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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Learning from Telephony
There are so many lessons we have, and can still learn, from techniques used since a century ago. One question as we struggle to exceed 100 m (330 ft) in Ethernet is: in POTs (plain old telephone system), how did we ever achieve up to 5.5 km runs using low-grade (AWG 26), single-pair wire?
First, the current consumption was very low. A typical old nonelectronic (carbon microphone based) phone could work down to at least about 20 mA of current (with no connection to the AC mains of course). In fact, Bell Telephone specifications stated that three telephones should work in parallel on a 20-mA loop. Note that European telephone companies often state that phones working in parallel is “technically impossible” and is therefore ...

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