Standard LAN Protocols - PowerPoint by zug10789

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									Standard LAN Protocols
                       The IEEE 802 Standards

   The IEEE 802 Standards (also known as ISO 8802)
    lay down a set of guide lines as to how a range of
    common networks should work.

   The IEEE 802.1 Standard describes the architecture,
    general management, addressing and internetworking
    of IEEE 802 networks.

   The IEEE 802.2 Standard describes the interface
    between the Network Layer and the Data Link Layer.
    – This standardised interface is called Logical Link Control
      (LLC) and allows the software from the Network Layer
      upwards to be ported from one IEEE 802 LAN to another.
                  IEEE 802.2: Logical Link Control

   Logical Link Control (LLC) is essentially a sub-layer
    in the Data Link Layer of IEEE 802 networks.
    – It forms the upper half of the Data Link Layer while
      Medium Access Control (MAC) forms the bottom half.




   The Network Layer passes its packets to the Data
    Link Layer using the LLC access primitives.
    – The LLC adds its own header containing sequence numbers
      and piggy-backed acknowledgement numbers.
    – The resulting structure is then passed to the MAC.
          IEEE 802.2: Logical Link Control
   The LLC offers three types of service to
    the Network Layer:
    – Unreliable datagram service - this type of connectionless
      service does not bother with acknowledgements. It is
      mainly used for sending status information.
    – Acknowledge datagram service - still connectionless but
      this time packets are acknowledge and retransmitted if they
      are corrupted or go missing.
    – Reliable connection-oriented service - packets are passed
      up to the Network Layer in the order they were transmitted.
      Packets are acknowledge and are retransmitted if they are
      corrupted or go missing.
                     LLC is based on an older protocol called
                       High-level Data Link Control (HDLC)
                  Medium Access Control

   IEEE 802.3, 802.4 and 802.5 describes
    the Medium Access Control (MAC) for
    CSMA/CD Bus, Token Bus and Token Ring LANs
    respectively.

   The roles MAC sub-layer are to determine when a
    frame can be transmitted, transmit the frame and to
    extract incoming frames from the bit stream (presented
    to it by the Physical Layer).

   The MAC sub-layer is particularly important in
    broadcast networks. This is because the MAC layer
    deals with the problem of contention (i.e. the situation
    when two hosts want to transmit at the same time).
               Multiple Access (MA)
   To help us understand the sort of problems MAC may
    have to deal with, imagine a bus topology that uses a
    single cable to connect multiple hosts.
                               Communication Link




   If any host is allowed to transmit data over this cable
    then there is a multiple access (MA) channel between
    the hosts.

   Such a network is actually viable. Each host can send
    a frame at any time and, as long as the network is not
    being used by another host, the destination host can
    receive it.
           Carrier Sense, Multiple Access (CSMA)
      With just Multiple Access, we
    can make a cheap and simple                          Communication Link


    network. However, if a host
    transmits a frame while another
    host is transmitting then both
    frames will be garbled. This is known as a collision.
   We can improve the performance of our simple
    network greatly if we introduce carrier sensing (CS).
    With carrier sensing, each host listens to the data
    being transmitted over the cable.
    – A host will only transmit its own frames when it cannot
      hear any data being transmitted by other hosts.
    – When a frame finishes, an interframe gap of about 9.6sec
      is allowed to pass before another host starts transmitting its
      frame.
               Collision Detection (CD)
        Now and again two hosts will attempt to transmit a
    frame at exactly the same time. Even carrier sensing
    will not help in this case because both hosts will already
    be transmitting before they hear the other host’s data.
    – This happens more often than you would think. There is a
      small time lag as data propagates along the cable. This means
      there is a realistic window of opportunity for both hosts to
      start transmitting without detecting the other.

   Both frames will, of course, be garbled. The best thing
    to do is for both hosts to abandon the transmission of
    their frames and to try again later.
    – This is done by getting the hosts to listen to the data on the
      cable and comparing it to the data they are transmitting. If
      they are different then a collision must have occurred!
              Collision Detection (CD)
       To ensure that the other hosts knows a collision
    has occurred as soon as possible, the first host to
    detect a collision will transmit a 48-bit burst of
    random data called a jam sequence.
    – This is just to make doubly sure that any hosts currently
      transmitting know to abandon their transmissions.

