A MAC Throughput in the Wireless LAN

Over the past few years, mobile networks have emerged as a promising approach for future mobile IP applications. With limited frequency resources, designing an effective MAC (Medium Access Control) protocol is a hot challenge. IEEE 802.11b/g/a/n networks are currently the most popular wireless LAN products on the market [1]. The conventional IEEE 802.11b and 802.11g/a specification provide up to 11 and 54 Mbps data rates, respectively. However, the MAC protocol that they are based upon is the same and employs a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) protocol with binary exponential back-off. IEEE 802.11 DCF (Distributed Coordination Function) is the de facto MAC protocol for wireless LAN because of its simplicity and robustness [2,3]. Therefore, considerable research efforts have been put on the investigation of the DCF performance over wireless LAN [2]. With the successful deployment of IEEE 802.11a/b/g wireless LAN and the increasing demand for real-time applications over wireless, the IEEE 802.11n Working Group standardized a new MAC and PHY (Physical) layer specification to increase the bit rate to be up to 600 Mbps [3]. The throughput performance at the MAC layer can be improved by aggregating several frames before transmission [3]. Frame aggregation not only reduces the transmission time for preamble and frame headers, but also reduces the waiting time during CSMA/CA random backoff period for successive frame transmissions. The frame aggregation can be performed within different sub-layers. In 802.11n, frame aggregation can be performed either by A-MPDU (MAC Protocol Data Unit Aggregation) or A-MSDU (MAC Service Data Unit Aggregation). Although frame aggregation can increase the throughput at the MAC layer under ideal channel conditions, a larger aggregated frame will cause each station to wait longer before its next chance for channel access. Under errorprone channels, corrupting a large aggregated frame may waste a long period of channel time and lead to a lower MAC efficiency [4]. On the other hand, wireless LAN mobile stations that are defined as the stations that access the LAN while in motion are considered in this chapter. The previous paper analyzed the IEEE 802.11b/g/n MAC performance for wireless LAN with fixed stations, not for wireless LAN with mobile stations [5, 6, 7, 8, 9, 10]. On the contrary, Xi Yong [11] and Ha Cheol Lee [12] analyzed the MAC performance for IEEE 802.11 wireless LAN with mobile stations, but considered only IEEE 802.11 and 802.11g/a wireless LAN specification. So, this chapter summarizes all the reference papers and analyzes the IEEE 802.11b/g/a/n MAC performance for wireless LAN with fixed and mobile stations. In other words, we will present the analytical evaluation of saturation


PHY related amendments
Although not interoperable, the DSSS and FHSS PHY initially seemed to have equal chances in the market. The FHSS PHY even had a duplicate in the HomeRF group that aimed at integrated voice and data services. This used plain 802.11 with FHSS for data transfer, complemented with a protocol for voice that was very similar to the Digital Enhanced Cordless Telecommunications standard. Neither HomeRF nor 802.11 saw FHSS extensions, although plans for a second-generation HomeRF existed that targeted at 10 Mb/s. In contrast, the high-rate project 802.11b was started in December 1997 and boosted the data rates of the DSSS PHY to 11 Mb/s. This caused 802.11b to ultimately supersede FHSS, including HomeRF, in the market. Figure 1 provides an overview of the 802.11 PHY amendments and their dependencies [2].  DSSS-OFDM: This is a new physical layer that uses a hybrid combination of DSSS and OFDM. The packet physical header is transmitted using DSSS, while the packet payload is transmitted using OFDM. The scope of this hybrid approach is to cover interoperability aspects, as explained later. From the above four physical layers, the first two are mandatory; every IEEE 802.11g device must support them. The other two are optional. Column 2 of Table 3 summarizes the supported data rates for the different physical layers of the IEEE 802.11g specification.

