NetXpress LX & CM-30R Installation & Operation Manual
2
–
Functional Design
Version 1.1
2-16
GatesAir, Inc.
Intraplex Products
from a telecom network provider. Another source of accurate external timing signals is a GPS
receiver, using GPS timing at the stream timing sites (Figure 2-8). This solution guarantees the
two streams received at site D have compatible timing, allowing Site D to time from either
incoming stream.
Figure 2-8. Timing Synchronization with GPS
In this case, the fixed cost of purchasing and deploying GPS units at Sites A and C can be traded
against the recurring cost of bandwidth to receive a timing stream at Site C.
2.7.2.1 Backup Timing
In networks that generate revenue-critical programming, it may be prudent to design backup timing
scenarios. Each NetXpress LX unit can be programmed to switch to an alternative timing source in the
case of primary timing signal loss. If the primary timing source is an incoming stream, for example,
the secondary source could be a different stream; either an audio stream or a dedicated timing
stream. Another scenario could use Stratum-1 or GPS as the primary timing source with stream timing
as the backup. Once again, the trade-off would be the one-time cost of GPS receivers or Stratum-1
timing sources versus the recurring cost of bandwidth for the backup timing signal. If both the primary
and secondary timing sources fail, the NetXpress LX unit falls back to internal timing.
Sites that are receive-only can usually utilize stream timing since they only need to be synchronized
during the time they are receiving a stream. If receive streams at a receive-only site are changed
frequently, there could be an operational advantage to using external (stratum-1 or GPS) timing, in
that the operator would not need to consider whether a receive signal is present.
Certain receive sites may also generate off-air monitor streams back into the network (Figure 2-9). A
logical choice for timing in this network would be to use internal timing at Site A and stream timing at
Sites B and C. In this case, if the audio stream to Site C is lost, the monitoring stream back from Site
C is not properly timed for reception at other network sites, which can result in periodic jitter buffer
underflow or overflow at the receiver with each event causing a brief interruption or audio glitch. If it
is important to maintain smooth continuous reception of this off-air monitoring stream during periods
of primary stream loss, an external timing source or a backup timing stream is needed at Site C.
NetXpress LX
A
NetXpress LX
B
NetXpress LX
D
NetXpress LX
C
External Timing
External Timing
Site D can derive system timing
from either incoming steam.
GPS
Stream Timing
GPS
