Comtech CDM-625A Advanced Satellite Modem

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Comtech EF Data CDM-625A Advanced Satellite Modem

The Comtech EF Data CDM-625A Advanced Satellite Modem builds on a legacy of providing the most efficient satellite modems. With support for VersaFEC® Forward Error Correction (FEC), the revolutionary DoubleTalk® Carrier-in-Carrier® bandwidth compression, additional rolloffs and advanced packet processing, the Comtech EF Data CDM-625A Advanced Satellite Modem provides significant 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


  • Mobile Network Operators
  • Telecom Operators
  • Satellite Service Providers
  • Government & Military
  • Enterprise
  • Offshore


  • Mobile Backhaul
  • G.703 Trunking
  • IP Trunking
  • Offshore & Maritime Communications
  • Enterprise
  • Communications on-the-Move
  • Satellite News Gathering

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  • DoubleTalk Carrier-in-Carrier bandwidth compression
  • Carrier-in-Carrier Automatic Power Control
  • VersaFEC FEC with Adaptive Coding and Modulation (ACM)
  • 5%, 10%, 15%, 20%, 25% and 35% Filter Rolloff
  • Packet Processor with header compression, payload compression, advanced Quality of Service (QoS) and Managed Switch Mode with VLAN support
  • Integrated 4-port managed Ethernet switch with VLAN and QoS
  • Jumbo frame support
  • Dual Band Capability: 70/140 MHz and extended L-Band (950 – 2250 MHz) in same unit
  • Data Rate: 18 kbps to 25 Mbps
  • Symbol Rate: 18 ksps to 12.5 Msps
  • Modulation: BPSK, QPSK/OQPSK, 8PSK/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
  • IEEE 1588v2 Precision Time Protocol
  • Sub Mux to multiplex IP/Ethernet traffic with serial or G.703 traffic
  • Drop & insert for T1/E1
  • 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
  • Carrier ID using MetaCarrier® Technology
  • 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
  • L-Band: Advanced FSK for LPOD M&C
  • 1:1 and 1:10 redundancy switches available
  • Backwards compatible with CDM-625


DoubleTalk Carrier-in-Carrier, based on patented “Adaptive Cancellation” technology, allows transmit and receive carriers of a duplex link to share the same transponder bandwidth. 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 efficiencies, DoubleTalk Carrier-in-Carrier utilizing advanced signal processing techniques provides a new dimension in bandwidth 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. As DoubleTalk Carrier-in-Carrier allows equivalent spectral efficiency using a lower order modulation and/or code rate, it can reduce the power required to close the link thereby reducing CAPEX by allowing a smaller BUC/HPA and/or antenna. Alternatively, DoubleTalk Carrier-in-Carrier can be used to achieve very high spectral efficiencies E.g., DoubleTalk Carrier-in-Carrier when used with 16-QAM approaches the bandwidth efficiency of 256-QAM (8 bps/Hz).

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.

Carrier-in-Carrier Automatic Power Control (CnC-APC)

The patent-pending Carrier-in-Carrier Automatic Power Control (CnC-APC) mechanism enables modems on both sides of a CnC link to automatically measure and compensate for rain fade while maintaining the Total Composite Power. In addition to automatically compensating for rain fade, CnC-APC also enables the modems to share link margin, i.e. a modem in clear sky conditions can effectively transfer excess link margin to a distant end modem experiencing fade, thereby further enhancing overall availability.

VersaFEC Forward Error Correction

CDM-625A offers VersaFEC, a patent-pending system of high-performance LDPC codes designed to provide maximum coding gain while minimizing latency. VersaFEC is designed to support ACM and CCM mode of operation.

The Ultra Low Latency (ULL) codes provide even lower latency compared to standard VersaFEC codes.

Adaptive Coding & Modulation (ACM)

Satellite users have traditionally relied on worst case link margin to overcome rain fade which leads to significant inefficiencies. ACM can provide significant increase in throughput as well as availability. ACM converts the fade margin into increased capacity making it possible to more than double the throughput for Ku-Band operation. Even under deep fade, ACM may be able to maintain the link at the lower MODCOD thereby increasing availability. It is tightly integrated with packet processor QoS which allows higher priority, mission critical traffic to be maintained even during fade.

Low Density Parity Check Codes (LDPC) & Turbo Product Codes (TPC)

CDM-625A 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 that allows for acquisition and tracking at much lower Eb/No compared to 8PSK.

Dual Band Capability

CDM-625A supports 70/140 MHz and extended L-Band (950 – 2250 MHz) 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.

4-Port Managed Ethernet Switch with VLAN & QoS

CDM-625A incorporates a 4-port 10/100Base-T managed Ethernet switch with VLAN capability and priority-based Quality of Service. Access (Native) Mode and Trunk Mode are supported. Traffic can be prioritized using port-based priority or VLAN priority. The modem supports jumbo frames with maximum Ethernet frame size of 2048 bytes.

Packet Processor

The optional high-performance Packet Processor enables efficient IP networking and transport over satellite with low overhead encapsulation, header compression, payload compression and advance Quality of Service to the CDM-625A. The 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.

Packet processor supports Routed mode as well as Managed Switch Mode of operation. In managed switch mode, it operates as a layer 2 switch with VLAN support, enabling seamless integration with existing infrastructure while providing full optimization including low overhead Streamline Encapsulation, header compression and payload compression and advanced QoS.

Header Compression

The Packet Processor incorporates industry-leading header compression for Ethernet and IP traffic. In managed switch mode, header compression can reduce the 54 byte Ethernet/IP/UDP/RTP header to as little as 1 byte. For TCP/IP, the 54 byte header (including Ethernet) is reduced to as little as 3 bytes. For applications such as VoIP, header compression can provide bandwidth savings exceeding 65%. E.g. the 8 kbps G.729 voice codec requires 31.2 kbps once encapsulated into an Ethernet frame with IP/UDP/RTP. With header compression, the same voice call needs about 9 kbps – a saving of almost 70%. And, bandwidth requirements for typical Web/HTTP traffic can be reduced by 10% or more with TCP/IP header compression.

Payload Compression

The Packet Processor incorporates industry-leading GZIP based payload compression for IP/Ethernet traffic. Implemented in hardware for maximum throughput and efficiency, payload compression can typically reduce the required satellite bandwidth by 30-40%.

Streamline Encapsulation (SLE)

The Packet Processor incorporates Comtech EF Data’s patent-pending 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 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:

  • Max/Priority – Provides 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

A powerful classifier supports packet classification by Protocol, VLAN ID / range, ToS Byte, Source IP (or subnet), Destination IP (or subnet), Source Port (or Range), Destination Port (or Range) and DSCP (for DiffServ).


The CDM-625A 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.


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 multi-carrier backoff. The IP sub mux ratio ranges from 9:1 (IP data rate is 9 times that of the serial or G.703 data rate) to as low as 1:59. IP sub mux can also be used to provision an overhead IP channel for management when using non IP/Ethernet traffic interfaces.


The CDM-625A 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 automatically adjusts modem transmit power based on feedback from the distant end modem to maintain the desired Eb/No. AUPC and EDMAC are supported for point-to-point duplex links.


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


PTP has emerged as the key technology for frequency, time and phase synchronization over a packet network. The CDM-625A incorporates hardware support for PTP, thereby significantly improving synchronization accuracy for satellite backhaul.


The Advanced FSK allows for monitoring and control of LPOD through modem front panel menus, serial remote control and Telnet.


Enhancing the capability of the CDM-625A 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.

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Comtech CDM 625A Advanced Satellite Modem Datasheet

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