Data Communications

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Data Communications Networking

• CT101 - Computing Systems

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Contents

• Data Transmission Circuits
• Data Communications
• Parallel Data Transmission
• Serial Data Transmission
• Asynchronous Serial Transmission
• Synchronous Serial Transmission
• Data Multiplexing Modems
• Networks Topologies (bus, star and ring)
• Cabling (coaxial, UTP, fiber optic)
• Media Access Methods (CSMA/CD, CSMA/CA, Token
  Passing)
• LAN Architectures (Ethernet, Token Ring)
• Networking Devices
• OSI Model. OSI layer functions. OSI versus TCP/IP

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Data Transmission

• Data transmission is the transfer of data from
  point-to-point often represented as an
  electromagnetic signal over a physical
  point-to-point or point-to-multipoint
  communication channel
• A communication channel refers to the medium used
  to convey information from a sender (or
  transmitter) to a receiver, and it can use fully
  or partially the medium.
• Examples of channels copper wires, optical
  fibbers or wireless communication channels.

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Data Communication Channels

• The following is a discussion on the THREE main
  types of transmission circuits (channels),
  simplex, half duplex and full duplex.
• Simplex
• Data in a simplex channel is always one way.
  Simplex channels are not often used because it is
  not possible to send back error or control
  signals to the transmit end. An example of a
  simplex channel in a computer system is the
  interface between the keyboard and the computer,
  in that key codes need only be sent one way from
  the keyboard to the computer system.
• Half Duplex
• A half duplex channel can send and receive, but
  not at the same time. Its like a one-lane bridge
  where two way traffic must give way in order to
  cross. Only one end transmits at a time, the
  other end receives.
• Full Duplex
• Data can travel in both directions
  simultaneously. There is no need to switch from
  transmit to receive mode like in half duplex. Its
  like a two lane bridge on a two-lane highway.

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Parallel and Serial Data

• Parallel transmission (e.g. 8 bits)
• Each bit uses a separate wire
• To transfer data on a parallel link, a separate
  line is used as a clock signal. This serves to
  inform the receiver when data is available. In
  addition, another line may be used by the
  receiver to inform the sender that the data has
  been used, and its ready for the next data.

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Parallel and Serial Data

• Serial (e.g. 8 bits)
• Each bit is sent over a single wire, one after
  the other
• Usually no signal lines are used to convey clock
  (timing information)
• There are two types of serial transmission,
  essentially having to do with how the clock is
  embedded into the serial data
• Asynchronous serial transmission
• Synchronous serial transmission
• If no clock information was sent, the receiver
  would misinterpret the arriving data (due to bits
  being lost, going too slow).
• Parallel transmission is obviously faster, in
  that all bits are sent at the same time, whereas
  serial transmission is slower, because only one
  bit can be sent at a time. Parallel transmission
  is very costly for anything except short links.

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Asynchronous Serial Transmission(RS232 Example)

• Because no signal lines are used to convey clock
  (timing) information, this method groups data
  together into a sequence of bits (five to eight),
  then prefixes them with a start bit and a stop
  bit. This is the method most widely used for PC
  or simple terminal serial communications.
• In asynchronous serial communication, the
  electrical interface is held in the mark position
  between characters. The start of transmission of
  a character is signaled by a drop in signal level
  to the space level. At this point, the receiver
  starts its clock. After one bit time (the start
  bit) come 8 bits of true data followed by one or
  more stop bits at the mark level.
• The receiver tries to sample the signal in the
  middle of each bit time. The byte will be read
  correctly if the line is still in the intended
  state when the last stop bit is read.
• Thus the transmitter and receiver only have to
  have approximately the same clock rate. A little
  arithmetic will show that for a 10 bit sequence,
  the last bit will be interpreted correctly even
  if the sender and receiver clocks differ by as
  much as 5.
• It is relatively simple, and therefore
  inexpensive. However, it has a high overhead, in
  that each byte carries at least two extra bits a
  20 loss of line bandwidth.

