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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Telephone Cable to Ethernet Cable
We will summarize some of the key points to keep in mind about cabling and jacks before we move on. In Fig. 2.5, on the right side, we have the registered jacks (RJs) used in telephony since the 1970s. The simplest version could involve just one twisted pair per cable/RJ, and that is RJ-11, or a 6p2c telephone jack: referring to its 6 positions (available slots) and its 2 contacts (number of wires present). Another twisted pair could be added to the cable, and that would make the same jack into a 6p4c jack, called an RJ-14. One more twisted pair would make this a 6p6c jack, or an RJ-25. At this point, the physical place in this particular RJ would have run out, since it has a maximum of six positions available ...

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