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Chapter 6: Local Area Networks


MAC protocols classified based on. Centralized or Distributed Access Control ... Priority Arbitrated MAC. Nodes Arbitrate for Network Access Based on Local Priority ... – PowerPoint PPT presentation

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Title: Chapter 6: Local Area Networks

  • Chapter 6 Local Area Networks
  • Key Learning Points
  • Layered Protocols and Plus
  • IEEE 802
  • Topologies
  • MAC
  • Design Considerations

Protocol Layers and PDU Encapsulation
  • physical interface ? buffers
  • service access points ? function calls

  • Local Area Networks (LAN)
  • A Computer Network of Limited Geographic Span -
    No Routing
  • 2 classes guided media wireless LANs
  • private data networks installed maintained by
  • shorter distances limited operations ? higher
    data rates
  • ISO OSI model for private public networks
  • differs in network dependent layers
  • higher layers normally same

  • LANs relatively simple topologies, high data
    rates (Gbits/second)
  • isolated LANs need only Data Link Layer
    (embedded LANs)
  • generally longer frames, more sophisticated
    addressing thanHDLC
  • includes media access control (MAC)
  • bridges inter-connect private LANs ? must
    translate different
  • frame formats
  • - use MAC address (layer 2 address)
  • gateways/routers interconnect to different
    networks, addressing is
  • more sophisticated
  • - use IP address (layer 3 address)
  • virtually all wired LANs are baseband use a
    variant of TDMA
  • for channel access control

IEEE 802 LAN Model Widely Used Local Area
Network Model IEEE 802.1 Bridging and
Management IEEE 802.2 Logical Link Control IEEE
802.3 CSMA-CA (aka Ethernet) IEEE 802.5 Priority
Token Ring IEEE 802.11a,b,g Wireless LAN
IEEE 802 LAN Architecture
  • Structured Wiring
  • TP typically used to connect DTEs to HUB or
    wiring closet
  • Coax link floor wiring closets to main building
  • Fiber Link Building Hubs to main central hub

  • LAN Topologies
  • node interconnection
  • number of nodes
  • geographic span
  • (1) Ring Topology
  • (2) Star Topology
  • (3) Bus (Tree) Topology

Ring Topology
  • Nodes Interfaced to Media with Digital
  • Repeaters (R) Connected in Unidirectional
    Point-Point Ring
  • Ring Protocols
  • FDDI
  • IEEE 802.5
  • SAE AS 4075

  • Star Coupler -
    Switched Topologies
  • Intelligent Switch Controls Channels Between
  • PDU Transmitted Directly from Source to
  • Switching Protocols
  • Giga-Bit Ethernet
  • Fiber Channel
  • ATM

Star-Coupler Examples
  • private automatic branch exchange (PABX)
  • all external paths designed for limited BW
    analog voice
  • modems required for data transmission
  • most use digital switching techniques w/i
  • connection oriented
  • private digital exchanges (PDX)
  • cheap ICs used to perform A/D D/A conversions
  • common practice ?extend 64kbps digital
    transmission channel
  • to subscriber
  • provides switched communication path for
    integrated voice
  • data
  • often used in a local community for voice,
    email, store-forward,
  • teleconferencing

Bus/Tree Topology
  • common media used for data transmission
  • nodes connected to NIC that includes MAC
  • physical connection tap into media
  • can be extended into tree by using repeaters,
  • Bus Protocols Mil-std 1553b, CAN, 10 base2


  • Hubs
  • Broadcasts PDU to all connected Nodes
  • - topology similar to star
  • - practically works like bus - collisions
  • Hub Protocols
  • Giga-Bit Ethernet
  • 100 Base X Ethernet
  • 10 Base T Ethernet

Connectionless Logical Link Control Operation
SEND SIDE - receive application data - frame
data - insert addresses - wait for media access
  • MAC Sub Layer Operations
  • Assemble MAC Frame
  • Initiate Media Access Protocol
  • Transmit Frame when Access is Gained
  • preamble synchronize bits, bytes, frames
  • SOF start of frame
  • destination address link destination layer
  • source address link layer source address
  • length payload length
  • padding added data for collision detection
  • frame check sequence CRC remainder

  • MAC Protocols Control Access to the Media
  • baseband trasnsmisison implies TDMA access
  • only 1 transmitter on each physical segment at a
  • MAC protocols classified based on
  • Centralized or Distributed Access Control
  • Type of Access Control Algorithms
  • - Priority Based
  • - Time-Sliced
  • - Random
  • Mechanisms used to Gain Access
  • - Carrier Sense Multiple Access (CSMA)
  • - tokens
  • - bus master

