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Token ring ppt

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					                 Ring Network

class of shared-media network.
•      Token Rings (802.5)
•      FDDI (Fiber Distributed Data Interface)
               a newer, faster type of token ring
•      Resilient Packet Ring or 802.17
               is nearing completion
                  token ring network
•Consists of a set of nodes connected
  in a ring .

•Data always flows in a particular direction around the
ring

•Each node receiving frames from its up stream
neighbor and then forwarding them to its downstream
neighbor

•The ring is viewed as a single shared medium

•It does not behave as a collection of independent
point-to-point links that just happen to be configured in
a loop
                    key features
A token ring shares two key features with an Ethernet


1.   It involves a distributed algorithm that controls when
     each node is allowed to transmit.
2.   All nodes see all frames, with the node identified in the
     frame header as the destination saving a copy of the
     frame as it flows past.
             Token ring (802.5)
• Early form of ring network.
• The idea is that a token, which is really just a special
  sequence of bits, circulates around the ring; each node
  receives and then forwards the token.
• When a node that has a frame to transmit sees the
  token, it takes the token off the ring (i.e., it does not
  forward the special bit pattern) and instead inserts its
  frame into the ring.
• The token circulates around the ring, each node gets a
  chance to transmit.
• Nodes are serviced in a round-robin fashion.
Token Ring
    Token Ring Implementation

• Series of 150-ohm shielded
  twisted-pairs sections
• Output port on each station
  connected to input port on the
  next
• Frame is passed to each
  station in sequence
• Station function as a repeater
Token Passing – Token Ring (IEEE
                  802.5)
• Requires that stations take turns sending
  data
• Token passing coordinates process
• Token is a specially formatted three-byte
  frame that circulates; station wishing to
  transmit must first have possession
• Token passes from NIC to NIC in
  sequence; if station has data to send,
  station takes token and sends data frame;
  if not, passes to neighbor
         Token Ring (cont)
• Each station receives the frame one by
  one and examines the destination address
• If it matches, frame is copied; station
  checks the frame for errors; changes bits
  to indicate the frame was received and
  copied
• Packet continues around the ring and is
  passed back to originating station
          Token Ring (cont)
• Once the sender receives the frame and
  recognizes its address in the sender field,
  it examines the address-recognized bits
• If they are set, it knows the frame was
  received and copied
• Sender then discards the frame and
  releases the token back to the ring
          Physical Properties
• A ring topology is that any link or node failure
  would render the whole network useless.
• This problem is addressed by connecting each
  station into the ring using an electromechanical
  relay.
• As long as the station is healthy, the relay is
  open and the station is included in the ring.
• If the station stops providing power, the relay
  closes and the ring automatically bypasses the
  station.
          Relay used on a token ring:
(a) relay open—host active; (b) relay closed—
                host bypassed.
     Multi Station Access Unit.
• Several of these relays are usually packed into a
  single box, known as a multi station access unit
  (MSAU).
• A token ring actually look more like a star
  topology
• It also makes it very easy to add stations to and
  remove stations from the network,
• since they can just be plugged into or unplugged
  from the nearest MSAU, while the overall wiring
  of the network can be left unchanged.
Multi Station Access Unit
Token Ring Media Access Control
     The network adaptor for a token ring contains a
 receiver, a transmitter, and one or more bits of data
 storage between them.
      When none of the stations connected
to the ring has anything to send, the token circulates
 around the ring.
    The ring has to have enough ―storage capacity‖ to
 hold an entire token.

 (designated station, called the monitor,
add some additional bits of delay to the
ring)
      Token Ring Maintenance
• Lost tokens - timer is issued each time a frame or token
  is generated
• If no frame is received within time period, new token is
  generated by a monitor station
• Orphan frames result if a sending station neglects to
  remove a used data frame from the ring
• Monitor sets a bit in the AC field in each frame; as frame
  passes, bit is set; if the frame passes again, the monitor
  discards, will remove it, and generate a new token
• the detection of dead stations
      Priority and Reservation
• Higher priority stations may access the
  token sooner,
• Every station has a priority code
• As token passes by, station waiting to
  transmit can place its priority code in the
  access control (AC) field of the token or
  data frame
• Higher priority stations may remove a
  lower priority reservation; if stations have
  equal priority, it’s first-come, first-served
                         Issues
• Once a station has the token, it is allowed to send one or
  more packets—exactly how many more depends on
  some factors.
• how much data a given node is allowed to transmit each
  time it possesses the token, or said another way, how
  long a given node is allowed to hold the token. this the
  token holding time (THT).
• no lower-priority packets get sent when higher-priority
  packets are waiting-lower-priority packets to be locked
  out of the ring for extended periods if there is a sufficient
  supply of high-priority packets.
• when the sending node releases the token.
Token Ring Frame

				
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