Wireless concepts: Modulation to Aggregation


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My first comprehensive wlan presentation in the draft-11n days in 2008.

Note: Lots of text and pictures are used from across the web, author doesn't claim any copyright on them. In case of issues/feedback please email: chaitanya.mgit@gmail.com

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Wireless concepts: Modulation to Aggregation

  1. 1. 802.11 Wireless LAN Concepts By Chaitanya T K
  2. 2. To repeat what others have said,requires education; to challenge it, requires brains. Mary Pettibone Poole, A Glass Eye at a Keyhole, 1938
  3. 3. Objectives:  Evolution-Why Wireless ?  Overview of 802.11 standards  Basics of RF Medium and wireless concepts  Different RF Access Technologies used in 802.11 PHY layer  Wireless Standards(802.11 b/a/g/n)  802.11 MAC layer in Detail  Roaming  802.11e-QOS
  4. 4. Evolution of Networks:  PC-PC wired, LAN (Ethernet-Fast Ethernet-Gigabit Ethernet), MAN and WAN  PC-PC wireless, WLAN,WiMAX and LTE Why Wireless?  Advantages  Disadvantages
  5. 5. Evolution of wireless:
  6. 6. Wireless Standard Bodies  FCC– Frequency,BW,Usage and EIRP (Only U.S and some others)  ITU-T– RF spectrum management through 5 standard bodies per continent (Global)  IEEE– Developing standards for PHY and MAC layers (L2 and L1 only)  Wi-fi Alliance – Maintaining interoperability b/w vendors  ISO- For developing a general framework for data communication (OSI Model)
  7. 7. Overview of wirelessStandards:  802.11 Legacy-IR/DSSS/FHSS - Up to 2 Mbps  802.11b- CCK-HR_DSSS-Up to 11 Mbps  802.11a-OFDM-Up to 54 Mbps (5Ghz)  802.11g-OFDM-up to 54 Mbps (2.4 GHz)  802.11n-MIMO-OFDMA-Up to 600 Mbps(Both 2.4 and 5 GHz)  802.11y-OFDM-up to 54 Mbps (3.7 GHz)
  8. 8. Comparison Table:
  9. 9. RF medium Basics
  10. 10. Wireless Terminology: Key Parameters in understanding wireless channels Delay Spread Coherence bandwidth Doppler spread Coherence time
  11. 11. Welcome to PHY layer
  12. 12. Modulation techniques:  PHY layer Modulation techniques used in WLAN  Infra Red OFDM  DHSS OFMDA  FHSS Scalable-OFDMA  HR-DHSS MIMO  CCK MIMO-OFDM(A)  CCK-OFDM and other common  PBCC QPSK/BPSK/QAM..
  13. 13. Spread Spectrum techniques:  Highly resistant to narrowband interference.  Spread-spectrum signals are difficult to intercept , it appears as a white noise  BW utilization by sharing the BW with zero interference.  Two Types:  Direct Sequence spread spectrum (DSSS)  Frequency Hopping spread spectrum (FHSS)
  14. 14. Direct Sequence spreadspectrum (DSSS)  It phase-modulates a sine wave pseudo randomly with a continuous string of pseudo-noise (PN) code symbols called "chips“.  Multiplies the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.
  15. 15. Tx in DSSS:
  16. 16. Frequency Hopping spreadspectrum (FHSS)  The initiating party sends a request via a predefined frequency or control channel.  The receiving party sends a number, known as a seed.  The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used.
  17. 17.  Most often the period of the frequency change is predefined, as to allow a single base station to serve multiple connections. The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence. The communication begins
  18. 18. Complementary Code Keying:  Complementary codes, first introduced by Golay in 1961.  They are sets of finite sequences of equal length, such that the number of pairs of identical elements with any given separation in one sequence is equal to the number of pairs of unlike elements having the same separation in the other sequences.
  19. 19. Binary Complementary Codes:
  20. 20. Mathematics Involved:  complementary codes are characterized by the property that their periodic auto correlative vector sum is zero everywhere except at the zero shift.
