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QFX5110 Network Cable and Transceiver Planning

Determining Transceiver Support for the QFX5110

The port panel of the QFX5110-48S supports 48 logical 10-Gigabit Ethernet ports. These data ports (0 through 47) support either 1-Gigabit small form-factor pluggable (SFP) or 10-Gigabit Ethernet Ethernet small form-factor pluggable plus (SFP+) transceivers. You can also use SFP+ DAC cables and 10-Gigabit active optical cables (AOC) in any access port.

The remaining 4 QSFP28 ports (48 through 51) support speeds of 40 Gbps or 100 Gbps. Each port can be configured as an independent 100-Gigabit Ethernet port or as an independent 40-Gigabit Ethernet port. These port are usually used as uplinks. In 40-Gigabit Ethernet mode, these ports can be channelized using QSP+ to SFP+ DAC breakout (DACBO) cables.

You can find information about the optical transceivers supported on your Juniper device by using the Hardware Compatibility Tool. In addition to transceiver and connection type, the optical and cable characteristics–where applicable–are documented for each transceiver. The Hardware Compatibility Tool enables you to search by product, displaying all the transceivers supported on that device, or category, by interface speed or type. The list of supported transceivers for the QFX5110 is located at https://pathfinder.juniper.net/hct/product/#prd=QFX5110.

CAUTION:

If you face a problem running a Juniper Networks device that uses a third-party optic or cable, the Juniper Networks Technical Assistance Center (JTAC) can help you diagnose the source of the problem. Your JTAC engineer might recommend that you check the third-party optic or cable and potentially replace it with an equivalent Juniper Networks optic or cable that is qualified for the device.

Cable Specifications for QSFP+ and QSFP28 Transceivers

The 40-Gigabit Ethernet QSFP+ and 100-Gigabit Ethernet QSFP28 transceivers that are used in QFX Series switches use 12-ribbon multimode fiber crossover cables with socket MPO/UPC connectors. The fiber can be either OM3 or OM4. These cables are not sold by Juniper Networks.

CAUTION:

To maintain agency approvals, use only a properly constructed, shielded cable.

Tip:

Ensure that you order cables with the correct polarity. Vendors refer to these crossover cables as key up to key up, latch up to latch up, Type B, or Method B. If you are using patch panels between two QSFP+ or QSFP28 transceivers, ensure that the proper polarity is maintained through the cable plant.

Table 1 describes the signals on each fiber. Table 2 shows the pin-to-pin connections for proper polarity.

Table 1: QSFP+ and QSFP28 Optical Module Receptacle Pinouts

Fiber

Signal

1

Tx0 (Transmit)

2

Tx1 (Transmit)

3

Tx2 (Transmit)

4

Tx3 (Transmit)

5

Unused

6

Unused

7

Unused

8

Unused

9

Rx3 (Receive)

10

Rx2 (Receive)

11

Rx1 (Receive)

12

Rx0 (Receive)

Table 2: QSFP+ MPO Fiber-Optic Crossover Cable Pinouts

Pin

Pin

1

12

2

11

3

10

4

9

5

8

6

7

7

6

8

5

9

4

10

3

11

2

12

1

Understanding QFX Series Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion

To determine the power budget and power margin needed for fiber-optic connections, you need to understand how signal loss, attenuation, and dispersion affect transmission. The QFX Series uses various types of network cables, including multimode and single-mode fiber-optic cables.

Signal Loss in Multimode and Single-Mode Fiber-Optic Cables

Multimode fiber is large enough in diameter to allow rays of light to reflect internally (bounce off the walls of the fiber). Interfaces with multimode optics typically use LEDs as light sources. However, LEDs are not coherent light sources. They spray varying wavelengths of light into the multimode fiber, which reflect the light at different angles. Light rays travel in jagged lines through a multimode fiber, causing signal dispersion. When light traveling in the fiber core radiates into the fiber cladding (layers of lower refractive index material in close contact with a core material of higher refractive index), higher-order mode loss occurs. Together, these factors reduce the transmission distance of multimode fiber compared to that of single-mode fiber.

Single-mode fiber is so small in diameter that rays of light reflect internally through one layer only. Interfaces with single-mode optics use lasers as light sources. Lasers generate a single wavelength of light, which travels in a straight line through the single-mode fiber. Compared to multimode fiber, single-mode fiber has a higher bandwidth and can carry signals for longer distances. It is consequently more expensive.

For information about the maximum transmission distance and supported wavelength range for the types of single-mode and multimode fiber-optic cables that are connected to the QFX Series, see the Hardware Compatibility Tool. Exceeding the maximum transmission distances can result in significant signal loss, which causes unreliable transmission.

