Chapter 8 Hardware Layers Wireless Local Area Networks Networking in the Internet Age by Alan Dennis Copyright 2002 John Wiley Sons Inc 1 Copyright John Wiley Sons Inc All rights reserved Reproduction or translation of this work beyond that named in Section 117 of the United States Copyright Act without the express written consent of the copyright owner is unlawful Requests for further information should be addressed to the Permissions Department John Wiley Sons Inc Adopters of the textbook are granted permission to make back up copies for their own use only to make copies for distribution to students of the course the textbook is used in and to modify this material to best suit their instructional needs Under no circumstances can copies be made for resale The Publisher assumes no responsibility for errors omissions or damages caused by the use of these programs or from the use of the information contained herein 2 Chapter 8 Learning Objectives Understand the major components of WLANs Understand 802 11b WLANs Understand 802 11a WLANs Understand 802 11g WLANs Be familiar with Bluetooth WLANs Understand the best practice recommendations for WLAN design 3 Chapter 8 Outline Introduction IEEE 802 11b Topology Media Access Control Error Control Message Delineation Data Transmission in the Physical Layer IEEE 802 11a Topology Media Access Control Error Control Message Delineation Data Transmission in the Physical Layer IEEE 802 11g Bluetooth Topology Media Access Control Error Control Message Delineation Data Transmission in the Physical Layer The Best Practice WLAN Design Effective Data Rates Costs Recommendations 4 Introduction 5 Introduction Wireless LANs transmit data through the air using radio frequency transmissions Several standards for WLANs have recently emerged facilitating market to take off Currently the three principal WLAN technologies are 802 11b low speed 802 11a a higher speed protocol and Bluetooth An emerging WLAN standard that may prove more important in the future is 802 11g 6 WLAN in the workplace WLANs are popular because they Eliminate cabling and make network access possible from a variety of locations Facilitate computing for mobile workers at different office locations or as those workers move around the office Are increasingly used in hospitals because they enable doctors and nurses access patient records Are becoming popular in airports because they enable business travelers to access the Internet while waiting for their flights to leave 7 IEEE 802 11b 8 Wireless Ethernet IEEE 802 11b The IEEE 802 11b also called wireless Ethernet is now the dominant WLAN standard Two version of IEEE 802 11b exist Frequency hopping spread spectrum FHSS with data rates of 1 and 2 Mbps and Direct sequence spread spectrum DHSS with data rates of 1 2 5 5 and 11Mbps which dominates the market due to its higher speed 9 Types of Wireless Ethernet Two forms of the IEEE 802 11b standard currently exist Direct Sequence Spread Spectrum DSSS uses the entire 2 4 GHz WLAN frequency band to transmit information DSSS is capable of data rates of up to 11 Mbps with fallback rates of 5 5 2 and 1 Mbps Lower rates are used whenever interference or congestion occurs Frequency Hopping Spread Spectrum FHSS divides the frequency band into a series of channels and then changes its frequency channel about every half a second using a pseudorandom sequence FHSS is more secure but is only capable of data rates of 1 or 2 Mbps since the frequency band gets divided up into a number of channels IEEE 802 11a is another Wireless LAN standard that is still being defined It will operate in the 5 GHz band and be capable of data rates of up to 54 Mbps but will probably average about 20 Mbps in practice 10 IEEE 802 11b Wireless LAN Topology WLANs use a physical star logical bus topology Fig 8 1 Each WLAN computer uses a wireless NIC that transmits radio signals to the AP WLAN network access is through devices called access points APs which have a maximum transmission range of about 100 500 feet AP also connect into the wired LAN The AP acts as a repeater by retransmitting frames from client computers over the wired network Multiple APs are needed to make wireless access possible in most areas of a building IEEE 802 11 also uses 3 separate radio channels allowing APs with overlapping ranges to be set up without interfering with each other s signals 11 Figure 8 1 A wireless Ethernet access point connected into an Ethernet Switch 12 WLAN Media Access Control Wireless LANs use CSMA CA where CA collision avoidance CA With CA a station waits until another station is finished transmitting then continues to wait an additional random period of time before sending anything thus ensuring that the network is truly not being used Two WLAN MAC techniques are now in use Distributed Coordination Function DCF Point Coordination Function PCF 13 Distributed Coordination Function DCF With DCF also known as physical sense carrier method a node that wants to send first listens to make sure that the transmitting node has finished then waits a random period of time longer During transmission each frame is sent using the Stop and Wait ARQ so by waiting the listening node can detect that the sending node has finished and can then begin sending its transmission With Wireless LANs ACK NAK signals are sent a short time after a frame is received Stations wishing to send a frame wait a somewhat longer time ensuring that no collision will occur 14 Point Coordination Function PCF When a computer on a Wireless LAN is near the transmission limits of the AP at one end and another computer is near the transmission limits at the other end of the AP s range both computers may be able to transmit to the AP but can not detect each other s signals This is known as the hidden node problem When it occurs the physical carrier sense method will not work The virtual carrier sense method solves this problem by having a transmitting station first send a request to send RTS signal to the AP If the AP responds with a clear to send CTS signal the computer wishing to send a frame can then begin transmitting 15 IEEE 802 11b Message Delineation IEEE 802 11b frames Figure 8 3 have 5 sections preamble which contains a preamble used to mark the start of the frame physical layer convergence protocol PLCP header enables the NIC and AP to determine the best data rate to use The length field also gives the length of the payload portion of the frame payload header contains