25G Direct Attach Cables - An Overview (DAC & AOC)

With the proliferation of sites and mobile applications that hog and exploit more bandwidth, the demand for higher communication bandwidth has drastically fueled. The greater the bandwidth, the faster the data transfer capability will be between the transmission source and the display.

Videos with high quality and resolutions, like 4K/8K/10K, as well as using AR and VR technologies require more bandwidth to send more data packets at one time and play videos without buffering. And let’s face it, no one wants to get stuck with that small white circle spinning endlessly on their display screens.

To support today’s use of server virtualization, high-quality streaming, and bandwidth requirements, Direct Attach Cables (DAC) are the optimal solutions.

An Introduction to Direct Attach Cables

Direct Attach Cables are designed from Twinax copper and consist of two copper-based transceiver modules for faster electrical signals and data transmission at short distances. It’s a high-performance and low-latency solution that establishes an integrated duplex data link between the source and receiver to enable secure, reliable bidirectional communication. The copper-based SFP transceivers terminated at both ends can’t be removed. DACs are faster and high-performance Twinax cable solutions and can be boiled into two types.

Direct Attach Cables: Why Are They Becoming Popular?

This type of cable is becoming a norm for Ethernet connectivity across data centers and data operators. DACs are built to meet the design, efficiency, and performance specs of IEEE, MSA, and SFF. A direct attach cable can effectively function in accordance with multiple communication protocols, from Ethernet to Fiber Channel to VPI to InfiniBand. They reduce the operational costs and lower the power consumption in the short-reach applications. It won’t be wrong to say that DACs are a cost-effective and better alternative to the optical transceiver. Ideal for short-range applications, direct attach cables are an economical option to increase the upload and download speed and transfer large amounts of data faster. Managed IT providers use DACs to achieve low-latency and high transfer rates without having to break their bank.

Direct attach cables offer connectors to connect fixed-length solution, leaving out the expensive lasers and other electronic components associated with traditional optical transceiver. These assemblies fully comply with the defined industrial standards for memory maps and EEPROM. This ensures not only proper interoperability but also accurate identification of a system. The direct attach cables offer speed of 10G, 15G, 25G – all the way up to 400G in the industry-standard form factors.

Two Types of 25G Direct Attached Cables

There are two types of 25G DAC high-speed cables available in the market, which are:

1. 25G SFP28 Direct Attach Copper Cable
2. 25G SFP28 Active Optical Cable

Both cables offer optical inter-connectivity for short-distance and can transmit data at a speed of 25Gbps. The direct attach copper cable can either designed in the active version or passive version, while AOCs are always active.

If they both are capable of transmission and providing fiber-connectivity at the same speed, then what exactly the differences are between them. Below, we have explained how the direct attach copper cables (DAC) differ from active optical cables (AOC).

25G SFP 28 Direct Attached Copper Cable

25G DAC cable, a high-speed copper cable, is designed with an insulation core and twin-axial copper cable with the SFP+ form factor and two modules, one at each end. These transceiver semiconductors establish a direct electrical connection between the ports. This kind of cable can be used to connect active equipment, routers, switches, servers, hubs, and network interface cards.

It is designed to offer low latency, high attenuation performance, and more bandwidth in data centers. The link length of an active direct attach cable is longer than the length of a passive DAC. An active DAC can be between 5 m and 10 m and can carry and transmit an Ethernet signal on a long-distance line, as opposed to a passive DAC. With that said, passive copper cable solutions are ideal for shorter distance interconnections, usually 0.5 m to 5 m, in Ethernet applications. An active direct attach cable consists of electronics or electrical components in the connectors for transmission signal conditioning. On the other hand, a passive DAC doesn’t contain any active or electrical components required to boost Ethernet signals.

