What is SFP+ Direct Attach Copper Cable?

wzt10gtek

SFP+ DAC 电缆成为云计算与云存储中的主流主要连接工具，但是关于什么是SFP+DAC cable，不同的SFP+ DAC cable的生产厂商对此也有不同的介绍和描述，今天，我们整理了一些相关的资料，主要说说什么是SFP+ DAC cable及其它的背景，同时也会谈到有源和无源的SFP+ DAC cable之间的优缺点和SFP+ DAC cable与10GBASE-T的比较。

SFP+ Direct Attach Copper Cable background （SFP+ DAC cable 的背景）
The SFP+ interface offers the highest density, lowest cost, and lowest power 10 Gigabit Ethernetsolution commercially available today. Further SFP+ enables mass migration of the 1GbE network to10 GbE by offering a very low cost 10GSFP+Cu (direct attached copper cable assembly) option for short reach interconnects along with multimode and single mode fiber options. The SFP+ interface accepts along with hot-pluggable, small-footprint, optical transceivers, a Direct Attach Cable “10GSFP+Cu”, which replaces two optical modules and a connectorized optical fiber with a twinaxial copper cable assembly. The 10GSFP+Cu option is defined for 10GbE applications over passive copper with a reach of 8.5m. 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. Electrical and mechanical specifications for SFP+ optical modules, 10GSFP+Cu, and hosts are defined in the SFF-8431 specification developed by the SFF Committee, with broad industry participation.

What is SFP+ Direct Attach Copper Cable?（SFP+ DAC Cable 的介绍）
10GSFP+Cu (SFP+ Direct Attach) is a copper interconnect using a passive twin-ax cable assembly that connects directly into an SFP+ housing. It has a range of 10 meters and like 10GBASE-CX4, is low power, low cost and low latency with the added advantage of having the small form factor of SFP+, and smaller, more flexible cabling.

SFP+ Direct Attach is known as the successor technology to 10GBASE-CX4. SFP+ Direct Attach, as implied in the name, uses SFP+ MSA and by using the inexpensive copper twinaxial cable with SFP+ connectors on both sides, provides 10 Gigabit Ethernet connectivity between devices with SFP+ interfaces. SFP+ Direct Attach has a 10 meter distance limitation, thus the target application is interconnection of top-of-rack switches with application servers and storage devices in a rack.

Pasive and  Active SFP+ DAC Cables？（有源和无源的SFP+ DAC Cables）
Passive cables are much less expensive but require the host to do the work of driving it properly.
– Benefits:
• Lower Costs
• Higher Reliability
– Fewer Components (No Active Tx /Rx Components)
– Only Capacitors, Resistors, EEPROM, Cable
– Tradeoffs:
• No LOS
• No TX Disable
• No Interrupts
• Limited Management Interface
• Host must drive Cu cable

Active cables offer the added benefit of being "optical-module" like.
Benefits:
• Enhanced Signal Integrity
• Longer Cable Lengths
• Transmit Pre-emphasis
• Active/Adaptive Receive Equalization
• Tx Disable
• Loss of Signal (LOS)
• Interrupts
• Management Interface
• Looks like an optical module
• Don’t have to worry about host Tx/Rx for Cu cables
– Tradeoffs:
• Higher Cost

How does SFP+ Direct Attach Copper compare with 10GBASE-T?
10GBASE-T will work with Cat 6 and Cat 7 cables that are commonly used for 1Gb network connections and also support much longer distances than SFP+ Direct Attach Copper. 10GBASE-T is currently not supported for OneConnect adapters due to power requirements and high latency of 10GBASE-T PHY chips. 10GBASE-T PHY technology continues to evolve and could be suitable for adapter products by the end of 2010.