University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science IEEE g “Wi-Fi” Ravi Teja Kundeti KU ID: th April 2008

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science Outline History and Background Overview and basic features of a and b g Differences between g and b Summary References Latest Developments 2

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science History and Background – suite Since g shares the same basic protocols and architecture, this presentation explains the and b systems in some detail. Then the differences between b and g are explored to understand why some decisions are taken , popularly known as “Wi-Fi TM ”, is a suite for specifications for wireless Ethernet or wireless local area network. It operates in 5GHz or 2.4 GHz public spectrum bands. All of the specifications use the same basic protocols. Security was originally purposefully weak. Mainly for the corporate LANs inside a building. 3

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science History and Background – Highlights of specifications 4 ProtocolRelease Date Op. Frequency (GHz) Data Rate Max – Typical (Mbit/s) Modulation Technique Range (Indoor – Outdoor) meters Legacy – 0.9DSSS a OFDM b – 4.3DSSS38 – g OFDM38 – n2009 (est) 2.4 and – yJune 08 (est) – 5000

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy Covers MAC and Physical layers. One single Mac with three Physical Layers. Based on cellular architecture. Cells called the Basic Service Set (BSS), controlled by Access Point (AP). Normally APs are connected by Distribution System, usually Ethernet, could be wireless. Whole set is seen as a single 802 network called Extended Service Set. Adhoc networks (IBSS) possible without AP with reduced features. Mac Layer uses two access methods a) Distributed Coordination Function (DCF) –mostly used b) Point Coordination Function (PCF). 5

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy – Typical Configuration 6

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy (Cont) DCF is basically a CSMA/CA with exponential backoff. Waits for Distributed Inter Frame Space (DIFS) –medium free time before transmitted its packet. Receiver gives an ack – success, else retransmit. Uses virtual carrier sense to avoid problem of indirect collision. PCF – optional, used for time-bounded services. Uses higher priority that AP may gain by PIFS asAP issues polling requests thus controlling access. Must leave enough time for distributed access. 7

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy – DCF working SIFS – Short Inter Frame Space, separate transmissions belonging to a single dialogue and is minimum Inter Frame size <DIFS and thus will have priority. Slot Time 8

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy (Cont) Allows for fragmentation and reassembly as shorter frames are beneficial. Synchronization through periodic “Beacon Frames”. To join an existing BSS a) Passive Scanning – wait for Beacons b) Active Scanning – send Probe Request Frames. Then Authentication and Association. Roaming similar to cellular but with differences. 9

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy (Cont) Frame Structures Fragmentation in MSDU – MAC Service Data Unit 10

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy (Cont) Frame Structures Three types of Frames Data - used for data Control - used to control access to medium (RTS, CTS,ACK) Management - Frames transmitted the same way as data frames to exchange management info in the same layer. 11 Frame in All frames in follow the above structure. Preamble – 96 bits – 80 bits of synch + 16 bits of SFD PLCP Header – PLCP_PDU Length word + PLCP signalling field + Header Error Check Field (16 Bit CRC)

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science legacy (Cont) Frame Structures MAC Data 12 Frame Control - Protocol Version + type of packet + whether from AP + Power Management + more Duration/ID - normally used for NAV calculation /station ID in poll messages Address fields – 1-recepient, 2-transmitter, 3- original source/destination, 4- special case, when (AP to AP) Sequence Control – order of different fragments

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science a An amendment to the IEEE specification that added a higher throughput of up to 54 Mbit/s by using the 5 GHz band, usually mid-20 Uses 52 OFDM subcarriers, 48 are for data and 4 are pilot subcarriers with a carrier separation of MHz (20 MHz/64). OFDM advantage in a multipath environment. Not a over crowded frequency but has weak Penetration of walls by frequency compared to 2.4 GHz. Had initial regulation issues and also timing and compatibility problems. Not reverse compatible with or b except for dual-band. 13

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science b b has a maximum raw data rate of 11 Mbit/s, typically 4.5 Mbit/s. Uses the same CSMA/CA access method. Uses exclusively DSSS (Direct-sequence spread spectrum) using CCK (Complementary code keying) or PBCC (packet binary convolutional coding) algorithm modulation scheme. slowest maximum speed; home appliances may interfere on unregulated frequency band but signal range is good and not easily obstructed Introduced optional support to Short PLCP PPDU format Made some changes to Long PLCP PPDU format 14

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science b –Long PLCP PPDU format 15 Changed the speed of signal rate Changed some uses of service field (Basically the same as )

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science b –Short PLCP PPDU format (Optional) 16 Observe that the preamble has been reduced to half

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science Motivation for g b has a maximum raw data rate of 11 Mbit/s, typically 4.5 Mbit/s, while a can provide up to 54Mbit/s. As days progressed 11Mbit/s was too small Want the same speed at 2.4MHz Be backward compatible to b Wish to take advantage of OFDM modulation scheme of a In short, need for a convergence of a and b at frequency range of 2.4MHz 17

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g g has a maximum raw data rate of 54 Mbit/s, typically 19 Mbit/s. Operates at 2.4MHz and is backward compatible to b. Can take advantage of OFDM modulation scheme. Observe typical of a is 23Mbit/s – the difference is due to legacy overhead for backward compatibility. Problem : The presence of even one b element in an other wise g network can drastically reduce performance. Similar to b, not compatible with a unless dual band. Today many of the products are dual-band/triple mode for compatibility. 18