Features of DAC Cables:

1. The direct attach cables are capable of supporting higher data rates and performance for various applications compared to a traditional copper cable. A DAC cable can generally support and handle a speed rate from 4Gbps to 10 Gbps per channel.
2. With the advancements in copper cable technology, a DAC cable also comes with an interchangeable function. That means the fiber optic module or transceiver can be interchanged and hot-swapped with the DAC cable.
3. A DAC cable is a cost-effective solution compared to optical fiber Ethernet solutions. The reason being that a DAC cable doesn’t include expensive electronic components such as optical lasers. In addition, it’s capable of supporting multiple communication protocols impeccably.
4. A DAC cable is extremely reliable, durable, and dependable for shorter Top of Rack (ToR) or adjacent racks. When plugged or connected directly with active equipment ports, a DAC cable enables high performance and high-density data connections between them while lowering the power consumption.
5. DAC cables are completely measured up to the SFP+ MSA specifications and are RoHS compliant.
6. The direct attach cables (DAC) use the same connection ports as a traditional optical transceiver but drastically reduce the operational costs and latency.

25G SFP28 Active Optical Cable

AOC is another cost-effective, reliable alternative to traditional, costly optical transceivers. This cable is an amalgamation of optical components and electronics or active electrical components. Similar to an active DAC, the 25G AOC uses two strands of optical transceiver assemblies in bi-directional transmission mode. It uses a fiber cable and silicon chip and provides an electrical-to-optical connection to boost the signal and performance of the cable.

25G AOC features multimode optical fibers of different lengths. The multimode optical fiber used in this cable could be OM3 or OM4, making it highly-efficient and giving it higher light-gathering capability. AOCs can be a little expensive than DACs.

Features of an Active Optical Cable:

1. An AOC cable uses converts standard electrical current or data signals into light signals, making them insusceptible to the effects of electromagnetic interference.
2. It uses dual SFP28 connectors at both ends, which help to establish an integrated duplex data link for bidirectional communications as opposed to traditional copper or optical cables.
3. An AOC offers longer reach capability that enables smoother signal and data transmission over long distances, giving it an upper hand over short reach solutions.
4. This type of cable is compatible with and widely used in data centers, storage area networks, hyperscale, HPCs, and enterprise storage systems.
5. Since an active optical cable comes with EMI immunity capability, it’s less prone to degradation, system failure, and response errors, compared to copper cables.
6. It’s ideal for long-range and high-performance computing applications as it can reach up to 100 to 200 meters. Since it’s not limited to a 5 to 10-meter reach, it’s ideal for different applications, including ToR, Middle of Row (MoR), End of Row (EoR), and zone-to-zone. An AOC can handle both small and large clusters.
7. The assemblies within an active optical cable can support high transfer rates up to 100 Gbit per second and help to achieve low latency in data centers and HPC applications. Based on the form factor, an AOC can offer a speed of 10 GB/s, 25 GB/s, 40 GB/s, and 100 GB/s.
8. AOCs conform to the RoHS, SFP+MSA, IEEE P802.3ba, and SFF-8432 Standards with outstanding reliability and EMI performance.

Conclusion: DAC vs. AOC

With progressive advancements in fiber technology, some operators might conclude that copper technology has, or will, become obsolete soon. However, that’s not the case with a direct attach cable. DACs have both copper and optical fiber solutions that are cheaper than optical transceivers and other fiber alternatives. If the aim is to deal with small clusters, direct attach copper cables will be the ideal solution for that. On the other hand, active optical cables can easily tackle the challenges associated with larger or continuously growing data centers and achieve high data rates.

A direct attach copper cable, with electronics embedded within its connectors, can support longer transmission distance, as compared to passive DAC and traditional copper cables. An active optical cable, conversely, has electronics and optics within the connectors, which is why it can reach and support the highest transmission distance than both active and passive DAC.

Direct active copper cables are bulky and heavy, while active optical cables are light-weight and easy to manage.

Based on your budget, distance reach, transmission speed, and other requirements, you can choose the solution that fits you best!