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COMMUNICATIONS PROTOCOLS D-17
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The DCP protocol physical level requires a 4-wire data link. Therefore, the data link must be composed of
at least two twisted pairs. The physical interface may include additional wire pairs for spares and auxiliary
power functions. Typically, 4-pair inside building wiring and 8-pin modular jacks are used.
The physical interface consists of 22-, 24-, or 26-gauge inside building wiring. The protocol supports
nonrepeatered cable up to a maximum distance of 5000 feet with 24-gauge wire and up to 4000 feet with
26-gauge wire. However, the maximum distance for specific terminals may be further constrained by other
limitations (e.g., depending on the number of associated modules).
Figure D-5 depicts the DCP frame structure. Each frame consists of four fields. The first is a framing field.
It is a 3-bit field which is used to establish frame boundaries. The second is a 1-bit signaling field. It is
used to transmit terminal and call signaling information between the switch and terminals. The third and
fourth are 8-bit information (I) fields. The I-fields are used to transport customer-generated data and PCM-
coded voice between the digital endpoint and the digital switch.
S
F
FRAMING
INFORMATION
SIGNALING
INFORMATION
I2
20 BITS
1 FRAME
125 MICROSECONDS
FIELD
FIELD
FIELD (1)
FIELD (2)
I1
Figure D-5. DCP Frame Format
Table D-5. DCP versus ISDN-BRI protocol parameters comparison
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PARAMETER DCP ISDN-BRI
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Aggregate Bit Rate 160 kbps 192 kbps
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Frame Rate 8 kHz 8 kHz
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Bits per Frame 20 24
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Information Channels Two (I1,I2) Two (B1,B2)
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Info Channel Rate 64 kbps 64 kbps
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Signaling Rate 8 kbps 16 kbps
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Type Signaling Stimulus Functional
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Configuration point-to-point point-to-point
or multipoint
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