A Token Ring Network is a local area network in which the stations are connected
in series and the first is linked to the last.
The data on a Token Ring Network is transmitted unidirectional
in a serial fashion, bit-by-bit, in variable-length frames from
station to station round the ring. Each station regenerates and repeats
the data that is being transmitted. The addressed destination station(s) copies the
information as it passes.
A station gains the right
to transmit its information onte the medium when it detects a token passing on the medium.
The token is a control signal comprised of a unique signaling sequence that circulates on
the medium following each information transfer. Finally, the station that transmitted the information
strips the information from the ring and generates a new free token. If the recipient of the token
has nothing to send, it passes the token directly to its successor.
Token Ring has facilities for assigning priorities to frames in the ring.
In order for a station to transmit a frame, the received token must have a
priority less than or equal to that of the data. If the token priority is greater than the
priority of the data, or a frame is repeated on the ring before a token is received, the
station requests a token of appropriate priority by setting the R reservation bits in the AC
field of the frame or token.
The Token Ring protocol is based on the presence of a monitor. Every ring has exactly
one station which takes on the functions of an active monitor. Normally, the token procedure
operates without further intervention. However, when tokens are lost, missing or duplicated,
intervention by a monitor is required.
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By using the applet TokenRingTrainer a student can test, train and enlarge
his knowledge on Token Ring Networks solving theoretical and computation exercises.
When starting the applet the student is asked to login. If he does not enter
his username he will not be able to submit his results to the server.
He can start a new exercise using the button "New Exercise". Whenever the user
solves an exercise correctly the difficulty level will rise. The information needed
to solve the exercise is shown in the sections "Exercise" and "Settings".
For a theoretical exercise the user has to enter his solution into the list below
the graphical animation. He has to describe every event using the two to four checkboxes
beneath the list and will add it to the list using the "Add"-Button. To change the order
of the events or to delete a single event already entered into the list the student can use
"move up", "move down" and "delete" buttons which are on the right side of the list.
By pressing the "test" button the entered events are checked for validity. Now no more changes
can be done to the list. If the user has logged in, he can transmit his results to the server
using the "send" button. For better understanding of the solution the user may run the animation
using the "start", "pause", "continue" and "stop" buttons which are located between the graphical
animation and the list.
Generally all events that occur have to be added into the list. If difficulty level is above 5
there is no neede for the user to enter the event of repeat a frame/token.
The solution of the computation exercises has to be entered into the text field which is located
on the lower left side. The expected unit of the solution value is written next to the text field.
Similar to the theoretical exercise the user can use the "test" button to check the his solution and
can transmit it with the "send" button to the server. By pressing the "show solution" button
the correct solution is entered into the text field. In exercises where the user has to compute the time
he can use the animation to show the events with every time stamp in which thay occur.
In the status field the user can see the topic, the current difficulty level, the time between pressing
the "new exercise" button und the "test" button also the valuation of the user's skills so far.
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Example
Three stations A, B, C. Token is by A, A has one PDU addressed to B to transmit.
Enter the occuring steps beginning with the arrivial of the token at the station A
until all stations have sent all their data and a new free token is generated.
Solution
A transmit its own frame
B copies and repeats the frame
C repeats the frame
A receives its own frame
A generates a new token
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Example
Three stations A, B, C. Token is by A, A has one PDU addressed to C to transmit.
The PDU will not be striped by station from the ring.
This is the error frame. The retransmission of the PDU is succesfull. B is the active monitor.
Enter the occuring steps beginning with the arrivial of the token at the station A
until all stations have sent all their data and a new free token is generated.
Solution
A transmits its own frame
B sets the monitor bit
C copies and repeates the frame
B receives a frame with M = 1
B transmits a ring purge frame
B strips the ring purge frame from the ring
B generates a new token
A transmits its own frame
B sets the monitor bit
A receives its own frame
A generates a new token
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Example
Three stations A, B, C. Token is by A, A has one PDU with P=2 to C to transmit.
B has one PDU with P=1 to C to transmit. C is the active monitor.
Enter the occuring steps beginning with the arrivial of the token at the station A
until all stations have sent all their data and a new free token is generated.
Solution
A transmits its own frame with P = 0 and R = 0
B sets the reservation bits R = 1
C sets the monitor bit
C copies and repeates the frame
A receives its own frame with P = 0 and R = 1
A becomes a stacking station
A generates a new priotoken with P = 1 and R = 0
B transmits its own frame with P = 1 and R = 0
C sets the monitor bit
C copies and repeates the frame
B receives its own frame with P = 1 and R = 0
B generates a new priotoken with P = 1 and R = 0
C sets the monitor bit
A receives a priotoken with P = 1 and R = 0
A discontinues its role as a stacking station
A generates a new token
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Frame Size [Byte] = THT [s] * Data Signaling Rate[Bit/s] / 8
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| Network Total Size [m] | |
| Ring Capacity [Bit] = Staition Count + | ------------------------------------------------------------ |
| Signaling Speed [m/s] / Data Signaling Rate [Bit/s] |
Example
Three stations A, B, C. Token is by A. A transmits one PDU and generates a new token.
The 1-Bit-Delay is to be ignored.
Distance between stations 1100 m, Data Signaling Rate 16 MBit/s,
Signaling Speed 2*108 m/s, The frame size is equivalent to 10 ms
and a frame contains one PDU.
How long does it take untill A has sent its data and
a new free token is generated?
Solution
| Time [msec] = | 10 ms | //Frame Transmission Time |
| + 3*(1100m / 2*108m/s)*1000ms/s | //Time for 3*1100m | |
| + 3*1Bit-Delay | //can be ignored | |
| + (24Bit / 16*106Bit/s)*1000ms/s | //Token Transmission Time | |
| = 10,018 ms |