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Synchronous Serial Transmission (PS2 Example)

• The PS/2 mouse and keyboard implement a
  bidirectional synchronous serial protocol.
• The bus is "idle" when both lines are high
  (open-collector).  This is the only state where
  the keyboard/mouse is allowed begin transmitting
  data.  The host has ultimate control over the bus
  and may inhibit communication at any time by
  pulling the Clock line low.
• The device (slave) always generates the clock
  signal.  If the host wants to send data, it must
  first inhibit communication from the device by
  pulling Clock low.  The host then pulls Data low
  and releases Clock.  This is the
  "Request-to-Send" state and signals the device to
  start generating clock pulses.
• Summary Bus StatesData high, Clock high
   Idle state.Data high, Clock low
   Communication Inhibited.Data low, Clock
  high  Host Request-to-Send

• Data is transmited 1 byte at a time
• 1 start bit.  This is always 0.
• 8 data bits, least significant bit first.
• 1 parity bit (odd parity - The number of 1's in
  the data bits plus the parity bit always add up
  to an odd number. This is used for error
  detection.).
• 1 stop bit.  This is always 1.
• 1 acknowledge bit (host-to-device communication
  only)

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Serial Communication
Name Sync/Async Type Duplex Max devices Maxspeed(Kbps) Maxdistance(feet) Pincount (not including ground)
RS-232 async peer full 2 115.2 30 2 (or 4 with HW handshake)
RS-422 async multi-drop half 10 10000 4,000 1 (unidirectional only, additional pins for each bidirectional comm.)
RS-485 async multi-point half 32 10000 4,000 2
I2C sync multi-master half Limitation based on bus capacitance and bit rate 3400 lt10 2
SPI sync multi-master full Limitation based on bus capacitance and bit rate gt1000 lt10 31(Additional pins needed for every slave if slave count is more than one)
Microwire sync master/slave full Limitation based on bus capacitance and bit rate gt625 lt10 31(Additional pins needed for every slave if slave count is more than one)
1-Wire async master/slave half Limitation based on bus capacitance and bit rate 16 1,000 1
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Data Communication Terminology

• Channel
• A channel is a portion of the communications
  medium allocated to the sender and receiver for
  conveying information between them. The
  communications medium is often subdivided into a
  number of separate paths, each of which is used
  by a sender and receiver for communication
  purposes.
• Baud Rate
• Baud rate is the same as symbol rate and is a
  measure of the number of line changes which occur
  every second. Each symbol can represent or convey
  one (binary encoded signal) or several bits of
  data. For a binary signal of 20Hz, this is
  equivalent to 20 baud (there are 20 changes per
  second).
• Bits Per Second
• This is an expression of the number of data bits
  per second. Where a binary signal is being used,
  this is the same as the baud rate. When the
  signal is changed to another form, it will not be
  equal to the baud rate, as each line change can
  represent more than one bit (either two or four
  bits).
• Bandwidth
• Bandwidth is the frequency range of a channel,
  measured as the difference between the highest
  and lowest frequencies that the channel supports.
  The maximum transmission speed is dependant upon
  the available bandwidth. The larger the
  bandwidth, the higher the transmission speed.

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Data Multiplexing

• A multiplexer is a device which shares a
  communication link between a number of devices
  (users).
• Rather than provide a separate circuit for each
  device, the multiplexer combines each low speed
  circuit onto a single high speed link. The cost
  of the single high speed link is less than the
  required number of low speed links.
• It does this by time or frequency division.

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Time Division Multiplexing

• In time division, the communications link is
  subdivided in terms of time.
• Each sub-circuit is given the channel for a
  limited amount of time, before it is switched
  over to the next user, and so on
• In the picture bellow it can be seen that each
  sub-channel occupies the entire bandwidth of the
  channel, but only for a portion of the time

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Frequency Division Multiplexing

• In frequency division multiplexing, each
  sub-channel is separated by frequency (each
  sub-channel is allocated part of the bandwidth of
  the main channel)
• The speed or bandwidth of the main link is the
  sum of the individual sub-channel speeds or
  bandwidth.

