Comtech EF Data MDX420 SkyWire Manual - Manual (Page 118)

MDX420 SkyWire™ Satellite Network Gateway SkyWire QoS
MN-MDX420 Revision 6 C–8
C.4 Additional capabilities
In addition to the aforementioned QOS capabilities, the SkyWire Ethernet interface provides the
following additional functionality.
C.4.1 Automatic Learning and Aging
The Automatic Learning and Aging function allows SkyWire to be connected to a network without
requiring extensive setup and programming by the operator. The learning process insures that
local LAN traffic stays on the local LAN and that only traffic that is not destined for other
equipment on the local LAN gets forwarded over the satellite link.
The aging process allows equipment to be shut off, moved, or even completely removed from the
network without tying up resources or requiring operator intervention. For instance, a particular
user might carry a laptop from remote office to remote office and use it to check their e-mail back
at the corporate office. The apparent movement of the laptop's MAC address from one position
in the network to another is automatically handled by SkyWire.
The learned address database maintained by SkyWire can hold up to 1024 MAC addresses.
When a new LAN address is learned, it is placed in the database and an aging timer is started.
Then, every time this learned address is heard from, its aging timer is reset and restarted.
Finally, when the learned address has not been heard from for more than 5 minutes, it is removed
from the database. All of the processing required for learning, searching, and aging is handled at
line speed and without any unnecessary delay at startup.
SkyWire also performs this automatic learning and aging function on its satellite WAN side.
Learning the MAC addresses it receives via the satellite, and only outputting those destined for its
LAN and those whose destination is still unlearned. This feature is particularly useful in mesh
applications as it prevent unnecessary traffic from flooding every LAN in the mesh.
C.4.2 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 Ethernet wrapper. When a CRC error is encountered, the packet is
discarded by the router, switch, hub, or other device in which the error was detected.
SkyWire uses a similar methodology over the satellite link where bit errors result in a corrupted
packet with a bad CRC. And for the majority of the 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, SkyWire
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 pass through any routers or switches in between the
gateway and the receiving device.