4. 12.4
12-1 RANDOM ACCESS
12-1 RANDOM ACCESS
In
In random access
random access or
or contention
contention methods, no station is superior to another station and
methods, no station is superior to another station and
none is assigned the control over another. No station permits, or does not permit,
none is assigned the control over another. No station permits, or does not permit,
another station to send. At each instance, a station that has data to send uses a
another station to send. At each instance, a station that has data to send uses a
procedure defined by the protocol to make a decision on whether or not to send.
procedure defined by the protocol to make a decision on whether or not to send.
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
Topics discussed in this section:
Topics discussed in this section:
5. ALOHA
Norman Abramson at University of Hawaii, in 70’s wanted to connect computer centers of all
the islands of Hawaii.
Hawaii is a collection of islands and it was not possible to connect them with telephone lines.
Joining islands with wires laid on seabed was very expensive, so they started thinking about
wireless solution.
Solution: ALOHA
◦ Using short range radios.
◦ Half duplex by nature. At a time, only can send or receiver. Switching also takes time.
◦ Two different frequencies, one for sending, another for receiving.
◦ But, problem of collision, how to solve it?
◦ Solution: Let the users communicate, if signals collide, not acknowledged and so, sender
resends data.
◦ Adding randomness reduces the chance of collision.
◦ Algorithm is called Binary Exponential Back-off Algorithm.
◦ Also had problem: While transmitting, sender can not sense collision.
◦ In ALOHA, maximum 18 out of 100 packets pass without collision if ALOHA works with
optimum speed.
7. Slotted ALOHA
Solution: Slotted ALOHA
◦ Robert, in 1972 proposed a scheme.
◦ Packets are vulnerable to collide with only those packets which were transmitted
before, but not during the lifetime.
◦ He divided timeslots equal to lifetime of packets.
◦ Packet can be transmitted only in beginning of next slot only.
◦ Slotted ALOHA introduces additional delay.
◦ Eg : B is to be transmitted during A’s lifetime, B will be delayed till next slot.
◦ Thus, reducing collision probability to half and performance is doubled.
◦ In slotted ALOHA, 36 out of 100 packets are delivered without collision at optimum
speed.
◦ In slotted ALOHA time is divided into discrete intervals, each corresponding to one
frame.
◦ A computer is not permitted to send whenever it has data to send.
◦ Instead it is required to wait for the next available slot.
Well, it still needs improvement.
See next figures that explain ALOHA and Slotted ALOHA.
18. Carrier Sense Multiple Access
(CSMA)
• Protocols in which stations listen for a carrier (i.e.
transmission) and act accordingly are called carrier
sense protocols.
1. 1-persistent CSMA
Channel Busy Continue sensing until free and then grab.
Channel Idle Transmit with probability 1.
Collision Wait for a random length of time and try again.
2. Non-persistent CSMA:
Channel Busy Does not continually sense the channel. Wait for a random
length of time and try again.
Channel Idle Transmit.
Collision Wait for a random length of time and try again.
18
19. 3. P-persistent CSMA:
Channel Busy Continue sensing until free (same as idle).
Channel Idle Transmit with probability p, and defer transmitting until the
next slot with probability q = 1-p.
Collision Wait for a random length of time and try again.
• Analysis:
• The non-persistent CSMA has better channel utilization but longer delays than 1-
persistent CSMA.
• CSMA are an improvement over ALOHA because they ensure that no station
begins to transmit when it senses the channel busy.
• Another improvement is for stations to abort their transmissions as soon as they
detect a collision.
• Quickly terminating damaged frames saves time and bandwidth.
• This protocol is called CSMA/CD (CSMA with Collision Detection).
BY: DR. BHARGAVI H. GOSWAMI, 9426669020,
19
21. CSMA/CD
Carrier Sense: Ethernet card listen to channel before transmission and differ to
transmit if somebody else is already transmitting.
Multiple Access: More than one user needs channel access.
Collision Detection: Protocol listen when transmission is going on and find stop
transmitting when it finds colliding.
Interframe gap: As soon as channel becomes free, it waits for small interframe gap
and then transmits. Interframe gap is idle time between frames. After a frame has
been sent, transmitters are required to transmit a minimum of 96 bits (12 octets) of
idle line state before transmitting the next frame.
Maximum distance limitation: Frame size min 64 bytes.
Minimum frame size limitation: Frame length min 250 m.
Both, distance and size can not be increased together.
More bandwidth deteriorates performance.
If first 64 bytes are successfully received, means later there would be no collision.
