4.4 WIRELESS LANS

• In infrastructure mode, each client is associated with an AP (Access Point) that is in turn connected to the other network. The client sends and receives its packets via the AP.
• Sev eral access points may be connected together, typically by a wired network called a distribution system, to form an extended 802.11 network. In this case, clients can send frames to other clients via their APs.
• The other mode, shown in Fig. 4-23(b), is an ad hoc network. This mode is a collection of computers that are associated so that they can directly send frames to each other. There is no access point.
• If the signal is clear, the highest rate can be used. This adjustment is called rate adaptation.
• The spreading sequence used by 802.11b is called a Barker sequence. It has the property that its autocorrelation is low except when the sequences are aligned
• The first transmission method we shall look at is 802.11b. It is a spread-spectrum method that supports rates of 1, 2, 5.5, and 11 Mbps, though in practice the operating rate is nearly always 11 Mbps.
• To send at 2 Mbps, it is used with QPSK modulation to send 2 bits per 11 chips. The higher rates are different. These rates use a technique called CCK (Complementary Code Keying) to construct codes instead of the Barker sequence.
• Next, we come to 802.11a, which supports rates up to 54 Mbps in the 5-GHz ISM band. You might have expected that 802.11a to come before 802.11b, but that was not the case
• The 802.11a method is based on OFDM (Orthogonal Frequency Division Multiplexing) because OFDM uses the spectrum efficiently and resists wireless signal degradations such as multipath.
• Not content to stop there, the IEEE committee began work on a high-throughput physical layer called 802.11n. It was ratified in 2009
• Fortunately, in May 2002, the FCC dropped its long-standing rule requiring all wireless communications equipment operating in the ISM bands in the U.S. to use spread spectrum, so it got to work on 802.11g, which was approved by IEEE in 2003.
• The signals of the streams interfere at the receiver, but they can be separated using MIMO (Multiple Input Multiple Output) communications techniques.
• Another recent 802.11 standard is 802.11ad. This one operates in the 60 GHz band (57–71 GHz), which means the radio wav es are very short: only 5 mm long.
• An improvement to this standard, increasing the bandwidth by a factor of four, is the 802.11ay standard
• Now we come to 802.11ax, sometimes referred to high-efficiency wireless. The consumer-friendly name for the standard is WiFi 6
• You might get 1 Gbps, though. In 802.11ax OFDMA, a central scheduler allocates fixed-length resource units to each of the transmitting stations, thus reducing contention in dense deployments. 802.11ax also provides support for spatial spectrum reuse, through a technique called coloring,
• The standard also supports smarter scheduling through a feature called target wake time, which allows a router to put devices in the home on transmission schedules to minimize collisions. T
• Instead, 802.11 tries to avoid collisions with a protocol called CSMA/CA (CSMA with Collision Avoidance). This protocol is conceptually similar to Ethernet’s CSMA/CD
• Second, acknowledgements are used to infer collisions because collisions cannot be detected. This mode of operation is called DCF (Distributed Coordination Function) because each station acts independently, without any kind of central control.
• The standard also includes an optional additional mode of operation called PCF (Point Coordination Function) in which the access point controls all activity in its cell, just like a cellular base station.
• With virtual sensing, each station keeps a logical record of when the channel is in use by tracking the NAV (Network Allocation Vector).
• Alternatively, 802.11 allows frames to be split into smaller pieces, called fragments, each with its own checksum. The fragment size is not fixed by the standard, but is a parameter that can be adjusted by the AP.
• The basic mechanism for saving power builds on beacon frames.
• Clients can set a power-management bit in frames that they send to the AP to tell it that they are entering power-save mode
• Another power-saving mechanism, called APSD (Automatic Power Save Delivery), was added to 802.11 in 2005.
• Five intervals are depicted in Fig. 4-28. The interval between regular data frames is called the DIFS (DCF InterFrame Spacing)
• The shortest interval is SIFS (Short InterFrame Spacing).
• The two AIFS (Arbitration InterFrame Space) intervals show examples of two different priority levels.
• The last time interval, EIFS (Extended InterFrame Spacing), is used only by a station that has just received a bad or unknown frame, to report the problem. The idea is that since the receiver may have no idea of what is going on, it should wait a while to avoid interfering with an ongoing dialog between two stations
• while to avoid interfering with an ongoing dialog between two stations. A further part of the quality of service extensions is the notion of a TXOP or transmission opportunity
• It is a stiff penalty for the fast sender. This issue is known as the rate anomaly
• First comes the Frame control field, which is made up of 11 subfields. The first of these is the Protocol version, set to 00
• The To DS and Fr om DS bits are set to indicate whether the frame is going to or coming from the network connected to the APs, which is called the distribution system.
• The More fragments bit means that more fragments will follow. The Retry bit marks a retransmission of a frame sent earlier.
• The Power management bit indicates that the sender is going into power-save mode.
• The More data bit indicates that the sender has additional frames for the receiver.
• The Protected Frame bit indicates that the frame body has been encrypted for security
• The Order bit tells the receiver that the higher layer expects the sequence of frames to arrive strictly in order.
• The second field of the data frame, the Duration field, tells how long the frame and its acknowledgement will occupy the channel, measured in microseconds
• The Sequence field numbers frames so that duplicates can be detected. Of the 16 bits available, 4 identify the fragment and 12 carry a number that is advanced with each new transmission.