Transcript Ch 13- 14 • Module 4.2: Media Access Control

Module 4.2: Media Access Control
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The Media Access Control (MAC) sublayer
– Random Access (CSMA), IEEE 802.3
– Token Passing, IEEE 802.5
Ch 13- 14
K. Salah
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MAC Concepts
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MAC sublayer provides the protocols that define the manner in which
nodes share the singly physical transmission media. The protocols
define the who, the how, the when, and the for how long of channel
allocation. These protocols are sometimes called contention protocols.
Random Access
– Stations access medium randomly
Contention
– Stations contend for time on medium
Random Access Protocols
– Pure ALOHA
– Slotted ALOHA
– 1-persistent CSMA
– Nonpersistent CSMA
– P-persistent CSMA
– CSMA/CD
– CSMA/CA
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ALOHA
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Pure ALOHA
– Packet Radio
– When station has frame, it sends
– Station listens (for max round trip time)plus small increment
– If ACK, fine. If not, retransmit
– If no ACK after repeated transmissions, give up
– Frame check sequence
– If frame OK and address matches receiver, send ACK
– Frame may be damaged by noise or by another station transmitting at
the same time (collision)
– Any overlap of frames causes collision
– Max useful throughput (frames/s) is 18%
Slotted ALOHA
– Time in uniform slots equal to frame transmission time
– Need central clock (or other sync mechanism)
– Transmission begins at slot boundary
– Frames either miss or overlap totally
– Max useful throughput (frames/s) is 37%
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Carrier Sense Multiple Access
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Propagation time is much less than
transmission time
All stations know that a transmission
has started almost immediately
First listen for clear medium (carrier
sense)
If medium idle, transmit
If two stations start at the same
instant, collision
Max utilization depends on
propagation time (medium length)
and frame length
– Longer frame and shorter
propagation gives better
utilization
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CSMA
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1-persistent CSMA
– Similar to telephone with persistent redial feature.
– P=1 means the probability of transmitting when sensing idle channel.
Nonpersistent CSMA
– If channel is busy, wait a random period of time and rechecks the channel.
P-persistent CSMA
– Uses slotted channel
– If channel is idle, transmit with a probability of p. Wait one time slot and sense
again idle channel with probability of 1-p. Next time slot, transmit with p
probability.
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CSMA/CD
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Step 1. “Listen before talking”.
Step 2. If channel is quiet for a certain time, called
interframe gap (IFG), then transmit. “Talk if quiet”.
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Step 3.
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Step 4.
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Step 5.
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Step 6.
If channel is busy, monitor the channel
until it is quiet for IFG period before transmitting.
“Wait for quiet before talking.”
Monitor the channel continuously during
transmission to detect collisions. “Listen while
talking”.
If collision occurs, the first node
recognizes it will send a jam signal to ensure that all
other stations detect the collision. Transmitters
should stop immediately, receivers should reject
data, and others wishing to transmit should
recalculate their backoff period. “A buzzer sounds off
indicating we have more than one talker at a time.”
All nodes wish to transmit must now wait
a random period called “backoff” and attempt again
to spread out collisions. “Backoff” is based on BEB
algorithm (Binary Exponential Backoff). After 16
collisions, drop the frame to be transmitted.
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Notes
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On baseband bus, collision
produces much higher signal
voltage than signal
Collision detected if cable signal
greater than single station signal
Signal attenuated over distance
Limit distance to 500m (10Base5) or
200m (10Base2)
For twisted pair (star-topology)
activity on more than one port is
collision
Why largest network diameter for
Ethernet LAN 10Base5 is 2500
meters?
– 5-4-3 Repeater Placement
Rule.
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Token Ring (802.5)
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MAC protocol
– Small frame (token) circulates
when idle
– Station waits for token
– Changes one bit in token to
make it SOF (Start Of Frame)
for data frame
– Append rest of data frame
– Frame makes round trip and
is absorbed by transmitting
station
– Station then inserts new
token when transmission has
finished and leading edge of
returning frame arrives
– Under light loads, some
inefficiency
– Under heavy loads, round
robin
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