part1 Managing user workstation

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					Managing user workstation
        Juhani Merilinna

               Juhani Merilinna
   Networking
       Introduction
       Basics
       Topology
       Media
       Ethernet
       Protocols

                       Juhani Merilinna
   Networks carry data between computers.
       Files, voice, video, email, web pages
   A network includes:
       Computers: Workstations and servers
       Printers, scanners etc.
       Protocols
       Networking programs
       Media (cabling)‫‏‬

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   Local area network (LAN)‫‏‬
       Connects computers, printers etc.
       Restricted size (<1km)‫‏‬
       Usually owned by the user
   Wide area network
       Connects computers to networks or networks
       Can be worldwide
       Usually owned by an operator
   Less used names:
       MAN metropolitan area network
                         Juhani Merilinna
      Network Components

Workstations, servers, printers etc. are called nodes

      Workstation                  Server                       Router

              Cable                                        Connecting devices
              • coaxial cable                              • hub
              • Twisted pair                               • switch
              • Optical cable                              • repeater
              • Radio waves, infrared                      • bridge

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    Network topology
   Topology tells how nodes are connected in a
   Topology can be:
       Physical: what the network looks like or how
        the cables are installed
       Logical: how the network works or how data
        moves in the network

                         Juhani Merilinna
     Topology: bus

   Seldom used in modern networks
   Physical
      All nodes are connected to the same cable or

      This type of a network is unreliable

      One fault and the whole network stops working

   Logical
      All nodes receive the same transmission

      “one sends all hear” Juhani Merilinna
   Cable means one piece of cable. The
    network has normally many cables
    connected together = segment
   A bus type of network needs terminators
    in both ends of the segment.

                      Juhani Merilinna
    Topology: ring

   Not used in modern networks
   Physical
       The cables form a ring
   Logical
       A node sends data only to the next node in the
        ring              Juhani Merilinna
    Topology: star

   Physical
       Nodes are connected to a device in the middle
   Logical
       ??
                          Juhani Merilinna
    Topology: tree


   Tree and star almost the same.
   Tree is needed when there are a lot of
    computers in the same network.

                      Juhani Merilinna

   Mesh
       Multiple routes between two points
            How to select the route?
       Telephone network
       Internet

                               Juhani Merilinna
    Media: where the data travels
   Network can be
       Wired
            Copper cable (coaxial, twisted pair)‫‏‬
            Optical cable (fiber)‫‏‬
       Wireless
            Radio waves
            Infrared light

                                Juhani Merilinna
    Coaxial cable

   Coaxial cable was popular in the first
    networks. Nowadays twisted pair and optical
    cables have replaced it.
   Original Ethernet used two types of coaxial
       Thinnet (10BASE2 network) for 100 m or less
       Thicknet (10BASE5network) for 500 m or less
   Maximum speed used was 10 Mbps (Mega
    bits per second)‫‏‬
                         Juhani Merilinna
    Coaxial cable
   Coaxial cable was used in bus type of networks.
   A bus needs a terminator in both ends of the
       A segment is one or more cables connected      BNC

   Terminators suppress reflections in the cable.
       The signal bounces back from the end of the
        cable (or segment)‫‏‬
   If the cable cut the whole network stops
   Coaxial cables use BNC connectors                 Terminator

                             Juhani Merilinna
    Twisted pair cable

   Pair of wires are twisted together
   Telephone cables have two pairs
   Network cables have four pairs
   The cable can be
       Shielded (STP)‫‏‬
            Used in difficult environments
       Unshielded (UTP)‫‏‬
            Most commonly used

                                Juhani Merilinna
     Twisted pair

   Twisted pair has different categories (quality
       Cat 3 10 Mbps
       Cat 5 100 Mbps
       Cat 5e and Cat 6 1000 Mbps
   Twisted pair cable connects connects usually
    only two devices together (star)‫‏‬
       No need to worry about terminators
   Maximum length 100 m
                     Juhani Merilinna
    Twisted pair
   RJ-45
   4 pairs
   Inner two pairs are the same
    as in telephone connector (RJ-
   10 and 100 Mbps networks
    use only outer two pair.
   The same cable can carry
    both LAN and telephone

                            Juhani Merilinna
    Optical cables
   Optical cable are faster and can be longer
    than copper cables.
   The cable is not very expensive but
    connections are difficult to make and
   Used in backbones
       Between networks
       Between hubs
       Seldom used as workstation cables

                         Juhani Merilinna
    Optical cable
   Thinner cable is better!!
   Multimode
       Cheaper
       Shorter distances
       Thicker
       Mostly plastic
   Single mode
       Length can be kilometers (in telephone networks)‫‏‬
       Very thin 8-10 μm
       Mostly glass
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  Optical cable

Multi mode
Signal reflects inside the cable
Different light rays travel different distances!

