AT&T DS1/DMi/ISDN-PRI manual Availability of Synchronization Sources, Misconception, Fact

5-28SYNCHRONIZATION OF DIGITAL FACILITIES

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, 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.
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