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Phones OFF Please


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Title: Phones OFF Please

Phones OFF Please
Local Areal Networks Parminder Singh Kang Home Email
  1. LAN Characteristics.
  2. LAN Topologies.
  3. Topologies and LLC Protocols.
  4. Topologies and MAC Protocols (Star, Bus, Ring,
  5. Hubs/Switches (intelligent Hubs).
  6. Network Access.
  7. Internetworks (Bridges, Routers and Gateways)
  8. Distributed Environments
  9. What Network to buy?

  • 1. Why LANS?
  • widespread distribution of cheap stand-alone
    microcomputer systems
  • Users need to share resources such as
  • relatively expensive peripherals
  • information such as common data files and shared
  • A network allows computer systems to communicate
    directly with each
  • other over a common communications system
  • connect terminals PCs to powerful host
  • access to shared resources, e.g. high quality
    printers, large disks
  • transfer information, e.g. programs, data, mail.

  • 2. LAN characteristics
  • transmission medium is shared by all devices
  • connected by a common cable
  • transmission by one device is received by all
    others, i.e. a broadcast network
  • transmission in the form of packets
  • limited distribution of machines up to 10 km
    and typically around 1km
  • typically restricted to a single site, e.g. an
    industrial plant
  • high data rate typically 10 times faster than
  • sharing of resources, e.g. users accessing
    common fileservers,
  • printers, plotters, etc.,

  • single ownership of all elements of the network.
  • connection of incompatible equipment to the
    network, i.e. machines
  • and software from different manufacturers
  • Since the communication lines are not owned by
    the PTT their bandwidth is
  • not limited artificially and the error
    incidence is lower.
  • Thus much higher data rates can be maintained
    and the protocols can be
  • simpler since efficiency is not as crucial a
  • Further, not using PTT lines automatically
    restricts the network to
  • a single private site and hence restricts the
    overall diameter of the network.

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  • Note that the topologies shown above are
    LOGICAL topologies
  • physical layout may be different.
  • bus networks are usually physically wired as a
    backbone spine off .
  • which individual cable segments are dropped
    by repeaters or bridges,
  • e.g. on each floor.
  • Often the shorter segments are of lower
    specification but there are strict
  • specifications on segment lengths, number of
    stations per segment,
  • number of repeaters, etc.
  • Meaning of Physical and Logical Topologies?
  • Network Segment Limit?

  • 4. MAC and LLC protocols
  • The lower three layers (Physical, Data Link and
    Network) of the ISO 7 layer
  • model are usually called the subnet deal with
    the communications aspects
  • of the network.
  • Application layers (4, 5, 6 and 7) are looked
    after by the stations
  • Physical Layer is the physical node to node link
    transfers raw data (bits)
  • Data Link Layer manages the node to node data
    transmission (flow control,
  • error correction, etc) which transfers data
  • Network Layer manages the network and the
    station to node link. Data transfer is in the
    form of packets.

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  • The majority of LANs are broadcast networks all
    stations share a common
  • communication medium all receive the
    transmitted signal.
  • In this case the Data Link Layer is split into
    two protocols
  • MAC (Medium Access Control protocol)
  • determines which device has access to the medium
    (cable) at any time.
  • for example bus topology.
  • LLC (Logical Link Control protocol)
  • manages the link, i.e. flow control, error
    correction (corrupt frames, lost frames,
    multiple frames), etc.

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  • Host computers connected to NIUs (Network
    Interface Units) which
  • handle the communication across the network.
  • In broadcast networks the MAC protocol also
    deals with physical
  • NIU addressing.
  • LAN network layer manages the host/NIU interface
    and splitting the
  • host messages up into packets.
  • In the case of a WAN addressing and routing is a
    function of the Network layer.

  • 5. Topologies and MAC protocols 
  • 5.1 Star configuration
  • Similar in concept to telephones connected to a
    an exchange.
  • Common in the early 1980's when hard disks were
    very expensive the
  • machine at the centre of the star acting as a
    fileserver for the others.
  • Today many networks are physically wired as
    stars using network hubs
  • although their logical structure may be bus or

  • 5.2 Bus configuration e.g. Ethernet, IEEE 802.3
  • Machines physically tap into a common cable
  • Stations multi dropped off the cable with
    transmission by one station
  • propagating in both directions along the bus.
  • All other stations 'hear' this transmission and
    the intended destination copies
  • the data content.
  • The signals are absorbed by terminators at each
    end of the segment.
  • packet identification by intended destination?

