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Packet Transmission (Part II)


Packet Transmission (Part II) Local Area Networks (LANs) – PowerPoint PPT presentation

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Title: Packet Transmission (Part II)

Packet Transmission (Part II)
  • Local Area Networks
  • (LANs)

Classification Terminology
  • Network technologies classified into three broad
  • Local Area Network (LAN)
  • Metropolitan Area Network (MAN)
  • Wide Area Network (WAN)
  • LAN and WAN most widely deployed


Local Area Network (LAN)
  • Interconnect a wide range of devices over short
    distances, e.g., within the same floor, building
    or campus (typically up to 10 km diameter).
  • Extremely popular
  • High throughput, low delay
  • Many LAN technologies exist

Local Area Network (contd)
  • Various LAN standards have been developed with
    Ethernet and FDDI are the most popular ones
  • LAN standards are collectively known as the IEEE
    802 standards
  • All hosts on a LAN share a common medium. Thus
    they operate on a broadcast mode
  • No intermediate (routing/switching) system
    required on a single LAN

Local Area Network (contd)
  • LAN parameters
  • topology
  • shared medium (twisted pair, coaxial, fiber)
  • medium access control technique
  • Medium Access Control (MAC)
  • govern the access to the LAN transmission medium.
  • provide the basis for the IEEE LAN specifications

  • Mathematical term
  • Roughly interpreted as geometry for cured

Network Topology
  • Specifies general shape of a network
  • Handful of broad categories
  • Often applied to LAN
  • Primarily refers to interconnections
  • Hides details of actual devices

Star Topology
  • Central component of network known as hub
  • Hub repeats incoming signal to all outgoing
  • Each computer has separate connection to hub

Ring Topology
  • No central facility
  • Connections go directly from one computer to
  • In practice, there is a short connector cable
    from the computer to the ring

Bus Topology
  • Single cable connects all computers
  • Each computer has connector to shared cable
  • Computers must synchronize and allow only one
    computer to transmit at a time

Bus vs. Ring Topologies
  • Bus configurations require taps passive
    interfaces that allow stations to listen to the
    cable to transmit data on the bus
  • Two stations may try to transmit simultaneously
  • Ring LANs use repeaters to pass on the received
    data (copy bit-by-bit). Links are unidirectional.
  • A ring-LAN can be physically configured as a star
    where the hub is replaced by a wire-center
  • The hub in a star-LAN can become a bottleneck.
    Redundancy is required should it fail.

LAN Protocol Layering
IEEE 802. Protocol Layers
  • Control hosts access to the shared medium
  • Frames are broadcast over the LAN but only the
    destination copies them
  • Each station gets a fair amount of access
  • Collisions may occur when two or more LAN
    stations try to transmit simultaneously
  • Control can be exercised in a centralized (by a
    controller) or distributed (dynamically) fashion

MAC Frame Format
  • MAC control protocol information (e.g., priority
  • Destination MAC address physical address of the
    LAN destination
  • Source MAC address physical address of the
    source LAN host
  • LLC PDU LLC data
  • CRC error-control field

MAC Protocols
  • The ALOHA protocol (Univ. of Hawaii, 1970s)
  • Slotted ALOHA (1970s)
  • CSMA (mid 70s)
  • Collision-free protocols
  • Wireless protocols
  • CDMA
  • GSM, etc.

The ALOHA Protocol
  • Multiple users share a single broadcast channel
  • Users do not test the channel before they
    transmit. They transmit fixed-size frames at
    arbitrary times
  • If a collision occurs (i.e., by overlapping
    transmissions), the colliding frames get garbed
    and thus discarded
  • Retransmission by upper-layer protocols
  • Maximum throughput is 18.4

Slotted ALOHA
  • Pure Aloha performance can be substantially
    improved if we require frame transmissions to
    occur on time slot boundaries
  • Time slots are the same as the frame size
  • Assume that all stations are globally
  • The frame will be scheduled for transmission at
    the start of the next time slot
  • User sends the frame without testing the state of
    the channel
  • If a collision occurs, the colliding frames
  • Retransmission by upper-layer protocols
  • Maximum throughput is 37

CSMA/CD Paradigm
  • Multiple Access (MA)
  • Multiple computers attach to shared media
  • Each uses same access algorithm
  • Carrier Sense (CS)
  • Wait until medium idle
  • Begin to transmit frame
  • Simultaneous transmission possible

CSMA/CD Paradigm (contd)
  • Two simultaneous transmissions
  • Interfere with one another
  • Called collision
  • CSMA plus Collision Detection (CD)
  • Listen to medium during transmission
  • Detect whether another stations signal
  • Back off from interference and try again

The CSMA Protocols
  • 1-persistent CSMA
  • listens to the channel prior to its transmission.
  • If it is idle then it sends its frame.
  • If a collision occurs the station waits for a
    random time and tries again.
  • It it is busy it waits for the ongoing
    transmission to finish
  • Non-persistent CSMA
  • Waits for a random period before it sends the
    frame if the channel is in use
  • P-persistent CSMA
  • Transmits with probability p when the channel is
  • Obviously improves over ALOHA protocols

IEEE 802.x LAN Standards
  • 10 Mbps CSMA/CD (802.3)
  • 100 Mbps CSMA/CD (802.3u)
  • 1000 Mbps CSMA/CD (802.3z)
  • Token Bus (802.4)
  • Token Ring (802.5)
  • And many more

Example Bus Network Ethernet
  • Most popular LAN
  • Widely used
  • IEEE standard 802.3
  • Several generalizations
  • Same frame format
  • Different data rates
  • Different wiring schemes

Illustration Of Ethernet Transmission
  • Only one station transmits at any time
  • Signal propagates across entire cable
  • All stations receive transmission
  • CSMA/CD 1-persistent media access scheme

Manchester Encoding
  • Uses rising and falling edges to encode data
  • Falling edge to encode 0, rising edge to encode 1
  • Use preamble for synchronization
  • Preamble consists of 64 alternating 1s and 0s

Ethernet Transmission
  • If channel is idle, send frame immediately
  • If channel is busy, wait until it is free and
    then transmit (1-persistent)
  • If collision occurs, using binary backoff

Carrier-Sensing Effects
  • LAN end-to-end propagation delay

Binary Backoff
  • When collision occurs
  • Wait random time
  • Use CSMA and try again
  • If second collision occurs
  • Wait random time
  • Double range for each successive collision
  • Called exponential backoff

  • Local Area Networks
  • Designed for short distance
  • Use shared media
  • Many technologies exist
  • Topology refers to general shape
  • Bus
  • Ring
  • Star

Summary (contd)
  • MAC protocols
  • ALOHA and Slotted ALOHA
  • CSMA protocols
  • Ethernet
  • CSMA/CD 1-persistent
  • Manchester encoding
  • Binary backoff