Collision and Collision Free Protocols
Collision Protocols
Pure ALOHA a simple broadcast protocol
- radio broadcast
- variable length frames
- bad packets are detect using a CRC and are discarded
- sender retransmits after random time
- effective maximum channel utilization is 18%
Slotted ALOHA a better broadcast protocol
- slotted time
- maximum frame size determined by slot size
- collision destroys only those packets transmitted at the beginning of a slot
- increased transmission delay on a lightly loaded network due to wait for slot
- effective maximum channel utilization is 37%
P-Persistent Carrier Sense Multiple Access (CSMA)
- station listens before transmitting
- if channel is busy, station waits until it is idle and then transmits immediately with a probability of P, otherwise it waits a random period of time before transmitting
- increase propagation delay results in increased collisions and decrease utilization
- decreased persistence results in increased utilization
- effective maximum channel utilization is 53% for P=1.0
Non-persistent CSMA
- if the channel is busy, the station waits a random period of time before attempting to transmit
- P-persistence CSMA with P=0.0
- Effective maximum channel utilization approaches 100%
CSMA with Collision Detection
- non-persistent
- station listens before transmitting
- station aborts transmission when a collision is detected
- station transmits a jamming signal to announce a collision to all stations
- transmitter waits a random amount of time after a collision
- effective maximum channel utilization
- Ethernet
Ethernet IEEE 802.3
uses a bus topology
uses a contention-based media access control technique called CSMA/CD carrier sense multiple access with collision detection
wait until the bus is free and transmit
collision detection waits a random amount of time before transmitting to prevent collisions
two types of Ethernet
Baseband
- Uses digital signaling and treats the cable as one single channel so it can only carry a single transmission at any one moment
Broadband
- Uses analog signaling and splits the cable into many different channels using FDM
Collision Free Protocols
Bit-Map Method Reservation
a n-slotted bit map is circulated to the n stations on the network
each station wanting to transmit flips its corresponding bit
in order, each waiting station transmits its frame
after all stations have transmitted, the bit map is re-circulated
every station is guaranteed a timeslot based on reservation
Slotted Ring Round Robin
a constant number of fixed length frames circulate
a station can load a frame if the frame is empty
ring is regarded as a continuous stream of time slots that circulate on the ring
Token Ring IEEE 802.5
uses a ring topology
Media Access Control - token passing
A computer with a message to transmit waits until it receives what is called a free token
The computer changes the free token into a busy token and attaches its message to it
The computer retransmits the token and the message on the circuit to the next computer in the sequence
If the next computer wants to transmit, it must wait because the token is busy so it just forwards the busy token
Only one message can be attached to the token at one time
Problems
Network fails if the ring is broken
Token Bus IEEE 802.4, Round Robin
a token circulates in a logical ring
like Token ring
all stations on ring are primary stations which means they have the ability to close the bus
FDDI Fiber Distributed Data Interface, Round Robin
a token-passing ring network that operates at 100 Mbps over a fiber optic cable
- a token ring with two fibers: send and receive
- maximum 1000 stations and 200 km with repeater every 2 km
- uses two counter-rotating rings called the primary ring and the secondary ring
- DAS dual attachment station on both rings
- In case of network breakage, it re-routs traffic from primary ring to secondary ring
- SAS single attachment station on primary ring
- 100 Mbps or higher
- Media Access Control Similar to token passing but more efficient
- early release mechanism after data is transmitted, it released the token
- when a computer receives the token with a message attached, it removes the token from the ring and transmits all messages that were attached to it
- as long as computer holds token, no other computer can transmit so no collisions
- many messages can be transmitted
- Types
- FDDI-C with copper medium
- Uses two pairs of either shielded or unshielded category 5 twisted-pair cable instead of fiber optic cable
- FDDI-II permits transmission of voice and video over the same cable of normal FDDI token passing data.
- Uses TDM to split 100 Mbps channel into 17 channels: 1 channel for token passing data circuit and 16 for either voice and video OR token passing