   After a suitable interval (the time it takes for the jam
    sequence to propagate along the whole network plus
    the interframe gap) the hosts can attempt to retransmit
    their frames.
    – But hang on! If the collision was due to two hosts
      transmitting at exactly the same time, what is to stop them
      from transmitting at the same time again and again?
        Binary Exponential Back-off Algorithm

     Rather than having two hosts (or occasionally more)
    attempting to retransmit their frames immediately after
    the interval, they wait for a random period of time.
    – It is unlikely that both hosts will wait for exactly the same
      time period and so a stalemate situation can be avoided.

   After the first collision, a host will typically wait 0 or 1
    time units (usually 1 unit = 51.2sec).
   If a collision occurs again, it will wait 0,1,2 or 3 time
    units.
    – And so on. After n collisions a host will randomly choose to
      wait between 0 and 2n-1 time units.
    – After 10 successive collisions, a host will typically give up
      and report to its user than it cannot transmit the data.
          IEEE 802.3: CSMA/CD Bus LAN
      The 802.3 standard describes the operation of the
    MAC sub-layer in a bus LAN that uses carrier sense,
    multiple access with collision detection (CSMA/CD).
    – Beside carrier sensing, collision detection and the binary
      exponential back-off algorithm, the standard also describes the
      format of the frames and the type of encoding used for
      transmitting frames.
    – The minimum length of frames can be varied from network to
      network. This is important because, depending on the size of
      the network, the frames must be of a suitable minimum length.
    – The standard also makes some suggestions about the type of
      cabling that should be used for CSMA/CD bus LANs.

   The CSMA/CD Bus LAN is also called Ethernet.
                   IEEE 802.3: Cable Types
     There are four types of cable use for CSMA/CD
    bus LANs. The most common are 10Base2 (a.k.a.
    thin Ethernet) and 10Base-T (a.k.a. category 5
    UTP).


    (a) 10Base5
    (b) 10Base2
    (c) 10Base-T
                                SEG.     NODES/
     NAME       CABLE           LENGTH   SEG.     CONNECTORS         ADVANTAGES
     10Base5    thick coaxial   500m     100        vampire taps     Good backbone
     10Base2    thin coaxial    200m     30         BNC (T-junc)     Cheap
     10Base-T   twisted pair    100m     1024       telephone like   Easy maintenance
     10Base-F   fibre optic     2000m    1024       expensive        Low noise
                IEEE 802.3: Frame Format
       Regardless of the type of cable used in the
    CSMA/CD Bus LAN, the format of the frame
    generated by the MAC sub-layer is the same.
Bytes:      7       1       2 or 6     2 or 6     2      0-1500   0-46      4
         Preamble       Destination   Source     Data    Data     Pad    Checksum
                         address      address   Length

                  SOF
                Delimiter


     – Frames are transmitted using Manchester Encoding.
     – The preamble contains the pattern 10101010… (a square
       wave) lasting for 5.6sec. When a network card hears that
       pattern, it gets ready to listen to the address information.
     – The Start Of Frame (SOF) byte has the pattern 10101011
       (continuing the square wave until the last bit). This change
       indicates that the destination address follows.
          IEEE 802.3: MAC Addresses
        Every network card in the world has a unique 46-
    bit serial number called a MAC address. The IEEE
    allocates these numbers to network card
    manufacturers who encode them into the firmware of
    their cards.
    – The destination and source address fields of the MAC frame
      have 48 bits set aside (the standard also allows for 16-bit
      addresses but these are rarely used).
    – The most significant bit is set to 0 to indicate an ordinary
      address and 1 to indicate a group address (this is for
      multicasting, which means that frames are sent to several
      hosts). If all 48 bits are set to 1 then frames are broadcast
      to all the hosts.
    – If the two most significant bits are both zero then the 46
      least significant bits contain the MAC addresses of the
      source and destination hosts.
                 IEEE 802.3: The Other Frame Fields
Bytes:       7       1     2 or 6       2 or 6     2      0-1500   0-46      4
          Preamble   S   Destination   Source     Data    Data     Pad    Checksum
                     O
                     F    address      address   Length


        The data length field contains the number of bytes of
         data (up to a maximum of 1500 bytes).
        The data field contains the LLC data structure, which
         in turn contains the Network Layer packet.
        If the data field is less that an appropriate minimum
         length (usually 46 bytes but this can be changed if
         necessary) then the pad field is filled will extra bytes
         to ensure the frame is long enough.
        The checksum field (a 32-bit cyclic redundancy code)
         is tagged onto the end of the frame so that the
         receiving host can check it for errors.
           IEEE 802.3: Minimum Frame Length
       When a host transmits a frame, there is a small
    chance that a collision will occur. The first host to
    detect a collision transmits a 48-bit jam sequence.

