IEEE 802.11n Frame Aggregation and Block ACK Mechanisms

Before the introduction of frame aggregation mechanisms, there are some terms that readers should know. The IEEE Std 802.11™-2007 [7] defines:

 Medium access control (MAC) service data unit (MSDU): information that is delivered as a unit between MAC service access points (SAPs).

 Medium access control (MAC) protocol data unit (MPDU): the unit of data exchanged between two peer MAC entities using the services of the physical layer (PHY).

Figure 2.2The Maximum Throughput and the Throughput Upper Limit for IEEE 802.11a in [5] Y. Xiao, “IEEE 802.11n: Enhancements for Higher Throughput in Wireless

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 PLCP protocol data unit (PPDU): the actual frame send on the wireless medium.

2.3.1. Aggregated-Mac Service Data Unit

Figure 2.3 illustrates the concept of A-MSDU aggregation. As one can see, this method aggregates the MSDU sending from the upper layer of MAC protocol layer into one big A-MSDU frame. Only those MSDUs that has the same destination address (DA) and source address (SA) as the receiver address (RA) and transmitter address (TA) can be aggregated into one A-MSDU. To completely form an A-MSDU either when the size of the waiting packets reaches the maximal A-MSDU threshold or the maximal delay of the oldest packet reaches a pre-assigned value. After the aggregation, the MSDUs become sub frames of the A-MSDU and send in to mac protocol layer. In the mac protocol layer, an A-MSDU should be encapsulated into a single MPDU. All the A-MSDU sub frames will share the same MAC header after being aggregated into one MPDU. One important point is that an A-MSDU in a MPDU should not be fragmented and the destination address field in mac header of the MPDU carrying an A-MSDU should be set to an individual address. One should know there is a size limitation of A-MSDU. In IEEE 802.11n [8], the maximum size of A-MSDU is ether 3389 bytes or 7935 bytes. A mobile station should not transmit an A-MSDU to another mobile station that exceeds its maximum A-MSDU length capability.

The A-MSDU aggregation technique reduces the overhead of the mac header by using a mac header to all the A-MSDU sub frames. A bigger improvement to the IEEE 802.11 mac protocol is it reduces the transmission overhead by sending the aggregated MSDU together rather than sending them one at a time.

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2.3.2. Aggregated-Mac Protocol Data Unit

Figure 2.4 illustrates the concept of A-MPDU aggregation. The idea of A-MPDU aggregation is to join multiple MPDU sub frames with a single leading PHY header. The A-MPDU aggregation is done after the MAC header encapsulation process. The A-MPDU aggregation restriction factor doesn’t contain the TID of the MPDU frame which is different from A-MSDU aggregation. Still, all the aggregated MPDUs must be addressed to the same receiver address. There is no waiting time to form an A-MPDU which means an A-MPDU is formed by the packets already in the transmission queue at the aggregation period. The maximum length of an A-MPDU is 65,535 bytes and the maximum number of sub frames that it can hold is 64 because a block ACK bitmap field is 128 bytes in length, where each frame is mapped using two bytes. The two bytes are for the acknowledgement of up to 16 fragments but because A-MPDU does not allow fragmentation, these extra bits are excessive. There is a

Figure 2.3Aggregated-Mac Service Data Unit

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new variant so called compressed block ACK which has a bitmap field only eight bytes long.

One can see there is a set of fields, known as MPDU delimiters in Figure 2.4. The delimiters are inserted before each MPDU and a set of padding bits length from 0 – 3 bytes are added at the tail. The delimiter header is used to define the MPDU position and length inside the aggregated frame. The cyclic redundancy check (CRC) field in the delimiter verifies the authenticity of the 16 preceding bits. The padding bytes are added such that each MPDU is a multiple of four bytes in length, which can assist sub frame delineation at the receiver side. In other words, the MPDU delimiters and PAD bytes determine the structure of the A-MPDU.

After the A-MPDU is received, a de-aggregation process initiates. First it checks the MPDU delimiter for any errors based on the CRC value. If it is correct, the MPDU is extracted, and it continues with the next sub frame until it reaches the end of the PSDU. Otherwise, it checks every four bytes until it locates a valid delimiter or the end of the PSDU. The delimiter signature has a unique pattern to assist the de-aggregation process while scanning for delimiters.

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2.3.3. Two-Level Aggregation (TLA)

Using A-MSDU and A-MPDU together called two-level frame aggregation. Figure 2.5 illustrates the concept of two-level frame aggregation. In first level, when a MSDU coming down from the upper layer, it first buffered in the A-MSDU provisional storage area and check if it satisfy the A-MSDU aggregation condition explained in the previous related subsection. If a data unit satisfied the aggregation condition, it can be compacted in to an A-MSDU. If condition is not satisfy such as TID is different, all these aberrant frames can move to the next level where they will be packed together with any A-MSDUs derived from the first level or other single MSDUs by using A-MPDU aggregation.

A two-level frame aggregation comprises a blend of A-MSDU and A-MPDU over two stages.

In Figure 2.5 we illustrate how this new scheme can be achieved. The basic operation is explained as follows: In the first stage, if any MSDUs that are buffered in the A-MSDU Figure 2.4 Aggregated-Mac Protocol Data Unit

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provisional storage area justify the A-MSDU constraints explained in the previous related subsection, these data units can be compacted into a single A-MSDU. If the TIDs are different, all these aberrant frames can move to the second stage where they will be packed together with any A-MSDUs derived from the first stage or other single MSDUs by using A-MPDU aggregation. However, it must be mentioned that given that the maximum MPDU length for an A-MPDU data frame is limited to 4095 bytes, then A-MSDUs or MSDUs with lengths larger than this threshold cannot be transmitted. Conjointly, any fragments from an A-MSDU or MSDUs also cannot be included in an A-MPDU. In the following section, we evaluate how this synthesis is more efficient in most of the cases than A-MPDU and A-MSDU aggregation operating alone.

2.3.4. Block ACK Mechanism

The Block ACK mechanism is proposed in IEEE 802.11e in [7]. It improves the channel efficiency by saving the time wasted from ACK transmission. The station can transmit several

Figure 2.5Two-Level Aggregation

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PPDU separated by SIFS and acknowledged by a single aggregated ACK frame, i.e. the Block ACK (BA) frame. The BA acknowledges the received frame with a bitmap field which

contains the information about the reception of corresponding MPDUs. The BA is reply after receives a block ACK request (BAR) message. Both the block ACK request and block ACK frame are transmitted at the same transmission rate that is used for the corresponding data frame transmission. The A-MPDU aggregation should work with Block ACK mechanism in order to transmit the PPDU frames burst.

Figure 2.6 Illustration of Block ACK operation.

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