Last Post: February 10, 2005:
hi, i like to how what is the maximum sixe for PSDU data for 802.11g, in the standerd the said its from 1-4095 octects and i found on some other site its 2346 so which one is the maximum and how is the frame look like when we use WPA, how many bits we will reservie for WPA PSK .
i hope to help me in that
IEEE 802.11 clause 19 ERP PSDU maximum size is 4095 octets.
IEEE 802.11 MSDU maximum size is explicitly 2304 octets. In practice MSDU size seldom exceeds 1508 because of the need to bridge with Ethernet.
The maximum MAC header and FCS overhead is 34 octets, but only frames between access points over a wireless distribution system (WDS) use all MAC header fields.
The encryption overhead is WEP 8 octets, TKIP 20 octets, and CCMP 16 octets. In consequence of all the above the number 2346 is implicitly the maximum size of a WEP encrypted WDS frame.
When WEP was the only encryption method the number 2346 appeared in 802.11 Annex D MIB description as a default setting for fragmentation threshold to mean "do not fragment". This number was given major visibility by the IEEE 802.11 Handbook and since immortalized by wireless authors, trainers, and testers.
In light of TKIP and CCMP introduced by 802.11i-2004, the obscure Annex D MIB reference needs to be amended; 802.11e draft 8 also published in 2004 offers to amend 2346 to an arbitrary 3000.
Unfortunately we will see 2346 for a long time. My advice is to memorize MaxMSDU=2304, maximum MAC header=34, and the three encryption overheads of 8, 20, and 16. These are the real magic numbers. And if it is any consolation the first three of these add up to 2346!
Disclaimer: I had no part in creating the 802.11 standards. I have only read them carefully and become rather good with using Adobe Reader to search the documents for key words. Consider all of the above to be my opinion subject to error.
I need to add one additional comment here....
Criss is dead on the money on all counts. Memorize exactly what he mentioned, and you'll be just dandy for the CWAP exam as far as frame sizes go. MSDU max = 2304 is very important, payload additions due to encryption overhead with WEP, TKIP, and CCMP are also important. Use of the fourth MAC address field due to WDS is important (since it's used in repeating, bridging, etc, but NOT used in typical client/AP communications).
i like to present my thanks for your information at
in fact , as you know the length of SPDU 4095 octets, but you told me the size is 2346, 8 for wep , TKIP 20 , and CCMP 16 octets.
i like to know what will happen for the rest octets , i mean what are exactly the 4095 octect, for example in 11b we haveMPDU ( payload data 1-2312 and the mac header with FCF 36 octetes), what is the 802.11g 4095
how many octetes for payload and how many octets for payload data or the 4095 the payload ??
i hope to help me in that
The standard requires that the MAC layer be able to exchange data units with the LLC layer above it that are no larger than 2304 octets.
The MAC layer adds or removes octets for MAC header and encryption. Currently the largest data unit the MAC layer can construct to pass to the physical layer below it is 2304 + 34 + 20 = 2358 octets.
The standard requires that the ERP physical layer be able to exchange data units with the MAC layer above it that are as large as 4095 octets.
But at present the physical layer will never receive data units from the MAC layer that are larger than 2358 octets.