Wireless LANS Part 2 Justin Champion Room C208 - Tel:

Wireless LANS  Contents  Speed  HIPERLAN  Wi-Fi  Issues with radio communications

Wireless LANS  Speed  Measure in bits per second (bps)  Kilo (Kbps)  1000 bits per second  Mega  1000 Kbps  Giga (Gbps)  1000 Mbps  Terra (Tbps)  1000 Gbps  Limited optical networks work at this speed and certainly nothing which is wireless  Remember that in data transfer a Kilo = 1000 not the 1024 used for data storage

Wireless LANS

 HIPERLAN  HIgh PERformance Local Area Network  HIPERLAN 1  20 Mbps  HIPERLAN 2  54 Mbps  Developed by ETSI  European Telecommunication Standards Institute

Wireless LANS  Common terms  Throughput  Is how much data is passing through a network in a given time  Bandwidth  Is the amount of data that could be transferred in a given time

Wireless LANS  HIPERLAN 2  Features  QOS  Power Saving built into the technology  Operates in the 5 GHz range  Strong Security using per session keys or long term key usage  DES or Triple-DES used  Increased Throughput over other wireless technologies  Allows convergence with other backbone technologies  ATM – This was originally the primary use of this technology  Ethernet  3G

Wireless LANS Why is HIPERLAN 2 good?

Wireless LANS  Central Control  This is referred to as the Access Point (AP)  Responsible for  Allows packets to be sent from a backbone to the wireless device  Informs devices which frequency to operate on  This allows for the optimum frequency to be selected, based on what else is happening with the interface  Each channel will be divided by 20 MHz giving 19 channels  Each of the channels will be divided into 52 sub carriers  48 of these are usable for data the other 4 are for synchronization

Wireless LANS  Communications  Multicast and Broadcast are supported  Communications are connection orientated  This gives a short setup time for communications to take place  Movement within the network is allowed  A device which recognises a stronger signal form another AP will connect to that AP  All connections will be moved from the 1 st AP to the 2 nd.  During this process packets may be lost and the application must request them again.

Wireless LANS  HIPERLAN 2 Layers Physical Data Link Control Layer Convergence Layer Higher OSI Layers

Wireless LANS  Physical  Different encoding methods used for different rates  If the signal to noise ratio becomes higher a lower transmission speed will be selected ModeModulation Code Rate Bit Rate Mbps 1BPSK¾6 2BPSK¾9 3QPSK½12 4QPSK¾18 516QAM9/ QAM¾36 764QAM¾54

Wireless LANS  Physical  Data transferred in several different sub carriers  Referred to as  Orthogonal Frequency Division Multiplex (OFDM)  The original bit pattern is re assembled at the destination  This technique allows  Better error handling  Reduction of multi-path propagation  Multi-Path Propagation  When a signal is sent it may bounce off several items before getting to the receiver  Indicating that the same signal can be received more than once or at a time when it would cause a corruption of a packet  The further the distance travelled for the signal the higher the likely hood of this happening.

Wireless LANS  Physical  Encoding the bits for transmission  Binary Phase Shift Keying (BPSK)  Any change in the carrier wave indicates a 1 else 0  The less data encoded the less chance there is of error Carrier BPSK

Wireless LANS  Physical  QPSK  Quadrature Phase Shirt Keying  Allows for streams of data to be encoded into the carrier wave  Shifts the carrier by either 90 or 180 degrees  16 QAM  Quadtrative Amplitude Modulation  By using Phase shifting and Amplitude changes encodes 4 bits at once  64 QAM  Same as 16 QAM but encoding 6 bits bb.watch.impress.co.jp/column/infra/2001/09/26/16qam.gif

Wireless LANS  Data Link Control Layer  Responsible for  MAC Access control  Responsible for the sharing of the radio link  Minimising the amount of time the link is required  Ultimately the responsibility of this is controlled by the access point  Logical Link Controller  Carried out error detection  Retransmission of lost packets  Forward Error recovery FEC, with a number of redundant bits sent it is possible to recover small amounts of corrupted data  HIPERLAN uses Reed Solomon codes to carry this out  Essential on a unreliable interface like radio

Wireless LANS  Convergence Layer  Adapts data sent from a lower/higher layer to the correct format.  This layer is the reason why HIPERLAN, is able to communicate with other technologies  As in transfer data from  ATM – Cell based convergence  Ethernet – Packet based convergence  3G - Packet based convergence

Wireless LANS  Transmission Medium  Uses Time Division duplex  Transmission Packet Format  Each packet which is transferred is a defined size of 2 ms in length