   To ensure that any hosts involved with the collision
    realise that the jam sequence is associate with their
    frame, they must still be transmitting when the jam
    sequence arrives. This means that the frame must be of
    a minimum length.

   The worse case scenario is if the two hosts are at far
    ends of the cable. If host A’s frame is just reaching
    host B when it begins transmitting, host B will detect
    the collision first and send a jam signal back to host A.
          IEEE 802.3: Minimum Frame Length
       The longest time between starting to transmit a
    frame and receiving the first bit of a jam sequence is
    twice the propagation delay from one end of the cable
    to the other. A Packet starts at B
                                                            A   Packet at time tp-   B
               (a)           time 0                   (b)



                         Collision occurs                        Jam sequence gets
                     A      at time tp       B              A    back to A at 2tp    B
               (c)                                    (d)

                                                           
                                            Jam sequence        Jam sequence

   This means that a frame must have enough bits to last
    twice the propagation delay.
    – The 802.3 CSMA/CD Bus LAN transmits data at the
      standard rate of r = 10Mbps.
    – The speed of signal propagation is about v = 2108m/s.
          IEEE 802.3: Minimum Frame Length
      In order to calculate the minimum frame length,
    we must first work out the propagation delay from one
    end of the cable to the other.
    – Say the cable is d = 400m long.
    – The propagation delay time tp = dv.
      In our example tp = 400  (2108) = 2 10-6 or 2sec.
    – The round-trip propagation delay is, of course, twice this.
      Thus the round trip delay is 2tp = 4sec.
    – With a data rate of r = 10Mbps, each bit has duration
       tb = 1/r = 1 / 10,000,000 = 0.1sec.
    – The number of bits we can fit into a round-trip propagation
      delay is 2tp  tb = 4  0.1 = 40 bits.
    – The minimum frame length is thus 40 bits (5 bytes). A
      margin of error is usually added to this (often to make it a
      power of 2) so we might use 64 bits (8 bytes).
          IEEE 802.3: Minimum Frame Length
      The standard frame length is at least 512 bits (64
    bytes) long, which is much longer than our minimum
    requirement of 64 bits (8 bytes).
    – We only have to start worrying when the LAN reaches
      lengths of more than 2.5km.


   802.3 CSMA/CD bus LANs longer than 500m are
    usually composed of multiple segments joined by in-
    line passive repeaters, which output on one cable the
    signals received on another cable.
    – When we work out the minimum frame length for these
      longer LANs, we also have to take the delays caused by the
      passive repeaters (about 2.5sec each) into account as well.
              IEEE 802.3: Non-Deterministic
       The 802.3 CSMA/CD bus LAN is said to be a
    non-deterministic network. This means that no host
    is guaranteed to be able to send its frame within a
    reasonable time (just a good probability of doing so).
    – When the network is busy, the number of collisions rises
      dramatically and it may become very difficult for any hosts
      to transmit their frames.

   A real-time computing application (such as an
    assembly line) will demand that data is transmitted
    within a specified time period.
    – Since the 802.3 bus LAN cannot guarantee this, its use for
      real-time applications may not only be undesirable but
      potentially dangerous in some situations.
    802.4 - Standard for Token Buses

   The IEEE 802.4 standard describes the operation
    of token buses.
   Unlike CSMA/CD, token buses have a guaranteed
    response time.
   Designed for use on factory floors, token buses are
    designed to be robust and reliable.
   The token bus uses a common media for sending
    frames (just like that used in CSMA/CD).
       Tokens




   Unlike CSMA/CD, each host must have
    permission before it can send a frame.
   Permission is given in the form of a token (a
    special packet that is sent to the host).
   There is only ever one token on the network,
    which is passed from host to host.
          The 802.4 frame
   The 802.4 frame is slightly different from that used by
    the 802.3 specification.




