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CHAPTER 6
ACCESS METHODS
TYPES OF LINKS
Data communication may involve single transmitter
with 1 or more receivers, or multiple receivers with 1
or more receivers.
 Communication channel that connects adjacent
computers along the communication path is called a
LINK.
 2 types of network links:

Point-to-point link
 Broadcast link

POINT TO POINT LINK
Involves only ONE transmitter and ONE receiver or
a link which directly connects transmitter and
receiver.
 Data transmission on this kind of link is called pointto-point transmission.
 Sender only transmits data that is intended to be
used by specific receiver

BROADCAST LINK
Involves single transmitter with multiple receivers or
multiple transmitters with multiple receivers.
 Data transmission between this link is called
broadcast transmission.
 Sends signal to all direction, all receivers whether
they use it or not. They are simple & quick to
implement
 Ex: TV stations transmitting signal from its tower to
thousand of homes.

LAN MEDIA ACCESS METHODS
Data on network media must follow certain rules to
avoid collisions with other data.
 Multiple devices & computers in a LAN cannot talk
simultaneously over same media at same time as it
would result in media contention(conflict)
 Access methods ensure that the sending and receiving
of network data is an orderly process(regulate the flow).
 Access methods;

Carrier Sense Multiple Access with Collision Detection
(CSMA/CD)
 Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA)
 Token Passing
 Demand Priority

CARRIER SENSE MULTIPLE ACCESS WITH
COLLISION DETECTION (CSMA/CD)
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When 2 computers send data over the media at the same
time, it will cause collision & the data become unusable.
To avoid it use CSMA/CD (IEEE 802.3) protocol.
CSMA/CD forces computers to ‘listen’ to the Ethernet
media to make sure no other computer on the same
media is transmitting data before sending their own data.
If a collision is heard, both computers will send a jam
signal over the Ethernet which will indicate the jam to all
other computers on the same LAN so that no one will
send data on the same media.
Then they will wait a random amount of time before
repeating the entire process & it helps to avoid the same
from happening again.
HOW CSMA/CD WORKS?
All computers stay quiet if another computer is talking
 Any computer that wants to talk, have to listen to the
media first & wait for a predetermined time
 If after waiting, the media is silent, then they can send
data
 If both computer wait & both talk together, a collision
will occur.
 Other computers recognize the collision & re-enforce it
by talking
 Then, all computers stop talking
 Each computer picks a random number & counts
backward to zero.
 When either one finish first, and begins talking, it will
cause other computers to wait.

CARRIER SENSE MULTIPLE ACCESS WITH
COLLISION AVOIDANCE(CSMA/CA)
Specified by IEEE 802.11 standard especially for
wireless LANs.
 It is not possible for wireless LANs to listen & send
data at the same time, therefore collision detection is
not possible in wireless LANs, therefore CSMA/CA is
used
 Other reason is the hidden terminal problem in which
one computer may not know whether the other
computer is transmitting or not if they are not in range.
 Backoff factor = determines the amount of the time the
computer must wait until its allowed to transmit data.
 Not suitable for radio frequency transmissions.

CONTROL TOKEN
Ina token network (either token bus / token ring)
control token is send over the network, & only the
computer having the token can send data over the
network.
 Once its done sending data, it releases the token over
the network & other computers will poses it.
 2 types of control tokens are:

Token Bus
 Token Ring

CONTROL TOKEN ( TOKEN RING – IEEE
802.5)
Works by passing a single token from one computer to
another over the network.
 To send data, a computer must wait for the token to
reach it, then attach the data to the token & returns
both the data & token to the network.
 Token passing are Deterministic, which means that its
possible to calculate the maximum time that will pass
before the any end station capable of transmitting.
 Ideal for applications in which delay must be
predictable, & robust network operation is important
 Ex: factory automation

CONTROL TOKEN ( TOKEN BUS – IEEE
802.4, ISO 8802/4)
Physically the network is a bus, however logically its
treated as a ring.
 Only the computer with the token has the right to
transmit data for a certain time.
 Once the time ends, it passes the token to the
following station.
 No collision & full bandwidth utilization.
 Workstations also has priority system in which data is
categorized according to its priorities, so when the
station has the right, it will transmit the data with
highest priority ie, audio, video
 The end of the networks never meets!

