Introduction to 40G QSFP+ Cabling Assemblies

Today’s high-performance computing environments featuring by switching and routing, cloud computing and virtualization require higher network speeds, greater scalability, and higher levels of performance and reliability in data centers. Some bandwidth-hungry applications, like video streaming applications, also drive data rates to higher points. These all boost the need for a migration to 40G and 100G interfaces as 1 and 10G can’t meet the bandwidth needs well. 40G interface is QSFP (Quad Small Form-factor Plable) which has several standards requiring different connectors to fit cabling infrastructure, so as to achieve network connectivity. Do you know what cabling infrastructure is needed to support 40G applications? MPO/MTP cable, direct attach cable (DAC), or LC fiber patch cable? Have any ideas? Follow this article and find the answer.

 

MTP/MPO Cable
MTP is a registered trademark of US Conec used to describe the connector, and MPO stands for multi-fiber push-on or also multi-path push-on. Actually, the former product is 100% compatible with the latter. Thus, only MTP is written for simplicity in the following paragraphs. In 2010, the IEEE 802.3ba standard specifies MTP connectors for standard-length multi-mode fiber (MMF) connectivity. Its small, high-density form factor makes MTP cable ideal for higher-speed 40G networks in data centers.

To support 40G applications, a 12-fiber MPO connector is needed. The typical implementations of MTP plug-and-play systems split a 12-fiber trunk into six channels that run up to 10 Gigabit Ethernet (depending on the length of the cable). 40G system uses 12-fiber trunk to create a Tx/Rx link, dedicating 4 fibers for 10G each of upstream transmit, and 4 fibers for 10G each of downstream receive, leaving the middle 4 fibers unused. The upgrade path for this type of system entails simply replacing the cassette with an MTP-to-MTP adapter module.

 

Direct Attach Cable
Besides MTP cable, many data centers also like to choose DACs for 40G cabling infrastructure. 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. The “transceivers” on both ends of DACs are not real optics and their components are without optical lasers, thus DACs are much cheaper, preferable for 40G data center applications. As such, the fiber connectivity cost is significantly reduced by using either direct attach copper cables or active optical cables (AOCs) instead of costly fiber transceivers and optical cables.

 

Direct Attach Copper Cable
Direct attach copper cables are designed in either active or passive versions for short-reaches in data center. Compared with active optical cables, these copper cables are less expensive. Nowadays, there are many twinaxial cables available to support 40G (10G x four channels), in QSFP+ to QSFP+ (ie. EX-QSFP-40GE-DAC-50CM) version or in QSFP to 4 SFP+ cable assembly (eg. QSFP-4SFP10G-CU5M).

 

The issue is that copper cable is stiff and bulky, thus consuming precious rack space and blocking critical airflow. But with the advancing technology, manufactures produce a thinner, uniquely shielded ribbon-style twinaxial cable that can support speeds of 10G per channel while addressing many of the concerns associated with round, bundled cable. And the ribbon-style twinaxial cable is significantly slimmer than its round counterparts. Even better, the cable can be folded multiple times and still maintain signal integrity, allowing for higher density racks and space savings.

 

Active Optical Cable
Being a form of DAC, AOC integrates single-mode fiber (SMF) or MMF cable terminated with a connector and embedded with transceivers. It uses electrical-to-optical conversion on the cable ends to improve speed and distance performance of the cable. AOCs can reach a longer distance copper cables, and use the same interfaces as copper cables, typically used in data center. Similar to direct attach copper cables, AOCs are also available in QSFP+ to QSFP+ (eg. QSFP-4X10G-AOC20M) and QSFP+ to 4 SFP+ cabling (ie. QSFP-4X10G-AOC10M) versions.

Since 40G AOC connectors are factory pre-terminated, 40G AOC is easier for installation and thus less affected by the repeating plug during daily use than MTP cable. In case there was a fault in the interconnection, for AOC, you can just replace it with another AOC.

LC Fiber Cable
Certainly, LC fiber cable can also be the cabling solution for the long-reach 40G QSFP+ modules (40GBASE-LR4). That is, 40GBASE-LR4 QSFP+ uses a duplex LC connector as the optical interface, able to support transmission distance up to 10km over single-mode fiber (SMF).