Attenuation and Dispersion in Fiber-Optic Cable

An optical data link functions correctly provided that modulated light reaching the receiver has enough power to be demodulated correctly. Attenuation is the reduction in strength of the light signal during transmission. Passive media components such as cables, cable splices, and connectors cause attenuation. Although attenuation is significantly lower for optical fiber than for other media, it still occurs in both multimode and single-mode transmission. An efficient optical data link must transmit enough light to overcome attenuation.

Dispersion is the spreading of the signal over time. The following two types of dispersion can affect signal transmission through an optical data link:

  • Chromatic dispersion, which is the spreading of the signal over time caused by the different speeds of light rays.

  • Modal dispersion, which is the spreading of the signal over time caused by the different propagation modes in the fiber.

For multimode transmission, modal dispersion, rather than chromatic dispersion or attenuation, usually limits the maximum bit rate and link length. For single-mode transmission, modal dispersion is not a factor. However, at higher bit rates and over longer distances, chromatic dispersion limits the maximum link length.

An efficient optical data link must have enough light to exceed the minimum power that the receiver requires to operate within its specifications. In addition, the total dispersion must be within the limits specified for the type of link in the Telcordia Technologies document GR-253-CORE (Section 4.3) and International Telecommunications Union (ITU) document G.957.

When chromatic dispersion is at the maximum allowed, its effect can be considered as a power penalty in the power budget. The optical power budget must allow for the sum of component attenuation, power penalties (including those from dispersion), and a safety margin for unexpected losses.

Calculating Power Budget and Power Margin for Fiber-Optic Cables

Use the information in this topic and the specifications for your optical interface to calculate the power budget and power margin for fiber-optic cables.

Tip:

You can use the Hardware Compatibility Tool to find information about the pluggable transceivers supported on your Juniper Networks device.

To calculate the power budget and power margin, perform the following tasks:

How to Calculate Power Budget for Fiber-Optic Cable

To ensure that fiber-optic connections have sufficient power for correct operation, you need to calculate the link's power budget, which is the maximum amount of power it can transmit. When you calculate the power budget, you use a worst-case analysis to provide a margin of error, even though all the parts of an actual system do not operate at the worst-case levels. To calculate the worst-case estimate of power budget (PB), you assume minimum transmitter power (PT) and minimum receiver sensitivity (PR):

PB = PT – PR

The following hypothetical power budget equation uses values measured in decibels (dB) and decibels referred to one milliwatt (dBm):

PB = PT – PR

PB = –15 dBm – (–28 dBm)

PB = 13 dB

How to Calculate Power Margin for Fiber-Optic Cable

After calculating a link's power budget, you can calculate the power margin (PM), which represents the amount of power available after subtracting attenuation or link loss (LL) from the power budget (PB). A worst-case estimate of PM assumes maximum LL:

PM = PB – LL

PM greater than zero indicates that the power budget is sufficient to operate the receiver.

Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion, connectors, splices, and fiber attenuation. Table 3 lists an estimated amount of loss for the factors used in the following sample calculations. For information about the actual amount of signal loss caused by equipment and other factors, refer to vendor documentation.

Table 3: Estimated Values for Factors Causing Link Loss

Link-Loss Factor

Estimated Link-Loss Value

Higher-order mode losses

Single mode—None

Multimode—0.5 dB

Modal and chromatic dispersion

Single mode—None

Multimode—None, if product of bandwidth and distance is less than 500 MHz-km

Connector

0.5 dB

Splice

0.5 dB

Fiber attenuation

Single mode—0.5 dB/km

Multimode—1 dB/km

The following sample calculation for a 2-km-long multimode link with a power budget (PB) of 13 dB uses the estimated values from Table 3 to calculate link loss (LL) as the sum of fiber attenuation (2 km @ 1 dB/km, or 2 dB) and loss for five connectors (0.5 dB per connector, or 2.5 dB) and two splices (0.5 dB per splice, or 1 dB) as well as higher-order mode losses (0.5 dB). The power margin (PM) is calculated as follows:

PM = PB – LL

PM = 13 dB – 2 km (1 dB/km) – 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB

PM = 13 dB – 2 dB – 2.5 dB – 1 dB – 0.5 dB

PM = 7 dB

The following sample calculation for an 8-km-long single-mode link with a power budget (PB) of 13 dB uses the estimated values from Table 3 to calculate link loss (LL) as the sum of fiber attenuation (8 km @ 0.5 dB/km, or 4 dB) and loss for seven connectors (0.5 dB per connector, or 3.5 dB). The power margin (PM) is calculated as follows:

PM = PB – LL

PM = 13 dB – 8 km (0.5 dB/km) – 7(0.5 dB)

PM = 13 dB – 4 dB – 3.5 dB

PM = 5.5 dB

In both examples, the calculated power margin is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.