IEEE 802 11 Wireless LAN Draft Standard Professor

IEEE 802. 11 Wireless LAN Draft Standard Professor R. A. Carrasco

Introduction • IEEE 802. 11 Draft 5. 0 is a draft standard for Wireless Local Area Network (WLAN) communication. • This tutorial is intended to describe the relationship between 802. 11 and other LANs, and to describe some of the details of its operation. • It is assumed that the audience is familiar with serial data communications, the use of LANs and has some knowledge of radios.

802. 11 Data Frame Bytes 2 Frame Control Bits 2 6 6 2 Duration Address 1 Address 2 Address 3 Seq Address 4 2 2 4 Version Type Subtype 1 1 1 To From MF DS DS 1 1 Re. Pwr More W try O 1 1 1 0 -2312 4 Data Checksum Frame Control

Contents • • • Glossary of 802. 11 Wireless Terms Overview 802. 11 Media Access Control (MAC) Frequency Hopping and Direct Sequence Spread Spectrum Techniques 802. 11 Physical Layer (PHY) Security Performance Inter Access Point Protocol Implementation Support Raytheon Implementation

Glossary of 802. 11 Wireless Terms • Station (STA): A computer or device with a wireless network interface. • Access Point (AP): Device used to bridge the wireless-wired boundary, or to increase distance as a wireless packet repeater. • Ad Hoc Network: A temporary one made up of stations in mutual range. • Infrastructure Network: One with one or more Access Points. • Channel: A radio frequency band, or Infrared, used for shared communication. • Basic Service Set (BSS): A set of stations communicating wirelessly on the same channel in the same area, Ad Hoc or Infrastructure. • Extended Service Set (ESS): A set BSSs and wired LANs with Access Points that appear as a single logical BSS.

Glossary of 802. 11 Wireless Terms, cont. • BSSID & ESSID: Data fields identifying a stations BSS & ESS. • Clear Channel Assessment (CCA): A station function used to determine when it is OK to transmit. • Association: A function that maps a station to an Access Point. • MAC Service Data Unit (MSDU): Data Frame passed between user & MAC. • MAC Protocol Data Unit (MPDU): Data Frame passed between MAC & PHY. • PLCP Packet (PLCP_PDU): Data Packet passed from PHY to PHY over the Wireless Medium.

Overview, IEEE 802, and 802. 11 Working Group • IEEE Project 802 charter: – Local & Metropolitan Area Networks – 1 Mb/s to 100 Mb/s and higher – 2 lower layers of 7 Layer OSI Reference Model • IEEE 802. 11 Working Group scope: – Wireless connectivity for fixed, portable and moving stations within a limited area – Appear to higher layers (LLC) the same as existing 802 standards • Transparent support of mobility (mobility across router ports is being address by a higher layer committee)

Overview, IEEE 802. 11 Committee • Committee formed in 1990 – Wide attendance • Multiple Physical Layers – Frequency Hopping Spread Spectrum – Direct Sequence Spread Spectrum – Infrared • 2. 4 GHz Industrial, Scientific & Medical shared unlicensed band – 2. 4 to 2. 4835 GHz with FCC transmitted power limits • 2 Mb/s & 1 Mb/s data transfer • 50 to 200 feet radius wireless coverage • Draft 5. 0 Letter Ballot passed and forwarded to Sponsor Ballot – Published Standard anticipated 1997 • Next 802. 11 - November 11 -14, Vancouver, BC – Chairman - Victor Hayes, v. hayes@ieee. org

Overview, 802. 11 Architecture ESS Existing Wired LAN AP STA BSS STA Infrastructure Network STA Ad Hoc Network STA BSS STA Ad Hoc Network

Overview, Wired vs. Wireless LANs • 802. 3 (Ethernet) uses CSMA/CD, Carrier Sense Multiple Access with 100% Collision Detect for reliable data transfer • 802. 11 has CSMA/CA (Collision Avoidance) – Large differences in signal strengths – Collisions can only be inferred afterward • Transmitters fail to get a response • Receivers see corrupted data through a CRC error

