01 wireless networks
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  • In a conventional wireless network, the coverage area is divided into a number of cell sites. Each cell site is served by a single base station that provides wireless connectivity to the terminals in its vicinity. Base stations are interconnected in a fixed wired (or wireless) network. Thus, base stations act as central control units that carry out the MAC functions. An MTSO is in charge of keeping track of wireless subscribers and connects the wireless network to other wide-area networks, such as the PSTN or ISDN networks. <br />

01 wireless networks 01 wireless networks Presentation Transcript

  • Mobile Computing COE 446 Introduction Tarek Sheltami KFUPM CCSE COE http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm Principles of Wireless Networks K. Pahlavan and P. Krishnamurth February 15, 2014 1
  • Introduction Background:    # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access two important (but different) challenges   communication over wireless link handling mobile user who changes point of attachment to network February 15, 2014 2
  • Cellular Subscribers February 15, 2014 3
  • ..Cellular Subscribers February 15, 2014 4
  • Characteristics of selected wireless link standards 54 Mbps 5-11 Mbps 1 Mbps 802.11{a,g} 802.11b .11 p-to-p link 802.15 3G UMTS/WCDMA, CDMA2000 384 Kbps 2G IS-95 CDMA, GSM 56 Kbps Indoor Outdoor Mid range outdoor Long range outdoor 10 – 30m 50 – 200m 200m – 4Km 5Km – 20Km February 15, 2014 5
  • Introduction- Conventional Wireless Communications February 15, 2014 6
  • Components of cellular network architecture MSC cell  connects cells to wide area net  manages call setup (more later!)  handles mobility (more later!)  covers geographical region  base station (BS) analogous to 802.11 AP  mobile users attach to network through BS  air-interface: physical and link layer protocol between mobile and BS Mobile Switching Center Public telephone network, and Internet Mobile Switching Center wired network February 15, 2014 7
  • Cellular networks: the first hop Two techniques for sharing mobile-to-BS radio spectrum  combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots frequency  CDMA: code division bands multiple access February 15, 2014 time slots 8
  • Cellular standards: brief survey 2G systems: voice channels   IS-136 TDMA: combined FDMA/TDMA (north america) GSM (global system for mobile communications): combined FDMA/TDMA   most widely deployed IS-95 CDMA: code division multiple access February 15, 2014 9
  • Cellular standards: brief survey 2.5 G systems: voice and data channels  for those who can’t wait for 3G service: 2G extensions  general packet radio service (GPRS)  evolved from GSM  data sent on multiple channels (if available)  enhanced data rates for global evolution (EDGE)  also evolved from GSM, using enhanced modulation  Date rates up to 384K  CDMA-2000 (phase 1)  data rates up to 144K  evolved from IS-95 February 15, 2014 10
  • Cellular standards: brief survey 3G systems: voice/data   Universal Mobile Telecommunications Service (UMTS)  GSM next step, but using CDMA CDMA-2000 February 15, 2014 11
  • Cellular standards: brief survey 3G+ systems: voice/data  High Speed Downlink Packet Access  Hybrid Automatic Repeat Request  Fast cell site selection  Adaptive Modulation and Coding February 15, 2014 12
  • ?Why HSDPA  Comparison Between 3G & 3.5G.  Data Rate ( 2Mbps -----> 10 Mbps)  Modulation ( QPSK -----> QPSK&16QAM)  Transmission Time Interval (TTI) ( 10ms ----> 2ms )
  • GSM: indirect routing to mobile home network HLR 2 home MSC consults HLR, gets roaming number of mobile in visited network correspondent home Mobile Switching Center 1 VLR 3 Mobile Switching Center 4 home MSC sets up 2nd leg of call to MSC in visited network mobile user visited network February 15, 2014 Public switched telephone network call routed to home network MSC in visited network completes call through base station to mobile 15
  • GSM: handoff with common MSC  VLR Mobile Switching Center old routing  Handoff goal: route call via new base station (without interruption) handoff initiated by old BSS new routing old BSS February 15, 2014 new BSS 16
  • GSM: handoff between MSCs  home network correspondent Home MSC anchor MSC MSC PSTN MSC  MSC  (a) before handoff February 15, 2014 anchor MSC: first MSC visited during call  call remains routed through anchor MSC new MSCs add on to end of MSC chain as mobile moves to new MSC IS-41 allows optional path minimization step to shorten multi-MSC chain 17
  • GSM: handoff between MSCs  home network correspondent Home MSC anchor MSC MSC PSTN MSC  MSC  (b) after handoff February 15, 2014 anchor MSC: first MSC visited during cal  call remains routed through anchor MSC new MSCs add on to end of MSC chain as mobile moves to new MSC IS-41 allows optional path minimization step to shorten multi-MSC chain 18
