Wireless_firstnotes.doc

334 views
323 views

Published on

0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
334
On SlideShare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
Downloads
5
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Wireless_firstnotes.doc

  1. 1. Wireless Networking Sends radio-frequency signals between your computers An access point which is a wired controller that receives and transmits data to the wireless adapters installed in each computer Four types: Bluetooth IrDA HomeRF (SWAP) WECA (Wi-Fi) Bluetooth • communicates on a frequency of 2.45 gigahertz, which has been set aside by international agreement for the use of industrial, scientific and medical devices (ISM) • spread-spectrum frequency hopping -a device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis. • Transmitters change frequencies 1600 times every second, meaning that more devices can make full use of a limited slice of radio spectrum • create a personal-area network (PAN), or piconet, that may fill a room or may encompass no more distance than that between the cell phone on a belt-clip and the headset on your head • Once a piconet is established, the members randomly hop frequencies in unison so they stay in touch with one another and avoid other • Bluetooth can send data at more than 64,000 bits per second in a full- duplex link • The devices in a piconet share a common communication data channel. The channel has a total capacity of 1 megabit per second (Mbps). Headers and handshaking information consume about 20 percent of this capacity. • In the United States and Europe, the frequency range is 2,400 to 2,483.5 MHz, with 79 1-MHz radio frequency (RF) channels. In practice, the range is 2,402 MHz to 2,480 MHz. In Japan, the frequency range is 2,472 to 2,497 MHz with 23 1-MHz RF channels. • A data channel hops randomly 1,600 times per second between the 79 (or 23) RF channels. • Each channel is divided into time slots 625 microseconds long. • A piconet has a master and up to seven slaves. The master transmits in even time slots, slaves in odd time slots. • Packets can be up to five time slots wide. • Data in a packet can be up to 2,745 bits in length. • There are currently two types of data transfer between devices: SCO (synchronous connection oriented) and ACL (asynchronous
  2. 2. connectionless). • In a piconet, there can be up to three SCO links of 64,000 bits per second each. To avoid timing and collision problems, the SCO links use reserved slots set up by the master. • Masters can support up to three SCO links with one, two or three slaves. • Slots not reserved for SCO links can be used for ACL links. • One master and slave can have a single ACL link. • ACL is either point-to-point (master to one slave) or broadcast to all the slaves. • ACL slaves can only transmit when requested by the master. • IrDa • Infrared Data Association • communicate using infrared light pulses • is a standard for devices to communicate using infrared light pulses; depend on direct line of sight • data speeds up to 4 megabits per second (Mbps) • downside is need access points in each room because need direct line of sight _______________________________ Institute of Electrical and Electronics Engineers (IEEE) original standard wireless-Ethernet specification IEEE 802.11 designated 2 ways of communicating between devices allowed for speeds up to 2Mbps Direct-sequence spread spectrum (DSSS) Frequency-hopping spread spectrum (FHSS) • Both use frequency-shift keying (FSK) technology • Both based on spread-spectrum radio waves in 2.4 gigahertz (GHz) range Spread-spectrum = data is sent in small pieces over a number of discrete frequencies available for use at any time in the specified range DSSS • communicate by splitting each byte of data into several parts and sending them concurrently on different frequencies • uses lot of available bandwidth, about 22megahertz (MHz) FHSS • send a short burst of data, shift frequencies (hop), and then send another short burst • devices using FHSS agree on frequencies to hop to, and use each
