Comtech EF Data CDM-625 Advanced Satellite Modem

  • DoubleTalk Carrier-in-Carrier
  • 70/140 MHz and L-Band capabilities
  • Data Rates to 25 Mbps
  • Extensive backwards compatibility
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Comtech EF Data CDM-625 Advanced Satellite Modem

The Comtech EF Data CDM-625 Advanced Satellite Modem builds on Comtech EF Data’s legacy of providing the most efficient satellite modems. It is the first modem to combine advanced Forward Error Correction (FEC) such as VersaFEC® and Low Density Parity Check (LDPC) codes with the revolutionary DoubleTalk® Carrier-in- Carrier® bandwidth compression, allowing for maximum savings under all conditions. This combination of advanced technologies enables multi-dimensional optimization, allowing satellite communications users to:

  • Minimize operating expenses (OPEX)
  • Maximize throughput without using additional transponder resources
  • Maximize availability (margin) without using additional transponder resources
  • Minimize capital expenses (CAPEX) by allowing a smaller BUC/HPA and/or antenna
  • Or, a combination to meet specific business needs

Comtech EF Data CDM-625 Features

  • DoubleTalk Carrier-in-Carrier bandwidth compression
  • Adaptive Coding and Modulation (ACM)
  • IP Packet Processor with header compression, payload compression and advanced Quality of Service (QoS)
  • Dual Band Capability: 70/140 MHz and L-Band in same unit
  • Data Rate: 18 kbps to 25 Mbps
  • Symbol Rate: 18 ksps to 12.5 Msps
  • Modulation: BPSK, QPSK/OQPSK, 8-PSK/8- QAM, 16-QAM
  • FEC: Viterbi, Sequential, Concatenated Reed Solomon, TCM, Turbo Product Code (TPC) (IESS-315 Compliant), LDPC Code and VersaFEC (low-latency LDPC)
  • Widest Range of data interfaces: EIA-422/530, V.35, G.703 T1, G.703 E1, G.703 T2, G.703 E2, Quad G.703 E1, ASI, LVDS, HSSI, 4-port 10/100Base-T Ethernet
  • 4-port managed Ethernet switch with VLAN and QoS
  • Sub Mux to multiplex IP/Ethernet traffic with serial or G.703 traffic
  • Drop & insert for T1/
  • Enhanced D&I++ for single T1/E1 & quad E1
  • Management: 10/100Base-T Ethernet with SNMP, Distant End SNMP Proxy, HTTP ,Telnet and EIA-232/EIA-485
  • Embedded Distant-end Monitor and Control (EDMAC)
  • Automatic Uplink Power Control (AUPC)
  • Engineering Service Channel (ESC/ESC++)
  • Standard high -stability internal reference (± 6 x 10-8)
  • 5-tap Adaptive Equalizer
  • L-Band TX: 10 MHz reference for BUC, FSK communications and optional BUC power supply
  • L-Band RX: 10 MHz reference and LNB power supply
  • Open network modes
  • CDM-600/L emulation mode
  • 1:1 and 1:10 redundancy switches available
  • Backwards compatible with CDM-500/CDM-550, CDM-550T, CDM-570/L and CDM-600/L Satellite Modems
  • Interoperable with many Comtech EF Data Satellite Modems: CDM-Qx/L, SDM-8000, 300A, and 300L3

The advanced technologies and features of CDM-625
are covered by a number of U.S. patents including
7,254,188, 7,353,444, 7,415,659 and other pending patents.

Doubletalk Carrier-in-Carrier

DoubleTalk Carrier-in-Carrier, based on patented “Adaptive Cancellation” technology, allows transmit and receive carriers of a duplex link to share the same transponder space.

Figure 1 shows the typical full duplex satellite link, where the two carriers are adjacent to each other. Figure 2 shows the typical DoubleTalk Carrier-in-Carrier operation, where the two carriers are overlapping, thus sharing the same spectrum.