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  • Most bus networks are based on Ethernet
    developed by Xerox.
  • The medium was usually coaxial cable which has
    been replaced over
  • the past few years by twisted pair cable.
  • The bus is a passive mechanism
  • when no device is transmitting the cable is idle
    (no signal present)
  • if any node crashes the remainder of the network
    remains operational.
  • No need to remove messages from the bus
    resistors absorb signals.
  • Message needs to be acknowledged explicitly by
    the receiving device,
  • e.g. using a stop and wait protocol.
  • Most bus networks are based on the Ethernet
    specification and use a
  • contention MAC protocol for sharing access to
    the cable, see next section.

  • 4.2.1 Bus MAC protocols
  • Carrier Sense Multiple Access (CSMA)
  • CSMA is an extension of a simple protocol
    developed at the University
  • of Hawaii in the late 1960s.
  • large number of islands which makes connection
    by cable very difficult
  • use radio - all devices used the same frequency
  • any device would send its message whenever it
    felt like it.
  • If only one device sent, the signal would get
    through but if two or more sent
  • at the same time, the signals would collide to
    produce garbage.
  • If the signal was not corrupted the receiver
    would send an acknowledgement
  • which, assuming no collision, would be
    received by the transmitter.

  • This protocol is called ALLOHA and is very
  • if traffic is very low, this inefficiency
    doesn't matter.
  • CSMA is a simple extension of this principle
    which improves the efficiency
  • no point in transmitting if someone else is
    already doing so
  • then, how to improve on ALLOHA?

  • Therefore
  • any station wishing to transmit first listens to
    see if cable is busy.
  • If it is idle, the station sends its message
    otherwise it defers sending.
  • How quickly it tries again is called the
    persistence it may
  • keep sensing for an idle cable continuously
    (1-persistent) or
  • back off for a random amount of time before
    trying again
  • (non-persistent) or
  • generate a random number 0ltrlt1 and retry only if
    r is greater
  • than some number p (p-persistent).  Each time
    it finds the cable
  • busy the random wait increases.

  • Sensing that the cable is idle does not
    guarantee that two stations
  • will not send at the same time  causing a
    collision, (WHY?)
  • i.e. two signals corrupt each other.
  • The LLC (Logical Link Control) protocol would
    resend the frame if an
  • acknowledgement of successful receipt has is
    not been received in a
  • given time, i.e. it will timeout and
    retransmit the frame.

  • Carrier Sense Multiple Access/ Collision
    Detect (CSMA/CD) protocol
  • There is no point in continuing to send if a
    collision has occurred
  • CSMA/CD - station listens to its own signal and
    aborts if it hears a collision
  • Transmits a short jamming signal and then waits
    a random amount 
  • of time before trying again.
  • Ethernet detects a collision by the interface
    monitoring the signal 
  • level, i.e. if gt maximum collision has
  • Attenuation is a problem so bus networks are
    restricted in length. 
  • Length also limited due to possibility of
    missing collision due to
  • signal propagation time, see notes.
  • Message may still be corrupt (noise on line) so
    LLC protocol would
  • provide an acknowledgement of receipt of the

  • Throughput on CSMA/CD
  • The  protocol  outlined  above is called 1
    persistent CSMA/CD in  which 
  • stations  wishing  to transmit wait for the
    line to become free then transmit.
  • this has the problem that as  network traffic 
    increases  collisions  are 
  • more frequent  and  unpredictable 
    performance  degradation occurs  when 
  • the traffic reaches approximately 30 of the
    bandwidth capacity.
  • In non-persistent CSMA/CD the stations wait a
    random amount of time
  • before even looking to see if the line is
    clear then apply random delays if it is busy. 
  • This results in improved line utilization but at
    the cost of greater average delays
  • in particular at low line utilization.

