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Breakout Cables Are Great – Extending Breakout to the Metro Is Even Better!

portrait of Fady Masoud

June 25, 2024
By Fady Masoud
Senior Director, Solutions Marketing

Breakout cables are a powerful solution that enables network operators to efficiently utilize high-speed ports in the network. They consist of breaking down a high-speed, channelized port on a network element into multiple lower-speed ports. For example, in a 4 x 100G breakout, a router/switch with 400G ports is connected to four 100G ports in different network elements.

Physically, breakout cables are built using cables or transceivers. Direct attach cable (DAC) breakouts are made of multiple copper twinaxial cables that are fixed in length, generally 1 to 10 m long. Active optical cables (AOCs) provide longer distances, generally up to 25 m, using active modules at the hub and at each end of fiber optic cables. While both AOC and DAC breakout cables have the modules permanently fixed to each cable, in transceiver-based breakouts, the length is dictated by the type of the transceiver or “pluggable” (e.g., SR, LR, etc.) used at each end of the fiber optic cable, as depicted in Figure 1.

Transceiver-based 4x100G Breakout cables
Figure 1: Transceiver-based 4 x 100G breakout cables

Breakout cables provide point-to-multipoint connectivity and are widely used inside data centers and central offices (CO) for IP over DWDM (IPoDWDM) and traffic aggregation applications. Their simplicity, cost-effectiveness, and ease of management lead to network simplification and a reduction in CapEx and OpEx. Breakouts are particularly valuable to:

  • Maximize the utilization of router ports by using high-bit-rate pluggables at hub router and avoiding stranded capacity at end sites
  • Enhance redundancy by providing different data paths
  • Enable incremental upgrades as each host device can be upgraded independently from the rest of the network

Breakout cables are also very well understood by the IP community and deep-rooted into current operating processes.

One might ask, can the same concept be extended to much longer distances while retaining all the benefits mentioned above?

If so, how do we set a clear demarcation point now that the hub and the end points are tens or hundreds of kilometers apart?

Leveraging XR Optics and Its Subcarrier Technology to Extend Breakout Applications

XR optics, driven by the Open XR Optics Forum, is the next major inflection point in optical transceiver technologies. XR optics utilizes digital signal processing to subdivide the transmission and reception of a given wavelength spectrum into a series of smaller-frequency channels called digital subcarriers. These digital subcarriers can be independently managed and assigned to different destinations, enabling the industry’s first scalable point-to-multipoint, direct low-speed to high-speed optical transceiver connectivity. Leveraging the digital signal processor (DSP), a single 400G XR optics hub module generates 16 x 25 Gb/s digital subcarriers. One or multiple digital subcarriers can be combined and assigned to a specific destination to provide the required bandwidth, thus enabling a dynamic assignment of capacity between the hub and each endpoint. This concept of point-to-multipoint leveraging digital subcarriers enables XR optics to be operated in a breakout mode. While it is possible to have a breakout mode in any granularity equal to or greater than the serializer/deserializer (SERDES) lane interfaces on the client side of the optics, the most common implementation example is a 4 x 100G breakout. In this mode, 4 x 25 Gb/s digital subcarriers are assigned to each of the four endpoints, creating a 4 x 100 Gb/s coherent breakout mode (Figure 2).

XR Optics 4x100 Gb/s Breakout Mode
Figure 2: XR optics 4 x 100 Gb/s breakout mode

XR optics supports a dynamic 4 x 100 Gb/s breakout mode in amplified and unamplified networks, where the host port to endpoint (or leaf) port association can be remotely configured, reducing on-site intervention and truck rolls. XR optics also provides much greater reach (~80 km unamplified and ~500 km amplified) between the host and all four endpoints/leaves, thus expanding geographical coverage while simplifying the network. Furthermore, XR optics provides a clear demarcation point at each endpoint as they spread across the metro network, in addition to streaming telemetry for real-time network health. Table 1 summarizes the differences between conventional transceiver-based and XR optics-based breakout solutions.

Comparison between conventional transceiver-based and XR optics 4 x 100 Gb/s breakout solutions
Table 1: Comparison between conventional transceiver-based and XR optics 4 x 100 Gb/s breakout solutions

If the splitter/combiner in the XR optics-based solution is located close to the end sites, one fiber pair is needed between the XR optics pluggable and the splitter, enabling network operators to maximize the utilization of existing fiber, as depicted in Figure 3.

Conventional Transceiver-based Breakout vs. XR Optics-based breakout
Figure 3: Conventional transceiver-based breakout vs. XR optics-based breakout

As the distance between the hub and each of the end sites can vary (from a few kilometers to hundreds of kilometers ), XR optics coherent breakout provides two options to close each link and meet the link budget requirements. These solutions are completely passive (unamplified) and require no modification to the network:

  • Flexible modulation: If one of the end sites requires extended reach, XR optics can be software-configured to operate in QPSK modulation, so each digital subcarrier operates at 12.5 Gb/s of capacity instead of 25 Gb/s. This will add 3 dB to the link budget but will limit the capacity at the hub to 200 Gb/s instead of 400 Gb/s. Using QPSK increases optical performance/reach while keeping the same spectral width.
  • Dynamic power management: While most end sites are close to the main aggregation site or hub, some can be quite far away. XR optics provides the ability to apply per-digital-subcarrier power emphasis. This feature enables the XR optics module to distribute the optical transmit power across the subcarriers in a non-symmetric way, where lower power is allocated to the digital subcarriers requiring a smaller link budget and higher power to subcarriers that need to satisfy a larger link budget. For example, network operators can lower the subcarrier transmit power assigned to End Site 1 (short link) by 2 dB and apply that to End Site 4 (long link), as depicted in Figure 4. This can increase the unamplified downlink link budget without the use of amplification. This capability can be planned into the link budget on day one or used in outage/restoration scenarios where fiber loss increased due to splices/fiber additions during the repair process.
Dynamic Power Management in XR Optics Coherent Breakout Mode
Figure 4: Dynamic power management in XR optics coherent breakout mode

Infinera’s ICE-X 400G XR intelligent coherent pluggables leverage XR optics technology to offer efficient and highly flexible 4 x 100G coherent breakout solutions that help network operators:

  • Reduce operating costs and network complexity through a significant reduction in the number of router ports
  • Maintain operational consistency with current breakout cable deployment
  • Enhance network redundancy by creating alternative data paths
  • Expand service coverage and simplify network architecture by leveraging coherent optical performance and reach
  • Enhance network flexibility by leveraging dynamic power allocation
  • Accelerate service turn-up and troubleshooting by providing a clear demarcation point

ICE-X’s coherent breakout solution can be used in a wide set of applications over different network topologies (ring, horseshoe, linear), including metro aggregation, router interconnect/IPoDWDM, campus aggregation, and 100G wavelength extensions.

Breakout cables can certainly be extended to a much larger geographical area using XR optics-based coherent pluggables, as they provide an efficient and highly flexible solution to optimize network resources, enhance redundancy, and simplify the network.

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