DoubleTalk Carrier-in-Carrier is complementary to all advances in modem technology, including advanced FEC and modulation techniques. As these technologies approach theoretical limits of power and bandwidth efficiency, DoubleTalk Carrier-in-Carrier utilizing advanced signal processing techniques provides a new dimension in bandwidth and power efficiency.

When observed on a spectrum analyzer, only the Composite is visible. Carrier 1 and Carrier 2 are shown in Figure 2 for reference only.

DoubleTalk Carrier-in-Carrier allows satellite users to achieve spectral efficiencies (i.e. bps/Hz) that cannot be achieved with traditional links. E.g., DoubleTalk Carrier-in-Carrier when used with 16-QAM approaches the bandwidth efficiency of 256-QAM (8bps/Hz).

As DoubleTalk Carrier-in-Carrier allows equivalent spectral efficiency using a lower order Modulation and/or FEC Code, it can simultaneously reduce CAPEX by allowing a smaller BUC/HPA and/or antenna.

When combined with VersaFEC or LDPC/TPC, it can provide unprecedented savings in transponder bandwidth and power utilization. This allows for its successful deployment in bandwidth-limited and power-limited scenarios, as well as reduction in Earth Station BUC/HPA power requirements.


CDM-625 is the first modem to offer VersaFEC, a patented (covered by U.S. patents 7,353,444 and 7,415,659; other patents pending) system of high performance short-block lowlatency LDPC codes designed to support latency-sensitive applications, such as cellular backhaul over satellite. VersaFEC provides excellent coding gain with lowest possible latency. VersaFEC’s Eb/No performance is similar to that of DVB-S2 (short block) or LDPC (16k block) with 70-90% lower latency. Compared to TPC, VersaFEC can provide coding gain of 1.0 dB or more.


Satellite users have traditionally relied on worst case link margin to overcome rain fade which leads to significant inefficiencies. ACM converts the fade margin into increased throughput – gain of 100% or more is possible. The CDM-625 with VersaFEC was specifically architected to support ACM for IP/Ethernet traffic. ACM maximizes throughput under all conditions – rain fade, inclined orbit satellite operation, antenna mis-pointing, noise, interference and other impairments.

VersaFEC ACM can provide almost 85% reduction in latency compared to DVB-S2 (short block).

ACM can also be used with DoubleTalk Carrier-in-Carrier.


CDM-625 offers an integrated LDPC and 2nd Generation TPC codec. LDPC is an advanced Forward Error Correction technique capable of providing performance much closer to Shannon limit. The current LDPC implementation can provide 0.7 to 1.2 dB additional coding gain compared to an equivalent TPC code.

In order to take full advantage of the increased coding gain provided by LDPC, Comtech EF Data has developed a patented 8-QAM modulation (U.S. patent 7,254,188) that allows for acquisition and tracking at much lower Eb/No compared to 8- PSK.


CDM-625 supports 70/140 MHz and L-Band capability in the same unit with independently selectable transmit and receive IF. This simplifies sparing and stocking in networks requiring 70/140 MHz and L-Band units.


CDM-625 base modem incorporates a 4-port 10/100Base-T managed Ethernet switch with VLAN capability and prioritybased Quality of Service (QoS). Access (Native) Mode and Trunk Mode are supported. Traffic can be prioritized using portbased priority or VLAN priority. The maximum Ethernet frame size is 1536 bytes.


The IP Packet Processor enables efficient IP networking and transport over satellite by adding routing capability with very low overhead encapsulation, header compression, payload compression and Quality of Service to the CDM-625. The advanced QoS combined with header and payload compression ensures the highest quality of service with minimal jitter and latency for real-time traffic, priority treatment of mission critical applications and maximum bandwidth efficiency.

