AT&T 2500 series, 7500 series manual Availability of Synchronization Sources, Misconception, Fact

B-28SYNCHRONIZATION OF DIGITAL FACILITIES

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AVAILABILITY OF SYNCHRONIZATION SOURCES

Misconception 1	The local exchange company (LEC) can always provide the synchronization source.
Fact 1	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.
Misconception 2	AT&T communications can provide the synchronization source.
Fact 2	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.
Misconception 3	Synchronization is not necessary for DS1 networks that only transmit voice.
Fact 3	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, Generic 1 and Generic 2 DS1s
	do not have separate transmit and receive equipment and therefore must use a single
	reference frequency to prevent overrunning or underrunning the buffers.
Misconception 4	The OIU-2 is not required in a D4-channel bank since both Generic 1 and
	Generic 2 are D4 compatible.
Fact 4	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