Medium Access Control Sublayer Notes | EduRev

: Medium Access Control Sublayer Notes | EduRev

 Page 1


Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 1
Gursharan Singh Tatla
mailme@gursharansingh.in
www.eazynotes.com
Medium Access Control 
Sublayer
31-Mar-2011 1 www.eazynotes.com
Introduction
? In broadcast networks, several stations share a 
single communication channel.
? The major issue in these networks is, which station 
should transmit data at a given time.
? This process of deciding the turn of different stations 
is known as Channel Allocation.
? To coordinate the access to the channel, multiple 
access protocols are required.
? All these protocols belong to the MAC sublayer.
31-Mar-2011 2 www.eazynotes.com
Introduction
? Data Link layer is divided into two sublayers:
? Logical Link Control (LLC)
? Medium Access Control (MAC)
? LCC is responsible for error control & flow control.
? MAC is responsible for multiple access resolutions.
31-Mar-2011 3 www.eazynotes.com
Network Layer
Logical Link Control (LLC)
Medium Access Control (MAC)
Physical Layer
Data Link Layer
Channel Allocation Problem
? In broadcast networks, single channel is shared by 
several stations.
? This channel can be allocated to only one 
transmitting user at a time.
? There are two different methods of channel 
allocations:
? Static Channel Allocation
? Dynamic Channel Allocation
31-Mar-2011 www.eazynotes.com 4
Static Channel Allocations
? In this method, a single channel is divided among 
various users either on the basis of frequency or on 
the basis of time.
? It either uses FDM (Frequency Division Multiplexing) 
or TDM (Time Division Multiplexing).
? In FDM, fixed frequency is assigned to each user, 
whereas, in TDM, fixed time slot is assigned to each 
user.
31-Mar-2011 www.eazynotes.com 5
Dynamic Channel Allocation
? In this method, no user is assigned fixed frequency 
or fixed time slot.
? All users are dynamically assigned frequency or time 
slot, depending upon the requirements of the user.
31-Mar-2011 www.eazynotes.com 6
Page 2


Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 1
Gursharan Singh Tatla
mailme@gursharansingh.in
www.eazynotes.com
Medium Access Control 
Sublayer
31-Mar-2011 1 www.eazynotes.com
Introduction
? In broadcast networks, several stations share a 
single communication channel.
? The major issue in these networks is, which station 
should transmit data at a given time.
? This process of deciding the turn of different stations 
is known as Channel Allocation.
? To coordinate the access to the channel, multiple 
access protocols are required.
? All these protocols belong to the MAC sublayer.
31-Mar-2011 2 www.eazynotes.com
Introduction
? Data Link layer is divided into two sublayers:
? Logical Link Control (LLC)
? Medium Access Control (MAC)
? LCC is responsible for error control & flow control.
? MAC is responsible for multiple access resolutions.
31-Mar-2011 3 www.eazynotes.com
Network Layer
Logical Link Control (LLC)
Medium Access Control (MAC)
Physical Layer
Data Link Layer
Channel Allocation Problem
? In broadcast networks, single channel is shared by 
several stations.
? This channel can be allocated to only one 
transmitting user at a time.
? There are two different methods of channel 
allocations:
? Static Channel Allocation
? Dynamic Channel Allocation
31-Mar-2011 www.eazynotes.com 4
Static Channel Allocations
? In this method, a single channel is divided among 
various users either on the basis of frequency or on 
the basis of time.
? It either uses FDM (Frequency Division Multiplexing) 
or TDM (Time Division Multiplexing).
? In FDM, fixed frequency is assigned to each user, 
whereas, in TDM, fixed time slot is assigned to each 
user.
31-Mar-2011 www.eazynotes.com 5
Dynamic Channel Allocation
? In this method, no user is assigned fixed frequency 
or fixed time slot.
? All users are dynamically assigned frequency or time 
slot, depending upon the requirements of the user.
31-Mar-2011 www.eazynotes.com 6
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 2
Multiple Access Protocols
? Many protocols have been defined to handle the 
access to shared link.