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Modems

• Modems are devices which allow digital data
  signals to be transmitted across an analogue
  link.
• Modem stands for modulator/demodulator. A modem
  changes the digital signal to an analogue
  frequency, and sends this tone across the
  analogue link. At the other end, another modem
  receives the signal and converts it back to
  digital.

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Modulation Techniques

• Modulation techniques are methods used to encode
  digital information in an analogue world.
• There are three basic modulation techniques
• AM (amplitude modulation)
• FM (frequency modulation)
• PM (phase modulation)
• All 3 modulation techniques employ a carrier
  signal. A carrier signal is a single frequency
  that is used to carry the intelligence (data).
• For digital, the intelligence is either a 1 or 0.
  
• When we modulate the carrier , we are changing
  its characteristics to correspond to either a 1
  or 0.

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Amplitude Modulation

• Modifies the amplitude of the carrier to
  represent 1s or 0s
• a 1 is represented by the presence of the carrier
  for a predefined period of 3 cycles of carrier.
• Absence or no carrier indicates a 0
• Pros
• Simple to design and implement
• Cons
• Noise spikes on transmission medium interfere
  with the carrier signal.
• Loss of connection is read as 0s.

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Frequency Modulation

• Modifies the frequency of the carrier to
  represent the 1s or 0s.
• a 0 is represented by the original carrier
  frequency
• a 1 by a much higher frequency ( the cycles are
  spaced closer together)
• Pros
• Immunity to noise on transmission medium.
• Always a signal present. Loss of signal easily
  detected
• Cons
• Requires 2 frequencies
• Detection circuit needs to recognize both
  frequencies when signal is lost.

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Phase Modulation

• Phase Modulation modifies the phase of the
  carrier to represent a 1 or 0.
• The carrier phase is switched at every occurrence
  of a 1 bit but remains unaffected for a 0 bit.
• The phase of the signal is measured relative to
  the phase of the preceding bit. The bits are
  timed to coincide with a specific number of
  carrier cycles (3 in this example 1 bit)
• Pros
• Only 1 frequency used
• Easy to detect loss of carrier
• Cons
• Complex circuitry required to generate and detect
  phase changes

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What is a network

• A network as a "group of computers and associated
  devices that are connected by communications
  facilities."
• A network provides two principle benefits the
  ability to communicate and the ability to share.
• A network supports communication among users in
  ways that other media cannot.
• Sharing involves not only information (database
  records, e-mail, graphics, etc.), but also
  resources (applications, printers, modems, disk
  space, scanners, etc.) Through its ability to
  share, a network promotes collaboration
• A network can consist of two computers connected
  together on a desk or it can consist of many
  Local Area Networks (LANs) connected together to
  form a Wide Area Network (WAN) across a continent.

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Network Classifications

• Scope
• Local area network (LAN)
• Metropolitan area (MAN)
• Wide area network (WAN)
• Ownership
• Closed versus open
• Topology (configuration)
• Bus (Ethernet)
• Star (Wireless networks with central Access
  Point)
• Ring

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Network Topologies

• A topology refers to the manner in which the
  cable is run to individual workstations on the
  network.
• the configurations formed by the connections
  between devices on a local area network (LAN) or
  between two or more LANs
• There are three basic network topologies (not
  counting variations thereon) the bus, the star,
  and the ring.
• It is important to make a distinction between a
  topology and an architecture.
• A topology is concerned with the physical
  arrangement of the network components.
• In contrast, an architecture addresses the
  components themselves and how a system is
  structured (cable access methods, lower level
  protocols, topology, etc.). An example of an
  architecture is 10baseT Ethernet which typically
  uses the star topology.

