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Wireless Networks

Wireless Networks. IEEE 802.11 And IEEE 802.16. The Electromagnetic Spectrum. Properties of the Electromagnetic Spectrum. LF and MF radio waves follow the ground and have a maximum range of about 1000 km. These waves penetrate through buildings easily. (maritime, short-wave radio).

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Wireless Networks

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  1. Wireless Networks IEEE 802.11 And IEEE 802.16

  2. The Electromagnetic Spectrum

  3. Properties of the Electromagnetic Spectrum LF and MF radio waves follow the ground and have a maximum range of about 1000 km. These waves penetrate through buildings easily. (maritime, short-wave radio) HF and VHF waves tend to be absorbed by the Earth (and do not easily penetrate buildings). These waves can be refracted by the ionosphere and bounce several times. (FM, Broadcast TV) Microwaves travel in nearly straight lines and can be narrowly focused. Repeaters are needed (on tall towers) to retransmit signals due to the curvature of the Earth limiting the distance a linear signal can travel and be received at ground level. Infrared and millimeter waves are used for short-range communication. These waves do not pass through solid walls and do not interfere with systems in adjacent rooms. These bands are unlicensed

  4. 802.11 (WI-FI) Standards Unlicenced (ISM) Microwave bands 2.4 GHz – fewer channels, less attenuation 5 GHz – more channels (greater bandwidth), higher attenuation 802.11 Standards 802.11a – up to 54 Mb/s uses OFDM on the 5 GHz band 802.11b – 5.5 or 11Mb/s uses DSSS on the 2.4 GHz band 802.11g – up to 54 Mb/s uses DSSS or OFDM on the 2.4 GHz band 802.11n – 600 Mb/s uses MIMO and Dual band Most 802.11 LANs are 802.11b with 802.11g gaining in popularity. 802.11b and 802.11g can interoperate.

  5. FHSS – Frequency Hopping Spread Specturm Uses 79 channels, each 1 MHz wide, starting at the low end of the 2.4 GHz band. (This is the ISM band) A pseudorandom number generator is used to produce the sequence of frequencies hopped to. The dwell time is the amount of time spent at each frequency – it must be less than 400 ms. It provides good resistance to multipath fading and Is relatively insensitive to radio interference It provides a modicum of security, since an intruder would have to know the hopping sequence and dwell time to eavesdrop. Disadvantage – low bandwidth

  6. DSSS -- Direct Sequence Spread Spectrum Restricted to 1 or 2 Mbps Uses an 11-chipBarker Sequence to transmit each bit. It uses phase shift modulation at 1 Mbaud, transmitting 1 bit per baud when operating at 1 Mbps, and 2 bits per baud when operating at 2 Mbps. Similar to CDMA (next slide), but with differences. High Rate Direct Sequence Spread Spectrum (HR-DSSS) is used in 802.11b has supported data rates of 1, 2, 5.5, and 11 Mbps. It uses 11 million chips per second to achieve 11Mbps. Its data rate is slower than that for 802.11a (next method), but its range is 7 times greater.

  7. Example of DSSS Using Binary Phase Shift Keying Where PN bits are the pseudorandom noise chipping sequence Signal sent/received is (input XOR PN chips) BPSK is used to produce a 180 degree phase shift on every transition

  8. DSSS Example (cont.)

  9. CDMA – Code Division Multiple Access • Divide each bit into M slots. All transmissions start at the beginning • of a bit frame, which consists of M slots. • Each sender/receiver pair is given a code word, cm, selected from the set • of orthogonal code words of length M at the time of connection. • For each bit, bi, that a sender needs to transmit, the sender will transmit • bi * cmj over the next M slots where 1 ≤ j ≤ M. • The receiver will receive the sum of all coded bits, T, sent over a bit frame • period, and perform a (dot product) calculation: • bi = T cm = (1/M)1≤j≤MTj * cmj • which extracts the transmitted bit on the receiver’s “code channel” • from the background of transmissions from other wireless units.

  10. CDMA Example Example If M =4, the orthogonal code vectors consist of the following: c1 = (1, 1, 1, 1), c2 = (1, 1, -1, -1), c3 = (1, -1, -1, 1), and c4 = (1, -1, 1, -1). Suppose that there are three active connections using c1, c2, and c4 and sending the bit patterns (1, 0, 1), (1, 1, 0), and (0, 0, 1) respectively. (Note! 0’s will be sent as the 1’s complement of a 1 bit – or effectively by multiplying a (-1) by the code vector).

