Tag: CSMA CD

Polling in Networking | Access Control Method

Access Control in Networking-

 

Before you go through this article, make sure that you have gone through the previous article on Access Control.

 

We have discussed-

  • Access Control is a mechanism that controls the access of stations to the transmission link.
  • Broadcast links require the access control mechanism.
  • There are various access control methods-

 

 

  1. Time Division Multiplexing
  2. Polling
  3. CSMA / CD
  4. Token Passing
  5. Aloha

 

In this article, we will discuss about polling.

 

Polling-

 

In this access control method,

  • A polling is conducted in which all the stations willing to send data participates.
  • The polling algorithm chooses one of the stations to send the data.
  • The chosen station sends the data to the destination.
  • After the chosen station has sent the data, the cycle repeats.

 

Example-

 

 

Here-

  • Tpoll = Time taken for polling
  • Tsend = Time taken for sending the data = Transmission delay + Propagation delay = Tt + Tp

 

Efficiency-

 

Efficiency (η) = Useful Time / Total Time

 

  • Useful time = Transmission delay of data packet = Tt
  • Useless time = Time wasted during polling + Propagation delay of data packet = Tpoll + Tp

 

Thus,

 

 

Advantages-

 

  • Unlike in Time Division Multiplexing, no slot is ever wasted.
  • It leads to maximum efficiency and bandwidth utilization.

 

Disadvantages-

 

  • Time is wasted during polling.
  • Link sharing is not fair since each station has the equal probability of winning in each round.
  • Few stations might starve for sending the data.

 

Important Formulas-

 

  • Efficiency (η) = Tt / (Tpoll + Tt + Tp)
  • Effective Bandwidth / Bandwidth Utilization / Throughput = Efficiency(η) x Bandwidth
  • Maximum Available Effective Bandwidth = Total number of stations x Bandwidth requirement of 1 station

 

To gain better understanding about Polling Access Control Method,

Watch this Video Lecture

 

Next Article- CSMA / CD | Access Control Method

 

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Time Division Multiplexing | Access Control

Access Control in Networking-

 

Before you go through this article, make sure that you have gone through the previous article on Access Control.

 

We have discussed-

  • Access Control is a mechanism that controls the access of stations to the transmission link.
  • Broadcast links require the access control mechanism.
  • There are various access control methods-

 

 

  1. Time Division Multiplexing
  2. Polling
  3. CSMA / CD
  4. Token Passing
  5. Aloha

 

In this article, we will discuss about Time Division Multiplexing (TDM).

 

Time Division Multiplexing-

 

In Time Division Multiplexing (TDM),

  • Time of the link is divided into fixed size intervals called as time slots or time slices.
  • Time slots are allocated to the stations in Round Robin manner.
  • Each station transmit its data during the time slot allocated to it.
  • In case, station does not have any data to send, its time slot goes waste.

 

Example-

 

 

Size Of Time Slots-

 

The size of each time slot is kept such that each station gets sufficient time for the following tasks-

  • To put its data packet on to the transmission link
  • Last bit of the packet is able to get out of the transmission link

 

Thus,

 

Size of each time slot = Tt + Tp

 

where-

  • Tt = Transmission delay
  • Tp = Propagation delay

 

NOTE-

 

To keep the size of time slots constant,

  • We have assumed that all the stations want to send the packets of same size.
  • This keeps Tt constant for all the stations.
  • We have considered the worst case when both the stations are present at the two extreme ends.
  • This ensures Tp will be maximum and all the stations will get sufficient time to propagate their data.

 

Efficiency-

 

Efficiency (η) = Useful Time / Total Time

 

  • Useful time = Transmission delay of data packet = Tt
  • Useless time = Propagation delay of data packet = Tp

 

Thus,

 

 

Important Formulas-

 

  • Size of each time slot in Time Division Multiplexing = Tt + Tp
  • Efficiency (η) = 1 / (1+a) where a = Tp / Tt
  • Effective Bandwidth / Bandwidth Utilization / Throughput = Efficiency(η) x Bandwidth
  • Maximum Available Effective Bandwidth = Total number of stations x Bandwidth requirement of 1 station

 

Disadvantage-

 

  • If any station does not have the data to send during its time slot, then its time slot goes waste.
  • This reduces the efficiency.
  • This time slot could have been allotted to some other station willing to send data.