MAC related amendments
A key element to the 802.11 success is its simple MAC operation based on the DCF protocol. This scheme has proven to be robust and adaptive to varying conditions, able to cover most needs sufficiently well. Following the trends visible from the wired Ethernet, 802.11's success is mainly based on overprovisioning of its capacity. The available data rate was sufficient to cover the original best effort applications, so complex resource scheduling and management algorithms were unnecessary.However, this may change in the future. Because of the growing popularity of 802.11, WLANs are expected to reach their capacity limits. Moreover, applications like voice and video streaming pose different demands for quality of service. Therefore, traffic differentiation and network management might become inevitable. In the following we explain 802.11 MAC related extensions of the amendments introduced in the previous section and those shown in Fig. 2

Wireless LAN access network
This section shows infrastructure-based and ad hoc-based operation of wireless access architecture in the 802.11b/a/g/n-based mobile LAN. The protocols of the various layers are called the protocol stack. The TCP/IP protocol stack consists of five layers: the physical, data link, network, transport and application layers. This section is focused on physical layer and data link layer which consists of MAC and LLC (Logical Link Control) sub-layers. An ad hoc network might be formed when people with laptops get together and want to exchange data in the absence of a centralized AP (Access Point). Wireless LAN topology is ad hoc-based or infrastructure-based as shown in Fig. 3   IEEE 802.11 MAC protocol supports the DCF and the PCF . The DCF uses the CSMA/CA mechanism for contention-based access, while the PCF provides contention-free access. The two modes are used alternately in time. IEEE 802.11 MAC protocol defines five timing intervals. Two of them are the SIFS and the slot time that are determined by the physical layer. The other three intervals are the PIFS (Priority InterFrame Space), DIFS and EIFS (Extended InterFrame Space) that are defined based on the above two intervals. But the PCF is restricted to infrastructure network configurations. Therefore, the DCF is widely assumed under the consideration of ad hoc-based wireless LAN. Fig. 6 shows two access schemes. IEEE 802.11 DCF stations access the channel via a basic access method or the four-way handshaking access method with an additional RTS/CTS message exchange. In the basic access method, the CSMA mechanism is applied. Stations wait for the channel to be idle for a DIFS period of time and then execute backoff for data transmission. Stations choose a random number between 0 and CW (Contention Window)-1 with equal probability as a backoff timer. When the backoff timer reaches zero, the data frame is transmitted. The receiver replies an ACK message upon successfully receiving a data packet. In the four-way handshaking access method, when the backoff timer of station reaches zero, the station first transmits a RTS frame. Upon receiving the RTS frame, the receiver replies with a CTS frame after a SIFS period. Once the RTS/CTS is exchanged successfully, the sender then transmits its data frame. The RTS and CTS frames carry a duration field, information of time interval to transmit the www.intechopen.com packet. Any station receiving RTS or CTS frames can read the duration field information. That information is then used to update a NAV (Network Allocation Vector) value that indicates to each station the amount of time that remains before the channel will become idle. Therefore, a station detecting the RTS and CTS frames suitably delays further transmission, and thus avoids collision. The NAV is thus referred to as a virtual carrier sensing mechanism. The main purpose of the RTS/CTS handshaking is to resolve the socalled hidden node problem.

