Editing Interplanetary Internet (section)

==Protocol==
The Consultative Committee for Space Data Systems ([[CCSDS]]) packet [[telemetry]] standard defines the protocol used for the transmission of spacecraft instrument data over the deep-space channel. Under this standard, an image or other data sent from a spacecraft instrument is transmitted using one or more packets.

=== CCSDS packet definition ===
A packet is a block of data with length that can vary between successive packets, ranging from 7
to 65,542 bytes, including the packet header.
* Packetized data is transmitted via frames, which are fixed-length data blocks. The size of a frame, including frame header and control information, can range up to 2048 bytes.
* Packet sizes are fixed during the development phase.
Because packet lengths are variable but frame lengths are fixed, packet boundaries usually do not coincide with frame boundaries.

=== Telecom processing notes ===
Data in a frame is typically protected from channel errors by error-correcting codes.
* Even when the channel errors exceed the correction capability of the error-correcting code, the presence of errors is nearly always detected by the error-correcting code or by a separate error-detecting code.
* Frames for which uncorrectable errors are detected are marked as undecodable and typically are deleted.

=== Handling data loss ===
Deleted undecodable whole frames are the principal type of data loss that affects compressed data sets. In general, there would be little to gain from attempting to use compressed data from a frame marked as undecodable.
* When errors are present in a frame, the bits of the subband pixels are already decoded before the first bit error will remain intact, but all subsequent decoded bits in the segment usually will be completely corrupted; a single bit error is often just as disruptive as many bit errors.
* Furthermore, compressed data usually are protected by powerful, long-blocklength error-correcting codes, which are the types of codes most likely to yield substantial fractions of bit errors throughout those frames that are undecodable.

Thus, frames with detected errors would be essentially unusable even if they were not deleted by the frame processor.

This data loss can be compensated for with the following mechanisms.
* If an erroneous frame escapes detection, the decompressor will blindly use the frame data as if they were reliable, whereas in the case of detected erroneous frames, the decompressor can base its reconstruction on incomplete, but not misleading, data.
* However, it is extremely rare for an erroneous frame to go undetected.
* For frames coded by the CCSDS [[Reed–Solomon error correction|Reed–Solomon code]], fewer than 1 in 40,000 erroneous frames can escape detection.
* All frames not employing the Reed–Solomon code use a [[cyclic redundancy check]] (CRC) error-detecting code, which has an undetected frame-error rate of less than 1 in 32,000.