Connectrix: How to Interpret SFP Transceiver TX and RX Power Levels as Part of Troubleshooting the SFP Transceiver (2023)


  • Adding ISL or trunk connections
  • Expanding distance on existing ISL connections
  • End device HBA or SFP replacement or upgrade
  • Customer infrastructure (cabling, patch panel, DCM, CWDM) issues or changes
  • Replacing old hardware with new hardware and using the same physical infrastructure (Cables, patch panels.)

Physical layer issues on connections can lead to connection instability causing:

  • End devices to log in and out of the fabric
  • Can cause impact to how the link performs.
  • Stops End device logging in
  • Causes issues with fabric expansion
  • SFP TX and RX power readings are not correctly interpreted.
  • SFP is incorrectly replaced based on false interpretations.
  • Interpret show interface fcx/xx transceiver details TX and RX Power.
  • Interpret sfpshow 8/22 f TX and RX Power.


  • Wrong interpretation of the TX or RX counters of the SFP outputs causes unnecessary SFP replacements.
  • Incorrect switch port SFPs with the cable length.
  • Incorrect cable
  • Customer infrastructure (cabling, patch panel, DCM, CWDM) issues or changes.
  • Failing switch port SFP.


This article is intended to assistwith the interpretation of the SFP transceiver TX and RX power readings available from the CLI.

It must also be used as an aid tothe following articles:

  • Pre SFP Replacement Procedure
  • Connectrix: How to troubleshoot Fibre Channel node to switch port or SFP communication problems by elimination.See Dell KB article28863:Connectrix: How to troubleshoot Fibre Channel node to switch port or SFP communication problems by means of elimination?

Common for Brocade and Cisco is the basic structure of the SFP:

Connectrix: How to Interpret SFP Transceiver TX and RX Power Levels as Part of Troubleshooting the SFP Transceiver (1) Connectrix: How to Interpret SFP Transceiver TX and RX Power Levels as Part of Troubleshooting the SFP Transceiver (2)

As can be seen the SFP has two receptacles into which the FC fibre cable is connected, each receptacle on the SFP represents its own channel, one channel for sending TX and the other for receiving RX making the SFP module a device that functions as both at transmitter and a receiver that is combined in the same housing that is otherwise known as a transceiver.

(Video) Brocade switch initial configuration setup

Logically the FC connection looks as follows:

Connectrix: How to Interpret SFP Transceiver TX and RX Power Levels as Part of Troubleshooting the SFP Transceiver (3)

In the example above, you could change the SFP with an HBA and it follows the same structure but for explanation we use an ISL connection. On the left is a switch port SFP which is connected using a cable to the right of another switch port SFP.

The red channel left TX transceiver is the source of the light signal. Looking at the TX power on this SFP, you are viewing the source. Looking at the power level on the right SFP RX power, you are viewing the light energy that is created by the left SFP as it passes through the cable and reaches the right SFP and the opposite way for the green channel.

How to view the light levels

Brocade CLI command:

  • Sfpshow [slot]/port fadmin> sfpshow 8/22 -fIdentifier: 3 SFP


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 Alarm Warn low high low highTemperature: 46 Centigrade -5 85 0 75Current: 7.428 mAmps 2.500 12.000 2.000 11.500Voltage: 3295.6 mVolts 3000.0 3600.0 3130.0 3460.0RX Power: -3.3 dBm (466.6 uW) 31.6 uW 1258.9 uW 31.6 uW 794.0 uWTX Power: -2.5 dBm (560.4 uW) 126.0 uW 1258.9 uW 251.0 uW 794.0 uW

Cisco CLI command

  • Show interface fc module/port transceiver details.
# show interface fc1/12 transceiver details


---------------------------------------------------------------------------- Alarms Warnings High Low High Low---------------------------------------------------------------------------- Temperature 47.05 C 89.00 C -9.00 C 85.00 C -5.00 C Voltage 3.30 V 3.60 V 3.00 V 3.50 V 3.10 V Current 5.47 mA 10.00 mA 2.00 mA 10.00 mA 2.00 mA Tx Power -4.87 dBm 1.00 dBm -13.50 dBm -3.00 dBm -9.50 dBm Rx Power -7.69 dBm 4.00 dBm -21.02 dBm 0.00 dBm -16.99 dBm Transmit Fault Count = 0---------------------------------------------------------------------------- Note: ++ high-alarm; + high-warning; -- low-alarm; - low-warning

As shown in red at the end of both command outputs are the TX and RX power readings, looking from left to right, the first value = the measured value the next two values are a high low limit where if the measured the value exceeds, the switch sends analarm and the next two values are high low limits that are considered to be warning.

Units of measurement

As seen in the brocade sfpshow output, the light level is represented in dBm and uW.

