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White Paper
looptelecom.com 1 August 13, 2007
Signal Names for Transmit & Receive
By Dr. John W. Pan
Circuit Switched Voice BandFor circuit switched telephone
networks consisting of switches and transmission facilities, the
labeling ofsignals as transmit or receive is from the point of view
of the switch. In the diagram below, a signal originatesfrom West
Switch, traverses a Carrier, and terminates in East Switch, with
the return path in reverse.
West
Switch
East
Switch
West
Carrier
Terminal
TxEast
Carrier
Terminal
Rx
Rx Tx
The traditional label for the signal from the Switch to the
Carrier Terminal is Transmit(Tx), with the oppositesignal, from the
Terminal back to the switch, as Receive(Rx). The signal originating
as Tx becomes Rx at
the destination. From the Carrier Terminal point of view, Tx is
actually received for transport. At the farend,that signal out of
the Carrier Terminal, now Rx, is transmitted to the Switch.
Using this rule, a signal coming out froma piece of equipment
will be labeled Tx or Rx depending on thatequipment being switching
or transmission, and conversely for a signal going toa piece of
equipment.
Packet Switched Ethernet
Ethernet equipment often contains both switching and
transmission functions. For naming of pins for twocommon connectors
used in Ethernet, RJ48, which serves a single port, and RJ21, which
serves 12 ports,two manufacturers chose different names for the
same signal, shown below.
Connector Adapter Panel by Cornet Technology, Inc.RJ45 Pin outs
have pins 1 & 2 as TxRJ21 Pin outs have pins 1 & 26 as
Tx
Ethernet Switching Module by 3-COMRJ45 Pin outs have pins 1
& 2 as RxRJ21 Pin outs have pins 1 & 26 as Rx
