Radyne OM20 Specifications download pdf (Page 172)

DMD1050 Satellite Modem Board Ethernet Data Interface

F.8.5 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, 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. 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, the EEI 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 modem

and the receiving device.

For both sets of customers, the EEI maintains and reports satellite link statistics on total packets,

erred packets, and packet error rate (PER).

F.8.6 Automatic Learning and Aging

The Automatic Learning and Aging function allows the EEI 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 the EEI.

The learned address database maintained by the EEI 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.

F.8.7 Internal Buffer and Throttle

The Enhanced Ethernet Interface provides 120K bytes of internal data storage for buffering of

Ethernet traffic. When this buffer is almost full, industry standard backpressure or an IEEE

802.3x Pause Frame is used to prevent the buffer from being overrun. Together the two provide

an effective rate exchange and throttle between the Ethernet port and the satellite link.

MN-DMD1050 F–10

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Radyne OM20 Specifications Page 172