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Network Technology CSE3020 Week 7


Network Technology CSE3020 Week 7 Local Area Network (LAN) LAN Applications LAN Applications LAN Architecture Protocol architecture. Topologies. – PowerPoint PPT presentation

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Title: Network Technology CSE3020 Week 7

Network Technology CSE3020 Week 7
Local Area Network (LAN)
  • Stations are near to each other (short
    propagation delay). ?
  • Number of stations per LAN generally small (easy
    to manage).
  • Stations access the network occasionally
    (bandwidth sharing).
  • Size increases from time to time (flexible
  • Connection Broadcasting.
  • Bandwidth Allocation Dynamic.
  • Protocol Service Connectionless.

LAN Applications
  • Personal computer LANs
  • Around small businesses and homes (share
    printers, disk, internet access).
  • Built with Hubs and/or Switches
  • Data rate 10/100 Mbps
  • Back end and storage area LANs
  • Interconnecting large systems - mainframes and
    large storage devices.
  • Data rate, 100 Mbps, and high speed parallel I/O
    interface or Fibre Channel
  • Limited distance and limited number of devices
    Computer room

LAN Applications
  • High speed office LANs
  • Use hubs and/or switches
  • Desktop image processing transfer of high
    quality images.
  • High capacity local storage.
  • 100Mbps
  • Backbone LANs
  • Use switches
  • Interconnect low speed local LANs.
  • Improves reliability, capacity and cost of a
    single LAN.
  • 100Mbps

LAN Architecture
  • Protocol architecture.
  • Topologies.
  • Logical Link Control.
  • Media Access Control.

Protocol Architecture
  • Higher layer protocols (above layer 3) are
    independent of network architecture.
  • LAN protocols concern with lower layers of the
    OSI model.
  • IEEE 802 reference model
  • Logical link control (LLC).
  • Media access control (MAC).
  • Physical.

IEEE 802 Layers
  • Logical Link Control
  • Interface to higher levels.
  • Flow and error control.
  • Media Access Control
  • Assembly of data into frame with address and
    error detection fields.
  • Disassembly of frame, Address recognition and
    Error detection.
  • Govern access to transmission medium.
  • For the same LLC, several MAC options may be
  • Physical
  • Encoding/decoding of signal.
  • Preamble generation for synchronization.
  • Bit transmission/reception.
  • Specification of Transmission medium and topology

LAN Protocols - Encapsulation
LAN Topologies
Star and Extended Star are ubiquitous
Bus and Tree Topologies
  • Bus is a special case of the tree only one trunk
    and no branches.
  • Multipoint medium.
  • Transmission propagates throughout the medium.
  • Broadcast - heard by all stations.
  • Need to identify target station - each station
    has unique address.
  • Data transmitted in small blocks frames.
  • Full duplex connection between station and its
    tap, channel for
  • Transmission of outgoing frame
  • Collision detection or reception of incoming
  • Need to regulate transmission collisions,
  • Terminator absorbs frames at end of medium
    avoid signal reflection.

Bus LAN Frame Transmission
Ring Topology
  • Repeaters joined by point-to-point links in
    closed loop.
  • Simple device.
  • Receive data on one link and retransmit on
  • Links unidirectional.
  • Stations attach to the repeaters.
  • Data is transmitted in frames.
  • Circulate frames past all stations.
  • Destination recognizes address and copies frame.
  • Frame circulates back to source where it is
  • Media access control determines when station can
    insert frame.

Ring LAN Frame Transmission
Star Topology
  • Each station is connected directly to a common
    central node.
  • Usually via two point-to-point links.
  • One for transmission and one for reception.
  • Central node can broadcast to all stations.
  • Physically a star, but logically bus.
  • Only one station at a time may successfully
  • Central node can act as frame switch.
  • Incoming frames are buffered at the central node.
  • Then, retransmitted on an outgoing link to the

Media Access Control
  • Controlling access to the transmission medium for
    an orderly and efficient use of the networks
    transmission capacity.
  • Where control is exercised
  • Central a controller grants access to the
  • Greater control (e.g., priority, overrides and
    guaranteed capacity).
  • Simple access logic at station.
  • Avoids problems of co-ordination among peer
  • Creates a single point of network failure.
  • It may act as a potential bottleneck, reducing
  • Distributed stations collectively control access.

