Five BiDi SFP Questions Answered to Improve Performance of C-band Cell Sites

Anritsu Company

February 28, 2022

Timelines for C-band site installation and commissioning are tight, with little margin for error and delay. The installation of mismatched Bi-Directional (BiDi) small form-factor pluggables (SFPs) leads to unnecessary waste in time and materials. Mobile operators and contractors need to initiate proactive testing of BiDi SFPs to achieve successful and timely C-band site deployment.

On the surface, installing BiDi SFPs appears to be straightforward. Field technicians simply plug them in and they will work. Like so many things associated with wireless networks, however, it is not always as it seems. Hence, the need for testing.

Effective and efficient BiDi SFP testing requires the proper test solutions that combine ease-of-use and accuracy in durable housing. It also needs an understanding of the components, their role in 4G and 5G base stations, and the measurements that need to be conducted. In this post, we answer five key questions that can help create an effective testing process when installing BiDi SFPs.

1. What are SFPs and What are They Used for?

An SFP is a small modular transceiver that plugs into a network element to enable a high-speed communications link to another network element. For example, it could be a communications link between a network router and a server. Another example is that an SFP is used to link communications between a Baseband Unit (BBU) and a Remote Radio Head (RRH) in a 4G/5G system.

In the optical world, there are many types of SFPs. They can be used with either single mode (SM) or multimode (MM) fiber, and over a wide range of data rates. Communication between standard 1 Gbps SFPs, which are typically used at LTE cell sites, is simplex. That means the communications link between SFPs has the same wavelength. For 4G base stations that is usually 1270 nm.

To establish two-way communications, two fibers are connected between the SFPs. One is in the transmit direction while the other is in the receive direction. In the example shown in figure 1, that means two fibers are communicating with each other in a single direction at one wavelength.

Example of a simplex SFP.
Figure 1: Example of a simplex SFP.

2. Should SFPs be Tested in the Field?

There is occasion when SFPs are tested in the field. A common reason is to verify wavelength or data rate. In general, however, they are not tested because SFPs are relatively low cost. If there are concerns about an SFP’s performance, typically one or both SFPs are replaced.

 3. What are BiDi SFPs?

Wave Division Multiplexing (WDM) diplexers are used by BiDi SFPs to transmit and receive signals over a single fiber. This has the advantage of reducing the number of fiber runs at a cell site while increasing port density. Today, BiDi SFPs are commonly used in data centers and in 5G deployments such as C-band.

WDM diplexers are able to combine and separate data transmitted over a single fiber based on wavelengths of light. That means the BiDi SFP has one port to transmit at say 1310 nm while, receiving at 1550 nm. The BiDi SFP at the other end of the optical link will communicate by receiving at 1310 nm and transmitting at 1550 nm.

4. Should BiDi SFPs Be Tested?

Unlike standard SFPs, BiDi SFPs must be deployed in matched pairs with their diplexers tuned to match the expected wavelength of the transmitter and receiver. Any mismatch in wavelengths, for example two BiDi SFPs transmitting at 1270 nm and receiving at 1330 nm (figure 2), will result in a loss of communication.

BiDi SFPs transmitting at 1270 nm and receiving 1330 nm.
Figure 2: BiDi SFPs transmitting at 1270 nm and receiving 1330 nm.

One solution to resolve the mismatch is to replace a suspected faulty BiDi SFP. Unlike standard SFPs, however, BiDi SFPs are expensive. They typically cost hundreds of dollars per unit. The potential to increase waste and expense during equipment installations rises considerably, as a result. It also raises the added possibility that the new transceiver may not work, as its not part of a matched pair.

Another mismatch scenario is a BiDi SFP that has previously been installed in a radio at the top of a tower without knowledge of its data rate or wavelength. The choice for mobile operators and/or contractors is to climb the tower, or call a tower crew, to physically check or replace the installed transceivers. The result is added expense and delay to site commissioning.

5. How do I Test BiDi SFPs?

Testing a BiDi SFP requires a test set capable of making measurements at the data rate of the transceiver. For a 5G C-band site installation, this means the test set would typically need the ability to test at the 25G data rate of the BiDi SFP.

The Anritsu Network Master™ Pro MT1000A all-in-one portable field tester can be configured to test optical transceivers at data rates of up to 100G (QSFP). To test BiDi SFPs, the MT1000A uses 2 x 25G SFP ports. In the scenario where both BiDi SFPs are tested in the MT1000A, a fiber must be connected between the transceivers to establish a communications link.

Once turned on, the field technician can verify the data rate and wavelength of each BiDi SFP. In figure 3, which is a display of the MT1000A, the SFP under test is in Port 3.1. If the light in this test doesn’t turn green, there’s a communication failure. This could mean the wavelengths are not compatible or there’s a broken fiber.

BiDi SFP measurement display from the Anritsu MT1000A
Figure 3: BiDi SFP measurement display from the Anritsu MT1000A

If needed, the results from this test can be saved as a report and included in a site closeout package, as well as for future reference. The MT1000A can also automate this test, reducing user error and test time.

Testing SFPs is only one measurement capability of the MT1000A for mobile networks. To learn more about optical testing of 5G base stations, download this 5G Mobile Network Quality Evaluation leaflet.

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