? These protocols are organized in three different 
groups.:
? Random Access Protocols
? Controlled Access Protocols
? Channelization Protocols
31-Mar-2011 www.eazynotes.com 7 31-Mar-2011 www.eazynotes.com 8
Multiple 
Access 
Protocols
Multiple 
Access 
Protocols
Random Access 
Protocols
Random Access 
Protocols
ALOHA ALOHA
Pure 
ALOHA
Pure 
ALOHA
Slotted 
ALOHA
Slotted 
ALOHA
CSMA CSMA
1-
Persistent
1-
Persistent
Non-
Persistent
Non-
Persistent
P-
Persistent
P-
Persistent
CSMA/CD CSMA/CD CSMA/CA CSMA/CA
Controlled Access 
Protocols
Controlled Access 
Protocols
Reservation Reservation Polling Polling
Token 
Passing
Token 
Passing
Channelization 
Protocols
Channelization 
Protocols
FDMA FDMA TDMA TDMA CDMA CDMA
Random Access Protocols
? It is also called Contention Method.
? In this method, there is no control station.
? Any station can send the data.
? The station can make a decision on whether or not to 
send data. This decision depends on the state of the 
channel, i.e. channel is busy or idle.
? There is no scheduled time for a stations to transmit. 
They can transmit in random order.
31-Mar-2011 www.eazynotes.com 9
Random Access Protocols
? There is no rule that decides which station should 
send next.
? If two stations transmit at the same time, there is 
collision and the frames are lost.
? The various random access methods are:
? ALOHA
? CSMA (Carrier Sense Multiple Access)
? CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
? CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
31-Mar-2011 www.eazynotes.com 10
ALOHA
? ALOHA was developed at University of Hawaii in early 
1970s by Norman Abramson.
? It was used for ground based radio broadcasting.
? In this method, stations share a common channel.
? When two stations transmit simultaneously, collision 
occurs and frames are lost.
? There are two different versions of ALOHA:
? Pure ALOHA
? Slotted ALOHA
31-Mar-2011 www.eazynotes.com 11
Pure ALOHA
? In pure ALOHA, stations transmit frames whenever 
they have data to send.
? When two stations transmit simultaneously, there is 
collision and frames are lost.
? In pure ALOHA, whenever any station transmits a 
frame, it expects an acknowledgement from the 
receiver.
? If acknowledgement is not received within specified 
time, the station assumes that the frame has been 
lost.
31-Mar-2011 www.eazynotes.com 12
Page 3


Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 1
Gursharan Singh Tatla
mailme@gursharansingh.in
www.eazynotes.com
Medium Access Control 
Sublayer
31-Mar-2011 1 www.eazynotes.com
Introduction
? In broadcast networks, several stations share a 
single communication channel.
? The major issue in these networks is, which station 
should transmit data at a given time.
? This process of deciding the turn of different stations 
is known as Channel Allocation.
? To coordinate the access to the channel, multiple 
access protocols are required.
? All these protocols belong to the MAC sublayer.
31-Mar-2011 2 www.eazynotes.com
Introduction
? Data Link layer is divided into two sublayers:
? Logical Link Control (LLC)
? Medium Access Control (MAC)
? LCC is responsible for error control & flow control.
? MAC is responsible for multiple access resolutions.
31-Mar-2011 3 www.eazynotes.com
Network Layer
Logical Link Control (LLC)
Medium Access Control (MAC)
Physical Layer
Data Link Layer
Channel Allocation Problem
? In broadcast networks, single channel is shared by 
several stations.
? This channel can be allocated to only one 
transmitting user at a time.
? There are two different methods of channel 
allocations:
? Static Channel Allocation
? Dynamic Channel Allocation
31-Mar-2011 www.eazynotes.com 4
Static Channel Allocations
? In this method, a single channel is divided among 
various users either on the basis of frequency or on 
the basis of time.
? It either uses FDM (Frequency Division Multiplexing) 
or TDM (Time Division Multiplexing).
? In FDM, fixed frequency is assigned to each user, 
whereas, in TDM, fixed time slot is assigned to each 
user.
31-Mar-2011 www.eazynotes.com 5
Dynamic Channel Allocation
? In this method, no user is assigned fixed frequency 
or fixed time slot.
? All users are dynamically assigned frequency or time 
slot, depending upon the requirements of the user.
31-Mar-2011 www.eazynotes.com 6
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 2
Multiple Access Protocols
? Many protocols have been defined to handle the 
access to shared link.