  21. 21. Example:
  22. 22. Polyphase ComplementaryCodes:  Polyphase complementary code is a sequence having complementary properties, the elements of which have phase parameters.  Eg: c = {1,– 1, j,j,–j,j ,– 1,-1}
  23. 23. Representation of CCK:  Uses I/Q Modulator and M-Ary Signaling schemes
  24. 24. I/Q Modulators:  Quadrature modulators are used to conserve bandwidth for a given data rate. This is accomplished by modulating two orthogonal data streams onto a common carrier. If the phases and amplitudes of both data stream (in-phase "I" and quadrature "Q"), then one of the sidebands is completely cancelled out. If there is no DC bias feed through, then the carrier itself is completely cancelled out.
  25. 25.  In practice, complete cancellation is never accomplished, but without too much work, achieving 40 dB of sideband cancellation is not hard to do. Even 60 dB is relatively easy; however, preventing drift due to thermal and mechanical effects is not so easy, and the result is that a "textbook" quadrature alignment during alignment can look pretty bad over time.
  26. 26. I/Q Modulation:
  27. 27. CCK modulator: I OUT 3 Pick One 1 of 8 Walsh Functions MBOK 3 Pick One 1 Q OUT of 8 Walsh Functions 1 Data Input MUX 1:8 1 1.375 MHz 11 MHz Modulation is Bi-orthagonal keying on both I and Q channels Data Rate = 8 bits/symbol * 1.375 MSps = 11 MBps I OUT Pick One of Differential Mod 6 1 64 Complex CCK Codes Q OUT 1 1 MUX Data Input 1:8 1 Code Set is defined by formula: 1.375 MHz c = e j(ϕ + 2 + 3 + 4 ) , e j (ϕ + 3+ 4 ) , e j (ϕ + 2 + 4 ) , { 1 ϕ ϕ ϕ 1 ϕ ϕ 1 ϕ ϕ 11 MHz − j(ϕ + 4 ) , e j (ϕ + 2 + 3 ) , e j(ϕ + 3 ) , − j (ϕ + 2 ) , e jϕ } e 1 ϕ 1 ϕ ϕ 1 ϕ e 1 ϕ 1 Data Rate = 8 bits/symbol * 1.375 MSps = 11 MBps
  28. 28. Comparison: Modulation Technique and Data rates c = e j(ϕ+ 2+ 3+ 4) , e j(ϕ+ 3+ 4) , e j(ϕ+ 2+ 4) , { 1ϕ ϕ ϕ 1 ϕ ϕ 1 ϕ ϕ − j(ϕ + 4) , e j(ϕ + 2+ 3) , e j(ϕ+ 3) , − j(ϕ+ 2) , e jϕ } e 1ϕ 1 ϕ ϕ 1 ϕ e 1ϕ 1 11 Bit Barker Word 22 MHz Code set 802.11 DSSS BPSK 802.11 DSSS QPSK 5.5 MBps 11 MBps CCK CCK 1 MBps 2MBps Barker Barker BPSK QPSK 1 bit used to 2 bits encoded to 6 bits encoded to 2 bits used to 64 complex code BPSK code word 4 complex code QPSK code word words; 2-QPSK words; 2-QPSK I, Q I, Q I, Q I, Q 11 chips 11 chips 8 chips 8 chips 1 MSps 1 MSps 1.375 MSps 1.375 MSps
  29. 29. Packet Binary ConvolutionalCoding: In its simplest form, PBCC works by reducing overhead (bandwidth used to transmit non-data) through the removal of extraneous information and optimizing transmission by using smaller data packets,cutting the response time in processing those packets. Thus, allowing for a greater amount of data to be transmitted between networked devices. Specific examples of overhead include:
  30. 30. • Protocol headers – carry information for controlling the device• Frame interval times – allow users to gain access to the frequency band• Error and flow control – ensure the integrity of the wireless transmission• Received message acknowledgements – verify that messages were received correctly or need to be resent due to errors and/or message collisions
  31. 31. OFDM:  Modulation and Multiplexing  Orthogonality  Sub carriers  Sub carrier spacing  Multipath and ISI  Cyclic Prefix  Flat Fading and Performs well in frequency Selective fading  High PAPR  Local Oscillator Frequency Offset
  32. 32. OFDM Symbol Time andFrequency Representation:
  33. 33. Transmission of OFDMSymbols:
  34. 34. Cyclic Prefix:
  35. 35. MIMO:
  36. 36. Different MIMO techniques:
  37. 37. ST Coding
  38. 38. SF Coding
  39. 39. SF Coding:Rate-2
  40. 40. STF Coding
  41. 41. Different MCS indices:
  42. 42. Protocol Stack:
  43. 43. Protocol Stack for 802.11:  PHYsical Layer:  PLCP (Physical Layer Convergence Protocol)  PMD (Physical Medium Dependent) sub layer.  The PMD takes care of the wireless encoding.
