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ECE 4331, Fall, 2009

ECE 4331, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 27 Nov. 24 th , 2009. Cellular Systems and MAC. Random Access. Packet Radio Protocols Multihop radio network that carries packets Not circuit oriented like GSM, CDMA, etc. Example Protocols Pure Aloha

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ECE 4331, Fall, 2009

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  1. ECE 4331, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 27 Nov. 24th, 2009

  2. Cellular Systems and MAC

  3. Random Access Packet Radio Protocols Multihop radio network that carries packets Not circuit oriented like GSM, CDMA, etc. Example Protocols Pure Aloha Slotted Aloha CSMA Protocols 1-persistent CSMA non-persistent CSMA p-persistent CSMA CSMA/CD Reservation Protocols Reservation Aloha PRMA Others MACA, MACAW IEEE 802.11 MAC

  4. Pure Aloha Algorithm: A mobile station transmits immediately whenever is has data. It then waits for ACK or NACK. If ACK is not received, it waits a random amount of time and retransmits. Ignoring the propagation delay between mobiles and base station: B The time difference between the time a mobile send the first bit of packet and the time the base station receives the last bit of the packet is given by 2T. T = C/P T: packet time. C: channel data rate (bps) P: packet length (bits) Ack/Nack Data M3 M1 M2 During this 2T period of time, the packet may collide with someone else packet.

  5. Contention for Aloha

  6. Throughput of Aloha Normalized Throughput ~0.185 0.5 Normalized Channel Occupancy

  7. Slotted Aloha

  8. Reservation Protocols Reservation Aloha Packet Reservation Multiple Access

  9. PRMA Packet Reservation Multiple Access A combination of TDMA and reservationALOHA Ask channel resource in the talkspurt Release channel resource in the silent gap Permission probability Effect of voice activity detector

  10. PRMA • NC-PRMA (Non-Collision Packet Reservation Multiple Access) • The existing users inform the BS about their demands in a non-collision manner (time-frequency signaling scheme)

  11. Improvements of PRMA Wong and Goodman Allow data transmission to reserve a limited number of slots with reservation threshold Frame Reservation Multiple Access (FRMA) Divide a frame into voice and data slots Ratio of the number of voice and data slots is dynamic PRMA++ Separates the request and data channels C-PRMA QoS guarantee Schedules the uplink transmission (Earliest due date (EDD))

  12. CSMA: Carrier Sense Multiple Access Aloha does not listen to the carrier before transmission. CSMA listen to the carrier before transmission and transmits if channel is idle. Detection delay and propagation delay are two important parameters for CSMA Detection delay: time required to sense the carrier and decide if it is idle or busy Propagation delay: distance/speed_of_light. The time required for bit to travel from transmitter to the receiver.

  13. CSMA Variations 1-persistent CSMA: A station waits until a channel is idle. When it detects that the channel is idle, it immediately starts transmission Non-persistent CSMA: When a station receives a negative acknowledgement, it waits a random amount of time before retransmission of the packet altough the carrier is idle. P-persistent CSMA P-persistent CSMA is applied to slotted channels. When a station detects that a channel is idle, it starts transmission with probability p in the first available timeslot. CSMA/CD Same with CSMA, however a station also listen to the carrier while transmitting to see if the transmission collides with someone else transmission. Can be used in listen-while-talk capable channels (full duplex) In single radio channels, the transmission need to be interrupted in order to sense the channel.

  14. MACA – Medium Access with Collision Avoidance CSMA protocols sense the carrier, but sensing the carrier does not always releases true information about the status of the wireless channel There are two problems that are unique to wireless channels (different than wireline channels), that makes CSMA useless in some cases. These problems are: Hidden terminal problem Exposed terminal problem.

  15. Hidden Terminal Problem C’s cell A’s cell A B C Hidden terminal • A is transmitting to B. • C is sensing the carrier and detects that it is idle (It can not hear A’s transmission). • C also transmits and collision occurs at B. • A is hidden from C.

  16. Exposed Terminal Problem B’s cell C’s cell A B C D Exposed terminal • B is transmitting to A. C is hearing this transmission. • C now wants to transmit to D. It senses the existence of carrier signal and • defers transmission to D. • However, C can actually start transmitting to D while B is transmitting to A, • Since A is out of range of C and C’s signals can not be heard at A. • C is exposed to B’s transmission.

  17. MACA Solution Concept Ali, lets talk! I am available. Can Can, I want to talk to you! Can, I want to talk to you! Biltepe Mountain Ali Veli

  18. MACA Protocol When a station wants to transmit data It sends an RTS (Ready-to-Send) packet to the intended receiver The RTS packet contains the length of the data that needs to be transmitted Any station other than the intended recipient hearing RTS defers transmission for a time duration equal to the end of the corresponding CTS reception The receiver sends back CTS (Clear-to-Send) packet back to sender if it is available to receive. The CTS packet contains the length of the data that original sender wants to transmit Any station other than the original RTS sender, hearing CTS defers transmission until the data is sent. The original sender upon reception of the CTS, starts transmitting.

