MA28140 Datasheet, PDF (21/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
4.4 SEGMENTATION LAYER
Overview of the Layer
The segmentation layer provides the means to distribute
several distinct streams of variable-length data units (e.g. the
TC packets) to different applications by providing a number of
service access points called the Multiple Access Points
(MAPs). The data flow on each stream can be controlled by the
receiving application using handshake control.
A TC segment consists of three distinct protocol data
elements:
- an 8-bit segment header, the purpose of which is to
identify the MAP connection and flag the sequential
position of the segment relative to the complete TC
Packet,
- a segment data field, of maximum length 248 octets,
which contains all or a portion of a TC Packet,
- the 9-octet Segment Trailer specific to authenticated
segments is removed by the authentication layer.
Standard Data Structures Within the Layer
The structure of the TC segment is given below:
SEGMENT DATA
SEGMENT HEADER
FIELD
Sequence
MAP
Flags
Identifier
2 bits
6 bits
variable
<--------------- 1 octet ------------><- from 0 to 248 octets ->
Segment Header
The Segment Header is the first octet (octet 0) of the TC
segment structure. The Segment Header is divided into two
major fields as follows:
- Sequence Flags (bits 0 & 1): this field is used by the
segmentation protocol to indicate the sequential position
of the segment relative to the complete data unit (e.g. the
TC Packet). The flags are interpreted as follows:
Bit 0 (MSB)
0
0
1
1
Bit 1
1
0
0
1
Interpretation
First segment
Continuation segment
Last segment
Unsegmented
Segment Data Field
The segment data field may vary from 0 to 248 octets
maximum. When the optional Segment Trailer is used, the
maximum length of the segment data field will be reduced by 9
octets.
Standard Procedures Within the Layer
The following segmentation layer functions are
implemented in the PTD:
- the back-end buffer for the accepted TC segment. The
back-end buffer is shared between the Transfer Layer
and the Segmentation Layer.
- the MAP interface.
Upon reception of a new segment the Segment Layer
performs the following operations:
- Checks whether the segment is authenticated or not.
- Starts the AU process if the segment is authenticated
and if the AU is not disabled. The Segment Layer waits
for the completion of the AU process (internal or external).
A security mechanism is implemented, in case of AU
locking mechanism the user can stop the AU process by
activating the AU disable signal. In this case, the segment
layer stops waiting for the AU completion process and the
content of the back end buffer is lost.
- Checks if the frame is a CPDU command (MAP 0). In
this case, the CPDU layer is activated and no data is
output on the MAP interface.
- Checks if the frame is an AU command (MAP 63) and
the AU is not disabled. In this case no data is output on
the MAP interface.
- For a MAP 1 to 62 and for MAP 63 if the AU is disabled,
the data is provided in serial or in parallel via the MAP
interface. The MAP output frequency for serial MAP is
selectable by reading a value associated with each MAP
in the external ROM (see section 5.2).
When the flags are set to 11 this means that the TC
Segment Data Field contains an entire TC Packet. Except for
the CPDU described in section 4.5, these flags are ignored by
the PTD.
- Multiplexed Access Point (MAP) Identifier: this 6-bit field
enables up to 64 MAP connection addresses to be associated
with a single Virtual Channel. The PTD supports MAP 1 to 63
as externally available MAPs. MAP 0 is dedicated to the
CPDU. MAP 63, when AU is enabled, is reserved for AU
commands; when the AU is disabled, MAP 63 is processed by
the segment layer like a standard MAP (see section 4.3).
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