Encryption has been evolving over the last few years. For a long time, the basic interoperable scrambling system (BISS) was the standard used within the broadcast industry. The National Institute of Standards and Technology (NIST) determined that BISS was not as strong as it could be and superseded the standard with the Advanced Encryption Standard (AES) as described by FIPS-192. FIPS-192 described how the core encryption is performed but lacked how it is applied to digital video. This led manufacturers to develop proprietary implementations that were generally referred to as AES. To address interoperability problems, an organization within the Department of Justice (DOJ) developed a standard describing how to apply the core AES algorithm to digital video. This standard is referred to as BCRYPT. BCRYPT is the only published standard applying AES to digital video applications. To maximize the utilization of your downlink, the transmitter needs to conform to BCRYPT.
BCRYPT encryption supports AES 128 and AES 256 at its core. The 128 and 256 describes the length of the programming key. In practice, many transmitters have the capability of preprogramming a set of five “keys” for different applications. To support interoperability these “keys” would need to be loaded into the downlink transmitter and shared with the multitude of receivers.
A digital standard definition video signal requires 270 mbps, while a high definition signal requires 1544 mbps. A robust digital downlink transmitter can only transport between 4 and 16 mbps. This is a fraction of what is needed to transport uncompressed video. Therefore, the bit rate of the video feed must be reduced, which is a function of a video encoder or compression system. Video carries a lot of redundant information; if these redundancies can be removed and processed, the minimum required bit rate can be reduced drastically.
Over time, the algorithms have improved and the standards have evolved. MPEG-2 was developed in 1994 and has been adopted for broadcast TV. It has largely focused on standard definition applications. Using MPEG-2, an SD signal can be compressed to around 6 mpbs from 270 mbps, and an HD signal to 18 mpbs from 1544 mbps. As the Internet evolved, the demands for a more efficient scheme also increased. They were met with MPEG-4 part 10 encoding, which is also referred to as advanced video coding (AVC) and H.264. All three references are synonymous with one another. Using MPEG-4, the same SD signal can be compressed to 3 mpbs and the HD signal to less than 9 mpbs.
There are many ground systems available with various capabilities. To support these systems it is recommended that the downlink transmitters support both MPEG-2 and MPEG-4.
Auxiliary or Ancillary Data
To support tactical operations, an aerial platform is usually placed within a few miles of the ground mobile receivers. These require small omni-directional antennas. However, these operations are often at great distances from the tower-based receive sites used to support strategic needs. Many of the legacy receive sites still utilize tracking antennas requiring GPS location of the aerial platform. This data is passed over the downlink transmitter via a general-purpose RS232 link. Due to the lack of a unified standard, each manufacturer devised its own proprietary method for moving this data. When interoperability was needed, the audio channel was used to transport the RS232 data. To solve this interoperability dilemma, the DOJ issued a standard outlining Ancillary Data Encapsulation in MPEG Transport Streams.
Success in Interoperability
The industry has recognized the incompatibility issues surrounding the airborne downlink systems and has agreed that new downlink transmitters should, at a minimum, support the following operating modes:
• The 6.5-GHz frequency plan outlined in Table 1.
• DVB-T 8-MHz COFDM as a standard modulation.
• BCRYPT AES 128/256.
• MPEG-2 as a minimum requirement.
o Support for MPEG-4 is recommended.
• DOJ implementation of Ancillary Data Encapsulation in MPEG Transport Streams.
This should be the model for all the new airborne downlink systems and receive equipment purchases. Adhering to this minimum will guarantee interoperability and long-term protection of your capital investment.
In mainland USA XM / Sirius uses COFDM towers in urban areas to overcome loss of satellite links due to high rise buildings.
But the quality of COFDM goes a little beyond good video quality. It also affords better penetration into buildings. Though Sirius / XM is basically an audio service, the digital stream allows video streaming to back seat passengers or hotels / motels can use it to distribute video within their facility.