QFX5220 Network Cable and Transceiver Planning
Determining QFX5220 Optical Interface Support
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 QFX5220 is located at https://apps.juniper.net/hct/product/#prd=QFX5220.
The Juniper Networks Technical Assistance Center (JTAC) provides complete support for Juniper-supplied optical modules and cables. However, JTAC does not provide support for third-party optical modules and cables that are not qualified or supplied by Juniper Networks. If you face a problem running a Juniper device that uses third-party optical modules or cables, JTAC may help you diagnose host-related issues if the observed issue is not, in the opinion of JTAC, related to the use of the third-party optical modules or cables. Your JTAC engineer will likely request that you check the third-party optical module or cable and, if required, replace it with an equivalent Juniper-qualified component.
Use of third-party optical modules with high-power consumption (for example, coherent ZR or ZR+) can potentially cause thermal damage to or reduce the lifespan of the host equipment. Any damage to the host equipment due to the use of third-party optical modules or cables is the users’ responsibility. Juniper Networks will accept no liability for any damage caused due to such use.
For interoperability with other QFX Series switches, ensure autonegotiation on the QFX5220 is disabled.
Cable Specifications for QSFP+, QSFP28, and QSFP-DD Transceivers
The 40-Gigabit Ethernet QSFP+, 100-Gigabit Ethernet QSFP28, and 400G (QDD-400G-DR4 and QDD-400G-SR4P2) transceivers that are used in QFX Series switches use 12-ribbon multimode fiber crossover cables with socket MPO-12 (UPC/APC) connectors. The fiber can be either OM3 or OM4. These cables are not sold by Juniper Networks.
To maintain agency approvals, use only a properly constructed, shielded cable.
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.
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) |
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
- Attenuation and Dispersion in Fiber-Optic Cable
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.
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:
Calculate Power Budget for Fiber-Optic Cables
To ensure that fiber-optic connections have sufficient power for correct operation, you need to calculate the link's power budget (PB), 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 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 Cables
After calculating a link's PB, you can calculate the power margin (PM), which represents the amount of power available after subtracting attenuation or link loss (LL) from the 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.
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 |
Faulty 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 PB of 13 dB uses the estimated values from Table 3. This example calculates 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 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 PB of 13 dB uses the estimated values from Table 3. This example calculates 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 pPM 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 the examples, the calculated PM is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.