IEEE 802.11n PHY/MAC layer
The key requirement that drove most of the development in 802.11n is the capability of at least 100 Mb/s MAC throughput. Considering that the typical throughput of 802.11a/g is 25 Mb/s (with a 54 Mb/s PHY data rate), this requirement dictated at least a fourfold increase in throughput. Defining the requirement as MAC throughput rather than PHY data rate forced developers to consider the difficult problem of improving MAC efficiency. The inability to achieve a throughput of 100 Mb/s necessitated substantial improvements in MAC efficiency when designing the 802.11n MAC. Two basic concepts are employed in 802.11n to increase the PHY data rates: MIMO and 40 MHz bandwidth channels. Increasing from a single spatial stream and one transmit antenna to four spatial streams and four antennas increases the data rate by a factor of four. The term spatial stream is defined in the 802.11n standard as one of several bitstreams that are transmitted over multiple spatial dimensions created by the use of multiple antennas at both ends of a communications link. However, due to the inherent increased cost associated with increasing the number of antennas, modes that use three and four spatial streams are optional. And to allow for handheld devices, the two spatial streams mode is only mandatory in an AP. 40 MHz bandwidth channel operation is optional in the standard due to concerns regarding interoperability between 20 and 40 MHz bandwidth devices, the permissibility of the use of 40 MHz bandwidth channels in the various regulatory domains, and spectral efficiency. However, the 40 MHz bandwidth channel mode has become a core feature due to the low cost of doubling the data rate from doubling the bandwidth. Almost all 802.11n products on the market feature a 40 MHz mode of operation. Other minor modifications were also made to the 802.11a/g waveform to www.intechopen.com increase the data rate. The highest encoder rate in 802.11a/g is 3/4. This was increased to 5/6 in 802.11n for an 11 percent increase in data rate. With the improvement in RF (Radio Frequency) technology, it was demonstrated that two extra frequency subcarriers could be squeezed into the guard band on each side of the spectral waveform and still meet the transmit spectral mask. This increased the data rate by 8 percent over 802.11a/g. Lastly, the waveform in 802.11a/g and mandatory operation in 802.11n contains an 800 ns guard interval between each OFDM symbol. An optional mode was defined with a 400 ns guard interval between each OFDM symbol to increase the data rates by another 11 percent. Another functional requirement of 802.11n was interoperability between 802.11a/g and 802.11n. The TG decided to meet this requirement in the physical layer by defining a waveform that was backward compatible with 802. 11a/g waveform. This includes the 802.11a/g short training field, long training field, and signal field. This allows 802.11a/g devices to detect the 802.11n mixed format packet and decode the signal field. Even though the 802.11a/g devices will not be able to decode the remainder of the 802.11n packet, they will be able to properly defer their own transmission based on the length specified in the signal field. The remainder of the 802.11n Mixed format waveform includes a second short training field, additional long training fields, and additional signal fields followed by the data. These new fields are required for MIMO training and signaling of the many new modes of operation. To ensure backward compatibility between 20 MHz bandwidth channel devices (including 802.11n and 802.11a/g) and 40 MHz bandwidth channel devices, the preamble of the 40 MHz waveform is identical to the 20 MHz waveform and is repeated on the two adjacent 20 MHz bandwidth channels that form the 40 MHz bandwidth channel. This allows 20 MHz bandwidth devices on either adjacent channel to decode the signal field and properly defer transmission. The preamble in 802.11a has a length of 20 μs; with the additional training and signal fields, the 802.11n mixed format packet has a preamble with a length of 36 μs for one spatial stream up to 48 μs for four spatial streams. Unfortunately, MIMO training and backward compatibility increases the overhead, which reduces efficiency. In environments free from legacy devices (termed greenfield) backward compatibility is not required.
By eliminating the components of the preamble that support backward compatibility, the greenfield format preamble is 12 μs shorter than the mixed format preamble. This difference in efficiency becomes more pronounced when the packet length is short, as in the case of VoIP traffic. Therefore, the use of the greenfield format is permitted even in the presence of legacy devices with proper MAC protection, although the overhead of the MAC protection may reduce the efficiency gained from the PHY. Range was considered as a performance metric in the PAR and comparison criteria. To increase the data rate at a given range requires enhanced robustness of the wireless link. 802.11n defines implicit and explicit TxBF   www.intechopen.com The possible timing sequences for A-MPDU and A-MSDU in the uni-directional transfer case are shown in Fig. 11. If RTS/CTS (Request To Send/Clear To Send) is used, the current transmission sequence of RTS-CTS-DATA (Data frame)-ACK (Acknowledgement) only allows the sender to transmit a single data frame. The DATA frame represents either an A-MPDU or an A-MSDU frame. The system time can be broken down into virtual time slots where each slot is the time interval between two consecutive countdown of backoff timers by non-transmitting stations. The 802.11n also specifies a bi-directional data transfer method. Fig. 11. IEEE 802.11n Uni-directional RTS/CTS Access Scheme [14][9] In the bi-directional data transfer method, the receiver may request a reverse data transmission in the CTS control frame. The sender can then grant a certain medium time for the receiver on the reverse link. The transmission sequence will then become RTS-CTS-DATAf-DATAr-ACK. This facilitates the transmission of some small feedback packets from the receiver and may also enhance the performance of TCP (Transmission Control Protocol) which requires the transmission of TCP ACK segments. BACK (Block Acknowledgement) can be used to replace the previous ACK frame. The BACK can use a bit map to efficiently acknowledge each individual sub-frame within the aggregated frame. For the bi-directional data transfer, the reverse DATAr frame can contain a BACK to acknowledge the previous DATAf frame. In this subsection, we briefly mention the most important MAC enhancements with a more detailed explanation on frame aggregation, which maximizes throughput and efficiency. Aggregate exchange sequences are made possible with a protocol that acknowledges multiple MPDUs with a single block ACK in response to a block acknowledgment request (BAR). Another key enhancement that the 802.11n specifies is the bidirectional data transfer method over a single TXOP, known as reverse direction. This feature permits the transportation of data frames, even aggregates, in both directions in one TXOP. Until now, when the sender STA is allocated with a TXOP, it informs surrounding STAs about how long the wireless medium will be engaged. However, this approximation of channel use is not always accurate, and often the transmission ends sooner. As a result, contended STAs assume that the channel is still occupied when this is not the case. With reverse direction, the initial receiver STA is allowed to send any packets available that are addressed to the sender for the remaining TXOP time. This fits especially well with TCP because it allows a TCP link to piggyback TCP ACK collection onto TCP data transmission. The long-NAV (Network Allocation Vector) is another enhancement that improves scheduling, given that a station that holds a TXOP may set a longer NAV value intended to protect multiple PPDUs. Another mandatory feature is PCO (Phased Coexistence www.intechopen.com Operation) which protects stations using either 20 MHz or 40 MHz channel spectrum at the same time. Finally, the RIFS (Reduced IFS) is proposed to allow a time interval of 2 μs between multiple PPDUs, which is much shorter than SIFS as defined in the legacy standards.

