What are the most used interconnections within a data center?

In data center environment there are many interconnections needed from regular large numbers of ports of access switches to the large bandwidth capacity ports of backbone routers with particular cases of ports facing the connections towards storage access networks.

All these ports needs reliable connections implementations in form of  patchcords on short distances. There are two main forms of interconnection patches: AOC - Active Optical Cables and DAC-Direct Attached Cables. The target application is interconnection of top-of-rack switches with application servers and storage devices in a rack.

AOC - patch cords are constructed using optical fibers and have attached on the ends active optical components such as transceivers. Their advantage consists in protection to electromagnetic perturbations, greater bandwidth capabilities, management functions embedded in the end active parts. All these features come with a proportional higher price than twinax cables.

DAC - Direct attach cables use the twinax copper wires to transmit signals.Twinax cable is almost similar with CATV coaxial cable but not the same having two conducting wires inside covered by protective shield. They are suitable up to 10Gbps data rates and distances up to 7 meters.

Twinax versus Cat5 or Cat6 Ethernet Cables
The advantage of twinax over short distance consist in lower delay 0.1 μs versus 1.5 to 2.5 μs for current implementations of SFP+ DAC cables vs 10GBASE-T.

The power draw of Twinax with SFP+ is around 0.1 watts, which is also much better than 4–8 watts for 10GBASE-T. Cables must not be bent below their minimum bend radius, which depends upon cable size as expressed in AWG - American Wire Gauge.The 10GSFP+Cu link utilizes a receive equalizer in the host PHY/SerDes in order to compensate for the Inter Symbol Interference (ISI) introduced by the cable.

Active vs Passive DAC Cables
There are some versions of DAC which have some active modules at the ends of them performing signal re-shaping and amplification making them suitable for longer distances. The usual distance reach is within 10 meters for data center connectivity. Also, the active components permits data rates up to 40G.

The versatility of the protocols used, besides 10G Ethernet, the support for Fibre Channel, Fibre Channel over Ethernet and Infiniband, low consumption for active parts, low prices, make them competitive alternative for AOC Cables.

Passive cables are much less costly but require the host to do the work of driving it properly.

Benefits:

Tradeoffs:

  • Lower Costs
  • Higher Reliability
  • No LOS
  • No TX Disable
  • No Interrupts
  • Limited Management Interface
  • Host must drive Cu cable

 

100G Direct Attach Cables

The 40GBASE-CR4 and 100GBASE-CR10 physical layers using 7 m twin-axial cable are being developed as part of 100 Gbit Ethernet specifications by IEEE 802.3bj workgroup.

IEEE 802.3bj define a 4-lane 100 Gbit/s backplane PHY for operation over links consistent with copper traces on with lengths up to at least 1 m and a 4-lane 100 Gbit/s PHY for operation over links consistent with copper twinaxial cables with lengths up to at least 5 m.

SC282801LXM30 - BlueLAN© 100GBASE-CR4 QSFP28 Direct Attach Cable (passive), 1 to 2 Meter, AWG 30, this item is the current offer that is able to transmit a total of 100Gps troughput.