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3.4 TYPICAL EXAMPLE Figure 3-4 shows an example of how the TM Space Data Link Protocol provides users with
services for transferring various type of data over a space link.
In this example, there are three sending entities onboard a spacecraft and three receiving
entities on the ground, each receiving entity corresponding to one of the sending entities. (1)
An onboard entity of the Space Packet Protocol sends urgent and non-urgent Packets to a
ground entity of the same protocol over the space link. Onboard the spacecraft and on the
ground, there may be other entities of the Space Packet Protocol that are not shown in the
figure, but they are connected via onboard and ground links (or sub-networks), respectively,
and how they communicate within these links (or sub-networks) is outside the scope of this
document. (2) An onboard process for calibrating the onboard clock needs to send the current
value of the onboard clock to the corresponding process on the ground so that the ground
process can calibrate the onboard clock against the ground clock. (3) A stream of encrypted
data whose structure is unknown to the TM-SDLP needs to be sent over the space link. CCSDS 130.2-G-1 Page 3-11 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS Sending Users Onboard a Spacecraft Receiving Users On the Ground Space
Transfer VC Access
Service VC Access
Service TM Space Data
Link Protocol Clock
Service VC 2
VC 0 Figure 3-4: Example of How TM-SDLP Provides Services
The sequence of urgent and non-urgent Packets are transferred with two instances of the
Packet Service, each on a different Virtual Channel (urgent Packets on VC 0 and non-urgent
Packets on VC 1 in this case). It will be explained in 4.2 how the traffic on these two Virtual
Channels is handled and how their quality of service is controlled.
Since the clock calibration data is short and of a fixed length, it is transferred with the
VC_OCF Service, one of the services for transferring a sequence of short, fixed-length
SDUs. In this example, VC 0 is chosen for transferring clock calibration data.
The mechanism used in this example for transferring a stream of encrypted data, multiplexed
with the other data, is to use the VCA Service on VC 2, which is dedicated to the transfer of
encrypted data. The sending user generates a sequence of chunks of fixed length from the
stream of encrypted data and supplies them to the service provider as SDUs, which are
delivered to the receiving user for decryption. CCSDS 130.2-G-1 Page 3-12 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS 4 WHAT DO THE SPACE DATA LINK PROTOCOLS PERFORM? FROM DEVELOPERS’ PERSPECTIVE 4.1
4.1.1 TRANSFER FRAMES
GENERAL The Protocol Data Units exchanged between entities of the Space Data Link Protocols are
called Transfer Frames. Tr...
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This document was uploaded on 03/06/2014.
- Spring '14