Collision Free Protocol
Computer Engineering Department
• Pure ALOHA, slotted ALOHA, CSMA and
CSMA/CD are contention based protocols
– try. If collide, retry.
– No guarantee of performance.
– What happens if the network load is high?
• Collision free protocols:
– pay constant overhead to achieve performance
– Good when network load is high
Reservation Polling Against Corruption
Collision Free World
• Provides in order access to shared medium so
that every station has chance to transfer (fair
• Eliminates collision completely
• Three methods for controlled access:
– Token Passing
Reservation – Bit Map
• Stations take turns transmitting a single
frame at a full rate ( R ) bps
• Each cycle begins with a reservation interval
that consists of (N) mini-slots. One mini-slot
for each of the N stations
Bit Map - Reservation Based
• When a station needs to send a data frame, it
makes a reservation in its own mini-slot.
• By listening to the reservation interval, every
station knows which stations will transfer
frames, and in which order.
• The stations that made reservations can send
their data frames after the reservation frame.
• Stations take turns accessing the medium
• One device is assigned as primary station and the
others as secondary stations
• All data exchanges are done through the primary
• When the primary has a frame to send it sends a
select frame that includes the address of the
• When the primary is ready to receive data it send a
Poll frame for each device to ask if it has data to send
or not. If yes, data will be transmitted otherwise NAK
• Polling can be done in order (Round-Robin) or based
on predetermined order
• Each station sends the address bits in some
order (from highest order bit to the lowest
• The bits in each position from different
stations are ORed.
Example – Binary Countdown
Limited Contention Protocols
• Collision based protocols (ALOHA,CSMA/CD)
are good when the network load is low.
• Collision free protocols (bit map, binary
Countdown) are good when load is high.
• How about combining their advantages --
limited contention protocols.
– Behave like the ALOHA scheme under light load
– Behave like the bitmap scheme under heavy load.
Adaptive Tree Walk Protocol
• Trick: partition the group of station and limit the
contention for each slot.
• Under light load, every one can try for each slot
• Under heavy load, only a small group can try for
• how do we do it
– treat stations as the leaf of a binary tree.
– first slot (after successful transmission), all stations
(under the root node) can try to get the slot.
– if no conflict, fine.
– if conflict, only nodes under a subtree get to try for
the next one. (depth first search)
3 4 5 6
A B C* D E* F* G H*
Slot 0: C*, E*, F*, H* (all nodes under node 0 can try), conflict
slot 1: C* (all nodes under node 1 can try), C sends
slot 2: E*, F*, H*(all nodes under node 2 can try), conflict
slot 3: E*, F* (all nodes under node 5 can try), conflict
slot 4: E* (all nodes under E can try), E sends
slot 5: F* (all nodes under F can try), F sends
slot 6: H* (all nodes under node 6 can try), H sends.
Review - 1
• Sixteen stations, numbered 1 through 16, are
contending for the use of a shared channel
• By using the adaptive tree walk protocol. If all
the stations whose addresses are prime
numbers suddenly become ready at once,
• how many bit slots are needed to resolve the
• Stations 2, 3, 5, 7, 11, and 13 want to send. Eleven slots
are needed, with the
– contents of each slot being as follows:
– slot 1: 2, 3, 5, 7, 11, 13
– slot 2: 2, 3, 5, 7
– slot 3: 2, 3
– slot 4: 2
– slot 5: 3
– slot 6: 5, 7
– slot 7: 5
– slot 8: 7
– slot 9: 11, 13
– slot 10: 11
– slot 11: 13