AT&T DS1/DMi/ISDN-PRI manual 2SYNCHRONIZATION of Digital Facilities, Transmit

5-2SYNCHRONIZATION OF DIGITAL FACILITIES

Figure 5-1-A, Options for Synchronization, shows one possible connection between a pair of D4- channel banks. Such a connection (using D4-channel banks) can typically be found with a pair of analog switching systems connected by T1-carrier facilities. For this arrangement, the transmitting portion of each channel bank independently determines the clock rates. The receiving portion of each channel bank derives its clock from the incoming digital bit stream. In this arrangement, the channel banks convert the received digital signal directly to analog, and there is no requirement that the two clock frequencies precisely match as the channel bank does not interface to another digital system.

		TRANSMIT
	CHANNEL	F
ANALOG		0
	BANK
		RECEIVE

F
0		RECEIVE	CHANNEL
			BANK
F		TRANSMIT
1		F
DIGITAL BIT	STREAM	1

A: Not Synced

ANALOG

	DIGITAL	TRANSMIT
	SWITCHING
DIGITAL	SYSTEM
	F	RECEIVE
	0

	DIGITAL	TRANSMIT
	SWITCHING
DIGITAL	SYSTEM
	F	RECEIVE
	0

REFERENCE CLOCK RATE F 0

F 0

F0

DIGITAL BIT STREAM

B:Loop Timed F0

F 0

DIGITAL BIT STREAM

C: Externally Synced

RECEIVE
	CHANNEL
TRANSMIT
	BANK
F
0

RECEIVE	DIGITAL
	SWITCHING
TRANSMIT	SYSTEM
	F
	0
	REFERENCE
CLOCK RATE F
	0

DIGITAL

D I G I T A L

Figure 5-1.Options for Synchronization

Figure 5-1-B, Options for Synchronization, shows a different connection between a channel bank and a digital switching system. For this configuration, the digital switch transmits a digital bit stream at the rate (F0) that is determined by its internal clock. The digital switch must receive the incoming digital bit stream at this same rate (F0). Otherwise, the switch’s receiving buffer may eventually overrun or underrun.

NOTE: Switching is done by placing the signals from individual time slots on one link into the time slots on other links using a process called time-slot interchange (TSI). For this process to work properly, bit synchronization must be maintained on all links terminating on the digital switching node, no matter where the links originate.

If the average transmit rate is faster than the average receive rate, the receive buffer will eventually overrun. If the average receive bit-clock rate is faster than the average transmit bit-clock rate, the receive buffer will eventually underrun. It is necessary to prevent overruns (deletions) and underruns (repetitions) by synchronizing the network properly. Improper synchronization results in buffers repeating or deleting bits in 1-frame increments.