by http://www.fiber-mart.comIn 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 need reliable connections implementations in form of connection cables on short distances. There are two main forms of interconnection patches: AOC – Active Optical Cables and DAC – Twinax Direct Attach Cables. The target application is interconnection of top-of-rack switches with application servers and storage devices in a rack or across adjacent racks.
Active Optical Cable (AOC) – The optical Solution AOC – Cables are constructed using optical fibers and have attached on the ends active optical components in form of optical transceivers. Their advantage consists in protection to electromagnetic perturbations, greater bandwidth capabilities and management functions embedded in the end active parts. All these features come with a proportional higher price than twinax cables. Direct Attach Cable (DAC) – The Twinax Copper Solution DAC – 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 10 Gbps data rates and distances up to 15 meters. At the end also a transceiver is used as a plug connector.
The Comparison: Twinax versus Cat5 or Cat6 Ethernet Cables The advantage of twinax over short distance consists in lower transmission delay 0.1μs versus 1.5 to 2.5μs 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. What is the difference between Active and Passive DAC Cables? There are some versions of DAC which have some active modules at the respective ends. The active 10G SFP+ CU/CR DAC link utilizes a receive equalizer in the host PHY/SerDes in order to compensate for the Inter Symbol Interference (ISI), performing signal re-shaping and amplification introduced by the cable making them suitable for longer distances.
The usual distance to reach of passive DAC cables is within 7 meters for data center connectivity. The active components are available for data rates of up to 40G, passive types up to 100G. Passive cables are much less costly but require the host to do the work of driving it properly. The new Generation: 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 G Ethernet specifications by IEEE 802.3bj workgroup. IEEE 802.3bj define a 4-lane 100Gbit/s backplane PHY for operation over links consistent with copper traces on with lengths up to at least 1 Meter and a 4-lane 100 Gbit/s PHY for operation over links consistent with copper twinax cables with lengths up to at least 5 Meter. The SC282801LXM30 – BlueLAN© 100GBASE-CR4 QSFP28 Direct Attach Cable (passive), 1 to 2 Meter, AWG 30 – This item is currently the DAC solution with the highest bandwidth with a total throughput of 100 gigabits per second.