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 CSMA/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 Service Data 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.
mrn-cciew 
Nice Article! very well written.
Thanks Pete
Ok here is myvery obviously ignorant question(I am pastor not engineer) can I enable both the MSDU and MPDU on the wifi settings for my internet at home? I ask because just got a smartphone to call India and though I sit right in front of my router it will not connect. so I noticed the MPDU was enabled and MSDU was not. Sorry if it seems like stupid question, but hey, I get them all the time in my line!
You can give it a try & see, but I doubted that will help.
What you can do is check what is the security protocol configuration of your wireless router. If you both enable AES & TKIP with WPA2, disable TKIP. Only leave AES with WPA2.
That should help if you have any Apple devices connecting difficulty to your wireless network.
HTH
Rasika
Thankyou for respondiong. I did get it worked out late last night by closing everything, shutting everythign off, the router esp. , and then re start the routerand the phone, don’t know what single action did it, but am on. I am also keepign your answewr for future knowledge. Thankyu very much. You ever want a look into scripture yu will never get from the standard pastor/teacher, you have my contact info. Baraka Bashad
Great article! lucidly written, brief and to the point. 🙂
Thanks Ghosh..
@guitarrayRaymond Nassar : It dont have relation with A-MPDU or A-MSDU, May be your MODEM or Router dint kick out the earlier MAC Address of your Device from the Association List, coz of that the Client Rejection May happened. So during Hard Reboot all the entries will be cleared so that it will help to re-associate to that MODEM.
Hi Rasika,
I have a question for you about frame aggregation. The standard for 802.11n makes it mandatory for all HT radios to use Frame aggregation. However, 802.11n also introduces RIFS (Reduced Inter-frame Space), an alternative to the legacy SIFS(Short Inter-frame Space). I don’t understand the need for RIFS when Frame aggregation is a requirement in 802.11n, whereas, RIFS/SIFS is used only between transmissions of individual frames.
I hope my question is clear.
Thanks
Pooja
Hi Pooja,
I have to read more about 802.11n stanadard to give you a good answer. Give me couple of weeks
Rasika
It is not mandatory to use frame aggregation, far from every frame is aggregated, hence use of RIFS. It is mandatory to support frame aggregation while this MAC-layer enhancements adds to the speeds of 802.11n.
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Great Article very well explained….
Thanks for the feedback 🙂
Thank you for the Article, I am wondering can the aggregate MPDU correspond to multiple different users? Thanks…
No. Refer this post as it gives bit more detail.
All MPDUs within A-MPDU should have same receiver address
HTH
Rasika
Hi Rasika,
I have a big problem with one way audio.
Sometimes just there is One-Way audio.
The Qos for wired and wireless is fine configured.
The IP phone 7925.
I would be very happy if you could provide me a little help =(.
TPC Config:
TPCv1
-Automatic power level assignment ever 600 secs
Thank you
Oneway Audio is mainly due to power mismatch between AP and Client.
What power level this AP operates ? (11dBm or 14dBm )
HTH
Rasika
Hi sir,
thanks for giving lot of info but i need small help from you
1.how to see whether frames are getting aggregated or not in 802.11n using sniffer capture if you can explain it wikk be so helpful for me
2.how to verify wheather channel bonding is working or not in 802.11n ap?
3.performance trouble shooting steps if there is similar ap’s but giving diffrent through put values?
sravangandavaram@gmail.com
pls help me sir
thanks and regards,
sravan
@sravan kumar
1.how to see whether frames are getting aggregated or not in 802.11n using sniffer capture if you can explain it wikk be so helpful for me.
1. After the station association you can see the ADDBA Request and Response later the Block Ack Request and Response. I suggest you to capture the Packets using Wireshark or Omni Pick and thgen try to filetr based on the Device MAC and THE AP BSSID MAC. so that you can see the packet exchange between only STA and AP.
2.how to verify wheather channel bonding is working or not in 802.11n ap?
In the Beacon Frame under HT or VHT Capabilities you can see the Channel Width.
From Practical Perspective ideally the throughput amount will be doubled if you are bonding two channels.
3.performance trouble shooting steps if there is similar ap’s but giving diffrent through put values?
Please Validate the CPU Processing Load, Memory and the Packet Drop in each Interface, that would give you a better idea where the packets are getting drop.
Thanks,
Deepak D
Hi Deepak,
Can we see the presence of A-MPDU or A-MSDU using any filters in Wireshark, if yes can you please tell me?
Also can you please the funda of Block Ack request and response here, is this specific to ampdu & amsdu?
Thanks in advance
Hi Rasika,
Great post.Thanks for sharing !
I have a question.Does the presence of 802.11 g client affect 802.11 n clients speed which is connected to 11 n AP(mixed mode)?
Thanks.
Yes in more than one way. One way is 802.11g clients send at lower data rates meaning transmissions take longer and this impacts 802.11n clients. Just like driving a Ferrari behind a truck on the freeway.
Hi,
I need a help.
1.How to capture A-msdu and A-mpdu packets.
2.A-mpdu over A-msdu
3.A-msdu over A-mpdu
In real time How above three methods of aggregation will make difference.
Thanks in Advance
Thank you for your brilliant blog!
This sentence may need completion (?):
> The main distinction between MSDU & MPDU is the former corresponds to upper part of MAC sublayer where as MPDU.
Hi Rasika,
Could you please write one blog for 802.11ac data transmission ?
After a firmware upgrade on Cisco 8821 wireless phones, there is intermittent loss of connectivity to CUCM. Cisco TAC are recommending that AMSDU be disabled so that the Cisco 8821 phones won’t drop out (Bug ID: CSCvd06463). What’s the impact on clients by disabling A-MPDU aggregation on 802.11n? The WLC (5508, v7.4.110.0) services the following clients Ascom wireless phones, Cisco 7925 wireless phones which form 98% of the devices using that WLAN.
Hi Rasika, great post thanks.
My question : on a WLAN network (Cisco vWLC 8.2 and 7925G phones) default A-MSDU is enable all except priority 6 and 7.
Best practices for 7925g is setting Qos for platinum (voice) profile at 5 or 6.
So it is better ti enable A-MSDU for priority 6 ?
Hi Rasika,
Is its possible to disable 802.11b&g but keep 802.11n for 2.4 enabled.
Hi Ahmed,
I do not think you can do it. You have to enable 11g support at least.
HTH
Rasika
In what cases will you enable A-MSDU & A-MPDU at the same time?
Wouldn’t A-MPDU be better (less overhead, more aggregation), so I think it doesn’t make sense to enable A-MSDU at the same time.
I’m in correct on my assumption ?
how do we configure 256 AMPDU size in Windows 10 clients (AX chipset). is there any registry settings.