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Bus Topology

• A bus topology connects each computer (node) to a
  single segment trunk.
• A trunk is a communication line, typically coax
  cable, that is referred to as the bus.  The
  signal travels from one end of the bus to the
  other.
• A terminator is required at each end to absorb
  the signal so it does not reflect back across the
  bus.
• In a bus topology, signals are broadcast to all
  stations. Each computer checks the address on the
  signal (data frame) as it passes along the bus.
  If the signals address matches that of the
  computer, the computer processes the signal. If
  the address doesnt match, the computer takes no
  action and the signal travels on down the bus.
• Only one computer can talk on a network at a
  time. A media access method (protocol) called
  CSMA/CD is used to handle the collisions that
  occur when two signals are placed on the wire at
  the same time.
• The bus topology is passive. In other words, the
  computers on the bus simply listen for a
  signal they are not responsible for moving the
  signal along.
• A bus topology is normally implemented with
  coaxial cable.

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Bus Topology

• Advantages of bus topology
• Easy to implement and extend
• Well suited for temporary networks that must be
  set up in a hurry
• Typically the cheapest topology to implement
• Failure of one station does not affect others
• Disadvantages of bus topology
• Difficult to administer/troubleshoot
• Limited cable length and number of stations
• A cable break can disable the entire network no
  redundancy
• Maintenance costs may be higher in the long run
• Performance degrades as additional computers are
  added

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Star Topology

• All of the stations in a star topology are
  connected to a central unit called a hub.
• The hub offers a common connection for all
  stations on the network. Each station has its own
  direct cable connection to the hub. In most
  cases, this means more cable is required than for
  a bus topology. However, this makes adding or
  moving computers a relatively easy task simply
  plug them into a cable outlet on the wall.
• If a cable is cut, it only affects the computer
  that was attached to it. This eliminates the
  single point of failure problem associated with
  the bus topology. (Unless, of course, the hub
  itself goes down.)
• Star topologies are normally implemented using
  twisted pair cable, specifically unshielded
  twisted pair (UTP). The star topology is probably
  the most common form of network topology
  currently in use.

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Star Topology

• Advantages of star topology
• Easy to add new stations
• Easy to monitor and troubleshoot
• Can accommodate different wiring
• Disadvantages of star topology
• Failure of hub cripples attached stations
• More cable required (more expensive to wire a
  building for networking)

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Ring Topology

• A ring topology consists of a set of stations
  connected serially by cable. In other words, its
  a circle or ring of computers. There are no
  terminated ends to the cable the signal travels
  around the circle in a clockwise (or
  anticlockwise) direction.
• Note that while this topology functions logically
  as ring, it is physically wired as a star. The
  central connector is not called a hub but a
  Multistation Access Unit or MAU. (Dont confuse a
  Token Ring MAU with a Media Adapter Unit which
  is actually a transceiver.)
• Under the ring concept, a signal is transferred
  sequentially via a "token" from one station to
  the next. When a station wants to transmit, it
  "grabs" the token, attaches data and an address
  to it, and then sends it around the ring. The
  token travels along the ring until it reaches the
  destination address. The receiving computer
  acknowledges receipt with a return message to the
  sender. The sender then releases the token for
  use by another computer.
• Each station on the ring has equal access but
  only one station can talk at a time.

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Ring Topology

• In contrast to the passive topology of the bus,
  the ring employs an active topology. Each
  station repeats or boosts the signal before
  passing it on to the next station.
• Rings are normally implemented using twisted pair
  or fiber-optic cable
• Advantages of ring topology
• Growth of system has minimal impact on
  performance
• All stations have equal access
• Disadvantages of ring topology
• Most expensive topology
• Failure of one computer may impact others
• Complex

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Choosing a Topology

• The following factors should be considered when
  choosing a topology
• Installation
• Maintenance and troubleshooting
• Expected growth
• Distances
• Infrastructure
• Existing network
• As a general rule, a bus topology is the cheapest
  to install, but may be more expensive to maintain
  because it does not provide for redundancy.
• Various topologies can be mixed on the same
  network.
• One very common example is a large Ethernet
  network with multiple hubs. Usually the hubs are
  located on different floors in a building or
  perhaps outside in another building. Each hub is
  wired in the typical star configuration. However,
  the hubs are connected together along a bus,
  typically referred to as a backbone.
• The backbone between hubs might consist of fiber
  optic cable while the workstations are wired to
  each individual hub with UTP (unshielded twisted
  pair) cable.

