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Packet Telecommand Features

This page describes some of the packet telecommand features supported by the Telecommand Encoder Shell and Telecommand Decoder Shell. For the definition of the features described here, see the standards and recommendations documents.

NOTE: In common with older issues of the standards and recommendations, we use the term Packet Telecommand. It refers to the protocols for the transport of data units, such as packets, in TC Transfer Frames. However, the protocols are not limited to packets and recent issues of the standards no longer use the term.

Telecommand Services

The CCSDS Recommendations for the TC Space Data Link Protocol define seven telecommand services.

MAP Packet (MAPP), where the Service Data Unit is a packet
MAP Access (MAPA), where the Service Data Unit is a MAP_SDU
Virtual Channel Packet (VCP), where the Service Data Unit is a packet
Virtual Channel Access (VCA), where the Service Data Unit is a VCA_SDU
Virtual Channel Frame (VCF), where the Service Data Unit is a transfer frame
Master Channel Frame (MCF), where the Service Data Unit is a transfer frame
COP Management (COPM), used for passing directives to COP-1

The Telecommand Encoder Shell and Telecommand Decoder Shell support any valid combination of use of the services.

Packets

The Telecommand Encoder Shell and Telecommand Decoder Shell handle the following types of packets on the MAP Packet (MAPP) and Virtual Channel Packet (VCP) services:

- CCSDS Space Packets (also known as Telecommand Packets or Source Packets)
- CCSDS SCPS NP Datagrams
- IPv4 Datagrams
- CCSDS Encapsulation Packets

MAP-SDU

The MAPA service handles MAP_SDUs, which are privately formatted service data units of variable length. The MAPA service can therefore be used for many applications which have a non-CCSDS packet format.

VCA-SDU

The VCA service handles VCA_SDUs. A VCA_SDU is a service data unit to be placed in the Transfer Frame Data Field of a transfer frame.

Type-A service and Type-B service

The CCSDS Recommendations define two service types for handling telecommand data: the Type-A service and the Type-B service.

The Type-A service uses the COP-1 "go-back-n" automatic retransmission protocol and provides delivery of transfer frames in-sequence, without gaps, and without duplication. This is sometimes referred to as the Type-A service guarantee.
The Type-B service provides no automatic retransmission, so there is no guarantee that sequences of transfer frames will be delivered without gaps.

The service types have a number of alternative names:

The Type-A service is also known as the Sequenced Controlled Service, the A Service and the AD Service.
The Type-B service is also known as the Expedited Service, the B Service and the BD Service.

The Telecommand Encoder Shell and Telecommand Decoder Shell support any combination of use of the two service types. The service type is a parameter for each individual packet or service data unit submitted to the Telecommand Encoder Shell for the MAPP, MAPA, VCP or VCA service.

Directives

The COP Management service is used for passing directives to COP-1 for managing the Type-A service. There are directives to initiate and terminate the Type-A service. Other directives set parameters such as timers and transmission limits.

The Telecommand Encoder Shell supports all the directives defined for the COP Management service.

Virtual Channels (VCs)

Multiple Virtual Channels (VCs) can share the capacity of a single Physical Channel. The Telecommand Encoder Shell and Telecommand Decoder Shell support the use of up to 64 Virtual Channels. The VC configuration is set at run time.

The transfer frames from the Virtual Channels have to be multiplexed onto the Physical Channel. The Telecommand Encoder Shell supports a number of different VC multiplexing algorithms:

FIFO
round robin
polling vector
absolute priority
special channel priority

The special channel priority algorithm gives an absolute high priority to one VC and uses round robin for the rest. The VC multiplexing algorithm and its parameters are configured at run time.

MAPs - Multiplexer Access Points

Telecommand packets from different sources may be multiplexed together so that they share one Virtual Channel. This is accomplished by using Multiplexer Access Points (MAPs).

The Telecommand Encoder Shell and Telecommand Decoder Shell support the use of up to 64 MAPs on each Virtual Channel. The MAP configuration is set at run time.

In the Telecommand Encoder Shell, the input for the MAPs is multiplexed onto the Virtual Channel using a FIFO algorithm.

Segmentation of packets and MAP_SDUs

Segmentation is the process of breaking long packets and MAP_SDUs into segments and transmitting each segment in a separate transfer frame. On arrival, the packet or MAP_SDU is reassembled from the segments. Segmentation makes it possible to send data units which are too big to fit in a single transfer frame. Segmentation is available for the MAPP, MAPA and VCP services and is usually limited to data units transmitted on the Type-A service.

