VBrick Systems VB5000 - Manual (Page 41)

MPEG-2 Configuration
VBrick MPEG-2 Appliance Admin Guide 33
Packet Payload Size Controls the amount of MPEG data within each UDP packet. The
default value is 4136. Since Ethernet networks are limited to a
packet size of 1500 bytes, multiple IP packets are required to span
one UDP packet. When choosing a packet payload size, special
attention must be paid to the network capability and topology. The
allowed range is 1316 to 8872 bytes per packet, even numbers only.
When the MTU is set to the default of 1500, the VBrick appliance
will not produce fragmented UDP packets at or below a payload size
of 1472. Note that MPEG-2 video playback on Vista PCs may be
suboptimal with small packet payload settings.
If you wish to select something other than the default (4136),
VBrick recommends that the Packet Payload Size be a number in the
vicinity of the Actual Transport Rate divided by 800—but no
smaller than 1316 and no larger than 8872. This recommendation
represents a compromise: smaller payloads provide smoother
delivery to the decoder and improve the decoder performance;
larger payloads impose less demand on the encoder processing
capabilities. This recommendation becomes increasingly significant
as the transport bitrate decreases and falls below 3.3 Mbps; 4136 is a
better value for Packet Payload Size when the transport bitrate is 3.3
Mbps or higher. It is also beneficial but not necessary for the Packet
Payload Size to be a multiple of 188.
Note that when you adjust the Packet Payload Size and video Bit
Rate on the encoder, the decoder video may be affected. For
example, when the Packet Payload Size on the encoder is set to 7000
or above and Bit Rate on the Configuration: Encoder > Video page
is set from 1–1.4 Mbps, the decoder receiving the video may go into
a continual reset loop (every 10–60 seconds depending on the
settings). To stop the resetting, set the Bit Rate to 1.5 Mbps or
higher.
Unicast Poll This parameter is used to instruct the VBrick to poll whether the
unicast destination VBrick is configured to accept this stream. If the
unicast destination VBrick is not configured to accept this stream,
this VBrick will not send the stream. This feature is designed to
eliminate unwanted unicast video traffic across the network.
Unicast Ping Feature for primary destination can be enabled, disabled. If the
VBrick is designated to transmit a transport stream to any
destination (whether another VBrick or not) in unicast mode, this
parameter is used to instruct the VBrick to ping the destination
periodically to test connectivity before sending the stream. If
Unicast Ping is not enabled, and the destination goes off-line, the
unicast stream may be broadcast to all destinations causing flooding
on the network. This scenario largely depends on network
architecture and may or may not occur. This feature should only be
disabled if the network has a particular requirement that makes it
unworkable.