Dolby Laboratories DP503 - Manual (Page 88)

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Regional Bell Operating Company. There are seven RBOCs, or local phone
companies, throughout the U.S. that provide local telephone service and
exchange calls with the IXCs (long distance carriers).
Terminal Adapter (TA)
The Terminal Adapter is a device or modem which connects to the ISDN
lines. The TA formats the data into the correct format and originates the
dialing and signaling required by the ISDN network.
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