  • Segmenting the Telecom Market Narrowband Broadband Mobile Cellular WiMAX 3G Local WiFi Cordless Fixed POTS Dialup DSL / Cable The Evolution from Audio to Video 19
  • WiMAX: A new paradigm +3G WIMAX Incumbent Operator Any Operator Voice and Data VoIP, Data, Video Mbps 30 Mbps 100 Handsets $200 Consumer Products Telecom ITU Internet IEEE Qualcomm Intel & Others month / $70 - $50 month / $40 - $20 20
  • Networks Potential of networking:  move bits everywhere , cheaply , and with desired performance characteristics  Break the space barrier for information  Network provides “connectivity”  February 15, 2014 21
  • What is “Connectivity ” ?   Direct or indirect access to every other node in the network Connectivity is the media needed to communicate if you do not have a direct pt-pt physical link.  Tradeoff: Performance characteristics worse than true physical link! February 15, 2014 22
  • .Connectivity   Building Blocks  links: coax cable, optical fiber...  nodes: general-purpose workstations... Direct connectivity:  point-to-point  multiple access February 15, 2014 23
  • Connectivity..  Indirect Connectivity  switched networks => switches  inter-networks => routers February 15, 2014 24
  • Connectivity …   Internet:  Best-effort (no performance guarantees)  Packet-by-packet A pt-pt physical link:  Always-connected  Fixed bandwidth  Fixed delay  Zero-jitter February 15, 2014 25
  • Wired and Wireless Multiple Access      Most multiple access were originally developed for wired networks Requirements for wired & wireless networks are different The main difference between wired and wireless channels are availability of BW and reliability of transmission The wired medium is moving toward optical media with enormous BW and very reliable transmission BW of wireless systems always limited because of the air medium February 15, 2014 26
  • Wired and Wireless Multiple ..Access   Wireless medium always suffers from multi-path and fading, which causes serious threat to reliable data transmission over the communication link Wireless have evolved around voice and data application Wireless Networks Voice Oriented February 15, 2014 Data Oriented 27
  • Wired and Wireless Multiple Access..    Voice oriented networks are designed for relatively long telephone conversation as the main application, therefore exchange of several Mbytes of information in both directions Data oriented networks are designed for bursts of data (packet switching) Wireless networks assigns a time slot, a portion of frequency, or a code to user preferably for the entire length of the conversation. February 15, 2014 28
  • ALOHA-Based Wireless Random Access ( Techniques (Pure ALOHA      MT transmits an information packets when the packet arrives from the upper layers of the protocol stack MTs say “hello” to the air interface as the packet arrives Each packet is encoded with an error-detection code The BS checks the parity of the received packet, if it is OK, it sends a short ACK packet If no ACK received the packet is assumed lost in a collision and it is transmitted again with randomly selected delay to avoid repeated collisions February 15, 2014 29
  • ALOHA-Based Wireless Random Access (.. Techniques (Pure ALOHA   Advantages  Simple  No synchronization between MTs Disadvantage  Low throughput under heavy load conditions  Max throughput for pure ALOHA 18% What is the max throughput of pure ALOHA network with large number of users and transmission range of 1 Mbps? Max Throughput = 1 Mbps X 18% = 180 Kbps February 15, 2014 30
  • ALOHA-Based Wireless Random Access (.. Techniques (Slotted ALOHA      Transmission time is divided into time slots BS transmits beacon signal for time and all MTs is divided into time slots to this beacon signal When MT generates a packet, it is buffered and transmitted at the start of the next time slot Assuming equal length packet, either we have a complete collision or no collision Throughput of slotted ALOHA = 36%, which is still low February 15, 2014 31
  • ALOHA-Based Wireless Random Access (.. Techniques (R-ALOHA   Time slots are divided into contention periods and contention free periods During contention interval, an MT uses very short packets to contend for the upcoming contention free intervals that will be used for transmission of the long information packets February 15, 2014 32
  • ALOHA-Based Wireless Random Access ..Techniques  Disadvantages of ALOHA-based Random Access:    The main drawback of ALOHA based contention is the lack of efficiency caused by collision and retransmission Users don’t take into account what other users are doing when they attempt to transmit data packets There is no mechanisms to avoid collision February 15, 2014 33
  • ALOHA-Based Wireless Random Access (.. Techniques (Pure ALOHA February 15, 2014 34
  • ALOHA-Based Wireless Random Access (.. Techniques (Slotted ALOHA February 15, 2014 35
  • ALOHA-Based Wireless Random Access (.. Techniques (R-ALOHA February 15, 2014 36