Too Slow? 25G Speeds Up Data Centers and Campus Backbones

Written by Craig Pasek, Product Manager, Transceiver Modules Group, Cisco

With the massive increase in demand for data, equipment providers are responding with 25Gbps edge devices that require more bandwidth than can be provided on a traditional 10Gbps interface. Whether it’s a server or a campus backbone, high speed data needs to be delivered cost-effectively in a small and low-power package.

In these bandwidth-intensive applications, the choice to go with 25G is clear. To get the same or better bandwidth, the number of 10G interfaces must be 3x (6x for redundancy) or the application needs to move to the larger, more expensive and power-hungry 40G QSFP.

SFP28: For 25G the dominant form factor is SFP28. The SFP28 standard relies on the 10G SFP+ (Small Form Factor Pluggable) standard for mechanical specifications, and the electrical specifications have been improved from one 10Gbps lane that operates at 10.312Gbps to one 28Gbps lane that operates at 25Gbps + error correction. 25G transceivers can be plugged into SFP+ sockets and 10G transceivers can be plugged into SFP28 sockets because they have the same electrical and mechanical pin-out, however the associated host needs to have the software support for associated devices.

Cisco’s 25G transceiver choices include 25G Copper DAC (Direct Attached Cables), 25G AOC (Active Optical Cables) and 25G SR-S (Short Reach) transceivers.

These 25G devices are plugged into Cisco’s data center, campus and service provider switches and routers to provide high speed 25Gbps connectivity. See Cisco’s 25G compatibility matrix for currently supported devices .

Why DAC?

25G DACs are generally used in data center applications and provide the lowest cost fixed length interconnect for TOR (Top of Rack) switches to high-performance servers. Depending upon the bandwidth and distance, DACs can be either passive or active and are generally based on Twin-AX cable. For 25G, DACs can generally operate up to 5 meters without active components in the data path. Up to 2 meters, no FEC (Forward Error Correction) is needed. For 3 meters FC-FEC (Fire Code Forward Error Correction) is needed, and for 5 meters RS-FEC (Reed Solomon Forward Error Correction) is needed to correct errors. Generally, at 25Gbps beyond 5 meters, active components are needed in the data path to amplify and correct the signal. These components drive up cost which causes network designers to consider optical interfaces.

Why AOC?

25G AOCs also provide a cost effect solution for those same data center applications that require longer distances than 5m. Generally, AOCs are provided in standard lengths of 1m, 2m, 3m, 5m and 10m. However, they are usually limited to about 25 meters because of inventory stock and slack storage issues. Often a data center will be wired with only AOCs for consistency reasons, instead of a combination of AOCs and DACs.

Why SR?

25G-SR is used with standard OM3 or OM4 multimode fiber and is suitable for:

• Data centers that require up 100 meters over OM4 fiber or 70 meters over OM3 fiber for interconnect between TOR switches and leaf or spine switches.

• Breakout configurations in conjunction with 100G-SR4 transceivers where the distances are less than 100 meters for OM4 fiber or 70 meters for OM3 fiber.

• Campus backbones, where the distances between distribution and aggregation switches are less than 100 meters for OM4 fiber or 70 meters for OM3 fiber.