Unformatted text preview: OTOCOLS 5
5.1 FREQUENTLY ASKED QUESTIONS ON THE SPACE DATA LINK
HOW DO THE SPACE DATA LINK PROTOCOLS DIFFER FROM HDLC? One difference between the Space Data Link Protocols and HDLC is that the former assume
that a powerful error coding method is used to protect data from errors induced while the
physical signals are being transmitted across the space link, and the frame structure is
designed in such a way that synchronization and decoding of received data can be performed
in an efficient way; while the latter does not take such things in account. Therefore, care
must be taken if HDLC is to be used over a noisy space link so that its performance is not
degraded too much by the errors that occur on the space link.
Another difference is that the frame structures used by the Space Data Link Protocols are
upper-compatible with those of traditional (time division multiplexing) frame formats so that
the base-band portion of the conventional receiving systems can be used to support the Space
Data Link Protocols without any major modification.
5.2 ARE THE SPACE DATA LINK PROTOCOLS SECURE PROTOCOLS? No. Either the SDUs carried by these protocols or the entire Protocol Data Units generated
by these protocols may be encrypted, but how it should be performed is not specified by
these protocols. See reference  for how security may be implemented with these
5.3 IS THERE A STANDARD METHOD FOR MANAGING PARAMETERS SUCH
AS (TM AND AOS) TRANSFER FRAME LENGTHS? CCSDS is developing a standard method for managing such parameters as part of the Space
Link Extension Service Management Recommended Standard. Please check the CCSDS
web site for information on its development.
5.4 CAN COP-1 BE APPLIED TO DEEP SPACE COMMANDING? Yes. In deep space, however, a special technique must be used to transmit a sequence of
Transfer Frames reliably over a space link with a very long round trip time delay. If reliable
transmission of a sequence of Transfer Frames is required and an error that cannot be
corrected by the Synchronization and Channel Coding Sublayer occurs on the space link
during transmission, the erroneous Transfer Frame and all the subsequent Transfer Frames
must be retransmitted by the sender, but this takes at least one additional round-trip lighttime if the standard procedure provided by COP-1 is used.
The most commonly used technique for avoiding retransmission is to transmit the same
sequence of Transfer Frames multiple times using the Sequence-Controlled Service of CCSDS 130.2-G-1 Page 5-1 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS COP-1 (see 22.214.171.124). This can be done by adjusting the initial value of the timer used by the
sending end to determine whether retransmission is necessary (for details, see reference ).
This technique reduces the probability of retransmission by sacrificing the transmission
efficiency (because the same Transfer Frames are transmitted multiple times). It should be
noted that it cannot complete...
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- Spring '14
- ........., CCSDS