  11. CDMA Example (cont.)

  12. Orthogonal Frequency Division Multiplexing Uses the 5 GHz ISM band to deliver up to 54 Mbps. • Divides the band into 52 channels – 48 data and 4 for synchronization. • Transmissions are present on multiple frequencies at the same time. • At 54 Mbps, 216 data bits are encoded into 288-bit symbols (This provides compatibility with the European HiperLAN/2) • Splitting the signal into many narrow bands has the following advantages: • Better immunity to narrowband interference • Provides the possibility of using non-contiguous bands

  13. IEEE 802.X LAN Portal Distribution System AP AP STA4 Basic Service Set STA2 STA5 Basic Service Set STA6 STA3 STA1 IEEE 802.11 Architecture Extened Services Set

  14. Logical link control Contention-free Service Contention Service Point coordination function (PCF) Distributed coordination function (DCF) 2.4 GHz FHSS 1 Mbps 2Mbps 2.4 GHz DSSS 1 Mbps 2 Mbps Infrared 1 Mbps 2 Mbps 5 GHz OFDM 6, 9, 12, 18, 24, 36, 48, 54 Mbps 2.4 GHz DSSS 5.5 Mbps 11 Mbps 2.4 GHz DSSS 6, 9, 12, 18, 24, 36, 48, 54 Mbps 802.11 Protocol Architecture 802.11 802.11a 802.11b 802.11g

  15. Multiple Access with Collision Avoidance for Wireless (MACAW) • A sends a short (30-byte) RTS (Request to Send) frame to B. • This frame contains the length of the data frame to follow. • B replies with a 30-byte CTS (Clear to Send) frame that confirms • the received data length. • A begins transmission. • Collision avoidance is achieved by the following set of rules. • Any station hearing the RTS is close to A and must remain silent • long enough to allow a CTS to be received. • Any station hearing the RTS and CTS must remain silent during • the upcoming data transmission, whose length it learns from the • length field in the CTS. • A station hearing only the CTS will also remain silent during the period learned from the CTS.

  16. Illustration of the MACAW collision avoidance protocol

  17. Illustration of the MACAW collision avoidance protocol (cont) In the previous picture C hears the RTS from A, but not the CTS returned from B. It must remain silent for a period long enough for a CTS to be transmitted. Station D did not hear the original RTS from A but hears the CTS and remains silent for the duration of the frame transmission period. Station E hears both the RTS and CTS and must remain silent for both the time to transmit a CTS and then, upon hearing it, for the duration of the frame transmission period. After each successful data frame has been transmitted, the receiver returns an ACK frame. If A does not does not receive a CTS within a timeout period, it uses a binary exponential backoff algorithm to determine when to retry sending its RTS. Carrier sensing is used to determine if another station in A’s transmission range is sending an RTS at the same time as another nearby station – again binary exponential backoff is used to resolve this collision.

  18. DCF mode – Distributed Coordination Function In the above example A is sending an RTS to B, C hears the RTS from A, D does not hear the RTS, but hears the CTS from B. Upon hearing the RTS (with the size of the request), C sets a NAV (non-activity vector for the length of the request + ACK. D does the same thing upon hearing the CTS.

  19. 802.11 Supported modes of operation PCF – Point Coordination Function (optional) The base station controls all activity within its cell DCF – Distributed Coordination Function (required) • Within DCF there are also two modes of operation • Using CSMA/CA • Using MACAW

  20. Coexistence of DCF and PCF within a single cell Central and Distributed control coexist within the same cell by defining four inter-frame time intervals SIFS = time period for parties in current dialog to go first PIFS = reserved for base station to send a beacon or poll frame DIFS = any station may attempt to acquire the channel to send a new frame EIFS = used by a station to report an unknown or bad frame (given lowest priority)

  21. Wait for frame to transmit Medium Busy? Wait IFS Wait IFS Wait until current transmission ends Still Idle? Still Idle? Transmit Transmit 802.11 Access Control Logic Yes No No No Yes Yes Use Binary Exponential Backoff while meium idle

  22. The SIFS – (Short Inter-frame Spacing) interval permits a party to break up a frame into multiple fragments and send each in sequence without having to resend an RTS, CTS pair before each. Fragmentation increases throughput by restricting retransmissions to small fragments instead of longer frames.