LAN - Media Access Control
  • How constrained by topology, cost, performance
    and complexity.
  • Synchronous MAC
  • Specific capacity dedicated to connection (like
    FDM and TDM).
  • Not optimal in LANs as the needs of the stations
    are unpredictable.
  • Asynchronous MAC
  • Allocate capacity in response to demand
  • Three categories
  • round robin
  • reservation
  • contention.

Asynchronous MAC - LAN
  • Round robin
  • Each station in turn is given the opportunity to
  • Control of sequence may be centralized or
  • Very efficient, if many stations have data to
    transmit over an extended period.
  • Considerable overhead if only a few stations have
    data to transmit.
  • Reservation
  • A station dynamically reserves future time slots
    for transmission.
  • Reservations may be made in a centralized or
    distributed fashion.
  • Good for stream traffic voice, video, telemetry
  • Contention (the most common)
  • No control to determine the turn - all stations
    contend for time.
  • Good for bursty traffic - short, sporadic
    transmission terminal traffic.
  • Simple to implement and efficient under moderate
  • May collapse under heavy load.

MAC Frame Format
  • MAC layer receives data from LLC layer.
  • Responsible for medium access and transmitting
  • MAC layer detects errors and discards frames.
  • LLC optionally retransmits unsuccessful frames.

Logical Link Control
  • Transmission of link level protocol data units
    (PDUs) between two stations
  • Must support multi-access, shared medium.
  • Relieved of some link access details by MAC
  • Addressing involves specifying source and
    destination LLC users.
  • Referred to as service access points (SAP).
  • User typically higher level protocol or network
    management function.

LLC Services and Protocol
  • Operation and format is based on HDLC.
  • Unacknowledged connectionless service (type 1
  • Best Effort Service - delivery of frame not
    guaranteed, higher layer deals with reliability
  • No prior logical connection is set up
  • No flow and error control.
  • Unnumbered information (UI) frame used to
    transfer data
  • Error detection and discard at the MAC level

LLC Services and Protocol
  • Connection-mode service (type 2 operation)
  • Reliable Delivery Service
  • Logical connection is set up
  • Flow and error control.
  • Uses HDLC in asynchronous balanced mode (ABM)
  • Acknowledged connectionless service (type 3
  • Frames acknowledged
  • No prior logical connection is set up

Extended Star LANs
  • Use unshielded twisted pair wire UTP Cat5
  • Stations are attached to a central active hub
    (act as a repeater).
  • Two links
  • Transmit and receive.
  • Hub repeats incoming signal on all outgoing
  • Link lengths limited to about 100m.
  • Fiber optic - up to 500m.
  • Logically bus - with collisions.

Extended Star LANs Two Level Topology
  • One header hub (HHUB) and one or more
    intermediate hubs (IHUB).

Extended Star LANs Hubs and Switches
  • Hub Shared medium
  • Central hub and a star wiring arrangement.
  • Hub retransmits incoming signal to all outgoing
  • To avoid collision, only one station can transmit
    at a time.
  • Switch
  • Incoming frame switched to appropriate outgoing
  • Unused lines can also be used to switch other

Extended Star LANs Hubs and Switches
Extended Star LANs Switches
  • No change to software or hardware of devices to
    change from shared to switched.
  • Each device has dedicated capacity.
  • Scales well.
  • Two types of switching hubs
  • Store and forward switch
  • Accept input, buffer it check CRC, then output.
  • Cut through switch
  • Take advantage of the destination address being
    at the start of the frame.
  • Begin repeating incoming frame onto output line
    as soon as address recognized.
  • Highest possible throughput.
  • May propagate some bad frames since no CRC check

  • Ability to expand beyond single LAN.
  • Provide interconnection to other LANs/WANs using
  • Bridge.
  • Router.
  • Bridge is simpler
  • Connects similar LANs.
  • Identical protocols for physical and link layers.
  • Minimal processing.
  • More sophisticated bridges are capable of mapping
    from one MAC format to another.
  • To connect an Ethernet and a Token ring LAN.
  • Router more general purpose
  • Interconnect various LANs and WANs.
  • Covered in higher layers of the protocol stack.

  • Why use a bridge rather than a single LAN?
  • Reliability A fault on the network may disable
    all devices.
  • Performance Reduce the number of devices on a
    single LAN.
  • Security Keep different levels of secure
    information on separate physical media.
  • Geography Multiple LANs separated by
    geographical distances.
  • Functions of a Bridge
  • Read all frames transmitted on one LAN and accept
    those addresses to any station on the other LAN.
  • Using MAC protocol for second LAN, retransmit
    each frame.
  • Do the same the other way round.