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Cabling

• Coaxial Cable
• Thinnet looks like regular TV cable. It is about
  1/4 inch in diameter and is very flexible and
  easy to work with.
• Thicknet is about 1/2 inch in diameter and not
  very flexible. Thicknet is older and not very
  common anymore except as a backbone within and
  between buildings. Coax transmits at 10 Mbps.. 
• Twisted Pair. Twisted pair looks like telephone
  wire and consists of insulated strands of copper
  wire twisted together. There are two versions of
  twisted pair cable
• Shielded Twisted Pair (STP). STP is commonly used
  in Token Ring networks
• Unshielded Twisted Pair (UTP). UTP is used in
  Ethernet networks. Transmission rates vary
  between 10-100-1000-10000 Mbps.
• Fiber-Optic Cable. Fiber-optic cable consists of
  a thin cylinder of glass surrounded by glass
  cladding, encased in protective outer sheath.  
  Fiber-optic cable is very fast (over 1Gbps).  It
  can transmit over long distances (2 km ) but is
  expensive.

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Cabling

• Top Unshielded Twisted Pair and Shielded Twisted
  Pair Cable
• Bottom Coaxial and Optical Fiber Cable

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Media Access Methods

• A media access method refers to the manner in
  which a computer gains and controls access to the
  networks physical medium (e.g., cable).
• Common media access methods include the
  following
• CSMA/CD
• CSMA/CA
• Token Passing
• One of the primary concerns with media access is
  how to prevent packets from colliding when two or
  more computers try to transmit simultaneously.
  Each of the methods listed above takes a
  different approach to this problem.
• Data transmitted over a network is sent one bit
  at a time. A bit is either a 1 or a 0 represented
  by a voltage change (on or off) or a light pulse.
  If two stations are transmitting at the same
  time, it is possible that the signals may
  overlap, resulting in garbled data. Such
  overlapping is referred to as a "collision."

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CSMA/CD

• CSMA/CD stands for Carrier-Sense Multiple Access
  with Collision Detection.  It is a media access
  method which means it defines how the network
  places data on the cable and how it takes it off.
  
• CSMA/CD specifies how bus topologies such as
  Ethernet handle transmission collisions. A
  collision occurs when two or more computers
  transmit signals at the same time.
• "Carrier Sense" means that each station on the
  LAN continually listens to (tests) the cable for
  the presence of a signal prior to transmitting.
• "Multiple Access" means that there are many
  computers attempting to transmit and compete for
  the opportunity to send data (i.e., they are in
  contention). 
• "Collision Detection" means that when a collision
  is detected, the stations will stop transmitting
  and wait a random length of time before
  retransmitting.
• CSMA/CD works best in an environment where
  relatively fewer, longer data frames are
  transmitted. This is in contrast to token passing
  which works best with a relatively large amount
  of short data frames.
• Because CSMA/CD works to control or manage
  collisions rather than prevent them, network
  performance can be degraded with heavy traffic. 
  The greater the traffic, the greater the number
  of collisions and retransmissions.
• CSMA/CD is used on Ethernet networks.

