Wireless LANs. The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Services 802.

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1 Wireless LANs The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Services The Working Group The IEEE was formed in July 1990 to develop CSMA/CA, a variation of CSMA/CD (Ethernet)-based wireless LANs. The working group produced the first standard in 1997, which specifies wireless LAN devices capable of operating up to 2 Mbps using the unlicensed 2.4-GHz band. Currently, the working group has nine basic task groups and each is identified by a letter from a to i. Following are the current task groups and their primary responsibilities: a. Provides a 5-GHz band standard for 54-Mbps transmission rate b. Specifies a 2.4-GHz band standard for up to 11-Mbps transmission rate c. Gives the required specific information to the ISO/IEC (IEEE 802.1D) standard The Family of Standards d. Adds the requirements and definitions necessary to allow wireless LAN equipment to operate in markets not served by the current standard e. Expands support for LAN applications with Quality of Service requirements f. Specifies the necessary information that needs to be exchanged between access points to support the P DS functions g. Develops a new PHY extension to enhance the performance and the possible applications of the b compatible networks by increasing the data rate achievable by such devices h. Enhances the current MAC and a PHY with network management and control extensions for spectrum and transmit power management in 5-GHz license exempt bands i. Enhances the current MAC to provide improvements in security refers to a family of specifications developed by the IEEE for wireless LAN technology. The original standard specifies an over-the-air interface between a wireless client and a base station or between two wireless clients. The IEEE accepted the specification for in The task groups within the working group have produced few extensions to the original specification. The products of these extensions are named after the task group and the original specification-for example, b is an extension developed by the task group b. The most popular extensions of specifications are b, a, and g 59 The Standard Details The standard specifies wireless LANs that provide up to 2 Mbps of transmission speed and operate in the 2.4-GHz Industrial, Scientific, and Medical (ISM) band using either frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS). The IEEE approved this standard in The standard defines a physical layer (PHY), a medium access control (MAC) layer, the security primitives, and the basic operation modes The Physical Layer The standard supports both radio frequency- and infrared-based physical network interfaces. However, most implementations of use radio frequency 60 61

2 Frequency Bandwidth standard-compliant devices operate in the unlicensed 2.4-GHz ISM band. Due to the limited bandwidth available when the electromagnetic spectrum is used for data transmission, many factors have to be considered for reliable, safe, and high-performance operation. These factors include the technologies used to propagate signals within the RF band, the time that a single device is allowed to have an exclusive transmission right, and the modulation scheme. For these reasons, FCC regulations require that radio frequency systems must use spread spectrum technology when operating in the unlicensed bands Spread Spectrum Technology The standard mandates using either DSSS or FHSS. In FHSS, the radio signal hops within the transmission band. Because the signal does not stay in one place on the band, FHSS can elude and resist radio interference. DSSS avoids interference by configuring the spreading function in the receiver to concentrate the desired signal, and to spread out and dilute any interfering signal Direct-Sequence Spread Spectrum (DSSS) In DSSS the transmission signal is spread over an allowed band. The data is transmitted by first modulating a binary string called spreading code. A random binary string is used to modulate the transmitted signal. This random string is called the spreading code. The data bits are mapped to a pattern of "chips" and mapped back into a bit at the destination. The number of chips that represent a bit is the spreading ratio. The higher the spreading ratio, the more the signal is resistant to interference. The lower the spreading ratio, the more bandwidth is available to the user. The FCC mandates that the spreading ratio must be more than 10. Most products have a spreading ratio of less than 20. The transmitter and the receiver must be synchronized with the same spreading code. Recovery is faster in DSSS systems because of the ability to spread the signal over a wider band. 64 Frequency-Hopping Spread Spectrum (FHSS) This spread spectrum technique divides the band into smaller subchannels of usually 1 MHz. The transmitter then hops between the subchannels sending out short bursts of data for a given time. The maximum amount of time that a transmitter spends in a subchannel is called the dwell time. In order for FHSS to work correctly, both communicating ends must be synchronized (that is, both sides must use the same hopping pattern), otherwise they lose the data. FHSS is more resistant to interference because of its hopping nature. The FCC mandates that the band must be split into at least 75 subchannels and that no subchannel is occupied for more than 400 milliseconds. Debate is always ongoing about the security that this hopping feature provides. 65 Frequency-Hopping Spread Spectrum (FHSS) Since there are only 75 subchannels available, the hopping pattern has to be repeated once all the 75 subchannels have been hopped. HomeRF FHSS implementations select the initial hopping sequence in a pseudorandom fashion from among a list of 75 channels without replacement. After the initial 75 hops, the entire sequence is repeated without any replacement or change in the hopping order. An intruder could possibly compromise the system by monitoring and recording the hopping sequence and then waiting till the whole sequence is repeated. Once the hacker confirms the hopping pattern, he or she can predict the next subchannel that hopping pattern will be using thereby defeating the hopping advantage altogether. HomeRF radios, for example, hop through each of the 75 hopping channels at a rate of 50 hops per second in a total of 1.5 seconds, repeating the same pattern each time, enabling a hacker to guess the hopping sequence in 3 seconds Frequency-Hopping Spread Spectrum (FHSS) Nevertheless, this technique still provides a high level of security in that expensive equipment is needed to break it. Many FHSS LANs can be colocated if an orthogonal hopping sequence is used. Since the subchannels in FHSS are smaller than DSSS, the number of colocated LANs can be greater with FHSS systems. The most commonly used standard based on FHSS is HomeRF 66 67

