As one kind of optical transceiver assembly, a direct attach cable (DAC) is used to connect one mobility access switch with another when forming a stack, which can be either passive or active. Since the passive DAC cable contains no active components, it provides a direct electrical connection between corresponding cable ends. This process can also be done by an active DAC cable that is considered active because there are optics and/or electronics embedded within the connectors. To put it simply. Active DAC vs. passive DAC: An Active DAC contains electronics for signal conditioning. While A Passive DAC does not contain electronics for signal conditioning. Then Why choose one implementation over the other? Just keep reading!
If a DAC contains the signal conditioning circuitry, it is an “Active DAC”. If the DAC does not contain this circuitry, it is a “Passive DAC”. For a passive DAC to work, the host networking equipment must support the signal processing functions. When an SFP+ is inserted, networking gear compatible with passive DAC reads the module type, and the signal conditioning is activated only when a passive DAC is detected. In all other cases, standard “machine level” signals are sent across the SFP+ edge connector.
More than 80% of all SFP+ equipped switches on the market today support passive DACs. The remaining less than 20% perform no signal conditioning and, therefore, must be equipped with substantially more expensive Active DAC if copper interconnection is desired. However, these switches, since they do not contain the signal conditioning on each SFP+ port, are considerably less expensive.
The decision regarding which way to go, a switch supporting Passive DAC or one requiring Active DACs, depends on several factors including:
Fiber Connectivity—If the application of a given switch is expected to utilize primarily/exclusively fiber SFP+ transceivers, the added cost of a switch that supports Passive DAC may not be warranted. On the other hand, if SFP+ ports are expected to be equipped primarily with DAC, the higher cost of the switch will be quickly recovered by the savings using low-cost Passive DAC versus higher priced Active DAC.
Length of DAC Spans—If the primary application of the use of DAC is limited to about 7 meters or less (e.g. within a rack or between adjacent racks), a solution supporting Passive DAC is indicated. However, if most of the DACs deployed with the switch are greater than 7 meters in length (e.g., ToR to EoR), Active DAC would be required regardless of the switch, so a low-priced Active-DAC-only switch may be the right choice. In fact, if a switch supporting signal conditioning detects an active DAC inserted, it will not use its internal signal conditioning circuitry but will leave that function to the active DAC itself.
FS.COM provides a full range of DACs, both passive and active, for all major and many minor brands of switches. The following is a sample of the popular 10G DACs offered by FS.COM:
| Part Number | Brand | Active/Passive |
| SFP-H10GB-CU1M | Cisco | Passive |
| SFP-H10GB-ACU5M | Cisco | Active |
| Force10 CBL-10GSFP-DAC-2M | Dell | Passive |
| EX-SFP-10GE-DAC-10MA | Juniper | Active |
| CAB-SFP-SFP-1.5M | Arista | Passive |
| 10G-SFPP-TWX-0701 | Brocade | Active |
| JD096C | HPE | Passive |
Conclusion
Besides the 10G DACs mentioned above, FS.COM offers a variety of high speed interconnect DAC assemblies including 10G SFP+ Cables, 40G QSFP+ Cables, and 120G CXP Cables to satisfy the demands from 10G to 100G interconnection. All of the FS.COM direct attach cables can meet the ever growing need to cost-effectively deliver more bandwidth, and can be customized to meet different requirements.
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