 

PRACTICE PROBLEM BASED ON TIME DIVISION MULTIPLEXING (TDM)-

 

Problem-

 

If transmission delay and propagation delay of a packet in Time Division Multiplexing is 1 msec each at 4 Mbps bandwidth, then-

  1. Find the efficiency.
  2. Find the effective bandwidth.
  3. How many maximum stations can be connected to the network if each station requires 2 Kbps bandwidth?

 

Solution-

 

Given-

  • Transmission delay (Tt) = 1msec
  • Propagation delay (Tp) = 1msec
  • Bandwidth = 4 Mbps

 

Part-01:

 

For a TDM Network,

 

Efficiency (η) = 1 / 1+a where a = Tp / Tt

 

Calculating Value Of ‘a’-

 

a = Tp / Tt

a = 1 msec / 1 msec

a = 1

 

Calculating Efficiency-

 

Efficiency (η)

= 1 / (1+a)

= 1 / (1 + 1)

= 1 / 2

= 0.5

= 50%

 

Part-02:

 

We know-

 

Effective Bandwidth = Efficiency (η) x Bandwidth

 

Thus,

Effective Bandwidth

= 0.5 x 4 Mbps

= 2 Mbps

 

Part-03:

 

We know-

 

Maximum Effective Bandwidth

= Total number of stations x Bandwidth requirement of 1 station

 

Let the total number of stations that can be connected be N.

Then, we have-

2 Mbps = N x 2 Kbps

N = 1000

Thus, maximum 1000 stations can be connected.

 

To gain better understanding about Time Division Multiplexing (TDM),

Watch this Video Lecture

 

Next Article- Polling | Access Control Method

 

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Access Control in Networking | Methods

Types of Communication Links-

 

In computer networking,

  • Communication links enable the stations to communicate with each other.
  • Stations may communicate using the following types of links-

 

 

  1. Point to Point Link
  2. Broadcast Link

 

1. Point to Point Link-

 

  • Point to Point link is a dedicated link that exists between the two stations.
  • The entire capacity of the link is used for transmission between the two connected stations only.
  • Depending upon the Type Of Channel, the data flow takes place between the stations.

 

Example-

 

 

2. Broadcast Link-

 

  • Broadcast link is a common link to which multiple stations are connected.
  • The capacity of the link is shared among the connected stations for transmission.

 

Example-

 

 

Access Control-

 

Access Control is a mechanism that controls the access of stations to the transmission link.

 

  • Broadcast links require the access control.
  • This is because the link is shared among several stations.

 

Need of Access Control-

 

To prevent the occurrence of collision or if the collision occurs, to deal with it.

 

Consider a situation where-

  • Multiple stations place their data packets on the link and starts transmitting simultaneously.
  • Such a situation gives rise to a collision among the data packets.
  • Collision of data packets causes the data to get corrupt.

 

Example-

 

Consider the following scenario-

 

 

Here,

  • Two stations A and D starts transmitting their data packets simultaneously.
  • This situation gives rise to a collision between the data packets transmitted by them.
  • Thus, to prevent the collision or to deal with it, access control is needed.

 

Access Control Methods-

 

Access control methods are the methods used for providing access control.

 

  • They prevent the collision or deal with it and ensures smooth flow of traffic on the network.
  • They are implemented at the data link layer of the OSI reference model.

 

Various access control methods used are-

 

 

  1. Time Division Multiplexing
  2. Polling
  3. CSMA / CD
  4. Token Passing
  5. Aloha

 

We will discuss all these methods one by one in detail.

 

To gain better understanding about Access Control,

Watch this Video Lecture

 

Next Article- Time Division Multiplexing (TDM)

 

Get more notes and other study material of Computer Networks.

Watch video lectures by visiting our YouTube channel LearnVidFun.