  • CSMA/CD protocol - offered load vs. net
  • The use of intelligent hubs and bridges (see
    next section) to break up a
  • large network can help to alleviate the

24 IEEE 802.3 CSMA/CD bus standards
parameter 10BASE5 10BASE2 10BASET 100BASET 10BROAD36
Transmission medium  coaxial cable  (50 ohm)  coaxial cable  (50 ohm)  Unshielded twisted pair  Unshielded twisted pair  coaxial cable  (75 ohm)
Signaling technique  Baseband  (Manchester)  Baseband  (Manchester)  Baseband  (Manchester)  Baseband  (Manchester)  Broadband  (DPSK)
data rate (Mbps)  10 10  10  100  10 
maximum segment length  500 185  100  100  1800 
 network span (meters)  2500  (4 repeaters)  925  (4 repeaters) 500  (4 repeaters) 500  (4 repeaters) 3600  (1 repeater)
 Nodes/segment  100 100  -  -  100 
 cable diameter (mm) 10  Thick Ethernet  5  Thin Ethernet 0.4 - 0.6 0.4 - 0.6 0.4 - 1.0
10BASE5 Was the original 802.3 standard based on the Ethernet bus using thick high quality (and expensive) co-axial cable. Due to the cable being high-quality it has low attenuation and the maximum segment length is 500 meters which can be extended to 2500 using a maximum of 4 repeaters (otherwise the collision window would become too large).
10BASE2 this is similar to 10BASE5 but uses thinner lower cost coaxial cable (sometimes called thin Ethernet or Cheapernet). Due to the lower quality cable the number of stations and the segment length is reduced (but is usually adequate for small office networks).
10BASET the T stands for unshielded twisted pair cable (cheap telephone cable) and, by sacrificing maximum segment length (100 meters), runs at a data rate of 10 Mbps (in the early 1980's this tended replace 10BASE2 in many networks).
100BASET is the latest addition to 802.3 runs at a data rate of 100 Mbps (this option is used where network load is high, e.g. systems using multimedia).
10BROAD36 is a broadband option providing support for more stations over a wider area but at greater cost.
  • Token passing bus
  • Stations are connected physically by a common
    bus but with the stations
  • logically organized (by the network software)
    as a ring, e.g. ARCnet.
  • A token passing protocol (described below) is
    then used to control access to the bus.
  • The software of token passing busses is very
    complex (handling such problems
  • as nodes joining and leaving the network).
  • They tend to be use for specialized applications
    where the advantages of
  • both bus and ring are required, e.g. factory

  • 5.3 Ring configuration
  • A ring network consists conceptually of a single
    loop of cable along which traffic
  • flows in one direction.

  • Cable passes through each node which repeats the
    signal so that its
  • strength is continuously maintained.
  • hence a ring can cover a larger distance than a
  • the ring is an active mechanism
  • if the cable is broken or a node crashes, the
    ring would be disabled
  • so implementations contain duplicate cable
    loops and interface circuits
  • Bus is passive mechanism WHY?

  • Since a message is continuously regenerated by
    each station it must
  • be explicitly removed.
  • convention is that the sender removes its
    message when it returns round the ring
  • an acknowledgement of receipt can be
    'piggybacked' onto the end of
  • the original message by the receiver.
  • However, interference on the ring could result
    in the transmitter not
  • recognizing its message.
  • the message would then circulate endlessly
  • a special station, the monitor, had
    responsibility for
  • clearing garbage messages.

  • 5.3.1 Ring MAC Protocols
  • Token passing ring (IEEE 802.5)
  • A special token packet (e.g. 11111111)
    circulates around the ring.
  • If a station wants to transmit, it must wait
    until it receives the token.
  • It seizes the token by flipping the last bit and
    converting it to a connector
  • (11111110) and follows this by inserting its
    message packet.
  • This transmitted bit stream then passes through
    each node on the ring.
  • No other station can send since it hasn't got
    the token.

  • Each station looks at the address in the message
    to see if it is addressed to it
  • if not it ignores it and passes it on.
  • If the address is that of the station it copies
    the packet into its buffers and
  • flips an acknowledgement bit at the end of the
    packet to indicate receipt.
  • The packet is repeated by each node and
    eventually arrives back at
  • the sender node which
  • takes it off the ring.
  • recreates the token and sends it on
  • Thus a station may only use the token once
    before passing it on ensuring
  • fair access to the network.
  • Packets are normally kept small to prevent long
    messages hogging the ring.

