SFP: Add-in module delivers diminutive performance, flexibility

Want to network-connect your gear using prevalent wired Ethernet? Or interference-impervious optical fiber? How about a bandwidth boost? Or a range extension? SFP and its siblings do it all.

One recent sequential-coverage cadence of mine just wrapped up, with part 4 of the “Debugging intermittent Comcast” run done. Another, my longstanding TP-Link smart plug teardown series, is nearing the finish line, with its next entry queued up to appear on EDN a week from next Monday and its final entry scheduled for next-month publication.

But if indeed all good things must sooner or later come to an end, other good things can also emerge in their stead. That’s what I hope will be the case for the small form-factor pluggable (SFP) module teardown series set to start next Monday. I first learned of SFP through my efforts to galvanically isolate my various LAN devices from lightning EMI-prone Ethernet and coax cables running outside of my residence.

Multi-gig gains ascendancy

I’ve subsequently become intrigued (also with pending editorial-coverage consequences) with the increasing (and dramatically so) cost-effectiveness of mainstream network switches, routers, and other devices based on 2.5 GbE technology. Take a look, for example, at this enterprise managed switch from the late 2000s, which cost several thousand dollars when brand new.

Granted, it has 24 primary Ethernet ports, but they “only” offer 10/100 Mbps. At far left are two more GbE Ethernet ports. And in-between the two RJ-45 arrays are two 1 Gbit SFP ports.

Fast-forward to today. This switch is admittedly unmanaged and has only eight RJ-45 ports.

But those RJ-45 ports are 2.5 GbE. The SFP ports are next-gen SFP+, 10 GbE. And the price tag? $41.39 at Amazon as I write these words.

Or this one:

Four fewer RJ-45s, albeit still 2.5 GbE. Once again, two 10 GbE SFP+ ports. And the price? $31.99. One of them is on an Amazon delivery truck headed to me later today as we speak, in fact. And in a near-future planned post, I’ll even detail how it’s possible to (and I in fact did) transform one into a fully user-managed variant using hacked factory firmware and/or open-source software.

Why 2.5 GbE (along with, to a lesser extent, 5 GbE) has become mainstream is a topic for another post another day (soon). Similarly, I’ll save for the near future more discussion on why 10 GbE SFP+ ports are appearing on mainstream gear like this. Today, in advance of a plethora of teardowns to come on a diversity of module variants I’ve been collecting in recent weeks, I just want to focus on what SFP is, along with its predecessor and siblings.

Without further ado, and focusing predominantly on the “flavors” most commonly used in consumer and workgroup settings, therefore in highest production volume, which typically translates to lowest cost (if you feel like your head’s about to explode after absorbing the full suite of SFP implementation options documented on Wikipedia, it’s perfectly understandable!)…

Mechanical form factors

Before SFP, there was GBIC, the gigabit interface converter, initially defined in 1995 and used with Gigabit Ethernet and Fibre Channel. As Wikipedia notes, “By standardizing on a hot swappable electrical interface, a single gigabit port can support a wide range of physical media, from copper to long-wave single-mode optical fiber, at lengths of hundreds of kilometers.”

Keeping in mind inevitable bandwidth extrapolation, thanks to further technology evolution, the same basic definition applies to 20-pin SFP, therefore explaining its alternative name, mini-GBIC.

Quad SFP (QSFP), as the name implies, supports four simultaneous bidirectional data lanes (therefore the 38-pin connector). The first picture above is of a standalone transceiver; the second shows an active optical cable (AOC) version conceptually like, albeit of course more complex than, the SFP-based ones I’m currently using in my network for galvanic isolation purposes.

The module is of the same height (8.5 mm/0.33 in.) as SFP, as is the XFP module I’ll discuss next. But it’s wider than SFP (18.35 mm/0.722 in. vs 13.4 mm/0.53 in.), although adapters can allow SFP modules to fit in QSFP sockets. And it’s also deeper than SFP; 72.4 mm/2.85 in. vs 56.5 mm/2.22 in.

Last, and least common nowadays, is another SFP precursor, aforementioned 30-pin XFP, dating from 2002. It’s even deeper than QSFP, 78.0 mm/3.07 in. The above photo is of it alongside SFP.

System interfaces

Commonplace SFP interfaces run at 100 Mbps and 1, 2.5 and 5 Gbps. The bitrate similarly to Ethernet counterparts is not accidental SFP+ leverages the same SFP mechanical form factor discussed earlier but runs at 10 Gbps and 25 Gbps, the latter alternatively known as SFP28. Less common 50 and 100 Mbps SFFP+ variants (SFP56 and SFP112) are also available, as are “DD” double density flavors which leverage up to 8 data lanes. Higher speed SFP+ versions migrate from non-return-to-zero (NRZ) modulation to four-level pulse-amplitude modulation (PAM-4).

Interconnect options

SFP modules connect to each other, as well as directly to system in some cases, via three main cable material and associated transceiver options: fiber optics in conjunction with electro-optical converters, RJ-45 Ethernet, and basic copper wire.

