IEEE 802.11 PHY layer specification concentrates mainly on wireless transmission and concurrently perform secondary functions like assessing the state of wireless medium & reporting back to the MAC sublayer.
IEEE 802.11 MAC architecture is based on the logical coordination functions that determine who and when to access the wireless medium at any time.It supports fragmentation & encryption and act as an interface between Logical Link Control-LLC sublayer & PHY layer.
In legacy 802.11 (prior to 802.11n) standards, there are two type of access schemes
1. DCF- Distributed Coordination Function (mandatory) based on CAMA/CA.
2. PCF- Point Coordination Function (Optional) based on poll-response.
Due to lack of QoS in above implementation 802.11e proposed HCF (Hybrid Coordination Function) with contention-based channel access known as EDCA (Enhanced DCF Channel Access). In addition to the differentiation & prioritization, 802.11e offers TXOP (Tx Opportunity) an interval of time in which multiple data frames can be transferred from one station to another as way of improving MAC efficiency. Also it introduce a way of ACK multiple received data frames known as “block ACK”.
Basic operation of DCF is illustrated in Figure1 below.
After a frame, also known as MAC Service Data Unit (MSDU) arrives from LLC at the head of the transmission queue, the DCF operation istruct to MAC to wait DIFS (Distributed Interframe Space) before any other action. Once frame received by a station check for errors & no errors are found send ACK after SIFS (Short Interframe Space). If errors detected or communication disturbed, it will start re-transmission process.
In 802.11n standard, Frame Aggregation was the most important MAC enhancement proposed which maximize throughput and efficiency. There are two method available to perform frame aggregation.
1. Aggregate MAC Protocol Service Unit (A-MSDU)
2. Aggregate MAC Protocol Data Unit (A-MPDU)
The main distinction between MSDU & MPDU is the former corresponds to upper part of MAC sublayer where as MPDU.
A-MSDU: The concept of A-MSDU is to allow multiple MSDUs to be sent to the same receiver concatenated in a single MPDU. This supporting function for A-MSDU within 802.11n is mandatory at the receiver. Due to Destination Address (DA) and sender Address (SA) in the subframe header must match to same receiver address (RA) and the transmitter address (TA) in the MAC header, A-MSDU cannot be used for broadcast & multicast.
A-MPDU: The concept of A-MPDU aggregation is to join Multiple MPDU sub frames with a single leading PHY header. A key difference from A-MSDU aggregation is that A-MPDU functions after the MAC header encapsulation process. This method offer higher MAC throughput compare to A-MSDU.
Figure 2 shows these two frame aggregation methods used in 802.11n protocol standard. (Two level aggregation comprise a blend of A-MPDU & A-MSDU over two stage also defined, not describe in this post)
So that’s basic theory about these two terms. How you configure this on a Cisco WLC ? You can check the settings of these two parameters by issuing “show 802.11a|802.11b” CLI commands
(4402-a) >show 802.11a 802.11a Network.................................. Disabled 11nSupport....................................... Enabled 802.11a Low Band........................... Enabled 802.11a Mid Band........................... Enabled 802.11a High Band.......................... Enabled . 802.11n Status: A-MPDU Tx: Priority 0............................... Enabled Priority 1............................... Disabled Priority 2............................... Disabled Priority 3............................... Disabled Priority 4............................... Enabled Priority 5............................... Enabled Priority 6............................... Disabled Priority 7............................... Disabled A-MSDU Tx: Priority 0............................... Enabled Priority 1............................... Enabled Priority 2............................... Enabled Priority 3............................... Enabled Priority 4............................... Enabled Priority 5............................... Enabled Priority 6............................... Disabled Priority 7............................... Disabled (4402-a) >show 802.11b 802.11b Network.................................. Disabled 11gSupport....................................... Disabled 11nSupport....................................... Enabled . 802.11n Status: A-MPDU Tx: Priority 0............................... Enabled Priority 1............................... Disabled Priority 2............................... Disabled Priority 3............................... Disabled Priority 4............................... Enabled Priority 5............................... Enabled Priority 6............................... Disabled Priority 7............................... Disabled A-MSDU Tx: Priority 0............................... Enabled Priority 1............................... Enabled Priority 2............................... Enabled Priority 3............................... Enabled Priority 4............................... Enabled Priority 5............................... Enabled Priority 6............................... Disabled Priority 7............................... Disabled
As you can see above A-MSDU & A-MPDU configured per radio band and per 802.1p priority. Therefore in the exam if you asked to configure A-MPDU only for 802.1p valued 4,5,6,7 frames & enable A-MSDU on all frames, you have to modify the above default settings. You have to disable 802.11a|802.11b network prior to any configuration. Remember this can be only configure via CLI. Command syntax would be config {802.11a|802.11b} 11nsupport {a-mpdu|a-msdu} tx priority {0-7} {eanble|disable)
(4402-a) >config 802.11a disable network (4402-a) >config 802.11a 11nsupport ? a-mpdu Configure 802.11n-5Ghz A-MPDU mode a-msdu Configure 802.11n-5Ghz A-MSDU mode antenna Configure 802.11n - 5 GHz antenna selection disable Disable 802.11n-5Ghz support enable Enable 802.11n-5Ghz support guard_interval Configure 802.11n-5Ghz guard interval mcs Configure 802.11n-5Ghz MCS rates rifs Configure 802.11n-5Ghz rifs (4402-a) >config 802.11a 11nsupport a-mpdu tx ? priority Configure 802.11n-5 GHz A-MPDU transmit priority (4402-a) >config 802.11a 11nsupport a-mpdu tx priority ? <0-7/all> 0-7/all (4402-a) >config 802.11a 11nsupport a-mpdu tx priority 6 enable (4402-a) >config 802.11a 11nsupport a-mpdu tx priority 7 enable (4402-a) >config 802.11a 11nsupport a-mpdu tx priority 0 disable (4402-a) >config 802.11a 11nsupport a-mpdu tx priority 1 disable ! (4402-a) >config 802.11a 11nSupport a-msdu tx priority 6 enable (4402-a) >config 802.11a 11nSupport a-msdu tx priority 7 enable (4402-a) >config 802.11a enable network
You can do the same on the 802.11b network. Once you do that you can verify settings by using “show 802.11a|b” command as shown in the above.
Here is the reference IEEE article I used for this post. Please go through it if you are really interested on deeper information on this topic.
