In Praise of The Bidirectional SFP Transceiver

Every time I step into someone else’s data center or inspect a colleague’s optical infrastructure, I’m consistently struck by their overwhelming reliance on duplex SFP transceivers. What’s even more surprising is that many consider this the ‘norm’ and haven’t ventured into the realm of bidirectional transceivers. I try to rationalize it as ‘legacy technology that is operationally hard to replace.’ However, when I hear about the nightmare support scenarios, I once again fail to understand why they are kept.

But before I delve into either criticism or praise, let’s take a moment to understand what I’m addressing.

SFP (Small Form-Factor Pluggable) connectors are transceivers used in networking equipment to connect fiber optic or copper cables. They are commonly found on network switches or routers that need high-speed connections or the reliability of the optical fiber. These connectors can be classified into two main types based on their communication mode: simplex and duplex.

1. Duplex SFP Connectors:
   • Duplex SFP connectors support communication in both directions but using different cables for transmission and reception. They consist of two fiber optic cables with identical connectors on each end, allowing for bidirectional data transfer.
   • Advantages:
     • Only one connector type: There is only one connector type. As long as RX is connected to TX on the other connector, all is fine.

1. Simplex or Bidirectional SFP Connectors:
   • Simplex SFP connectors support bidirectional communication (transmission and reception) in a single fiber, by using separate wavelengths for each. Physically, they use a single fiber optic cable with matching connectors at each end. To achieve this, they use WDM (wavelength division multiplexing) technology.
   • For example, one connector might use 1310TX/1490RX wavelengths while the matching connector uses 1490TX/1310RX.
   • Advantages:
     • Simplicity: Since they involve only one fiber, they are simpler to install and manage.
     • Error free connection: With only one fiber to connect, it is impossible to erroneously swap transmission with reception.
     • Cost: Simplex connectors cost about the same as their duplex counterparts. However, since they use only one fiber, cabling is cut in half.
     • Efficiency: By using a single fiber, the bulk in a bundle of cables is cut in half, making it easier to manage
   • Standard naming:
     • BX – Single Fiber Bi-Directional Gigabit SFP Transceivers, paired as BX-U and BX-D for uplink and downlink respectively.
     • Maximum transmission distance is coded in kilometers, as BX10 for 10 Km. modules, BX20 for 20 Km., Bx40, etc.
     • Upload and Download sides are coded depending on wavelength and bandwidth using this color coding:
       

       	Side A		Side B
       Name	Color	TX wavelength	Color	TX wavelength
       1000BASE-BX	Blue	1310 nm	Purple	1490 nm
       1000BASE-BX	Blue	1310 nm	Yellow	1550 nm
       10GBASE-BX 25GBASE-BX	Blue	1270	Red	1330 nm
       10GBASE-BX	White	1490	White	1550 nm

As a practical tip, use SFPs in the same order according to their placement relative to the core switch or the entry point of the network. For example, on the core, you would use only BX-D SFP; at the other end of the cable, in the access switch, there would be a BX-U SFP. Technically, there is no difference, but preserving this standard makes it easier to pair simplex SFPs in the field, particularly if both ends are separated by a few kilometers, as in a campus setting.

Reconsiderations over time

At the turn of the century, the introduction of ITU-T G.983 for FTTX applications marked the first practical use of bi-directional communications over fiber. It became more or less commonplace in metropolitan telecommunications infrastructure, where distances between 1 to 40 km needed to be covered with the then-expensive optical fiber cabling. Besides the other benefits of choosing passive optical networks, the expense of optical fiber justified attempting to save half the cost by using one fiber strand instead of two, especially when single-mode was more expensive than multi-mode fiber.

Meanwhile, in the datacenter, the most popular optical cabling format was duplex multi-mode. Leaving the single-mode/multi-mode discussion aside, using a single cable instead of two (duplex) can be justified in the datacenter because of the short hauls.

Coming to our days, optical fiber, especially single-mode, has become cheap, and equivalent simplex and duplex SFP connectors cost about the same. This change in circumstances makes it opportune to re-evaluate the use of bi-directional transceivers everywhere.

In summary, nowadays, there is no good reason to use duplex connectors. The primary difference between simplex and duplex SFP connectors lies in their ability to handle data transmission with a single fiber instead of two. Cost and complexity lower when dealing with a single-stranded cable.