Cabling For Data Centers: AOC

According to a report from Communications Industry Researchers (CIR), the active optical cable (AOC) market for data centers will reach US$4.2 billion in 2020. Active Optical Cable are used in limited range interconnect applications in data center. It can be used on rack-to-rack, shelf-to-shelf interconnect, switches, servers, switch to server.

  • For short distance (<90m@10G and <10m@40G), copper cable is the cheapest option.
  • For medium distance (<500m@10G and 150m@40G), Multimode VCSEL (Vertical-cavity surface-emitting laser) transceiver is the most suitable choice, and AOC is one of them.
  • More than 500m@10G, 150m@40G and 100m@100G, there are a variety of single-mode transceivers to choose from, and their costs are gradually increasing.

In data center integrated wiring system, the server architectures include TOR (Top of Rack) architecture and EOR (End of Row) architecture.

Top of Rack (TOR)

Top of Rack (TOR) architecture is where the cabling between switch and server stays within a rack. The servers are directly connected to the switch on the top through short patch cables, and then connected to the core switch from the uplink port of the switch via copper/fiber optic cables. The term “top of rack” has been coined for this design however the actual physical location of the switch does not necessarily need to be at the top of the rack. Other switch locations could be bottom of the rack or middle of rack, however top of the rack is most common due to easier accessibility and cleaner cable management.

TOR

End of Row (EOR)

In the End of Row (EoR) architecture, each server in individual racks is connected to a common EOR (End of Row) Aggregation Switch directly, without connecting to individual switches in each rack.

Server cabinets (or racks) are typically lined up side by side in a row. Each row might contain, for example, 12 server cabinets. The term “End of Row” was coined to describe a rack or cabinet placed at either end of the “server row” for the purpose of providing network connectivity to the servers within that row. Each server cabinet in this design has a bundle of twisted pair copper cabling containing as many as 48 (or more) individual cables routed to the “End of Row”. The End of Row network racks may not necessarily be located at the end of each actual row. There may be designs where a handful of network racks are placed in a small row of their own collectively providing EOR copper connectivity to more than one row of servers.

EOR

As for the EoR, with fewer switches used, more cables are needed between racks resulting in the higher possibility of cable mess and higher cost for higher performance cables. Besides, it’s difficult and more expensive to upgrade cabling infrastructure to support higher speed network.

The TOR wiring method simplifies the wiring between the server cabinet and the network cabinet. There are fewer cables fibers from each server cabinet to the EOR/MOR network cabinet, and it also greatly shortens the distance between the server and the switch. Therefore, TOR will be more likely to be adopted by more data center cabling. In this cabling method, the transmission distance is mostly medium distance, so it will be used to a large number of active optical cables. Moreover, this design enables easily upgrade from 1GE/ 10GE network to 10GE/ 40GE network in the future with minimum costs and changes to cabling.

AOC is usually used in the following locations in data centers:

Server cabinets: up to 40 servers are connected to the top-of-rack switch (TOR). Each server has one or two Ethernet ports connected to the switch.

Main network area: AOC can be used in Spine, Leaf or core switching area. In these areas of today's network, there are a large number of discrete switches, and they all need to be connected to each other to create a large switch architecture. These interconnections are usually implemented using AOC. In some data centers, the switch architecture can occupy multiple cabinets, or even an entire row in the data center.

Of course, choosing AOC is not only due to the distance factor, but also has many of the following advantages, which makes it the first choice for data center cabling.

Compared with DAC:

  • The transmission power on the system link is lower;
  • The volume is about half of the copper cable, and the weight is only a quarter of the DAC cable;
  • The bending radius of the optical cable is smaller than that of the copper cable;
  • The bit error rate of the product transmission performance is also better, and the BER can reach 10^-15.

Compared with Fiber optical transceiver:

  • The optical interface of the aoc cable is not exposed, that is, there is no problem of clean and contaminated optical interface;
  • Higher system stability and reliability;
  • Make the network system more convenient to manage and maintain;
  • Greatly reduce operation and maintenance costs and improve efficiency.

From the perspective of the future development trend of data centers, we will advocate all-optical networks in the future and aim to increase the network speed. With the continuous improvement of network speeds, active optical cable will be widely used in data centers.

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