Wireless Networks 120719.ppt
- 2. Wireless? • A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier. • The last link with the users is wireless, to give a network connection to all users in a building or campus. • The backbone network usually uses cables
- 3. Common Topologies The wireless LAN connects to a wired LAN • There is a need of an access point that bridges wireless LAN traffic into the wired LAN. • The access point (AP) can also act as a repeater for wireless nodes, effectively doubling the maximum possible distance between nodes.
- 4. Common Topologies Complete Wireless Networks • The physical size of the network is determined by the maximum reliable propagation range of the radio signals. • Referred to as ad hoc networks • Are self-organizing networks without any centralized control • Suited for temporary situations such as meetings and conferences.
- 5. How do wireless LANs work? Wireless LANs operate in almost the same way as wired LANs, using the same networking protocols and supporting the most of the same applications.
- 6. LAN
- 7. Reference model Physical layer: conversion of stream of bits into • signals – carrier generation - frequency selection • – signal detection – encryption Data link layer: accessing the medium – • multiplexing - error correction – synchronization Network layer: routing packets – addressing - • handover between networks Transport layer: establish an end-to-end • connection – quality of service – flow and • congestion control Application layer: service location – support • multimedia – wireless access to www
- 8. Wireless Network The difference between wired and wireless is the • physical layer and the data link layer Wired network technology is based on wires or • fibers Data transmission in wireless networks take place • using electromagnetic waves which propagates • through space (scattered, reflected, attenuated) Data are modulated onto carrier frequencies • (amplitude, frequency) The data link layer (accessing the medium, • multiplexing, error correction, synchronization) • requires more complex mechanisms.
- 9. How are WLANs Different? • They use specialized physical and data link protocols • They integrate into existing networks through access points which provide a bridging function • They let you stay connected as you roam from one coverage area to another • They have unique security considerations • They have specific interoperability requirements • They require different hardware • They offer performance that differs from wired LANs.
- 10. Physical and Data Link Layers Physical Layer: • The wireless NIC takes frames of data from the link layer, scrambles the data in a predetermined way, then uses the modified data stream to modulate a radio carrier signal. Data Link Layer: • Uses Carriers-Sense-Multiple-Access with Collision Avoidance (CSMA/CA).
- 11. Integration With Existing Networks • Wireless Access Points (APs) - a small device that bridges wireless traffic to your network. • Most access points bridge wireless LANs into Ethernet networks, but Token-Ring options are available as well.
- 12. Integration With Existing Networks
- 13. Roaming • Users maintain a continuous connection as they roam from one physical area to another • Mobile nodes automatically register with the new access point. • Methods: DHCP, Mobile IP • IEEE 802.11 standard does not address roaming, you may need to purchase equipment from one vendor if your users need to roam from one access point to another.
- 14. Security • In theory, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used • The IEEE 802.11 standard specifies optional security called "Wired Equivalent Privacy" whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures.
- 15. Interoperability • Before the IEEE 802.11 interoperability was based on cooperation between vendors. • IEEE 802.11 only standardizes the physical and medium access control layers. • Vendors must still work with each other to ensure their IEEE 802.11 implementations interoperate • Wireless Ethernet Compatibility Alliance (WECA) introduces the Wi-Fi Certification to ensure cross- vendor interoperability of 802.11b solutions
- 16. Hardware • PC Card, either with integral antenna or with external antenna/RF module. • ISA Card with external antenna connected by cable. • Handheld terminals • Access points
- 17. Hardware CISCO Aironet 350 series Wireless Handheld Terminal Semi Parabolic Antenna BreezeCOM AP
- 18. Performance • 802.11a offers speeds with a theoretically maximum rate of 54Mbps in the 5 GHz band • 802.11b offers speeds with a theoretically maximum rate of 11Mbps at in the 2.4 GHz spectrum band • 802.11g is a new standard for data rates of up to a theoretical maximum of 54 Mbps at 2.4 GHz.
