Underlying Technologies by F2Qe57

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									 Underlying
Technologies



  Chapter 2
        Introduction
• This chapter
  –Introduces basic packet-switching
   concepts and terminology
  –Reviews some of the underlying
   network hardware technologies
   used in tcp/ip internets
   Connection-oriented
       approach
• Circuit-switched
• Traditional telephony
• Guaranteed capacity
• Fixed circuit costs
 –You pay even when you are not
  talking
      Connectionless
• Data is divided into small pieces
  called packets
  –Hence, packet-switched
• Packets carry identification
  that allows network hardware
  to send it to the appropriate
  destination
Connectionless (contd)
• Multiple communications can
  proceed concurrently
• As activity increases, a given
  pair of hosts receives less of
  the network capacity
Connectionless (contd)
• Benefits are cost and
  performance
• The term network is usually
  applied to connectionless
  technology
   Wide Area Networks
• Terminology is applied loosely
• Wan, long haul, networks
  –No limit on distance
  –Up to 2.4 Gbps
  –Delays vary from milliseconds to
   several tenths of a second (and
   more)
  Local Area Networks
• High speed without the ability to
  span long distances
• A single building or small
  campus
• 100 Mbps – 10 Gbps
• Lower delays than wans
       Lans (contd)
• Each computer carries a nic,
  network interface card, that
  connects to the network
• The network need not contain
  much intelligence
• Wans have packet switches
  connected by long distance
  lines
    Network hardware
       addresses
• Each host attached to a
  network must be assigned a
  unique address, usually an
  integer
• Some technologies use
  compatible schemes, some do
  not
  Ethernet technology
• Invented at xerox parc in the
  early 1970s
• Standardized in 1978
  –Coaxial cable
  –Twisted pair – 10Base-T
  –10 mbps
• Fast and gigabit ethernet
10/100/1000 Ethernet
• Requires automatic negotiation
  Power over Ethernet
• PoE
• Can send a small amount of
  power over the same copper
  cable without degrading data
  transmission
• Enough power for small
  devices
Properties of an Ethernet

• Shared bus
• Broadcast technology
• Best-effort delivery
 –Sender receives no information
  that the packet has been received
Properties of an Ethernet

• Access control is distributed
  –No central authority
• Carrier sense multiple access
 with collision detect
  –Csma/cd
  –Each host senses whether the
   network is idle
  Collision detection and
         recovery
• Signal travels at 70% of the
  speed of light
• Two hosts may sense an idle
  network
• Signals become scrambled
• Wait a random time and resend
• Increase wait time if necessary
   Wireless networks
• Ethernet is ieee 802.3
• 802.11b, 802.11a, 802.11g
 –Access technologies (access
  point)
 –Point-to-point
     Ethernet hardware
         addresses
• Each nic is assigned a 48-bit
  number by the manufacturer
• Manufacturers purchase
  numbers in blocks
  –Part of the number indicates the
   manufacturer
  –Hardware, physical, media access
   (mac), or layer 2 addresses
    Ethernet hardware
        addresses
• Computers do not have
  hardware addresses, nics have
  hardware addresses
• The nic compares its address to
  the destination address in a
  packet
• Unicast, broadcast, multicast
     Ethernet hardware
         addresses
• All 1’s is the broadcast address
• Multicast
  –Participating computers instruct
   their nics to listen for a given
   multicast address
• Promiscuous mode
Ethernet Frame Format
• Ethernet frames
 –Between 64 and 1518 octets
 –Preamble, daddr, saddr, type, data,
  crc
 –Preamble – 64 bits of alternating
  0’s and 1’s
 –Cyclic redundacy check is
  computed
Ethernet Frame Format
• Frame type – 16 bits
  –Frames are self-identifying
  –Allows multiple protocols
  –The type determines which
   protocol software will handle
   the data
• http://www.calvin.edu/~lave/f
  igure-2.2.pdf
            bridges
• Connect two Ethernets passing
  frames between them
  –Cable modem, dsl
  –Hide details
  –A set of bridged segments acts as
   a single ehernet
• Adaptive or learning bridges
           bridges
• A bridge learns which hosts are
  on which side and only forward
  when necessary
• Most actual bridges are more
  sophisticated than this
• Learn the network topology
  and use a distributed spanning-
  tree algorithm
   Switched Ethernet
• An ethernet switch provides
  multiple connections, ports
• A multi-port bridge
• Learns which hosts are on
  which port
Asynchronoous Transfer
        Mode
• Special-purpose hardware
• High-speed switches
• Fixed-size frames called cells
  –53 octets (sort of)
• Connection-oriented
  –Manual or discovered
  –Entries in tables in switches
   Point-to-point wans
• Data circuits leased from
  telephone companies
• http://www.calvin.edu/~lave/f
  igure-2.3.pdf
• http://www.calvin.edu/~lave/f
  igure-2.4.pdf

								
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