  19. Solution for Hidden Terminal Problem A is transmitting to B. C’s cell A’s cell CTS(n) RTS(n) RTS(n) X A B C CTS(n) C defers transmission for duration of n bytes of data transmission. Node A is no longer hidden from C effectively. X defers transmission until expected CTS reception time by RTS sender. Data(n) Waiting time of node X is much smaller than waiting time of node C.

  20. Solution for Exposed Terminal Problem B is transmitting to A B’s cell C’s cell RTS(n) RTS(n) A B C D RTS(m) CTS(n) CTS(m) Data(n) Data(m) • C defers transmission upon hearing B’s RTS until B could get CTS from A. • After that C can start transmission to D. For that it first sends an RTS. • C is not longer exposed to the data transmission of B.

  21. IEEE 802.11b MAC IEEE 802.11b: High Data-rate Wireless LAN standard. Operates in 2.4-2483 MHz ISM RF Band. 83 MHz spectrum width Max data-rate: 11Mbps simplex. Spectrum Usage: FHSS or DHSS Modulation Technique: CCK with QPSK For 11Mbps: Symbol rate = 1,375 MSps Number of symbol states = 8 One symbol can encode 3 bits of information. Range: around 100m.

  22. 802.11b Works in Two Operational Modes Infrastructure Mode Ad-Hoc Mode Infrastructure Mode Access Point Access Point Wireless Link Wireless Link Wireless Link Mobile Station Extended Service Set (ESS) Basic Service Set (BSS) All traffic has to go through access points Access point provides connectivity to the wired backbone

  23. 802.11b Ad-Hoc Mode Independent Basic Service Set (IBSS) Mobile Stations can talk directly with each-other. All stations in an IBSS need to be in the range of each-other.

  24. 802.11b MAC Sublayer Support two different MAC modes depending on the operational mode of the Wireless LAN 1) DCF: Distributed Coordination Function Based on CSMA/CA Carrier Sensing: Physical and Virtual. 2) PCF: Point Coordination Function Connection oriented Contention free service Polling based

  25. 802.11b PHY Layer Can support data rates at: 1,2,5.5,11 Mbps FHSS 2.4 GHz band is divided into 75 one-MHz subchannels. The sender and receiver hops through this 75 channels in a synchronized manner using a hopping pattern. Can not support more than 2 Mbps data-rate.

  26. DSSS Divides the 2.4 GHz band into 14 twenty-two MHz channels Adjacent channels can overlap partially. 3 of 14 channels are completely non-overlapping Data is sent over one 22 MHz channel without hoppling using DSSS technique (chipping and code words are used like CDMA) Each access point uses a different 22 MHz channel if possible. All mobiles in the coverage of the access point uses the channel that is used by the access point. 802.11b MAC is used to coordinate the access to the shared 22 MHz channel. Original 802.11 systems use 11 bit chipping (code words of length 11). Later 802.11b systems use 8 bit chipping (code words of length 8 bits). Defines 64 different codewords from a space of 256.

  27. DSSS Channels 25 MHz 25 MHz 2.412 GHz 2.437 GHz 2.462 GHz Channel 1 22 MHz Channel 11 22 MHz Channel 6 22 MHz 2.400 GHz 2.484 GHz Spectrum Allocated for 802.11b Channel 1, 6, and 11 are non-overlapping.

  28. Channel Assignment and Registration In multi-access environment, the operator should try to allocate non-overlapping channels to the physically adjacent channels. If adjacent access points use overlapping channels, then interference can be high. A mobile station periodically tunes to all channels and evaluates the signal strength received over each channel Depending on the signal strength received over the channels, a mobile selects an access point and registers with that provided that the access points accepts the mobile. This is also called association. Re-association with a new access point occurs when the mobile moves away from the current access point. When the signal conditions changes between the mobile and current access point. When there are a lot of users associated with the current access point.

  29. Re-association at the PHY layer. Access Point (AP) A Access Point (AP) B Signal from A Signal from B Associated withAccess Point B Associated withAccess Point A Mobile tunes to the channel of AP B when it moves into its range.

  30. An example 3-cell Reuse scheme for WLAN deployment 1 11 11 6 6 1 1 1 11 11 6 6 An access point is located in the center of each hexagon.

