MDS 05-4055A01, Rev. A
MDS entraNET 900 System Guide (Preliminary)
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• The bridge code in the transceiver operates and makes decisions
about packet forwarding just like any other bridge. The bridge
code builds a list of source MAC addresses that it has seen on
each of its ports. There are a few general rules that are followed
when a packet is received on any port:
• If the destination address is a multicast or broadcast address,
forward the packet to all other ports.
• If the destination address is not known, forward the packet to all
other ports.
• If the destination address is known, forward the packet to the
port that the destination is known to be on (usually the RF port).
• The bridge code uses Spanning Tree Protocol (STP) to prevent
loops from being created when connecting bridges in parallel.
For example, connecting two remotes to the same wired LAN
could create a loop if STP was not used. Every bridge running
STP sends out Bridge Protocol Data Units (BPDU's) at regular
intervals so that the spanning tree can be built and maintained.
BPDU's are 60-byte multicast Ethernet frames.
2. The wireless MAC has two settings that can be adjusted.
• Fragmentation threshold is the threshold in bytes, which
causes the MAC to fragment a packet.
• RTS threshold is the threshold in bytes that causes the MAC to
use RTS/CTS before sending the packet.
3. Throughput calculations must take into account all overhead.
The following is an example of the overhead at each layer for a
100-bytes of data over UDP:
• Data: 100 bytes
• UDP header: 8 bytes
• IP header: 20 bytes
• Ethernet header: 14 bytes
• 802.11 header 24 bytes
• LLC and SNAP header: 8 bytes
• FHSS header and FCS: 16 bytes
Total over-the-air frame size=190 bytes
If the frame is directed (for example: not multicast/broadcast), the
802.11 ACK frame must be accounted for:
• 14 bytes—802.11 ACK
• 30 bytes—Over-the-air ACK frame (added 16 the FHSS PHY)
If the 802.11 encapsulated Ethernet frame (NOT the UDP or Ethernet
frame) exceeds the RTS threshold, then the overhead for RTS/CTS
frames must also be accounted for.
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