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An Introduction to Computer Networks

An Introduction to Computer Networks

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An Introduction to Computer Networks

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  1. An Introductionto Computer Networks Lecture 8: Wirless Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani

  2. Outline • Why wireless Networks • What is special on wireless networks • Challenges • Bluetooth • Zigbee • 802.11 • 802.11 mac Introduction to computer Network

  3. Why wireless networks? • Mobility: to support mobile applications • Costs: reductions in infrastructure and operating costs: no cabling or cable replacement • Special situations: No cabling is possible or it is very expensive. • Reduce downtime: Moisture or hazards may cut connections.

  4. Why wireless networks? (cont) • Rapidly growing market attests to public need for mobility and uninterrupted access • Consumers are used to the flexibility and will demand instantaneous, uninterrupted, fast access regardless of the application. • Consumers and businesses are willing to pay for it

  5. The Two Hottest Trends inTelecommunications Networks Millions Mobile Telephone Users Internet Users Year Introduction to computer Network Source: Ericsson Radio Systems, Inc.

  6. Growth of Home wireless

  7. Why is it so popular? • Flexible • Low cost • Easy to deploy • Support mobility

  8. Applications ? • Ubiquitous, Pervasive computing or nomadic access. • Ad hoc networking: Where it is difficult or impossible to set infrastructure. • LAN extensions: Robots or industrial equipment communicate each others. Sensor network where elements are two many and they can not be wired!. • Sensor Networks: for monitoring, controlling, e

  9. Ad hoc networks • Collection of wireless mobile nodes dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. • Hop-by-hop routing due to limited range of each node • Nodes may enter and leave the network • Usage scenarios: • Military • Disaster relief • Temporary groups of participants (conferences)

  10. Sensor networks • Deployment of small, usually wireless sensor nodes. • Collect data, stream to central site • Maybe have actuators • Hugely resource constrained • Internet protocols have implicit assumptions about node capabilities • Power cost to transmit each bit is very high relative to node battery lifetime • Loss / etc., like other wireless • Ad-hoc: Deployment is often somewhat random

  11. Summary • Need to be connected from everywhere and anytime. • Need to be connected on movement • Need to good quality service on those situation. • Interworking with the existing networks

  12. Classification of Wireless Networks • Mobility: fixed wireless or mobile • Analog or digital • Ad hoc (decentralized) or centralized (fixed base stations) • Services: voice (isochronous) or data (asynchronous) • Ownership: public or private

  13. Classification of Wireless Networks • Area: wide (WAN), metropolitan (MAN), local (LAN), or personal (PAN) area networks • Switched (circuit- or packet-switched) or broadcast • Low bit-rate (voice grade) or high bit-rate (video, multimedia) • Terrestrial or satellite

  14. What is special on wireless? • Mobility in the network elements • Very diverse applications/devices. • Connectivity and coverage (internetworking) is a problem. • Maintaining quality of service over very unreliable links • Security (privacy, authentication,...) is very serious here. Broadcast media. • Cost efficiency

  15. Big issues! • Integration with existing data networks sounds very difficult. • It is not always possible to apply wired networks design methods/principles here. • Layering is not work very well, mostly we need cross layer design

  16. Wireless Differences 1 • Physical layer: signals travel in open space • Subject to interference • From other sources and self (multipath) • Creates interference for other wireless devices • Noisy  lots of losses • Channel conditions can be very dynamic

  17. Wireless Differences 2 • Need to share airwaves rather than wire • Don’t know what hosts are involved • Hosts may not be using same link technology • Interaction of multiple transmitters at receiver • Collisions, capture, interference • Use of spectrum: limited resource. • Cannot “create” more capacity very easily • More pressure to use spectrum efficiently

  18. Wireless Differences 3 • Mobility • Must update routing protocols to handle frequent changes • Requires hand off as mobile host moves in/out range • Changes in the channel conditions. • Coarse time scale: distance/interference/obstacles change • Other characteristics of wireless • Slow

  19. Growing Application Diversity Collision Avoidance:Car Networks Mesh Networks Wired Internet Access Point Sensor Relay Node Ad-Hoc/Sensor Networks Wireless Home Multimedia

  20. Challenge: Diversity • New architectures must accommodate rapidly evolving technology • Must accommodate different optimization goals • Power, coverage, capacity, price Wireless Edge Network INTERNET INTERNET Wireless Edge Network 2005 2010

  21. Other Challenges • Performance: Nothing is really work well • Security: It is a broadcast media • Cross layer interception • TCP performance

