Question

1. What are the three persistence protocols that are used with CSMA. (10pt.) 2. Name the two factors that can have influence on flow control. (10pt.) What are the differences between? a: asynchronous and synchronous transmission mode (5 pt.) 3. b:RS232 and null modem. (5 pt.) 4. Consider the packet delay in pure ALOHA versus SLOTTED ALOHA, at low traffi one is less? Explain. (10pt.) Briefly define the differences between IEEE 802.3. IEEE 802.11. (10p.). UG 5. Using CRC digit method show the step that is needed to check the error for the dig where P-8. (10pt). 6.

Answer 1

1)What are the three persistence protocols that are used with CSMA?

Ans:

There Are Three Different Type of CSMA Protocol

(i) I-persistent CSMA :

• In this method, station that wants to transmit data continuously senses the channel to check whether the channel is idle or busy.

• If the channel is busy, the station waits until it becomes idle.

• When the station detects an idle-channel, it immediately transmits the frame with probability 1. Hence it is called I-persistent CSMA.

• This method has the highest chance of collision because two or more stations may find channel to be idle at the same time and transmit their frames.

• When the collision occurs, the stations wait a random amount of time and start allover again.

Drawback of I-persistent  

• The propagation delay time greatly affects this protocol. Let us suppose, just after the station I begins its transmission, station 2 also became ready to send its data and senses the channel. If the station I signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision.

Even if propagation delay time is zero, collision will still occur. If two stations became .ready in the middle of third station's transmission, both stations will wait until the transmission of first station ends and then both will begin their transmission exactly simultaneously. This will also result in collision.

(ii) Non-persistent CSMA

• In this scheme, if a station wants to transmit a frame and it finds that the channel is busy (some other station is transmitting) then it will wait for fixed interval oftime.
• After this time, it again checks the status of the channel and if the channel is.free it will transmit.

• A station that has a frame to send senses the channel.

• If the channel is idle, it sends immediately.

• If the channel is busy, it waits a random amount of time and then senses the channel again.

• In non-persistent CSMA the station does not continuously sense the channel for the purpose of capturing it when it detects the end of previous transmission.

Advantage of non-persistent

• It reduces the chance of collision because the stations wait a random amount of time. It is unlikely that two or more stations will wait for same amount of time and will retransmit at the same time.

Disadvantage of non-persistent

• It reduces the efficiency of network because the channel remains idle when there may be stations with frames to send. This is due to the fact that the stations wait a random amount of time after the collision.

  

(iii) p-persistent CSMA  

• This method is used when channel has time slots such that the time slot duration is equal to or greater than the maximum propagation delay time.

• Whenever a station becomes ready to send, it senses the channel.

• If channel is busy, station waits until next slot.

• If channel is idle, it transmits with a probability p.

• With the probability q=l-p, the station then waits for the beginning of the next time slot.

• If the next slot is also idle, it either transmits or waits again with probabilities p and q.

• This process is repeated till either frame has been transmitted or another station has begun transmitting.

• In case of the transmission by another station, the station acts as though a collision has occurred and it waits a random amount of time and starts again.

Advantage of p-persistent

• It reduces the chance of collision and improves the efficiency of the network.

3)

A).What are the differences between?
a: asynchronous and synchronous transmission mode?

And:

Difference of asynchronous and synchronous transmission mode

Basis	Synchronous Transmission	Asynchronous Transmission
Meaning	In synchronous transmission data is send in the form of blocks or frames
Cost	Synchronous transmission is expensive	Asynchronous transmission is economical
Rate of transmission	The rate of transmission of data is fast in Synchronous Transmission
Examples	Examples of synchronous transmission then face to face interactions, video conferencing, telephonic conversations and chat rooms.	The examples of asynchronous transmission are letters, emails, forums, televisions and radios.