OM20 Universal Outdoor Modem Theory of OperationTM116 – Rev. 1.2 4-15External reference port input is specified at 0 to +6 dBm.4.7 RS422/V.35 Interface J3 (Standard option)Data must be clocked into the modem by either the SCTE or SCT Source. If SCTE is selected asthe Tx Clock Source, then SCTE must be supplied to the modem on the EIA-530 port. The outputof the dejitter buffer will be clocked with this source. SCT should be used if SCTE has excessivejitter.4.7.1 G.703 Interface (Optional)G7103 is an optional interface option. The G703 option supports both balanced and unbalancedinterface types. he modem support If the G.703 Interface is selected, then the Tx Clock Sourcewill default to SCTE and the Clock Polarity will default to Auto.Loop timing with a G.703 Interface or Asymmetrical Data Rates requires external equipment atthe remote end that is capable of using the recovered RD Clock as source timing for (SCTE) SD.The modem will not manipulate the clock frequency. Therefore, the transmit and receive clockrates must be equal in order for the modem to perform loop timing.4.7.2 Ethernet Data Interface J4 (Optional)The modem support a 4 port 10/100 Base-T or a 10/100/1000 Base T Interface. When thisinterface is selected additional menus will be displayed. Refer to Appendix J for interface set upand description of supporting features.When Ethernet Data Interface is selected, the Tx Clock Source will default to SCTE and the ClockPolarity will default to Normal. In addition, the Buffer Clock will default to RxSat and the BufferClock Polarity will default to Normal.4.8 Reed-Solomon CodecUtilizing a Reed-Solomon (R-S) Outer Codec concatenated with a Convolutional Inner Codec isan effective way to produce very low error rates even for poor signal-to-noise ratios whilerequiring only a small increase in transmission bandwidth. Typically, concatenating an R-SCodec requires an increase in transmission bandwidth of only 9 – 12% while producing a greaterthan 2 dB improvement in E b/N o. R-S is a block Codec where K data bytes are fed into theencoder which adds 2t = (N – K) check bytes to produce an N byte R-S block. The R-S decodercan then correct up to “t” erred bytes in the block. Refer to Figures 4-8, 4-9 and Table 4-14.8.1 Reed-Solomon Operation in the OM20When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section of the OM20where it is scrambled, formed into blocks, R-S encoded, and interleaved. Unique words areadded so that the blocks can be reformed in the Receiving Modem. Data is then sent to themodulator where it is convolutionally encoded, modulated, and transmitted to the satellite.