   Even the modulation is different - there are an extra
    three signals other than those that represent 0, 1 and
    idle.
   These signals are used to indicate the start and ends of
    frames.
        Frame Fields

   Preamble lengths are different and there is no
    data length field.
   The 802.4 frame includes a frame control field
    that specifies the type of frame being sent.
    There are frames for sending data, passing
    tokens and management frames.
   The source address, destination address and
    checksum (CRC) fields are the same as in the
    802.3 standard.
         802.5 - Standard for Token Rings
   The IEEE 802.5 standard describes the
    operation of token rings.
   Like token buses, these have a guaranteed
    response time depending on network size.
            Structure of Token Ring
   Unlike bus networks and token buses, token
    rings do not use a single media.
   Instead, each ring interface (NIU) is connected
    to the next by a separate link.
   The links are arranged in a circle.
   Each ring interface has a 1-bit buffer (which
    introduces a 1-bit delay).
   The interface can either store-and-forward the
    bit or send a different bit.
                     The Token

   In a token ring, the token is a special bit
    pattern that circulates around the ring.
   There can be only one token so only one host
    can transmit when it has the token.
   The bits propagate around the ring until a host
    decides it wants to send and removes the token
    from the network.
    – Removing the token is done by inverting just one
      bit in the token to turn it into a frame header.
                    Sending Data
   Once a host has the token, it can transmit a
    frame. In fact it can transmit several frames so
    long as it can do so within the token-holding
    time (10msec).
   The frame propagates around the token ring
    where it is seen by the destination ring
    interface, which gives a copy of it to the
    destination host.
   The frame propagates around the whole token
    ring and is removed by the sending host.
                Acknowledgements
   Acknowledgements are sent back by modifying
    bits in the frame status field.
   If left unmodified, the sender knows that the
    destination host never received the frame (in
    which case the sender will retransmit).
   When the last frame has been sent, the
    transmitting host reconstructs the token and then
    the next host can take it.
       The 802.5 Token/Frame




   The token is made of 3 fields: SD,AC,ED.
   The SD field indicates the start of a frame.
   The ED field indicates the end of a frame.
   The AC field contains a number of flags.
   The frame also starts SD,AC, …
      The 802.5 Frame

   The frame uses the FC field to distinguish a
    data frame from other management frames
    (e.g. for electing a new monitor station if the
    existing one fails).
   The destination and source address fields are
    the same as in 802.3 and 802.4 frames.
   There is no limit to the amount of data sent as
    long as it is within the token-holding time.
   The checksum is a 32-bit CRC.
     The Frame Status Field
   The frame status field is used for
    acknowledgements. There are two flags in
    this field, called A and C, which are set to
    A=0 and C=0 when the frame is
    transmitted.
   The receiving station can change these bits.
    When read by the sender, they mean:
    – A=0, C=0: destination not found
    – A=1, C=0: frame not acknowledged (=NAK)
    – A=1, C=1: frame acknowledged (=ACK)
        The Access Control Field
   The access control field of the frame contains
    flags for indicating priorities, differentiating
    between tokens and data frames, and a
    monitor bit.

                P P P T M R R R

   When T is 1, it indicates that the frame is a
    token. When it is 0, it is a data frame.
             The Monitor Bit of the
              Access Control Field
   The monitor bit M is used to mark frames as
    having passed the monitor station.
   One station on the network is nominated as the
    monitor when the network is switched on.
   Any frame passing through the monitor is
    marked. If a frame passes through the monitor
    a second time, it is deleted.
                 The Priority Bits
   The 802.5 standard allows 8 levels of priority
    (0 being the lowest and 7 being the highest) as
    indicated in the PPP bits.
   A host with an equal or higher priority than the
    token may capture the token.
   A host may also attempt to reserve a token by
    setting the RRR bits of a passing frame to its
    priority. This will then be copied to the new
    token’s PPP bits.
            Benefits of Token Rings
   Unlike CSMA/CD, the efficiency of token
    rings is actually at its best when the network is
    being used most heavily.
   A token ring can be more difficult to maintain
    and expand than a bus network or a token bus
    network but it is no less reliable than a token
    bus and, in heavy usage environment, much
    better than a bus network.
         802.2 - Logical Link Control
   It is useful to hide the differences between
    networks by using a common protocol for
    controlling them.
   This is what the Logical Link Control does (its
    like an interface used by hosts for talking to the
    network regardless of type).
   The IEEE 802.2 standard describes Logical Link
    Control for all 802.x networks.
       Logical Link Control (LLC)