Bridges Operation
Bridges Design Aspects
  • No modification to content or format of frame.
  • No encapsulation.
  • Exact bitwise copy of frame.
  • Enough buffering to meet peak demand.
  • Contains routing and address intelligence
  • Must be able to tell which frames to pass.
  • May be more than one bridge to cross.
  • May connect more than two LANs.
  • Bridging is transparent to stations
  • Appears to all stations on multiple LANs as if
    they are on one single LAN.

Bridges Protocol Architecture
  • Function at MAC level.
  • IEEE 802.1D defines the protocol architecture.
  • Station address is at this level.
  • Bridge does not need LLC layer.
  • It is relaying MAC frames.
  • Can pass frame over external communication
  • e.g. WAN link.
  • Capture the MAC frame and encapsulate it in the
    appropriate packaging.
  • Forward it across link to the target bridge.
  • Target bridge removes encapsulation and forward
    over LAN link.

Bridges Connection of Two LANs
Bridges Fixed Routing
  • Complex large LANs need alternative routes
  • Load balancing.
  • Fault tolerance.
  • Bridge must decide whether to forward frame.
  • Bridge must decide which LAN to forward frame on.
  • Routing selected for each source-destination pair
    of LANs
  • Done in configuration.
  • Usually least hop route is selected.
  • Only changed when the topology changes.
  • A central routing matrix to be created.
  • Each bridge needs one routing table (derived from
    routing matrix).
  • Simple and minimal processing requirements.
  • Too limited for complex network.

Bridges Spanning Tree
  • Bridge automatically develops routing table.
  • Automatically update in response to changes.
  • Frame forwarding.
  • Address learning.
  • Loop resolution.

Bridges Frame forwarding
  • Bridge maintains a forwarding database for each
  • List station addresses reached through each port.
  • For a frame arriving on port X
  • Search forwarding database to see if MAC address
    is listed for any port except X.
  • If address not found, forward to all ports except
  • If address listed for port Y, check port Y for
    blocking or forwarding state.
  • Blocking prevents port from receiving or
  • If not blocked, transmit frame through port Y.

Bridges Address Learning
  • Can preload forwarding database into each bridge
    (as in fixed routing).
  • Bridges can learn forwarding database.
  • When frame arrives at port X, it has come form
    the LAN attached to port X.
  • Use the source address to update forwarding
    database for port X to include that address.
  • Timer on each entry in database entry removed
    when timer expires.
  • Each time frame arrives, source address checked
    against forwarding database.
  • If the element is already in database, update
    entry and reset timer.
  • Otherwise, a new entry is created, with its own

Bridges Spanning Tree Algorithm
  • Address learning works for tree layout (no closed

Bridges Spanning Tree Algorithm
  • For any connected graph, there is a spanning tree
    that maintains connectivity but contains no
    closed loops.
  • Spanning tree algorithm developed by IEEE 802.1.
  • Each bridge assigned unique identifier and costs
    is assigned to each port.
  • Brief exchange of messages among bridges to
    establish minimum-cost spanning tree.

Required Reading
  • W. Stallings, Data and Computer Communications
  • gtgt Ch13 6E, Ch15 7E
  • The End

  • Following Slides for Interest ONLY

Bus LANs
  • Multipoint Configuration more than two devices.
  • Signal balancing
  • Signal must be strong enough to meet receivers
    minimum signal strength requirements.
  • Strong enough to maintain adequate signal to
    noise ratio.
  • Not so strong that it overloads transmitter.
  • Must satisfy these for all combinations of
    sending and receiving station on bus.
  • Divide network into small segments.
  • Link segments with amplifies or repeaters.