? These protocols are organized in three different 
groups.:
? Random Access Protocols
? Controlled Access Protocols
? Channelization Protocols
31-Mar-2011 www.eazynotes.com 7 31-Mar-2011 www.eazynotes.com 8
Multiple 
Access 
Protocols
Multiple 
Access 
Protocols
Random Access 
Protocols
Random Access 
Protocols
ALOHA ALOHA
Pure 
ALOHA
Pure 
ALOHA
Slotted 
ALOHA
Slotted 
ALOHA
CSMA CSMA
1-
Persistent
1-
Persistent
Non-
Persistent
Non-
Persistent
P-
Persistent
P-
Persistent
CSMA/CD CSMA/CD CSMA/CA CSMA/CA
Controlled Access 
Protocols
Controlled Access 
Protocols
Reservation Reservation Polling Polling
Token 
Passing
Token 
Passing
Channelization 
Protocols
Channelization 
Protocols
FDMA FDMA TDMA TDMA CDMA CDMA
Random Access Protocols
? It is also called Contention Method.
? In this method, there is no control station.
? Any station can send the data.
? The station can make a decision on whether or not to 
send data. This decision depends on the state of the 
channel, i.e. channel is busy or idle.
? There is no scheduled time for a stations to transmit. 
They can transmit in random order.
31-Mar-2011 www.eazynotes.com 9
Random Access Protocols
? There is no rule that decides which station should 
send next.
? If two stations transmit at the same time, there is 
collision and the frames are lost.
? The various random access methods are:
? ALOHA
? CSMA (Carrier Sense Multiple Access)
? CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
? CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
31-Mar-2011 www.eazynotes.com 10
ALOHA
? ALOHA was developed at University of Hawaii in early 
1970s by Norman Abramson.
? It was used for ground based radio broadcasting.
? In this method, stations share a common channel.
? When two stations transmit simultaneously, collision 
occurs and frames are lost.
? There are two different versions of ALOHA:
? Pure ALOHA
? Slotted ALOHA
31-Mar-2011 www.eazynotes.com 11
Pure ALOHA
? In pure ALOHA, stations transmit frames whenever 
they have data to send.
? When two stations transmit simultaneously, there is 
collision and frames are lost.
? In pure ALOHA, whenever any station transmits a 
frame, it expects an acknowledgement from the 
receiver.
? If acknowledgement is not received within specified 
time, the station assumes that the frame has been 
lost.
31-Mar-2011 www.eazynotes.com 12
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 3
Pure ALOHA
? If the frame is lost, station waits for a random amount 
of time and sends it again.
? This waiting time must be random, otherwise, same 
frames will collide again and again.
? Whenever two frames try to occupy the channel at 
the same time, there will be collision and both the 
frames will be lost.
? If first bit of a new frame overlaps with the last bit of a 
frame almost finished, both frames will be lost and 
both will have to be retransmitted.
31-Mar-2011 www.eazynotes.com 13
Pure ALOHA
31-Mar-2011 www.eazynotes.com 14
Slotted ALOHA
? Slotted ALOHA was invented to improve the efficiency of 
pure ALOHA.
? In slotted ALOHA, time of the channel is divided into 
intervals called slots.
? The station can send a frame only at the beginning of the 
slot and only one frame is sent in each slot.
? If any station is not able to place the frame onto the 
channel at the beginning of the slot, it has to wait until the 
next time slot.
? There is still a possibility of collision if two stations try to 
send at the beginning of the same time slot.
31-Mar-2011 www.eazynotes.com 15
Slotted ALOHA
31-Mar-2011 www.eazynotes.com 16
Carrier Sense Multiple Access (CSMA)
? CSMA was developed to overcome the problems of 
ALOHA i.e. to  minimize the chances of collision.
? CSMA is based on the principle of “carrier sense”.
? The station sense the carrier or channel before 
transmitting a frame.
? It means the station checks whether the channel is 
idle or busy.
? The chances of collision reduces to a great extent if 
a station checks the channel before trying to use it.
31-Mar-2011 www.eazynotes.com 17 31-Mar-2011 www.eazynotes.com 18
Carrier Sense Multiple Access (CSMA)
Page 4


Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 1
Gursharan Singh Tatla
mailme@gursharansingh.in
www.eazynotes.com
Medium Access Control 
Sublayer
31-Mar-2011 1 www.eazynotes.com
Introduction
? In broadcast networks, several stations share a 
single communication channel.
? The major issue in these networks is, which station 
should transmit data at a given time.
? This process of deciding the turn of different stations 
is known as Channel Allocation.
? To coordinate the access to the channel, multiple 
access protocols are required.