  44. 44.  The PLCP presents a common interface for higher-level drivers to write to and provides carrier sense and CCA (Clear Channel Assessment), which is the signal that the MAC (Media Access Control) layer needs so it can determine whether the medium is currently in use.
  45. 45. PLCP frame format:
  46. 46. PHY concepts:  The PLCP consists of a 144 bits preamble that is used for synchronization to determine radio gain and to establish CCA.  The preamble comprises 128 bits of synchronization, followed by a 16 bits field consisting of the pattern 1111001110100000.  This sequence is used to mark the start of every frame and is called the SFD (Start Frame Delimiter).
  47. 47.  The next 48 bits are collectively known as the PLCP header. The header contains four fields: signal, service, length and HEC (header error check). The signal field indicates how fast the payload will be transmitted (1, 2, 5.5 or 11 Mbps). The service field is reserved for future use. The length field indicates the length of the ensuing payload, and the HEC is a 16 bits CRC of the 48 bits header.
  48. 48.  In a wireless environment, the PLCP is always transmitted at 1 Mbps. Thus, 24 bytes of each packet are sent at 1 Mbps. The PLCP introduces 24 bytes of overhead into each wireless Ethernet packet before we even start talking about where the packet is going. Ethernet introduces only 8 bytes of data. Because the 192 bits header payload is transmitted at 1 Mbps, 802.11b is at best only 85 percent efficient at the physical layer.
  49. 49. PLCP formats in 11n:
  50. 50. PHY Tx Procedure:
  51. 51. PHY Rx Procedure:
  52. 52. MAC Layer Concepts
  53. 53. Modes of WLAN:
  54. 54. Working of Different Modesin WLAN:  802.11 MAC functions much the same way in both ADHOC (or)independent and Infrastructure modes.  Only difference is a centralized controller is to elected in the absence of an Access point.
  55. 55. BSS and ESS:
  56. 56. Modes of Access Point:  Root Mode,  Bridge Mode,  Repeater Mode  BSSID  ESSID  SSID
  57. 57. MAC coordination functions:
  58. 58. Frame Processing:
  59. 59. Carrier Sensing:  Physical Sensing-Hardware-PHY  Virtual sensing-Software-MAC-NAV
  60. 60. Carrier Sensing Algorithm:
  61. 61. Clear Channel AssessmentModes (DSSS):
  62. 62. Clear Channel AssessmentModes (CCK):
  63. 63. Scanning Procedures:  Active Scanning -Use of Probe Request -Can be a unicast/multicast/broadcast - Optional  Passive Scanning - Waits for beacon - It may miss a beacon - Mandatory
  64. 64. Inter frame Spacing:
  65. 65. Inter frame Spacing(Contd..):
  66. 66. Fragmentation:
  67. 67. MAC Framing:
  68. 68. Addresses:
  69. 69. 802.11 frame: addressing Internet H1 R1 router AP R1 MAC addr AP MAC addr dest. address source address 802.3 frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame
  70. 70. Summary Of MAC rules:
  71. 71. PCF contention Free:
  72. 72. Problems and Solutions inMAC:
  73. 73. Basic handshakes:
  74. 74. Channel Placement in 2.4 &5 GHz bands:
  75. 75. 5GHz band:
  76. 76. 5GHz band(Contd..):
  77. 77. 802.11n:  Requirements:  MAC layer throughput > 100Mbps  Support 20MHz Channel  Support 5GHz Band  Backward compatible with 802.11g (if support 2.4GHz Band) and 802.11a  Support 802.11e (QoS)  Spectral Efficiency >= 3 bps / Hz  Control of support for legacy STA from 802.11n AP
  78. 78. 11n Enhancements:
  79. 79. Enhancements in detail-I:
  80. 80. Enhancements in detail-II:
  81. 81. MAC enhancements:
  82. 82. MSDU Aggregation:
  83. 83. MPDU aggregation:
  84. 84. BLOCK ACK bitmap:
  85. 85. BLOCK ACK Bitmap Analysis:
  86. 86. Timing Diagram ForAggregation:
