What Are the Types of 100G QSFP28 Cables?

From providing flawless, high-flow media to real-time connectivity with customers around the world, today's data centers require ultra-high-speed data transmission, promoting the development of the whole optical communication market. QSFP28 cable is a necessity for data transmission. How many types of QSFP28 cables are available on the market and what are they? Find answers in this article.

QSFP28 Cables Basics

QSFP28 cable contains two forms: one is a form of high-speed cable with QSFP28 connectors on either end, transmitting and receiving 100Gbps data over a thin twinax cable or a fiber optic cable, and the other is a form of breakout cable that can split one 100G signal into 4x 25G (QSFP28 to SFP28) or 2x 50G, enabling connectivity between network devices with different speed ports, while fully utilizing port bandwidth.

Types of 100G Passive DAC

Generally speaking, there are two kinds of 100G DAC cables: 100G Active DAC and 100G Passive DAC. And 100G Passive DAC is generally divided into three types: 100G QSFP28 to 100G QSFP28 Passive DAC, 100G QSFP28 to 4× 25G SFP28 Passive DAC, and 100G QSFP28 to 2x 50G QSFP28 Passive DAC.

100G QSFP28 to 100G QSFP28 Passive DAC

100G QSFP28 to 100G QSFP28 Passive DAC is a 4-channel parallel passive copper cable that combines four 28 Gbps SFP channels into one high-density cable. It offers 4 independent data transmitting channels and 4 data receiving channels via copper cable, providing a cost-effective way to establish a 100-Gigabit link between QSFP-100G ports of switches within racks and across adjacent racks, ideal for data centers, high-end servers and enterprise wiring closets. QSFP28 DAC is usually applied for Top of Rack (ToR) interconnections between 100G ToR switch and server or the stacking of 100GbE switches (as the figure below).

Figure 1: Application of 100G QSFP28 Passive Direct Attach Cable

100G QSFP28 to 4× 25G SFP28 Passive DAC Cable

100G QSFP28 to SFP28 Passive DAC Cable is a breakout cable providing a hybrid transition from a QSFP28 on one end to four individual SFP28 on the other end. It offers four parallel, bi-directional channels each operating at up to 25Gps. Featuring low crosstalk, small bend radius and low power consumption, the 100G to 25G breakout cable meets the ever-increasing demand for higher channel density with high-level signal integrity in high-performance computing, top-of-rack switching, and network storage deployments.

100G QSFP28 to 2x 50G QSFP28 Passive Direct Attach Cable

100G QSFP28 to 2x 50G QSFP28 breakout DAC is a 4-channel parallel copper direct attach cable offering 4 independent data transmission channels and 4 data receiving channels via the cable. The aggregate data rate of 100Gbps over 5m transmission can be achieved with this product. Designed for use in cost-effective 100GbE to 2x 50GbE Ethernet connectivity solution to meet the growing needs for higher bandwidth in data centers, this QSFP28 cable is suitable for the Infiniband EDR and 128G Fiber Channel.

Figure 2: 100G DAC Breakout Cables

Types of 100G AOC

AOC (Active Optical Cable) is a communication cable that needs to use external energy to convert electrical signals into optical signals or conversely in the communication process. There are commonly two types of 100G AOC: 100G QSFP28 to 100G QSFP28 AOC and 100G QSFP28 to 2x 50G QSFP28 AOC.

100G QSFP28 to 100G QSFP28 AOC

100G QSFP28 to 100G QSFP28 AOC cable is composed of multi-mode optical fiber and parallel optical modules. The QSFP28 transceivers at both ends provide photoelectric conversion and optical transmission functions to increase the transmission rate and distance of the optical cable without weakening the compatibility with the standard electrical interface. With no insertion loss and return loss, 100G AOC is mainly used for short-distance multi-channel data communication applications in several locations in the data center like ToR, EoR and MoR (as the figure below).

Figure 3: Application of 100G QSFP28 AOC

100G QSFP28 to 2x 50G QSFP28 AOC

100G QSFP28 to 2x 50G QSFP28 breakout AOC is a high data rate parallel AOC, to overcome the bandwidth limitation of traditional copper cable, terminated with a QSFP28 module at one end and two 50G QSFP28 modules at the other. It provides a cost-effective solution for data center interconnecting using devices with 100G QSFP28 ports and 50G QSFP28 ports, which is also ideal for High Performance Computing (HPC) and network storage deployments.

Figure 4: 100G AOC Breakout Cables

FAQs About 100G DAC/AOC Cables

What is the difference between 100G DAC and AOC?

A: 100G DAC is composed of Twinax copper cables with connectors on both ends, while 100G AOC is MMF cable with SFP connectos on both ends. There are several differences between these two cables. In general, 100G DAC consumes less power and is more cost-effective than 100G AOC. By contrast, 100G AOC supports higher performance in transmission distance and is also more suitable for data center deployment. For more information on the difference between DAC and AOC cables, please read Data Center Cabling Solution: DAC Cables vs AOC Cables.

Q: What is the structure of 100G AOC?

A: 100G AOC mainly consists of two optical modules and an optical cable jumper. The transceivers at both ends are composed of different lengths of OM3 or OM4 multimode fiber.

Q: Can 100G QSFP28 cables be used for 40G?

A: Yes, QSFP28 cables can be used for 40G, but not vice versa. There is a rule about mixing optical transceivers at a different speed: both ends of the two modules have to match and the form factor needs to match as well. Additionally, port speed needs to be equal or greater than the optic used.

Q: What does the breakout mode of QSFP28 cable mean?

A: Breakout mode refers to running a 100G port as 4 separate channels of 25G or 2 separate channels of 50G. When breaking out a single 100G port to 4x 25G or 2x 50G links, the Forward Error Correction (FEC) mode on both ends of the link must be the same to ensure link-up.

Q: Can customers use third-party QSFP28 Cable?

A: Yes, third-party 100G passive copper cables can be used but all the cables are required to comply with the associated IEEE specifications, as well as SFF-8636 management interface / EEPROM specifications to allow them to be correctly identified and recognized by the switch/router.