- 19. What is 802.11? • A family of wireless LAN (WLAN) specifications developed by a working group at the Institute of Electrical and Electronic Engineers (IEEE) • Defines standard for WLANs using the following four technologies • Frequency Hopping Spread Spectrum (FHSS) • Direct Sequence Spread Spectrum (DSSS) • Infrared (IR) • Orthogonal Frequency Division Multiplexing (OFDM) • Versions: 802.11a, 802.11b, 802.11g, 802.11e, 802.11f, 802.11i
- 20. 802.11 - Transmission • Most wireless LAN products operate in unlicensed radio bands • 2.4 GHz is most popular • Available in most parts of the world • No need for user licensing • Most wireless LANs use spread-spectrum radio • Resistant to interference, secure • Two popular methods • Frequency Hopping (FH) • Direct Sequence (DS)
- 21. Frequency Hopping Vs. Direct Sequence • FH systems use a radio carrier that “hops” from frequency to frequency in a pattern known to both transmitter and receiver • Easy to implement • Resistance to noise • Limited throughput (2-3 Mbps @ 2.4 GHz) • DS systems use a carrier that remains fixed to a specific frequency band. The data signal is spread onto a much larger range of frequencies (at a much lower power level) using a specific encoding scheme. • Much higher throughput than FH (11 Mbps) • Better range • Less resistant to noise (made up for by redundancy – it transmits at least 10 fully redundant copies of the original signal at the same time)
- 22. 802.11a • Employs Orthogonal Frequency Division Multiplexing (OFDM) • Offers higher bandwidth than that of 802.11b, DSSS (Direct Sequence Spread Spectrum) • 802.11a MAC (Media Access Control) is same as 802.11b • Operates in the 5 GHz range
- 23. 802.11a Advantages • Ultra-high spectrum efficiency • 5 GHz band is 300 MHz (vs. 83.5 MHz @ 2.4 GHz) • More data can travel over a smaller amount of bandwidth • High speed • Up to 54 Mbps • Less interference • Fewer products using the frequency • 2.4 GHz band shared by cordless phones, microwave ovens, Bluetooth, and WLANs
- 24. 802.11a Disadvantages • Standards and Interoperability • Standard not accepted worldwide • No interoperability certification available for 802.11a products • Not compatible or interoperable with 802.11b • Legal issues • License-free spectrum in 5 GHz band not available worldwide • Market • Beyond LAN-LAN bridging, there is limited interest for 5 GHz adoption
- 25. 802.11a Disadvantages • Cost • 2.4 GHz will still has >40% cost advantage • Range • At equivalent power, 5 GHz range will be ~50% of 2.4 GHz • Power consumption • Higher data rates and increased signal require more power • OFDM is less power-efficient then DSSS
- 26. 802.11a Applications • Building-to-building connections • Video, audio conferencing/streaming video, and audio • Large file transfers, such as engineering CAD drawings • Faster Web access and browsing • High worker density or high throughput scenarios • Numerous PCs running graphics-intensive applications
- 27. 802.11a Vs. 802.11b 802.11a vs. 802.11b 802.11a 802.11b Raw data rates Up to 54 Mbps (54, 48, 36, 24,18, 12 and 6 Mbps) Up to 11 Mbps (11, 5.5, 2, and 1 Mbps) Range 50 Meters 100 Meters Bandwidth UNII and ISM (5 GHz range) ISM (2.4000— 2.4835 GHz range) Modulation OFDM technology DSSS technology
- 28. 802.11g • 802.11g is a high-speed extension to 802.11b • Compatible with 802.11b • High speed up to 54 Mbps • 2.4 GHz (vs. 802.11a, 5 GHz) • Using ODFM for backward compatibility • Adaptive Rate Shifting
- 29. 802.11g Advantages • Provides higher speeds and higher capacity requirements for applications • Wireless Public Access • Compatible with existing 802.11b standard • Leverages Worldwide spectrum availability in 2.4 GHz • Likely to be less costly than 5 GHz alternatives • Provides easy migration for current users of 802.11b WLANs • Delivers backward support for existing 802.11b products • Provides path to even higher speeds in the future
- 30. 802.11e Introduces Quality of Service • Also know as P802.11 TGe • Purpose: • To enhance the 802.11 Medium Access Control (MAC) to improve and manage Quality of Service (QoS) • Cannot be supported in current chip design • Requires new radio chips • Can do basic QoS in MAC layer
- 31. 802.11f – Inter Access Point Protocol • Also know as P802.11 TGf • Purpose: • To develop a set of requirements for Inter-Access Point Protocol (IAPP), including operational and management aspects
- 32. 802.11b Security Features • Wired Equivalent Privacy (WEP) – A protocol to protect link-level data during wireless transmission between clients and access points. • Services: • Authentication: provides access control to the network by denying access to client stations that fail to authenticate properly. • Confidentiality: intends to prevent information compromise from casual eavesdropping • Integrity: prevents messages from being modified while in transit between the wireless client and the access point.
- 33. Authentication Means: • Based on cryptography • Non-cryptographic • Both are identity-based verification mechanisms (devices request access based on the SSID – Service Set Identifier of the wireless network).
- 35. Privacy • Cryptographic techniques • WEP Uses RC4 symmetric key, stream cipher algorithm to generate a pseudo random data sequence. The stream is XORed with the data to be transmitted • Key sizes: 40bits to 128bits • Unfortunately, recent attacks have shown that the WEP approach for privacy is vulnerable to certain attack regardless of key size
- 36. Data Integrity • Data integrity is ensured by a simple encrypted version of CRC (Cyclic Redundant Check) • Also vulnerable to some attacks
- 37. Security Problems • Security features in Wireless products are frequently not enabled. • Use of static WEP keys (keys are in use for a very long time). WEP does not provide key management. • Cryptographic keys are short. • No user authentication occurs – only devices are authenticated. A stolen device can access the network. • Identity based systems are vulnerable. • Packet integrity is poor.
