Скачать презентацию Wireless Environments Reference r Overview of Скачать презентацию Wireless Environments Reference r Overview of

8630246aac7852602116f8cd565d6b1c.ppt

  • Количество слайдов: 41

Wireless Environments Wireless Environments

Reference r Reference r " Overview of the IEEE 802. 11 b wireless Ethernet standard , " 3 Com r William Stallings " IEEE 802. 11: moving closer to practical wireless LANs, " IT Professional, Volume: 3 Issue: 3 , May. June 2001, pp. 17 -23. r J. Haartsen, " The Bluetooth radio system, " IEEE Personal Communications, Volume: 7 Issue: 1 , Feb. 2000, pp. 28 -36 r Jaap Haartsen, Mahmoud Naghshineh, Jon Inouye, Olaf J. Joeresson, and Warren Allen, " Bluetooth: Vision, Goals, and Architecture " ACM Mobile Computing and Communications Review, Volume 2, Number 4, October 1998, pp. 38 -45. r " Wi-Fi (802. 11 b) and Bluetooth: An Examination of Coexistence Approaches , " Mobilian Corporation, 2001.

Wired vs. Wireless r Wired m Wired • • Can have a point to Wired vs. Wireless r Wired m Wired • • Can have a point to point connection Not a scare medium Reliable Plentiful power r Wireless m Broadcast medium (within range) m Scare medium m Unreliable m Communication devices have scare power r We use a mixture of wired and wireless devices

Wireless LAN r Replaces Wired LANS or can be a “gateway” to the wired Wireless LAN r Replaces Wired LANS or can be a “gateway” to the wired world. r LAN-sized distance r No flow guarantees r Device to (wired) router backbone to device

IEEE 802. 11 Wireless LAN r Standard enjoying the most widespread r r deployment IEEE 802. 11 Wireless LAN r Standard enjoying the most widespread r r deployment is the IEEE 802. 11 b. 2. 4 -5 GHz unlicensed radio spectrum Up to 11 Mbps Uses CSMA/CA for multiple access Has a base-station and ad-hoc network version

Base Station Approach r Wireless host communicates with a base station m base station Base Station Approach r Wireless host communicates with a base station m base station = access point (AP) r Basic Service Set (BSS) (a. k. a. “cell”) contains: m wireless hosts m access point (AP): base station r BSS’s combined to form distribution system (DS)

Ad Hoc Network approach r No AP (i. e. , base station) r Wireless Ad Hoc Network approach r No AP (i. e. , base station) r Wireless hosts communicate with each other m To get packet from wireless host A to B may need to route through wireless hosts X, Y, Z r Applications: m “laptop” meeting in conference room, car m interconnection of “personal” devices m battlefield r IETF MANET (Mobile Ad hoc Networks) working group

Personal Area Networks (PAN) r Networks that connect devices within a small range m Personal Area Networks (PAN) r Networks that connect devices within a small range m Typically on the order of 10 -100 meters r Intended to replace cables between devices r Lost cost r One device in multiple networks r To make service free, must operate within an unlicensed band -- 2. 4 GZ band.

PAN r Application areas m Data and voice access points • Real-time voice and PAN r Application areas m Data and voice access points • Real-time voice and data transmissions m Cable replacement • Eliminates need for numerous cable attachments • Hook your laptop to your PDA, headphones, mouse, keyboard, printer, camera, etc. m Ad hoc networking • Device with PAN radio can establish connection with another when in range

Application Scenarios r The Briefcase Trick m Access e-mail while the laptop is still Application Scenarios r The Briefcase Trick m Access e-mail while the laptop is still in the briefcase. m When the laptop receives an e-mail message, you are notified by your mobile phone. m The mobile phone can be used to browse incoming e-mail and read messages. r Interactive Conferences m At meetings and conferences, information can be shared “instantly” with other participants. r Instant Digital Postcard m Connect a camera cordlessly to a mobile phone or any wire -bound connection. m Add comments and send.

Application Scenarios r Cordless desktop m Connect your desktop/laptop computer cordlessly to printers, scanner, Application Scenarios r Cordless desktop m Connect your desktop/laptop computer cordlessly to printers, scanner, keyboard, mouse, and the LAN. r Automatic Synchronizer m Automatic synchronization of data on your desktop, notebook, personal digital (PDA) and mobile phone. m Example: As you enter your office the address list and calendar in your notebook will automatically be updated to agree with the one in you desktop.

Application Scenarios r Wirelessly connect computer/stereo components. r Control household devices with PDA and Application Scenarios r Wirelessly connect computer/stereo components. r Control household devices with PDA and house hold devices communication information to PDA. r Cell-phone/PDA to vending machine

System Challenges r Support for both voice and data m Physical medium must support System Challenges r Support for both voice and data m Physical medium must support good quality real-time voice. r Able to establish ad hoc connections r Able to withstand interference from other r r sources in an unlicensed band. Worldwide use Similar amount of protection compared to cable Small size to accommodate integration into a variety of devices. Negligible power consumption compared with device in which the radio is used. Ubiquitous deployment of the technology.

