Comtech EF Data CDM-625 - Manual (Page 38)

CDM-625 Advanced Satellite Modem Revision 15
Introduction MN-CDM625
1–2
• It can be equipped with a DoubleTalk ® Carrier-in-Carrier ® option card that can save up to 50%
transponder bandwidth.
• It can be equipped with an optional IP Packet Processor card that, in addition to providing
Layer 3 functionality, incorporates a number of key features for Wide Area Network (WAN)
bandwidth optimization: very low overhead Streamline Encapsulation (SLE), Header and
Payload Compression, Advanced Quality of Service (QoS), and Advanced Encryption Standard
(AES) Encryption.
• IEEE-1588v2 Precision Timing Protocol (PTP) and Jumbo Frame Support are available options
(either requires the CDM-625 Rev 2 Hardware configuration).
• Carrier ID is a patent pending carrier identification (CID) technique that uses MetaCarrier®
spread spectrum technology to embed a unique carrier identification sequence for the
transmitted carrier.
1.2 Functional Description
The unit has two fundamentally different types of interface – IF and data:
• The IF interface provides a bidirectional link with the satellite via the uplink and downlink
equipment.
• The data interface is a bidirectional path that connects with the customer’s equipment
(assumed to be the DTE) and the modem (assumed to be the DCE).
Transmit data is received by the terrestrial interface where line receivers convert the clock and
data signals to CMOS levels for further processing. A small FIFO follows the terrestrial interface
to facilitate the various clocking and framing options. If framing is enabled, the transmit clock
and data output from the FIFO pass through the framer, where the overhead data (IDR, IBS, D&I
or EDMAC) is added to the main data; otherwise, the clock and data are passed directly to the
Forward Error Correction encoder.
In the FEC encoder, the data is differentially encoded, scrambled, and then convolutionally or
block encoded. Following the encoder, the data is fed to the transmit digital filters, which
perform spectral shaping on the data signals. The resultant I and Q signals are then fed to the
BPSK, QPSK/OQPSK, 8PSK, 8-QAM, or 16-QAM modulator.
The carrier is generated by a frequency synthesizer, and the I and Q signals directly modulate
this carrier. For L-Band applications, the directly modulated signal comprises the main output.
For IF applications (50–180 MHz), the L-Band signal is mixed down and filtered to produce the
desired output. The Rx-IF signal at L-Band is processed by a dual IF superheterodyne receiver.
For IF applications (50–180 MHz), the signal is first mixed up to the first IF frequency. The
second conversion is a complex mix, resulting in the signal once more being split into an in-
phase (I) and a quadrature (Q) component, producing an output at near-zero frequency.