# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 4 Aeronautical Mobile Telemetry
### Channel Coding in the 21 st Century
#### 4.2.4 Packet Telemetry Downlink  
Packet telemetry downlink is a provision in IRIG 106 that facilitates the inclusion of packet data, such as Ethernet packets, in continuous framed PCM transmissions, TmNS messages (see Section 4.2.3), or \"Chapter 11\" packets (see Chapter 11 of IRIG 106-23 [163]). When used, the data occupies one minor frame of the PCM stream. (This is a generalization of the simple PCM example illustrated in Figure 21). A preliminary version appeared in Chapter 7 of the 2015 version of IRIG 106-15. The current fully-developed version first appeared in Chapter 7 of the 2017 version of IRIG 106-17 [209].  
The standard defines three packet layers illustrated in Figure 27. The packet produced by its source is called a source packet (SP). These packets usually come with their own headers. Each SP is encapsulated in an encapsulation packet (EP). Finally, the EP is encapsulated in a fixed-length transport packet (TP). The illustration in Figure 27 is simplified for illustration. SPs are usually not the same length as a PCM minor frame. For example, when an SP is longer than a minor frame, it is partitioned into an integer number of EPs.  
The packet type and its length are identified in its header (the SP Header in Figure 27). The EP header also encodes the packet type and its length. The TP header contains sequencing information. Because errors in the SP, TP, and EP headers can lead to lengthy  
Figure 27: The three packet layers defined in the packet telemetry downlink standard in IRIG 106, Section 7[163].  
Figure 26: The structure of the iNET RF MAC frame and the RF burst. Adapted from Chapter 27 of IRIG 106-23 [163].  
data outages, critical information in these headers are protected by the (24,12) Golay code. The Golay code, in systematic form, is defined by the generator matrix  
\[\mathbf{G}=\begin{bmatrix}100000000000&1100&01110&101\\ 01000000000&0110&001110&111\\ 00100000000&11110&11010&1000\\ 00010000000&011110&110110&100\\ 00001000000&0011110&110110&1010\\ 00000100000&110110&1001110&1001\\ 00000001000&00110110&10111\\ 00000000100&00110&11110&00111\\ 00000000100&1010110&1110&00111\\ 00000000010&1010110&10010&1111\\ 000000000010&1010&1010&10111\\ 00000000001&10&1010&101111\\ 00000000001&10&10011110&1110\\ 000000000&10&001110&10110&1111\\ 000000000&10&001110&10110&1111\\ 000000000&10&001110&10110&1111\\ 00000000&001&100&011110&1110\\ 00000000&001&100&01110&10111\\ \end{bmatrix} \tag{16}\]  
The standard modifies the SP headers for TmNS messages and Chapter 11 packets. The modified Chapter 11 SP Header comprises 4 (24,12) Golay code codewords followed by 96 bits (12 bytes) of unprotected information. The modified TmNS message SP Header comprises 8 (24,12) Golay code codewords plus a variable number of unprotected bits. The standard does not modify the SP header for Ethernet packets. The EP Header comprises 2 (24,12) Golay code codewords and no unprotected bits. The TP comprises one (24,12) Golay code codeword plus 8 bits (1 byte) of unprotected data. The (24,12) Golay code was chosen because encoding and decoding algorithms were readily available, simple to implement, and the right size for protecting critical header information [210,211].  
Additional coding to protect the payload portion of the PT data frames was considered when the standard was being written. The length-255 Reed-Solomon codes defined in CCSDS 131.0.B-2 [50] was of special interest [210,211]. Additional coding was not adopted in the standard due to the encryption issue described in Section 2. When the IRIG 10 LDPC codes described in Section 4.2.2 became commonly available, the need for additional coding in SOQPSK-TG systems diminished [211]. However, for PCM/FM systems (where LDPC codes are currently not available), at least one manufacturer applies length-255 CCSDS Reed-Solomon codes to the PTFR payload data [211,212].