The development of cabling technology in data center is driven by the ever-increasing bandwidth needs. The battle in data center between cabling performance, transmission distance and cost has never stopped and now it becomes fiercer with 40G becomes the target of most data center. In this battle, the cabling used to interconnect switches, connectors and storage devices are directly impacted by the increasing bandwidth. However, cost is also a critical factor as thousand of cables may be used within a 40G data center network. At the same time, many other factors are affecting the final decision.
Currently the direct attached cable (DAC) is the accepted effective and cost-save solution for data center interconnection. There are three types of 40G DAC: active optical cable (AOC), passive copper cable and active copper cable. For data center where a lot of expensive components are used, finding the best solution of the best is always reasonable and necessary. Which should be the one for connecting signals across the rack in data center? Comparing between these DACs from performance to cost, this post will recommend the most suitable rack interconnection solution in 40G data center.
In the past, when the data rate required is less than 5 Gbps, the passive copper cables are used for interconnection. This type of DAC connects two SFP connectors by a copper cable, providing direct connection between cable ends via copper wire. Thus they are not expensive and robust with reliability. As it is passive, they cost no power generally. However, when it comes to 40Gbps, they cannot satisfy such data rate in most cases. The passive copper cable with QSFP connectors attached on both ends can only support transmission 40 Gbps over very short distance. Thus passive copper cable is not suggested for regular 40G interconnection, unless 40G transmission in very short distance.
Then active optical cable is used to over come the 40G challenges. Optical cables are thinner, flexible and can reach much longer distance up to 100 meters or more, which is much longer than that of the passive copper cable. However, AOCs are usually very expensive with the connectors attached on the active optical cable are embedded with optics and/or electronics.
The connectors of the active copper cable are embedded with electronics. Although they cannot support transmission distance as long as AOC, active copper cable can support longer transmission distance than that of the passive copper cable via copper wire. 40G transmission distance of active copper cable is about 15 meters.
In this round of battle over 40G transmission distance. Active optical cable wins with a transmission distance up to 100 meters. The active copper cable got the second place. And passive copper cable is at the last place. It is only suggested for 40G transmission over really short distance.
The main reason why active copper cable and AOC can support longer transmission distance than passive cooper cable can do is that they are supported with active electronics. Passive copper cable requires no power. For 40G transmission, the power required for active copper cable is about 440mW, which is much less than that of AOC—2W. Thus, passive copper cable wins the battle over power consumption.
Cooling is always important for data center, as it is closely related to the data center reliability and life of use. During DAC cable selection, two main factors can affect the cooling of data center. One is cable size. the other is the power consumption of direct attached cable.
For the former factor, the thinner the cable is, the better dispersion devices in data center would have. Among the three types of DACs, optical cable is the thinnest among the three types of DAC. Active copper is also thinner than passive cooper cable. The relation between power consumption and data center cooling is easy to understand. Higher power consumption can generate more heat in data center, which will load more burden on the data center cooling system. As mentioned. Active optical cable needs the highest power and passive copper cable needs the lowest.
In this round of battle, which one is better to be selected cannot be decided easily. It depends on the practical and specific situations in the data center.
Cost, one thing you cannot ignore during DAC selection for 40G data center. Sometimes, cost is the main factor that affects your choice. It is clear that copper is much cheaper than optical cable. AOC, with connectors embedded with electronics and/or optics is the most expensive one. Although it supports the highest transmission distance, many data centers won’t choose it as it is too expensive. Passive copper cable is much cheaper than AOC. However, it cannot support 40G transmission in most cases in data center. Active copper cable is less expensive than AOC and can support longer transmission than passive copper cable seems to be a good choice.
In the battle over cost, passive copper cable wins. But it is not suggested for 40G transmission in most cases.
After several rounds of battle, the price and performances of the three DACs are clear. What to be considered next is the practical application. In most data center, most server to server connections require 2-5 meters, while rack to rack connections are required to be connected by 8-12 meters cables. For 40G transmission, the application of passive copper cable is limited because it only supports really short transmission distance. AOC has the best transmission performance. However, it costs most in both material and daily use. It’s expensive and needs more power. And in daily maintenance, they need careful management, because the optical cables are fragile. If cost is not a problem, then AOC is suggested for 40G transmission.
Active copper cable, which is less expensive and can support 40G transmission up to 15 meters with low power consumption, can satisfy the regular interconnection requirement for distance and cooling in most data center. For the cost, you also don’t need to worry about, as it is much cheaper than AOC and it is stronger than AOC during cabling and daily maintenance. If you are looking for an economical solution to upgrade your data center to 40G, then active copper cable won’t let you down. The above picture shows an active copper cable connecting to a switch in data center. The following chart can illustrate the advantages of active copper cable clearly.
|DAC Type||40G Transmission Distance||Power Consumption||Price|
|Passive Copper Cable||5m Reach||0W||Inexpensive|
|Active Copper Cable||15m Reach||440mW||Moderately Priced|
|Active Optical Cable||100M+ Reach||2W+||Expensive|
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