Bus LAN Transmission Media
  • Twisted pair
  • Not practical in shared bus at higher data rates.
  • Baseband coaxial cable
  • Used by the original Ethernet and IEEE 802.3
  • Broadband coaxial cable
  • More expensive and difficult to install and
  • Included in 802.3 specification but no longer
  • Optical fiber
  • Expensive, difficulty with availability, not
    commonly used.
  • Few new installations
  • Replaced by star based twisted pair and optical

Bus LAN Baseband Coaxial Cable
  • Uses digital signaling Manchester or
    Differential Manchester encoding.
  • Entire frequency spectrum of cable used (single
    channel on cable).
  • Bi-directional transmission over few kilometer
  • Original use for Ethernet (basis for 802.3) at
  • 50-ohm cable (less reflection and noise than the
    standard CATV 75-ohm cable).
  • 10Base5 (10 Mbps, baseband, 500 m cable)
  • Ethernet and 802.3 originally used 0.4 inch
    diameter cable.
  • 10Base2 (10 Mbps, baseband, 200 m cable (185m))
  • Cheapernet using 0.25 inch cable.
  • More flexible, easier to bring to workstation,
    cheaper electronics.
  • Greater attenuation and lower noise resistance.

Bus LAN Repeaters
  • Transmits in both directions.
  • Joins two segments of cable.
  • No buffering.
  • No logical isolation of segments.
  • If two stations on different segments send at the
    same time, packets will collide.
  • Only one path of segments and repeaters between
    any two stations.

Ring LANs
  • Each repeater connects to two others via
    unidirectional transmission links to create a
    single closed path.
  • Data transferred bit by bit from one repeater to
    the next.
  • Repeater regenerates and retransmits each bit.
  • Repeater performs data insertion, data reception,
    data removal.
  • Repeater acts as attachment point.
  • Packet removed by transmitter after one trip
    round ring.
  • Permits automatic acknowledgement.
  • Permits multicast addressing.
  • MAC protocol for determining how and when packets
    are inserted.
  • Ring repeater - Pass all data that comes its
    way. - Provide access point to the
    attach stations to send and
    receive data.

Ring LANs Repeater States
  • Listen State Functions
  • Scan passing bit stream for patterns.
  • Address of attached station.
  • Token for permission to transmit.
  • Copy incoming bit and send it to attached
    station, while forwarding each bit.
  • Modify a bit as it passes by.
  • To indicate a packet has been copied

Ring LANs Repeater States
  • Transmit State Functions
  • Station has data and the repeater has permission
    to send.
  • Repeater receives bits from the station and
    retransmit them on its outgoing link.
  • May receive incoming bits during transmission
  • If ring bit length shorter than packet.
  • Pass back to station for checking (ACK).
  • May be more than one packet on ring.
  • Buffer for retransmission later.
  • Bypass State
  • Signals propagate past repeater with no delay
    (other than propagation delay).
  • Partial solution to reliability problem.
  • Improved performance by eliminating repeater
    delay for inactive station.

Ring LANs Transmission Media
  • Twisted pair.
  • Baseband coaxial.
  • Fiber optic.
  • Not broadband coaxial.
  • Would have to receive and transmit on multiple
    channels, asynchronously.

Ring LANs Timing Jitter
  • Clocking included with signal
  • e.g. differential Manchester encoding.
  • Clock recovered by each repeaters.
  • To know when to sample signal and recover bits.
  • Use the clocking for retransmitting the signal to
    the next repeater.
  • Clock recovery deviates from mid-bit transitions
  • Due to noise during transmission.
  • Imperfections in receiver circuitry.
  • Known as timing jitter.
  • Retransmission without distortion but with timing
  • Cumulative effect is that bit length varies.
  • Limits number of repeaters in a ring.

Ring LANs Solving Timing Jitter
  • Cannot be entirely overcome.
  • Two approaches are used in combination.
  • Repeater uses phase locked loop.
  • Uses feedback to minimize deviation from one bit
    to the next.
  • Use buffer at one or more repeaters.
  • Hold a certain number of bits.
  • Expand and contract to keep bit length of ring
  • Significant increase in maximum ring size.

Ring LANs Potential Problems
  • Break in any link disables the network.
  • Repeater failure disables the network.
  • Installation of new repeater to attach new
    station requires identification of two
    topologically adjacent repeaters.
  • Timing jitter.
  • Method of removing circulating packets required.
  • With backup in case of errors.
  • Mostly solved with star-ring architecture.

Ring LANs Star Ring Architecture
  • Feed all inter-repeater links to single site
    (wiring concentrator).
  • Provides central access to signal on every link.
  • Easier to find faults.
  • Can launch message into ring and see how far it
  • Faulty segment can be disconnected and repaired
  • New repeater can be added easily.
  • Bypass relay can be moved to the wiring
  • Can lead to long cable runs.
  • Can connect multiple rings using bridges