802. 11 Media Access Control • Carrier Sense: Listen before talking • Handshaking to infer collisions – DATA-ACK packets • Collision Avoidance – – – RTS-CTS-DATA-ACK to request the medium Duration information in each packet Random Backoff after collision is determined Net Allocation Vector (NAV) to reserve bandwidth Hidden Nodes use CTS duration information

802. 11 Media Access Control, cont. • Fragmentation – Bit Error Rate (BER) goes up with distance and decreases the probability of successfully transmitting long frames – MSDUs given to MAC can be broken up into smaller MPDUs given to PHY, each with a sequence number for reassembly • Can increase range by allowing operation at higher BER • Lessens the impact of collisions • Trade overhead for overhead of RTS-CTS • Less impact from Hidden Nodes

802. 11 Media Access Control, cont • Beacons used convey network parameters such as hop sequence • Probe Requests and Responses used to join a network • Power Savings Mode – Frames stored at Access Point or Stations for sleeping Stations – Traffic Indication Map (TIM) in Frames alerts awaking Stations

802. 11 Protocol Stack Upper Layers Logical Link Control Data Link Layer MAC Sublayer 802. 11 Infrared 802. 11 FHSS 802. 11 DSSS 802. 11 a OFDM 802. 11 b HR-DSSS 802. 11 g OFDM Physical Layer

Performance of IEEE 802. 11 b MAC Header 30 Bytes CRC 4 Bytes Data MPDU Backoff PLCP Preamble PLCP Header MPDU SIFS PLCP Preamble Header DIFS Ack 14 Bytes

Performance of IEEE 802. 11 b • Successful transmission of a signal frame • PLCP = physical layer convergence protocol preamble Header transmission time (varies according to the bit rate used by the host SIFS = 10 sec (Short Inter Frame Space) is the MAC acknowledgement transmission time (10 sec if the selected rate is 11 Mb/sec, as the ACK length is 112 bits

Performance of IEEE 802. 11 b • DIFS = = is the frame transmission time, when it transmits at 1 Mb/s, the long PLCP header is used and = If it uses 2, 5. 5 or 11 Mb/s, then = (Short PLCP header)

Performance of IEEE 802. 11 b • For bit rates greater than 1 Mb/s and the frame size of 1500 Bytes of data (MPDU of total 1534 Bytes), proportion p of the useful throughput measured above the MAC layer will be: • So, a signal host sending long frames over a 11 Mb/s radio channel will have a maximum useful throughput of 7. 74 Mb/s

Performance of IEEE 802. 11 b • If we neglect propagation time, the overall transmission time is composed of the transmission time and a constant overhead Where the constant overhead

Performance of IEEE 802. 11 b • The overall frame transmission time experienced by a single host when competing with N – 1 other hosts has to be increased by time interval tcont that accounts for the time spent in contention procedures

Performance of IEEE 802. 11 b So the overall transmission time Where is the propagation of collision experienced for each packet successfully acknowledged at the MAC

Performance of IEEE 802. 11 b • Consider how the situation in which N hosts of different bit rate compete for the radio channel. N-1 hosts use the high transmission rate R = 11 Mb/s and one host transmits at a degraded rate R = 5. 5, 2, or 1 Mb/s Where is the data frame length in bits

Performance of IEEE 802. 11 b • The MAC layer ACK frame is also sent at the rate that depends on the host speed, thus we denote by and Let the associated overhead time be the overall transmission time for a “fast” host transmitting at rate R

Performance of IEEE 802. 11 b • Similarly, let Ts be the corresponding time for a “slow” host transmitting at rate T: We can express the channel utilization of the slow host as where

Performance of IEEE 802. 11 b • Study: The UDP traffic & TCP traffic. Flows in IEEE 802. 11 WLANs

