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EX4200 Network Cable and Transceiver Planning

Pluggable Transceivers Supported on EX4200 Switches

Optional uplink modules for EX4200 switches support SFP, SFP+, or XFP transceivers. You can find the list of transceivers supported on EX4200 switches and information about those transceivers at the Hardware Compatibility Tool page for EX4200.

Note:

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

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.

The Gigabit Ethernet SFP, SFP+, or XFP transceivers installed in EX4200 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.

Note:

The transceivers support DOM even if they are installed in uplink module ports configured as Virtual Chassis ports.

SFP+ Direct Attach Copper Cables for EX Series Switches

Small form-factor pluggable plus transceiver (SFP+) direct attach copper (DAC) cables, also known as Twinax cables, are suitable for in-rack connections between servers and switches. They are suitable for short distances, making them ideal for highly cost-effective networking connectivity within a rack and between adjacent racks.

Note:

We recommend that you use only SFP+ DAC cables purchased from Juniper Networks with your Juniper Networks device.

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

EX  Series switches support SFP+ passive DAC cables. The passive Twinax cable is a straight cable with no active electronic components. EX  Series switches support 1  m, 3  m, 5  m, and 7  m long SFP+ passive DAC cables. See Figure 1.

Figure 1: SFP+ Direct Attach Copper Cables for EX  Series SwitchesSFP+ Direct Attach Copper Cables for EX  Series Switches

The cables are hot-removable and hot-insertable: You can remove and replace them without powering off the switch or disrupting switch functions. A cable comprises a low-voltage cable assembly that connects directly into two 10-Gigabit Ethernet ports, one at each end of the cable. The cables use high-performance integrated duplex serial data links for bidirectional communication and are designed for data rates of up to 10  Gbps.

List of DAC Cables Supported on EX  Series Switches

For the list of DAC cables supported on EX  Series switches and the specifications of these cables, see:

Standards Supported by These Cables

The cables comply with the following standards:

Understanding 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 cable, 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 cladding (layers of lower refractive index material in close contact with a core material of higher refractive index), higher-order mode loss (HOL) 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.

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 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 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 to 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, even though all the parts of an actual system do not operate at the worst-case levels.

To calculate the worst-case estimate for 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). For example, here, (PT) and (PR ) are measured in decibels, and decibels are referred 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 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 losses 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 attenuation or link loss (LL) has been subtracted 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 all the parts of an actual system do not 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 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.