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The IEEE 802.11x set of standards, also known as Wi-Fi Wireless Fidelity, a trademark of Wi-Fi Alliance, are the most known wireless local area network (WLAN) protocols. The initial version of IEEE 802.11 was released in 1997 and clarified in 1999. The specification defined three distinct physical layers (PHY), the Frequency Hopping Spread Spectrum (FHSS), the Direct Sequence Spread Spectrum (DSSS) and the Infrared (IR). The first two layers are designed to operate over the Industrial, Scientific and Medical band (ISM) of 2.4 MHz, with a bandwidth of 20 MHz, and data bitrates of 1 and 2 Mbps. The infrared is now obsolete.
IEEE released in 1999 the 802.11b and the 802.11a specifications. which they offered improver physical layers over the 2.4 ISM and the 5 GHz bands, respectively. The IEEE 802.11b offers improved PHY over the 2.4 MHz ISM band and theoretical throughputs 5.5 and 11 Mbps using DSSS and the IEEE 802.11a gives 6/9/12/18/24/36/48/54 Mbps using Orthogonal Frequency Division Multiplexing (OFDM) over the Unlicensed National Information Infrastructure (U-NII) band. Both continue to use 20 MHz bandwidth, as the legacy 802.11. In 1999 IEEE released 802.11g, a major improvement of IEEE 802.11b, which uses the same band of 2.4MHz, the same bandwidth of 20 MHz and combines OFDM and Complementary Code Keying (CCK) for backward compatibility.
Fig. 1. Sub-layers and IEEE 802.11 variants
Essentially the IEEE 802.11x set belongs to the wider IEEE 802.x family, which includes wireless and wired protocols like the IEEE 802.15 Personal Area Networks (PAN), or the IEEE 802.3 Ethernet. The IEEE 802.11x family defines only the two lower layers, the Physical and the Data Link layer (Fig. 1). The last comprises of the Logical Link Control (LLC) sub-layer, which is common in several 802.x protocols, and the Media Access Control, which is common among the several 802.11x protocols.
Fig. 2. An Extended Service Set with Ethernet DS.
The IEEE 802.11 standard defines two distinct network topologies, the Basic Service Set (BSS) and the Independent Basic Service Set (IBSS). The first implementation, also known as Infrastructure mode, comprises of several client Stations (STA) associated with a single Access Point (AP) and serves all the uplink (STA to AP) and downlink (AP to STA) traffic via the AP. In the second implementation, also known as Ad-hoc mode, every station becomes independent and exchanges traffic with his neighboring stations. Both WLANs are identified by a unique name, called Service Set Identifier (SSID), which is transmitted as a 6-byte (48-bit) identifier in beacon frames. When two or more APs, which form two or more BSSs, are connected through Distribution System (DS) like Ethernet, they create an Extended Service Set (ESS), as shown in Fig. 2. The Extended Service Set ID, which is a 32-character identifier (in ASCII), is transmitted in beacon frames from all the APs that form the ESS.
The IEEE 802.11 MAC sub-layer specification defines two operation modes, the Distributed Coordination Function (DFC), including the RTS/CTS sub-mode, and the Point Coordination Function (PDF). In the DCF all client stations contend equally to have access to the wireless channel, using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), a distributed random access protocol. In the PCF mode, the coordination function is locate to the AP, and controls the access to the wireless channel with a polling mechanism. The associated STAs respond to AP polls and the coordinator grand them access time, according to their demand, without contention. The PCF mode is often called Contention Free Period (CFP) in contrast with DFC mode, which is Contention Period (CP).
The following chapter discusses about DCF because it is the legacy mode where the Enhanced Distribution Channel Access (EDCA) mode of the IEEE 802.11e specification is based, to provide QoS extensions.