  87. 87. Backward Compatibility:
  88. 88. 802.11y:  Operates in 3650-3700 MHz band  Novel Licensing  High Power compared to normal 20 Watts EIRP.  Hence Higher coverage area (5 Km)  3 Major changes compared to traditional 802.11:  Contention based protocol  Extended Channel switch announcement  Dependent station enablement
  89. 89. Terminology in 802.11y:  Dependent STA: A station that must receive and successfully decode an enabling signal before it is allowed to transmit.  Registered STA: A station that is certified to operate at a given location after registering with the required authorities. Registered STAs do not need to receive an enabling signal before commencing operation.
  90. 90. Terminology in 802.11y(Contd..):  Fixed STA: a station that is authorized to operate only at a specific location.  Enabling point(EP): A fixed station that transmits coded signals that contain the necessary information and authorization for a dependant STAs to transmit in a restricted band for the purpose of engaging in the DSE process.
  91. 91. Terminology in 802.11y(Contd..):  Dependent STA enablement (DSE): The process by which a dependant STA gains permission to transmit on a restricted channel.
  92. 92. Roaming in WLAN
  93. 93. Roaming in WLAN:  Seamless Roaming  Nomadic Roaming  A roaming domain is defined as APs that are in the same broadcast domain and configured with the same SSID.  Stated another way, a client can only roam between APs in the same VLAN and with the same SSID.  As WLAN deployments expand within an organization, roaming domains might need to scale beyond a single Layer 2 VLAN.
  94. 94. Roaming Types:
  95. 95. Layer 2 Roaming:  The client must decide to roam—Roaming algorithms (signal strength, frame acknowledgment, missed beacons..)  The client must decide where to roam—  preemptive AP discovery,  roam-time AP discovery.  The client initiates a roam—The client uses 802.11 (re-)association frames to associate to a new AP.  The client can resume existing application sessions.
  96. 96. Roam Types: Preemptive AP discovery: Effects application Throughput Reduces roaming time Better use for Power save clients Not suitable for fast roaming client Roam-time AP discovery: High roaming delay
  97. 97. Roaming Processes:  The previous AP must determine that the client has roamed away from it.  The previous AP should buffer data destined for the roaming client.*  The new AP should indicate to the previous AP that the client has successfully roamed. This step usually happens via a unicast or multicast packet from the old AP to the new AP with the source MAC address set to the MAC of the roaming client.*  The previous AP should send the buffered data to the new AP.
  98. 98.  The previous AP must determine that the client has roamed away from it. The AP must update MAC address tables on infrastructure switches to prevent the loss of data to the roaming client.Note: * Tasks are not mandatory because they are not specified in the 802.11 standard.
  99. 99. Effects of roaming on DataFlow:
  100. 100. Layer 3 Roaming:
  101. 101. Mobile IP to the rescue:  Mobile node (MN)—The MN is the roaming station.  Home agent (HA)—The HA exists on routers or Layer 3 switches and ensures that a roaming MN receives its IP packets.Foreign agent (FA)—The FA exists on router or Layer 3 switches and aids the MN notifying the HA of the new MN location by receiving packets from the HA destined for the MN.  Care-of address (CoA)—The CoA is a locally attached router that receives packets sent by the HA, destined for the MN. Co-located care-of address (CCoA)—A CoA that exists on the mobile node itself.