Fig. 12. One-level frame aggregation: a) A-MSDU; b) A-MPDU [10]
A-MSDU -The principle of the A-MSDU (or MSDU aggregation) is to allow multiple MSDUs to be sent to the same receiver concatenated in a single MPDU. This definitely improves the efficiency of the MAC layer, specifically when there are many small MSDUs, such as TCP acknowledgments. This supporting function for A-MSDU within the 802.11n is mandatory at the receiver. For an A-MSDU to be formed, a layer at the top of the MAC receives and buffers multiple packets (MSDUs). The A-MSDU is completed 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. Its maximum length can be either 3839 or 7935 bytes; this is 256 bytes shorter than the maximum PHY PSDU length (4095 or 8191 bytes, respectively), as predicted space is allocated for future status or control information. The size can be found in the HT capabilities element that is advertised from an HT STA in order to declare its HT status. The maximal delay can be set to an independent value for every AC but is usually set to 1 μs for all ACs. There are also certain constraints when constructing an A-MSDU:  All MSDUs must have the same TID value  Lifetime of the A-MSDU should correspond to the maximum lifetime of its constituent elements  The DA (Destination Address) and SA (Sender Address) parameter values in the subframe header must match to the same RA (Receiver Address) and TA (Transmitter Address) in the MAC header.
Thus, broadcasting or multicasting is not allowed. Figure 12a describes a simple structure of a carrier MPDU that contains an A-MSDU. Each subframe consists of a subframe header followed by the packet that arrived from the LLC and 0 ~ 3 bytes of padding. The padding size depends on the rule that each subframe, except for the last one, should be a multiple of four bytes, so the end receiver can approximate the beginning of the next subframe. A major drawback of using A-MSDU is under error-prone channels. By compressing all MSDUs into a single MPDU with a single sequence number, for any subframes that are corrupted, the entire A-MSDU must be retransmitted. Additional frame structures or optimum frame sizes have been proposed to improve performance under noisy channels.