1.2 The Distributed Coordination Function
The DCF mode is based on the CSMA/CA protocol. The function of the CSMA/CA is similar to the CSMA/CD, which is the base of the IEEE 802.3 Ethernet protocol, with a major difference. The wireless transceiver is incapable to operate in full duplex mode, which means that it cannot receive while transmitting, so it functions only as half-duplex. If another transceiver transmits the same time in the range of the receiver, the collision will not be detected by both transceivers. The sender assumes that a collision has happened when it does not receives acknowledge from the receiver. Therefore the protocol tries to avoid the collisions, as much as possible.
The CSMA/CA operates on the MAC layer. When the LLC layer delivers a packet or a MAC Service Data Unit (MSDU), to the MAC layer the station scans the wireless channel (Carries Sensing, CS) for an active broadcast. If the channel is found IDLE for a minimum time period, called DCF Inter Frame Space (DIFS), the station begin transmission and the rest of the stations in range become idle until the sending completes. The idle period is called Net Allocation Vector (NAV) and is calculated from the duration field of the transmitting frame. When the destination station successfully receives the frame is sends an acknowledge (ACK) signal back to the sender, after a fixed period, called Short Frame Inter Frame Space (SIFS), and the transaction is completed. Fig. 3 displays the above procedure.
Fig. 3. The basic DCF access method.
If a sending station finds the medium BUSY, it defers transmission and enters into BACKOFF period, which will be explained later in detail. Consequent transmissions are separated with one of the three inter-frame spaces IFS), according to the priority of the frame exchange. The period between the transmission of DATA and the ACK from the receiver has the highest priority, and it lasts SIFS, which is the shortest period. DIFS is the longest period and has the lowest priority. Therefore stations waiting for DIFS before transmission are unable to interrupt a successful frame exchange between two stations.
The third IFS is called PCF Inter-Frame Space (PIFS), it lies between SIFS and DIFS, and is used by the PCF mode. As mentioned before, the point coordinator function resides on the AP and it polls the individual stations sequentially, with higher priority than the distributed function. The values of the three IFS depend on the specific PHY, which the 802.11x uses, and are multiples of a slot time.
Fig. 4. Inter-Frame Spaces relationships.
The slot time depends of the physical layer attributes, e.g. the propagation delay, the transmitter delay, etc. Fig. 4 depicts the IFS relationships. It is:
The 802.11 transceiver performs two types of carrier sensing. On the first type, the physical layer before transmitting it performs Clear Channel Assessment (CCA) on the wireless channel, and reports the status to the MAC layer. On the second, when the MAC layer receives a frame which is not destined for the specific station, it reads the duration field and defers any transaction for the specific time (NAV). The concept of virtual CCA will be discussed more on the 802.11e protocol.
1.3 The Collision Avoidance Function
When two stations try to transmit after a successful CCA, it will lead to a collision, and both signals will be lost. The CSMA/CA tries to avoid collisions, therefore every station waits an additional time after the DIFS time either the channel is busy or not. Essentially the station defers the transmission by a period called random backoff, which is an integer multiple of the time slot. The value of the period is chosen from a uniform distribution of the interval [0, CW]. The quantity CW is the Contention Window, a constant number equal to the initial default number of time slots for the DCF, which are 31. The CW is randomly selected to avoid two or more stations begin to transmit the moment when their timers expire, which it will guide to a new collision.
All the deferred stations after the DIFS period and the successful selection of a backoff interval start to decrement their timers every time slot by one until be zeroed. It is obvious that the station with the smallest selected value will expire earlier and will transmit first. Then the rest of the stations pause their timers and enter to NAV state, which is accurately calculated. After the completion of the transmission, the stations which hold data, enter to DIFS period again and after they continue to decrement their backoff timers until zeroed or interrupted one more time by an earlier timeout.
