
About Line Protocols
115391-A Rev. A 2-19
T1 Overview
T1 services use digital signals to allow two pairs of wires to carry 24 voice or data
transmissions. T
able 2-3 shows the specification for a T1 line.
Frame Formats
T1 uses two types of frame formats:
•D4
• Extended superframe (ESF)
D4
D4 is the original T1 frame format. A D4 frame (Figure 2-8) comprises
• One framing bit
•A DS0 timeslot for each channel on the line
A DS0 timeslot is an 8-bit sample from a channel.
A T1 line generates 8000 D4 frames/second.
Table 2-3. Specification for T1 Line
Attribute Value
Framing rate 8000 frames/s
Channels per frame 24
Line speed 1.544 Mb/s
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