The Telecommand Encoder Shell and Telecommand Decoder Shell support segmentation and reassembly. It is a run-time configuration option which can be set for each individual MAP. Additional parameters control segmentation on the Type-A and Type-B services within a Virtual Channel.

The Telecommand Encoder Shell and Telecommand Decoder Shell also include a run-time configuration option to support the ESA-specified Packet Assembly Controller (PAC) and its associated MAP Reset command.

Packet Aggregation or Blocking

Packet aggregation is the process of placing multiple packets together so that they are all transmitted in a single transfer frame. The aggregated packets must be complete: for example, it is not allowed to aggregate three whole packets with part of a fourth.

Packet aggregation is also known as blocking.

The Telecommand Encoder Shell and Telecommand Decoder Shell support packet aggregation on the MAPP and VCP services. It is a run-time configuration option. There are configuration parameters to set the aggregation timer and to limit the number of packets that may be placed in a segment. Individual packets may carry a "do not aggregate" parameter.

Maximum Frame Length

The CCSDS recommendations define a maximum transfer frame length of 1024 octets. The Telecommand Encoder Shell and the Telecommand Decoder Shell support transfer frames of all valid lengths up to the maximum of 1024 octets. There is a run-time configuration option to set a lower maximum length for a Virtual Channel or a Physical Channel.

Frame Error Control

The CCSDS recommendations define an optional Frame Error Control Field that may be included at the end of every transfer frame. It uses a CRC mechanism, which enables the receiver to detect errors in the frame.

The Telecommand Encoder Shell and the Telecommand Decoder Shell support the use of Frame Error Control. It is a run-time configuration option.

CLCW - Communications Link Control Word

The CCSDS recommendations define the CLCW (Communications Link Control Word or Command Link Control Word), which carries the return part of the COP-1 protocol data. CLCWs are carried in telemetry frames.

The Telecommand Decoder Shell generates CLCWs on demand. The Telecommand Encoder Shell expects CLCWs extracted from the downlink telemetry.

CLTU - Communications Link Transmission Unit

The Coding Layer in the ground end of the Packet Telecommand system converts transfer frames into Communications Link Transmission Units (CLTUs), also known as Command Link Transmission Units. The CLTUs are designed for handling by RF systems.

The data from the frame is encoded into a set of equal-length codeblocks. The last codeblock in a CLTU may include fill bits to bring it up to the correct length.

The Telecommand Encoder Shell generates and delivers CLTUs. The Telecommand Decoder Shell accepts and processes CLTUs.

BCH code

In a CLTU, the last octet of each codeblock contains a BCH code, which enables the receiver to detect (and possibly correct) errors.

The Telecommand Encoder Shell generates the BCH code in each codeblock. The Telecommand Decoder Shell accepts and processes candidate BCH codeblocks.

The Telecommand Decoder Shell has a run-time configuration parameter to select the processing to be performed on the BCH field of each codeblock. The parameter may be set for triple error detection or for single error correction.

CLTU Randomisation

The CCSDS Recommendations define an option for randomisation of the data in each Communications Link Transmission Unit (CLTU). The Telecommand Encoder Shell and Telecommand Decoder Shell support three alternatives for CLTU randomisation. It is a run-time configuration option. The three alternatives are:

No randomisation (default). The data in the CLTUs is not randomised.
The data portion of each CLTU is randomised, but any fill bits in the final codeblock of the CLTU are not randomised.
The data portion of each CLTU is randomised, and any fill bits in the final codeblock of the CLTU are also randomised.

CLTU Tail Sequences

Every Communications Link Transmission Unit (CLTU) has a tail sequence. The Telecommand Encoder Shell and Telecommand Decoder Shell support three alternatives for the CLTU tail sequence. It is a run-time configuration option. The three alternatives are:

8-octet tail sequence and an idle octet. All the octets contain hexadecimal 55. This follows the original ESA recommendation of June 1990.
8-octet tail sequence of hexadecimal 55, but without the idle octet. This follows the original CCSDS recommendation of January 1987.
8-octet tail sequence consisting of seven octets of hexadecimal C5 and an eighth octet containing hexadecimal 79. This is the current CCSDS recommendation and the current ESA recommendation.

ESA Telecommand Authentication

The Telecommand Encoder Shell and Telecommand Decoder Shell support ESA Telecommand Authentication as an optional extra.

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