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science Differences between g and b The major differences are The provision of four different physical layers The mandatory support of the short preamble type The ERP network attribute Newly defined protection mechanisms that deal with interoperability aspects The CTS-to-self mechanism 19

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g – Four Physical Layers ERP-DSSS/CCK (Mandatory): Old physical layer used by IEEE b. DSSS technology is used with CCK modulation. The data rates provided are those of IEEE b. ERP-OFDM (Mandatory): New physical layer, introduced by IEEE g. OFDM is used to provide IEEE a data rates at the 2.4 GHz band. ERP-DSSS/PBCC (Optional): Introduced as an option in IEEE b and provided the same data rates as the DSSS/CCK physical layer. IEEE g extended the set of data rates by adding 22 and 33 Mb/s (earlier 2,5.5,11 Mb/s). DSSS-OFDM (Optional): This is a new physical layer that uses a hybrid combination of DSSS and OFDM. The packet physical header is transmitted using DSSS, while the packet payload is transmitted using OFDM. The scope of this hybrid approach is to cover interoperability aspects. 20

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g – Four Physical Layers 21 Parameters of the different IEEE g physical layers.

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g – Mandatory support of Short Preamble It was clear even for b that the long preamble was too big, so they had introduced the short preamble g makes it mandatory. When the preamble and header are transmitted using DSSS (this happens at all physical layers except the ERP-OFDM), short and long types of preamble and header are defined. For the ERP-OFDM physical layer there is only one type of preamble and header, the format of which is almost identical to that of the IEEE a standard. 22

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g – The ERP network attribute Slot time =20 micro seconds, min contention window =31 slots in b. These values are good for data rates of b. For backward compatibility, g adapted them. However these values are too big for 6-54Mb/s, especially OFDM with only 20 micro seconds for preamble. The best values are from a which are 9 micro seconds and 15 slots g has dynamic adjustments of these values using a flag “ERP network attribute”, sent via a beacon frame. For BSS, if ERP attribute enabled, the slot time = 9 micro, mcw=15 and all frame exchanges use ERP-OFDM data rates. 23

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g – Interoperability and Protection Mechanisms Choice of 14 data rates, four physical rates and then Different stations ERP stations – basically g non-ERP supporting short preamble – newer b non-ERP without short preamble – older b Non-ERP stations do not detect ERP-OFDM from ERP. Solution1: Use of DSSS-OFDM, where every one can detect the PLCP preamble Solution2: Use of RTS/CTS frames to protect the OFDM packets and use of only ERP-DSSS physical layer for those. 24

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science g – CTS to Self Mechanism Problem of hidden node. 25

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science Conclusions 27 OFDM has been adopted as the mandatory high rate waveform in the 2.4 GHz band, so as to speeds up to 54Mb/s. Backward compatibility with b was assured. mandatory use of OFDM for data rates >20 Mbps, there are two optional waveforms: CCK/OFDM and PBCC. the case of the optional PBCC waveform, the peak data rate is 33 Mbps as compared to 54 Mbps for OFDM, i.e. the optional PBCC waveform is actually slower than the peak data rates for the mandatory OFDM waveform. OFDM already implemented for a, so for dual-band, it is very easy support for g.

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science References i) - site from which I showed the location of IEEE g standard document. ii) the standard document iii)http//ieeexplore.ieee.org/iel5/65/31204/ pdf?arnumber= The IEEE g Standard for High Data Rate WLANs iv) v) IEEE g New Draft Standard Clarifies Future of Wireless LAN vi) vii) 06.pdf IEEE g Explained 27

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science References viii) ix) A technical tutorial on IEEE protocol by Pablo Brenner. x) xi) xii) xiii) and other wiki pages xiv) 28

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science Other Developments in As originally the security in was low, it was improved by i. In July 2007, a new release of the standard that includes amendments a, b, d, e, g, h, i & j was made called the IEEE or ma n is trying to improve the data rate up to 300 Mb/s using MIMO antennas, expected to finalize June 09. On the other hand, keeping the data rate constant at 54Mb/s but increasing the distance to 5000 m, is y, using contention based protocol and a “lite licensing” scheme from FCC. This is expected this June.

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science a 27 The major problem in a was delay spread. With the then technology, the ceiling was around 20Mbps. It uses a modulation technique known as COFDM (coded OFDM). COFDM sends data in a massively parallel fashion, and slows the symbol rate down so each symbol transmission is much longer than the typical delay spread. A guard interval is inserted at the beginning of the symbol transmission to let all delayed signals "settle" before the baseband processor demodulates the data. COFDM slows the symbol rate while packing many bits in each symbol transmission, making the symbol rate substantially slower than the data bit rate.

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science a 27 It maps the data signal to be transmitted into several lower-speed signals, or subcarriers, which then are modulated individually and transmitted in parallel. IEEE a uses only the PLCP (physical layer convergence protocol) preamble which contains 10 short and 2 long symbols

University of Kansas | School of Engineering Department of Electrical Engineering and Computer Science a Frame PLCP preamble Section1 for synchronization Section2 for channel estimation.