? All these protocols belong to the MAC sublayer.
31-Mar-2011 2 www.eazynotes.com
Introduction
? Data Link layer is divided into two sublayers:
? Logical Link Control (LLC)
? Medium Access Control (MAC)
? LCC is responsible for error control & flow control.
? MAC is responsible for multiple access resolutions.
31-Mar-2011 3 www.eazynotes.com
Network Layer
Logical Link Control (LLC)
Medium Access Control (MAC)
Physical Layer
Data Link Layer
Channel Allocation Problem
? In broadcast networks, single channel is shared by 
several stations.
? This channel can be allocated to only one 
transmitting user at a time.
? There are two different methods of channel 
allocations:
? Static Channel Allocation
? Dynamic Channel Allocation
31-Mar-2011 www.eazynotes.com 4
Static Channel Allocations
? In this method, a single channel is divided among 
various users either on the basis of frequency or on 
the basis of time.
? It either uses FDM (Frequency Division Multiplexing) 
or TDM (Time Division Multiplexing).
? In FDM, fixed frequency is assigned to each user, 
whereas, in TDM, fixed time slot is assigned to each 
user.
31-Mar-2011 www.eazynotes.com 5
Dynamic Channel Allocation
? In this method, no user is assigned fixed frequency 
or fixed time slot.
? All users are dynamically assigned frequency or time 
slot, depending upon the requirements of the user.
31-Mar-2011 www.eazynotes.com 6
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 2
Multiple Access Protocols
? Many protocols have been defined to handle the 
access to shared link.
? These protocols are organized in three different 
groups.:
? Random Access Protocols
? Controlled Access Protocols
? Channelization Protocols
31-Mar-2011 www.eazynotes.com 7 31-Mar-2011 www.eazynotes.com 8
Multiple 
Access 
Protocols
Multiple 
Access 
Protocols
Random Access 
Protocols
Random Access 
Protocols
ALOHA ALOHA
Pure 
ALOHA
Pure 
ALOHA
Slotted 
ALOHA
Slotted 
ALOHA
CSMA CSMA
1-
Persistent
1-
Persistent
Non-
Persistent
Non-
Persistent
P-
Persistent
P-
Persistent
CSMA/CD CSMA/CD CSMA/CA CSMA/CA
Controlled Access 
Protocols
Controlled Access 
Protocols
Reservation Reservation Polling Polling
Token 
Passing
Token 
Passing
Channelization 
Protocols
Channelization 
Protocols
FDMA FDMA TDMA TDMA CDMA CDMA
Random Access Protocols
? It is also called Contention Method.
? In this method, there is no control station.
? Any station can send the data.
? The station can make a decision on whether or not to 
send data. This decision depends on the state of the 
channel, i.e. channel is busy or idle.
? There is no scheduled time for a stations to transmit. 
They can transmit in random order.
31-Mar-2011 www.eazynotes.com 9
Random Access Protocols
? There is no rule that decides which station should 
send next.
? If two stations transmit at the same time, there is 
collision and the frames are lost.
? The various random access methods are:
? ALOHA
? CSMA (Carrier Sense Multiple Access)
? CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
? CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
31-Mar-2011 www.eazynotes.com 10
ALOHA
? ALOHA was developed at University of Hawaii in early 
1970s by Norman Abramson.
? It was used for ground based radio broadcasting.
? In this method, stations share a common channel.
? When two stations transmit simultaneously, collision 
occurs and frames are lost.
? There are two different versions of ALOHA:
? Pure ALOHA
? Slotted ALOHA
31-Mar-2011 www.eazynotes.com 11
Pure ALOHA
? In pure ALOHA, stations transmit frames whenever 
they have data to send.
? When two stations transmit simultaneously, there is 
collision and frames are lost.
? In pure ALOHA, whenever any station transmits a 
frame, it expects an acknowledgement from the 
receiver.
? If acknowledgement is not received within specified 
time, the station assumes that the frame has been 
lost.
31-Mar-2011 www.eazynotes.com 12
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 3
Pure ALOHA
? If the frame is lost, station waits for a random amount 
of time and sends it again.
? This waiting time must be random, otherwise, same 
frames will collide again and again.
? Whenever two frames try to occupy the channel at 
the same time, there will be collision and both the 
frames will be lost.
? If first bit of a new frame overlaps with the last bit of a 
frame almost finished, both frames will be lost and 
both will have to be retransmitted.