  3. 3. frequency for short period of time (less than 400 milliseconds) before moving on, several independent FHSS networks can exist in same physical area without interfering with each other • use only 1MHz or less of available bandwidth • less prone to interference than DSSS because use any given frequency for such short period of time • FHSS-based devices are easier and cheaper to produce HomeRF (radio frequency) and SWAP HomeRF is alliance of businesses that developed standard called Shared Wireless Access Protocol (SWAP) • Includes 6 voice channels bases on Digital Enhanced Cordless Telecommunications (DECT) and 802.11 wireless-Ethernet specification for data • SWAP devices make 50 hops per second • Transmit at 1 Mbps • Can get up to 2Mbps if very little interference Advantages of SWAP: Here are the advantages of SWAP: * It's inexpensive ($70 to $200 per device). * It's easy to install. * It requires no additional wires. * It has no access point. * It uses six full-duplex voice channels and one data channel. * It allows up to 127 devices per network. * It allows multiple networks in the same location. * You can use encryption to make your data secure. Here are the disadvantages of SWAP: * It's not very fast (normally 1 Mbps). * It has a limited range (75 to 125 ft / 23 to 38 m). * It's not compatible with FHSS devices. * Physical obstructions (walls, large metal objects) can interfere with communication. * It's difficult to integrate into existing wired networks. WECA and Wi-Fi Wireless Ethernet Compatibility Alliance (WECA)
  4. 4. • Targeted more at office use than home networks • Wi-Fi stands for “wireless fidelity” like hi-fi for “high fidelity” • Compliant with IEEE 802.11b • Specification drops FHSS and focuses on DSSS because of higher data rate • Speed of 11Mbps whenever possible • Here are the advantages of Wi-Fi: * It's fast (11 Mbps). * It's reliable. * It has a long range (1,000 ft / 305 m in open areas, 250 to 400 ft / 76 to 122 m in closed areas) * It's easily integrated into existing wired-Ethernet networks. * It's compatible with original 802.11 DSSS devices. Here are the disadvantages: * It's expensive. * It requires an access point. * It can be difficult to set up. • Speed can fluctuate significantly. Airport has to be connected to an Apple computer, but will accept signals from any 802.11b-compatible wireless-network card Tyson, Jeff. “How Wireless Networking Works.” Marshall Brain’s HowStuffWorks. pp1-6, http://www.howstuffworks.com/wireless-network.htm/printable Franklin, Curtis. “How Bluetooth Works” Marshall Brain’s HowStuffWorks. pp. 1-7. http://www.howstuffworks.com/bluetooth.htm
  5. 5. Gast, Matthew S. 802.11 Wireless Networks: The Definitive Guide. Advantages of wireless networks --- • Mobility • Ease and speed of deployment – no need to mess with wires • Flexibility • Cost – “Over time, point-to-point wireless links are far cheaper than leasing capacity from the telephone company.”(p. x) • 1997 802.11 standard • speeds increased from 2 – 11 – 54 Mbps • network medium is a form of electromagnetic radiation o infrared light – limitations – easily blocked by walls, partitions, and other office construction o radio waves – can penetrate most office obstructions and offer wider coverage ---most use radio wave physical layer!! ISM (Industrial, Scientific, and Medical) – frequency bands (microwave oven) – where 802.11 devices operate 802.11b devices specifically in S-band ISM – license-free!! 802.11 1 & 2 Mbps, 2.4 GHz, first standard (1997) – both frequency-hopping and direct-sequence modulation techniques 802.11a up to 54 Mbps 5 GHz, second standard (1999), but products not released until late 2000 802.11b 5.5 & 11 Mbps, 2.4 GHz, third standard, but second wave of products and most common equipment 802.11g up to 54 Mbps, 2.4 GHz, not yet standardized IEEE 802 focus on lowest 2 layers of OSI model 802.11b HR/DSSS physical layer – High Rate, Direct-sequence Layer 802.11a OFDM physical layer – Orthogonal Frequency Division Multiplexing access point --- “If one mobile station in an infrastructure BSS needs to communicate with a second mobile station, the communication must make 2 hops. First, the originating mobile station transfers the frame to the access point. Second, the access point transfers the frame to the destination station.”(p. 11)
  6. 6. so all mobile stations must maintain a distance to the access point – useful because otherwise physical layer complexity would be increased due to the need for maintenance of neighbor relationships (without access point) stations must associate with an access point – association is the process by which mobile stations join 802.11 network wireless bridge 802.11 sometimes called “wireless Ethernet” because core elements similar o stations id by 48-bit MAC address o frames are delivered based on MAC address o frame delivery is unreliable services o distribution – access point o integration – with non-IEEE 802.11 network o association –mobile stations register or associate with access points o reassociation –if moving around, and station finds access point with stronger signal o disassociation—detach from network o