MAC Sublayer
COMPUTER NETWORKS
Data-link Layer
(The Medium Access Control Sublayer)
MAC Sublayer
Questions to be answered ?
• In broadcast networks, How the
channel is divided between competing
users?
• What is Medium Access Control
(MAC)?
• What protocols are used for
allocating a multiple access
channel ?
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MAC Sublayer
the need for determine who
gets to use the channel
- When there are two or more users
trying to use a shared single
channel there should be an algorithm
to control this access.
- This problem occurs in broadcast
networks which are known as
multiaccess channels.
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MAC Sublayer
What is MAC?
- Medium Access Control (MAC) is a
sublayer of the Data-link layer.
- The protocols used to determine who goes
next on a multiaccess channel belongs to a
MAC sublayer.
- MAC is important in LAN which use a
multiaccess channel as the basis for
communication.
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MAC Sublayer
The Channel Allocation Problem
•
There are two schemes to allocate
a single channel among competing users:
1) Static Channel Allocation.
2) Dynamic Channel Allocation
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MAC Sublayer
Static Channel Allocation:
In this scheme a Frequency Division
Multiplexing (FDM) is used for allocating
a single channel among competing users.
Example
if we have N users, the bandwidth will be
divided into N equal-size portions.
++ FDM is a simple and efficient allocation
mechanism.
- - Waste of resources when the traffic is
bursty, or the channel is lightly loaded.
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MAC Sublayer
Dynamic Channel Allocation:
Before the discussion of algorithms used for
dynamic allocation we need to consider the
following assumptions.
1) Station Model: N independent stations
generate frames for transmission.
(Generate >Block >Transmission)
2) Single channel Assumption: Single
channel is available for all communication.
3) Collision Assumption
4) Continuous Time, or Slotted Time
5) Carrier Sense, or No Carrier sense
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MAC Sublayer
Multiple Access Protocols:
- ALOHA is a system proposed for solving
the channel allocation problem.
- there are two versions of ALOHA:
1) Pure ALOHA; 2) Slotted ALOHA
The basic difference with respect to
timing is:
Pure ALOHA does not require global
time synchronization;
Slotted ALOHA does
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Pure ALOHA
In pure ALOHA, frames are transmitted at completely
arbitrary times.
MAC Sublayer
Pure ALOHA
The system is working as follows:
1- let users transmit whenever they
have data to be sent.
2- expected collisions will occur.
3- the collided frames will be destroyed.
4- using a feedback mechanism to
know about the status of frame.
5- retransmit the destroyed frame.
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Pure ALOHA (2)
Vulnerable period for the shaded frame.
MAC Sublayer
Pure ALOHA
The main disadvantage of Pure ALOHA
is a low channel utilization.
This is expected due to the feature that
all users transmit whenever they
want.
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Pure ALOHA (3)
Throughput versus offered traffic for ALOHA systems.
MAC Sublayer
Slotted ALOHA
In this method the proposal was to divide the
time into discrete intervals each interval
corresponding to one frame.
In Slotted ALOHA, a computer can not send
anytime, instead it is required to wait for
the beginning of the time slot.
The big advantage of Slotted ALOHA is the
increase in channel utilization.
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MAC Sublayer
Slotted ALOHA
There is a limit for the best channel utilization
using Slotted ALOHA.
To reduce the chance of collisions the
station should be able to detect what
other stations are doing.
In LAN networks this is possible, therefore
they can achieve better utilization than
Slotted ALOHA.
Carrier Sense Protocols are protocols in
which stations listen for a carrier.
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MAC Sublayer
Carrier Sense Multiple Access
(CSMA) Protocols
There are several versions of carrier sense
protocols:
- 1-persistent CSMA
- Non-persistent CSMA
- P-persistent CSMA
- CSMA with Collision Detection (CSMA/CD)
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MAC Sublayer
Carrier Sense Multiple Access
(CSMA) Protocols
- In 1-persistent CSMA, a station prior to
send data it listen to the channel to see if
anyone else is transmitting at that moment.
- if the channel is busy, the station waits
until it becomes idle.
- If the channel is idle, the station transmits
a frame.
- If a collision occurs, the station waits a
random amount of time and starts all over
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again.
MAC Sublayer
Carrier Sense Multiple Access
(CSMA) Protocols
- Although this protocol has disadvantages,
it is better than ALOHA and Slotted
ALOHA
-1) It’s performance depends on the
propagation delay.
-2) There is a chance when two
stations start transmission at the same
time.
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MAC Sublayer
Carrier Sense Multiple Access
(CSMA) Protocols
- In Non-persistent CSMA a station makes
conscious attempt to sense the channel.
- After the first attempt, if the channel is idle,
it sends, however, if the channel is already
in use, it waits a random period of time and
repeats the algorithm.
- (+ -)This algorithm has better utilization
but longer delays than 1-persistent
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CSMA.
MAC Sublayer
Carrier Sense Multiple Access
(CSMA) Protocols
- In p-persistent CSMA a station transmits
if the channel is idle with a probability p
and with probability q=1-p it waits until the
next slot.
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MAC Sublayer
Carrier Sense Multiple Access
(CSMA) Protocols
- The main advantage of persistent and
non-persistent over ALOHA is that they
ensure no station begins to transmit when
it senses the channel busy.
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MAC Sublayer
CSMA with Collision Detection
- It is important that stations should
terminate transmission as soon as they
detect a collision.
- This protocol is called CSMA/CD.
- It is widely used on LANs in the MAC
sublayer.
- It is the basis of the popular Ethernet
LAN.
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CSMA with Collision Detection
CSMA/CD can be in one of three states: contention,
transmission, or idle.
MAC Sublayer
CSMA with Collision Detection
- A collision can be detected by looking at
the power or pulse width of the received
signal and comparing it to the transmitted
signal.
- After a station detects a collision it
terminates its transmission, waits a
random period of time, and then tries
again, assuming that no other station has
started transmitting in the meantime.
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MAC Sublayer
CSMA with Collision Detection
- Collision detection is an analog process.
Therefore, special encoding is commonly
used.
- A sending station must continually
monitor the channel, listening for noise
bursts that might indicate a collision. For
this reason, CSMA/CD with a single
channel is inherently a half-duplex system.
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