Single mode
The fiber is so thin that there is now room for reflections

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   Infrared
       Short distances
       Needs line of sight
   Radio waves
       WLAN
       Bluetooth
       Can go through walls
       A computer can easily be moved
   Laser
       Needs direct line of sight. Between buildings.
       Fog, smoke, rain ?? Juhani Merilinna
        Network devices
   Repeater
       Connects two segments together.
       Amplifies signal.
       Needed if distances are long
   Bridge
       Like repeater but passes transmission only when needed
       Bridges can be used also to connect different types of
   Router
       Connects networks together
       Finds a route between networks

                                 Juhani Merilinna
         A       B                           C   D

   When A sends to B, the bridge does not pass
    the traffic to the other side
   If A sends to D, the bridge passes the signal
   Reduces traffic in one segment
       A can send to B and C to D at the same time

                          Juhani Merilinna
   bps = bits per second
       Usually means so called raw speed.
   Bps = bytes per second
       (byte = 8 bits)‫‏‬
       Usually means how fast data travels
       Smaller than raw speed
       Data trasmission includes additional information
        like error correction/detection code, addresses

                          Juhani Merilinna
   Medium - Devices attach to a common medium that
    provides a path along which the electronic signals will
    travel. Historically, this medium has been coaxial copper
    cable, but today it is more commonly a twisted pair or fiber
    optic cabling.
   Segment - We refer to a single shared medium as an
    Ethernet segment.
   Node, station - Devices that are attached to a network are
    stations or nodes. (Computers, printers, etc.)‫‏‬
   Frame - The nodes communicate in short messages called
    frames, which are variably sized chunks of information

                            Juhani Merilinna
   Data is never send as a continuous stream of
   In data networks data is divided into
       One transmission does not reserve the whole
       If an error occurs only one packet needs to be
        sent again
   Packets are also called frames or
    datagrams depending on the context
                          Juhani Merilinna

           Juhani Merilinna
   Ethernet is LAN standard (IEEE 802.3)‫‏‬
   Ethernet includes:
       Hardware:
           Cabling, connecting devices (hub, switch)‫‏‬
   Link level protocols
       Frame, MAC address
       CSMA/CD

                              Juhani Merilinna
    Ethernet, media
   Almost all local networks are Ethernet networks
   Original Ethernet used coaxial cables and bus topology
       Not used any more
       10BASE-2 (thin net) max 200 m
          Workstation cabling

       10BASE-5 (thick net) max 500 m
          Backbones, to connect thinnet segments together

   Links:

                             Juhani Merilinna
    Ethernet, media
   Modern ethernet networks use twisted pair or
    wireless (or optical cables)‫‏‬
   10BASE-T, 10Mbps, Cat 4 cable
   100BASE-T, 100Mbps, Cat 5 cable
   1000BASE-T, 1000Mbps, Cat 5e or Cat 6
   1000BASE-X, 1000Mbps, optical cable
       1000BASE-LX, single mode fiber, 5 km
       1000BASE-SX, multi mode fiber, 500m
   IEEE 802.11 wireless
                         Juhani Merilinna
    Ethernet, CSMA/CD
   In the original Ethernet all nodes share the same cable. (MA
    = multiple access)‫‏‬
   Only one node can send at any time. Others must wait their
       The node listens if the cable is free (CR = carrier sense)‫‏‬
       If the cable is free, the node starts to transmit.
   If two nodes start simultaneously, a collision happens. When
    a collision is detected, the nodes stop sending. (CD =
    collision detection). The nodes wait a random time and try
   In practice 1 Gbps ethernet does not use CSMA/CD, because
    switches make it useless.
   10 Gbps ethernet does not include CSMA/CD at all.
                               Juhani Merilinna
        Workstation1   Workstation2   Workstation3   Server

   Two send at the same time: collision
   Both wait a random time and try again
   The node that waits a shorter time sends first
   Collisions reduce the speed considerably if
    the network is busy.
                                  Juhani Merilinna
    Ethernet, MAC address
   Hardware address, physical address
   Every network interface has its own unique
   Network interface cards (NIC) have
    preprogrammed addresses. They should be
    unique. Most NICs allow you to change the
   Ethernet uses MAC addresses to separate

                      Juhani Merilinna
    MAC address
   When a node sends a message, it adds the
    receivers MAC address to the message.
   A node listens only messages where it finds
    its own address
   MAC address is a 48 bit number.
       Normally written as six numbers
       00-14-4A-B5-8F-BD

                          Juhani Merilinna
       Packets in Ethernet are called frames
       A frame includes:
            Receivers MAC address
            Senders MAC address
            Length of the frame
            Data
            Error correction code (CRC)‫‏‬
    n                   Length              Data    CRC

                                 Juhani Merilinna
   Maximum transmission unit
       Larges possible frame
       Normally 1500 bytes in Ethernet
   Minimum size is usually 64 bytes
       Shorter frames are considered faulty. For
        example a collision can cause shorter frames

                          Juhani Merilinna
   Broadcast is a special message which is sent
    to all nodes in the network.
       For example when the sender does not know
        receivers address
   Routers and bridges limit broadcast areas
       Broadcast messages work only inside a local area

                          Juhani Merilinna
   A message to everyone
   A router stops


                    Juhani Merilinna
    Ethernet, hub and switch
   Modern networks are ethernet networks with
    tree (or star) topology
   All nodes are connected with a cable to a

                     Juhani Merilinna
    Switch and hub
   A switch or a hub is a box with a lot of
    connectors (port)‫‏‬
   Number of ports is normally 4 - 72


                       Juhani Merilinna
    Hubs, switches
   Original hubs were simple repeaters.
       They send every packet to every node.
       Collisions
       Works like a bus = only one node can send
       Seldom used any more. Switches are as cheap
   Switching hub
       Most people call them simply switches
       Send a packet only to the right receiver
       No collisions
                          send simultaneously
        Several nodes can Juhani Merilinna
   When B sends to C, all others must wait



                C           D

                         Juhani Merilinna
   Node A can send to E at the same time as B
    sends to C
   What if A and B send to S ??


