A T1 frame is 193 bits. 192 bits (24X8) corresponds to data, and 1 bit is
used for synchronization. There are 8000 frames per second. T1 was
originally defined for PCM-encoded voice (8 bits per channel, but 1 bit for
signaling when needed - "robbed-bit signaling"), with 24 voice channels per
frame. It is actually unfortunate that "T-1" became a generic term for
anything that has a 1.544 Mbs bit-rate, because T1, DS1, and ISDN PRI should
be defined completely differently. T1 should have been reserved for
24-channel voice, DS1 for 24-channel data, and PRI for 23-channel
+1D-channel (ISDN).
Regardless T1 frame = 24(8-bit voice + signaling) channels + 1sync bit
DS1 frame = 24(8-bit digital data) + 1 sync bit
ISDN frame = 23(8-bit digital data) + 1(8-bit signaling) +
1 sync-bit (as logically represented on a T1 carrier)
Therefore, 193X8000 = 1544000 clock-rate for T1 (DS1).
192X8000 = 1536000 data-bits/sec for T1 (DS1).
1X8000 = 8000 sync-bits/sec for T1 (DS1).
Notice that there is no defined signaling channel. If multiple sites are
sharing a T1(DS1) link then without some form of common signaling there
would have to be in-band signaling, which would reduce the 8-bits per
channel to 7-bits per channel (ever heard of "switched-56K"?). Fractional T1
would allow common signaling for a higher user data-rate.
Therefore, assuming that some form of out-of-band signaling exists, T1(DS1)
provides 1.536 Mbs of bandwidth, with 8Kbs lost due to sync bits.
The question for PRI becomes even more confused. 23 B-channels + 1 D-channel
+ 1 sync-bit, but the D-channel is primarily used for common signaling. The
question is if there are 23 B-channels at 64Kbs each, how much of the
D-channel's bandwidth can be used for user data, if any (for example, by
running X25 on top of the signaling channel)? This would imply that PRI
would have only 23X64Kbs for user data plus 64Kbs for signaling. Although
ISDN is considered to be all digital, in reality PCM-encoded voice on a T1
line is a digital representation of an analog sample. ISDN still uses T1,
T3, etc. lines to bundle multiple facilities on a common line, but the
channels are interpreted differently. What is really fascinating is to see
how ISDN achieves these rates via the data format from the U-interface.
So the question is "which provides 1.544 Mbs of bandwidth, T1 or PRI?" is
nebulous at best. Neither can provide 1.544 Mbs for exclusive use for user
data. But both have a 1.544 Mbs clock-rate.
MLC
"NetEng" <[EMAIL PROTECTED]> wrote in message
8qvlk4$vtv$[EMAIL PROTECTED]">news:8qvlk4$vtv$[EMAIL PROTECTED]...
> Is there a difference between a PRI and a T-1? What has 23B + 1D? What one
> has 24B? The reason I ask is I have a practice question that asks, "what
> provides 1.544Mbs bandwidth." PRI and T-1 are answers, but only PRI is
the
> correct answer (according to the test). What's the final answer????
>
>
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