10G SFP+ Direct Attach Copper Media System-10GSFP+Cu

January 7, 2015

10G SFP+ Direct Attach Copper Media System-10GSFP+Cu

The escalating deployments of multi-core processor-based servers and demanding applications are driving the need for 10 Gigabit connections. Customers require flexible and scalable I/O solutions to meet the rigorous requirements of these deployments. 10GSFP+Cu addresses the demanding needs for today’s data center environments.

In recent year, the networking industry was gearing up for massive uptake of copper-based 10 Gigabit Ethernet. Now that 10GbE is booming with new applications accentuating the differences between Ethernet adapter designs, copper is back on the 10GbE menu. Moreover, price, performance and flexibility have made 10 GbE an attractive choice for the data center. Consequently, the copper option called 10GSFP+Cu also becomes popular.

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Basics of 10GSFP+Cu


10GSFP+Cu is also known as 10GBASE-CR, 10GBASE-CX1 and 10G SFP+ direct attach copper. 10GSFP+Cu is a copper interconnect using an inexpensive twinax copper cable assembly with SFP+ connectors on both sides based on SFP+ MSA (multi-source agreement). This standard has a transmission range of 10 m, and similar to 10GBASE-CX4, it has the advantage of low power, low cost and low latency. In addition, unlike 10GBASE-CX4, this standard has the added advantage of using less bulky cables and having the small form factor of SFP+.


SFP+ Direct Attach uses a copper 10GbE cable that comes as active or passive twinax cable assembly and connects directly into an SFP+ housing. 10G SFP+ active copper cable assemblies have a silicon chip to boost the performance of the cable. Without a chip, the cable is considered a “passive” cable. Passive copper cables are more susceptible to degradation due to attenuation and crosstalk. This active boosting allows cables to be more compact, thinner, longer, and transmits data faster than their passive equivalents. The benefit of embedding chip technology in cables is a reduction in the copper used in cable production, reducing the overall form factor and weight of the cable. Other benefits include longer reach and lower power consumption. Generally, SFP+ Direct Attach has a fixed-length cable. Twinax cables shorter than 7 meters are passive and those longer than 7 meters are active.

This low-cost 10G SFP+ direct attached copper cable assembly is useful for interconnecting a stack of top-of-rack switches and short-distance connections between switch ports and Ethernet interfaces on servers and storage devices in a rack. By incorporating SFP+ DACs into the physical infrastructure, organizations can achieve 10G performance without additional signal processing or conversion. This provides an ideal low power and low latency 10 Gbps server interconnect option for ToR switching deployments.

10GSFP+Cu Link

The 10GSFP+Cu link utilizes a receive equalizer in the host port physical layer PHY/SerDes in order to compensate for the Inter Symbol Interference (ISI) introduced by the cable. Electrical and mechanical specifications for SFP+ optical modules, 10G SFP+ DAC, and hosts are defined in the SFF-8431 specification developed by the SFF Committee, with broad industry participation. 10GSFP+Cu interoperability test can conduct with each host PHY or network interface card (NIC), which can demonstrate the 10GSFP+Cu specification’s consistency and host interoperability over various cable lengths, host channels, and PHYs.

10GSFP+CU Signaling Components

The direct attach twinax cables use the SFP+ connector module that is used on 10 Gbps optical fiber links. A SFP+ port may support either active or passive DAC cables, or both. There is no standard for this cable type, so you cannot assume that a direct attach port can support either cable type. The DAC cables and their SFP+ connector modules use an electrical signaling interface called SFI, which is defined as the “SFP+ high-speed serial electrical interface”. The SFI definition enables 10 Gbps operation over a single differential signaling path in each direction for a total of two pairs or four wire connections. The twinaxial cable includes the two pairs of signal-carrying wires in a coaxial cable format, providing high performance and stable signaling over the length of the cable.

10GSFP+CU Signal Encoding

The SFP+ MSA specifications for DAC cables note that 10GSFP+Cu connections can only be used on systems with common power grounds. The power supplies for the switches or any computers are connected to them over DAC cables. They must be connected to the same local power grid with a common ground between all devices. Otherwise, it may result in electrical damage to the interfaces or the devices.


