OSI Layer 1, part II: fiber

While part I covered copper, fiber standards differ. Also, fiber always uses a dedicated cable for each direction, so it’s always full-duplex. The official fiber standards for Ethernet (using small ‘x’ as a wildcard):

802.3j – 10BASE-F
One letter up from 10BASE-T is the standard for 10 Mbps over fiber. It’s never been widely adopted, most likely because fiber was (is) more expensive compared to existing (telephone) copper wiring so no new investments were done, just to get the same speed.

802.3u – 100BASE-FX
Yes, the same standard as copper, they were defined together. Note that 100BASE-SX products were also made by many vendors, but it was never officially made a standard. It was significantly cheaper compared to 100BASE-FX . The first uses lasers and can go up to 2 km on multi-mode fiber, while the latter often used cheaper LEDs but only went up to 550 m.

802.3z – 1000BASE-X
The gigabit standard for fiber was defined before the copper standard. The standard defines multiple different cables and wavelenghts, but generally speaking it allows multi-mode fiber up to 550 m and single-mode fiber up to 5 km. Longer distances are possible using higher quality fibers.

802.3ae – 10GBASE-xx
The standard defines multiple modes of operation. In multi-mode, most used standards are 10GBASE-SR (400 m) and 10GBASE-LRM (802.3aq, 220 m). Single-mode has 10GBASE-LR (10 km) and 10GBASE-ER (40 km).

802.3ba – 40GBASE-xR4, 100GBASE-xR4 & 100GBASE-xR10
One standard defining two different speeds. For 40 GE, the -xR4 means four different physical wires in each direction are used. These cables have eight or twelve smaller fiber cables inside (in case of twelve cables, four are currently unused), each running at 10 Gbps. Data is spread across these fibers in a sort of ‘layer 1 port-channel’ fashion.
There’s not much information on 100 GE cable types yet. It seems either 10 fiber strands are used in each direction, at 10 Gbps, or 4 fibers at 25 Gbps each.
The distance is the same for both: 100-125 m over multi-mode fiber (depending on the quality: OM3 or OM4) and 10 km over single-mode fiber.

Cable types
There are three different types of cables: multi-mode step index, multi-mode graded index, and single mode fiber.

Source: Wikipedia

Multi-mode step index is widely used: typically usable up to a few hundred meters, relatively cheap.  The graded index is similar, except due to the different (graded) densities of the glass inside the cable there’s no single reflection surface, but rather a ‘bending’ of the laser inside. This gives less attenuation (weakening) of the signal.
Single mode uses a very small core fiber, so the laser generally follows a more straight path towards the next device. This results in much less attenuation and allows the laser to cross a distance of multiple kilometers.

Propagation speed
A widely accepted thought is that fiber is faster compared to copper, because light propagates at 300,000 km/s and electrical signals at about 200,000 km/s.
However, in a recent session about ultra low latency designs, Lucien Avramov proved this to be a misconception: a typical multi-mode fiber has a refraction index of 1,5 because the lasers bounces (or bends) off the internal surface of the fiber, making the signals propagate at about… 200,000 km/s. Copper and fiber are the same in this regard, with signals travelling at 5 ns (nanoseconds) per meter. Fastest cable? A twinax cable, at 4.3 ns per meter, due to the higher quality metal inside, allowing for faster propagation. However, a twinax is only limited up to 5 meters in passive mode and 10 meters in active mode. Taking into account that a typical copper SFP connector and active twinax connector introduces more latency than a fiber SFP, fiber is still the best option for ultra low latency environments were you need to run more than 5 meters of cable.

Connectors
That’s the speed and cable types, but what about the connectors? Often fiber connectors aren’t present on a networking device, but rather plug-in connectors are used, in most cases hot-swappable. For 100 and 1000 Mbps, on older switches, GBIC connectors are used:

These are very wide and take up a lot of space. For this reason, Small Form-Factor Pluggable (SFP) connectors were made, on which mode gigabit fibers (and copper cables too) terminate these days:

For 10 Gbps, SFP+ modules are used, which look nearly identical to the SFP modules. An SFP module also fits in a SFP+ slot. These SFP and SFP+ interfaces are the same size as typical RJ-45 interfaces, so switches with 24 SFP ports are not uncommon.
40 Gbps currently uses no clearly defined module, but often these are used:

This is a 40 GE cable. The modules are attached to the cables. This is a thick cable as there are 8 or 12 smaller ones inside.
Finding an image of a 100 GE cable proves to be impossible for now, but for comparison an image of the 100 GE module of a Nexus 7000:

These are just two ports, yet they cover most of the front panel. Most likely, towards the future, smaller formats will be introduced.

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