made easy test series

Question

which of the following is false?

1. In ethernet, each nodes physical address is guaranteed to be globally unique.
2. the single parity check can detect any odd no of bit error in a transmitted codeword.
3. in ethernet , a node wishing to transmit might never be allowed to access the channel.
4. none of these.

Comments

Why C is not false????

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@mahendrapatel

Ethernet follows CSMA/CD,in which there are frequent collisions ,so there is a possibility that every time it tries to transmit there is a collision.

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My bad this explanation is wrong I didn’t read it properly
the explanation in the selected answer is perfect

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@JAINchiNMayits ok bro, but selected answer is wrong i guess,A and B are completely looks wrong

Answer 1

Option A) False. It is possible that 2 different devices have same Physical addresses (MAC addresses) on a global level. This happens because MAC addresses are given by manufacturers and they do reuse the addresses. Also, had the physical addresses been globally unique (and hierarchical) we could have just used those instead of IP addresses for global communication.

Option B) False. The entire parity check scheme can detect only odd bit errors. But here they’ve given SINGLE PARITY CHECK which can detect only 1 bit error.

Option C) False. There is no reason for a station to not get the channel access. All stations continuously sense the channel. Ethernet uses 1 -persistent method with CSMA/CD. Whenever channel is sensed idle, station transmits with 100% probability. While transmitting as well, it keeps on sensing, so even if if there is a collision, the station will go into back-off and try again later.

Comments

for option (A) As far as I know, MAC address have 3 parts -Vendor ID

                                                                                               Date of manufacture

                                                                                               serial no.

so different manufacturer will have different Vendor ID.

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MAC address can be overridden by software in some cases .

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Yes you are right, every manufacturer follows that format. But that is the exact reason why every manufacturer only has a limited number of mac addresses that it can assign to its manufactured devices. Once the Mac pool of the vendor is exhausted, the manufacturer reuses the Mac addresses under the assumption that it is highly unlikely that two devices with same mac will connect to the same network.

So, it is possible for two devices to have same Mac addresses on a global level.