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CSMA/CD Operation

• In its most simple form it operates as follows
• A station that wishes to transmit on the network
  checks to see if the cable is free.
• If the cable is free, the station starts
  transmitting.
• However, another station may have detected a free
  cable at the same instant and also start
  transmitting. The result is a "collision."
• Once the collision is detected, all stations
  immediately stop transmitting.
• Stations then wait a random length of time before
  checking the cable and then retransmit

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CSMA/CA

• CSMA/CA stands for Carrier-Sense Multiple Access
  with Collision Avoidance and is a media access
  method very similar to CSMA/CD.  
• The difference is that the CD (collision
  detection) is changed to CA (collision
  avoidance). Instead of detecting and reacting to
  collisions, CSMA/CA tries to avoid them by having
  each computer signal its intention to transmit
  before actually transmitting.   In effect, the
  transmitting computer gives a 'heads up' prior to
  transmitting.
• Although CSMA/CA can prevent collisions, it comes
  with a cost in the form of the additional
  overhead incurred by having each workstation
  broadcast it's intention prior to transmitting.
  Thus, CSMA/CA is slower than CSMA/CD.
• CSMA/CA is used on Apple networks and on WiFi
  (IEEE 802.11) networks.

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The hidden terminal problem
36
Token Passing

• Token passing is a media access method by which
  collisions are prevented. 
• Collisions are eliminated under token passing
  because only a computer that possesses a free
  token (a small data frame) is allowed to
  transmit. The token passing method also allows
  different priorities to be assigned to different
  stations on the ring. Transmissions from a
  stations with higher priority take precedence
  over stations with lower priority.
• Token passing works best in an environment where
  a relatively large number of shorter data frames
  are being transmitted. (As opposed to CSMA/CD
  which works best in an environment where
  relatively fewer, longer data frames are being
  transmitted.)
• Token passing is used on Token Ring networks

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Token Passing Operation

• In its most simple form it operates as follows
• A station that wishes to transmit on the network
  waits until it will receive a free token.
• The sending station transmits its data with the
  token.
• The data travels to the recipient without
  stopping at other stations (it is just relayed).
• The receiving station receives the data and
  returns the token to the sender as an
  acknowledgment.
• The sender receives acknowledgment and releases
  the token to next station.
• The token continues being passed along the ring
  until it is seized" by the next station that
  wants to transmit.

38
LAN Architectures

• Network architecture refers to the manner in
  which the hardware and software is structured.
  The architecture includes the cable access method
  (transmission), topology, and lower level
  protocols.
• The most common type of LAN architecture is
  Ethernet. Token Ring was also used in the past.
• These architectures are sometimes referred to as
  "lower-level protocols" because they represent
  the specifications for the IEEE802 model which
  encompasses the  physical (1st) and data link
  (2nd) layers of the OSI model (to be discussed
  latter)

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Ethernet

• Ethernet is a popular, relatively inexpensive,
  easy-to-install LAN architecture with the
  following characteristics
• Uses the CSMA/CD media access control.
• Data transmission normally occurs at 100 Mbps
  (10Mbps in the early forms and 10Gbps in the most
  recent forms).
• Typically implemented in a star topology (early
  versions used bus topology as well).
• Ethernet LANs are normally distinguished by the
  type of cable they use Twisted Pair (Thinnet or
  Thicknet were also used in the past).
• The Ethernet architecture conforms to most but
  not all of the IEEE 802.3 specification (the
  physical layers are identical but the MAC layers
  are somewhat different).
• An Ethernet LAN is often described in terms of
  three parameters transmission rate, transmission
  type, and segment distance or cable type.
• "100baseT" means
• 100 - transmission rate or through put of 100Mbps
  
• base - transmission type is baseband rather than
  broadband network (i.e., the signal is placed
  directly on the cable, one signal at a time)
• T the cable type (e.g. Twisted pair)
• Few types of Ethernet 10Base2, 10Base5, 10BaseT
  and 10BaseF, 100BaseT, 100BaseF, etc..