A-MPDU -The concept of A-MPDU aggregation is to join multiple MPDU subframes with a single leading PHY header. A key difference from A-MSDU aggregation is that A-MPDU
functions after the MAC header encapsulation process. Consequently, the A-MSDU restriction of aggregating frames with matching TIDs is not a factor with A-MPDUs. However, all the MPDUs within an A-MPDU must be addressed to the same receiver address. Also, there is no waiting/holding time to form an A-MPDU so the number of MPDUs to be aggregated totally depends on the number of packets already in the transmission queue. The maximum length that an A-MPDU can obtain -in other words the maximum length of the PSDU that may be received -is 65,535 bytes, but it can be further constrained according to the capabilities of the STA found in the HT capabilities element. The utmost number of subframes 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. Note that these two bytes are required to acknowledge up to 16 fragments but because A-MPDU does not allow fragmentation, these extra bits are excessive. As a result, a new variant has been implemented, known as compressed block ACK with a bitmap field of eight bytes long. Finally, the size of each subframe is limited to 4095 bytes as the length of a PPDU cannot exceed the 5.46-ms time limit; this can be derived from the maximum length divided by the lowest PHY rate, which is 6 Mb/s and is the highest duration of an MPDU in 802.11a. The basic structure is shown in Fig. 12b. A set of fields, known as delimiters are inserted before each MPDU and padding bits varied from 0 ~ 3 bytes are added at the tail. The basic operation of the delimiter header is to define the MPDU position and length inside the aggregated frame. It is noted that the CRC (Cyclic Redundancy Check) 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 subframe delineation at the receiver side. In other words, the MPDU delimiters and PAD bytes determine the structure of the A-MPDU. After the AMPDU 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 subframe till 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.

IEEE 802.11ac/ad PHY/MAC layer
The WiGig (Wireless Gigabit) Alliance was formed to meet this need by establishing a unified specification for wireless communication at multi-gigabit speeds; this specification is designed to drive a global ecosystem of interoperable products. The WiGig MAC and PHY Specification enables data rates up to 7 Gbps, more than 10 times the speed of the fastest Wi-Fi networks based on IEEE 802.11n. It operates in the unlicensed 60 GHz frequency band, which has much more spectrum available than the 2.4 GHz and 5 GHz bands used by existing Wi-Fi products. This allows wider channels that support faster transmission speeds. The WiGig specification is based on the existing IEEE 802.11 standard, which is at the core of hundreds of millions of Wi-Fi products deployed worldwide. The specification includes native support for Wi-Fi over 60 GHz; new devices with tri-band radios will be able to seamlessly integrate into existing 2.4 GHz and 5 GHz Wi-Fi networks. The specification enables a broad range of advanced uses,  The WiGig Alliance is also defining PALs (Protocol Adaptation Layers) that support specific data and display standards over 60 GHz. PALs allow wireless implementations of these standard interfaces that run directly on the WiGig MAC and PHY, as shown in Figure 14, and can be implemented in hardware. The initial PALs are audio-visual (A/V), which defines support for HDMI and DisplayPort, and input-output (I/O), which defines support for USB and PCIe.   is the ratio of direct-todiffuse signal power on the th i sub-channel.  has 0 in a pure Rayleigh fading channel and ranges from 0 to 10 in a composite Rayleigh/Ricean fading channel. bi  is the ratio of received average energy per bit-to-noise power spectral density on the th i sub-channel. The overall p is the average of the probability of bit error on each of the N OFDM sub-channels.
Note that for either no channel fading or for all sub-channels experiencing the same fading (that is, i    and for all is the ratio of received average energy per bit-to-noise power spectral density ,  is the ratio of direct-to-diffuse signal power. Now, using equation (6) in equation (3) or (4) and taking the result into equation (2), we obtain the performance of 64 QAM with HDD over Ricean fading channels. For basic access mechanism, a data packet including the PHY header and the MAC header needs retransmission if any one bit of them is corrupted. We define a variable P c which is the probability that a backoff occurs in a station due to bit errors in frames. We further assume that bit errors randomly appear in the frames. So frame error rate is represented by (7).
CSMA/CA is also used as the MAC scheme in IEEE 802.11n wireless LAN, and it has basic and RTS/CTS access scheme. Although there is a successful RTS/CTS transmission in the time slot, a frame have to be retransmitted when there is a bit error in a payload. For convenience, we define a variable P e which is the probability that a backoff occurs in a station due to bit errors in frames. We further assume that bit errors randomly appear in the frames and A-MSDU scheme is used. So frame error rate is represented by (8).
Where L is the aggregated MAC frame's size. For a convolutional code with a coding rate k c /n c , the bit error rate, denoted as q, can be approximated by   Where d free is the maximum free distance of the convolutional code and q b is the probability of a bit error for the M-QAM.

