Two Main Questions About Direct Attach Cables

The increasing bandwidth demands in data centers call for new cost-effective network solutions that are able to provide great bandwidth and improved power efficiency. As such, direct attach cables (DACs) are designed to replace expensive fiber optic cables in some Ethernet applications, like choosing SFP+ DACs and QSFP+ DACs accordingly as 10 Gigabit Ethernet (GbE) and 40GbE cabling solutions to achieve high performance. How much do you know about this kind of cable? Do you know its such basic information as classifications? If not, then you can follow this article to understand DAC in depth based on the two main questions.

Question 1: What Is DAC?

DAC, a kind of optical transceiver assembly, is a form of high speed cable with “transceivers” on either end used to connect switches to routers or servers. Often referred to as twin-ax, this direct attach twin-axial cable is very similar to coaxial cable, except for one additional copper conductor core. DACs are much cheaper than the regular optics, since the “transceivers” on both ends of DACs are not real optics and their components are without optical lasers. In some 10GbE and 40GbE infrastructures, DACs have been selected to replace fiber optic patch cord when the required link length is relatively short. And in storage area network, data center, and high-performance computing connectivity, DACs are preferable choice because of their low cost, low power consumption and high performances.

Question 2: How DAC Is Classified?

When it comes to DAC’s classifications, there exist two primary standards: Ethernet transmission rate, material of cables.

Based on Ethernet transmission rate and construction standard, 10G SFP+ DACs, 40G QSFP+ DACs, and 120G CXP+ DACs are all available, meaning that DAC can be used as transmission medium for 10GbE, 40GbE, and 120GbE applications when combined as transceivers. Typical DAC assemblies have one connector on each end of the cable. Take SFP-10G-AOC1M for example, this Cisco compatible SFP+ to SFP+ Direct-Attach Active Optical Cable assembly has one SFP+ connector on each end of the cable, designed for relatively short reach that is 1m.

SFP-10G-AOC1M, one SFP+ connector at each end

According to material of cables used, DACs are available in direct attach copper cables and active optical cables (AOCs).

Direct Attach Copper Cable

Direct attach copper cables are designed in either active or passive versions, providing flexibility with a choice of 1-, 3-, 5-, 7-, and 10-meter lengths. The former provides signal processing electronics to avoid signal issue, thus to improve signal quality. What’s more, the former can transmit data over a longer distance than the latter which offers a direct electrical connection between corresponding cable ends. Both direct attach passive copper cables and direct attach active copper cables have gained popularity in data centers. For instance, EX-QSFP-40GE-DAC-50CM, the Juniper 40G cabling product, hot-removable and hot-insertable, is the QSFP+ to  QSFP+ direct attach passive copper cable assembly, really suitable for short distances of up to 0.5m(1.6ft), appropriate for highly cost-effective networking connectivity within a rack and between adjacent racks.

EX-QSFP-40GE-DAC-50CM, for short reach

Active Optical Cable

AOC is also one form of DAC. It uses electrical-to-optical conversion on the cable ends to improve speed and distance performance of the cable while mating with electrical interface standard. Compared with direct attach copper cable, its smaller size, electromagnetic interference immunity, lower interconnection loss and longer transmission distance make it popular among consumers.

DACs offer great flexibility in cabling length choices, simplify server connectivity in top-of-rack deployments, and reduce the power needed to transmit data. More importantly, DACs ensure high system reliability after going through rigorous qualification and certification testing, helping network designers to achieve new levels of infrastructure consolidation while expanding application and service capabilities.

Conclusion

DACs are able to provide an end-to-end solution that is easy to maintain, thus helping improve the availability of networks that support mission-critical applications. Fiberstore offers a broad selection of DACs with high quality for state-of art performance, 10G SFP+ DACs, 40G QSFP+ DACs, and 120G CXP+ DACs all included. For more information about DACs, you can visit Fiberstore.

