Sunday, November 11, 2012

Notes - Example Networks

The following are notes from Computer Networks written by Tanenbaum 5th edition.

The Internet
  • Not really a network at all, vast collection of different networks that use common protocols, provide common services
  • The ARPANET
    • DoD wanted a command and control network that can survive nuclear war
    • switching offices, small redundancy
    • contract to RAND Corporation Paul Baran designed distributed fault tolerant design
    • ARPA (Advanced Research Projects Agency)
      • worked by issuing grants and contracts to agencies and universities
    • Subnets consist of minicomputers called IMPs(Interface Message Processsors)
      • connected by 56kbps transmission lines
      • connect to at least two other Imps so reroute possible
    • node consist of IMP and host
      • host can send messages 8063 bits
      • IMP breaks into packets of 1008 bits and forward them to destination
    • selected BBN to contract
      • Honeywell DDP-316 minicomputers
      • outside of subnet software needed
        • host-imp
        • host-host protocol
        • application software
    • Hosts need software too
    • Arpanet connected universities
  • Arpa announced contracts to build TCP/IP on different computer platforms
    • IBM, DEC, HP systems, UNIX
    • TCP/IP rewritten as sockets with the 4.2 BSD release of UNIX
  • DNS(Domain Name System)
NSFNET
  • NSF(National Science Foundation)
  • NSF funded CSNET 1981
  • NSF started backbone network to connect 6 supercomputers
    • given a little brother fuzzball
    • LSI-11 microcomputer
  • connected libraries, research labs, museums, universities
  • big success
  • NSF realized government cannot continue funding
    • encouraged MERIT MCI IBM
    • form nonprofit corporation ANS(Advanced Networks and Services)
  • Awarded 4 different network operators to establish NAP(Network Access Point)
  • packet has choice of backbone carriers
  • 1990s countries and regions followed suit, EuropaNET EBONE
  • Now WWW(World Wide Web)
Architecture of the Internet
  • Changed from beginnings
  • ISP(Internet service provider)
    • connects to home computer user purchases internet access or connectivity
    • connect to ISP with phone line
    • DSL(Digital Subscriber Line) reuses telephone line for data transmission
  • DSLAM(Digital Subscriber Line Access Multiplexer)
    • converts between signals and packets
  • Dial-up
    • use telephone line
    • Modem-Modulator Demodulator
  • Send over Cable TV System
    • Device at the home is cable modem
    • Cable headend is called CMTS (Cable Modem Termination System)
  • DSL and Cable access at rates small fraction of megabit/sec to multiple megabit/sec
  • rates much greater than dial up due to more bandwidth
  • broadband
  • FTTH(Fiber to the Home)
    • fiber optic transmission 45 Mbps
  • Location at which customer packets arrive at ISP Network PoP(Point of Presence)
  • Long distance transmission lines interconnect routers at PoP in different cities for ISP to serve users.
  • ISPs connect networks to exchange traffic at IXPs(Internet exchange points)
    • peer with each other
    • room full of routers to forward to ISP backbone
  • Small ISP may pay larger ISP for distant hosts, pay for transit
  • Path of packet depends on peer choices
  • AT&T/Sprint are large so do not pay for transit
  • Companies that provide large content, Google Yahoo, locate computers in data centers, designed for computers, server farm
  • Colocation, hosting
  • Intranets-companies self interconnected all internal networks
Third Generation Mobile Phone Networks
  • AMPS(Advanced Mobile Phone System)
    • Deployed 1982 first generation
  • Second Generation - voice in digital form offer text messaging
  • GSM(Global System for Mobile Communications)
    • deployed 1991 2G
  • UMTS(Universal Mobile Telecommunications System)
    • 3G systems provide rates of 2 Mbps for stationary, 384 kbps in moving vehicle
    • WCDMA(Wideband Code Division Multiple Access)
    • main 3G system deployed, 14 Mbps on downlink, 6 Mbps on uplink
    • scarcity of spectrum led to cellular network design
      • within a cell, users assigned channel that do not interfere with one another
      • allows for good reuse of spectrum
      • allows each cell to use all frequencies
    • Architecture
      • Air Interface
        • radio communication protocol over air between mobile and base station
        • CDMA
      • Base Station with controller forms
        • radio access network
        • Controller node RNC(Radio Network Controller)
        • Node B
      • Mobile phone network carries traffic for radio access
        • called core network
        • evolved from 2G GSM system that came before it
    • Connectionless/Connection Oriented
      • connectionless packet independently routed, dynamic reconfigure
        • potential router choking lose packets
      • circuit camp comes from telephone, stable system higher quality of service
        • rejected calls, busy signal
      • Older mobile phones use circuit switched core
      • UMTS network with legacy elements set up connections over PSTN(Public Switched Telephone Network)
        • MSC(Mobile Switching Center) 
        • GSMC(Gateway Switching Center)
        • MGW(Media Gateway)
    • GRPS(General Packet Radio Service) in GSM
    • UMTS core network nodes connect directly to packet switched network
      • SGSN (Serving GPRS Support Node)
      • GGSN(Gateway GPRS Support Node)
      • deliver data packets to and from  mobiles and interface to external packet networks like internet
      • Internet protocols used to set up VoIP, change to support quality of service
    • Mobility, user moves in and out of base station, handoff/handover to reroute data to new station
      • soft handover is when connect before disconnect possible
      • hard handover, disconnect before connection established
    • HSS(Home Subscriber Server)
      • knows location of each subscriber and authorization
  • Sim Card
    • Subscriber Identity Module defines user
    • authentication of user
  • 4G technologies
    • LTE(Long Term evolution)
  • WiMAX
    • 802.16 networks
Wireless LANs: 802.11
  • Laptop connected to internet easily
  • WiFi 802.11
    • operate in unlicensed band such as ISM bands defined by ITU-R
    • made up of clients and infrastructure
      • access points/base stations installed in buildings
      • clients talk to each other, ad hoc networks instead of access points
    • multipath fading
      • signals reflected off of solid objects causing fluctuations in signals
    • solve issue with path diversity
      • send along multiple independent paths so at least one good signal reaches destination
    • OFDM (Orthogonal Frequency Division Multiplexing)
      • divides wide band spectrum into narrow slices
      • CSMA(Carrier Sense Multiple Access)
        • determines if transmission band is free
    • ALOHA wireless network
      • setup in Hawaii
    • Client Mobility
      • moving away between access points, cell system implemented
    • Security
      • WEP(Wired Equivalent Privacy)
        • tried to make wireless security like wired security
        • broken scheme
      • WPA
        • Wifi protected access
        • now replaced by WPA2
RFID and Sensor Networks
  • RFID(Radio frequency Identification)
    • a tag that is embedded into an object or person to be tracked
  • passive RFID
    • no power, reflects signal
    • UHF RFID (Ultra High frequency RFID)
      • 902-928 Mhz
      • used on shipping and driver licenses
    • HF RFID(High Frequency RFID)
      • 13.56 Mhz
    • LF RFID (Low Frequency RFID)
  • active RFID
    • has power sends signal
  • Readers for RFID must solve issue of multiple tags in reading range
    • easy collision in return
  • Security for RFID
  • Sensor Network
    • monitor physical world
    • multihop network
      • relays messages between each other, self organization before sending to data collection point

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