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