  31. 802.11b PHY Layer MAC PLCP PLCP: Physical Layer Convergence Protocol PMD: Physical Medium Dependent Sublayer PHY Layer PMD Sublayer PLCP Frame Format SYNC(128) SFD(16) Signal(8) Service(8) Length(16) CRC(16) MPDU(Variable Length) SYNC: Synchronization fieldSFD: Start frame deliminerSignal: Indicated how fast the data will be transmittedService: ReservedLength: MAC Protocol Data Unit (MPDU) lengthCRC: used for error detecting on the frame

  32. 802.11b MAC Sublayer Supports both infrastructure and ad-hoc modes of operation. CRC is added to each MAC frame Packet fragmentation is supported to chop large higher layer (IP) packets into small pieces. Has advantages: Probability a packet gets corrupted increases with the packet size. In case of corruption, only a small fragment needs to be re-transmitted.

  33. Inter-frame Space 4 types of Inter-frame spaces: Short IFS (SIFS): period between completion of packet transmission and start of ACK frame Point Coordination IFS (PIFS): SIFS plus a slot time. Distributed IFS (DIFS): PIFS plus a slot time. Extended IFS (EIFS): longer IFS used by a station that has received a packet that it could no longer understand. Needed to prevent collisions.

  34. MAC Protocol 802.11b uses CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) MAC protocol. CSMA/CA is the protocol to implement the distributed coordination function (DCF) of the MAC sub-layer. RTS/CTS is used to avoid collisions. Use of RTS/CTS can be enabled or disabled depending on the traffic load (probability of collisions).

  35. CSMA – Transmission of MPDU (Data) without use of RTS/CTS DIFS Data Source SIFS ACK Destination Contention Window (Slot Times) DIFS Data Others Defer Access Backoff afterDefer A station backoffs a random number of slot times.

  36. CSMA/CA – Transmission of MPDU (Data) using RTS/CTS DIFS RTS DATA Source SIFS SIFS SIFS CTS ACK Destination Others DIFS Defer Access for NAV(RTS) Defer Access for NAV(CTS) Backoff afterDefer Defer Access for NAV(Data)

  37. CSMA/CA Collision Avoidance RTS/CTS is used to reserve channel forthe duration of the packet transmission. This prevents hidden and exposed terminalproblems ACK is required to understand if the packet is correctly received (without any collisions ) at the receiver. Ethernet does not require ACK to be sent, since the transmitter can detect the collision on the channel (cable) without requiring an explicit feedback from the receiver. A wireless transmitter can not detect collision, because:1) Transmit power is much larger than the received power: received signal is regarded as noise (not collision). 2) There could be a hidden terminal Access Point Mobile RTS CTS DATA ACK

  38. 802.11b Frame Format IEEE 802.11b MAC Frame Format FC(2 bytes) ID(2) Add1(6) Add2(6) Add3(6) SC(2) Add4(6) Data(0-2312 bytes) CRC(4) Frame Control Format (2 bytes) Protocol(2 bits) Type(2) Subtype(4) To DS(1) From DS(1) More Frag(1) Retry(1) Pw Mgt(1) More Data(1) WEP(1) Order(1) Protocol Version: version of 802.11 standardType: Management. Control, Data frameSubtype: RTS, CTS, ACK frameTo DS: 1 if frame is sent to Distribution System (DS)From DS: 1 if frame is received from Distribution SystemMore fragment: 1 if there are more fragments belonging to the same frame following the current frame. Retry: indicates that is fragment is retransmission of previously transmitted fragment. Power Management: the type of power management mode that the station will be after the transmission of the frame. More Data: indicates that there are more frames buffered at the sender for this station. WEP: indicates that frame body is encrypted according to WEP. Order: indicates that the frame is sent using the strictly-ordered service class. Frame Control (FC): protocol version and frame typeDuration/ID (ID): power-save poll message frame type and for NAV calculationAddress Fields: contains up-to 4 MAC addressesSequence Control: fragmentation and sequence number. Data: higher layer data that is maximum 2312 bytes. CRC: 32 bit cyclic redundancy check for detecting error on the frame.

  39. Mobility What happens when a station moves between access points Re-association function of the PHY layer associates a mobile with a new access point. Some vendor specific, layer-2 (datalink layer) solutions solves the mobility at layer. Solutions like Mobile IP needed to provide seamless mobility to higher layers (transport and application layers). DHCP is also a method but not as convenient as Mobile IP. We will see in the forthcoming classes how Mobile IP works.

  40. Final Words Submit your homework, projects and term project ON TIME! Still have one class 12/1 by video, exam on 12/3!!! Thank you for attending my class. I do appreciate a lot If you get A, you can ask me for reference. Good luck Interdisciplinary class: Hope to ring a bell to you Continue education after graduation Cutting edge technology Lucky to be an Engineering in US If you success, do not forget UoH

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