Random access and Polling

Jun 16 • General • 2252 Views • 2 Comments on Random access and Polling

Random access and Polling 

Random Access Description-

  • In case Random access protocol the channel is not divided but a complete bandwidth is given to all users. In case there are one or more  than one node transmitting at the same time then there is to happen a collision in the network. As in any moment when a node has something to transmit it sends it on channel at full data rate R. There is no prior coordination among the nodes and the nodes do not inform each other before transmitting  data on a channel. At  specific time when there are two or even  more transmission taking place simultaneously node collision will take place in the channel.
  • The random access protocol is basically used  for monitoring the collision. It detects collision and also  the way of recovery from that collision. If two collisions take place simultaneously then the protocol will detect them quickly and find the way to recover the lost data between these collisions and it is done by delayed re-transmission. If there is a collision in the channel then it is detected by protocol and due to present transmission then it signifies that some other node is also transmitting  then the protocol at this node  waits for some time and retransmits data. Some of the random access protocols are:

1.Slotted ALOHA



Slotted aloha

In case of slotted aloha same size of frames are created to send data. Here the time is divided into equal size slots which depends on the time taken to transmit one frame. The nodes start their transmission  at the beginning of slot here the  nodes are synchronized  in case two or more number of node transmitting at the same time then all nodes detect collision.


In this case there is no synchronization between the slots and so it is simpler. The nodes can start transmission as soon as they get the frame. In this case the collision probability  increases.

Carrier Sense Multiple Access (CSMA)

  • Carrier sense multiple access (CSMA) is called as  probabilistic media access control (MAC) protocol in which the node verifies the absence of other traffic before transmitting , such as an electrical bus, or a band of the electromagnetic spectrum.
  • “Carrier sense” means  the transmitter uses feedback from the receiver to determine whether another transmission is in progress prior sending the data it also tries to detect the presence of the  carrier wave from another station before transmitting. If the carrier is sensed then the station waits for the transmission in progress to finish before initiating it transmission thus  CSMA is based on the principle of “sense before transmitting” or “listen before speaking”.“Multiple access” signifies that multiple stations sends and also receives on a medium.


  • It is basically a Media Access Control method  and mainly uses the carrier sensing scheme.
  • The transmitting data station detects the another signal while transmitting a frame and stops transmitting that frame ,it transmits a jam signal, and before trying to resend the frame it waits for a random time interval. CSMA/CD is the modification of pure carrier sense multiple access (CSMA). CSMA/CD is mainly used to improve the CSMA performance by terminating the transmission as soon as a collision is detected , it shortens the time required before a retry can be attempted.
  • CSMA/CD is notable for its usage in early Ethernet installations, although this has fallen out of use as Ethernet switches have become commonplace.


  • Carrier sense multiple access with collision avoidance (CSMA/CA) in computer networking, is the network multiple access method in which carrier sensing is used, but the nodes attempt to avoid collisions by transmitting only when the channel is “idle”. When they do transmitting, the nodes transmit their packet data in its entirety.
  • It is particularly important for the wireless networks, where the collision detection of the alternative CSMA/CD is not reliable due to the hidden node problem.
  • CSMA/CA is a protocol and it operates the Data Link Layer (Layer 2) of the OSI model.

Token Passing

  • In telecommunication, the token passing is a channel access method where the signal is called a token and is passed between the nodes that is being authorized by node to communicate. The well-known examples are token ring and ARCNET.
  • This scheme provides round-robin scheduling where  the packets are equally sized and  the scheduling is max-min fair there is an advantage over contention based channel access which is the collisions are eliminated, and the channel bandwidth can also be fully utilized without idle time whenever the demand becomes heavy. The disadvantage which takes place only even when demand is light  a station wishing to transmit must wait for the token  due to which  latency increases.
  • Some types of token passing schemes do not need to be explicitly send a token between the systems because the process of “passing the token” is a implicit function. Example is the channel access method which is being used during “Contention Free Time Slots” in the power lines or phone lines and coaxial cable.


  • Polling, or polled operation in computer science which refers to actively sampling of status of an external device by a client program used as a synchronous activity. Polling is most often used in terms of input or output , and is also referred to as polled input or output and also software-driven Input or output..
  • It is sometimes used synonymously with busy-wait polling . In this situation, when an I/O operation is required, then the computer does nothing other than to check the status of the I/O device until it is ready, at which point the device is accessed. In other words, the computer waits until the device is ready.


  1. What are the uses of  Multiplexing and De-multiplexing?

ANS-In case of the  TCP/IP protocol suite there is one TCP but there are several application programs that want to use its services so to handle this situation TCP does multiplexing and de-multiplexing.

2.  What is a connection termination?

Ans – Any of the two parties involved in exchanging data can close the connection when the connection  in any  one direction is terminated then the other party can continue sending data in the other direction due to which both machines are needed to close the connection in both direction.

3. What is the format  used by UDP packet .Explain?

Ans- Header and data  which contains Source port number as well as the Destination port number and Total length as well as Checksum.

4.Describe the various fields of IP Header format?

Ans-The fields in the IP header that relate to fragmentation which are the identification number as well as the fragmentation flags and also the fragmentation offset.

5.  Give the importance and working of DHCP?

Ans-The DHCP has been originated to provide dynamic configuration it is an extension to BOOTP.

GATE syllabus-

1. Gate syllabus for Electronics and Communication 2014

2. Gate Syllabus for Computer science and Information technology 2014

3. Gate syllabus for Engineering science 2014

IES Syllabus-

1. IES Syllabus for Electronics and telecomm

2. IES Syllabus for General ability


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2 Responses to Random access and Polling

  1. Dinesh says:

    Random access is very good and newly idea arising technology witch will be useful for every one to study and research

  2. Amit Kumar says:

    blog on a very nice topic of networking, really very helpful for the students…

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