Frequency Hopping and Direct Sequence Spread Spectrum Techniques • Spread Spectrum used to avoid interference from licensed and other non-licensed users, and from noise, e. g. , microwave ovens • Frequency Hopping (FHSS) – Using one of 78 hop sequences, hop to a new 1 MHz channel (out of the total of 79 channels) at least every 400 milliseconds • Requires hop acquisition and synchronization • Hops away from interference • Direct Sequence (DSSS) – Using one of 11 overlapping channels, multiply the data by an 11 -bit number to spread the 1 M-symbol/sec data over 11 MHz • Requires RF linearity over 11 MHz • Spreading yields processing gain at receiver • Less immune to interference

802. 11 Physical Layer • Preamble Sync, 16 -bit Start Frame Delimiter, PLCP Header including 16 -bit Header CRC, MPDU, 32 -bit CRC • FHSS – 2 & 4 GFSK – Data Whitening for Bias Suppression • 32/33 bit stuffing and block inversion • 7 -bit LFSR scrambler – 80 -bit Preamble Sync pattern – 32 -bit Header • DSSS – – DBPSK & DQPSK Data Scrambling using 8 -bit LFSR 128 -bit Preamble Sync pattern 48 -bit Header

802. 11 Physical Layer, cont. • Antenna Diversity – – Multipath fading a signal can inhibit reception Multiple antennas can significantly minimize Spacial Separation of Orthoganality Choose Antenna during Preamble Sync pattern • Presence of energy • RSSI - Received Signal Strength Indication • Combination of both • Clear Channel Assessment – Require reliable indication that channel is in use to defer transmission – Use same mechanisms as for Antenna Diversity – Use NAV information

A Fragment Burst A B C D Frag 1 RTS CTS Frag 2 ACK NAV Time Frag 3 ACK

Security • Authentication: A function that determines whether a Station is allowed to participate in network communication – Open System (null authentication) & Shared Key • WEP - Wired Equivalent Privacy • Encryption of data • ESSID offers casual separation of traffic

Performance, Theoretical Maximum Throughput • Throughput numbers in Mbits/sec: – Assumes 100 ms beacon interval, RTS, CTS used, no collision – Slide courtesy of Matt Fischer, AMD

Background for broadband wireless technologies • UWB – Ultra Wide Band – High speed wireless personal area network • Wi-Fi – Wireless fidelity – Wireless technology for indoor environment (WLANS) – broader range that WPANs • Wi. MAX – Worldwide Interoperability for Microwave Access – Wireless Metropolitan Area Networks (WMANs) – For outdoor coverage in LOS and NLOS environment – Fixed and Mobile standards • 3 G – Third generation – Wireless Wide Area Networks (WMANs) are the broadest range wireless networks – High speed data transmission and greater voice capacity for mobile users

What is Wi. Max? • Wi. MAX is an IEEE 802. 16/ETSI Hiper. MAN based certificate for equipments fulfilling the interoperability requirements set by Wi. MAX Forum. • Wi. MAX Forum comprises of industry leaders who are committed to the open interoperability of all products used for broadband wireless access. • The technique or technology behind the standards is often referred as Wi. MAX

What is Wi. Max? • Broadband is thus a Broadband Wireless Access (BWA) technique • Wi. Max offers fast broadband connections over long distances • The interpretability of different vendor’s product is the most important factor when comparing to the other techniques.

The IEEE 802. 16 Standards • The IEEE 802. 16 standards family - broadband wireless wideband internet connection - wider coverage than any wired or wireless connection before • Wireless system have the capacity to address broad geographic areas without the expensive wired infrastructure • For example, a study made in University of Oulu state that Wi. Max is clearly more cost effective solution for providing broadband internet connection in Kainuu than x. DSL

The IEEE 802. 16 Standards • The IEEE 802. 16 standards family - broadband wireless wideband internet connection - wider coverage than any wired or wireless connection before • Wireless system have the capacity to address broad geographic areas without the expensive wired infrastructure • For example, a study made in University of Oulu state that Wi. Max is clearly more cost effective solution for providing broadband internet connection in Kainuu than x. DSL