Wireless LANS  Transmission Packet Format Mac Frame BCHFCHACHDL PhaseUL PhaseRCH’s 2 ms

Wireless LANS  Transmission Packet Format  Broadcast Channel (BCH)  Transmission Power  Starting point of the FCH and length  Starting point and Length RCH  Identifier for the AP and network  Frame Control Channel (FCH)  Description of how transmit and receive resources are allocated in this frame  Access Feedback Channel (ACH)  Give information on previous attempts at accessing the RCH  Downlink (DL) Phase & Uplink (UL) Phase  Allows the transmission/receipt of packets of 54 bytes in size  Random Access Channel (RCH)  Used to request transmission resources – uplink and downlink  Transmission at this time takes place based on the fact that nothing else is transmitting

Wireless LANS  Summary of HIPERLAN  Quality of service is given by controlling the amount of transmissions  Each device must request permission to transmit or receive  This is given on the basis of the quality of service required  Issues  Expensive at the moment due to the radio technology  Not widely available  Future  HiperMan  High speed access on a metropolitan wide basis  HiperPan  High speed access on a personal basis

Wireless LANS  Wireless Fidelity (Wi-Fi)  Is a generic name for a set of IEEE standards namely  IEEE  IEEE A  IEEE B  IEEE G  Any product with this logo is able to interoperate with each other  The products are certified by the Wi-Fi Alliance

Wireless LANS  Standards  IEEE A  54 Mbps  5 GHz range  Modulation OFDM  IEEE B  11 Mbps  2.4 GHz Range  Modulation of DSSS  IEEE G  20 Mbps  2.4 GHz Range  Modulation of OFDM

Wireless LANS  Frequency 2.4 GHz  More commonly known as the ISM band  Industrial Scientific Medical (ISM)  Intended as worldwide free usage radio band  No license required  Widely used for  Wireless LAN technology  Actual usage will differ between countries  Usage must be none commercial  Same frequency as  Microwave Ovens  Cordless Phones  Other wireless Devices

Wireless LANS  Wireless Fidelity (Wi-Fi)  Operates  In the same manner as the Ethernet networks  A device waits for silence on the radio frequency  Carrier Sense Multiple Access (CSMA)  Operates in  AD-Hoc mode or  Uses a Access point (AP) to allow connection to wired infrastructure

Wireless LANS  Wireless Fidelity (Wi-Fi) CSMA RTS CTS Data ACK Access Point RTS = Request to Send CTS = Clear to Send Data = ACK = Acknowledgment

Wireless LANS  Wi-Fi b transmissions  Carried out using Direct Sequence Spread Spectrum (DSSS)  The original signal is combined with a Pseudo random Number (Code Word)  The code word is referred to as the Baker Code  This allows improved reliability in data transmission rather than just sending data.  A wider bandwidth is required for the transmission  Each part of the packet is then sent on a different frequency  Total required bandwidth is 22 MHz  The receiver puts this information back together again  This method is used due to the ability to send large amounts of data at once

Wireless LANS  Wi-FI Data Packet  Only the Data part is sent at full speed  The rest is sent at 1 Mbps

Wireless LANS  Wi-Fi  Wired Equivalent Privacy (WEP)  Uses either 40 or 128 bit RS4 symmetric encryption  The standard does not define how to distribute the keys!  Discussion of cracking the encryption algorithm real-time!  )  When used 40 Bit encryption reduces throughput by 20%  Trying to avoid the Pringle situation, which was embarrassing for the technology and users of it  news.bbc.co.uk/1/hi/sci/tech/ stm, 2002) news.bbc.co.uk/1/hi/sci/tech/ stm  War Driving, users accessing wireless networks without permission  Wi-Fi is targeted with wall chalking indicating locations to connect  The technology is targeted as it is widely used and available  Radio Signal Propagation  Radio signals can not be restricted to a geographic area without expensive specially built buildings

Wireless LANS  Wi-Fi

Wireless LANS  Wi-Fi - Future  IEEE (WiMax)  a is intended as a wireless metropolitan technology  First devices should be appearing in Late 2004  Allows 120 Mbps  Uses the 10 to 66 GHz frequency  This will require no interference, so transmitters and receivers will need to be placed on the roof  Transmission up to 30 Miles  Currently a limit on receivers which is the hundreds  Standards Group  grouper.ieee.org/groups/802/16/index.html

Wireless LANS  Summary  HiperLan2  ISM  Wi-fi