31-Mar-2011 www.eazynotes.com 13
Pure ALOHA
31-Mar-2011 www.eazynotes.com 14
Slotted ALOHA
? Slotted ALOHA was invented to improve the efficiency of 
pure ALOHA.
? In slotted ALOHA, time of the channel is divided into 
intervals called slots.
? The station can send a frame only at the beginning of the 
slot and only one frame is sent in each slot.
? If any station is not able to place the frame onto the 
channel at the beginning of the slot, it has to wait until the 
next time slot.
? There is still a possibility of collision if two stations try to 
send at the beginning of the same time slot.
31-Mar-2011 www.eazynotes.com 15
Slotted ALOHA
31-Mar-2011 www.eazynotes.com 16
Carrier Sense Multiple Access (CSMA)
? CSMA was developed to overcome the problems of 
ALOHA i.e. to  minimize the chances of collision.
? CSMA is based on the principle of “carrier sense”.
? The station sense the carrier or channel before 
transmitting a frame.
? It means the station checks whether the channel is 
idle or busy.
? The chances of collision reduces to a great extent if 
a station checks the channel before trying to use it.
31-Mar-2011 www.eazynotes.com 17 31-Mar-2011 www.eazynotes.com 18
Carrier Sense Multiple Access (CSMA)
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 4
Carrier Sense Multiple Access (CSMA)
? The chances of collision still exists because of 
propagation delay.
? The frame transmitted by one station takes some 
time to reach the other station.
? In the meantime, other station may sense the 
channel to be idle and transmit its frames.
? This results in the collision.
31-Mar-2011 www.eazynotes.com 19
Carrier Sense Multiple Access (CSMA)
? There are three different types of CSMA protocols:
? 1-Persistent CSMA
? Non-Persistent CSMA
? P-Persistent CSMA
31-Mar-2011 www.eazynotes.com 20
1-Persistent CSMA
? In this method, station that wants to transmit data, 
continuously senses the channel to check whether 
he channel is idle or busy.
? If the channel is busy, station waits until it becomes 
idle.
? When the station detects an idle channel, it 
immediately transmits the frame.
? This method has the highest chance of collision 
because two or more stations may find channel to be 
idle at the same time and transmit their frames.
31-Mar-2011 www.eazynotes.com 21
Non-Persistent CSMA
? A station that has a frame to send, senses the 
channel.
? If the channel is idle, it sends immediately.
? If the channel is busy, it waits a random amount of 
time and then senses the channel again.
? It reduces the chance of collision because the 
stations wait for a random amount of time .
? It is unlikely that two or more stations will wait for the 
same amount of time and will retransmit at the same 
time.
31-Mar-2011 www.eazynotes.com 22
P-Persistent CSMA
? In this method, the channel has time slots such that 
the time slot duration is equal to or greater than the 
maximum propagation delay time.
? When a station is ready to send, it senses the 
channel.
? If the channel is busy, station waits until next slot.
? If the channel is idle, it transmits the frame.
? It reduces the chance of collision and improves the 
efficiency of the network.
31-Mar-2011 www.eazynotes.com 23
CSMA with Collision Detection 
(CSMA/CD)
? In this protocol, the station senses the channel 
before transmitting the frame. If the channel is busy, 
the station waits.
? Additional feature in CSMA/CD is that the stations 
can detect collisions.
? The stations abort their transmission as soon as they 
detect collision.
? This feature is not present in CSMA.
? The stations continue to transmit even though they 
find that collision has occurred.
31-Mar-2011 www.eazynotes.com 24
Page 5


Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 1
Gursharan Singh Tatla
mailme@gursharansingh.in
www.eazynotes.com
Medium Access Control 
Sublayer
31-Mar-2011 1 www.eazynotes.com
Introduction
? In broadcast networks, several stations share a 
single communication channel.
? The major issue in these networks is, which station 
should transmit data at a given time.
? This process of deciding the turn of different stations 
is known as Channel Allocation.
? To coordinate the access to the channel, multiple 
access protocols are required.
? All these protocols belong to the MAC sublayer.
31-Mar-2011 2 www.eazynotes.com
Introduction
? Data Link layer is divided into two sublayers:
? Logical Link Control (LLC)
? Medium Access Control (MAC)
? LCC is responsible for error control & flow control.
? MAC is responsible for multiple access resolutions.