authentication—security measure o deauthentication--termination o privacy—Wired Equivalency Protocol (WEP), encrypts frames as they go across air o MSDU delivery—MAC Service Data Unit delivery service Uses Carrier Sense Multiple Access (CSMA) scheme, but instead uses Collision Avoidance (not Collision Detection, because waste valuable transmission capacity) Chapter 5, Wired Equivalency Protocol Security has haunted wireless deployments since standardization. Clause 8.2 of 802.11 includes optional WEP standard…which can be used by stations to protect data as it traverses the wireless medium, but it provides no protection past the access point “WEP was initially marketed as the security solution for wireless LANs, though its design was so flawed as to make that impossible”. p. 86 problem with cryptographic cipher used by WEP
  7. 7. o RC4 cipher (symmetric (secret-key) stream cipher) o Uses stream of bits, keystream, and combined with message to produce ciphertext o To recover original text, receiver processes ciphertext w/ identical keystream o Key length only 40 bits Confidentiality, integrity, authentication WEP – frame body encryption = confidentiality; integrity check sequence = integrity; shared-key authentication=authentication IN REALITY, WEP falls short on all 3: o RC4 cipher compromises confidentiality o Integrity check poorly designed o Authentication is to MAC addresses, not to users themselves AirSnort Key Distribution: Type keys into your device drivers or access points BY HAND, which is the most non-scalable protocol in use o Keys cannot be considered secret, as they are statically entered o WEP cannot protect against authorized insiders who also have the key o Published keys for large organizations Problems with WEP Design flaws – Internet Security, Applications, Authentication and Cryptography (ISAAC) group o Manual key mgmt o 40 bit secret key o stream ciphers vulnerable when key stream reused o decryption dictionaries – due to infrequent rekeying o CRCs not cryptographically secure, so because of frame retransmissions, an attacker could replace a legitimate frame with something else o Access point Some vendors offer proprietary approaches that allow stronger public-key authentication and random session keys
  8. 8. 802.1x IETF’s Extensible Authentication Protocol (EAP) – Cisco’s lightweight EAP, LEAP, is based on EAP -802.11 does not provide a way to guarantee the authenticity and integrity of any frames on the wireless network
  9. 9. O’Hara, Bob and Al Petrick. IEEE 802.11 Handbook: A Designer’s Companion. IEEE 802.11 WLAN was designed to look and feel like any IEEE 802 wired LAN Drawbacks: Data carried by WLAN is not private, broadcast for all to hear… Vagaries of electromagnetic propagation o Both radio and infrared everything is either a reflector or an attenuator of the signal carrying the LAN data No simple way to deal with the change of a layer-3 network address should the mobile station cross from one part of the network to another that is connected by a router…solutions today using DHCP and Mobile-IP “notions such as the nearest network printer must be defined in a different way, when the physical location of network user may be constantly changing.” p. 5 1997 IEEE first standard for WLANs, IEEE Std. 802.11-1997 o Medium access control (MAC) sublayer o MAC mgmt protocols and services o 3 physical layers – infrared (IR) baseband PHY, a frequency hopping spread spectrum (FHSS) radio in the 2.4 GHz band, and a direct sequence spread spectrum (DSSS) radio in the 2.4 GHz band o both 1 & 2 Mbps operation o developing 2 new PHY layers – IEEE Std 802.11a is orthogonal frequency domain multiplexing (OFDM) radio in the UNII bands, delivering up to 54 Mbps data rates; IEEE Std 802.11b is extension of DSSS PHY in the 2.4 GHz band, delivering up to 11Mbps data rates Architecture o most decision-making distributed to mobile stations o flexible to support small, transient networks and large semipermanent or permanent networks o deep power-saving modes of operations (because mobile stations run on battery) AP – provides buffering of traffic for a mobile station while that station is operating in very low power state IBSS BSS ESS (Extended Service Set) DS – Distribution System may be wired or unwired
