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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