DANZ – Tap Aggregation optics / transceivers selection

This articles clarifies certain criteria that are important to consider in the design of a Network Packet Broker (NPB) aggregating traffic from various sources.

For distance reasons, the main type of media used in tap aggregation is optical (multimode or single mode), therefore this article mainly focuses on these media.

 

1) Understanding Optical Budgets

Multiple factors contribute to the degradation of optical signals

  • Fiber attenuation
  • Insertion loss (e.g. connectors, patch panels and splices)
  • Fiber type mismatch (e.g. connecting 50/125MMF to 62.5/125MMF)
  • Over-bending of fibre plant
  • Intermediate passive devices (e.g. taps, attenuators or mode filters)

 

Media Type Approximate Loss *
50/125 MMF with 850nm signal 3 dB/km
9/125 SMF with 1310nm signal 0.5 dB/km
LC Connector (MMF) 0.3 dB
MTP Connector (MMF) 0.5 dB
Optical Taps Insertion and split losses vary significantly – refer to vendor

* While link budget calculations may be estimated, in-situ testing with appropriate equipment is recommended to ensure link integrity.

For more on Arista transceivers, you may visit https://www.arista.com/en/products/transceivers-cables

 

2) Estimating Insertion Losses

The following diagram illustrates the loss of optical power along the succession of different common passive elements in a fibre path:

optical power graph

 

 

3) Optical Splitter Operation

 

An optical splitter sits inline between two switches. It is a passive device that diverts a percentage of the signal to a second pair of ports. The percentage diverted can vary from 10% to 50%. The splitter also introduces insertion loss. The resulting signal must be within the optical budget of both the monitored link and the receiving ports on the tap aggregator.

Splitter operations

Splitter operations

 

 

Before installing the splitter, the optical levels should be checked to verify there In EOS, this can be done with “show interfaces transceiver”

 

switch#show interfaces transceiver
If device is externally calibrated, only calibrated values are printed.
N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).
                              Bias      Optical   Optical                
         Temp       Voltage   Current   Tx Power  Rx Power  Last Update  
Port      (Celsius)  (Volts)   (mA)      (dBm)     (dBm)     (Date Time)  
-----     ---------  --------  --------  --------  --------  -------------------
Et1        28.80      3.27      5.86     -2.45     -0.60     2014-11-05 10:24:58
Et2        N/A        N/A       N/A       N/A       N/A      N/A       
Et3        29.84      3.27      6.05     -2.37     -1.03     2014-11-05 10:24:58
Et4        N/A        N/A       N/A       N/A       N/A      N/A       
Et5        29.84      3.27      5.87     -2.37     -0.58     2014-11-05 10:24:58


This information is also available via SNMP.

 

4) Port usage on Tap Agg Switch

All the ports on the tap aggregation switch support both transmit and receive, while the optical splitter sends two unidirectional streams. These are connected to the RX side of two ports. So long as the splitter (or any intermediate device) does not send a loss of signal error, the ports will link up on the switch.

Arista Tap Aggregator's port in tap mode, comes up with Rx only

Arista Tap Aggregator’s port in tap mode, comes up with Rx only

 

 

 

5) Overcoming optical loss with wideband optics on tap ports

Certain Ethernet standard accommodate for greater optical sensitivity than some others, normally in relation to the reach the standard achieves.

Additionally, while the electronic/photonics receiver used for long reach is centered on 1310 nm or 1550 nm, it can also receive signals on other wavelengths. Wideband refers to that transceiver’s ability to detect a wider range of wavelengths, and also at the additional benefit of greater sensitivity, meaning that it is capable to receive and comprehend weaker signal.

 

What about standard compatibility?

In an Arista Tap Aggregator, the ports receiving signal are using their receiver only (Rx, on tap ports), the Tx is not employed. While the signal received on an Arista tap port might originally be issued by short range transceivers and multimode media (such as SR or SR4), the Tap Aggregator’s tap port would never send signal back towards the tap or original source, so its phased-out TX ability is irrelevant. The receiving-only transceiver could be a longer reach transceiver, normally employed on singlemode fibre (such as LR or LR4) without any problem.

Under the same considerations, the tap ports on the Arista tap aggregator should never set to negotiate, since traffic would never reach to original traffic sender/receivers. For the link to come up, it should not be negotiating speed. By default the port would be configured for auto-negotiation.

This is not a problem at 10G since 10G only operates at fixed 10G (even with an apparent default of auto-neg). There is no speed negotiation at 10G.

However for 1G SFP, there would be negotiation exchanges. If listening at negotiation, the tap port might miss it, since it’s not participating at all in the A-B link negotiation; the tap port would not have a link partner. You should disable speed negotiation on the 1G port with: “speed forced 1000full” (only needed for 1G SFP)

This is obviously different for the tool ports (towards analyzers) that do send traffic: a tool port must be bi-directionally compatible with the remote end (analyzer).

 

Details on wideband optics

  • The use of Wideband optics in the tap aggregator allow for splitter ratios that preserve more of the signal for the production link
  • Wideband optics have a greater receive sensitivity
  • Wideband optics, while centered on 1310 nm or 1550 nm, can receive signals on other wavelengths.
  • ‘ER’ and ‘ZR’ Transceivers are capable of receiving ‘LR’ (1310nm) signals

 

Highlight of the wideband optics benefits:

  • Use less aggressive optical taps to preserve more link budget for production data (e.g. 70:30 vs 50:50)
  • Recover weaker signals at the aggregator without impacting optic choice on production links (e.g. retain SR/SR4 in production, using LR on the Tap Aggregator)
  • Overcome losses in long links between splitter/tap and aggregator

 

 

6) Tapping High Speed Links (40G / 100G)

 

40 and 100 Gigabit links use multiple lanes delivering serialized data. These lanes can be provisioned as a fiber pair per lane – typically used with multimode – or as multiple wavelengths on a single pair of fiber. Arista has a FAQ that covers the details of the cabling required:

http://www.arista.com/assets/data/pdf/40G_FAQ.pdf

Network taps are available that support the MTP cabling used for 40 and 100 Gbps links. When connecting to a 7150 running in tapagg mode, these taps would be connected to a QSFP port on the DCS-7150-64 or, using an MTP to LC splitter cable, to 4 SFP+ ports configured as an agile port (a feature unique to the 7150 that allows the user to configure adjacent blocks of 4xSFP+ interfaces as a single 40GbE link).


Duplex fiber can be connected to the QSFP+ ports. Supported optics include -LR, -LRL and -UNIV adapters. While the universal optics have a tighter optical budget than the LR optics, they use the same wavelengths and so can be used with any optical tap that supports 40G duplex links.

 

 

by Alexis Dacquay and Edmund Roche-Kelly