PAN Standards r Standards are needed for interoperability. r Main standards: Bluetooth; IEEE 802. PAN Standards r Standards are needed for interoperability. r Main standards: Bluetooth; IEEE 802. 15 (for interoperability with 802. 11 b).

Bluetooth Standard r Universal short-range wireless capability r Name Bluetooth comes from Harald Blaatand, Bluetooth Standard r Universal short-range wireless capability r Name Bluetooth comes from Harald Blaatand, Danish Viking King 940 -981 united Denmark and Norway r Bluetooth standardization began in 1998 (currently over 1500 pages) r Specifies the physical, link, and MAC layers of the protocol stack r Sponsors m m Initial: Ericsson, Nokia, IBM, Toshiba, and Intel Expanded in 1999 to include 3 Com, Lucent, Microsoft, and Motorola r Goals of system design m m m Global operation No fixed infrastructure required for network set-up or maintenance Voice and data connections Small, low power radio Low cost: $5 -$10 per node

Bluetooth Architecture r The basic unit of networking in Bluetooth is a piconet. m Bluetooth Architecture r The basic unit of networking in Bluetooth is a piconet. m Consists of a master and from one to seven active slave devices r A device in one piconet may also exist as part of another piconet and may function as either a slave or master in each piconet. This form of overlapping is called a scatternet.

Piconet r Master connected to <= 7 slaves Piconet r Master connected to <= 7 slaves

Topologies Topologies

Radio Spectrum r Defined in a global band (2. 45 GHz ISM (Industrial, Scientific, Radio Spectrum r Defined in a global band (2. 45 GHz ISM (Industrial, Scientific, Medical) band) r This is the same spectrum used by microwave ovens, WLANs and cordless phones which makes it crowded with radio signals. r Can be divided into 80 1 MHz channels r Devices within 10 m can share up to 720 kbps of capacity.

Multiple Access Scheme r Frequency hopping occurs by jumping from one physical channel to Multiple Access Scheme r Frequency hopping occurs by jumping from one physical channel to another in a pseudorandom sequence. r The master makes the determination of the channel (frequency-hopping sequence) and phase (timing offset i. e. , when to transmit) that shall be used by all devices in the piconet. r Different logical channels (i. e. , different hopping sequences) can simultaneously share the bandwidth.

Medium Access Control r The hop rate is 1600 hops/second so that each physical Medium Access Control r The hop rate is 1600 hops/second so that each physical channel is occupied for a duration of 0. 625 ms. r Packets can be 1, 3 or 5 slots in length. r Bluetooth radios communicate using a time division duplex (TDD) discipline. m m Transmit and receive in alternate time slots Master-slave architecture • Master transmits in a slot • Slave transmits in the following slot r Master schedules all traffic m Master must poll slaves

Bluetooth Frequency Hopping Bluetooth Frequency Hopping

Bluetooth Polling Bluetooth Polling

Medium Access Control r Transmission of a packet starts at the beginning of a Medium Access Control r Transmission of a packet starts at the beginning of a slot. r Packet lengths requiring 1, 3 or 5 slots are allowed. r For multislot packets, the radio remains at the same frequency until the entire packet has been sent. r In the next slot after the multislot packet, the radio returns to the frequency required for its hopping sequence.

Multiple Access Scheme r Why not just use FDMA? m Static division is difficult Multiple Access Scheme r Why not just use FDMA? m Static division is difficult in a dynamic environment. m We need to minimize the effects of interference from other signals by hopping to a new frequency after transmitting or receiving a packet.

Medium Access Control r Why TDD? m Prevents crosstalk between transmit and receive operations. Medium Access Control r Why TDD? m Prevents crosstalk between transmit and receive operations. m Access is contention free. m A contention-based access scheme would provide too much overhead and is thus not efficient.

Hop Selection r The frequency hopping sequence is determined by the master and is Hop Selection r The frequency hopping sequence is determined by the master and is a function of the master’s Bluetooth address and the clock on the Bluetooth device. r The function is a rather complex mathematical operation involving permutations and exclusive-OR (XOR) operations.

Hop Selection and Contention r Different piconets in the same area will have different Hop Selection and Contention r Different piconets in the same area will have different masters and thus will use different hop sequences. r This means that most of the time, transmissions on two devices on different piconets in the same area will be on different physical channels.

Hop Selection and Contention r Occasionally, two piconets will use the same physical channel Hop Selection and Contention r Occasionally, two piconets will use the same physical channel during the same time slot, causing a collision and lost data. r Should be infrequent r Can use ARQ (Automated Repeat re. Quest) m m m Stop-and-wait Suppose slave A sends packet to master Master checks a specific field to determine if the header contains errors. If so, master’s next packet to A sets ARQ bit to NARQ (negative acknowledgement). Slave A’s next packet to Master is a retransmission of previous corrupted packet.