Three Common High Density Host Ports – SFP+, QSFP+ and CXP

The push behind users’ requests for high-quality video content, whether for live Internet video or video downloads from servers, is the principal driver of extremely high growth of Internet traffic. Besides, more and more complex technical computing applications are demanding even greater bandwidth. In such cases, these SFP+, QSFP+ and CXP high density host ports are used to increase the bandwidth, enabling the high speed networking connections. This article gives an overview of these three ports, including their cabling solutions and bandwidth density.

High Speed Solutions: SFP+, QSFP+ and CXP Ports

Leading companies and industry organizations related to telecommunications have made their great efforts to develop specifications to assure commonality, compatibility and networking functionality of hardware connections, signaling and software communications. These specifications for high speed networking solutions include SFP+, QSFP+ and CXP links.

SFP+ Ports

In today’s data center, SFP+ links are supplanting SFP links for both Ethernet and Fibre Channel. Using the same board space as SFP, SFP+ provides a 10x bandwidth improvement over SFP for Ethernet (10Gb/s vs. 1Gb/s) and 2x improvement for Fibre Channel (8.5Gb/s vs. 4.25Gb/s). The SFP+ system also offers capability to freely designate or configure any available system port with either copper- or fiber-based cabling as dictated by the specific installation environment.

QSFP+ and CXP Ports

The other two high speed parallel link specifications which allow for even higher bandwidth are QSFP+ and CXP systems. The QSFP+ system uses a 4 x 10 Gb/s link configuration for a 40Gb/s port. Similarly, the CXP system provides 12 lanes that can be deployed to support 100 to 120Gb/s aggregated port bandwidth. QSFP+ and CXP are specified for 4x and 12x Infiniband Quad Data Rate (QDR) interconnect links. CXP ports can also be used for 40G links.

Comparison Among SFP+, QSFP+ and CXP Ports

The comparison among these three ports starts form their cabling solutions, then bandwidth density.

Three Common Cabling Solutions for SFP+, QSFP+ and CXP

The SFP+, QSFP+ or CXP host ports can accept either a passive copper-based cable solution for generally cable lengths of 5 to 7 meters, an active copper-based cable solution for typical cable lengths up to 15meters (or longer depending on the acceptance criteria), or a plug-in optical transceiver module with an optical connector on the rear of the module to accept passive fiber optic cable assemblies to enable even longer cable lengths. These cabling approaches enable flexibility to configure the cables needed to cater to different working environments. Take QSFP+ copper cabling solutions for example, Intel XLDACBL5 is the QSFP+ to QSFP+ passive copper cable assembly designed for 40-gigabit links with the distance up to 5m. Fiberstore compatible Intel XLDACBL5 is shown below.

A Fourth Cabling Solution for QSFP+

With the widespread use of QSFP+ for Ethernet transmission in high performance computing systems, there emerged a fourth cabling solution: active optical cable (AOC) assembly. In an AOC, the optical fiber is terminated directly to an optical transceiver that is sealed within the metal backshell on each end of the cable assembly. The integrated electro-optical assembly lowers cost in component reduction and presents an electrical interface to the outside world. Like 721070-B21 module, Fiberstore compatible HP 721070-B21 is the QSFP+ to 4SFP+ breakout AOC assembly used for 40G links.

Bandwidth Density

The SFP+, QSFP+ or CXP host ports can increase I/O port bandwidth density along the edge of a switch line card. A single SFP+ port operating at 10Gb/s provides about 16 Gb/s bandwidth per inch, QSFP+ offers 3x improvement to 48Gb/s per inch, and CXP offers a further 2.3x improvement to 113 Gb/s per inch. The port configurations give system designers options to achieve even higher linear bandwidth density with
some port types.

Conclusion

These high density SFP+, QSFP+ and CXP ports can provide increased communications bandwidth for data center networking. Fiberstore offers various SFP+, QSFP+ and CXP ports, and their cabling solutions. These cabling modules are fully compatible with major brands, like Intel (XLDACBL5), HP (721070-B21), Dell and Force 10 (CBL-QSFP-40GE-PASS-1M). You can visit Fiberstore to know more about SFP+, QSFP+ and CXP ports.