Comtech EF Data CMR-8500 Manual - Manual (Page 81)

CMR-8500 DVB IP ENCAPSULATOR Revision 2
Appendix B. IP Routing Support MN-CMR8500
B.3 802.1Q VLAN Support
802.1Q VLAN support allows the encapsulator to route traffic based on the VLAN tags. The
configuration of the VLAN tag is done on the Advanced Routing page on the terminal interface and is
also available on the web interface. VLAN tags are supported when LLC_SNAP Ethernet frames are
being used. Normal
Ethernet frames utilize IEEE 802.3 framing with a maximum frame size of 1,518 bytes; however,
LLC_SNAP frames contain four extra bytes (on the frame header) that may be used for VLAN tags, so
the Ethernet frame size is up to 1,522 bytes. The VLAN tag allows the CMR-8500 to route packets based
on the presence of a route that matches the VLAN tag. The VLAN tag is 12 bits in length and valid values
from 0 to 4095 are accepted.
B.4 Section Packing
Section packing is a technique to provide better utilization of the MPEG-2 Transport Stream (TS) packet
structure. When section packing is not used, each TS packet (containing the MPE section) can carry no
more than a single section. This often results in wasted payload capacity when the sections are less than
184 bytes. The section packing feature allows more than a single MPE section to be carried by an MPEG-
2 TS packet, thereby minimizing wasted payload capacity. In the case of a large MPE section that spans
multiple MPEG-2 packets, the ending of an MPE section may occur in the middle of an MPEG-2 packet.
In this instance, with section packing, a new MPE section begins immediately after the first MPE section
has finished without any ‘fill’ (wasted) bytes having to be inserted before the start of the next MPEG-2
packet.
The CMR-8500 provides an option to set how much time is allocated to hold packets to ensure filling an
MPEG frame before transmitting the frame.
B–3