Wireless LANs(IEEE802.11) Architecture

Wireless LANs(IEEE802.11) Architecture

1. UNIT 04 4.1 Wireless LANs(IEEE802.11) Architecture

2. A wireless LAN (or WLAN, for wireless local area network) is one in which a mobile user can connect to a local area network ( LAN ) through a  wireless  (radio) connection The  IEEE   802.11  group of standards specify the technologies for wireless LANs. 802.11 standards use the  Ethernet   protocol  and  CSMA/CA  (carrier sense multiple access with collision avoidance) for path sharing and include an encryption method, the  Wired Equivalent Privacy   algorithm .

3. Architectural Comparison

4. 3. Connection to other network

5. Each computer, mobile, portable or fixed, is referred to as a station in 802.11 [ Wireless Local Area Networks ] . When two or more stations come together to communicate with each other, they form a Basic Service Set (BSS). The minimum BSS consists of two stations. 802.11 LANs use the BSS as the standard building block.

6. A BSS that stands alone and is not connected to a base is called an Independent Basic Service Set (IBSS) or is referred to as an Ad-Hoc Network. An ad-hoc network is a network where stations communicate only peer to peer. There is no base and no one gives permission to talk. Mostly these networks are spontaneous and can be set up rapidly. Ad-Hoc or IBSS networks are characteristically limited both temporally and spatially.

7. Extend Service Set (ESS) an extended service set is created by joining two or more basic service sets (BSS) having access points (APs ). These extended networks are created by joining the access points of basic services sets through a wired LAN known as distribution system . There are two types of stations in ESS: ( i )  Mobile stations : These are normal stations inside a BSS. (ii)  Stationary stations : These are AP stations that are part of a wired LAN.

8. Extend Service Set (ESS)

9. IEEE 802.11 defines three types of stations on the basis of their mobility in wireless LAN. These are: 1 .  No-transition Mobility : These types of stations are either stationary  i.e.  immovable or move only inside a BSS. 2.  BSS-transition mobility : These types of stations can move from one BSS to another but the movement is limited inside an ESS. 3.  ESS-transition mobility : These types of stations can move from one ESS to another. The communication mayor may not be continuous when a station moves from one ESS to another ESS.

10. MAC sublayer Functions 802.11 support two different modes of operations. These are: 1. Distributed Coordination Function (DCF) 2. Point Coordination Function (PCF)  

11. Distributed Coordination Function   The DCF is used in BSS having no access point. DCF uses CSMA/CA  protocol  for transmission. The following steps are followed in this method.

12. 1When a station wants to transmit, it senses the channel to see whether it is free or not. 2. If the channel is not free the station waits for back off time. 3. If the station finds a channel to be idle, the station waits for a period of time called distributed inter frame space (DIFS). 4. The station then sends control frame called request to send (RTS) as shown in figure. 5. The destination station receives the frame and waits for a short period of time called short inter frame space (SIFS). 6. The destination station then sends a control frame called clear to send (CTS) to the source station. This frame indicates that the destination station is ready to receive data.

13. 7. The sender then waits for SIFS time and sends data. 8. The destination waits for SIFS time and sends acknowledgement for the received frame. Collision avoidance • 802.11 standard uses Network Allocation Vector (NAV) for collision avoidance.

14. Point Coordination Function   PCF method is used in infrastructure network. In this Access point is used to control the network activity. It is implemented on top of the DCF and it is used for time sensitive transmissions. PCF uses centralized, contention free polling access method. The AP performs polling for stations that wants to transmit data. The various stations are polled one after the other.

15. Frame Format of 802.11

16. Subtype: RTS, CTS, ACK

17. To: DS Frame is going to distributed system   D . It stands for duration and is of 2 bytes. This field defines the duration for which the frame and its acknowledgement will occupy the channel. It is also used to set the value of NA V for other stations. 3.  Addresses . There are 4 address fields of 6 bytes length. These four addresses represent source, destination, source base station and destination base station. 4.  Sequence Control (SC).  This 2 byte field defines the sequence number of frame to be used in flow control. 5.  Frame body . This field can be between 0 and 2312 bytes. It contains the information. 6. FCS. This field is 4 bytes long and contains 'cRC-32 error detection sequence.

18. Physical layer functions • physical layer is responsible for converting data stream into signals, the bits of 802.11 networks can be converted to radio waves or infrared waves. • These are six different specifications of IEEE 802.11. These implementations, except the first one, operate in  industrial, scientific  and  medical (ISM)  band. These three bands are unlicensed and their ranges are 1.902-928 MHz 2.2.400-4.835 GHz 3.5.725-5.850 GHz

19. IEEE 802.11 FHSS  IEEE 802.11 FHSS  • IEEE 802.11 uses Frequency Hoping Spread Spectrum (FHSS ) method for signal generation. • This method uses 2.4 GHz ISM band. This band is divided into 79 subbands of 1MHz with some guard bands. The allowed data rates are 1 or 2 Mbps.

20. 14. 25 Figure 14.15 Physical layer of IEEE 802.11 FHSS

21. IEEE 802.11 DSSS   This method uses Direct Sequence Spread Spectrum (DSSS) method for signal generation. Each bit is transmitted as 11 chips using a Barker sequence. • DSSS uses the 2.4-GHz ISM band. • It also allows the data rates of 1 or 2 Mbps.