  • FDDI Network (ANSI X3T9.5)
  • FDDI makes use of the high bandwidth and noise
    immunity of fiber optic
  • cable to operate at speeds greater than
    100Mbit/sec and over distances
  • of hundreds of kilometers.
  • FDDI uses a Token passing protocol except that
    when a station waiting to
  • transmit captures the token it transmits
    packets of information and then issues a
  • new token which the next station can capture
    for its messages.
  • Thus packets of information are circulating
    followed by a token.
  • This slight change makes better use of the very
    high network bandwidth.
  • If the network is broken the two rings reform to
    become a single ring.

The FDDI (Fiber Distributed Data Interface)
FDDI operating as a single ring when a break
occurs in a link between nodes
  • Up to 1000 nodes may be connected to an FDDI
    ring with a maximum
  • spacing of 2km between nodes giving a maximum
    circumference of 200km.
  • The major problem with FDDI is that it is
    expensive - tends to be used to
  • interconnect buildings on a large site with
    high speed networks based on
  • 100baseT used within the buildings.

  • 6.1 Hubs (or wiring centers)
  • 10Base2 networks using coaxial cables are
    usually multi-dropped from the
  • cable using coaxial T connectors so that the
    physical and logical
  • representations look similar, e.g.

  • 10BaseT connections are usually taken to a
    wiring centre called a
  • hub which organizes the network into a bus. In
    this case the physical
  • layout looks like a star although the logical
    layout is still a bus

  • Rings often use a similar hub system with the
    stations connected on 'petals', e.g.
  • This still operates as a logical ring since all
    traffic follows the path
  • A - B - C - D - E - F - G - H - A - etc.
  • Using wire centers makes networks much easier to
    install and to identify
  • and isolate faulty loops, e.g. a broken cable,
    bad cable connector or faulty station.

  • 6.2 Intelligent Hubs
  • In cases where the load on a CSMA/CD bus network
    is heavy and many
  • collisions would occur an intelligent (or
    active) hub many be used.
  • In this case the hub has a processor and
    allocates RAM memory to buffer
  • data going to/from the stations, removing the
    possibility of collisions,
  • e.g. when the destination line is free the
    packets are transmitted.
  • In addition the fileserver connection may be
    faster (e.g. 100BaseT) than
  • the workstations (e.g. 10BaseT) to improve
  • Concept of store-and-forward?
  • Broadcast and Collision Domain?

  • 7. Network Access
  • Probabilistic or Deterministic
  • Probabilistic access occurs when devices compete
    for access
  • CSMA/CD protocol - there is only a certain
    probability that a particular
  • device will be granted access.
  • There is no guarantee of access within a
    specified time and under heavy
  • network loads a device may never get access.
  • Deterministic access occurs when the protocol
    pre-determines when a
  • device will be granted access
  • token passing protocol access is granted to each
    device in strict rotation.
  • Thus a device will always get access (within a
    time which can be
  • calculated from network speed, number of
    stations, etc.)

  • Effect on applications
  • The application area may well determine what
    type of network
  • and/or protocol may be used.
  • For example, in real time applications (such as
    process control, real-time
  • voice, etc.) need to have guaranteed access
    within specified time limits.
  • When attempting to use Ethernet to transmit
    real-time voice conversations one
  • has no idea when the data will get through
    (there may be long breaks
  • in the conversation).
  • In a real-time control systems (e.g. car,
    nuclear power station, etc.) it is very
  • important that critical messages can get
    through within a specified time.
  • Multimedia applications which involve
    transferring large files over a network
  • can also cause sever problems if there are
    bottlenecks either on the servers
  • or the network.

  • Contention busses (such as Ethernet) are fine
    for applications where
  • traffic is relatively low and there are no
    real-time applications running.
  • Rings react evenly to heavy traffic but have the
    problem of
  • maintaining physical ring integrity.
  • The token passing bus has the advantages of both
    (easier to install
  • than rings but with deterministic access)
  • but the software is very complex (commonly used
    in factory automation).