  102. 102. Steps in Layer 3 Roaming: A station is on its home subnet if the stations IP address belongs to the subnet of the HA. As the MN roams to a foreign subnet, the MN detects the presence of the FA and registers with the FA or with the MN CCoA. The FA or MN CCoA communicates with the HA and establishes a tunnel between the HA and a CoA for the MN.
  103. 103.  Packets destined to the MN are sent to the HA (via normal IP routing. The HA forwards the packets via the tunnel to the MN. Any packets the MN transmits are sent via the FA as if the MN were local on the subnet. (A "reverse tunnel" mode is available when the edge routers use ingress packet filtering.)
  104. 104. Effect on data flow in L3roaming:
  105. 105. Overview of Mobile IP inWLAN:  Agent discovery:  Uses IRDP (ICMP+router discovery Protocol) Multicast/Broadcast  The agent advertisement contains two fields that allow the MN to determine whether it has roamed to a new subnet:  The lifetime field from the agent advertisement  The prefix-length extension
  106. 106.  Upon determining it is on a foreign subnet, the MN gleans the CoA from the agent advertisement. The CoA can take two forms: The address of the FA. CCoA (Note that the CCoA is not advertised by the FA, but it is probably acquired by the MN as a Dynamic Host Configuration Protocol [DHCP] option.)
  107. 107. MN Registration Process:  The MN sends a registration request to the FA. If the MN has a CCoA, this step is skipped.  The FA processes the registration request and forwards the request to the HA.  The HA accepts or declines the registration and sends a registration reply to the FA.  The FA processes the registration reply and relays it to the MN.
  108. 108. Understanding 802.11e- QoS
  109. 109. 802.11e:  HCF: Hybrid of DCF & PCF  NO ACK  BLOCK ACK  TX OP
  110. 110. EDCA:  EDCA contention access is an extension of the legacy CSMA/CA DCF mechanism to include priorities
  111. 111. EDCA (Contd..):  After resolving internal and external collisions an STA is given a TXOP.  If the frame exchange sequence has been completed, and there is still time remaining in the TXOP, the QSTA can may extend the frame exchange sequence by transmitting another frame in the same access category. The QSTA must ensure that the transmitted frame and any necessary acknowledgement can fit into the time remaining in the TXOP.
  112. 112. EDCA Admission control:  A station specifies its traffic flow requirements (data rate, delay bounds, packet size, and others) and requests the QAP to create a TSPEC by sending the ADDTS (add TSPEC) management action frame  The QAP calculates the existing load based on the current set of issued TSPECs
  113. 113.  If the TSPEC is denied, the high priority access category inside the QSTA is not permitted to use the high priority access parameters, but it must use lower priority parameters instead. Admission control is not intended to be used for the "best effort" and "background" traffic classes
  114. 114. HCCA:  The HCF controlled channel access (HCCA) mechanism uses a hybrid coordinator (HC) to centrally manage medium access. The intent of HCCA is to increase efficiency by reducing the contention on the medium.  Delay for HC will be the least  HC control Admission control using TSPEC and grants the TXOP
  115. 115. Optional 802.11e features:  Contention-free bursts, -When time remains in a given TXOP -To save 11g from 11b clients by TXOP(g)=1 frame length of 11b  Block acknowledgements,  Direct link protocol,  Active power mode save delivery -Automatic Schedule in terms of beacon periods -APSD operation is invoked by a station by establishing a TSPEC with the APSD flag set.
  116. 116. Block acknowledgements: Immediate BLOCK ACK Delayed BLOCK ACK
  117. 117. Direct Link protocol:
  118. 118. My method is to take the utmosttrouble to find the right thing to say, and then to say it with the utmost levity. . George Bernard Shaw, "Answers to Nine Questions" Irish dramatist & socialist (1856 - 1950)