Decibels (dB) is the ratio of output power to input power, expressed as 10*log (power ratio).
dBm = power in decibels relative to 1mW (1000uW). Areading of0 dBm is 1mW.
If the power reading is plus (+) dBmthe poweris >1mW, minus (-) is <1mW.

For example, in the Brocade sfpshow output above, 466uW is 10*log(466/1000) = -3.3dBm.

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Also displayed is the light power in uW.

uW = micro Watts which is the actual measure of the light power (not a ratio)

Cisco only gives the reading in dBm.

Understand readings

Once the TX and RX power readings are within the limits that are provided in the command output the SFP is receiving and transmitting light within acceptable levels, because they receive power as measured on an SFP is subject to several external factors:

  • Fibre infrastructure
    • Patch panel health
    • Cabling health
    • Cabling length
    • Number of splices causing losses due to refraction
  • Temperature affects the light measurement.

Because of this, there are wide but clearly defined set of limits that all conform to IEEE standards BER (bit error ratio).

If RX and TX power readings are of any concern andRx power is lower than expected, we would first look to the above factors and apply the Pre SFP replacement procedure (see above) with the additional step of collecting the SFPshow -f (Brocade) or show interface transceiver details (Cisco) command outputs as follows:

  1. Before the cable move
  2. After the cable move
  3. Move the cable back to the original port in step 1.

Comparing the RX power at each step.

  • If the RX power is low in step 1, is ok in step 2 and is low in step 3 using the same cabling, then it indicates an SFP issue where SFP cleaning should be applied (see Dell KB article 45672:Connectrix, Symmetrix, CLARiiON, Celerra, VNX, RecoverPoint, and VPLEX: Use cases for Fibre Channel inspection and cleaning kit deployment.)and the test carried out again after cleaning, if retest yields the same outcome replace the SFP.
  • If the RX power is low in step 1, ok for step 2, ok for step 3 it would indicate a dirty cable connector or SFP and during the test the debris causing the original issue was dislodged or move this would still be a candidate for cable and or SFP cleaning and retest.
  • If the RX power is low in step 1, low in step 2 and low in step 3 this suggests theissue external to the switch port SFP and indicates anissue with the cabling infrastructure or end device.

In cases where the SFP TX power is low (outside the thresholds that are provided in the command output) because the SFP is the source of the TX power this suggests SFP issues and is a candidate for SFP replacement.

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The number 1 issue related to link related problems are dirty cables and optics, many times it is incorrectly assumed to be a faulty optic.

Example would be when in a scenario where a new higher speed (16 Gb) switch is rolled into an existing environment and then old and existing hosts and targets are installed with existing fiber cables.

This is such a wide spread and common event that there is an optic cleaning kit available with the Dell field team to alleviate these issues. In other words, do not assume it is the optic. More information about the cleaning kit can be found in Dell KB article45672:Connectrix, Symmetrix, CLARiiON, Celerra, VNX, RecoverPoint, and VPLEX: Use cases for Fibre Channel inspection and cleaning kit deployment.

Additional Information

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How do you interpret SFP transceiver Tx and Rx power levels? ›

The optical TX power is the signal level leaving from that device, which should be within the transmitter power range. The RX sensitivity is the incoming signal level being received from the far end device and it should fall within the receive power range.

What is RX and TX in SFP? ›

RX and TX stand for receiver and transmitter sensitivity which is an important part of the optical power calculation.

How do you test SFP transceiver? ›

The simplest way to test an SFP transceiver is with the FiberLert™ live fiber detector, which lights up and beeps when placed in front of an active fiber or port.

What is good Tx and Rx power? ›

Transmit power is typically good when it is in the 6 dB range between -1 and -7 dBm. Receive power is normally expected between - 1 and -9.9 dBm. When we see a Rx power around -14 dBm or lower there is typically some sort of fault in the cable plant (bad splice, dirty connector, poorly seated jumper etc.)

What is RX and TX rate? ›

Tx/Rx rate – displays the transmit/receive data rates (in Mbps). Tx/Rx CCQ, % – displays the wireless Client Connection Quality (CCQ). The value (in percentages) shows how effectively the bandwidth is used with respect to the maximum possible bandwidth.

What is SFP transceiver and how does it work? ›

An SFP port is a slot on a network device or computer into which small form-factor pluggable (SFP) transceivers are inserted. An SFP transceiver, also known as an SFP module, is just a hot-swappable, pinky-sized metal component that, when connected to another device using a cable, allows for the transmission of data.

How do I check SFP module optical signal strength? ›

To view the optical signal strength of the SFP module on the Cisco switches, run the show interface transceiver details command. To view the optical signal strength of the SFP module on the Brocade switches, run the sfpshow portnum command to view the SFP information of a port.

Which side is Tx on SFP? ›

SFP optics are used in communication networks and have a transmitting side (Tx) and a receiving side (Rx). The transceiver has a laser which communicates to the receiving side of the other optic on the other side.

