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EX9200 Network Cable and Transceiver Planning

Pluggable Transceivers Supported on EX9200 Switches

The line cards in EX9200 switches support 1-gigabit Ethernet small form-factor pluggable (SFP), 1-gigabit Fast Ethernet SFP, 10-gigabit small form-factor pluggable plus (SFP+), 40-gigabit quad small form-factor pluggable plus (QSFP+), and 100-gigabit C form-factor pluggable (CFP) transceivers.

Note:

We recommend that you use only optical transceivers and optical connectors purchased from Juniper Networks with your Juniper Networks device.

CAUTION:

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.

The Gigabit Ethernet SFP, SFP+, and QSFP+ transceivers and the 100GBASE-LR4 CFP transceivers installed in EX9200 switches support digital optical monitoring (DOM): You can view the diagnostic details for these transceivers by issuing the operational mode CLI command show interfaces diagnostics optics.

Overview of EX Series Switches: 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. EX Series switches use various types of network cables, including multimode and single-mode fiber-optic cable.

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

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 reflects 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), higher-order mode loss (HOL) occurs. (Cladding consists of layers of lower-refractive index material in close contact with a core material of higher refractive index.) 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. Single-mode fiber is consequently more expensive than multimode fiber.

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 transmissions. 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 usually limits the maximum bit rate and link length. Chromatic dispersion or attenuation is not a factor.

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 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, you can consider its effect 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 power loss.

Calculate the Fiber-Optic Cable Power Budget for EX Series Devices

To ensure that fiber-optic connections have sufficient power for correct operation, calculate the link's power budget when planning fiber-optic cable layout and distances. This planning helps you ensure that fiber-optic connections have sufficient power for correct operation. The power budget is the maximum amount of power the link can transmit. When you calculate the power budget, you use a worst-case analysis to provide a margin of error. You use a worst-case analysis even though not all the parts of an actual system operate at the worst-case levels.

To calculate the worst-case estimate for a fiber-optic cable power budget (PB) for the link:

  1. Determine values for the link's minimum transmitter power (PT) and minimum receiver sensitivity (PR). In the following example, we measure both (PT) and (PR ) in decibels relative to one milliwatt (dBm).

    PT = – 15 dBm

    PR = – 28 dBm

    Note:

    See the specifications for your transmitter and receiver to find the minimum transmitter power and minimum receiver sensitivity.
  2. Calculate the power budget (PB) by subtracting (PR) from (PT):

    – 15 dBm – (–28 dBm) = 13 dBm

Calculating the Fiber-Optic Cable Power Margin for EX Series Devices

Before calculating the power margin, calculate the power budget (see Calculating the Fiber-Optic Cable Power Budget for EX Series Devices).

Calculate the link's power margin when planning fiber-optic cable layout and distances to ensure that fiber-optic connections have sufficient signal power to overcome system loss and still satisfy the minimum input requirements of the receiver for the required performance level. The power margin (PM) is the amount of power available after you subtract attenuation or link loss (LL) from the power budget (PB).

When you calculate the power margin, you use a worst-case analysis to provide a margin of error, even though not all parts of an actual system operate at worst-case levels. A power margin (PM ) greater than zero indicates that the power budget is sufficient to operate the receiver and that it does not exceed the maximum receiver input power. This means that the link will work. A (PM) that is zero or negative indicates insufficient power to operate the receiver. See the specification for your receiver to find the maximum receiver input power.

To calculate the worst-case estimate for the power margin (PM) for the link:

  1. Determine the maximum value for link loss (LL) by adding estimated values for applicable link-loss factors—for example, use the sample values for various factors as provided in Table 1 (here, the link is 2 km long and multimode, and the (PB) is 13 dBm):
    Table 1: Estimated Values for Factors Causing Link Loss

    Link-Loss Factor

    Estimated Link-Loss Value

    Sample (LL) Calculation Values

    Higher-order mode losses (HOL)

    • Multimode—0.5 dBm

    • Single mode—None

    • 0.5 dBm

    • 0 dBm

    Modal and chromatic dispersion

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

    • Single mode—None

    • 0 dBm

    • 0 dBm

    Connector

    0.5 dBm

    This example assumes 5 connectors. Loss for 5 connectors:

    (5) * (0.5 dBm) = 2.5 dBm

    Splice

    0.5 dBm

    This example assumes 2 splices. Loss for two splices:

    (2) * (0.5 dBm) = 1 dBm

    Fiber attenuation

    • Multimode—1 dBm/km

    • Single mode—0.5 dBm/km

    This example assumes the link is 2  km long. Fiber attenuation for 2 km:

    • (2 km) * (1.0 dBm/km) = 2 dBm

    • (2 km) * (0.5 dBm/km) = 1 dBm

    Clock Recovery Module (CRM)

    1 dBm

    1 dBm
    Note:

    For information about the actual amount of signal loss caused by equipment and other factors, see your vendor documentation for that equipment.

  2. Calculate the (PM) by subtracting (LL) from (PB):

    PB – LL = PM

    (13 dBm) – (0.5 dBm [HOL]) – ((5) * (0.5 dBm)) – ((2) * (0.5 dBm)) – ((2 km) * (1.0 dBm/km)) – (1 dB [CRM]) = PM

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

    PM = 6 dBm

    The calculated power margin is greater than zero, indicating that the link has sufficient power for transmission. Also, the power margin value does not exceed the maximum receiver input power. Refer to the specification for your receiver to find the maximum receiver input power.