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LAN & WAN Basics

Data and Phone Patch Panels

Ever wonder why Ethernet cables and connectors are wired the way they are? Turns out it's not just to make them more difficult to terminate, but to be backward compatible with phone wiring conventions. The EIA / TIA T568 A/B standard was designed so that one wiring pattern could be used to handle telephone, ISDN and data applications.

The "pair" numbers in Figure 6 refer to the different colored twisted pairs of wires in an eight conductor CAT5 cable, with each "pair" carrying a single phone line.

T-568 A/B pinouts and pairs

Figure 6: T-568 A/B pinouts and pairs
(image courtesy of A.P.T. Communications)
(click image to enlarge)

Note that since pair #1 ends up on the two center contacts (4 and 5) of the eight-conductor RJ45 jack, they will make proper connection to a phone's 4-conductor RJ11 plug. Note that the lower T-568A pinout puts the second wire pair on contacts 3 and 6 of the jack for proper connection to a two-line telephone. This is why most wiring guides recommend its use instead of the alternative T-568B pinout. Since Ethernet relies on just the orange and green pairs (pins 1, 2 and 3, 6) being connected and doesn't care which is which, it doesn't matter whether T-568A or B is used for pure Ethernet applications.

With the decision of T-568A / B out of the way, I next had to decide how I was going to terminate all the CAT5e at the wiring panel. The cheapest way to go was to not use a patch panel at all, but instead just crimp an RJ45 plug onto the end of each cable. After all, the data lines will just end up getting plugged into a switch port, so why have the extra layer of interconnect? I almost went this way, but remembered how much I hate getting the wires lined up properly to crimp into an RJ45 plug. So despite the added cost, I decided to use a patch panel.

The question I next had to face was whether to use a patch panel with RJ45 jacks (Figure 7), telephone style "punch down" panel (Figure 8), or one of each for data and phone drops respectively.

CAT5e patch panel

Figure 7: CAT5e patch panel
(images courtesy of Deep Surplus)

110 block with terminated cables

Figure 8: 110 block with terminated cables
(click image to enlarge)
(image courtesy of Leviton Voice & Data Division)

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