  23. 802.11 Frame Structure The Frame control field is in the first 2 bytes of an 802.11 MAC frame. It consists of 11 subfields: Version (DCF or PCF or other?) Type data, control, management Subtype RTS, CTS, ACK, … ToDS and FromDS Indicates whether frame is going to or coming from the inter-cell distribution center. MF More fragments will follow Retry Retransmission of a frame sent earlier Pwr used by the base station to put the receiver to sleep or to wake it up More sender has additional frames to send W frame body has been encrypted using WEP (Wired Equivalent Privacy) O notifies the receiver that a sequence of frames with this bit on must be processed strictly in order

  24. 802.11 Frame Format (cont) In addition to the Frame Control Field in the 802.11 header there was a 2-byte Duration Field, 4 802 address fields, a 2-byte Sequence Number field, up to 2312 bytes of data and a 4-byte Checksum The Duration Field informed the stations within range of the time that this sender would be occupying the channel with data + ACK Four Address fields identify the Source and Destination plus the two adjacent Base Stations in adjacent cells. In the 16-bit Seq field, 12 bits are used to identify the frame and 4 to identify the fragment Example: A9C-1, A9C-2 would indicate two fragments of a frame numbered hex A9C

  25. WiMAX IEEE 802.16 A Metropolitan Area Network (MAN) for connecting “subscriber stations” (for instance Corporate LANs operating in separate buildings), it operates over a distance of several miles (up to about 30 miles) The subscriber stations in 802.16 are fixed, although an extension of the original WiMAX protocol (IEEE 802.16e) has been developed to incorporate mobile stations. WiMAX defines several physical layer protocols designed to work in the 10 – 66 GHz range. In this range waves travel in straight lines, limiting communication to line of sight. WiMAX (unlike Ethernet and WI-FI) is connection oriented. It can thus offer QoS guarantees on properties such as latency and jitter. It is designed to handle real-time traffic such as high-volume multimedia and to compete with DSL as a “last mile” technology for providing these services.

  26. WiMAX Physical Layer Standards Two ways of dividing the bandwidth between upstream (subscriber to base station) and downstream traffic: • Time Division Duplexing (TDD) • Frequency Division Duplexing (FDD) In TDD, the upstream and downstream traffic takes turns using the same frequency. Like STDM, the proportion of time allocated to each stream is dynamically adjusted by the base station. In FDD, the two streams use different frequency bands. Sharing the upstream and downstream channels is based upon dividing them into fixed-size time slots. In the upstream the connections are awarded slots based upon their QoS requirements. Some connections get a fixed number of slots. Others get polled to determine how many slots they currently need, and some must request slots by contending for reservations. Contention is based upon a binary exponential backoff algorithm to limit collisions.

  27. Bluetooth IEEE 802.15.1 Used for very short-range communication. It is an alternative to connecting pairs of devices by wire. It is characterized as being a Personal Area Network (PAN). Bluetooth operates in the ISM band at 2.45 GHz. It has a range of about 10 meters It uses FHSS with 79 channels and a dwell time of 625 micro sec. The dwell time is the “natural” time slot for Bluetooth to use for Synchronous TDM. The Bluetooth piconet consists of a master device and up to 7 active slave devices. Slaves can only communicate with the master and only when requested to do so. A frame can take up 1, 3 or 5 time slots. Only the master can transmit in odd-numbered slots. A slave can start to transmit in an even slot only in response to a request from the master during a previous slot. To save battery power, a slave can be parked (made inactive) and can only communicate on the piconet when reactivated by the master. A piconet can have up to 255 parked devices in addition to the active “slaves”,

  28. Bluetooth Bluetooth has a suite of protocols beyond the Link Layer. It defines application protocols for such things as synchronizing a PDA with a PC or giving a mobile computeraccess to a wired LAN. Bluetooth equipment is simpler and hence cheaper than Wi-Fi and it promotes extending battery life. It’s range and data rate is much less than that of a Wi-Fi LAN.

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