54 Network Design and Engineering Guide 2010-10-26General Carrier DesignFrom User Traffic to Satellite Link CarriersGenerally a selection of 0.2 for the roll-off factor will save bandwidth on the satellite transpond-er. Smaller roll-off factors mean increased signal power ripples in the time domain which mightpose a problem if transmitters on the earth stations or the satellite are operated close to thesaturation power level. In these cases a higher roll-off factor may be necessary.2.5.1 SummaryThe following picture is a graphical summary of the signal preparation steps as discussed be-fore. For each step the additional overhead is specified explicitly. Note the two different defini-tions of data rates:- User data rate: The data rate of user traffic received on the terrestrial interfaces (LAN, se-rial ports). Note that the LAN data does not include the Ethernet header and CRC check-sum as this will be stripped before forwarding to the satellite link.- Modem data rate: The data rate of all information bits which will be encoded by the forwarderror correction procedure. Besides the user data rate itself it also includes all necessaryheaders for SLL framing and gross container encapsulation. Additionally the signaling bitstransported in reference and request bursts are included. Note that the error correction bitsare not included in the modem data rate.Figure 2-37 Signal Preparation - Summary