22. Collision Detection &
Avoidance
Collision garble the frames.
Collision Detection:
◦ Let collision happen and then solve it.
◦ If sender detects collision, it can stop sending and restart later by following
‘binary back-off algorithm’.
◦ Need a mechanism to listen to channel.
◦ Used by classic Ethernet.
Collision Avoidance:
◦ See that collision do not occur by carefully avoiding it.
◦ Here, it is possible to extract any component signal from collided signal. So
retransmission is not needed. We just extract what we need from the received
signals.
◦ Preferred by 802.11 wireless LANs.
◦ CDMA Code Division Multiple Access is used in Mobile phones.
23. CSMA/CA
Collision Avoidance with Career Sense Multiple Access.
On Wireless Networks
Strategies:
◦ 1. Inter-frame Spacing (IFS)
◦ 2. Contention Window – Binary Exponential Back off Algorithm
◦ 3. Acknowledgement
24. Wireless LAN Protocol
•Because signal strength is not uniform throughout the space in which wireless
LANs operate, carrier detection and collision may fail in the following ways:
- Hidden nodes:
◦ Hidden stations: Carrier sensing may fail to detect another station. For
example, A and D.
◦ Fading: The strength of radio signals diminished rapidly with the distance
from the transmitter. For example, A and C.
- Exposed nodes:
◦ Exposed stations: B is sending to A. C can detect it. C might want to send
to E but conclude it cannot transmit because C hears B.
◦ Collision masking: The local signal might drown out the remote
transmission.
•The result scheme is carrier sensing multiple access with collision avoidance
(CSMA/CA).
24
25. Wireless LAN Protocols
A wireless LAN.
(a) A transmitting. (b) B transmitting.
25
• Hidden station problem: A is transmitting to B. C cannot hear A.
If C starts transmitting, it will interfere at B.
• Exposed station problem: B is transmitting to A. C concludes that
it may not send to D but the interference exists only between B and
C.
26. MACA and MACAW
MACA: Multiple Access with Collision Avoidance:
The sender transmits a RTS (Request To Send) frame.
The receiver replies with a CTS (Clear To Send) frame.
Neighbors
◦ see CTS, then keep quiet.
◦ see RTS but not CTS, then keep quiet until the CTS is back to the sender.
The receiver sends an ACK when receiving an frame.
◦ Neighbors keep silent until see ACK.
Collisions
◦ There is no collision detection.
◦ The senders know collision when they don’t receive CTS.
◦ They each wait for the exponential backoff time.
MACAW (MACA for Wireless) is a revision of MACA which introduced ACK
mechanism.
Till ACK are seen, other stations remain silent.
26
27. Wireless LAN Protocols (2)
The MACA protocol.
(a) A sending an RTS to B.
(b) B responding with a CTS to A.
BY: DR. BHARGAVI H.
GOSWAMI,
9426669020, 2
34. 12.34
In CSMA/CA, the IFS can also be used to
define the priority of a station or a
frame.
Note
35. 12.35
In CSMA/CA, if the station finds the
channel busy, it does not restart the
timer of the contention window;
it stops the timer and restarts it when
the channel becomes idle.
Note
37. NAV – DIFS – SIFS – PIFS – EIFS – CTS - RTS
12.37
network allocation vector (NAV) that shows how much time must pass before
these stations are allowed to check the channel for idleness.
38. 12.38
12-2 CONTROLLED ACCESS
12-2 CONTROLLED ACCESS
In
In controlled access
controlled access, the stations consult one another to find which station has the
, the stations consult one another to find which station has the
right to send. A station cannot send unless it has been authorized by other stations.
right to send. A station cannot send unless it has been authorized by other stations.
We discuss three popular controlled-access methods.
We discuss three popular controlled-access methods.
Reservation
Polling
Token Passing
Topics discussed in this section:
Topics discussed in this section:
42. 12.42
12-3 CHANNELIZATION
12-3 CHANNELIZATION
Channelization
Channelization is a multiple-access method in which the available bandwidth of a link
is a multiple-access method in which the available bandwidth of a link
is shared in time, frequency, or through code, between different stations. In this
is shared in time, frequency, or through code, between different stations. In this
section, we discuss three channelization protocols.
section, we discuss three channelization protocols.
Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
Code-Division Multiple Access (CDMA)
Topics discussed in this section:
Topics discussed in this section:
43. 12.43
We see the application of all these
methods in Chapter 16 when
we discuss cellular phone systems.
Note
