Token Rings and

1

• Token Rings and
• Network Adaptors

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Token Ring (IEEE 802.5)
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Properties

• The ring is a single, shared medium, not point
  to point links in a loop.
• All nodes see all frames.
• A distributed algorithm determines when a node
  can transmit.
• Data always flows in one direction.

Basic idea A token circulates around the ring.
When a host has a frame to transmit, it seizes
the token and injects the frame on the medium.
The frame is forwarded by intermediate nodes
until arriving at the destination. The
destination puts the frame back after receiving
it, but with a special mark. The frame
circulates back to the sender.
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Physical Properties

• Question What happens if a node on the ring
  fails?

As long as a node provides power to the relay,
the relay stays open. When the node fails, the
relay closes bypassing the node.
Multi-Station Access Unit Several relays
packaged together. Multiple MSAUs can be plugged
together to make a larger network.
Typical token ring data rates go from 4 Mbps to
16 Mbps. Number of stations can be as high as
260.
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Medium Access Control (MAC)
network adaptor
Each host holds part of the token (1 bit). A
designated monitor host may hold more bits than
others.
TX
RX
data storage
As the token circulates around the ring, a host
needing to send data drains it off the ring. Each
TXed frame contains the receivers address.
Frames are not removed from the ring they flow
through the NIC and the receiver copies it into a
buffer. When the frame gets back to where it
started, the sender drains it off the ring.
Question How long should a host hold the token?
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MAC Performance

• Token holding time (THT) How long a node is
  allowed to hold the token (indirectly, how much
  data a node can transmit).
• Token rotation time (TRT) The amount of time a
  token takes to traverse the ring.
• Ring latency Time for the token to circulate
  around the whole ring when no host has data to
  send.

Early release token goes back out following
frame.
Delayed release token goes back after frame has
gone around and been removed.
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Token Ring Maintenance

• Monitor host Ensure the health of the ring.
  Periodically announces its presence when it
  fails, another host assumes the role.
• Becoming the monitor Host circulates special
  token, if it comes back, it can assume the role
  of monitor. What if more than one node wants to
  become monitor?

Question What can cause a token to disappear in
a ring?
Detecting a missing token Watch for a passing
token, count maximum rotation time (MRT).
Question What else should the monitor to do?
Detect and drain corrupted and orphaned
frames. Detect dead hosts send beacon frame to
suspected failed host, see how far it goes in the
ring.
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Network Adaptors
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Basic Features
Control and status No carrier, missed incoming
packet, interrupt enable, received packet
interrupt, transmitter packet interrupt, memory
error, no carrier, initialization done, RX on, TX
on, start, stop, initialize.
Interrupts, DMA, and PIO Why use interrupts with
a network adaptor? How does Direct Memory Access
work? How does Programmed I/O work?
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Scatter-read and Gather-write
Organization of a frame and the several protocol
headers it contains in addition to a messages
data
Scatter-read separate incoming frames are put in
separate buffers, but a single frame may be
scattered across multiple buffers. Gather-write
an outgoing frame is built piece-by-piece by each
protocol layer attaching its header/trailer.
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Device Drivers and Memory
Discussion Why do we write device drivers? What
is the relationship between hardware and device
driver? What is the relationship between
operating system and device driver?
Discussion What can you say about the
performance of the combined computer system and
networking hardware?