   Basically, the network layer hands packets
    to the LLC (upper part of data link layer).
   The LLC adds some control data to the
    packet and passes it to the data link layer
    proper.
   The data link layer encapsulates the packets
    inside data frames that are then transmitted
    on the network.
               LLC service options

   The LLC can operate in several different modes:
    – unreliable datagram service is useful for transmitting
      video and voice signals. There are no
      acknowledgements and no sequence numbers.
    – acknowledged datagram service includes
      acknowledgements and sequence numbers.
    – connection-oriented service provides an apparently
      reliable error-free connection.
                   Some Jargon

   A Bridge is a connection that links two similar
    LANs. Packets not meant for the current LAN are
    passed to the bridge and sent on to the other LAN.

   A Router connects several similar networks and
    uses the destination address to decide which
    network the packet should be sent to.

   A Gateway connects dissimilar networks and
    performs protocol conversions on packets.
        Revisiting the OSI Model

   The ISO (International Standards
    Organization) have devised a reference
    model for computer networks.
   Such a model is meant to help network
    designers design networks and protocols.
   The model is called the OSI (Open Systems
    Interconnection) reference model.
   It has 7 layers with distinct functions.
                The OSI Reference Model
             Host A                                 Host B          Sends data in form of:

        Application Layer                      Application Layer    APDU

        Presentation layer                     Presentation layer   PPDU

          Session Layer                         Session Layer       SPDU

         Transport Layer                       Transport Layer      TPDU
                                 Router

         Network Layer       Network Layer      Network Layer       Packet

         Data Link Layer     Data Link Layer   Data Link Layer      Frame

                             Physical Layer                         Bit




   We imagine that each layer is the servant of the layer
    above it.
   Data and instructions are passed down through the
    layers until the data is physically transmitted by the
    physical layer. The data is then passed up through the
    layers of the destination.
         Peer-to-peer Communication

   We imagine that each layer in the transmitting
    host is talking directly to the equivalent layer
    (or peer) in the receiving host.
   It is normal to say things like ‘I was talking to
    Fred on the phone’. In fact, it was the
    telephone I was talking to, not Fred!
   Fred is my peer (another person) but my voice
    was processed by the telephone and then sent
    via the telephone network.
                   The Layers

   The physical layer transmits raw bits over a
    communication channel.
   The data link layer uses link flow control
    and error correction to make the link appear
    free of errors.
   The network layer controls the local sub-net
    and decides on which direction packets
    should be routed.
                   The Layers

   The transport layer turns the packet based
    communication into a stream of data. It
    deals with assembly of packets and the
    disassembly of message into packets. It also
    contains end-to-end flow control.

   The session layer establishes and maintains
    a logical connection between sender and
    receiver.
                   The Layers

   The presentation layer can perform various
    functions. Typically it might convert ASCII
    characters into UNICODE characters.

   The application layer represents the
    programs that use network communication
    such as FTP,TELNET, Web Browsers etc...
                 The TCP/IP Reference Model

   Not every network is based on the OSI
    reference model. Many systems are based on
    the TCP/IP model.
            Host A                                Host B

       Application Layer                     Application Layer

       Transport Layer                       Transport Layer
                                Router

        Network Layer       Network Layer     Network Layer

       Data Link Layer     Data Link Layer   Data Link Layer

                           Physical Layer




   TCP stands for Transmission Control Protocol
    and IP stands for Internet Protocol.
             How TCP/IP differs from OSI

   TCP/IP has 5 layers rather than 7 (actually it
    traditionally has 4 layers, the data link layer and
    physical layer being combined in the Host-to-network
    layer).




   There are no
    session or
    presentation
    layers (their roles
    being performed by the application layer if necessary).
                The TCP/IP Layers

   The TCP/IP layers that exist are essentially the
    same as those in the OSI reference model.
   The host-to-network layer is responsible for
    sending bits across the network and for link error
    control and link flow control (i.e. data link layer
    and physical layer combined).
   The Internet layer switches packets around the
    network and places packets on (or removes them
    from) the network using a packet format called
    IP (Internet Protocol).
    Finally….The TCP/IP Layers

   The transport layer accepts data (and
    instructions) from the application layer and breaks
    the data up into packets. It also reassembles
    received packets into data. It is also responsible
    for end-to-end flow control.

   The application layer contains all the higher level
    protocols such as TELNET, FTP, SMTP (email)
    etc... that are used by applications.

								
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