Multiple access protocol

MULTIPLE ACCESS PROTOCOL

PRESENTED BY,
J.MERLIN FLORRENCE
I M.Sc

AGENDA
INTORDUCTION
TYPES OF NETWORK LINK

CHANNEL PARITIONING PROTOCOLS
RANDOM ACCESS
Slotted ALOHA

ALOHA
CSMA
TAKING-TURNS PROTOCOLS
LOCAL AREA NETWORKS

TYPES OF NETWORK LINK
Network link

Point-to-point link

Broadcast link

Point-to-point link:
It consists of a single sender at one end of
the link and a single receiver at the other end of
the link.

BROADCAST-LINK
•It can have multiple sending and receiving nodes
all connected to the same, single shared broadcast
channel.
•Examples:
Ethernet, Wireless LAN

Multiple Access Problem:
How to coordinate the access of multiple
sending and receiving to a shared broadcast channel.

Multiple access protocol
Multiple access protocol is used to coordinate
access to the link.
Nodes can regulate their transmission onto the
shared broadcast channel by using Multiple
access protocol.
It is used both wired and wireless local area
Text
network and satellite network.
All nodes are capable of transmitting frame,
more than two nodes can transmit frames at the
same time.
If so, the transmitted frames collide at all of the
receivers.

When there is a collision, none of the receiving
nodes can make any sense of any of the frames that
were transmitted;
In a sense, the signals of the colliding frames
become inextricably tangled together.

Thus, all frames involved in the collision are lost,
and the broad cast channel as wasted during the
collision interval.
To rectify this problem Multiple access protocol
was implemented

Various multiple access channels

Classification of MAP
Multiple Access
Protocol

Channel-Partitioning
protocols

Random Access
Protocol
Taking-turns
Protocols

Channel Partitioning Protocols
Techniques used to partition a broadcast channel’s
are,
• Time-division multiplexing
• Frequency-division multiplexing
For example, if the channel send N nodes and that
the transmission rate of the channel is R bps.
TDM divides time into time frames and further
divides each time frame into N time slots.
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Each slot time is then assigned to one of the N
nodes.
whenever a node has a packet to send, it transmits
the packet’s bits during its assigned time slot in the
revolving TDM frame.

TDM is appealing because it eliminates collisions and
is perfectly fair : Each node gets a dedicated
transmission rate of R/N bps during each frame time.
DRAWBACKS:
A node is limited to an average rate of R/N bps even
when it is the only node with packets to send.
A node must always wait for its turn in the
transmission sequence-again, even when it is the only
node with frame to send.

A Four-node TDM and FDM example
TDMA: time division multiple access
Example:

Frequency

4 users

time
FDMA: frequency division multiple access

Frequency

time

CDMA: code division multiple access
 Same frequency and time but different codes.

11

FDM
While TDM shares the broadcast channel in time, FDM divides
the R bps channel into different frequencies and assigns each
frequency to one of the nodes.
FDM creates N smaller channels of R/N bps out of the single,
larger R bps channel.

CDMA:
It is Code Division Multiple Access.
While TDM and FDM assign time slot and frequencies
respectively, to the nodes, CDMA assigns a different code to each
node.

RANDOM ACCESS PROTOCOL
When node has packet to send
Sense the channel.
If it is busy, wait for random amount of time and then
retry.
no a prior coordination among nodes.
All nodes use the same time, frequency and code.

Two or more transmitting nodes ➜ “collision”
Random access MAC protocol specifies how to recover
from collisions -> Exponential backoff.

Examples of random access MAC protocols:
CSMA, CSMA/CA, CSMA/CD

Cont…
Most commonly used random access protocols,
ALOHA
CSMA(Carrier Sense Multiple Access Protocols)
Slotted ALOHA
we assume,
All frames consist of exactly L bits.
Time is divided into slots of size L/R seconds
Nodes start to transmit frames only at the beginning of slots
The nodes are synchronized so that each node knows when the
slots begin.
If two or more frames collide in a slot ; then all the nodes detect
the collision event before the slot ends

Let p be a probability ,that is, a number between o and 1.
the operation of slotted ALOHA in each node is.
When the node has a fresh frame to send, it waits until
the beginning of the next slot and transmits the entire
frame in the slot.
If there isn’t a collision, the node has successfully
transmitted its frame and thus need not consider
retransmitting the frame.
If there is a collision, the node detects the collision
before the end of the slot. The node retransmits its frame
in each subsequent slot with probability p until the frame
is transmitted without a collision

Key:
C : Collision
E : Empty set
S : successful slot

Nodes 1,2 and 3 collide in the first slot. Node 2 finally
succeeds in the fourth slot, node 1 n the eight slot, and
node 3 in the ninth slot

Pros and Cons of Slotted ALOHA
Pros
• single active node can continuously transmit at
full rate of channel
• highly decentralized: only slots in nodes need to
be in sync
• Simple protocol

Cons
• collisions, wasting slots
• idle slots
• nodes may be able to detect collision in less than time to
transmit packet
• clock synchronization

ALOHA
It required that all nodes synchronize their
transmissions to start at the beginning of a slot.

