Cisco Cisco DWDM Transceiver Modules 白皮書

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White Paper

Monitor Links with Passive Optical Taps and
Cisco QSFP 40G BiDi

What You Will Learn

In this white paper, we discuss passive optical monitoring of networks that employ dedicated hardware monitor
equipment and Cisco’s Quad Small Form-Factor Pluggable (QSFP) bidirectional fiber-optic communication

technology. Bidirectional (BiDi) optical communication means that both transmit and receive optical signals travel in

the same fiber without interference from each other. The QSFP BiDi design uses this technique to provide 20-Gbps

bandwidth in each fiber of a dual-fiber multimode fiber (MMF) cable, resulting in a 40 Gigabit Ethernet transceiver.

The BiDi technique allows instant gains in fiber capacity by using existing MMF infrastructure originally designed

for 10GBASE-SR links. This is a quadrupling of fiber capacity. To monitor this MMF network, Cisco has created the

Cisco

QSFP BiDi Monitor module, which allows network monitoring hardware such as the Cisco Nexus

Data

Broker to monitor the 40-Gbps bidirectional traffic in a link.

Traditional Network Monitoring

In optical transmission, full duplex is required: one path for transmit and one path for receive. In standardized 40

Gigabit Ethernet protocols, there is typically a pair of fibers, one for each of the two directions of transmission.

Network monitors can use the same transceivers to receive tapped signals for processing and analysis. The
transmit signal from the network monitor’s transceiver is unused, since the monitor’s only purpose is to receive.

This configuration is shown in Figure 1.

Figure 1. Configuration Used in Traditional Network Monitoring

In the case of QSFP BiDi links, the transmit and receive signals travel within the same fiber. This leads to a

potential issue if one attempts to optically tap and monitor the link. The transmitted light from the monitor ports

propagates back through the tap and into the live link path, thereby interfering with the live link itself. This is shown

as the red and pink signals in Figure 2.