How do you use Tx and Rx? ›


Remember these LEDs don't flash if serial communication is done through 0(RX),1(TX) pins on your board. These two pins are designated for connecting your own serial device whether the USB serial cable is connected or not. TX led blinking means the board is sending something via Serial. print() function.

What is RX and TX signal? ›

TX (transmit) is how loud you're talking. RX (receive) is how loud you think the other person is talking. If Tx is really high, and RX is really low, it's like being at a conference (etc.)

What is the purpose of SFP module? ›

SFP modules facilitate high-speed communication between switches and network components such as routers and other devices. It is mainly used with copper or fiber optic cables. Its small form factor makes it ideal for areas that may not be very accessible.

What is the difference between SFP and transceiver? ›

SFP and SFP+ transceivers are virtually identical in size and appearance. The primary difference is that SFP+ is an updated version that supports higher speeds up to 10Gbps. The difference in data rate also accounts for a difference in transmission distance—SFP typically has a longer transmission distance.

How do you know if its SFP or cable failure? ›

How to determine SFP or cable faulty, review the porterrshow output from the switch. enc out errors alone imply primarily a cable issue. enc out and crc err in combination primarily imply a GBIC/SFP problem. disc c3 combined with c3 timeout tx suggest SFP faulty.

What should my TX power level be? ›

The higher the transmission power, the more accurate it is, but it will also drain the battery quicker. Most accuracy is gained with power level 7, but often power level 3-4 is sufficient. We suggest starting testing with Tx power at maximum and then you may be able to reduce it down to 3-4 after your tests.

What should TX power be set to? ›

6. The TX Power of the radio should be set to 83% to get an RSSI of -60 dBm at a distance of 10 meters from the access point.

What is maximum Tx power? ›

Maximum Transmit Output Power in the ISM bands

Several of the FCC part 15 rules govern the transmit power permited in the ISM bands. Here is a summary of those rules: Maximum transmitter output power, fed into the antenna, is 30 dBm (1 watt). Maximum Effective Isotropic Radiated Power (EIRP) is 36 dBm (4 watt).

Does TX Go to Tx or Rx? ›

As such, serial devices should have two serial pins: the receiver, RX, and the transmitter, TX. It's important to note that those RX and TX labels are with respect to the device itself. So the RX from one device should go to the TX of the other, and vice-versa.

Can high Tx or Rx? ›

The Receive (Rx) signal of CAN is the signal receiving the data. This will be an input to the MCU and is connected to an external physical layer device. The Transmit (Tx) signal of CAN is the signal transmitting the data. This will be an output of the MCU and is connected to an external physical layer device.

What are the different types of SFP modules? ›

SFP modules have a wide range of applications and can be used with most modern networks. Most fall into four main categories: cable type, transmission range, transmission rate, and application.

How can you tell the difference between single mode and multimode SFP? ›

As with Singlemode SFPs, Multi-Mode SFPs are identified by the color of the Bale Clasp: Black color coded bale clasp designates a Multi-mode SFP. The color of the compatible fiber optic patch cord or pigtail is orange.

What is a SFP optical transceiver? ›

SFP Optical Module

Perle SFP Optical Transceivers are hot-swappable, compact media connectors that provide instant fiber connectivity for your networking gear. They are a cost effective way to connect a single network device to a wide variety of fiber cable distances and types.

What are acceptable light levels for fiber? ›

A good laser source for a singlemode link will have a power output of ~ +3 to +6 dBm - 2-4mw - coupled into the fiber. A VCSEL for multimode links should have a power around 0dBm - 1mw. And a LED, used in older multimode links, has a typical power of -10 dBm - 0.1mw or 100microwatts.

How much power does SFP use? ›

SFP modules come with LC connectors. Fiber patch cables are available to connect to distribution frames and convert to different connectors. Typical power consumption is 1 W.

How do you test fiber optic power? ›

Typically both transmitters and receivers have receptacles for fiber optic connectors; so to measure the power of a transmitter, attach a test cable to the source and measure the power at the other end. For receivers, disconnect the cable attached to the receiver receptacle and measure the output with the meter.

What is RX power low alarm? ›

If the RX power is low in step 1, low in step 2 and low in step 3 this suggests the issue external to the switch port SFP and indicates an issue with the cabling infrastructure or end device.

What is Tx output? ›

Output TX is a dual-buffered device designed to go at the very end of your signal chain. With two types of summing and an audio transformer it can handle any... Availability: In Stock.

Is downstream TX or RX? ›

Downstream (Rx) Receive Power Level:

This is the amount of signal received by the modem from the transmitter in the cable company head-end.