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Token Ring

• Token ring is a relatively expensive LAN
  architecture that was strongly influenced by IBM.
  It is very stable and can be expanded without a
  significant degradation in network performance.
• Token ring uses the token passing media access
  control. Data transmission normally occurs at 4
  or 16 Mbps depending on the cable.
• Token ring is normally implemented in a logical
  ring/physical star topology with a MAU
  (Multistation Access Unit) as the hub. The
  maximum number of stations on one ring is 260 for
  shielded twisted pair and 72 for unshielded
  twisted pair (UTP). There can be up to 33 MAUs
  per ring.
• Token Ring LANs normally use shielded twisted
  pair (STP) but may also use unshielded twisted
  pair (UTP) or fiber-optic cable. The maximum
  distance to the MAU from the workstation depends
  on the cable and varies from 45 meters for UTP to
  100 meters for STP.
• The Token Ring architecture conforms generally to
  the IEEEs 802.5 specification

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Ethernet vs. Token Ring

• Ethernet is generally less expensive and easier
  to install than Token Ring.
• Token Ring is generally more secure and more
  stable than Ethernet, but not used anymore in
  typical LAN configurations.
• It is usually more difficult to add more
  computers on a Token Ring LAN than it is to an
  Ethernet LAN. However, as additional computers
  are added, performance degradation will be less
  pronounced on the Token Ring LAN than it will be
  on the Ethernet LAN.
• Ethernet uses CSMA/CD media access control and
  Token Ring uses token passing. This makes
  Ethernet better suited in a situation where there
  are a large number of computers sending fewer,
  larger data frames. Token Ring is better suited
  for small to medium size LANs sending many,
  smaller data frames.

42
Connecting Networks

• Repeater Extends a network
• Bridge Connects two compatible networks, doesnt
  necessarily pass all the messages across the
  connection
• Switch Connect several compatible networks,
  allowing it to connect several busses rather than
  two.

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Building a large bus network from smaller ones
44
Connecting Networks

• When networks are connected via repeaters,
  bridges or switches, the result is a single large
  network.
• The entire system operates in the same way as the
  original smaller networks
• Sometimes the networks to be connected have
  incompatible characteristics (e.g. WiFi network
  to be connected with Ethernet network, etc..).
• When building networks of networks, the system is
  known as internet (note the small i, term that
  is distinct from the Internet which refers to a
  particular world wide internet).
• Router Connects two incompatible networks
  resulting in a network of networks - internet

45
Routers connecting two WiFi networks and an
Ethernet network to form an internet
46
Routers and internet addressing

• Routers purpose is to route (forward messages) in
  their proper directions.
• The forwarding process is based on an internet
  wide addressing system which all the machines in
  the internet (including the machines in the
  original networks as well as the routers) are
  assigned unique addresses.
• Thus each machine in an internet has two
  addresses its original local address within its
  own network and the internet address
• A machine wanting to send a message to a machine
  in a distant network, it will attach the internet
  address of the destination and will direct the
  message to its local router. From there it is
  forwarded to the proper direction (based on a
  forwarding table maintained by the router).

47
Client and Server

• The terms "client" and "server" are used to
  describe individual computers that are part of a
  network where computing resources and workload
  are shared.
• A server is a computer that makes its resources
  available to the network and responds to the
  commands of a client. The servers shared
  resources can be files (a file server) printers
  (a print server) processing power (an
  application server) etc
• A client is a computer that uses the resources
  made available by a server. The client must have
  sufficient processing power on its own to run
  applications that interact with the resources on
  the server.
• It is possible, and quite common, for an
  individual computer to function as both a client
  and a server.
• For example, if Bill queries a SQL Server
  database from his workstation for the data he
  needs to create an Excel spreadsheet, then his
  workstation is functioning as a client. However,
  if Sue then connects to Bills workstation from
  her computer and copies the spreadsheet, then
  Bills workstation is functioning as a server.

48
ISO/OSI Model

• The International Standards Organization (ISO)
  Open Systems Interconnect (OSI) is a standard set
  of rules describing the transfer of data between
  each layer in a network operating system. Each
  layer has a specific function (i.e. the physical
  layer deals with the electrical and cable
  specifications)
• The OSI Model clearly defines the interfaces
  between each layer. This allows different network
  operating systems and protocols to work together
  by having each manufacturer adhere to the
  standard interfaces. The application of the ISO
  OSI model has allowed the modern networks that
  exist today. There are seven layers in the OSI
  model.