SLOTTED RING

Basically a ring networks that allows
unidirectional data transmission between data
stations by transffering data in a predefined slots
in the transmission stream over one transmission
medium.
HOW SLOTTED RING NETWORK WORKS?
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Each station contains a shift register to introduce delay.
Monitor station initiates the ring by inserting a stream of
continuously circulating bits
The bit stream is divided into a fixed number of slots, each
capable of carrying a small frame
Initially all slots are marked empty by the monitor
When a station wants to transmit, it wait for the empty
slot, fills slot with frame containing source/destination
address & data
When frame passes destination station, it is copid into
buffer & the response bits at the end of the frame is
altered accordingly (received, ignored, selected)
The frame returns to source station which then marks the
slot as empty again.
DISADVANTAGES OF SLOTTED RING
Special (vulnerable) monitor node is required to
maintain the slot structure
 The transmission of a typical link level frame
requires several slots, since each slot only
contains 16 bit of data.

COMPARISON OF ETHERNET & TOKEN RING
Ethernet
Token ring
CSMA/CD offers simplest,
widely available mechanism
that gives lowest average
delays for a lightly loaded LAN
Token ring gives each station
equal opportunity on heavily
loaded LAN while CSMA/CD
will fail due to multiple collision
Access to channel on
CSMA/CD is probabilistic – it
has no priority mechanism
Token passing has average
longer delay, but its
deterministic under heavy loads.
Hence used in MAP & real time
applications
CSMA/CD uses a passive
transmission medium
(regenerators are not required
at each station) therefore
inherently more reliable
Token ring interface is active –
hence require mechanism to
cope with stations being
switched off / failing
DEMAND PRIORITY (IEEE 802.12)
New access method designed for the 100Mbps
Ethernet standard known as 100VG-AnyLAN.
 Its based on the fact that repeaters & end notes are
the two components that make up all 100VGAnyLAN networks.
 Repeaters manages network access by doing roundrobin searches for requests to send from all nodes on
the network.
 Repeater/hub is responsible for noting all addresses,
links & end-nodes and verifying that they are all
functioning.

COMPARISON OF DIFFERENT MEDIUM
CONTROL ACCESS METHODS
Feature/
function
CSMA/CD CSMA/CA Token
Passing
Demand
Priority
Communicati
on
Broadcast
based
Hub based
Access
method
Contention Contention NonContention
Contention
Network
Ethernet
Broadcast
based
Local Talk
Wireless
Token
based
Token
Ring
ArcNet
100VG –
AnyLAN
MULTIPLEXING


3 types : TDMA, CDMA & FDMA
Time Division Multiple Access (TDMA)
 Method for allocating access to communication
channels.
 Allow large number of users to access a single-radiofrequency channel without interference.
 The frequency band is divided into a number of
channels, which are stacked onto short time units, so
several users can share single channel without
interfering.
 Used in Global System for Mobile
Communications(GSM) & Personal Digital Cellular
(PDC) systems.
 Also used in satellite systems, LANs, physical security
systems & other combat-net radio systems.
MULTIPLEXING

Frequency Division Multiple Access (FDMA)
 Oldest & most important of the 3 main ways for
multiple radio transmitters to share the radio
spectrum.
 Other methods are TDMA & Code Division Multiple
Access(CDMA)
 Given radio frequency is divided into smaller
frequency
 A control mechanism is used to ensure 2 or more
earth stations do not transmit in the same
subdivision at the same time, so phones use different
allocated frequency at the same time.
MULTIPLEXING

Code Division Multiple Access (CDMA)
 Also called spread spectrum
 Transmitter encodes the signal using a pseudorandom sequence.
 The sequence is known by the receiver to decode the
received signal.
 Each different random sequence corresponds to
different communication channel.
 CDMA is not a modulation scheme like TDMA or
FDMA, so it does not divide the channel by time or
frequency.
 Used in GPS and satellite system.