Datasheet

Configuring Voice Ports
Configuring Digital Voice Ports
VC-67
Cisco IOS Voice, Video, and Fax Configuration Guide
If the timing source is internal, timing derives from the onboard phase-lock loop (PLL) chip in the digital
voice interface. If the timing source is line (external), then timing derives from the PBX or PSTN CO to
which the voice port is connected. It is generally preferable to derive timing from the PSTN because their
clocks are maintained at an extremely accurate level. This is the default setting for the clocks. When two
or more controllers are configured, one should be designated as the primary clock source; it will drive
the other controllers.
The line keyword specifies that the clock source is derived from the active line rather than from the
free-running internal clock. The following rules apply to clock sourcing on the controller ports:
• When both ports are set to line clocking with no primary specification, port 0 is the default primary
clock source and port 1 is the default secondary clock source.
• When both ports are set to line and one port is set as the primary clock source, the other port is by
default the backup or secondary source and is loop-timed.
• If one port is set to clock source line or clock source line primary and the other is set to clock source
internal, the internal port recovers clock from the clock source line port if the clock source line port
is up. If it is down, then the internal port generates its own clock.
• If both ports are set to clock source internal, there is only one clock source: internal.
This section describes the five basic timing scenarios that can occur when a digital voice port is
connected to a PBX or CO. In all the examples that follow, the PSTN (or CO) and the PBX are
interchangeable for purposes of providing or receiving clocking.
• Single Voice Port Providing Clocking—In this scenario, the digital voice hardware is the clock
source for the connected device, as shown in Figure 17. The PLL generates the clock internally and
drives the clocking on the line. Generally, this method is useful only when connecting to a PBX, key
system, or channel bank. A Cisco VoIP gateway rarely provides clocking to the CO because CO
clocking is much more reliable. The following configuration sets up this clocking method for a
digital E1 voice port:
controller E1 1/0
framing crc4
linecoding hdb3
clock source internal
ds0-group timeslots 1-15 type e&m-wink-start
Figure 17 Single Voice Port Providing Clocking
• Single Voice Port Receiving Internal Clocking—In this scenario, the digital voice hardware receives
clocking from the connected device (CO telephony switch or PBX) (see Figure 18). The PLL
clocking is driven by the clock reference on the receive (Rx) side of the digital line connection.
Figure 18 Single E1 Port Receiving Clocking from the Line
E1 0
PBX
Clock
26919
E1 0
Clock
26920
PSTN