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
•Each node receiving frames from its up stream
neighbor and then forwarding them to its downstream
•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 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 Implementation
• Series of 150-ohm shielded
• Output port on each station
connected to input port on the
• Frame is passed to each
station in sequence
• Station function as a repeater
Token Passing – Token Ring (IEEE
• Requires that stations take turns sending
• 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
• 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
• 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
• 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
Relay used on a token ring:
(a) relay open—host active; (b) relay closed—
Multi Station Access Unit.
• Several of these relays are usually packed into a
single box, known as a multi station access unit
• A token ring actually look more like a star
• 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
Token Ring Maintenance
• Lost tokens - timer is issued each time a frame or token
• 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
• 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
• Higher priority stations may remove a
lower priority reservation; if stations have
equal priority, it’s first-come, first-served
• Once a station has the token, it is allowed to send one or
more packets—exactly how many more depends on
• 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