  10. 10. MAC (Medium Access Control) supplies the functionality required to provide a reliable delivery mechanism for user data over noisy, unreliable wireless media WEP – “The level of encryption chosen approximates the level of protection data might have on a wired LAN in a building with controlled access that prevents physically connecting to the LAN wiring without authorization.” p. 19 o CSMA/CA with binary exponential backoff – “listen before talk” o Unusual for wireless devices to receive and transmit simultaneously (why uses collision avoidance) o Unusual for all wireless devices in a LAN to be able to communicate directly with all other devices o Implements Network Allocation Vector (NAV) that is a value indicates to a station the amount of time that remains before the medium will become available o NAV kept current by duration values transmitted in all frames Timing intervals IEEE 802.11 different from others because has very specific management frame types 802.11 MAC can fragment its frames in an attempt to increase the probability that they will be delivered without errors induced by interference (microwaves) WEP is an encryption mechanism that takes the content of a data frame, its frame body, and passes it through an encryption algorithm---SO only frame body is data frames is encrypted o So WEP provides protection for content of data frames o Does not protect against other security threats, such as traffic analysis RC4 developed by Ron Rivest of RSA Data Security, Inc. (RSADSI, which is now part of Network Associates, Inc. Is symmetric stream cipher that supports variable length key Symmetric stream cipher is one that uses the same key and algorithm for both encryption and decryption
  11. 11. Kanellakis, Kelly. “White Paper: Enterasys on Standard’s Confusion.” 802.11 Planet.com. February 26, 2002 . http://www.80211planet.com/tutorials/article/0,,10724_981611,00.html Originally the plan was (for most vendors and customers) to move from an 802.11b technology to 802.11a technology. The new technology would deliver increased bandwidth (from 11Mbps to 54Mbps) and at the same time allow wireless data transmission to move to a less crowded area of the radio spectrum (from the crowded 2.4 GHz band to the less crowded 5 GHz band) In reality, WEP was designed to provide only the same level of privacy as a wired network. As part of the Wi-Fi standard, wireless networks can advertise (literally broadcast) their network names to make them easy to find and join. This advertisement is called the SSID. The first step in providing some form of security would be to not broadcast this name. Then it would be smart to choose a name which cannot easily be guessed. Many organizations that set up wireless networks do not turn on any sort of security, and they allow the name of their network to be broadcasted to anyone who wants to listen for it. TEN different options for wireless data transmission outside of 802.11b Other Wireless Technologies The next set of technologies that should be examined are ones that, while they still exist and are being promoted, can really be considered fringe technologies at this point. They are considered fringe technologies for the enterprise data network because they lack many of the features that an enterprise data network requires. These technologies include the likes of HyperLAN 1 and 2,
  12. 12. Bluetooth, Ultra-Wideband, Wide Band Frequency Hopping and HomeRF. While none of these technologies have a presence in the mainstream wireless market - with the possible exception of Bluetooth (and even that is a stretch) - each one is trying to gain wireless market share. Yet, all of these technologies have some technical merit, and may be a good solution in some cases. The problem is that because there are so many of them, they add to the confusion in the market. It is not expected that any of these will ever gain significant market share, although they will continue to exist in the market. Security concerns are being addressed by 802.11i committee, which can been seen as the security solution for 802.11a (which became a standard in 1999, but no hardware was built until now) 802.11i will probably use some form of widely accepted encryption like AES or something just as strong. For 802.11a to be truly accepted as a technology to be for use in the enterprise it must include 802.11i to provide a standards-based and strong security capability. The IEEE should ratify 802.11i sometime in the summer of 2002 timeframe. Interoperability – will be promoted by Wireless Ethernet Compatibility Alliance (WECA), which was the group responsible for the Wi-Fi specification for 802.11b, and now has specification for 802.11a (called Wi-Fi5) o Should be at least 2 chip manufacturers o Should be at least 3 vendor solutions Your solution (in this case 802.11a) needs to be able to detect these other radios and react by lowering its power output in order to keep them from interfering with each other.
