Token Ring
and
Fiber Distributed Data Interface
(FDDI)
1
IEEE 802.5 Token Ring
• Proposed in 1969 and initially referred to as a
Newhall ring.
Token ring :: a number of stations connected by
transmission links in a ring topology. Information
flows in one direction along the ring from source
to destination and back to source.
Medium access control is provided by a small frame,
the token, that circulates around the ring when all
stations are idle. Only the station possessing the
token is allowed to transmit at any given time.
2
Token Ring Operation
• When a station wishes to transmit, it must wait for
token to pass by and seize the token .
– One approach: change one bit in token which transforms it
into a “start-of-frame sequence” and appends frame for
transmission.
– Second approach: station claims token by removing it
from the ring.
• The data frame circles the ring and is removed by
the transmitting station.
• Each station interrogates passing frame. If destined
for station, it copies the frame into local buffer.
{Normally, there is a one bit delay as the frame
passes through a station.}
3
Figure 6.58Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
Token Ring Network
with star topology
4
Token Insertion Choices
• multi-token: insert token after station has
completed transmission of the last bit of the
frame.
• single-token: insert token after last bit of busy
token is received and the last bit of the frame is
transmitted.
• single-frame: insert token after the last bit of the
frame has returned to the sending station.
Performance is determined by whether more than
one frame is allowed on the ring at the same
time and the relative propagation time.
5