Frame error rate of mobile wireless channel
Mobile wireless channel is assumed to be flat fading Rayleigh channel with Jake spectrum. The channel is in fading states or inter-fading states by evaluating a certain threshold value of received signal power level. If and only if the whole frame is in inter-fading state, there is the successful frame transmission. If any part of frame is in fading duration, the frame is received in error. In the fading channel fading margin is considered and defined as ρ = R req /R rms , Where R req is the required received power level and R rms is the mean received power. Generally, the fading duration and inter-fading duration can be taken to be exponentially distributed for ρ<-10dB. With the above assumptions, let Tpi be the frame duration, then the frame error rate is given by (12).

Analysis of frame error rate under the Rayleigh/Rician fading channel with fixed stations
In the Fig. 15, P c (P, b  , K) shows PER(Packet Error Rate) due to b  , the ratio of received average energy per bit-to-noise power spectral density. K means Rician factor and P means payload size. and as expected, the PER (Frame Error Rate) performance improves with K and the smaller payload size is, the better performance is.
In the Fig. 16, q s (ρ,K) shows SER(Symbol Error Rate) and P e (K,ρ,n s ,P) shows PER(Packet Error Rate). K means Rician factor and as expected, the SER performance improves with K. Also, the PER performance improves with K and the smaller subframe' payload size is, the better performance is.

Analysis of frame error rate under the flat fading Rayleigh channel with mobile stations
In the Fig. 17(a) ~ Fig. 17(c), the symbol fer (,  , P) shows frame error rate of IEEE 802.11a/g. In the Fig. 17(d), the symbol fer (ns, ,  , P) shows frame error rate of IEEE 802.11n with the horizontal parameter of subframe' payload size. In the Fig. 17(e), the symbol fer (, ns,  , P) shows frame error rate of IEEE 802.11n using the number of subframes as the horizontal parameter. It is generally identified that the higher mobile speed is, the higher frame error rate is. In case of payload size, the same result mentioned above is also acquired.

DCF throughput analysis
The back-off procedure of the DCF protocol is modeled as a discrete-time, two-dimensional Markov chain. Fig. 18   We will present the analytical evaluation of saturation throughput with bit errors appearing in the transmitting channel. The number of stations n is assumed to be fixed and each station always has packets for transmission. In other words, we operate in saturation conditions, the transmission queue of each station is assumed to be always nonempty.
www.intechopen.com   [20] Let S be the normalized system throughput, defined as the fraction of time in which the channel is used to successfully transmit payload bits. tr P is the probability that there is at

