Lecture 9: Ethernet and Token Ring Networks - PowerPoint PPT Presentation

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Lecture 9: Ethernet and Token Ring Networks

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Title: Lecture 6: Framing Approaches (cont) Author: John C. Wolfe Last modified by: John C. Wolfe Created Date: 9/12/2000 9:55:36 PM Document presentation format – PowerPoint PPT presentation

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Title: Lecture 9: Ethernet and Token Ring Networks


1
Lecture 9 Ethernet and Token Ring Networks
2
Ethernet review
Network topology with two stars connected by a
point-to-point link
3
Ethernet Frame
MAX1500 bytes Min46 bytes Typedemux key to
identify application CRC-32
4
Addresses
  • Media Access Control (MAC) address
  • 48 bits/6 bytes-X1X2X3X4X5X6X7X8X9X10X11X12
  • Unique worldwide for all time
  • Belongs to adapter
  • Broadcast address
  • FFFFFFFFFFFF
  • Multicast address
  • 1xxxxxxxXXXXXXXXXX

5
Receiver Algorithm
  • Adaptor
  • Receives all frames on the ethernet
  • Only Accepts Frames Addressed to
  • Its unicast address
  • the broadcast address
  • A mult-cast address it has been programmed to
    accept
  • Accepts all frames in promiscuous
    (eavesdropping) mode

6
Transmitter Algorithm
  • If adapter has a frame to send
  • Line is idle-sends its frame-up to 1500 bytes
  • Line is busy-wait til line goes idle and send the
    frame
  • Collisions
  • These are detected as overvoltage AT AN ADAPTER
  • When collision is detected, receiver transmits 32
    bit jamming sequence and shuts down-purpose of
    jamming sequence is to cause a collision at the
    sender
  • Frame must be long enough for this collision to
    occur512 bits in classic ethernet

7
The ethernet advantage
  • Its simpleno routing tables to maintain
  • Its flexiblejust plug in your PC
  • Its cheap
  • Cable 0.12/ft
  • Adaptors 20
  • Hubs-- 30/port
  • Switch-- 50/port

8
Token Ring
  • Shared medium similar to ethernet
  • Distributed algorithm controls transmission
  • All nodes see all frames

Tokensmall (24 bit) circulating frame To send
Remove token and send frame Sender removes
message when it comes around and reinserts token
9
Multi-station access unit (MSAU) create a
star-like topology
10
Token Ring Media Access Control MAC
  • Network adaptor contains-receiver, transmitter,
    buffer gt1 bit
  • Need enough storage within the ring to store a 24
    bit token
  • If you have less than 24 stationsone has to
    provide the storagethis will be the monitor

11
MAC cont.
  • To seize the ring-change 1 bit in the second
    byte-becomes preamble to the data packet.
  • A host copies those packets addressed to it and
    retransmits it around the ring
  • Sender removes the packet as it comes aroundmay
    still be sending information

12
Key Concept
  • Token Holding Time (THT)gttransmission time of a
    packet
  • Token Rotation Time (TRT)lt ActiveNodes x
    THTRingLatency Note TRT is load dependent

13
Early release versus delayed release
THTFrame transmission time Ring Latency
THTFrame transmission time
14
Reliable Transmission
  • A-bit set to 1 by destination host when
    message is detected
  • C-bit set to 1 if successfully copied

15
Priority
  • Priority (3-bits)8 levels
  • Token priority, packet priority
  • To seize the ring Packet priority gt Token
    priority
  • Reservations Use reservation bits (3-bits) on a
    circulating packet to lock out low priority
    users.
  • Increase rbs to your packet priority.
  • Current token holder sets token priority to the
    reservation
  • All users with packet priority gt reservation may
    seize the ring.
  • New user must reset token priority to the
    original value

16
Token Ring Maintenance
  • Monitor roles
  • Ensure adequate delay
  • Check for token circulation Max TRTNumber of
    usersTHTRingLatency
  • Removes orphaned and corrupted frames
  • Detects dead stations-beacon frame

17
Death of a Monitor
  • Monitor sends control message periodically
  • Hosts watch for this signal-if timeout, they try
    to become the monitor by sending a claim token
  • If claim token returns-sender is the monitor
  • If another claim token returns first-arbitration
    determines the new monitor

18
Token ring frame
  • Manchester encoding
  • Illegal Manchester codes are used as delimiters
  • Frame status bits-A C
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