Relationship to Other Protocols

[FAQ's top]

Q: Can SCPS be used with the CCSDS link layer protocols? Specifically, packet telemetry downlink and CCSDS telecommand uplink?

Yes. The SCPS were designed to operate over CCSDS space-ground links,
 although use of other link layer protocols is possible. The figure below shows just one example of a protocol profile in which SCPS operates over CCSDS telemetry and telecommand frames. In this example, on-board applications can both send and receive CCSDS Packets. This data can be transferred as streams of packets (via SCPS-TP) or as files of packets (via SCPS-FP). End-to-end data protection is provided by SCPS-SP, and network addressing is provided by SCPS-NP. The use of an expected enhancement to telemetry multiplexing is assumed; this enhancement would provide for multiplexing of SCPS-NP or other network PDUs (e.g., internet IP packets) into the data zone of CCSDS telemetry frames (VCDUs).

img_55810aa9 

One Example of SCPS supported by CCSDS Link Protocols

Q: How are the SCPS and CCSDS protocols related to internet and OSI protocols?

In the figure below, OSI and the internet protocols FTP, TCP, and IP are shown along with CCSDS and SCPS. Within the CCSDS/SCPS family this figure shows some profile options not shown in the previous figure. For example, the option of running SCPS over the AOS path (packet) service via a convergence layer is shown.

scpstcpip-a
SCPS, TCP/IP, and CCSDS Protocol Profiles

Q: Which typical mission data operations tasks are performed by the SCPS and earlier CCSDS space link  protocols ?

The table below shows a number of tasks performed in most space missions, and lists the SCPS and other CCSDS protocols that may be used to support the task. Use of CCSDS link layer services is assumed in all cases.

Task

CCSDS

SCPS

Real-time data stream

PT-SP, AOS-Path

SCPS-TP
(UDP option)

Real-time in-sequence data stream

PT-SP, AOS-Path

SCPS-TP
(best-effort option; no re-xmit)

Near-real-time, nearly  complete data

M*

SCPS-TP
(best-effort option)

Reliable transfer of telemetry

M*

SCPS-TP
(complete option)

Guaranteed delivery of event-driven messages

M*

SCPS-TP

Uploading software files to subsystems/payloads

M*

SCPS-FP

Uploading software patches to subsystems/payloads (record operations)

M*

SCPS-FP

Uploading data files and memory contents to subsystems and payloads

M*

SCPS-FP

Downloading data files and memory contents from subsystems and

M*

SCPS-FP

Transmission of commands, or sequences of commands, from the ground user to the                   
 subsystems/payloads

Telecommand

SCPS-TP

Initiation, control, and monitoring of stored command programs

Telecommand

SCPS-TP

Interactive access from an onboard terminal to a ground data system or from a ground terminal to an onboard data system

  

SCPS-TP

Data base access and file transfer from ground to crew workstations

  

SCPS-FP

Secure end-to-end data transfer

  

SCPS-SP

*M => Accomplished through manual operations and/or mission-specific software.
PT-SP => Packet Telemetry - Source Packet Service  
AOS -Path => CCSDS Advance Orbiting Systems - Path Service

Q: Won't the Reed-Solomon and other FEC capabilities of the CCSDS link provide nearly perfect delivery without the overhead of transport and file transfer protocols?

There are several parts to this answer.

a)  Not all space links use such powerful error correction.

b)  Even with Reed-Solomon error correction, performance is better described as "usually perfect, but with occasional gaps"-almost all data is delivered, and the delivered data is error-free, but occasionally whole messages are lost because a link  layer frame is not decodable, due to weather effects or operational problems.

c)  Many user data traverse multiple sub-networks-not just the space link. Data can be lost during transit over LANs in end systems or in gateways due to buffer overflow. Trying to solve a high-level problem-Did all of it get to the final destination correctly?-at lower levels is futile1. No amount of confirmation, correction, or protection on a link-by-link or subnet-by-subnet level will do the job. On the other hand, it is not possible to communicate effectively without some reliability measures at lower layers, given the link characteristics and intermittent connectivity in space operations. A balance of upper-layer, confirmed, end-to-end services supported by links good enough to avoid excessive retransmission is the optimal solution.

[FAQ's top]
[Home] [Documents] [Reference S/W] [Contact Points] [FAQs]