Comtech EF Data VMS v3.12.x Vipersat - Manual (Page 342)

VMS Service Managers MN/22156, rev 12
6-36 VMS User Guide
An advanced switching table can be constructed for a remote modulator where
specified modulation types and FEC code rates are paired with set data rates.
Each data rate is associated with a Mod/Code and, as the system achieves the set
rate, the transmission is modified to the new higher- or lower-order modulation
setting specified for that rate. For each table entry, the VMS calculates an opti-
mized switching threshold that the system uses to assign the most efficient
bandwidth in an advanced switching environment.
As a switch request is processed, it is compared to the Advanced Switching
table. If the requested data rate crosses a threshold where the higher-order
modulation actually becomes more bandwidth efficient, the switch request will
go up to the higher-order modulation at the lowest bit rate that exceeds the
request. Thus, it is possible that a higher bit rate can be granted while actually
utilizing less bandwidth resources.
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 and code rate was specified in the Advanced Switching table entry
for this switch point, as shown in figure 6-44.
The following equations illustrate this scenario:
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
Figure 6-44 Advanced Switching Table for Remote (R_2)