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SYNCHRONIZATION OF DIGITAL FACILITIES
AVAILABILITY OF SYNCHRONIZATION SOURCES
Misconception 1
Fact 1
Misconception 2
Fact 2
Misconception 3
Fact 3
Misconception 4
Fact 4
The local exchange company (LEC) can always provide the synchronization
source.
The LECs are not always subscribers to the AT&T reference frequency. Many
end offices still use analog switches and D4-channel banks. In the past, these
channel banks have not had to be synchronized and probably have not been
equipped with an office interface unit (OIU-2); therefore, they do not provide a
lower- (3, 2, or 1) stratum reference.
Even if the CO provides a lower stratum reference, a T1 facility must be
terminated in equipment that is clocked by the lower stratum reference to access
this lower reference.
AT&T communications can provide the synchronization source.
AT&T communications is bound by FCC tariffs that currently do not require
the use of framing bits in the customer’s incoming T1 bit stream. Until framing
bits are required, you should not arbitrarily assume that the carrier’s T1
facilities provide a synchronization source.
Synchronization is not necessary for DS1 networks that only transmit voice.
The issue here is the use of a digital switch instead of channel banks. With
channel banks, the transmit and receive functions are separate and could have
different clock frequencies without slipping. With channel banks, it makes no
difference whether you transmit voice or voice-grade data. However, a Generic
1 and Generic 2 DS1 does not have separate transmit and receive equipment
and therefore must use a single reference frequency to prevent overrunning or
underrunning the buffers.
The OIU-2 is not required in a D4-channel bank since both Generic 1 and
Generic 2 are D4 compatible.
Both Generic 1 and Generic 2 are (when operating in D4 mode) compatible
with D4-formatted DS1 or T1 facilities. However, a DS1 was not designed to
be functionally the equivalent of a channel unit. D4-channel banks, as
described in Fact 3 above, use line-powered oscillators to provide their
clocking. Given the normal drift in an AC line, the transmit frequency of a
D4-channel bank can vary significantly. The OIU adds a stratum-4 clock to the
common equipment of a D4, allowing the transmit and receive sections to be
commonly synchronized to:
●
The incoming line (loop timed)
●
Free-running (stratum 4 accuracy)
●
An external reference (a link to the output of a lower stratum reference),
which applies to both DS1 span; you can select either DS1 span as the
reference for both