Comtech EF Data MDX420 SkyWire Manual - Manual (Page 26)

MDX420 SkyWire™ Satellite Network Gateway Introduction
MN-MDX420 Revision 6 1–10
Figure 1-10 QoS Priority Queuing
In certain circumstances it may be desirable to have the Ethernet Interface to operate in a FIFO like mode
with no reordering of packets. This can be done by using a single port and setting the Ethernet QoS type
to Port Base and the Ethernet QoS Queue to strict priority. This will allow packets to be transmitted in the
exact order in which they are received.
1.5.2 CRC Control Satellite Packet Error Checking
Packet error checking is a standard part of any terrestrial Ethernet system and is performed using the
CRC contained in the layer 2 Ethernet wrapper. The CRC is used as a checksum to detect alteration of
data during transmission. When a CRC error is encountered, the packet is typically discarded by the
router, switch, or hub in which the error was detected.
A similar methodology is utilized over the satellite link where bit errors result in a corrupted packet with a
bad CRC. For most situations, the standard process of discarding these packets when they are
encountered is the preferred methodology as the packet can simply be retransmitted.
However, there are some cases where the end device would rather receive the erred packet than no
packet at all. Scenarios involving cryptography or where the end device has additional error correction
capability are examples of two such situations. For these customers, the gateway provides the ability to
turn off the satellite packet error checking and packets with bit errors will be output with a valid CRC so
that they will properly pass to the connecting router or switch. Checking the CRC Control box in the
Sattelite Demod Configuration GUI (normal operation) will look for a valid CRC and if not found will drop
the Ethernet datagram. Unchecking the CRC control box will append a valid CRC to a potentially corrupt
Ethernet datagram and pass the datagram to the next device in the chain. Note: CRC control only
functions from the flow of satellite to the Ethernet LAN. Ethernet LAN to satellite flow will always drop bad
CRC datagrams regardless of selection.
Regardless of the configuration of the CRC Control function, enabled or disabled, SkyWire maintains and
reports satellite link statistics on total packets, erred packets, and packet error rate (PER).