What are the elements of the SFP and describe each of them? ›

Elements of the financial statements include Assets, Liabilities, Equity, Income & Expenses. The first three elements relate to the statement of financial position whereas the latter two relate to the income statement.

What causes SFP to fail? ›

The Main Causes of SFP Module Failures

The end face of the fiber connector and optical slots is exposed to the environment and contaminated by dust. Using the scratched or poor-quality fiber connector will cause the SFPs failures.

What is inside a SFP transceiver? ›

Transmitter Optical Sub Assembly (TOSA):

The transmitter optical sub-assembly consists of a laser diode, optical interface, monitor photodiode, metal and/or plastic housing, and electrical interface. The TOSA is an essential component of every fiber optic transceiver.

What is the difference between a transceiver and receiver? ›

In a wired telephone, the handset contains the transmitter (for speaking) and receiver (for listening). Despite being able to transmit and receive data, the whole unit is colloquially referred to as a "receiver". On a mobile telephone or other radiotelephone, the entire unit is a transceiver for both audio and radio.

Is SFP active or passive? ›

SFP+ cable is a twinax cable with SFP+ connector at each end. An active twinax cable has active electronic components in the SFP+ housing to improve the 10 Gig Ethernet signals quality. A SFP+ passive twinax cable is just a direct attach cable and contains no active components to boost signal.

How do I connect SFP transceiver? ›

Inserting SFP Modules
  1. Close the bale-clasp before inserting the SFP or SFP+ module.
  2. Line up the module with the port, with the printed label facing down. ...
  3. Gently slide the module into the port.
  4. Gently press the module into the port until it is firmly seated.
  5. Connect the cable.

How do you identify a fault in a fiber-optic cable? ›

There are two methods of detecting the problems in a fiber optic link, namely, the application of a time-domain reflectometer and a fiber optics visual fault locator. Out of which, a visual fault locator is frequently utilized by the fiber optics industry experts.

How do you locate faults in fiber-optic cable? ›

A visible fault locator is a fiber optic laser light tester that can be used to find problems and check continuity over lengths of only a few Km. It can also be used along with an OTDR tester to find a fault with greater accuracy.

What is RX level? ›

The Rx level is the strength of the signal that our mobile receives from a BTS (mobile tower). Here, the Rx level stands for Receiver Level. The mathematical formula to calculate the Rx level is: RxLev (dBm) = EIRP (dBm) - Path Loss (dB)

What does TX power stand for? ›

Transmit power is the ammount of power input into the signal to a device. " The high transmit power (TX power) of UniFi APs is great for single-AP installations, but can be problematic in enterprise/multi-AP deployments.

What power level means? ›

What Does Power Leveling (PL) Mean? Power leveling (PL) is the term widely used to define leveling where players spend hours or days playing a game with the sole intent of achieving a higher level in the game in as little time as possible.

Should transmit power be high or low? ›

The transmit power of an access point radio is proportional to its effective range. The higher the transmit power, the farther a signal can travel, and the more obstructions it can effectively penetrate.

How can we reduce TX power? ›

to decrease the TX power the easiest way to do so is to set a higher value in the antenna gain (in advanced mode). The unit will compute the maximum EiRP using this larger antenna gain and will reduce the TX power to compensate. E.g. antenna gain 10 more -> 10 dB less TX power.

Does high transmit power affect speed? ›

However, the relationship between the transmit power of a given Wi-Fi access point and the corresponding user experience isn't a 1:1 relationship. More power doesn't automatically mean you get better coverage or speed.

How is Tx power calculated in LTE? ›

RS Tx. Power (Avg) is calculated by summing the powers of all resource elements occupied by C-RS on the reference measurement channel in the Measurement Interval and dividing by the total number of resource elements in the Measurement Interval.

How does Tx and Rx work? ›

1. What's a TX or a RX port? TX: the data-sending end, generally plays the role as the transmitter, normally the TXD pin must be connected to the RXD Pin of other devices. RX: the data-receiving end, generally plays the role as the receiver, normally the RXD pin must be connected to the TXD Pin of other devices.

What does an Rx number tell you? ›

The first thing to note is the Rx#, which is unique to you. This number helps the pharmacy identify your prescription. Below the Rx# is your name, the medication name and how much of the drug you take per dose (1mg, in this case).

What does Rx mean in testing? ›

Search. Transmitter (Tx) and Receiver (Rx) Testing for High Speed Data Connectivity.

What is Tx and Rx in communication? ›

Rx and Tx refer to receiver and transmitter components, respectively.

How is transmission power calculated? ›

Effectively to get Watts (W) you just divide the number of Milliwatts (mW) by 1000 and to get Milliwatts (mW) you just multiple the number of Watts (W) by 1000.

What does TX mean transmitter? ›

Also used are Tx (Transmitter mode) and TRx(Transceiver mode - i.e. both receiver and transmitter in one connection/smpp bind connection)

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