Comtech EF Data DMD1050TS, Инструкция по установке и эксплуатации

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DMD1050TS Satellite Modem Board
Revision 1
Appendix F F–8 MN-DMD1050TS
F.8.3 Fair Weighted Queuing
A fair weighted priority scheme allows higher priority traffic to move quickly through the system,
while making sure lower priority traffic does not stall indefinitely. Fair weighted queues are the
default on the original Ethernet Bridge. When fair weighted queuing is selected, packets are
transmitted at rates of 8, 4, 2 and 1, from the highest to lowest priority queues, respectively.
F.8.4 Strict Priority Queuing
A strict priority scheme means that the highest priority traffic is always transmitted first, even if the
lower priority traffic is stalled indefinitely. When Strict Priority queuing is selected, packets in the
highest priority queue are transmitted until that queue is empty. Then, packets from the next
highest priority queue are transmitted until the queue is empty, or a higher priority packet arrives.
If a higher priority packet arrives before a lower priority queue is empty, that packet is the next
packet transmitted.
CAUTION
If Strict Priority Queuing is used, data in a lower priority queue might never be
transmitted. Use with care.
F.8.5 Satellite Packet Error Checking
Packet error checking is a standard part of any terrestrial Ethernet system. It uses the Cyclic
Redundancy Check (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 method is used over the satellite link, where bit errors result in a corrupted packet with a
bad CRC. For the majority of the situations, the standard process of discarding these packets
when found is preferred, because the packet can be retransmitted.
However, there are some cases where the end device must receive the erred packet, rather than
no packet at all. Examples of two such situations include cryptography use or an end device with
added error correction capability. For these customers, the EEI can turn off the satellite packet
error checking, and cause packets with bit errors to be output with a valid CRC, so that they pass
through any routers or switches between the modem and the receiving device.
For all customers, the EEI maintains and reports satellite link statistics on total packets, erred
packets and PER.
F.8.6 Automatic Learning and Aging
The Automatic Learning and Aging function allows the EEI to connect to a network without
requiring extensive setup and programming. The learning process makes sure that local LAN
traffic stays on the local LAN. Only traffic that is not destined for other equipment on the local
LAN is forwarded over the satellite link.
The aging process allows equipment to be shut off, moved or removed from the network
completely, without tying up resources or requiring operator intervention. For example, a user
might carry a laptop from remote office to remote office, and use it to check e-mail at the
corporate office. The EEI handles the apparent movement of the laptop's MAC address from one
position in the network to another, automatically.
The learned address database maintained by the EEI can hold up to 1024 MAC addresses.
When a new LAN address is learned, it is put in the database and an aging timer is started. The
aging timer is reset and restarted every time this learned address communicates. Finally, when
the learned address has been inactive for more than 5 minutes, it is removed from the database.
All of the processing needed for learning, searching and aging occurs at line speed, without any
unnecessary delay at startup.