31-Mar-2011 3 www.eazynotes.com
Network Layer
Logical Link Control (LLC)
Medium Access Control (MAC)
Physical Layer
Data Link Layer
Channel Allocation Problem
? In broadcast networks, single channel is shared by 
several stations.
? This channel can be allocated to only one 
transmitting user at a time.
? There are two different methods of channel 
allocations:
? Static Channel Allocation
? Dynamic Channel Allocation
31-Mar-2011 www.eazynotes.com 4
Static Channel Allocations
? In this method, a single channel is divided among 
various users either on the basis of frequency or on 
the basis of time.
? It either uses FDM (Frequency Division Multiplexing) 
or TDM (Time Division Multiplexing).
? In FDM, fixed frequency is assigned to each user, 
whereas, in TDM, fixed time slot is assigned to each 
user.
31-Mar-2011 www.eazynotes.com 5
Dynamic Channel Allocation
? In this method, no user is assigned fixed frequency 
or fixed time slot.
? All users are dynamically assigned frequency or time 
slot, depending upon the requirements of the user.
31-Mar-2011 www.eazynotes.com 6
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 2
Multiple Access Protocols
? Many protocols have been defined to handle the 
access to shared link.
? These protocols are organized in three different 
groups.:
? Random Access Protocols
? Controlled Access Protocols
? Channelization Protocols
31-Mar-2011 www.eazynotes.com 7 31-Mar-2011 www.eazynotes.com 8
Multiple 
Access 
Protocols
Multiple 
Access 
Protocols
Random Access 
Protocols
Random Access 
Protocols
ALOHA ALOHA
Pure 
ALOHA
Pure 
ALOHA
Slotted 
ALOHA
Slotted 
ALOHA
CSMA CSMA
1-
Persistent
1-
Persistent
Non-
Persistent
Non-
Persistent
P-
Persistent
P-
Persistent
CSMA/CD CSMA/CD CSMA/CA CSMA/CA
Controlled Access 
Protocols
Controlled Access 
Protocols
Reservation Reservation Polling Polling
Token 
Passing
Token 
Passing
Channelization 
Protocols
Channelization 
Protocols
FDMA FDMA TDMA TDMA CDMA CDMA
Random Access Protocols
? It is also called Contention Method.
? In this method, there is no control station.
? Any station can send the data.
? The station can make a decision on whether or not to 
send data. This decision depends on the state of the 
channel, i.e. channel is busy or idle.
? There is no scheduled time for a stations to transmit. 
They can transmit in random order.
31-Mar-2011 www.eazynotes.com 9
Random Access Protocols
? There is no rule that decides which station should 
send next.
? If two stations transmit at the same time, there is 
collision and the frames are lost.
? The various random access methods are:
? ALOHA
? CSMA (Carrier Sense Multiple Access)
? CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
? CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
31-Mar-2011 www.eazynotes.com 10
ALOHA
? ALOHA was developed at University of Hawaii in early 
1970s by Norman Abramson.
? It was used for ground based radio broadcasting.
? In this method, stations share a common channel.
? When two stations transmit simultaneously, collision 
occurs and frames are lost.
? There are two different versions of ALOHA:
? Pure ALOHA
? Slotted ALOHA
31-Mar-2011 www.eazynotes.com 11
Pure ALOHA
? In pure ALOHA, stations transmit frames whenever 
they have data to send.
? When two stations transmit simultaneously, there is 
collision and frames are lost.
? In pure ALOHA, whenever any station transmits a 
frame, it expects an acknowledgement from the 
receiver.
? If acknowledgement is not received within specified 
time, the station assumes that the frame has been 
lost.
31-Mar-2011 www.eazynotes.com 12
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 3
Pure ALOHA
? If the frame is lost, station waits for a random amount 
of time and sends it again.
? This waiting time must be random, otherwise, same 
frames will collide again and again.
? Whenever two frames try to occupy the channel at 
the same time, there will be collision and both the 
frames will be lost.
? If first bit of a new frame overlaps with the last bit of a 
frame almost finished, both frames will be lost and 
both will have to be retransmitted.
31-Mar-2011 www.eazynotes.com 13
Pure ALOHA
31-Mar-2011 www.eazynotes.com 14
Slotted ALOHA
? Slotted ALOHA was invented to improve the efficiency of 
pure ALOHA.
? In slotted ALOHA, time of the channel is divided into 
intervals called slots.