3 The Protocol Stack The Extensions Part of the protocol stack 68 The Working Group realized that the initial standard that was passed in 1997 would not be sufficient to attract implementers. Therefore, the working group established various task groups with the responsibilities to develop different extensions to the standard. The idea behind having different task groups is to develop standards for different types of usage scenarios that still conform to a basic set of operating rules and are still interoperable to a certain extent. The most promising standards at this time include b, a, g, and e b b b is an extension to that operates at speeds up to 11 Mbps transmission (with a fallback to 5.5, 2, and 1 Mbps) in the 2.4-GHz band and uses only DSSS b is also known as high rate or wireless fidelity (Wi-Fi) Now with operating speeds up to 11 Mbps, it is far more practical to use the wireless LANs than the conventional wired LANs. It is being used in Small Office Home Office (SoHo) environments, enterprises, and by Wireless Internet Service Providers (WISPs) b Limitations b is haunted by the possibility of interference in the 2.4-GHz frequency band in which it operates. The 2.4-GHz frequency is already crowded and will soon be more so. Microwave ovens operate at 2.4 GHz and can deter the performance of wireless networks. Many powerful cordless phones also operate at the 2.4-GHz frequency. If you use b networking products, forget about using these phones in the same area. Blue-tooth, the short-range wireless networking standard, which also operates in the 2.4-GHz range, is slated to coexist with wireless LANs a a is an extension to , which operates at speeds of up to 54-Mbps transmission rate (with a fallback to 48, 36, 24, 18, 12, and 6 Mbps) in the more recently allocated 5-GHz Unlicensed National Information Infrastructure (UNII) band a uses an Orthogonal Frequency Division Multiplexing (OFDM) encoding scheme as its spread spectrum technology Because a operates in 5 GHz, it can coexist with b networks without causing any interference a is being used to connect network backbones in small enterprise environments and the applications that a a networks are incompatible with the b networks due to the difference in the radio frequency band used by a (802.11b uses 2.4 GHz whereas a uses 5 GHz), and the speeds they operate at (802.11b has a maximum operating speed of 11 Mbps whereas a operates at up to 54 Mbps). The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multipath distortion require high bandwidth 72 73