- 38. WLAN Migration – Cutting The Cord • Essential Questions • Choosing the Right Technology • Data Rates • Access Point Placement and Power • Antenna Selection and Placement • Connecting to the Wired LAN • The Site Survey
- 39. Transmission Techniques • We would like to transmit data with: • Highest possible data rate • With the minimum level of the signal power • Minimum channel bandwidth • Reduced transmitter and receiver complexity • Wired • Data received from upper layers are line coded and the voltages are applied to the medium directly • Transmissions are often referred to as baseband transmission schemes • In voice-band modems, Digital Subscriber Line (DSL), and coaxial cable model applications, the transmitted signal is modulated over a carrier • Wireless: 3 categories • Pulse transmission techniques employed mostly in Infrared (IR) applications and, more recently, in the so-called impulse radio or ultra-wideband (UWB) transmission • Basic modulation techniques widely used in Time Division Multiple Access (TDMA) cellular, as well as a number of mobile data networks • Spread spectrum systems used in the Code Division Multiple Access (CDMA) and in wireless LANs operating in ISM frequency bands
- 40. Copyright © 2006, Dr. Carlos Cordeiro and Prof. Dharma P. Agrawal, All rights reserved. 40 ISM Frequency Bands Extremely Low Very Low Low Medium High Very High Ultra High Super High Infrared Visible Light Ultra- violet X-Rays Audio AM Broadcast Short Wave Radio FM Broadcast Television Infrared wireless LAN Cellular (840MHz) NPCS (1.9GHz) 5 GHz (IEEE 802.11a, HiperLAN, HiperLAN2) 22 MHz channel bandwidth Must implement spread spectrum technology Must operate at 500 mill-watts or less 902 - 928 MHz 26 MHz 2.4 - 2.4835 GHz 83.5 MHz (IEEE 802.11b, Bluetooth)
- 41. Copyright © 2006, Dr. Carlos Cordeiro and Prof. Dharma P. Agrawal, All rights reserved. 41 Infrared (IR) Remote controls for TVs, VCRs, DVD and CD players use IR technology A direct line of sight of the transmitter needed in order to successfully establish communication link New diffused IR technologies can work without LOS inside a room Systems are simple to design and are inexpensive Use the same signal frequencies used on fiber optic links Systems detect only the amplitude of the signal, so interference is greatly reduced IR transmission can be aimed and range extended to a couple of kilometers and can be used outdoors It also offers the highest bandwidth and throughput The other way is to transmit omni-directionally and bounce the signals off of everything in every direction, which reduces the coverage to 30 - 60 feet IR signals cannot penetrate opaque objects IEEE 802.11 standard for high-speed diffused IR employs pulse- position-modulation (PPM) with a wavelength of 850nm–950nm and data rates of 1 and 2 Mbps
- 42. Copyright © 2006, Dr. Carlos Cordeiro and Prof. Dharma P. Agrawal, All rights reserved. 42 Microwave and Radio Frequencies Microwave: Operate at less than 500 milliwatts of power (MW) Use narrow-band transmission with single frequency modulation mostly in the 5.8 GHz band Radio Frequency: To link appliances that are distributed throughout the house Can be categorized as narrowband or spread spectrum Narrowband technology includes microwave transmissions which are high-frequency radio waves that can be transmitted to distances up to 50 Km Not suitable for local networks, but could be used to connect networks in separate buildings Transmitted signal in Subscriber Station occupies a much larger bandwidth than the traditional radio modems Two basic techniques used: frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS)
- 43. Copyright © 2006, Dr. Carlos Cordeiro and Prof. Dharma P. Agrawal, All rights reserved. 43 Spread Spectrum Frequency Hopping Spread Spectrum: Splits the band into many small subchannels (e.g., 1 MHz) Signal then hops from subchannel to subchannel transmitting short bursts of data on each channel for a set period of time, called dwell time Hopping sequence must be synchronized at the sender and the receiver or else, the information is lost FCC requires that the band is split into at least 75 subchannels Extremely difficult to intercept and gives a high degree of security Direct Sequence Spread Spectrum: Transmission signal is spread over an allowed band Two-stage modulation technique: a random binary string, called the spreading code, is used to modulate the transmitted signal, and in the second stage, the chips are transmitted over a traditional digital modulator
- 44. Copyright © 2006, Dr. Carlos Cordeiro and Prof. Dharma P. Agrawal, All rights reserved. 44 Direct Sequence Spread Spectrum Physical layer of the IEEE 802.11 standard employing DSSS requires a spreading ratio of eleven If orthogonal spreading codes are used, then more than one LAN can share the same band However, because DSSS systems use wide subchannels, the number of co-located LANs is limited by the size of those subchannels As the transmitted chips are much narrower than data bits, the bandwidth of the transmitted DSSS signal is much larger than systems without spreading The DSSS systems provide a robust signal with better coverage area than FHSS The DSSS systems provide a robust signal with better coverage area than FHSS