? The station can send a frame only at the beginning of the 
slot and only one frame is sent in each slot.
? If any station is not able to place the frame onto the 
channel at the beginning of the slot, it has to wait until the 
next time slot.
? There is still a possibility of collision if two stations try to 
send at the beginning of the same time slot.
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Slotted ALOHA
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Carrier Sense Multiple Access (CSMA)
? CSMA was developed to overcome the problems of 
ALOHA i.e. to  minimize the chances of collision.
? CSMA is based on the principle of “carrier sense”.
? The station sense the carrier or channel before 
transmitting a frame.
? It means the station checks whether the channel is 
idle or busy.
? The chances of collision reduces to a great extent if 
a station checks the channel before trying to use it.
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Carrier Sense Multiple Access (CSMA)
Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 4
Carrier Sense Multiple Access (CSMA)
? The chances of collision still exists because of 
propagation delay.
? The frame transmitted by one station takes some 
time to reach the other station.
? In the meantime, other station may sense the 
channel to be idle and transmit its frames.
? This results in the collision.
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Carrier Sense Multiple Access (CSMA)
? There are three different types of CSMA protocols:
? 1-Persistent CSMA
? Non-Persistent CSMA
? P-Persistent CSMA
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1-Persistent CSMA
? In this method, station that wants to transmit data, 
continuously senses the channel to check whether 
he channel is idle or busy.
? If the channel is busy, station waits until it becomes 
idle.
? When the station detects an idle channel, it 
immediately transmits the frame.
? This method has the highest chance of collision 
because two or more stations may find channel to be 
idle at the same time and transmit their frames.
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Non-Persistent CSMA
? A station that has a frame to send, senses the 
channel.
? If the channel is idle, it sends immediately.
? If the channel is busy, it waits a random amount of 
time and then senses the channel again.
? It reduces the chance of collision because the 
stations wait for a random amount of time .
? It is unlikely that two or more stations will wait for the 
same amount of time and will retransmit at the same 
time.
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P-Persistent CSMA
? In this method, the channel has time slots such that 
the time slot duration is equal to or greater than the 
maximum propagation delay time.
? When a station is ready to send, it senses the 
channel.
? If the channel is busy, station waits until next slot.
? If the channel is idle, it transmits the frame.
? It reduces the chance of collision and improves the 
efficiency of the network.
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CSMA with Collision Detection 
(CSMA/CD)
? In this protocol, the station senses the channel 
before transmitting the frame. If the channel is busy, 
the station waits.
? Additional feature in CSMA/CD is that the stations 
can detect collisions.
? The stations abort their transmission as soon as they 
detect collision.
? This feature is not present in CSMA.
? The stations continue to transmit even though they 
find that collision has occurred.
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Gursharan Singh Tatla 31-Mar-2011
www.eazynotes.com 5
CSMA with Collision Detection 
(CSMA/CD)
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CSMA with Collision Detection 
(CSMA/CD)
? In CSMA/CD, the station that sends its data on the 
channel, continues to sense the channel even after data 
transmission.
? If collision is detected, the station aborts its transmission 
and waits for a random amount of time & sends its data 
again.
? As soon as a collision is detected, the transmitting station 
release a jam signal.
? Jam signal alerts other stations. Stations are not 
supposed to transmit immediately after the collision has 
occurred.
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CSMA with Collision Avoidance 
(CSMA/CA)
? This protocol is used in wireless networks because 
they cannot detect the collision.
? So, the only solution is collision avoidance.
? It avoids the collision by using three basic 
techniques:
? Interframe Space
? Contention Window
? Acknowledgements
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CSMA with Collision Avoidance 
(CSMA/CA)
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Interframe Space
? Whenever the channel is found idle, the station does not 
transmit immediately.
? It waits for a period of time called Interframe Space (IFS).
? When channel is sensed idle, it may be possible that 
some distant station may have already started 
transmitting.
? Therefore, the purpose of IFS time is to allow this 
transmitted signal to reach its destination.
? If after this IFS time, channel is still idle, the station can 
send the frames.
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Contention Window
? Contention window is the amount of time divided into 
slots.
? Station that is ready to send chooses a random number 
of slots as its waiting time.
? The number of slots in the window changes with time.
? It means that it is set of one slot for the first time, and 
then doubles each time the station cannot detect an idle 
channel after the IFS time.
? In contention window, the station needs to sense the 
channel after each time slot.
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