When a frame arrives, the node immediately
transmits the frame in it’s the frame in its entirely into
the broadcast channel.
If a transmitted frame experiences a collision with
one or more other transmissions, the node will then
immediately retransmit the frame with probability p.
Otherwise , the node waits for a frame transmission
time

Efficiency is the long-run fraction of successful slots when
there are many nodes, each with many frames to send
Suppose N nodes with many frames to send, each transmits in
slot with probability p
probability that node 1 has success in a slot = p(1-p)N-1
probability that any node has a success = Np(1-p)N-1
For max efficiency with N nodes, find p* that maximizes
Np(1-p)N-1

For many nodes, take limit of Np*(1-p*)N-1 as N goes to
infinity, gives 1/e = .37
Efficiency is 37%, even with optimal p

unsloted Aloha: simpler, no synchronization
when frame first arrives
transmit immediately
collision probability increases:
frame sent at t0 collides with other frames sent in
[t0-1,t0+1]

P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0,t0+1]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
choosing optimum p and then letting n ->
Efficiency = 1/(2e) = .18

...

Carrier Sense Multiple
Access(CSMA)
Rules of CSMA and CSMA/CD
Carrier sensing
Collision detection

CARRIER SENSING:
A node is listens to the channel before transmitting.
If a frame from another nodes currently being transmitted into
the channel, a node then waits a random amount of time and
then again senses the channel.
If the channel is sensed to be idle, the node then begins frame
transmission.
Otherwise, the node waits another random amount of time and
repeats this process.

COLLISION DETECTION:
A transmitting node listens to the channel while it is
transmitting.
If it detects that another node is transmitting an
interfering frame, it stops transmitting and uses some
protocol to determine when it should next attempt to
transmit

CSMA: listen before transmit. If channel is sensed
busy, defer transmission
Persistent CSMA: retry immediately when
channel becomes idle (this may cause instability)
Non persistent CSMA: retry after random interval
of time
Note: collisions may still exist, since two stations
may sense the channel idle at the same time ( or
better, within a “vulnerable” window = round trip
delay)
In case of collision, the entire packet transmission
time is wasted

Space-time diagram of two CSMA nodes will
colliding transmission
Collisions can still occur:
Propagation delay means
two nodes may not hear
each other’s transmission

Collision:
Entire packet transmission
time wasted

Note:
Role of distance & propagation
delay in determining collision
probability

CSMA CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
– collisions detected within short time
– colliding transmissions aborted, reducing channel
wastage
• collision detection:
– easy in wired LANs: measure signal strengths,
compare transmitted, received signals
– difficult in wireless LANs: receiver shut off
while transmitting
• human analogy: the polite conversationalist

CSMA/CD: like in CSMA
collisions are detected within a few bit times
Transmission is then aborted, reducing the
channel wastage considerably
persistent retransmission is implemented
Collision detection is easy in wired LANs:
can measure signal strength on the line
Collision detection cannot be done in wireless
LANs :
CSMA/CD can approach channel utilization =1 in
LANs:
low ratio of propagation over frame transmission
time

Comparison
Channel partitioning MAC protocols:
– share channel efficiently and fairly at high load
– inefficient at low load: delay in channel access, 1/N
bandwidth allocated even if only 1 active node
Random access MAC protocols
– efficient at low load: single node can fully utilize channel
– high load: collision overhead

Both these types of protocols have been used in sensor
networks depending on the application needs.
29

TAKING -TURNS PROTOCOL
Taking Turns MAC protocols:
(a) Polling: Master “invites” slaves
- Request/Clear overhead, latency, single
point of failure
(b) Token passing: token is passed from one node to
the next
+ Reduce latency, improve fault
tolerance
- elaborate procedures to recover from
lost token

Comparison
Token passing:
Polling:
• master node “invites”  control token passed from one
slave nodes to transmit node to next sequentially.
 token message
in turn
 concerns:
• Request to Send,
 token overhead
Clear to Send msgs
 latency
• concerns:
– polling overhead
– latency
– single point of failure
(master)



single point of failure (token)

LOCAL AREA NETWORKS(LANs)
It is a connection of nodes with in a campus.
The transmission rate of R, of most LANs very high.
Classes of LAN technology:
Token passing technology(including token ring)
FDDI
TOKEN RING LAN:

The N nodes of the LAN are connected in a ring by direct
links.
The topology of the ring defines the token-passing order.
When a node obtains the token and sends a frame, the frame
propagates around the entire ring, there by creating a virtual
broadcast channel.

FDDI
It was designed for geographically larger LANs,
including Metropolitan Area Network(MAN).
It is efficient to let a frame propagate back to the
sending node once the frame has passed the destination
node.
FDDI has the destination node remove the frame
from the ring.

Multiple access protocol

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