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The networking functions (color coded red in the FRM, Figure 3-1) provide four key attributes as described in the following subsections.
3.4.1 Command and Control Network
Standards for this functional area are:
The primary function of the command and control (C2) network is to transport and distribute near-real-time commands to control on-board sensors and other functional components in the FRM. C2 network functions are very similar to the multimedia network, the key difference being data throughput. In fact, the C2 network functions can be implemented on the same physical LAN as the multimedia network. For many applications, the FDDI network referenced below is more than adequate to accommodate the C2 information flow.
3.4.2 High-Speed Data Flow Network
Standards for this functional area are:
The high-speed data flow network provides the transport and distribution functions for real-time exchange of raw/pre-processed digital sensor data between processing components. In order to preserve real-time synchronization of vital sensor data, the high-speed (500 to 1000 megabits per second) data flow network must have a low, deterministic end-to-end latency.
The SIGINT community has selected ANSI X3.230, Fibre Channel Physical and Signaling Interface (FC-PH), as the lowest risk technology for the high-speed data flow network. It offers the following technical characteristics which should be adequate to meet the functional needs of IMINT and MASINT sensors as well the SIGINT applications for which it was selected as a technology standard.
- TCP/IP (Transport Control Protocol / Internet Protocol)
- Real-time and arbitrated rings/loops
- UDP (User Datagram Protocol) over IP
Standards for this functional area are:
The multimedia network provides the transport and distribution functions for near-real-time distribution of processed sensor data and metadata tagged to the sensor data (e.g., information about the sensors and platform). It is similar to the high-speed data flow network, but the multimedia network can introduce small non-deterministic delays (latency) since it does not have to support true real-time communications requirements. However, it must be designed (data rate, number of nodes, media access protocol) such that the maximum latency through the network meets the near-real-time requirements for networking multimedia data - spectral data, digital audio, video, graphics, imagery, and MASINT sensor data.
The ARTA recommends FDDI or Fast Ethernet for the multimedia network. Although ATM data rates may be preferable over FDDI and Fast Ethernet, especially for IMINT and multispectral applications, the lack of maturity of ATM LAN technology precludes its selection at this time.
The following are key FDDI technical characteristics.
Fast Ethernet is similar to FDDI in performance. Two key differences are that Ethernet doesn't use dual counter-rotating ring topology, and uses carrier-sense multiple access with collision detection access protocol (CSMA/CD) rather than the token ring protocol used in FDDI.
Standards for this functional area are:
The data link provides for near-real-time communications between the airborne platform and ground/surface functions. Note that the FRM (Figure 3-1) is notional, and different airborne reconnaissance system implementations may place specific functions on different sides of the data link interface. For example, the SAR image formation function is allocated to the digital signal processing (DSP) block in the FRM, but actual equipment performing this function could be implemented in ground/surface system in some instances. Similarly, operator oriented database functions, reach back, and other functions shown to be ground/surface based in the FRM can actually be implemented on-board manned aircraft.
An OASD (C3I) Policy Letter, dated 18 October 1994, requires the Common Data Link (CDL) be used for all primary data links in airborne reconnaissance systems. CDL is a full duplex, jam resistant, point-to-point, microwave communications system developed by the Government for use in imagery and signals intelligence systems. It provides an interoperable, high bandwidth, digital data link for air-to-ground, air-to-surface, and air-to-satellite (relay) communications in airborne reconnaissance systems. The term "CDL" actually refers to a family of interoperable data links offering alternative levels of capabilities for different specific applications. In other words, the "generic" CDL can be scaled and configured in various ways to provide data link capabilities required for specific airborne reconnaissance mission requirements. For example, configuration options include choices of operating RF band (X or Ku), data rate (up to 274 megabits per second on the return link), and transmission power (offering design trades for size, weight, power, and range). Interoperability among the CDL family of datalinks is achieved by specifying the data link waveform (RF and digital), controlling and coordinating hardware configuration options, and managing pre-planned product improvement and technology insertion efforts to maintain backwards compatibility with fielded systems. For detailed technical information on CDL refer to the System Description Document for CDL, Specification #7681996, 5 May 1993, and the System Specification for the CDL Segment, Class 1, Specification #7681990, draft-C, 11 April 1996.
CDL provides "standardized" command link and return link services. The command link operates at a 200 kilobits per second (spread spectrum) data rate and provides services for transmitting data to the airborne platform (e.g., commands to the platform and/or sensor equipment, secure voice, ranging and navigation corrections, and commander's tactical terminal (CTT) link data). The return link operates at a either 10.7, 137, or 274 megabits per second data rates and provides services for transmitting data from the airborne platform (e.g., sensor data, platform navigation data, secure voice, etc.).
CDL communications links through satellite utilize different data rates than the other line-of-sight CDLs. The return link operates at 2xT-1(3.088 megabits per second), T-1 (1.544 megabits per second), T/2 (772 kilobits per second), and T/4 (386 kilobits per second) data rates and provides services for transmitting data from the remoted airborne platform (e.g., sensor data, platform navigation data, secure voice, etc.). The command link operates from 200 bits per second up to 6 kilobits per second over X band DSCS satellites and up to 64 kilobits per second data rate over commercial Ku band satellites and provides services for transmitting data to the remoted airborne platform (e.g., command and control of sensors, platform (UAVs), secure voice, CTT data link).
The standardized channel assignments for data multiplexing on the command link and return link are shown in Figure 3-5 and Figure 3-6 respectively. These assignments reflect current design practice, but a more flexible scheme is needed to accommodate future requirements (such as variable data rates from sensor outputs). In this regard, the CDL will migrate to a LAN interface standard (such as ATM or Fast Ethernet) which will provide a more robust, flexible, and transparent interface between airborne and ground/surface networks.
Legacy video systems currently use analog components; however, digital
video is preferred and is the desired goal. To provide a clear migration
path toward an all-digital system, any upgrades to existing systems should,
at a minimum, support dual-capable analog/259M equipment. CDL will be used
for the primary data link and real-time dissemination to warfighters can
be supported by either the industry-standard DSS-based JBS/GBS or NTSC analog
broadcast methods.
Figure 3-5: CDL Channelization Standard - Command Link
Figure 3-6: CDL Channelization Standard- Return Link
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