                   C               D

                           Juhani Merilinna
    How a switch works
   The switch receives a packet to its memory
   The switch looks the receivers MAC address
    and send the packet to the right port
       The switch knows the MAC address of the node
        connected to a port
   If multiple nodes send at the same time, the
    switch handles one packet first and others
    wait in the memory
       No collisions!

                         Juhani Merilinna
   Switches use normally MAC addresses, but
    they can use also ip addresses

                     Juhani Merilinna

                B              C

   Note that in a tree type of a network one or
    more switches are used to connect other
   Between any two nodes there can be only 4
                         Juhani Merilinna
   Wireless networks (WLAN, Wifi) are also
• IEEE 802.11
• 802.11b was the first version to reach the marketplace. It
  is the slowest and least expensive of the three. 802.11b
  transmits at 2.4 GHz and can handle up to 11 megabits per
• 802.11a was next. It operates at 5 GHz and can handle up
  to 54 megabits per second.
• 802.11g is a mix of both worlds. It operates at 2.4Ghz
  (giving it the cost advantage of 802.11b) but it has the 54
  megabits per second speed of 802.11a.
                           Juhani Merilinna
   Latest version is 802.11n
       300 Mbps
       Not common yet but coming
       Backward compatible with previous versions

                         Juhani Merilinna
   WLAN can be:
       Managed.
           Nodes connected to an access point
       Ad hoc
           Two nodes (not access point) connected together

                             Juhani Merilinna

                Access point-1   Access point-2       Access point-3

   Nodes make a connection to an access point (base station)‫‏‬
   Access points can be connected to a wired network or
    wirelessly together
   Max distance depends on the environment
       Open area up to 300m (more with a special antenna)‫‏‬
       Inside a building walls etc. shorten the distance

                                   Juhani Merilinna
    WLAN, channels

               Access point-1   Access point-2   Access point-3

   The ranges of access points may overlap.
   Access points near each other must use different channel
   The number of channels depend on the area:
       11 in North America
       13 in Europe
       14 in Japan
                                      Juhani Merilinna
   To minimize interference access points with overlapping
    ranges should use only channels:
       1, 5, 9, 13 in Europe
       1, 6, 13 in USA
   If a client detects two access points with the same SSID, it
    usually selects the one having stronger signal

   Other devices use the same frequency and can interfere:
       Bluetooth
       Micro wave ovens
       etc

                                Juhani Merilinna
   SSID (service set identification) the name of
    the wireless network
   Encryption of the transmission
       Wep
       Wpa, wpa2
   Network authentication
   User authentication

                      Juhani Merilinna
    WLAN, security
   WLAN uses radio waves. So anyone close
    enough can listen
       Encryption and passwords help
   WEP
       Old but relatively easy to break
   WPA and WPA2
       Newer and better.
       Not all WLAN cards support
       Most people don't protect their home networks
            Settings are difficult
            Neighbor can use your Internet connection
                              Juhani Merilinna
    WLAN cannot detect collisions
   Sender waits an acknowledgment for every
    packet. If it does not receive
    acknowledgment, the sender retransmits the

                     Juhani Merilinna
   Haaga-Helia has many buildings.
       Is there one or more LANs
       a. in each building
       b. in Haaga-Helia
   Is hub also a node? What about a router?
   Are physical and logical topologies always the
   Why is a broadcast area limited?
   Does WLAN also use MAC addresses?
                         Juhani Merilinna
   Are there collisions when the network uses
    only switches?
   Not all workstations can use all security systems (WEP, WPA,
    WPA2) even if the access point could use them. What limits
    the choices?

                            Juhani Merilinna
  Home excercises
1. Why is mesh topology better in the Internet and other large
2. Why does Haaga-Helia use optical fibers between switches
   but not to connect individual workstations?
3. Why is MTU 1500 bytes and not longer for example 100 000
4. In tree type of a network one switch is a main switch where
   other switches are connected to. Does the main switch need
   to know MAC addresses (and who's addresses)?
5. What is the status of 802.11n? (are there access points and
   other devices? Is the standard ready?
6. How can you find out the MAC address of your computer (in
                           Juhani Merilinna
    Home exercises

                           Access point-2    Access point-3
        Access point-1

       SSID=MyNet                           SSID=MyNet
                                                                     Access point-3


   Which channels would you use for access points in the

                                                Juhani Merilinna

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