4 802.11g The g standard is still under development. The g standard operates in the 2.4-GHz band and provides speeds up to 54 Mbps (with a fallback to 48, 36, 24, 18, 11, 5.5, 2, and 1 Mbps). The g differs from b because it can optionally use OFDM (802.11g draft mandates that OFDM be used for speeds above 20 Mbps). Though g devices would provide higher speed than the currently available b devices, it still suffers the interference issue with other devices operating in the same RF band, primarily the Bluetooth devices The MAC The working group chose the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), a derivative of CSMA/CD, as the MAC protocol for the standard. CSMA/CA works as follows: The station listens before it sends. If someone is already transmitting, it waits for a random period and tries again. If no one is transmitting, then it sends a short message. This message is called the ready-to-send message (RTS). This message contains the destination address and the duration of the transmission. Other stations now know that they must wait that long before they can transmit The MAC Sublayer Protocol The MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem. 76 The use of virtual channel sensing using CSMA/CA 77 The MAC Sublayer Protocol The MAC Sublayer Protocol (4) A fragment burst. Because of the much higher probability of interference, a frame is sometimes split into Interframe spacing in smaller fragments 78 79

5 The Frame Structure Services The data frame Distribution Services Association Disassociation Reassociation Distribution Integration Intracell Services Authentication Deauthentication Privacy Data Delivery Services Association Used by mobile stations to connect themselves to a base station Disassociation Breaking the relationship to a base station Re-association Used by a mobile station to change it s base station Useful for stations moving from one cell to another Distribution Determines how to route frames sent to a base station Integration Handles translating the frames into non formats Broadband Wireless The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure The Protocol Stack QPSK Constellation 84 85

6 QAM-16 Constellation QAM-64 Constellation The Physical Layer The Physical Layer (2) Frames and time slots for time division duplexing MAC Sublayer The data link layer is subdivided into three sublayers In the security sublayer, the payload is encrypted, but the headers are not So, a snooper can see who is talking to whom, but not what is being said The MAC frame is composed of subframes They tell what is in which time slot and which time slots are free The MAC Sublayer Protocol The downstream channel has 4 classes of service Constant bit rate service (CBR) Real-time variable bit rate service (VRT) Non-real-time variable bit rate service (NVRT) Best efforts service The first 3 classes map directly to various classes of service in ATM The last class maps to an IP or Ethernet class of service All services are connection-oriented 90 91

7 The Frame Structure Bluetooth Bluetooth Architecture Bluetooth Applications The Bluetooth Protocol Stack The Bluetooth Frame Structure (a) A generic frame. (b) A bandwidth request frame Bluetooth Bluetooth came out of the efforts to define a Personal Area Network standard Realistically, it started because someone wanted to use his cell phone to connect to the Internet without a cable The IEEE came along afterwards and wrote the standard Which basically standardized what the manufacturers had already standardized on The hope is that by having an impartial standard, the technology may gain wider adoption as it won t be subject to corporate whims and politics 94 Bluetooth The basic unit is the Personal Area Network, or Piconet A Piconet consists of a master node and up to 7 active slave nodes A Piconet can have up to 255 non-active parked nodes All communications is between master and slave There are no slave-slave communications 95 Bluetooth Architecture Bluetooth Applications Two piconets can be connected to form a scatternet 96 97

8 The Bluetooth Protocol Stack The Bluetooth Frame Structure The version of the Bluetooth protocol architecture A typical Bluetooth data frame Data Link Layer Switching Bridges from 802.x to 802.y Local Internetworking Spanning Tree Bridges Remote Bridges Repeaters, Hubs, Bridges, Switches, Routers, Gateways Virtual LANs 100 Data Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN 101 Bridges from 802.x to 802.y Bridges from 802.x to 802.y Operation of a LAN bridge from to The IEEE 802 frame formats 103

9 Local Internetworking Spanning Tree Bridges A configuration with four LANs and two bridges 104 Two parallel transparent bridges 105 Spanning Tree Bridges (2) Remote Bridges (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree. 106 Remote bridges can be used to interconnect distant LANs 107 Repeaters, Hubs, Bridges, Switches, Routers and Gateways Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) (a) Which device is in which layer (b) Frames, packets, and headers 108 (a) A hub. (b) A bridge. (c) a switch 109

10 Virtual LANs Virtual LANs (2) A building with centralized wiring using hubs and a switch (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches The IEEE 802.1Q Standard The IEEE 802.1Q Standard (2) Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not The (legacy) and 802.1Q Ethernet frame formats

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