I’ll discuss fiber optics in more detail in the next section; for now, I’ll note the following:

Both plastic and glass cable construction material options are available. Plastic characteristics include (with glass characteristics essentially the exact opposite):
- Lower cost
- Greater flexibility and overall handling safety
- But much shorter usable distance due to high attenuation loss
- Low tolerance of temperature extremes
When the cable is permanently installed to SFP modules on both ends, it’s referred to (as alluded to earlier) as an active optical cable (AOC).

RJ-45 modules mate the SFP or SFP+ circuitry to an Ethernet transceiver. Speeds up to 10 GbE, such as with the module shown at the top of this section, are widely available. These modules tend to run “hotter” than fiber optical or basic wire alternatives, all other factors being equal.

Passive direct-attached-cable (DAC) wire harness-based cables are the most elementary version of this particular form factor, with the shortest effective range. Active copper cables (ACC), as a helpful white paper from NADDOD explains, “use a redriver chip architecture, employing continuous time linear equalization (CTLE) to boost signals on the receiver (Rx) side, acting as analog signal amplifiers.” And active electrical cables (AECs) “are more advanced, using a retimer chip architecture to amplify and equalize signals at both transmitter (Tx) and receiver (Rx) ends, with added clock data recovery (CDR) to reduce jitter, offering higher signal integrity and clearer data transmission.”

Wavelengths

SFP modules most commonly run at the following wavelengths (all are center frequencies):

850 nm (“multimode”)
1300 nm (“multimode”)/1310 nm (“single-mode”)
1550 nm (“single-mode”)

The distinction between multimode and single-mode fiber optics is important to comprehend and keep in mind, as the two technologies are not interchangeable (although some modules will work with both associated cable material types).

Multimode was historically much less expensive to implement, at the tradeoff of lower usable transmission distance. It features a comparatively larger cable core (50 to 62.5 microns) that lets multiple light signals travel down different paths at the same time, and it usually uses lower-cost LEDs or vertical-cavity surface-emitting lasers (VCSELs).

Single-mode features a comparatively tiny core (about 8 to 10 microns), which allows only a single ray of light to pass straight through without bouncing off the edges, and it uses focused lasers as a light source. Its historical cost disadvantage versus “multimode” has more recently decreased, due in part to the availability of non-proprietary, widely compatible modules. And as noted earlier, it generally specifies much longer usable transmission distances.

Cable tiers

We’ve already discussed fiber optic cable materials and construction options, along with associated light source and reception approaches. Each combination also has multiple quality tiers, which are commonly color-coded for ease of user recognition and interpretation.

Multimode cable comes in OM1 through OM5 options, with OM1 and OM2 now in legacy status and OM3 and OM4 most common nowadays. The fundamental tradeoffs between them involve lower cost (OM3) versus higher modal bandwidth and longer transmission spans (OM4).

For single-mode fiber optics, it’s simpler—OS1 and OS2—although the two types are incompatible in that they cannot be directly connected to each other. Cost, bandwidth and transmission distance are again the predominant evaluation criteria between them, although construction variances also tend to favor OS1 for indoor use and OS2 for outdoor applications.

Fiber connectors

Legacy GBIC deployments used the Standard (or Subscriber) Connector (SC) to mate cables to modules. Newer SFP-based implementations have switched to the much smaller Lucent Connector (LC). AOC fiber interconnect with SC plugs on one end and LC plugs on the other is also commonly available to bridge legacy and newer networking hardware.

Module flavors

As mentioned earlier, I’ve got a bunch of modules in hand, which I plan to tear down and internals-share with you in the coming months. As you can likely already imagine, the implementation diversity inherent in combining the numerous technology variables discussed in the previous sections results in oft-“interesting” module results. Here’s what I’ll be dissecting:

1 Gbit SR (short range, multimode) SFP module
1 Gbit LX (long range, usable with both single-mode and multimode cable at differing distances) SFP+ module
1 Gbit SFP to RJ45 transceiver module
5 Gbit ZX+ (extended long-haul range, single-mode) SFP module
10 Gbit IR (intermediate range, single-mode) SFP+ module
10 Gbit 0.3 meter/1 foot DAC cable
25 Gbit SR (short range, multimode) SFP+ (SFP28) module
40 Gbit SR QSFP+ module

The last one, whose image is at the top of this section, is particularly interesting (at least to me). It’s a 1 Gbit “BX” SFP module, with BX standing for bidirectional. Compared to the prior fiber-based modules, which use one strand for transmission and the other for reception (so you need to be sure when you hook them up that each strand’s transmission connection on one module end mates up with the other module’s receiver connection at the other end, and vice versa!), a BX module both transmits and receives across a common single cable strand.

The wavelengths employed by each module are vendor-specific, so you need to be careful in reading the specifications to ensure that you’ll end up with a transmit-and-receive wavelength matched pairing on both ends of the cable. Or just play it safe and buy all your modules from a single supplier, using a common model number.

And here’s a further “wrinkle” on the concept; in the above picture, since only a single strand is in use, there’s only one exposed optical connector site necessary. cSFP modules instead continue to use both fiber cable connectors, combining two bidirectional electro-optical subsystems in one module for doubled per-cable transfer rates. Tricky, eh?

That’s all I’ve got for you today. Look for my initial module teardown in the series, of the aforementioned 10 Gbit LX SFP+ module, to come early next week. And until then, I as always welcome your series-so-far thoughts in the comments!

—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.

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