  • Centralized Arbitration (Master-Slave) A Single
    Node Controls
  • All Data Flow
  • Predictable Traffic Flow
  • Single Point of Failure,
  • Tightly Coupled Network
  • Distributed Arbitration nodes are equals
    (peers),control is distributed
  • More difficult to predict traffic flow
  • Network Nodes are Decoupled
  • e.g. Timed or Prioritized Tokens
  • Carrier Sense Multiple Access Collision
    Detect (CSMA-CD)
  • Carrier Sense Multiple Access Collision
    Avoidance (CSMA-CA)

  • Time Slicing network capacity allocated in
    time slices
  • each node is assigned a time slice during which
    it can transmit
  • nodes surrender control of the network when time
    slice expires
  • or transmission is completed
  • optimizes on throughput minimal contention
  • individual message latency increased
  • Priority Driven Protocol (PDP) nodes contend for
    network access based on priorities
  • priority arbitration occurs before each frame is
  • highest priority frame always transmitted next
  • optimizes on low latency
  • long contention delays can impact throughput

  • Random nodes transmit randomly ? simultaneous
    transmission results in signal collision - lost
  • worst case latency is unpredictable
  • effective throughput drops significantly as
    loading exceeds 30
  • optimizes on simplicity

Time Sliced Protocols
  • Nodes are assigned a Time Slice During Which It
  • Time Slice Expires or Node Finishes ? Control is
  • Throughput Optimal No Contention Resolution
  • Latency Can Be High Priority Inversion
  • Requires Precise Timers On Each Node
  • Examples of Time Sliced Protocols
  • Time Triggered Protocol
  • Fiber Data Distributed Interface
  • Fiber Channel Arbitrated Loop

Priority Arbitrated MAC
  • Nodes Arbitrate for Network Access Based on
    Local Priority
  • Pending Frame with Highest Priority Always
    Transmits Next
  • Latency Optimal Minimum Priority Inversion
  • Throughput Penalty- Arbitration Cycle Before
    Each Frame
  • Transmission
  • Priority Arbitrated MAC Protocols
  • IEEE 802.5
  • SAE AS 4075

  • PDP Example CSMA CA - Controller Area Network
  • Priority Encoded in CAN Identifier frames 1st
    29 (or 11) bits
  • Pending Frame with Highest Priority will
    Transmit next
  • Latency Optimal Minimum Priority Inversion
  • Throughput Penalty- negligable (specified at 40m
    for 1Mbps)
  • Protocol Restriction nodes must simultaneously
    receive signal representing a bit
  • bit length bit time ? signal speed
  • bit length (3 ?108 m/s )( 1 ?10-6 s/bit)
  • ? theoretical maximum network span lt 200m
  • - ok for many embedded systems
  • - relaxed for slower data rates 100k bps ? 2000m
    max span

CAN Protocol Arbitration
bus state hi low
numeric value
Random Access Example CSMA-CD
  • Each Node Senses Transmission Media
  • - If Traffic Is Present ? Wait Until Network Is
  • - If No Traffic Detected ?Transmit a Frame
  • Collision Occurs when nodes initiate
    transmission during idle time
  • - often two modes waiting on another to finish
  • Each Node Monitors Transmitted Signal -
    Collision Detected As
  • Noise Burst

  • Nodes that Detect Collision Broadcast Jam
    Sequence to Reinforce
  • Collision
  • Nodes involved in Collision Pick Random Number
    x ? Set Timer
  • - Nodes Retransmit When Timer Expires
  • - time delay increases exponentially with each
    successive collision
  • to reduce likelihood of another collision
  • e.g. 23 8, 24 16

  • Ethernet Hub Bus Topologies have Collision
  • frames can collide with simultaneous media
    access attempt
  • transmitting nodes must hear collision while
    still transmitting
  • requires Minimum Frame Length 2 ?frame
    propagation time

Minimum frame length for 10 base T 512 bits _at_
2500 m - 10 Mbps - minimum frame length 512
bits ? 5000m - signal can travel 2500m, collide
and reflect back 2500m and transmitter will
still be transmitting - if a frame is lt 512 bits,
Ethernet adds extra bits Minimum Frame Size for
Gbit Enet 4096 bits - smaller frames are padded
Collisions can cause throughput to collapse at
loading gt 30 - collisions and retransmissions
cause more collisions - retransmissions are
exponentially delayed - difficult to provide
QoS, impossible to guarantee time constraints
Use of Full Duplex Switches in 100Mbps and 1Gbps
Ethernet removes collision domain - each node has
a dedicated forward and reverse path to switch -
minimum frame length still required to support
hub topologies
  • Ethernet classes
  • Older Variants
  • 10 base 2 bus topology - coax with max segment
    length 200m
  • 10 base 5 bus topology - coax with max segment
    length 500m
  • 10 base T/F Hub Topologies with TP or Fiber
  • Currently widely used Ethernet classes
  • 100 Mbps hub or switch topology 100 Mbps
  • - 100 base 4T 100 Mbps with four Cat 3 UTP
  • 3 pairs for signalling at 33.3 Mbps, 4th
    for collision detection
  • - 100 base X 100 Mbps with Cat 5 UTP
  • - 100 base F 100 Mbps with Optical Fiber
  • 1000 base T/F 1 Gbps hub or swtiched topology
    with fiber or TP

