Local Area Networks NETW 901

Transcription

1 Local Area Networks NETW 901 Lecture 4 Wireless LAN Course Instructor: Dr.-Ing. Maggie Mashaly maggie.ezzat@guc.edu.eg C

2 Contents What is a Wireless LAN? Applications and Requirements Transmission Techniques Spread Spectrum Technology OFDM 2

3 Wireless LANs A wireless LAN is one that makes use of wireless transmission medium Applications: 1. LAN Extension For buildings with large open areas or insufficient infrastructure as well as small offices where wired LAN installations are not economical 2. Mobile Access To connect a LAN hub to a mobile terminal with an antenna 3. Ad Hoc Networking A peer-to-peer network set up temporarily to meet some immediate need 3

4 Wireless LANs Requirements Global, seamless, simple operation Low power for long battery life No special permissions or licenses needed to use the LAN Easy to use for everyone, simple management Protection of investment in wired networks Security Transparency concerning higher layer protocols 4

5 Wireless LANs Transmission Techniques Modulation - Is the process of transforming digital information (1s and 0s) into analog signals - Performed by varying one or more properties of a periodic waveform called Carrier Signal 5

6 Wireless LANs Transmission Techniques Modulation Schemes 6

7 Wireless LANs Transmission Techniques Multiplexing - Is the process of sending multiple signals or streams of information over a communication link at the same time in the form of a single, complex signal - Can be based on time, frequency, code or space 7

8 Wireless LANs Frequency Division Multiplexing (FDM) Each user transmits over a narrow bandwidth frequency channel - Disadvantages: 1. Very narrow band is very sensitive to frequency variation 2. Frequency selective fading: different channels see different fading 3. Requires a frequency guard 8

9 Wireless LANs Frequency Hopping - Change the carrier frequency so that the transmission in one narrow band channel is done for a short time - Transmitter operates in one channel at a time for a fixed interval (e.g.: 300 ms in IEEE ) - A receiver has to be in synchronization with the sender s hopping pattern to pick up the message 9

10 Wireless LANs Frequency Hopping - Spreading Factor= Hopping Bandwidth/Narrow Band BW - Advantages: 1. Takes the average effect of interference 2. Takes the average effect of fading (in frequency) 3. Reduces the required fading and interference link margins 10

11 Wireless LANs - For a fast broadband channel, symbol duration becomes very small Symbol duration= 1/Symbol rate Symbol rate (Modulation rate) is the number of symbol/waveform changes across the transmission medium per time unit Effect of delay variation will become very evident resulting in high Intersymbol interference - Solution: Increase Symbol duration Use several links in parallel OFDM 11

12 Wireless LANs Orthogonal Frequency Division Multiplexing (OFDM ) - Uses one wide-frequency channel, breaks it up into N several slow narrow- band channels On each sub-channel symbol duration increases N times - Each sub-channel is used to transmit data - All slow sub-channels are then multiplexed into one combined fast channel - Sub-channels are closely spaced by removing guard bands without causing interference All sub-carriers are orthogonal (At the peak of each subcarrier the power of all other subcarriers is zero) 12

13 Wireless LANs OFDM - Peak of each signal frequency corresponds to the nulls of others 13

14 Wireless LANs OFDM - Reduces Inter-Symbol-Interference - Results in bandwidth saving 14

15 Wireless LAN Standards IEEE

16 IEEE Standard In 1990 IEEE working group was formed to develop a MAC protocol and physical medium specification dedicated to Wireless LANs Several versions have been developed since then: Original : 1 and 2 Mbit/s in a 2.45 GHz band Used either Frequency Hopping, or Direct Sequence Spreading b 11 Mbit/s in a 20 MHz Channel a Presented an alternative based on OFDM 54-Mbit/s, works in the 5 GHz band g - Used same modulation formats as a - Uses the 2.54 frequency band e - Provide modification to the MAC that allow ensuring certain levels of QoS More versions will be discussed through the course 16

17 IEEE Network Architecture Two architecture models: Infrastructure vs. Ad-hoc Networks infrastructure network AP AP: Access Point AP wired network AP ad-hoc network 17

18 IEEE Network Architecture 1. Architecture of an Infrastructure Network Station (STA) Terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) Group of stations using the same radio frequency Access Point Station integrated into the wireless LAN and the distribution system Portal Distribution System - Bridge to other wired networks - Interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS STA 1 ESS LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA LAN STA 3 18

19 IEEE Network Architecture 2. Architecture of an Ad-hoc Network Direct communication between nodes within a limited range Station (STA) Terminal with access mechanisms to the wireless medium and radio contact to other stations STA LAN IBSS 1 STA 3 Independent Basic Service Set (IBSS) Group of stations using the same radio frequency STA 2 IBSS 2 STA 5 STA LAN 19

