Friday, March 20, 2015

An overview of SFPs for interconnecting switches with fiber

Those of us who deploy Wi-Fi networks are often expected to understand the wired side of the network just as well as the wireless side.  Thus, as a primer, here is an overview of SFPs that I needed to put together for one of my clients.

For distances of more than 100 m / 328 ft, CAT5e / CAT6a is insufficient, and thus fiber is often used to interconnect switches in high-rise and multi-building deployments.  Most layer 2 managed and smart switches contain ports to handle one or more small form-factor pluggable (SFP) modules for interconnecting switches with fiber.  The SFP converts between an Ethernet network (copper) and an optical network (fiber), and contains a class 1 laser diode for the optical transmissions.  The SFP module has a receiver port and a transmitter port to make up one optical interface.  An example of an SFP module is shown in the figure.




Since these modules are based on an IEEE 802.3z specification, any 1 Gbps Cisco-compatible mini-GBIC fiber SFP module will generally work fine in any vendor's layer 2 managed or smart network switch.   

The specific SFP module selected depends on the type of fiber and the application.   For high rise building and multi-building applications, multimode fiber (MMF) is typically used with a diameter of either 50 microns or  62.5 microns.  Single mode fiber (SMF) is sometimes used in building or between neighboring buildings, but SMF is generally more expensive and is really intended for very long distance applications (i.e. many miles).   The fiber patch cable between the switch and the fiber patch panel should match whatever fiber type and diameter is used in the building(s).  Typically, most mini-GBIC fiber SFP modules have an LC interface.   The fiber patch panel could have an LC, SC, or ST interface, and fiber patch cables are available to interconnect the LC interface on the mini-GBIC to any of these interfaces.

They also come in different operating frequencies, depending on the application:


  • 850 nm  (Short wavelength, SX):   Used with multimode fiber for relatively short distances (220 m – 275 m / 722’ – 902’  for 62.5 micron, 500 m – 550 m / 1640’ – 1804’ for 50 micron)
  • 1300 nm (Long wavelength, LX or LH):  Used with either multimode or single mode fiber for distances up to 550 m / 1804’ (multimode) or 10 km / 6.2 miles (single mode)
  • 1550 nm :  (Extended distance, ZX):   Used with single mode fiber for 70 km – 100 km (43 – 62 miles)

On a network switch, most switches will implement a shared port, meaning that there will be both an RJ-45 and an SFP interface that share the same port, and either physical connection can be used on that port (but not both physical connections simultaneously).   Some vendors do manufacture switches with dedicated SFP ports.  Dedicated SFP ports are also common on core switches (e.g. HP Procurve 6200yl), where most or all of the ports on the switch are SFP ports for interconnecting with remote Ethernet switches.   To connect to these types of "SFP only" switches, there are SFP modules manufactured that provide an RJ-45 interface.

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