The IEEE 802. 16 Standards • • • 802. 16, published in April 2002 - A set od air interfaces on a common MAC protocol - Addresses frequencies 10 to 66 GHz - Single carrier (SC) and only LOS 802. 16 a, published in January 2003 - A completed amendment that extends the physical layer to the 2 to 11 GHz both licensed and lincensed-exempt frequencies - SC, 256 point FFT OFDM and 2048 point FFT OFDMA - LOS and NLOS 802. 16 -2004, published in July 2004 - Revises and replaces 802. 16, 802. 16 a and 802. 16 REVd. - This announcements marks a significant milestone in the development of future Wi. Max technology - P 802. 16 -2004/Corl published on 8. 11. 2005

IEEE 802. 16: Broadband Wireless MAN Standard (Wi. MAX) • An 802. 16 wireless service provides a communications path between a subscriber site and a core network such as the public telephone network and the Internet. This wireless broadband access standard provides the missing link for the "last mile" connection in metropolitan area networks where DSL, Cable and other broadband access methods are not available or too expensive.

Comparison Overview of IEEE 802. 16 a • IEEE 802. 16 and Wi. MAX are designed as a complimentary technology to Wi-Fi and Bluetooth. The following table provides a quick comparison of 802. 16 a with to 802. 11 b Parameters 802. 16 a (Wi. Max) 802. 11 (WLAN) 802. 15 (Bluetooth) Frequency Band 2 -11 GHz 2. 4 GHz Varies Range ~31 miles ~100 meters ~10 meters Data transfer rate 70 Mbps 11 Mbps – 55 Mbps 20 Kbps – 55 Mbps Number of Users Thousands Dozens

Protocol Structure -IEEE 802. 16: Standard (Wi. MAX) • IEEE 802. 16 Protocol Architecture has 4 layers: Convergence, MAC, Transmission and physical, which can be map to two OSI lowest layers: physical and data link

ALOHA and Packet Broadcasting Channel Prof. R. A. Carrasco School of Electrical, Electronic and Computer engineering 2006 University of Newcastle-upon-Tyne

Packet Broadcasting Related Works by Metcalfe and Abransom 1) 1970: N. Abramson, “The ALOHA System – Another alternative for computer communications. ”, in Proc. AFIPS Press, vol 37, 1970 2) 1973: R. M. Metcalfe, “Packet communication, ” MIT, Cambridge, MA, Rep. MAC TR-114, July 1973. 3) 1977: N. Abramson, “The Throughput of Packet Broadcasting Channels, ” IEEE Trans. Commun. , vol. COM -25, no. 10, Jan 1977 4) 1985: N. Abramson, “Development of the ALOAHANET, ” IEEE Trans. Info. Theory. , March 1985

IEEE Transactions on Information Theory, March 1985 • Development of the ALOHANET

ALOHA Project • Started In September 1968 • Goal – To build computer network in University of Hawaii. – To investigate the use of radio communications as an alternative to the telephone system for computer communication. – To determine those situations where radio communications are preferable to conventional wire communications

Problem • Limited Resource: Channel • Intermittent operation typical of interactive computer terminal don’t need point-topoint channels. (FDMA or TDMA) • Spread Spectrum is not appropriate to share the channel.

Approach • Packet Broadcasting Channels – Each user transmits packets over the common broadcast channel. – Key innovation of ALOHANET. There are basically two types of ALOHA systems --Synchronized or slotted and --Unsynchronized or unslotted

System Design • 1968, they decided main approach (Packet Broadcasting) for design simplicity. • Frequency Band: two 100 KHz bandwidth channels at 407. 350 MHz and 413. 475 MHz. • TCU (Terminal Control Unit): – – Formatting of the ALOHA packets. Retransmission protocol. A Terminal attached TCU by means of RS 232. Half duplex mode. (too expensive memory)

History • 1971: start operation in University of Hawaii. • 1971 -72: build additional TCUs. • 1972: connect to ARPANET using satellite channel. (56 kbps) • 1973: Metcalfe’s doctorial dissertation about packet broadcasting. • 1973: PACNET, international satellite networks. (9600 bits/s) • 1973 ~ : Many researches about “packet broadcasting”. • 1976: slotted ALOHA. • 1984: unslotted ALOHA in the UHF band by Motorola.