  13. 13. Brown, Bruce. “802.11a—Fast Wireless Networking”. Extremetech. December 3, 2001. http://www.extremetech.com/article2/0,3973,9151,00.asp we found 802.11a to be almost five times faster than 802.11b (the current hands down wireless network standard) at short distances 802.11a and b aren’t compatible because operate at different frequencies, 2.4 GHz for 802.11b, 5 GHz for 802.11a can coexist Wireless networking wasn't accepted initially for three reasons: throughput (1Mbps/2Mbps) was much too slow compared to the most prevalent (10Mbps) wired Ethernet standard; wireless adapters and access points were significantly more expensive than wired NICs and switches; and the first wireless products didn't work well together with wireless products from other vendors. Wi-Fi was probably the most significant, since it assured large volume enterprise buyers that they wouldn't be stuck with proprietary, dead-end solutions. isn't perfect. 802.11b has three major problems: limited bandwidth, radio interference from other devices and networks, and security concerns. can cut the actual throughput to 4 to 7Mbps (in our testing we measured actual file transfer performance below 3Mbps for 802.11b Bluetooth and 802.11b both use the 2.4GHz radio spectrum, as do microwave ovens and the most powerful widely available cordless phones, also legacy wireless devices and home control devices that use the X-10 standard both specifications were published in 1999 by the IEEE standards body - but 802.11b made it to market faster. Major advantage of a over b are higher throughput rates and increased channel support (both of which result in higher bandwidth) Nominal speed is 54Mbps, but with overhead actual max should range from 22-26Mbps
  14. 14. 802.11b's speeds are 11, 5.5, 2, and 1Mbps. 802.11a has a maximum rated speed of 54Mbps but drops back to 48, 36, 24, 18, 12, 9, and 6Mbps. A second reason for additional overall bandwidth with 802.11a is channel support. With 802.11b, three channels are available for simultaneous operation in the 2.4 to 2.4835GHz frequency band (there are eleven center frequencies specified 2.412, 2.417, 2.422, 2.427, 2.432, 2.437, 2.442, 2.447, 2.452, 2.457, and 2.462 GHz, but because there is a required 25MHz spacing between active channels, only three are typically used at one time). In 802.11a, however, eight channels can operate simultaneously in the two lower bands of the 5GHz spectrum used in the U.S., 5.15 to 5.25GHz and 5.25 to 5.35GHz. The center points for the eight channels, each of which is 20MHz wide and can support 52 carrier signals, are 5.18, 5.2, 5.22, 5.24, 5.26, 5.28, 5.30, and 5.32 GHz. The high band of the unlicensed 5GHz spectrum (5.725 to 5.825GHz), is available, but is more commonly used for building-to-building wireless applications.