IEEE 802.11n DCF throughput
The saturation throughput can be calculated as follows [9]. (1 ) (1 ) (1 ) (1 ) ( where p E is the number of payload information bits successfully transmitted in a virtual time slot, and t E is the expected length of a virtual time slot. e P is the error probability on condition that there is a successful RTS/CTS transmission in the time slot. idle P is the probability of an idle slot. s P is the probability for a non-collided transmission. err P is the transmission failure probability due to error (no collisions but having transmission errors). is the probability for a successful transmission without collisions and transmission errors. idle T , c T and succ T are the idle, collision and successful virtual time slot's length. e T is the virtual time slot length for an error transmission sequence. p L is the aggregated frame's payload length. In the RTS/CTS scheme, we obtain, OFDM are used. Three common packet sizes of 60 bytes(TCP ACK), 576 bytes(typical size for web browsing) and 1,500 bytes(the maximum size for Ethernet) are considered.
In the Fig.19, S( P , b  ,  , n ,  ) shows the saturation throughput over error-prone channel due to number of stations( n ) for common packet sizes on the condition that packet transmission probability( ), average energy per bit-to-noise power spectral density( b  ) and the ratio of direct-to-diffuse signal power(  ) are fixed. The larger payload size be, the higher saturation throughput be for error-prone channel. It is identified that there are optimum number of stations corresponding to maximum saturation throughput. The DCF saturation throughput of 802.11a is the highest for error-prone channel. Because system needs 25 dB minimum signal to noise ratio at the data rate of 54 Mbps, this paper used 23 dB and 28 dB as the signal to noise ratio.
We evaluated DCF throughput performance of IEEE 802.11n wireless LAN based on MIMO OFDM with the system parameter defined in Table 12. MCS (Modulation and Coding Scheme) index 15 is used to generate the physical data rate of 130 Mbps with 20 MHz bandwidth and long guard interval. And the two common packets passed down to the MAC layer are 576 bytes (typical size for web browsing) and 1,500 bytes (the maximum size for Ethernet) in length. S(K, ρ, n s , P, n, ) shows DCF throughput performance over the Rician fading channel. Fig. 20(a) shows DCF throughput on the condition that the subframe' payload size is 576 bytes, the number of stations is 10 and the packet transmission probability is 0.05. In that Fig. 20(a), it is identified most of the the ratio of received average energy per bit-to-noise power spectral density that the larger the Rician factor and the number of subframe are, the better the DCF throughput performance is. Fig. 20(b) has the same condition as Fig 20(a) except the packet transmission probability 0.05 replaced by 0.2. It is identified that if the packet transmission probability is lower, the DCF throughput performance is improved because packet collision probability is decreased. Fig 20(c) has the same condition as Fig. 20(a) except the number of stations 10 replaced by 30. Fig 20(d) is compared to Fig 20(a) about the DCF throughput performance for the subframe size, 576 bytes and 1,500 bytes. It is identified that the larger the subframe' payload size is, the better the DCF throughput performance is.
(a) P=576 bytes, n=10, τ=0.05 Ethernet) in length. In the IEEE 802.11n-based mobile LAN, the number of packets aggregated in one MAC frame varies from 1 to 100, which leads to an aggregated frame's payload length ( p L ) from 60, 576 and 1,500 bytes to 6, 57.6 and 150 Kbytes. In the Fig. 21(a) ~ Fig. 21(c), the symbol S ( P , ,  , n ,  ) shows the saturation throughput over error-prone channel according to the number of stations( n ) for common packet sizes ( P ) on the condition that packet transmission probability (  ), mobile velocity ( ) and fading margin () are fixed. In the Fig. 21(d) and Fig. 21(e), the symbol S ( ns , P, ,  , n ,  ) and S (P, ns , ,  , n ,  ) respectively shows the saturation throughput over error-prone channel according to the number of stations ( n ) and the typical number of packets aggregated in one MAC frame ( ns ) for two subframe length on the condition that packet transmission probability (τ), mobile velocity ( ) and fading margin() are fixed. For example, in the Fig. 21(a), if the number of stations is 7, packet transmission probability is 0.05, packet length is 1,500 and fading margin is Mbps. Also, Fig. 21(a ~ d) shows that the longer frame (or subframe) length is, the higher throughput is. And, for the same frame (or subframe) length, the higher speed is, the lower throughput is. As the results of evaluation, we also know that there is optimum number of stations to maximize saturation throughput under the error-prone channel. Specially, in Fig  21(e), the number of subframes is considered and it is identified that there is optimum number of subframes to maximize saturation throughput under the error-prone channel. . DCF throughput of IEEE 802.11a/g/n mobile LAN [12,22] In conclusion, we obtained the fact that there exist an optimal number of stations (or subframes) to maximize the saturation throughput under the error-prone channel. Also, we can identify that the larger payload (or subpayload) size be, the higher saturation throughput be. And if a mobile velocity of station is increased, the throughput is decreased a little. Out of the three different physical layers defined in this analysis with the maximum transmission rate of 54 Mbps, which are 802.11g ERP-OFDM, 802.11g DSSS-OFDM and 802.11a OFDM, The DCF saturation throughput of 802.11a OFDM is the highest at all the channel environments. In the case of 802.11n, because A-MSDU scheme is applied, it is identified that MAC efficiency of IEEE 802.11n is the best out of all four schemes.