  22. Ideal Wireless Area network? Wish List High speed (Efficiency) Low cost No use/minimal use of the mobile equipment battery Can work in the presence of other WLAN (Heterogeneity) Easy to install and use Etc Univ. of Tehran Computer Network 23

  23. Wireless LAN Design Goals Wireless LAN Design Goals Portable product: Different countries have different regulations concerning RF band usage. Low power consumption License free operation Multiple networks should co-exist Univ. of Tehran Computer Network 24

  24. Wireless LAN Design Alternatives Design Choices Physical Layer: diffused Infrared (IR) or Radio Frequency (RF)? Radio Technology: Direct-Sequence or Frequency-Hopping? Which frequency range to use? Which MAC protocol to use. Peer-Peer architecture or Base-Station approach? Univ. of Tehran Computer Network 25

  25. DSSS (Direct Sequence Spread Spectrum) XOR of the signal with pseudo-random number (chipping sequence) generate a signal with a wider range of frequency: spread spectrum tb user data 0 1 XOR tc chipping sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 = resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit period tc: chip period

  26. Radio Technology Spread Spectrum Technologies Frequency Hopping: The sender keeps changing the carrier wave frequency at which its sending its data. Receiver must be in synch with transmitter, and know the ordering of frequencies. Direct-Sequence: The receiver listens to a set of frequencies at the same time. The subset of frequencies that actually contain data from the sender is determined by spreading code, which both the sender and receiver must know. This subset of frequencies changes during transmission. Non-Spread Spectrum requires licensing Univ. of Tehran Computer Network 27

  27. Wireless Standards Univ. of Tehran Computer Network 28

  28. Distance vs. Data Rate Univ. of Tehran Computer Network 29

  29. Bluetooth Goals Ad-hoc wireless connectivity for everything! Original goal Low-cost replacement for annoying wire between cellphone and headset Result: Two modes of operation Point to point (serial wire replacement) Point to multipoint (ad-hoc networking) Univ. of Tehran Computer Network 30

  30. Bluetooth devices Cellphones Headsets PDAs Laptops Two-way pagers Pads, tabs, etc… Univ. of Tehran Computer Network 31

  31. Bluetooth design Specs Started with Ericsson's Bluetooth Project in 1994 ! Named after Danish king Herald Blatand (AD 940-981) who was fond of blueberries Radio-frequency communication between cell phones over short distances Intel, IBM, Nokia, Toshiba, and Ericsson formed Bluetooth SIG in May 1998 Version 1.0A of the specification came out in late 1999. IEEE 802.15.1 approved in early 2002 is based on Bluetooth Key Features: Lower Power: 10 μA in standby, 50 mA while transmitting Cheap: $5 per device Small: 9 mm2 single chips Univ. of Tehran Computer Network 32

  32. Bluetooth design Specs Frequency Range: 2402 - 2480 MHz (total 79 MHz band) 23 MHz in some countries, e.g., Spain Data Rate:1 Mbps (Nominal) 720 kbps (User) Channel Bandwidth:1 MHz Range: Up to 10 m can be extended further RF hopping: 1600 times/s => 625 μs/hop Security: Challenge/Response Authentication. 128b Encryption TX Output Power: Class 1: 20 dBm Max. (0.1W) – 100m Class 2: 4 dBm (2.5 mW) Class 3: 0 dBm (1mW) – 10m Univ. of Tehran Computer Network 33

  33. Piconet Piconet is formed by a master and many slaves Up to 7 active slaves. Slaves can only transmit when requested by master Up to 255 Parked slaves Active slaves are polled by master for transmission Each station gets a 8-bit parked address => 255 parked slaves/piconet The parked station can join in 2ms. Other stations can join in more time. A device can participate in multiple piconets => complex schedule Univ. of Tehran Computer Network 34

  34. Bluetooth Operational States Univ. of Tehran Computer Network 35

  35. Bluetooth Operational States (Cont) Standby: Initial state Inquiry: Master sends an inquiry packet. Slaves scan for inquiries and respond with their address and clock after a random delay (CSMA/CA) Page: Master in page state invites devices to join the piconet. Page message is sent in 3 consecutive slots (3 frequencies). Slave enters page response state and sends page response including its device access code. Master informs slave about its clock and address so that slave can participate in piconet. Slave computes the clock offset. Connected: A short 3-bit logical address is assigned Transmit: Univ. of Tehran Computer Network 36

