method also allows TS-Packets [ISO-MPEG] to be sent as GSE SNDUs. This may be
used to provide control information and/or Program Specific Information (PSI) for a
Multiplex.
The design rationales for the protocol were driven by the peculiarities of DVB-S2
Generic Streams which carries both several fixed sized packets as well as variable
sized packets in baseband frames (BBframe) of variable length, whose length is
determined by the amount of forward error correction information being added onto
physical layer frames. BB-frame sizes vary from 384 bytes to 7274 bytes. Adaptive
Coding and Modulation (ACM) allows for changing coding and modulation
(MODCOD) on-the-fly and in accordance with the link quality perceived at a certain
Receiver or group of Receivers. Consequently a specific Receiver will be able to
demodulate and decode only those BBframes whose MODCOD matches the
perceived link quality. Receivers that are able to demodulate and decode all BBframes
of the Generic Stream receive the same Generic Stream as continuous stream of BB-
frames as being sent from the Encapsulator, whereas Receivers with worse reception
will receive only reasonable protected BB-frames, hence, each MODCOD forms a
sort of virtual channel. However, Receiver hardware perceives these virtual channels
as a continuous stream of BBframes, potentially with different sizes. The DVB-S2
standard permits an Encapsulator to transmit different network layer packets destined
to a specific Receiver into BBframes with different MODCOD, and feedback from
the Receiver about its link quality may trigger MODCOD changes at any time. The
10B header of a BB-frame carries the length of the DataField, but, in difference to the
4B header of a Transport Stream Packet, does neither include a Payload Unit Start
Indicator (PUSI) nor a Transport Error Indicator (TEI), GSE will resemble its own
Start and End Indicator for reassembly of encapsulated units instead.
GSE can transport payload data units (PDUs) over DVB-S2. For this the PDU is
encapsulated with a TypeField and an optional label which can be used for receiver
filtering (e.g., a 6-byte MAC address). The GSE protocol defines Encapsulated
Payload Units (EPUs) consisting of a GSE header, a or fragment of a PDU/SNDU,
and a CRC-32 (in case that the EPU is the last EPU of a fragmented PDU/SNDU.
GSE can fragment an payload data units (PDUs) into fragments of any size. A DVB-
S2 DataField can carry several EPUs with typically sized network packets.
Fragmentation across BBframe boundaries in principle allows for cutting a network
packet at BB-frame border and for resuming it in the next BB-frame.
Because the ACM allows MODCOD changes at any time, a BB-frame fragmentation
requires a different MODCOD and some or all Receivers will not be able to decode it.
Manufacturer, hence, have requested to allow for arbitrary placement of complete or
fragmented network packets within BB-frames in order to meet potential QoS
requirements of specific IP flows and in order to optimize link utilization. Since
arbitrary fragment placements does not necessarily preserve fragment order, except
maybe within the same MODCOD, the identification of fragments is subtle. It shall be
noted that the maximum possible number of incomplete fragments may require
significant buffer space to be maintained in a receiver. In ULE, the receiver only has
the states IDLE or REASSEMBLING, whereas arbitrary placement in the worst case
requires N simultaneous REASSEMBLING states, where N is the number of not yet
completely reassembled PDUs.
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