Hop Selection r The hop selection mechanism satisfies the following requirements: m m m Hop Selection r The hop selection mechanism satisfies the following requirements: m m m The sequence cycle covers about 23 hours On average, all frequencies are visited with equal probability By changing the clock and/or identity, the selected hop changes instantaneously

Bluetooth States r Standby: waiting to join a piconet r Connecting m Inquire: find Bluetooth States r Standby: waiting to join a piconet r Connecting m Inquire: find existing nodes in the area m Page: connect to a specified radio r Active: actively on a piconet as a master or slave r Park/Hold/Sniff: low-power connected states.

Establishing a Connection r Standby Mode r Node listens to hop channels with a Establishing a Connection r Standby Mode r Node listens to hop channels with a low duty cycle m Listening for “Inquiry” or “Page” messages from other nodes (masters) • Inquiry: Used by a potential master to find other nodes in the area (address is unknown) • Page: Used to set up a connection with a specific node (address is known) • Inquiry messages are followed by page messages m Node “wakes-up” every so often (between 0 and 3. 84 s) and listens for close to 11 ms to a particular hop channel • Node listens to one of 32 “wake-up” channels out of 79 total channels • Pseudorandom subset of 79 channels • Wake-up sequence – Node will visit each channel within wake-up set once – Wake-up channel set and sequence determined by node’s ID and clock – Different nodes have different wake-up channel sets and sequences

Paging r Once a node has device ID and clock of a node, it Paging r Once a node has device ID and clock of a node, it can page a node with which it wants to establish a connection (paging node becomes Master) r Master pages node with node’s device ID m m Must know ID from an inquiry scan Preferable to know node’s native clock to determine node’s wake -up sequence and determine which hop channel node will be listening to during next wake-up r Time-frequency uncertainty m m Master does not know when next wake-up for node will occur Master transmits page message with node’s ID in expected hop channel as well as previous and next channels in wake-up sequence for 10 ms (16 hops) Short paging message can be transmitted several times on different hop channels during 10 ms page repeated until node responds or timeout

Paging (cont. ) r Node listens to current channel in wake-up sequence for 11 Paging (cont. ) r Node listens to current channel in wake-up sequence for 11 ms (page scan) r Node acknowledges master’s page with its device ID r Master responds to ACK by sending its device ID and Clock r Slave now sets hopping pattern based on master’s ID and clock master and slave connected

Inquiry r Node A (potential master) in standby wants to find other Bluetooth radios Inquiry r Node A (potential master) in standby wants to find other Bluetooth radios in the area r Node A transmits an “Inquiry” message r Other radios in the area currently doing inquire scans to listen for inquire messages r A generic Inquire identifier is used to generate the hop sequence for the Inquiry message. m All nodes listen to known inquiry channels for known inquiry ID r Node B hears Inquire message and responds with a message that has B’s device ID and clock r Node A can issue subsequent Inquire messages to find other nodes in the area r When other nodes hear Inquire message, they respond with their device ID and clock r If two nodes respond at once, collision occurs and nodes backoff

Inquiry (cont. ) r Nodes doing an Inquire scan respond if they hear the Inquiry (cont. ) r Nodes doing an Inquire scan respond if they hear the inquiry message m m m Each node returns its device ID and clock Information in return message allows inquiring node to perform a page If there is a collision, nodes wait random number of slots before responding to page inquire r After inquiry process complete, inquiring radio has Device IDs and Clocks of all radios in range r Inquiry message followed by a page message

Power Reduction Algorithms r Receiver can determine quickly if continued reception required or not Power Reduction Algorithms r Receiver can determine quickly if continued reception required or not m Correlate incoming packet with piconet access code • If code does not correlate (takes 100 ms), node can return to sleep for duration of receive slot as well as for transmit slot if node not a master – No packet sent – Packet corrupted by noise and not worth receiving • If code does correlate, node can decode slave address – If slave address matches, node continues receiving – Otherwise, packet not for node and can go to sleep for receive and transmit slots

Low Power States r Devices connected but not participating r Hold mode m If Low Power States r Devices connected but not participating r Hold mode m If no communication needed for some time, master can put slave in HOLD mode m Hold allows slave to • Go to sleep • Switch to another piconet • Perform scanning, inquiry or paging m After Hold expires, slave returns immediately to channel (synchronization remains during Hold period)

Low Power States (cont. ) r Park mode m Low duty-cycle mode low power Low Power States (cont. ) r Park mode m Low duty-cycle mode low power m Slave wakes up occasionally to resynchronize with master and check for broadcast messages m Parked slave gives up • Allows more than 7 slaves to be connected to a master m Master establishes beacon channel • Enables parked slaves to remain synchronized to piconet • Allows master to communicate with slaves m Slave cannot communicate until unparked

Low Power States (cont. ) r Sniff mode m Slave can skip some receive Low Power States (cont. ) r Sniff mode m Slave can skip some receive slots to save power m Master and slave agree on which slots slave will listen to channel

Bluetooth Security r Link layer security r Authentication m Challenge/response system r Encryption (privacy) Bluetooth Security r Link layer security r Authentication m Challenge/response system r Encryption (privacy) m Encrypts data between two devices m Stream cipher • Key management and usage m Configurable encryption key length (0 – 16 bytes) • Radios negotiate key size • Governmental regulations m Key generation algorithm