20 IEEE Layers and Functions mobile terminal fixed terminal application TCP IP LLC access point LLC infrastructure network application TCP IP LLC MAC MAC MAC MAC PHY PHY PHY PHY 20

21 Station Management IEEE Layers and Functions MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding LLC PHY Management channel selection, MIB Station Management coordination of all management functions MAC PLCP PMD MAC Management PHY Management 21

22 IEEE MAC Challenges Existing multiple PHYs Support single and multiple channel PHYs Support PHYs with different Medium Sense characteristics BW not licensed to users Multiple users can exist in the same area and need to share the medium and reuse the frequencies Co-channel interference Need mechanisms to deal with - Hidden Nodes - Privacy and Access Control 22

23 IEEE MAC Layer-Access Methods MAC Layer is divided into two layers: 1. Distributed Coordination Function (DCF) - The DCF is the fundamental access method used to support asynchronous data transfer on a best effort basis. - All stations must support the DCF. - The DCF operates solely in the ad hoc network - Either operates solely or coexists with the PCF in an infrastructure network. 23

24 IEEE MAC Layer-Access Methods MAC Layer is divided into two layers: 2. Point Coordination Function (PCF) - The PCF is an optional capability - It is connection-oriented, and provides contention-free (CF) frame transfer. - PCF relies on the point coordinator (PC) to perform polling, enabling polled stations to transmit without contending Contention Free Transmission? 24

25 IEEE CSMA/CA Carrier Sense Multiple Access with Collision Avoidance CSMA/CA No Start Station has Tx data No Yes Listen to the channel Channel Idle Yes Transmit Collision Wait a random time No Wait until Tx stops Yes Wait Random Time 25

26 IEEE CSMA/CA Access CSMA/CA Mechanism Collision detection/avoidance - In wireless media collision can not be detected by listening to the channel (due to large dynamic range of signals on the medium, a transmitting station cannot effectively distinguish between incoming weak noise signals and effects of its own transmission ) - Collision can only be detected by not receiving an ACK for the transmitted packets To ensure smooth and fair functioning of CSMA/CA, DCF includes a set of delays Inter-Frame Spacing Intervals 26

27 IEEE CSMA/CA Access Inter-Frame Spacing Intervals - IFS intervals are mandatory periods of idle time on the transmission medium. - Three IFS intervals are specified in the standard: Short IFS (SIFS) Shortest IFS, used for all immediate response actions (e.g: before ACK reply) Point Coordination Function IFS (PIFS) Mid-length IFS, used by the centralized controller in the PCF scheme when issuing polls Distributed Coordination Function IFS (DIFS) Longest IFS, used as a minimum delay for asynchronous frames contending for access - Priority access to the wireless medium is controlled through the use of interframe space (IFS) (Any station using SIFS to determine transmission opportunity has, in effect, the highest priority because it will always gain access in preference to a station waiting for PIFS or DIFS) 27

28 IEEE CSMA/CA Access CSMA/CA operates as follows A station ready to send starts sensing the medium, if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) If the medium is busy (either because the station initially finds the medium busy or because a transmission started during IFS), the station defers transmission and continues to monitor the medium until the current transmission is over. (collision avoidance) Once current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off using a binary exponential back-off scheme and again senses the medium. If the medium is still idle, the station may transmit. DIFS medium busy DIFS PIFS SIFS contention window (randomized back-off mechanism) next frame direct access if medium is free DIFS slot time (20µs) t 28

29 IEEE CSMA/CA Access But consider the following case - A starts transmission to B, C doesn t receive the information - C also wants to transmit to B and senses the medium, it finds it free, so it also starts sending to B - What happens at B? Collision A B C 29

30 IEEE CSMA/CA Access Hidden Terminal Problem Occurs when two nodes are outside each other s range perform simultaneous transmission to a node that is within the range of each of them, resulting in a collision A B C C is a hidden node for A & A is a hidden node for C However, CSMA/CA has a solution for this problem 30

31 IEEE CSMA/CA Access Two types of Carrier Sensing can be used: Physical Carrier Sensing Depending upon the PHY layer, it senses the availability of the carrier frequency. Virtual Carrier Sensing Logical carrier sensing at the MAC layer. Every packet (with some exceptions) announces the duration for which the current transmission will hold the channel using the Network Allocation Vector (NAV). All stations monitoring the channel read the MAC header, which contains the NAV. They all back off for NAV microseconds before starting the contention for the next transmission. 31

32 IEEE Virtual Channel Sensing Source station first sends (Request to Send) RTS control frame (After successfully contending for the channel) All stations hearing the RTS packet, read the duration field and set their NAVs accordingly. After an SIFS idle period, the destination station responds to the RTS packet with a (Clear To Send) CTS packet. Stations hearing the CTS packet look at the duration field and again update their NAV. Upon successful reception of the CTS, the source station is virtually assured that the medium is stable and reserved for successful transmission sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t 32