T 1 Frame (Figure)
o DS-1 requires 8 Kbps of overhead
o To understand this overhead, lets examine
format of a 24-voice channel frame
o Frame used on T-1 line is usually 193
bits divided into 24 slots of 8 bits each
+
1 bit for synchronization (24*8+1=193)
o 24 segments are interleaved in one frame
o If a T-1 carries 8000 frames, the data
rate is 1.544 Mbps (193 * 8000=1.544
Mbps)
which is capacity of the line
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Fractional T Lines (Figures)
o Many subscribers do not need the entire
capacity of the T line
|
o For example, a small business may need
only one-fourth of the capacity of a
T-1
line
o If four business of same size lie in same
building, they can share a T-1 line
o DSU/CSU allow the capacity of T-1 line to
be interleaved into 4 channels
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