Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 169)

Chapter 3 - VMS Configuration 3-69
MN/22156, rev 12 Network Manager Configuration
• FAST Codes — The modem/routers must have the appropriate FAST
codes to ensure compatible functionality.
• Encryption — A Remote set for using TRANSEC requires the Hub
device to use TRANSEC also.
Considerations for Using Guaranteed Bandwidth with Advanced Switching
Care should be taken when assigning Bandwidth Reservations to a Remote that
also uses Advanced Switching (refer to “Set InBand Modulation and Coding”
on page 3-70). The VMS does not guarantee a bit rate, per se. Rather, a band-
width reservation (frequency value) is assigned. This is why the option for edit-
ing FEC and Modulation settings is provided in the Reservations dialog for a
remote site.
The VMS attempts to assign the most efficient bandwidth utilization in an
advanced switching environment. If Advanced Switching is configured for a
Remote, a switch request that crosses the threshold where the higher-order
modulation actually becomes more bandwidth efficient will result in a step up to
the higher-order modulation at the lowest bit rate that exceeds the request.
For example, a site currently operating at QPSK 3/4 that generates a switch
request for 192 kbps will be switched up to 256 kbps at 8PSK 7/8, provided this
modulation code rate was specified in the Advanced Switching table entry for
this switch point. This scenario is illustrated using the following equations:
QPSK 3/4 @192 kbps @1.3 spacing:
Allocated Bandwidth = 192 x (1/2) x (1/.75) x 1.3 = 166.4 kHz
8PSK 7/8 @256 kbps @1.3 spacing:
Allocated Bandwidth = 256 x (1/3) x (1/.875) x 1.3 = 126.781 kHz
However, when a bandwidth reservation is added to this scenario, the end result
may differ. If the reservation specifies 192 kbps at QPSK 3/4, the VMS will
perform the same calculation as shown in the first equation above and the
reserved bandwidth will be 166.4 kHz. Since this falls within the range at which
the VMS would step up to 8PSK, the bit rate available with an allocated band-
width of 166.4 kHz would be provided, which is 336 kbps.
Thus, when a guarantee is set within the threshold range of advanced switching,
unexpected results may result. In this example, the result is that the guaranteed
data rate that is provided by the VMS (336 kbps) is actually greater than the
expected CIR that was entered as the bandwidth reservation (192 kbps). In addi-
tion, the advanced switching performance will also differ, resulting in a higher
data rate as well as higher bandwidth usage.