  • (4) Tokens Special control frame used to control
    access to media
  • Token Ring Topology uses Repeaters
  • Token Bus Topology
  • token (control frame) is transmitted - Node
    captures a token to
  • gain control of the media and transmit data

Priority Driven Token (IEEE 802.5)
  • (i) token reservation stage
  • a free token circulates over the network
  • any node wishing to transmit can reserve the
  • - token reservation encoding its priority in
    reservation field
  • - node can reserve token if its priority gt
    current reserved priority
  • (ii) token capture stage
  • free token circulates a second time with
    reservation field set
  • a node with priority ? reserved priority can
    capture token
  • - token is captured by setting the capture field
  • data frame is immediately sent followed by a
    free token

  • Time Sliced Token (FDDI)
  • each node request a time slice during
  • target token rotation time (TTRT) established
    based on all requests
  • each node can transmit for its time slice once
    it receives a free token
  • after transmitting, a node issues another free
  • control is passed voluntarily in a round-robin
  • (4) Master-Slave Mil Std 1553
  • used in many military/aerospace applications
  • Master Polls Slave
  • If slave wishes to transmit data is must wait
    until polled by master
  • Throughput inefficient 2 messages transmitted
    for 1 payload

Considerations for LAN Design
  • network demand
  • network capacity
  • support requirements
  • availability of technical support and components
  • planning and implementation
  • network modelling

  • network demand is determined from application
  • and support needs and environment
  • information systems client-server, peer-peer
  • real-time system
  • Quality of Service (QoS) specification,
  • reliable delivery- number of retransmits for
    lost/corrupt frames ?
  • time sensitivity of frames tardy delivery
  • importance of frames any critical data
  • throughput needs frame size inter-arrival
  • number of users and number of nodes
  • coverage area geographic span required/desired

  • network capacity is determined from
    characteristics and parameters of protocols
  • geographic span
  • - signal propagation delay
  • - inherent limitations
  • bit rate supported
  • media arbitration protocol (PDP, Time Sliced,
  • maximum frame size, overhead per frame

  • support requirements
  • scalability can the network scale to
    different/changing needs?
  • network reliability - what types of fault
    tolerance does the
  • network provide?
  • - transient faults temporaty fluctuations in
    effective throughput - - permanent faults media,
    hardware components
  • operational environment bit error rate,
    temperature, shock,
  • vibration, EMI, timing jitter
  • availability of technical support and components
  • (aka obsolescence management)
  • how will components be procured in the future
  • what is the upgrade path
  • who will provide technical support in out-years

Planning And Implementation documentation check
lists, sign-off sheets, specification, change
management project plan tasks, budget ,
schedule, deliverables, risk, plan B! performance
requirements policies how network is to be used
e.g. personnel access, procedures instruction on
how to perform specific actions adding
users vendor/support staff selection test plan
and test execution initial tests, intermediate
tests, acceptance tests
Schedules will slip and costs will grow plan
for it!
  • Network Modeling is important to understand
    network behavior
  • Models are Abstractions That Help Designers
  • Behavioral and Functional Characteristics
  • Common Network Issues that are Studied Using
  • - Throughput Requirements
  • - Robustness
  • - Scalability
  • - Transmission Media Performance (Bit Error Rate)
  • - Message Transmission Times
  • Types of Models
  • - Analytical Models (Data Flow Diagrams,
    Mathematical Models)
  • - Simulation Models

  • Analytical
  • Provide Detailed Data To Evaluate Some Specific
  • Usually for Some Assumed Conditions
  • Data Flow Diagram of a CAN Network Interface Card

Mathematical Modeling Demand vs
Capacity An Analytical Approach for Modeling
Throughput Requirements Demand Arises from the
Message Set Capacity is Inherent to a
Network Protocol
  • Message Inter-arrival Rates
  • Length of Messages
  • Temporal Parameters time sensitivity deadlines
  • Data Rate
  • Geographic Span
  • Maximum Frame Size, Overhead
  • MAC and Local Scheduling

  • Simulation
    Modeling Tools
  • Provide Extensive Analysis of Network Behavior
  • Statistical Techniques
  • Dynamic Conditions