Vcc Ground
Guest
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 Posted:
Fri Aug 19, 2005 4:05 pm Post subject:
Re: A simple question about T1framing |
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Nx64 is the T1 framing without in-band signalling, and Nx56 is the T1
framing with in-band signalling. When in-band signalling is used, the
signalling goes in the least significant channel bit of every sixth T1
frame (SF and ESF framing), leaving the effective throughput of those
channels at 56 kbit/s. Although in-band is still in use, signalling
(call setup, teardown, routing and status) is generally carried
out-of-band over a separate network using a protocol called signalling
system 7 (SS7). |
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James Carlson
Guest
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| Posted:
Fri Aug 19, 2005 10:00 pm Post subject:
Re: A simple question about T1framing |
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"Vcc Ground" <vccground@yahoo.co.in> writes:
| Quote: | Nx64 is the T1 framing without in-band signalling, and Nx56 is the T1
framing with in-band signalling. When in-band signalling is used, the
signalling goes in the least significant channel bit of every sixth T1
frame (SF and ESF framing), leaving the effective throughput of those
channels at 56 kbit/s. Although in-band is still in use, signalling
(call setup, teardown, routing and status) is generally carried
out-of-band over a separate network using a protocol called signalling
system 7 (SS7).
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There can be more to it than that.
Even if you're not doing in-band ABCD signaling, you can be limited to
Nx56 if the link is (mis)configured without B8ZS coding. This is
unfortunately not too uncommon in some areas.
If you're using ISDN (PRI), then one of the 24 channels may be stolen
for the signaling (the "D" [Delta] channel). The others are normally
Nx64, but could be Nx56 if there's a misconfiguration or an equipment
limitation in the path. But you can still have 24 channel lines with
PRI when using NFAS (Non-Facilities-Associated Signaling -- basically,
a D channel on some other line).
If the line is just used for data (a dedicated T1, perhaps carrying
ATM or HDLC), then there is no call signaling -- neither SS7 nor ISDN
-- and 24 channels of 64Kbps data is a typical configuration.
(And even then, there are unusual cases where AMI is used instead of
B8ZS, but 64Kbps operation per channel is still possible -- as when
using inverted HDLC. The underlying issue there is the run length
limitation that clock recovery requires, not signaling.)
The basic issue, though, is whether that 8th bit is (from the user's
perspective) stomped on by the hardware or available for ordinary
data. If it's the former, you're limited to 56K.
--
James Carlson, KISS Network <james.d.carlson@sun.com>
Sun Microsystems / 1 Network Drive 71.232W Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757 42.496N Fax +1 781 442 1677 |
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