Today’s networks require higher performance from the network cabling, as well as cost effective connectivity. Direct attach copper and fiber cables provide both of these with their factory terminated performance and reduction in costs associated with field terminations. Let’s begin with the types of direct attach cables on the market today, advantages and disadvantages to each, and which cables are best for various applications.
What is a high speed direct attach cable? A high speed direct attach cable is a type of factory terminated cable assembly used in data centers for point-to-point connections of active network equipment. These cable assemblies consist of fixed lengths of shielded copper coaxial or fiber optic cable with pluggable transceivers factory terminated on either end. Direct attach cables are available in popular transceiver form factors, including SFP, SFP+ and QSFP .
You will typically find high speed interconnect cables in data centers, storage area networks and high performance computing centers (HPC) due to the requirement for high bandwidth, connection density and low latency. There are three common types of direct attach cables: Passive DAC – Direct Attach CopperActive DAC – Active Direct Attach CopperAOC – Active Optical Cable Passive DACs DACs are the most basic form of direct attach cabling. DACs are constructed using shielded twin-axial copper cable in varying gauges from 24 to 30AWG. The length of the cable affects the signal attenuation which requires a specific gauge for the conductors. Longer cables require larger gauges in order to reduce the signal transmission loss through the cables. DACs are passive assemblies since they do not amplify or condition the signal in any way. Instead, signals are passed through and regenerated by the host network equipment.
The length limit for passive DACs (without amplification) is 7m.Although DACs are passive, the connectors in DACs do contain an “Electrically Erasable Programmable Read-Only Memory”, or EEPROM, that is used to store and provide information to host network equipment such as manufacturer name, serial number, part number, and date of manufacture. Technically, this EEPROM does consume a very small amount of power, around 0.15 W. Active DACs Active DACs, or Active Copper Cables (ACCs), are similar in construction to passive DACs but contain a microprocessor and other circuitry in the transceiver connectors to extend signal reach. The distance limit of an ACC is about 15m, which is a 2x improvement over a passive DACs limit. Also, the additional circuitry of the ACC does increase its power consumption to around 0.5-1.0 W, on average.
AOCs – Active Optical Cable AOCs are similar to the active DACs in that they consist of a duplex fiber optic cable terminated with pluggable transceiver connectors on either end. The cable used in an AOC is either multi-mode or single-mode optical fiber which provides advantages over DACs or DCCs, such as longer transmission distances, isolation from signal interference and crosstalk and higher signal transmission capacities (bandwidths). The connectors in AOCs are actually optical transceivers making them a bit more complex and expensive than passive or active DACs. The optical fiber and technology used in AOCs give them a reach of up to 100m or more. Of the three types of direct attach cables, AOCs consume the most power at around 1-2W. Advantages and Disadvantages of Using High Speed Direct Attach Cabling Over Transceivers When considering the use of direct attach cabling for a particular cabling infrastructure application, one must weigh the advantages and disadvantages.
The following list below highlights some of the advantages and disadvantages of using direct attach cabling over discreet transceivers attached with field-connected structured cabling. Advantages
• Lower Price – Direct attach cables are less costly than using discreet transceivers with field-connected structure cabling, because the interconnection is simplified. There aren’t as many connectors, adapters, patch panels, and other infrastructure elements along the path of communication channel.
• Lower Power Consumption – Particularly with passive DAC cables, power consumption is less when compared to the use of transceivers because they are “self contained” components and not bound by transmission specifications as transceivers. For example, transceivers designed to work with copper twisted pair structured cabling must have a maximum reach of 100m whereas an active DAC only needs to reach a maximum of 15m. As a result, the required internal circuitry and signal power can be simplified and reduced.
• Plug and Play Simplicity – DACs and AOCs are only one component to manage rather than multiple components that must be interconnected together. In addition, the installer does not have to be concerned with cleaning and inspecting optical fibers in the field before plugging the cables into the transceivers.
• Factory Terminated Performance – DACs and AOCs are terminated and 100% tested at the factory. This provides consistent and expected transmission performance levels for the channel. Disadvantages • Reduced Cable Flexibility – Passive and active copper DACs have a larger bend radius and weight than traditional structured cabling or AOCs, which can sometimes place additional demands on the cable management and airflow management within a rack or cabinet.
• Reduced Modularity – Structured cabling provides improved modularity through the use of patch panels and other components to make moves, adds and changes quicker and easier. DACs and AOCs are point-to-point cabling that require some additional labor since they need to be completely pulled out of racks, cable managers, cable tray, and other infrastructure elements.
• Limited Distance – Transceivers and structured cabling are designed to work together in a universal and cohesive system. Therefore, pluggable transceivers are required to reach 100m or beyond, while DACs and AOCs are not. Applications for Direct Attach Cables Direct attach cables can be used in a variety of applications and locations in a data center. In general, this pre-terminated solution is particularly effective for the following applications:
• Top of Rack/Adjacent Rack – Passive or active DACs are ideal for shorter ToR or rack-to-rack runs with cost-conscious budgets. AOCs will certainly work at shorter lengths (typically 5 feet), but the performance/cost trade-off may not be as compelling.
• Middle of Row – Active DACs may be a perfect solution in this application, as long as the runs are less than 15m. AOCs would also make a good solution for MoR deployments.
• End of Row – AOCs are most likely the best option for EoR configurations since the applicability of the active DACs reach their limit at around 15 meters in length.
• Zone-to-Zone – AOCs are the clear solution for longer zone-to-zone runs due to the advantages of using fiber optic cables as mentioned previously. DACs vs. AOCs vs. Transceivers & Structured Cabling The following shows a table summarizing the key differences between the use of DACs, AOCs and Transceivers: Conclusions Direct Attach Cables provide an excellent pre-terminated and factory assembled & tested solution for both copper and fiber optic cabling in data centers. Performance advantages and cost savings can be realized over field installed cabling by avoiding testing and inspection of individual connectors and cabling components in the link.