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Telephone:
pin RJ25 RJ14 RJ11 Pair T/R Color Old
1 X     3 T + white/green orange
2 X X   2 T + white/orange black
3 X X X 1 R - blue/white red
4 X X X 1 T + white/blue green
5 X X   2 R - orange/white yellow
6 X     3 R - green/white blue
T - Tip - ground
R - Ring - 48 volts
Ringing voltage is usually 88-110Vac, 20 Hz superimposed on -48Vdc.
Source: RJ11, RJ14, RJ25 - Wikipedia

Data - Ethernet
Standard Required Pairs 10M 100M 1000M Notes
10base-T 2 (1/2 and 3/6) yes yes no 100m support only if no cat 3/4 in run
100base-TX 2 (1/2 and 3/6) yes yes no 100m support only if no cat 3/4 in run
100base-T4 4 (1/2, 3/6, 4/5 and 7/8) yes yes yes  
1000base-T 4 (1/2, 3/6, 4/5 and 7/8) yes yes yes Functionally identical to 100base-T4
Source: Tech Info - LAN Wiring and Pinouts

Ethernet Wiring 8 Position 8 Contact (8P8C)
 (often incorrectly referred to as RJ45)
10Base-T and 100Base-TX
TIA/EIA  T-568B  - Most Common
pin color   function
1   wh/or   TxData +
2   or      TxData -
3   wh/grn  RecvData+
4   blu    
5   wh/blu 
6   grn     RecvData-
7   wh/brn 
8   brn     


TIA/EIA T568A (Transmit and receive colors reversed)
Based on original AT&T USOC (Universal Service Order Codes) standard
pin
1  TD+  white/green
2  TD-  green
3  RD+  white/orange
4       blue
5       white/blue
6  RD-  orange
7       white/brown
8       brown


A cable wired with 568A at one end and 568B at the other is a crossover cable
Standards:
- International Standards (ISO/IEC 11801:1995) for structured cabling systems
- Telecommunications Industry Association (TIA)
- The Electronic Industries Alliance (EIA) is a trade organization representing the United States high technology community, sponsoring technical standards

Links:
RJ - 45 wiring standard
Tech Info - LAN Wiring and Pinouts
568A v 568B


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last updated 14 Apr 2008