22. IEEE 802.11a OFDM • This method uses Orthogonal Frequency Division Multiplexing (OFDM ) for signal generation. • This method is capable of delivering data upto 18 or 54 Mbps. • In OFDM all the subbands are used by one source at a given time. • It uses 5 GHz ISM band. IEEE 802.11b HR-DSSS • It uses High Rate Direct Sequence Spread Spectrum method for signal generation. • HR-DSSS is similar to DSSS except for encoding method. • Here, 4 or 8 bits are encoded into a special symbol called complementary code key (CCK). • It uses 2.4 GHz ISM band. • It supports four data rates: 1,2,5.5 and 11 Mbps.

23. IEEE 802.11g OFDM IEEE 802.11g OFDM • It uses OFDM modulation technique. • It uses 2.4 GHz ISM band. • It supports the data rates of 22 or 54 Mbps. • It is backward compatible with 802.11 b.

24. 802.11n 802.11n operates on both the 2.4 GHz and the 5 GHz bands. Support for 5 GHz bands is optional. IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple output (MIMO) and 40 MHz channels to the PHY layer , 802.11ac with speed 6.93 Gbps

25. BLUETOOTH Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously. IEEE 802.15 standard ( Wireless PAN) Operates in the unlicensed ISM band at 2.4 GHz Gross data rate is 1Mb/s

26. Architecture Defines two types of network : Piconet and Scatternet . When more than two Bluetooth devices communicate with one another, this is called a  PICONET . A Piconet can contain up to seven slaves/ secondary clustered around a single master. The device that initializes establishment of the Piconet becomes the  master . Or primary

27. Scatternet   Scatternet   Scattemet is formed by combining various piconets . • A slave in one piconet can act as a master or primary in other piconet .

28. Bluetooth layers and Protocol Stack

29. Radio Layer • The Bluetooth radio layer corresponds to the physical layer of OSI model. • It deals with ratio transmission and modulation. • The radio layer moves data from master to slave or vice versa. • It is a low power system that uses 2.4 GHz ISM band in a range of 10 meters . Bluetooth hops 1600 times per second,  i.e.  each device changes its modulation frequency 1600 times per second.

30. Baseband Layer  • Baseband layer is equivalent to the MAC sublayer in LANs. • Bluetooth uses a form of TDMA called TDD-TDMA (time division duplex TDMA). • Master and slave stations communicate with each other using time slots.

31. in TDD- TDMA, communication is half duplex in which receiver can send and receive data but not at the same time. • In the piconet the master uses even numbered slots (0, 2, 4, ...) and the slave uses odd-numbered slots (1, 3, 5, .... ). Both master and slave communicate in half duplex mode. In slot 0, master sends & secondary receives; in slot 1, secondary sends and primary receives.

32. In Baseband layer, two types of links can be created between a master and slave. These are Asynchronous Connection-less (ACL) • It is used for packet switched data that is available at irregular intervals. • ACL delivers traffic on a best effort basis. Frames can be lost & may have to be retransmitted. • A slave can have only one ACL link to its master. • Thus ACL link is used where correct delivery is preferred over fast delivery. • The ACL can achieve a maximum data rate of 721 kbps by using one, three or more slots . ACL is multipoint connection between one master and many slaves.

33. Synchronous Connection Oriented (SCO) Synchronous Connection Oriented (SCO) SCO is Point to Point Connection between only one master and only one slave. sco is used for real time data such as sound. It is used where fast delivery is preferred over accurate delivery. • In an sco link, a physical link is created between the master and slave by reserving specific slots at regular intervals. • Damaged packet; are not retransmitted over sco links. • A slave can have three sco links with the master and can send data at 64 Kbps

34. Logical Link, Control Adaptation Protocol Layer (L2CAP) • The logical unit link control adaptation protocol is equivalent to logical link control sublayer of LAN. • The ACL link uses L2CAP for data exchange but sco channel does not use it. • The various function of L2CAP is: 1. Segmentation and reassembly • L2CAP receives the packets of upto 64 KB from upper layers and divides them into frames for transmission. • It adds extra information to define the location of frame in the original packet. • The L2CAP reassembles the frame into packets again at the destination. 2. Multiplexing • L2CAP performs multiplexing at sender side and demultiplexing at receiver side. •

35. At the sender site, it accepts data from one of the upper layer protocols frames them and deliver them to the Baseband layer. • At the receiver site, it accepts a frame from the baseband layer, extracts the data, and delivers them to the appropriate protocol1ayer Quality of Service (QOS) • L2CAP handles quality of service requirements, both when links are established and during normal operation. • It also enables the devices to negotiate the maximum payload size during connection establishment.

36. 15. 44 Figure 6.23: Frame format types

37.   Access Code : It is 72 bit field that contains synchronization bits. It identifies the master.   Header : This is 54-bit field. It contain 18 bit pattern that is repeated for 3 time. The header field contains following subfields:   Address : This 3 bit field can define upto seven slaves (1 to 7). If the address is zero, it is used for broadcast communication from primary to all secondaries . Type : This 4 bit field identifies the type of data coming from upper layers.   F : This flow bit is used for flow control. When set to 1, it means the device is unable to receive more frames.   A : This bit is used for acknowledgement. .

38. S : This bit contains a sequence number of the frame to detect retransmission. As stop and wait protocol is used, one bit is sufficient HEC: Header Error correction This 8 bit field contains checksum to detect errors in header. Data : This field can be 0 to 2744 bits long. It contains data or control information coming from upper layers Payload : This subfields can be 0 to 2740 bits long.