real time operating system

1. Real-time operating
system
Medium Access Control
Protocols for Broadcast
Networks
by
Dr. Vikas Goel
Dept. of IT

2. Contents
 Medium Access Protocol in CAN
 Medium Access Protocol in IEEE 802.5 Token
Ring
 Prioritized Access in IEEE 802.5 Token Ring
 Polling
 Timed Token medium Access Control Protocol

3. Medium Access Control Protocols
for Broadcast Networks
• The transmission medium of a broadcast network is a processor.
• A MAC protocol is a discipline for scheduling this type of processor.
• Scheduling the transmission medium is done distributedly by
network interface of hosts in the network.

4. Medium Access Protocol in CAN
(Controller Area Network)
• CANs are very small network.
• CANs are used to connect components of embedded controllers.
• An example is an automotive control system whose components
control the engine, brake, and other parts of an automobile.
• The end to end length of CAN must not exceed 100 meters.
• This means that within the fraction of a bit time, after a station
transmits, all stations should hear the transmission on the network.
• The output of all stations are wire-ANDed together by the bus.

5. • On the network, the bit is a logical 0 if the output of any station is 0
and logical 1 when the output of all stations is 1 during bit time.
• CAN MAC protocol is similar to the CSMA/CD (carrier sense Multiple
Access/ Collision Detection)
• A station with a packet to send, waits until it hears that the network
is idle and then commences to transmit the ID number of the
packet.
• At the same time, the station listens.
• Whenever it hears a 0 on the network while it is transmitting 1, it
interrupts its own transmission.
• Network contention is resolved in favor of the packet with the
smallest ID among all contending packets.

6. MAC in IEEE 802.5 Token Ring
• In a token ring network, packets are transmitted in one direction
along a circular transmission medium.
• A station transmits a packet by placing its packet on the output link
to the network.
• As the packet circulates around the network, the stations identified
by the destination address in the header copies the packet.
• When the packet returns to the source station, the station removes
the packet.

7. Prioritized Access in IEEE 802.5
Token Ring
• Prioritized access is made possible by using the two groups of 3 bits
each in AC field.
• Their value represents the token priority and reservation priority.
• Specially the 3 token priority bits give the priority of the token.
• A station can seize the free token only when its outgoing packet has an
equal or higher priority than the token priority.
• When a station seizes the token, it leaves the token priority unchanged
but set the reservation priority to the lowest priority of the network.
» It then marks the token busy and put the token in the
header of the packet and transmit the packet.

8. Polling
• Network contention is resolved by a polling mechanism called token
passing.
• For the purpose of polling, each packet has an 8-bit Access Control (AC)
field in its header.
• One of the bit in an AC field is called the token bit.
• By examining this bit in the current packet on the network, a station can
determine whether the network is busy.
• If the network is free the packet is polling packet.

9. • As a polling packet circulates around the ring, the stations are polled in
a round robin manner in order of physical locations on the ring.
• The polling packet is called free token or simple token.
• When a free token reaches a station that has outgoing packets waiting,
it can seize packets if it has the highest priority at that time.

10. Timed Token medium Access
Control Protocol
• When the transmission medium is bus, stations can poll each other
by circulating a polling packet / token according to a token
circulating list.
• The list gives the polling order.
• Each station polls the next station downstream according to the
circulating list by broadcasting a token that has the address of the
next station in the header.
• A station may transmit its data packets only when polled.
– When it completes its transmission, it transmit the token
to the next station downstream.

11. • The prioritized MAC method in IEEE 802.4 and FDDI standards is called
the timed token MAC method.
• the timed token MAC protocol divides all messages on the network into
two major classes:
– Synchronous class
• These messages are periodic messages and have deadlines
– Asynchronous class
• These messages are aperiodic messages
• Each station sets aside some network bandwidth for the transmission
of synchronous messages and transmits asynchronous
messages on a time available basis.

12. Thank
You