Strategic Theoretical Realities • An appreciation of the basic capacity of the channels and the matching of that capacity to the information rate of the signals. – In data network, distinguish between the average data rate and the burst data rate – Network design: to handle different kinds of signals from different source. • Deals with the problem of scaling for large system. – Packet broadcasting channel is more scalable than point-to-point channel or switching. • Theoretical analysis give good guide to design network, but the converse also is true. The operation of a real network can be a valuable guide to the selection of theoretical problems.

Packet Switching and Packet Broadcasting • Packet switching can provide a powerful means of sharing communication resources. • But it employ point-to-point channels and large switches for routing. • By use of packet broadcasting Elimination of routing and switches. System simplicity Some channels are basically broadcast channel. (satellite, . . ) • Needs unified presentation of packet broadcasting theory.

Packet Broadcasting Channel • Each user transmits packets over the common broadcast channel completely unsynchronized. • Loss due to the overlap. • How many users can share a channel?

Recovery of Lost Packets • Positive Acknowledgements. • Transponder Packet Broadcasting. • Carrier Sense Packet Broadcasting. • Packet Recovery Codes

ALOHA Systems and Protocols • We assume that the start time of packets/s that are transmitted is a Poisson point process • An average rate of λ packets • Let Tp denote the time duration of a packet • The normalised channel traffic G is defined G=λTp It also called the offered channel traffic

ALOHA Capacity • Errors reduce the ALOHA Capacity – Random noise errors – Errors caused by packet overlap. Statistical Analysis: S: Channel Throughput G: Channel Traffic Throughput is maximum 1/2 e when channel traffic equals 0. 5.

ALOHA Capacity • Meaning of the result – ALOHA: 9600 bits/s – Terminal: 5 bits/s – 9600 X 1/2 e = about 1600 bits/s – The channel can handle the traffic of over 300 active terminals and each terminal will operate at a peak data rate 9600 bits/s

Slotted ALOHA Channel Capacity • Each user can start his packet only at certain fixed instants. Statistical Analysis It increase throughput

Mixed Data Rates • • Unslotted ALOHA: Variable Packet Lengths = Long Packet Length/ Short Packet Length • • G 1 = Short Packet Traffic G 2 = Long Packet Traffic Total channel throughput can undergo a significant decrease.

Slotted ALOHA: Variable Packet Rates • Assume ALOHA used by n users with different channel traffic.

ALOHA • Meaning of the result – In a lightly loaded slotted ALOHA channel, a single user can transmit data at rates above the limit 1/e. : Excess Capacity. – Important for the network consisting of many interactive terminal users and small number of users who send large but infrequent files.

Question 1 • In a pure ALOHA system, the channel bit rate is 2400 bits/s. Suppose that each terminal transmits a 100 -bit message every minute on average. i) Determine the maximum number of terminals that can use the channel ii) Repeat (i) if slotted ALOHA is used

Question 2 • An alternative derivation for the throughput in a pure ALOHA system may be obtained from the relation G=S+A, where A is the average (normalised) rate of retransmission. Show that A=G(1 -e-2 G ) and then solve for S.

Question 3 • Consider a pure ALOHA system that is operating with a throughput S=0. 1 and packets are generated with a Poisson arrival rate λ. Determine: i) The value of G ii) The average number of attempted transmissions to send a packet.

Question 4 • Consider a CSMA/CD system in which the transmission rate on the bus is 10 Mτbits/s. The bus is 2 Km and the propagation delay is 5 μs/Km. Packets are 1000 bits long. Determine: i) The end-to-end delay d. ii) The packet duration Tp iii) The ratio d/Tp iv) The maximum utilization of the bus and the maximum bit rate.