  36. Bluetooth Packet Format Packets can be up to five slots long. 2745 bits. Access codes: Channel access code identifies the piconet Device access code for paging requests and response Inquiry access code to discover units Header: member address (3b), type code (4b), flow control, ack/nack (1b), sequence number, and header error check (8b) 8b Header is encoded using 1/3 rate FEC resulting in 54b Synchronous traffic has periodic reserved slots. Other slots can be allocated for asynchronous traffic 54b 0-2754b 72b Univ. of Tehran Computer Network 37

  37. Bluetooth Energy Management Three inactive states: Hold: No ACL. SCO (Sync data) continues. Node can do something else: scan, page, inquire Sniff: Low-power mode. Slave listens only after fixed sniff intervals. Park: Very Low-power mode. Gives up its 3-bit active member address and gets an 8-bit parked member address. Packets for parked stations are broadcast to 3-bit zero address. Sniff Univ. of Tehran Computer Network 38

  38. Bluetooth Protocol Stack RF = Frequency hopping GFSK modulation Baseband: Frequency hop selection, connection, MAC Univ. of Tehran Computer Network 39

  39. Baseband Layer Each device has a 48-bit IEEE MAC address 3 parts: Lower address part (LAP) – 24 bits Upper address part (UAP) – 8 bits Non-significant address part (NAP) - 16 bits UAP+NAP = Organizationally Unique Identifier (OUI) from IEEE LAP is used in identifying the piconet and other operations Clock runs at 3200 cycles/sec or 312.5 μs (twice the hop rate) Univ. of Tehran Computer Network 40

  40. Bluetooth Protocol Stack Logical Link Control and Adaptation Protocol (L2CAP) Protocol multiplexing Segmentation and reassembly Controls peak bandwidth, latency, and delay variation Host Controller Interface RFCOMM Layer: Presents a virtual serial port Sets up a connection to another RFCOMM Service Discovery Protocol (SDP): Each device has one SDP which acts as a server and client for service discovery messages IrDA Interoperability protocols: Allow existing IrDA applications to work w/o changes Univ. of Tehran Computer Network 41

  41. Bluetooth Protocol Stack IrDA object Exchange (IrOBEX) and Infrared Mobile Communication (IrMC) for synchronization Audio is carried over 64 kbps over SCO links over baseband Telephony control specification binary (TCS-BIN) implements call control including group management (multiple extensions, call forwarding, and group calls) Application Profiles: Set of algorithms, options, and parameters. Standard profiles: Headset, Cordless telephony, Intercom, LAN, Fax, Serial line (RS232 and USB). Univ. of Tehran Computer Network 42

  42. 802.11 LAN Architectures Distributed wireless Networks: also called Ad-hoc networks Centralized wireless Networks: also called last hop networks. They are extension to wired networks. Univ. of Tehran Computer Network 43

  43. Wireless LAN Architecture Access Point Access Point Ad Hoc Laptop Laptop Server DS Pager Laptop PDA Laptop Univ. of Tehran Computer Network 44

  44. Access Point Functions Access point has three components Wireless LAN interface to communicate with nodes in its service area Wireline interface card to connect to the backbone network MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently Univ. of Tehran Computer Network 45

  45. Performance Metrics Delay: ave time on the MAC queue Throughput: fraction used for data transmission. Fairness: Not preference any node Stability: handle instantaneous loads greater than its max. capacity. Robust against channel fading Power consumption: or power saving Support for multimedia Univ. of Tehran Computer Network 46

  46. Wireless LAN Architecture, Cont… Logical Link Control Layer MAC Layer: Consist of two sub layer, physical Layer and physical convergence layer • Physical convergence layer, shields LLC from the specifics of the physical medium. Together with LLC it constitutes equivalent of Link Layer of OSI Univ. of Tehran Computer Network 47

  47. 802.11 Features Power management: NICs to switch to lower-power standby modes periodically when not transmitting, reducing the drain on the battery. Put to sleep, etc. Bandwidth: To compress data Security: Addressing: destination address does not always correspond to location. Univ. of Tehran Computer Network 48

  48. Power Management Battery life of mobile computers/PDAs are very short. Need to save The additional usage for wireless should be minimal Wireless stations have three states Sleep Awake Transmit Univ. of Tehran Computer Network 49

  49. Power Management, Cont… AP knows the power management of each node AP buffers packets to the sleeping nodes AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when? The node is awake only when it is sending data, receiving data or listening to TDIM. Univ. of Tehran Computer Network 50

  50. IEEE 802.11 Topology Independent basic service set (IBSS) networks (Ad-hoc) Basic service set (BSS), associated node with an AP Extended service set (ESS) BSS networks Distribution system (DS) as an element that interconnects BSSs within the ESS via APs. Univ. of Tehran Computer Network 51