1. Answer the following questions related to MAC. (a) The maximurn channel utili

Question

1. Answer the following questions related to MAC. (a) The maximurn channel utilization of pure Aloha is about 1/2eR$18.4%, and that of slotted Aloha is about 1/e 36.8%. Explain why slotted Aloha can achieve higher channel utilization than pure Aloha. [2)] (b) Explain why CSMA is more efficient than slotted Aloba in local area networks. [2 (c) Explain why using Collision Detection (CD) can further improve the efficiency in local area networks. [2 (d) Give an example to show the hidden terminal problem in wireless multi-hop networks

Explanation / Answer

Both Pure ALOHA and SLOTTED ALOHA are the Random access Protocols, that are implemented on the Medium access control (MAC) layer, a sublayer of Data Link Layer.

The purpose of ALOHA protocol is to determine that which competing station must get the next chance of accessing the multiaccess channel at MAC layer.

The main difference between Pure ALOHA and Slotted ALOHA is that the time in Pure Aloha is continuous and Slotted Aloha is discrete.

Now coming to the questions given

(a)

In pure ALOHA ,it just allows every station to transmit the data whenever they have the data to be sent.

When every station transmits the data without checking whether the channel is free or not there is always the possibility of collision of data frames. If the frame is damaged or collided then the stations wait for random amount of time and retransmits the frame till it transmits successfully.

The formula to calculate throughput is S = G * e^ -2G ---> 1.

where G: no of stations who wants to transmit .

Here vulnerable time is 2Tt where Tt is the transmission time.

So by differentiating equation 1 with respect to G and equating to zero we get G=1/2

Now substituting G=1/2 in equation 1 => Smax =1/2 * (e^ -1) = 0.184 = 18.4%

SLOTTED ALOHA

In slotted Aloha the time is divided into discrete intervals called time slots. Each time slot corresponds to length of the frame.

In this it doesnot allow to transmit the data whenever it is available. The station should wait till the next time slot begins and allow each data frame to be transmitted in the new time slot.

The formula to calculate throughput is S = G * e^ -G ---> 2.

where G: no of stations who wants to transmit .

Here vulnerable time is Tt where Tt is the transmission time.

So by differentiating equation 2 with respect to G and equating to zero we get G=1

Now substituting G=1 in equation 2 => Smax =1 * (e^ -1) = 1/e = 0.368 = 36.8%

Finally the conclusion is => In S-ALOHA frames may only be sent within slots, so the period of vulnerability is half that of pure ALOHA

(b)

The main difference between Aloha and CSMA is that Aloha protocol doesnot try to detect whether the channel is free before transmitting but the CSMA protocol verifies that the channel is free before transmitting data.

Thus CSMA protocol avoids clashes before they happen while Aloha protocol detects that a channel is busy only after a clash happens. Due to this CSMA is more suitable for networks such as Ethernet (local area networks) where multiple sources and destinations use the same channel.

(c)

CSMA/CD adds collision detection to CSMA, so that the medium is freed as soon as a collision is detected rather than continuing to transmit a frame that has suffered a collision. If the time it takes to detect a collision is significantly less than the time to transmit a frame, then CD can really helpful in increasing efficiency.

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