MAC (Medium Access Control)
sublayer
MAC sublayer covers Medium Access
techniques and framing
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Types of MAC
Control:
Distributed.
Centralized.
How they coordinate medium access:
Round-robin.
Scheduled-access.
Contention-based.
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MAC Protocols
Contention-based
ALOHA and Slotted ALOHA.
CSMA.
CSMA/CD.
Round-robin : token-based protocols.
Token bus.
Token ring.
Scheduled Access Based:
Centralized or distributed scheduling based MAC
Ex. GSM, WIMAX, LTE, WiFi networks.
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The ALOHA Protocols
Developed at University of Hawaii in early 70s.
Packet radio networks.
Free for all: whenever station has a frame to send, it does
so.
Station listens for maximum RTT (Round Trip Time) for an ACK.
If no ACK, re-sends frame for a number of times and then gives up.
Receivers check FCS (Frame Check Sequence) and destination address
to ACK.
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Pure ALOHA
In pure ALOHA, frames are transmitted at completely
arbitrary times.
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Collisions
Invalid frames may be caused by channel noise or
Because other station(s) transmitted at the same time:
collision.
Collision happens even when the last bit of a frame
overlaps with the first bit of the next frame.
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Pure ALOHA: Performance
Vulnerable period for the shaded frame.
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ALOHAs Performance (Contd)
S = G e-2G, where S is the throughput (rate of successful
transmissions) and G is the mean offered load per frame
duration.
S = Smax = 1/2e = 0.184 for G=0.5.
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Slotted Aloha
Time divided in uniform slots
Need central clock (or other sync mechanism)
Frames can only be transmitted at beginning of slot: discrete
ALOHA.
Frames either miss or overlap totally
Vulnerable period is half of pure ALOHA.
S = G e-G.
S = Smax = 1/e = 0.368 for G = 1.
Doubles performance of pure ALOHA.
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ALOHA Protocols: Performance
Throughput versus offered traffic for ALOHA
systems.
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ALOHA Protocols: Summary
Simple.
poor utilization (network efficiency)
Applicable only in the network having very low user
density
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CARRIER SENSE MULTIPLE ACCESS
(CSMA)
CSMA protocol was developed to overcome the problem found in ALOHA
i.e. to minimize the chances of collision, so as to improve the performance.
CSMA protocol is based on the principle of carrier sense.
The chances of collision can be reduce to great extent if a station senses the
channel before trying to use it.
Although CSMA can reduce the possibility of collision, but it cannot eliminate
it completely.
The chances of collision still exist because of propagation delay.
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CSMA Protocol
1-Persistent CSMA
Continuous sensing
Transmits packet with probability 1 in immediate idle time slot
Collision rate is high
Non-Persistent CSMA
Senses carrier at random time
Network efficiency is comparatively high
Collision rate is also comparatively low
P-Persistent CSMA
Time divided into slots
if idle time sensed Transmits packet with probability p
Network efficiency is comparatively high
Collision rate is low
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CSMA /CD protocol
Carrier Sense Multiple Access/ Collision Detection
Used in Ethernet LAN
Transmitting device transmits packets and continuously detects
collision
If collision detected, differs transmission for random time
For Random time delay Binary exponential back off algorithm
is used.
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Throughput comparison of contention
based MAC protocols
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Collision Free Protocols
Reservation protocol
Like token base access
Bit Mapped Protocol using contention period
transmit time is divided in to contention and Frame time
In contention period one bit is reserved (0 nothing to transmit, 1 to
transmit)
Contention period followed by number of frames = No. of 1 in
contention period
Binary count down Protocol using contention period
Instead reserving one bit for all users in contention period user devices
transmits their binary addresses
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Binary Count down contention protocol
In contention period user nodes transmits their binary address (only those who wants
to transmit)
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Limited-Contention Protocol
Contention Protocol are more suitable for low load
As load increases contention delay also increases
As load increases packet collision probability also increases
Collision free protocols are more suitable for heavy load
As load increases channel efficiency also improves at the cost of
additional overhead
Limited-Contention Protocol combines the best properties of
contention and collision free protocols
At low load uses contention Protocol due to less delay
At high load uses collision free protocol due to good channel efficiency
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Limited-Contention Protocol Cont.
Adaptive Tree walk Protocol
Low load
Every body contends
Collision
Reduces number of stations
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Adaptive Tree walk algorithm
First contention all stations are permitted to contend
If collision then in next slot only stations under node 2 contends
If success then in next slot stations under node 3 contends
If collision then in next slot stations under node 4 contends
If success then in next slot stations under node 5 contends
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IEEE Communications standards*
(widely used & popular)
Standard
Also known as
Medium Access
Topology /frequency band
IEEE 802.3
LAN/Eathernet (Metcalfe /Xerox)
CSMA/CD
Star topology
IEEE 802.4
LAN (developed by General motors)
Token Bus
Bus topology
IEEE 802.5
LAN (developed by IBM)
Token Ring
Ring topology
IEEE 802.11 b/g/n
WLAN/WiFi
CSMA/CA
ISM bands 2.4 GHz & 5.8 GHz
IEEE 802.15.1
PAN/Bluetooth (developed by
Ericsson)
PAN/ZigBee/WSN (Wireless Sensor
Networks)
--
ISM band at 2.4GHz
IEEE 802.16d
IEEE 802.16e
IEEE 802.16j
IEEE 802.16m
BWA/Fixed WiMAX
BWA/Mobile WiMAX
BWA/Relay WiMAX
BWA/Very high data rate
OFDM
OFDMA
OFDMA
OFDMA
2 GHz to 66 GHz (initially 10 GHz
to 66 GHz was allocated for LOS
but to support NLoS 2 GHz to 11
GHz was allocated later)
IEEE 802.20
MBWA/Mobile-Fi ( Will support
connectivity up to 250 km/h mobile
speed)
--
Licensed Bands (below 3.5 GHz)
IEEE 802.15.4
ISM bands** RF & 2.4GHz
--
*IEEE provides communication standards for Physical &OPM
MAC layer only
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** ZigBee operating frequencies 868 MHz in Europe, 915 MHz in the USA and Australia, and 2.4 GHz rest of the world
Binary exponential back-off algorithm
Used for randomization
The contention slot is decided as 2 power n
Where n= 0,1,2,.N
For first time collision n=1, station transmits in slot 0 or 1.
For second time collision n=2, station transmits in any one slot
from 0 to 3.
For third time collision n=3, station transmits in any one slot from
0 to 7.
And so on.
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