Datasheet

Configuring Voice Ports
Configuring Digital Voice Ports
VC-68
Cisco IOS Voice, Video, and Fax Configuration Guide
The following configuration sets up this clocking method:
controller T1 1/0
framing esf
linecoding ami
clock source line
ds0-group timeslots 1-12 type e&m-wink-start
• Dual Voice Ports Receiving Clocking from the Line—In this scenario, the digital voice port has two
reference clocks, one from the PBX and another from the CO, as shown in Figure 19. Because the
PLL can derive clocking from only one source, this case is more complex than the two preceding
examples.
Before looking at the details, consider the following as they pertain to the clocking method:
– Looped-time clocking: The voice port takes the clock received on its Rx (receive) pair and
regenerates it on its Tx (transmit) pair. While the port receives clocking, the port is not driving
the PLL on the card but is “spoofing” (that is, fooling) the port so that the connected device has
a viable clock and does not see slips (that is, loss of data bits). PBXs are not designed to accept
slips on a T1 or E1 line, and such slips cause a PBX to drop the link into failure mode. While
in looped-time mode, the router often sees slips, but because these are controlled slips, they
usually do not force failures of the router’s voice port.
– Slips: These messages indicate that the voice port is receiving clock information that is out of
phase (out of synchronization). Because the router has only a single PLL, it can experience
controlled slips while it receives clocking from two different time sources. The router can
usually handle controlled slips because its single-PLL architecture anticipates them.
Note Physical layer issues, such as bad cabling or faulty clocking references, can cause slips.
Eliminate these slips by addressing the physical layer or clock reference problems.
In the dual voice ports receiving clocking from the line scenario, the PLL derives clocking from the
CO and puts the voice port connected to the PBX into looped-time mode. This is usually the best
method because the CO provides an excellent clock source (and the PLL usually requires that the
CO provide that source) and a PBX usually must receive clocking from the other voice port.
Figure 19 Dual E1 Ports Receiving Clocking from the Line
The following configuration sets up this clocking method:
controller E1 1/0 << description - connected to the CO
framing crc4
linecoding hdb3
clock source line primary
ds0-group timeslots 1-15 type e&m-wink-start
!
E1 0
E1 1
Looped time
Clock
Clock
26921
PSTN
PBX