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MAC Sub-Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021

Introduction
 On point-to-point networks, there are only singular sender and receiver pairs, eliminating transmission contention
 On broadcast networks, determining right to transmit is a complex problem
 Medium Access Control (MAC) sub-layer is used to assist in resolving transmission conflicts
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MAC Sub-layer
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Types of Channel Allocation Mechanisms
 Various methods exist for allocating a single broadcast channel amongst competing users
 Static Channel Allocation
 Dynamic Channel Allocation
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Static Channel Allocation
 Arbitrary division of a channel into segments and each user is allocated a dedicated segment for transmission
 Time Division Multiplexing (TDM)
 Frequency Division Multiplexing (FDM)
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Time Division Multiplexing
 TDM: users take turns on a fixed schedule  e.g. 2G mobile network
U1 U2 U3 U1 U2 U3
Guard Time
Time
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Frequency Division Multiplexing
 FDM shares the channel by placing users on different frequencies.
 e.g. TV and Radio; ADSL; 4G
Overall FDM channel
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Static Channel Allocation
 Usually good for fixed number of users
 Significant inefficiencies arise when:  Number of senders > allocated segments  Number of senders is not static
 Network traffic is bursty, but static methods TDM and FDM try to give consistent access to the network
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Dynamic Channel Allocation (1)
 Channel segmentation and segment allocation are dynamic
 Assumptions for dynamic channel allocation: 1) Single channel for all communication
2) Independent transmission stations
3) Simultaneous transmission results in damaged frames (collision)
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Dynamic Channel Allocation (2)
4) Time
 Continuous: Transmission can begin at any time
 Slotted: Transmission can begin only within discrete intervals
5) Carrier Sense
 Carrier Sense: Detection of channel use prior to transmission
 No Carrier Sense: No detection of channel use prior to transmission
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Multiple Access Protocols
 Contention
 ALOHA, Slotted ALOHA
 Carrier Sense Multiple Access
 Collision Free
 Limited Contention
 MACA/MACAW (for Wireless LANs)
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ALOHA
 Users transmit frames whenever they have data; retry after a random time if there are collisions (or no Ack is arrived)
 Requires no central control mechanism
 Efficient under low load but inefficient under high traffic loads
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Slotted ALOHA
 Allows the users to start sending only at the beginning of defined slots.
 Increase efficiency of pure ALOHA by reducing possibility of collisions
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Carrier Sense Multiple Access (CSMA)
 Require transmission state detection to determine transmission rights dynamically, there are specific protocols which are used  Persistent and Non-Persistent CSMA
 CSMA with Collision Detection
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Persistent and Non-Persistent CSMA (1)
CSMA: when a sender has data to transmit, first check channel to detect other active transmission
 1-persistent CSMA
 Continuously check, and wait until channel idle; transmit one frame and check collisions; if collision, wait for a random time and repeat
 Non-persistent CSMA
 If channel is busy, wait random period and check again; if idle,
start transmitting
 p-persistent CSMA
 If channel is idle, transmit with probability p, or wait with probability (1-p) and check again
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Persistent and Non-Persistent CSMA (2)
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CSMA Variants
 Comparison of the efficiencies (channel utilisations) for various protocols
CSMA outperforms ALOHA, and being less persistent is better under high load
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CSMA with Collision Detection
 Process: After collision detected, abort transmission, wait random period, try again
 Channel must be continually monitored
 Reduce contention times to improve performance
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Collision Free Protocols (1)
 Bit Map Protocol
 Reservation-based protocol  Overhead: 1 bit per station
 Division of transmission right, and transmission event – no collisions
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Collision Free Protocols (2)
 Binary Countdown Protocol
 Defines transmission order based on the binary station
addressing
 Higher numbered stations have a higher priority – no collisions
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Collision Free Protocols (3)
 Binary Countdown Protocol
 Stations send their address from high-order bit in contention slots (log2 N slots instead of N)
 Channel medium ORs bits; stations give up when they send a “0” but see a “1”
 The station that sees its full address is the next to send
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Contention vs. Collision Free
 2 strategies: contention and collision free
 Under low loads (collisions are rare), the collision free
is less attractive due to the overhead.
 Under higher loads, contention method is less attractive due to higher number of collisions.
 Both become inefficient at different points
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