The initial CW is set to a minimum CW size, which is CWmin. If two stations contend for the access to the channel and they randomly have chosen the same backoff value, their timers will expire concurrently and their transmissions will collide. Then both stations increase their CW to a new value, given by the equation: 2(CW+1)-1), which is an exponential function and it is often reffered as exponential backoff. If the two counters expire together again, then their CWs will continue to increase until they reach the maximum CW size, which is CWmax. Both minimum and maximum values depend on the physical layer. Fig. 5 depicts the exponential increments for the DSSS physical layer, with the seven consecutive increments of 31/63/127/255/511/1023. In the last two steps CWmax remains the same.
The effectiveness of the exponential backoff stands where a doubled CW decreases the probability for two counters to choose the same backoff value and to have a new collision. The DCF protocol defines the retry limit, which is the maximum number of unsuccessful retransmissions. If that value is exceeded then the frame is dropped. After the successful transmission the CW is restored to CWmin. The same delay of DIFS plus a random backoff is used again after a successful ACK, if there is another frame to be sent from the same transmitter. It is called post backoff and performs a form of fairness on the protocol, because it allows the rest of the paused timers to continue to countdown until zeroed, and the respective station to have access on the channel.
Fig. 5. The exponential backoff
When there are lots of stations in range, which exchange data, the number of collision increases and the frame drop rate. Eventually the protocol increases the length of the contention window up to the maximum limit. This reduces the number of collisions and consequently the frame drop rate. Unfortunately, this has a drawback because every frame is finally transmitted after a number of retries, introducing a significant amount of latency on the transmission, with the long term of reduced throughput and low channel efficiency.
1.4 A sample of the DFC operation
The following figure 6 is an illustrated example of a typical DCF operation. It supposes that there are three stations in range, operating in ad-hoc mode. Station 3 has data to send and perform during DIFS period a CCA. Channel is found IDLE and there are not any active backoff counters, so it starts transmitting immediately. During transmission the channel is found BUSY by the other stations. Actually stations 1 and 2 receive data from the upper layers so after the DIFS period they choose a random backoff value, 10 and 19 respectively from the window [0, CW]. Station 3 has more data to send and after the first ACK it enters to DIFS and a new random backoff, with value 25. It is clear that station's 1 timer will expire first, so station 1 will seize the channel and start transmitting. Station 2 and 3 pause at 9 and 15, respectively, and enter to NAV state during the activity of station 1.
Fig. 6. A sample of the DCF function.
After the completion of station's 1 transmission, all stations delay DIFS, station 1 chooses a new backoff value, which is 9, and station 2 and 3 continues to countdown their timers until station's 2 expire first. There it happen an unavoidable state, a collision between station's 1 transmission and station's 2. Both signal get lost and stations 1 and 2 double their CW and they choose a new value from [0, 63] windows, which are 52 and 43, respectively. After the DIFS periods all counters countdown and station's 3 twice paused counter, reaches zero after 6 time slots. Then station 3 seizes the channel and transmits.
1.5 The RTS/CTS Function
Wireless protocols that use CSMA/CA, encounter a functional problem called the hidden node problem. It happens when station B receives data from station A in range and a third station C, which is out of the range of station A, start to send data to station B. The result is that data from A collide with data from C on receiving station B, and the collision avoidance fails. The problem is reduced by introducing a handshaking function, where the station that has data to transmit, after the DIFS period, sends a Request to Send (RTS) signal (see Fig.7). All stations in range, which are not addressed to receive any data, interpret the frame by extracting the duration field, and hold their data up to the end of the transmission (including the SIFS period and the ACK).
Fig. 7. The RTS/CTS function.
Station B after receiving the RTS signal (plus the SIFS period) it replies the sender with a Clear to Send (CTS) signal. Similarly, all stations that receive CTS hold their data according to duration field and eventually, they do not interrupt the transmission until the entire handshaking (including SIFS intervals and final ACK) will complete. During the RTS/CTS handshake, the SIFS periods defers any transmission from other stations because the DIFS period (where the station perform CCA), is longer that SIFS and the BUSY channel will be detected.