MDS 05-2806A01, Rev. A MDS iNET 900 User’s Guide 57
• 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) that pre-
vents loops from being created when connecting bridges in par-
allel. 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. Thus, a large network of iNETs will have a lot of BPDU
traffic. Because of the nature of the iNET networks (i.e. they are
rarely connected in parallel) STP could be turned off without
consequences.
2. The wireless MAC has several settings that can be adjusted.
• Max Data Length is the largest packet that can be given to the
MAC.
• 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-byte
UDP packet:
• 100 bytes—UDP data
• 142 bytes— Ethernet frame (added 4-byte UDP header, 20-byte
IP header, 14-byte Ethernet header)
• 174 bytes— 802.11 encapsulated Ethernet (added 24 bytes—
802.11 header, 3 bytes—LLC, 5 bytes—SNAP, 4 bytes—
802.11 FCS)
• 190 bytes— Over-the-air frame size (added 16 bytes to the
FHSS PHY)
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.
• 20 bytes—802.11 RTS.
• 14 bytes—802.11 CTS.
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