Controlled Access Protocols

CONTROLLED ACCESS
 In controlled access, the stations consults each
other to find which station has right to send.
 Controlled access protocols grants permission to
send only one node at a time, to avoid collision of
messages on the shared medium.
 A station cannot send data unless it is authorized
by the other stations.

CONTROLLED ACCESS
 Now we will discuss three named controlled access
methods.
 1. Reservation.
Ex: cable modem, DQDB
 2.Polling
 Ex: HDLC(normal response mode)
 3.Token Passing
 Ex: Token Ring, Token Bus.

RESERVATION
 In the reservation method, a station needs to make
a reservation before sending data.
 Time is divided into intervals.
 In each interval, a reservation frame precedes the
data frames sent in that interval

RESERVATION
 If there are N stations in the system, there are
exactly N reservation mini slots in the reservation
frame.
 Each mini slot belongs to a station. When a station
needs to send a data frame, it makes a reservation
in its own mini slot.
 The stations that have made reservations can send
their data frames after the reservation frame

RESERVATION
 a situation with five stations and a five minislot
reservation frame.

 To impose order on a network of independent users
and to establish one station in the network as a
controller that periodically polls all other stations which
is called Polling.
 There are two general polling policies:
i. Round Robin Order
ii. Priority Order

 It works with topologies in which one device is
designated as a Primary Station and the other
devices are Secondary Stations.
 The Primary device controls the link, where as the
secondary follows it’s instructions.
 Exchange of data must be made through the
primary device even though the final destination is
secondary.

SELECT FUNCTION:
 Whenever primary has something to send, it sends
the message to each node.
 Before Sending the data, it creates and transmits a
Select(SEL) frame, one field of it includes the
address of the intended secondary.
 While sending, the primary should know whether the
target device is ready to receive or not.

 Hence, it alerts the secondary for the upcoming
transmission and wait for an acknowledgement
(ACK) of secondary’s status.

POLL FUNCTION:
 When the primary is ready to receive data, it must
ask (poll) each device if it has anything to send.
 If the secondary has data to transmit, it sends the
data frame. Otherwise, it sends a negative
acknowledgement(NAK) .
 The primary then polls the next secondary. When
the response is positive (a data frame), the primary
reads the frame and returns an acknowledgment
(ACK).

 There are two possibilities to terminate the
transmission: either the secondary sends all data,
finishing with an EOT frame, or the primary says
timer is up.

Advantages:
 Priorities can be assigned to ensure faster access
from some secondary .
 Maximum and minimum access times and data
rates on the channel are predictable and fixed.
Drawbacks:
 High dependence on the reliability of the controller.
 Increase in turn around time reduces the channel
data rate under low loads and it’s throughput.

FEATURES:
 A Station is authorized to send data when it receives a
special frame called a Token.
 Stations are arranged around a ring (physically or
logically)
- A Token circulates around a ring
 If a station needs to send data ,it waits for the token
 The Station captures the token and sends one or more
frames as long as the allocated time has not expired
 It releases the token to be used by the successor
station.
Token

Station Interface is in two states :
o Listen state: Listen to the arriving bits and check the
destination address to see if it is its own address. If yes the
frame is copied to the station otherwise it is passed through
the output port to the next station.
o Transmit state: station captures a special frame called free
token and transmits its frames. Sending station is responsible
for reinserting the free token into the ring medium and for
removing the transmitted frame from the medium.
bits are copied to the output bits
with a one bit delay Bits are inserted by the station
listen mode
1 bit delay
transmit mode
delay
to station from station
input
from
ring
output
to
ring
to station from station

Token Management :
 We need token management , if there is a loss of token or it
is destroyed when a station fails
 We can assign priorities as which station can receive the
token.
Network Topology :
o The way in which different systems and nodes are
connected and communicate with each other is
determined by topology of the network.
 Topology can be physical or logical.
Physical Topology is the physical layout of nodes,
workstations and cables in the network
logical topology is the way information flows between
different components.

TYPES OF LOGICAL RINGS :
 Physical Ring Topology
The way that the workstations are connected to the
network through the actual cables that transmit data is
same as the physical structure of the network .

DUAL RING TOPOLOGY :
 A network topology in which two concentric rings
connect each node on a network
 Typically, the secondary ring in a dual-ring topology is
redundant.
 It is used as a backup in case the primary ring fails.
 In these configurations, data moves in opposite
directions around the rings.

BUS RING TOPOLOGY :
 All the nodes are connected to the single cable called
bus. Every workstation communicates with the other
device through this Bus.
 A signal from the source is broadcasted and it travels to
all workstations connected to bus cable.
 only the intended recipient, whose MAC address or IP
address matches, accepts it.
 If the MAC /IP address of machine doesn’t match with the
intended address, machine discards the signal.

STAR TOPOLOGY :
 In Star topology, all the components of network are
connected to the central device called “hub” which
may be a hub, a router or a switch.
 Hub acts as a junction to connect different nodes
present in Star Network, and at the same time it
manages and controls whole of the network.

MESH TOPOLOGY :
 In a mesh network topology, each of the network node,
computer and other devices, are interconnected with
one another.
 Every node not only sends its own signals but also
relays data from other nodes.

TREE TOPOLOGY :
 Tree Topology integrates the characteristics of Star and
Bus Topology.
 number of Star networks are connected using Bus.

Controlled Access Protocols

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