  • 8 Internetworks
  • Bridges, Routers and Gateways
  • An internetwork is a collection of WANs and/or
    LANs that are connected
  • together via bridges, routers and gateways.
  • A bridge
  • is used to interconnect two similar LANs and
    acts as an address filter
  • to break up a large network into logical
  • contains addressing and routing intelligence and
    is aware of
  • which stations are on which network.
  • only packets destined to stations on the other
    network are passed across.
  • Splitting up a network into a number of
    semi-independent sections can

  • A gateway
  • is used is used to interconnect dissimilar
  • e.g. bus LAN to ring LAN.
  • it must contain protocol conversion software in
  • to addressing and routing intelligence.
  • A router is used in where more than two networks
    are to be interconnected.
  • In practice a bridge may also act as a router
    when interconnecting networks
  • of the same type otherwise a sophisticated
    gateway may be required.
  • The routing information is generally provided by
    static routing tables which
  • are set up by the network manager.

(No Transcript)
  • If an internetwork is configured correctly the
    majority of traffic on the network
  • is local to the semi-independent sections with
    the bridges and gateways
  • handling a (relatively) small amount of long
    distance traffic.

  • 9. Distributed Environments
  • Modern networks tend to form a distributed
  • The network communications hardware and
  • User workstations terminals, PCs and/or
    professional workstations
  • Servers powerful computer systems which provide
    general services, e.g.
  • fileservers hold general system files,
    compilers, user files, etc.
  • print servers
  • database servers holding centralized databases.
  • computational server provides computational
    power beyond the
  • capacity of workstations
  • X terminal server
  • Servers may be a high powered PC, a specialized
    workstation, a minicomputer,
  • a mainframe or supercomputer.

  • Bridges and gateways
  • in a distributed environment care must be taken
    not to overload the network
  • and to provide adequate security for sensitive
  • Splitting a large network up into a number of
    semi independent networks
  • linked by bridges and gateways can assist with
    these problems.

  • 9.1 Performance factors in a distributed
  • Distributed environments can be very complex and
    the overall performance
  • depends upon many factors.
  • Performance of the network or networks
  • This is dependent upon the physical network
    configuration and speed,
  • the communications protocol used, number of
    bridges and gateways, etc.
  • Network configuration
  • The number of user workstations, their
    distribution over the network(s)
  • together with servers, bridges, gateways, etc.
  • Modern network have hardware and software tools
    (which can keep
  • track of network traffic, files used, etc.) to
    assist with network configuration.

  • User workstations
  • Computational power, main memory size and disk
    size - high powered
  • workstations may be slowed down by bottlenecks
  • e.g. by a slow networks or overloaded servers.
  • Fileservers
  • The number and their power in terms of
  • computational power power to cope with network
    and disk I/O.
  • I/O bandwidth a good as possible to support
    disk and network I/O.
  • size of main memory very important - sufficient
    to buffer information.
  • disk speed and size

  • The distribution of software packages and user
  • around the fileservers is critical
  • complex intensive centralized tasks could well
    require a dedicated fileserver
  • spreading the end-user files around the
    fileservers prevents overloading
  • of particular fileservers .
  • Also, if a fileserver breaks down some users can
    still do their work.

  • 9.2 Common problems
  • Clearly great care is needed in configuring a
    distributed environment with
  • a slight error giving the impression of
    'clockwork' powered machines.
  • Common problems are
  • Network speed too slow for modern workstations
    together with
  • poor distribution of files.
  • too few fileservers for the number of user
    workstations and/or poor
  • distribution of fileservers across the network.
  • too little main memory on fileservers causing
    bottlenecks in the
  • accessing of centralized file systems.

  • 10. What network to buy?
  • There is no easy answer as to which type of
    network is most suitable
  • for a given application.
  • Networks based on baseband bus (e.g. Ethernet)
    were the earliest and are
  • still widely available and used.
  • The mechanism is robust and uses many well
    proven components.
  • e.g. from cable TV and telephone technology
  • Opponents would argue that the CSMA/CD
    contention protocols used
  • are unsuitable for high traffic loads, cannot
    support voice traffic and severely
  • limit the maximum length of the bus.
  • However, intelligent hubs can overcome some of
    these problems.

  • Rings are capable of covering greater distances
    and of reacting more evenly
  • to heavy traffic but are typically more
    expensive and have suffered
  • reliability problems in the past.
  • Stars have the great advantage that they can
    often be installed using existing
  • cabling but HAVE the disadvantage of all
    centralized networks dependence
  • on a master station or hub.
  • Broadband networks can handle enormous traffic
    flows at very high speeds.
  • they are the only effective mechanism at present
    which can mix data, voice
  • and image traffic but they are extremely