49
OSI Model

• The Physical Layer
• Establishes the physical characteristics of the
  network (e.g., the type of cable, connectors,
  length of cable, etc.) 
• Defines the electrical characteristics of the
  signals used to transmit the data (e.g. signal
  voltage swing, duration of voltages, etc.) 
• Transmits the binary data (bits) as electrical or
  optical signals depending on the medium.
• The Data Link Layer
• Defines how the signal will be placed on or taken
  off the NIC.  The data frames are broken down
  into individual bits that can be translated into
  electric signals and sent over the network. On
  the receiving side, the bits are reassembled into
  frames for processing by upper levels. 
• Error detection and correction  is also performed
  at the data link layer.  If an acknowledgement is
  expected and not received, the frame will be
  resent. Corrupt data is also identified at the
  data link layer.
• Because the Data-Link Layer is very complex, it
  is sometimes divided into sublayers (as defined
  by the IEEE 802 model). The lower sublayer
  provides network access. The upper sublayer is
  concerned with sending and receiving packets and
  error checking.

50
OSI Model

• The Network Layer
• Primarily concerned with addressing and routing. 
  Logical addresses (e.g., an IP address) are
  translated into physical addresses (i.e., the MAC
  address) for transmission at the network layer.
  On the receiving side, the translation process is
  reversed. 
• It is at the network layer where the route from
  the source to destination computer is determined.
  Routes are determined based  on packet addresses
  and network conditions. Traffic control measures
  are also implemented at the network layer.  
• The Transport Layer
• On the sending side, messages are packaged for
  efficient transmission and assigned a tracking
  number so they can be reassembled in proper
  order. On the receiving side, the packets are
  reassembled, checked for errors and acknowledged.
  
• Performs error handling in that it ensures all
  data is received in the proper sequence and
  without errors. If there are errors, the data is
  retransmitted.

51
OSI Model

• The Session Layer
• Is responsible for establishing, maintaining, and
  terminating a connection called a 'session'.
• A session is an exchange of messages between
  computers (a dialog). Managing the session
  involves synchronization of user tasks and dialog
  control (e.g., who transmits and for how long).  
  Synchronization involves the use of checkpoints
  in the data stream. In the event of a failure,
  only the data from the last checkpoint has to be
  resent.
• Logon, name recognition and security functions
  take place at the Session Layer.
• The Presentation Layer
• It is responsible for data translation
  (formatting), compression, and encryption.
• The Presentation Layer is primarily concerned
  with translation interpreting and converting the
  data from various formats. For example, EBCIDIC
  characters might be converted into ASCII.  It is
  also where data is compressed for transmission
  and uncompressed on receipt. Encryption
  techniques are implemented at the Presentation
  Layer.
• The redirector operates at the presentation layer
  by redirecting I/O operations across the
  network. 
• The Application Layer
• Provides the operating system with direct access
  to network services.
• It serves as the interface between the user and
  the network by providing services that directly
  support user applications. 

52
OSI Model
53
OSI Model

• Each layer may add a Header and a Trailer to its
  Data (which consists of the next higher layer's
  Header, Trailer and Data as it moves through the
  layers). The Headers contain information that
  specifically addresses layer-to-layer
  communication. For example, the Transport Header
  (TH) contains information that only the Transport
  layer sees. All other layers below the Transport
  layer pass the Transport Header as part of their
  Data.

54
OSI vs. TCP/IP
55
References

• Andrew S. Tanenbaum Computer Networks, ISBN
  0-13066102-3
• J Glenn Brookshear Computer Science An
  Overview, ISBN 0-321-54428-5
• Eugene Blanchard Introduction to Networking and
  Data Communications