wrong connector cannot be plugged in, just as a 10G module with the wrong connector cannot. A buyer who chooses QSFP28 must still match the reach to the link's distance, with margin, the fiber type to the installed fiber, and the connector to the installed connectors, because the higher rate does not relax any of those rules, and because a 100G module that mismatches on any of them will fail just as a 10G module would.

The representative pages on this site illustrate the point: the 100G pages transcribe their official title wording, with reaches of 40km, 80km, and 10km, and fiber types of single mode and multimode, and a buyer who reads those titles as specifications will overestimate what they evidence. Ask for the specification for the exact model, in writing, with the rate, the reach, the fiber type, and the connector, and compare it to the equipment's requirements, because a specification that matches on all four is a module that will serve.

Cost: count the total, not the module

A QSFP28 module costs more than an SFP+ module, but a 100G link may cost less than four 10G links, because one QSFP28 port replaces four SFP+ ports, saving switch ports, saving power, saving cabling, and saving management, and because the total cost of a link includes the ports, the power, the cabling, and the management, not just the module. A buyer who compares module prices alone will often choose 10G where 100G is cheaper in total, and a buyer who counts the total will often find that 100G, for a high-traffic link, is the cheaper choice, despite the higher module price.

Count the ports first: how many switch ports does each option need, and what do those ports cost, in hardware, in power, and in management. Count the cabling second: how many fibers does each option need, and what do those fibers cost, in installation and in maintenance. Count the power third: how much power does each option draw, and what does that power cost over the link's life. A buyer who counts all three will choose the cheaper total, not the cheaper module, and the cheaper total is the choice that matters.

Upgrading: fiber first, modules second

A network that plans to upgrade from 10G to 100G should install fiber that supports 100G today, even if it runs 10G modules today, because fiber is the expensive part to replace, while modules are the cheap part, and because a network whose fiber supports 100G can upgrade by swapping modules, while a network whose fiber supports only 10G must replace its fiber to upgrade, at far greater cost and disruption. Single mode fiber supports both 10G and 100G, and beyond, while multimode fiber's support depends on its type, and a buyer who installs the right fiber today has kept the upgrade path open.

A buyer who upgrades should also check the equipment: a switch whose ports are SFP+ cannot take QSFP28 modules, however good its fiber, and upgrading from 10G to 100G may require new switches, not just new modules. Plan the upgrade as equipment plus fiber plus modules, not as modules alone, because a module upgrade without an equipment upgrade is a module that cannot be plugged in, and an equipment upgrade without a fiber upgrade is a switch that cannot reach its links.

What to confirm, before ordering

Before payment, confirm the form factor, the rate, the reach, the fiber type, and the connector for the exact equipment, the minimum order quantity for that model, the lead time with its meaning, the warranty terms, the payment and shipping terms, and the certificates that apply to your market, by name, with documents attached. Name the equipment, its slot type, its port rate, the link distance, the fiber type, the connectors, the quantity, and the market, so the supplier can confirm for your installation, rather than for a general case.

A buyer who confirms the form factor first — SFP+ for 10G, QSFP28 for 100G — and then the rate, the reach, the fiber type, and the connector, in writing, before payment, has done the work that prevents the common failures: the wrong form factor, the wrong rate, the short reach, the wrong fiber, and the wrong connector. A buyer who skips that work has saved an hour and risked a shipment, and the hour saved is rarely worth the shipment risked.