  15. 15. Brown, Bruce. “Wireless Standards Up in the Air”. Extremetech. Dec. 3, 2001, http://www.extremetech.com/article2/0,3973,9164,00.asp one of the most anticipated proposed standards, 802.11e, the standard that promises to bring QoS (quality of service - essential for multimedia) to the 802.11 world, is still in committee November 15th (2001) the 802.11g specification was granted tentative approval based on a compromise between proposed specifications from Intersil and Texas Instruments. 802.11b Today 802.11b is the clear winner in business wireless networking. Operating in the 2.4GHz frequency range, 802.11b (aka Wi-Fi) has a nominal maximum data rate of 11Mbps, with the potential of three simultaneous channels. 802.11b has a great advantage in that it is accepted worldwide. One of the more significant disadvantages of 802.11b is that the frequency band is crowded, and subject to interference from other networking technologies, microwave ovens, 2.4GHz cordless phones (a huge market), and Bluetooth. There are drawbacks to 802.11b, including lack of interoperability with voice devices, and no QoS provisions for multimedia content. Interference and other limitations aside, 802.11b is the clear leader in business and institutional wireless networking and is gaining share for home applications as well. 802.11a 802.11a, which has just started to ship, is much faster than 802.11b, with a 54Mbps maximum data rate (actually increased to 72Mbps or 108Mbps in a non-standard double-speed mode depending on the chipset vendor and component manufacturer). 802.11a (and its recently announced interoperability standard called Wi-Fi5) operates in the 5GHz frequency range and allows eight simultaneous channels. One big advantage to 802.11a is that it isn't subject to interference from Bluetooth or any of the other 2.4GHz frequency denizens. One big disadvantage is that it is not directly compatible with 802.11b, and requires new bridging products that can support both types of networks--although if you don't mind spending the money for access points for both 11a and 11b, you can plug them into hubs or better yet, switches on your network and they'll work just fine. Other clear disadvantages are that 802.11a is only available in half the bandwidth in Japan (for a maximum of four channels), and it isn't approved for use in Europe, where HiperLAN2 is the standard. Another IEEE group, 802.11h, is working on technologies that will tweak 802.11a to work around
  16. 16. some of the 5GHz channels used by military in Europe. Like 802.11b, 802.11a has no provisions to optimize voice or multimedia content. 802.11g At first glance 802.11g, which operates in the 2.4GHz frequency with mandatory compatibility with 802.11b but with a maximum data rate of 54Mbps, would be an obvious step in the race to improve wireless networking performance while maintaining compatibility with Wi-Fi, but there's more to the story. Far from a rubber-stamping, the 802.11g proposal met significant resistance and many predicted it would be tossed out, leaving the field for high-speed wireless networking to 802.11a. But the compromise worked out last month based on proposals from Intersil and TI is moving things along. Note that it's still a tentatively approved specification (it won't have final approval until working versions are tested and 90% of the voting body votes affirmatively--that's when it gets actual "letter status"). So here's the deal on 802.11g technology. The standard operates entirely in the 2.4GHz frequency, but uses a minimum of two modes (both mandatory) with two optional modes. The mandatory modulation/access modes are the same CCK (Complementary Code Keying) mode used by 802.11b (hence the compatibility with Wi-Fi) and the OFDM (Orthogonal Frequency Division Multiplexing) mode used by 802.11a (but in this case in the 2.4GHz frequency band). The mandatory CCK mode supports 11Mbps and the OFDM mode has a maximum of 54Mbps. There are also two modes that use different methods to attain a 22Mbps data rate--TI's PBCC-22 (Packet Binary Convolutional Coding, rated for 6 to 54Mbps) and Intersil's CCK-OFDM mode (with a rated max of 33Mbps). The obvious advantage of 802.11g is that it maintains compatibility with 802.11b (and 802.11b's worldwide acceptance) and also offers faster data rates comparable with 802.11a (at least on paper, since working silicon isn't available). The number of channels available, however, is not increased, since channels are a function of bandwidth, not radio signal modulation - and on that score, 802.11a wins with its eight channels, compared to the three channels available with either 802.11b or 802.11g. Another disadvantage of 802.11g is that the 2.4GHz frequency will get even more crowded. 802.11g also gives up roughly one year to 802.11a-- products for the latter are already beginning to reach the market, although many products (those based on chipsets from companies other than Atheros) won't be out until mid year. Companies that want faster performance now may not have any choice but to upgrade to, or augment existing networks with 802.11a, which could make it harder for 802.11g to
  17. 17. succeed.

×