10GBASE-T is an IEEE 802.3an standard which supports the creation of technology that is capable of transmitting 10 Gigabit Ethernet up to 100 meters over unshielded or shielded twisted pair cables. Prior to the development of 10GBASE-T, SFP+ Direct Attach Copper is a low cost alternative to traditional fiber and twisted-pair copper cabling in data center deployments for 10 Gbps connection. SFP+ DAC provides better cable management for high-density deployments and enhanced electrical characteristics for the most reliable signal transmission. Here, we have summarized the advantages of 10GSFP+Cu over 10GBASE-T as follows:

First, the connection of 10GSFP+CU is able to transmit at 10 gigabits/second full duplex speed over 5 meter distances. Second, 10GSFP+CU offers 15 to 25 times lower transceiver latency than current 10GBASE-T Cat 6/Cat 6a/Cat 7 cabling systems: 0.1μs for twinax with SFP+ versus 1.5 to 2.5μs for current 10GBASE-T specification. Third, the power draw of twinax with SFP+ is around 0.1 watts, which is also much better than 4-8 watts for 10GBASE-T. Fourth, twinax copper cabling has BER better than 10-18, and therefore is acceptable for applications in critical environments.

10GSFP+Cu Deployment Considerations

Don’t Exceed Bend Radius of Your SFP+ DAC Cables

It is important to observe and maintain proper cable bend radius and provide adequate and secure strain relief on the cable. In order to help maintain proper bend radius, it is recommended to confirm the American wire gauge (AWG) of your SFP+ DAC cables first. SFP+ direct attach copper cables are offered in different wire AWG depending on length. Cables must not be bent below their minimum bend radius, which depends upon cable size as expressed in AWG. The table below summarizes minimum values typically admitted for SFP+ DAC cables sustained bend radiuses.

AWG Cable Size Sustained Bend Radius
24 1.3 inches (33 mm)
26 1.5 inches (38 mm)
28 1.0 inches (25 mm)
30 0.9 inches (23 mm)
Calculate the Length of Your SFP+ DAC Cables

When you deploy SFP+ cables within a single 84 inches 45 RU cabinet, conservatively the longest connection will be 7 ft. or 2.1m to reach from the top U to the bottom and approximately 1.5 ft. or 0.45m to route to any port on either end. Example cabinet with 2 top of rack switches and 20 2U servers with dual SFP+ NICs (Network Interface Cards) total of 40 SFP+ cables. Conservatively, longest cable required to reach farthest port is 2.1+2×0.45≈3m. A 3m cable should be adequate to connect any two ports within a cabinet.

Use Cable Management Tools to Bear Your Heavy SFP+ DAC Cables

To help manage the weight of bundled cable and ensure they do not sag over time, a strain relief bar should be installed to support SFP+ DAC cables and provide strain relief along the horizontal plane. Strain relief bars facilitate the correct alignment of cable and connector into the port, and help installers observe manufacturer and bend radius requirements of cable close to the connector. The strain relief bars also keep cables routed clear from spaces directly behind server and switch equipment, reducing thermal resistance through the equipment and promote effective cooling and airflow. Besides, cable ties should be used to bundle the cables together and tie them to the strain relief bars and cable managers. This should be done carefully to ensure the cables are firmly in place and will not move, but not so tight as to deform or stress the cable jacketing.

Final Thoughts

With the development of advanced technology, it is usual for a new Ethernet media system to operate a higher speed than the previous standard. Until now, media system have been evolving to carry 40 and 100 Gb/s Ethernet signals over short-range copper coaxial cables. 40 Gigabit Ethernet short copper cable media system (40GBASE-CR4) specifies a media system based on four lanes of PCS data carried over four twinaxial cables, referenced in the IEEE standard as small form factor specification SFF-8436. 100 Gigabit Ethernet short copper cable media system (100GBASE-CR10) specifies a media system based on 10 lanes of PCS data carried over 10 twinaxial cables or equivalent cabling that meet the electrical specifications, referenced in the IEEE standard as small form factor specification SFF-8642. As you can see, the Ethernet system has been reinvented to provide more flexible and reliable cabling, to accommodate the rapid increase in network traffic with higher speeds, and to provide more capabilities for today’s more complex network systems. Also, direct attach copper will continue to play its important role in the telecommunication industry.


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