Tag Archives: DAC

25G SFP28 Cable: The Most Economical Option for ToR Server Connection

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25 Gigabit Ethernet is proposed standard for Ethernet connectivity in a data center environment, developed by IEEE 802.3 task force P802.3by. The IEEE 802.3bj standard then uses technology defined for 100 Gigabit Ethernet implemented as four 25-Gbit/s lanes. These standards define:

  • a single-lane 25 Gbit/s 25GBASE-KR PHY for printed circuit backplanes
  • a single-lane 25 Gbit/s 25GBASE-CR-S PHY for 3 m twin-ax cables (in-rack)
  • a single-lane 25 Gbit/s 25GBASE-CR-L PHY for 5 m twin-ax cables (inter-rack)
  • a single-lane 25 Gbit/s 25GBASE-SR PHY for 100 m OM4 or 70 m OM3 multi-mode optical fiber

What’s 25G SFP28 Cable?
According to the above standards, the IEEE CFI is now focused on the SFP28 and QSFP28 direct attach copper twin-ax cables (DACs). SFP28 DAC refers to the 25G DAC cable using the SFP+ form factor, and QSFP28 DAC refers to the 100G DAC cable using the QSFP+ form factor. The maximum transmission distance of these cables is 5 meters.

There are two SFP28 DAC cable types: 25G SFP28 to SFP28 DAC and 100G QSFP28 to four SFP28 breakout DAC. The SFP28 to SFP28 passive copper cable is a high speed, cost-effective 25Gbp/s Ethernet connectivity solution designed to meet the growing needs for higher bandwidth in data centers. The QSFP28 to four SFP28 breakout DAC is used to connect 100G switches to four 25 Gigabit in cabinet or adjacent cabinet servers. Compared to 40G using four 10G lanes and 100G using 10 10G lanes, the 25G SFP28 DAC provides the low-cost copper server connection for Top of Rack (ToR) switches.

To more directly illustrate effectiveness of SFP28 to SFP28 DAC cable and QSFP28 to four SFP28 breakout DAC cable, let’s see a series of pictures displayed below:

Existing 10G Topology
Today’s volume topology for web-scale data centers

  • 48 servers/ToR
  • 3:1 oversubscription
  • Uses low-cost, thin 4-wire SFP+ DAC cable

sfp+ to sfp+ DAC

40G Topology
High-performance, low volume topology

  • Uses bulkier 16-wire QSFP+ DAC cable
  • Max. 24 servers/ToR with 3:1 oversubscription
  • Will transition to 100G

qsfp+ to qsfp+ dac

25G Direct Connect
Same topology as 10G

  • 48 servers/ToR
  • 3:1 oversubscription w/ 100G uplinks, non-blocking w/ 400G
  • Uses 4-wire SFP28 DAC cable

sfp28 to sfp28 dac

Existing 4x10G Topology
Commonly used topology in web-scale data centers

  • Permits non-blocking 10G mesh
  • 40G ports used as 4x10G with QSFP+ to SFP+ breakout cable
  • Same server network interface card (NIC) as 10G

qsfp+ to 4 sfp+ dac

4x25G Breakout
Same topology as 4x10G

  • Permits non-blocking 25G mesh
  • 100G ports used as 4x25G with QSFP28 to SFP28 break-out cable
  • Same server network interface card (NIC) as 25G direct connect

qsfp28 to 4 sfp28 dac

High Density 25G
Increased port switch port density

  • 64 servers in non-blocking architecture
  • 96 servers in a 3:1 oversubscription
  • 24-port 400G ToR
  • 192 servers in non-blocking architecture

100G qsfp28 to 4 sfp28 dac

Related articles:

Taking an In-depth Look at 25G SFP28

25G Vs. 40G Ethernet: Who Is the Winner?

How Far Can 25G Ethernet Go?

The Changing Network Architecture in 25G/100G Data Center