User`s guide

MDS 05-2806A01, Rev. E.1 MDS iNET 900 Series User’s Guide 89
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.
• 20 bytes—802.11 RTS.
• 14 bytes—802.11 CTS.
• 66 bytes—Total Over-the-air bytes for RTS/CTS with PHY
headers.
If the frame is TCP, then there is a 32-byte TCP header instead of the
8-byte UDP header.
• ARP requests, ARP replies and BPDU’s will affect throughput.
• ARP requests are 60-byte Ethernet frames. 142 bytes
over-the-air.
• ARP replies are 60-byte Ethernet frames. 142 bytes
over-the-air.
• BPDUs are 60-byte Ethernet frames. 142 bytes over-the-air.
Note that the overhead to put a single Ethernet frame
over-the-air is 82 bytes. If RTS/CTS is invoked, it is 148 bytes.
Therefore, the overhead for a minimal Ethernet frame
(60 bytes) is 128% and, as such, gives the transceiver a poor
small-packet performance.
If any radio in your network is connected to a large LAN, such as may
be found in a large office complex, there may be undesired multi-
cast/broadcast traffic over the air.
5. Station-to-Station Traffic
• When sending frames from an endpoint connected to one trans-
ceiver to another endpoint with a different transceiver, the
throughput will be halved at best. This is because all frames
must go through the AP and thus are transmitted twice over the
same radio system. Therefore, in the previous 100-byte UDP
example, the number of over-the-air bytes will be 380 bytes
(190 bytes x 2) if the frame has to go station-to-station.
6. Interference has a direct correlation to throughput.
• Interference could be caused by any unnecessary traffic on the
network from unrelated activities, or Radio Frequency Interfer-
ence in the wireless spectrum.