FED-STD-1OO5
January 20, 1977
I
FEDERAL STANDARD
TELECOMWNICATIONS: CODING ANO MODULATION
REQUIREMENTS FOR NONOIVERSITV 2400 BIT/SECONO MOOEMS
This standard is issued by the General services Administration pursuant tO the Federal
Property and Administrative Services Act of 1949, as amended.
1. a
1.1 Description. This standard establishes the coding and modulation requirements for 2400 bit/second
modems owned or leased by the Federal Goverment for use over analog transmission channels other than
those derived from high-frequency radio facilities.
1.2 w. This standard is to facilitate interoperability between telecommunication facilities and
systems of the Federal Government.
1.3 Application. This standard shall be used by all Federal agencies in the design and procurement of
non$iversity 2400 bit fsecond modems for use with nominal 4 KHz channels derived from either switched
networks or dedicated lines. Typically, such channels are derived from frequency division multiplex
equipment associated with microwave, cable, and satellite transmission systems.
2. Requirements
2.1 2400 Bit/second Operation
2.1.1 The transmit carrier frequency shall be 1800 ~1 Hz.
2.1.2 The data stream to be modulated is divided into pairs of consecutive bits (dibits). Each dibit is
encoded as a phase change of the 1800 Hz carrier relative to the phase of the carrier during transmission
of the immediately preceding dibit as indicated below.
m
The phase change is the actual phase shift i“ the tram ftlon region from the end of one signaling element
to the beginning of the following signaling element IS illustrated below.
+45”
+135”
+225” +315”
I I
1
,
At the demodulator, the dibits are decoded and the bits are reassembled in the correct order. The left-
hand digit of the dibit is the one occurring first in the data stream as it enters the modulator portion
of the modem.
2.1.3 Synchronization
2.1.3.1 Ourin the interval between Request to Send (RS) indication from an associated Oata Terminal
?
Equipment (OTE and the modem responding with a Clear to Send (CS) indication, synchronizationos ignals
will be enerated by the transmitting modem(s). Unless a scrambler is utilized, continuous 225 phase
?“”
changes dlblt
11) shall be transmitted for the duration of the synchronization period.
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