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MPDU size

1 posts by 1 authors in: Forums > CWDP & CWAP - Enterprise Wi-Fi Design & Analysis
Last Post: October 2, 2006:
  • By asoa
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    Dear Criss,

    "While using TKIP with fragmentation each fragment will increase by 12 octets not 20, as MIC will be fragmented as part of the original MSDU."

    The above statement was taken from Page 8 of the WLAN Analysis Pocket Guide by Vinay Saini which is available on http://www.4shared.com/dir/254863/b52b8fef/sharing.html which has a link in the CWAP forum too.

    In the last paragraph of Page 78 of CWAP book quotes.

    "So, an unfragmented MPDU is increased by 20 octets as we have mentioned, first by adding 8 octets to the MSDU then by adding 12 octets to the MPDU."

    "But each MPDU of a series of fragments is only increased by 12 octets! The extra 8 octets are inserted once for the series as though it were part of the original 'raw' MSDU."

    The confusion comes from above 1st statement of Page 78 along with the Figure 3.11 on Page 79 of CWAP book which shows Data Frame Body Format Using TKIP. (It doesn't specify whether the data frame is fragmented or not).

    The following two questions and explanations in the Practice Test also paves way for more confusion.


    Q. When using TKIP, the maximum available MSDU size that may be used by upper layer protocols is _______

    E. The maximum MSDU is always 2304 bytes regardless of whether encryption is used or not. When encryption is used, the MSDU is encapsulated inside the MPDU frame body which expands the overall length of the MPDU frame. Static WEP adds 8 bytes (IV & ICV), TKIP adds 20 bytes (WEP IV, WEP ICV, TKIP Ext IV, TKIP MIC), and 802.11i's CCMP adds 16 bytes (CCMP header and MIC).


    Q. What is the maximum MSDU size in an IEEE 802.11 data frame that is not using any form of encryption?

    E. 802.11-1999 Section 6.2.1.1.2 says, "The data parameter specifies the MSDU to be transmitted by the MAC sublayer entity. For IEEE 802.11, the length of the MSDU must be less than or equal to 2304 octets." The MPDU frame body is a maximum of 2304 bytes long when no encryption is used, and is expanded to 2312 bytes when WEP is in use, 2324 when TKIP is in use, and 2320 when CCMP is in use.

    I hope that helps.

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