Header Compression
The IP Packet Processor incorporates industry-leading header compression for IP traffic. Header compression can reduce the 40 byte IP/UDP/RTP header to as little as 1 byte. For TCP/IP, the 40 byte header is reduced to as little as 3 bytes. For applications such as VoIP, header compression can provide bandwidth savings exceeding 60%. E.g. the 8 kbps G.729 voice codec requires 24 kbps of IP

bandwidth once encapsulated into an IP/UDP/RTP datagram. With header compression, the same voice call needs about 8.5 kbps – a saving of almost 65%. And, bandwidth requirements for typical Web/HTTP traffic can be reduced by 10% or more with TCP/IP header compression.

Payload Compression
The IP Packet Processor incorporates industry-leading payload compression for IP traffic. Implemented in the hardware for maximum throughput and efficiency, payload compression can reduce the required satellite bandwidth by as much as 40-50%.

Streamline Encapsulation (SLE)
The IP Packet Processor incorporates Comtech EF Data’s patent-pending very low overhead Streamline Encapsulation (SLE). SLE can reduce the encapsulation overhead by as much as 65% compared to industry standard HDLC.

Advanced Quality of Service (QoS)
The IP Packet Processor incorporates multi-level QoS to ensure the highest quality service with minimal jitter and latency for real-time traffic, priority treatment of mission critical applications and maximum bandwidth efficiency.

Supported modes are:

  • DiffServ – Industry-standard method of providing QoS enabling seamless co-existence in networks that implement DiffServ
  • Max/Priority – Provides multi-level traffic prioritization with the ability to limit maximum traffic per priority class
  • Min/Max – Provides a Committed Information Rate (CIR) to each user defined class of traffic with the ability to allow a higher burstable rate depending on availability


The CDM-625 supports a Quad E1 interface that can aggregate up to four full or fractional E1s into a single carrier, with very low overhead. This provides significant CAPEX savings by reducing the number of modems and could possibly reduce the BUC/HPA size by eliminating the multi-carrier backoff. A proprietary, closed network Drop & Insert (D&I++) allows for Dropping or Inserting any combination of 1 to 31 time slots on each E1. D&I++ is supported for E1-CCS only.

For QDI operation, all E1s must have a common clock source.


The IP Sub Mux allows multiplexing IP/Ethernet traffic with serial or G.703 traffic into a single carrier. This is particularly useful for cellular backhaul when both E1 and IP backhaul is required. This reduces the number of modems and could possibly reduce the BUC/HPA size by eliminating the multicarrier backoff.


The CDM-625 supports EDMAC, EDMAC-2, EDMAC-3 and AUPC. EDMAC/EDMAC-2/EDMAC-3 can be used to monitor and control the distant end of a satellite link using a proprietary overhead channel. EDMAC-3 is also used for SNMP management of the distant end modem. AUPC enables automatic uplink power control for a duplex link.


The modem can be managed via the front panel, the remote M&C port (EIA-232/EIA-485), or the 10/100Base-T Ethernet port. With support for SNMP, HTTP and Telnet, the modem can be easily integrated into an IP-based management system.

The CDM-625 can also act as SNMP proxy for the distant end modem. This allows distant end modem management using SNMP without requiring an end-to-end IP link.


CDM-625 can be placed in CDM-600 or CDM-600L emulation mode. This permits easy integration into an existing CDM-600/L setup without changes to M&C platform or redundancy switches.


Enhancing the capability of the CDM-625 in the field is easy. Features that do not require additional hardware can be added on site, using FAST access codes purchased from Comtech EF Data.

More Information
Weight 10.8000
Country of Manufacture United States
Manufacturer Comtech EF Data
Input Power 100 - 240 VAC
Max Data Rate 18 kbps to 25 Mbps
Max Symbol Rate 18 ksps to 12.5 Msps
Modulation BPSK, QPSK, OQPSK, 8QAM, 16QAM
FEC VersaFEC, Reed-Solomon, Sequential, LDPC
Output Frequency 50 - 180 MHz, 950 - 2000 MHz
Warranty No
Dimensions 1.75” x 19.0” x 17.65” (4.4 x 48 x 44.8 cm)
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101 Eagle Road Building 7. Avon, Colorado. 81620.
T : (970)-748-4250.