About Data Link Layer Protocols

1. Data Link Layer Protocols always put CRC in there trailer rather than in the header. Why?

Ans: Data Link Layer Protocols always put CRC in there trailer rather than in the header because then you can compute the CRC as you transmit the packet, and then splat out the final CRC at the end. If you put it in the header, you would have to make one pass over the packet computing the CRC, then send the CRC, and then a second pass actually stuffing each byte out the interface.

2. Slotted aloha is an improved version on aloha protocol. On what factors the improvement is implemented in slotted aloha.

Ans: The ALOHA protocol is an OSI layer protocolfor LANnetworks withbroadcast topology. ALOHA is a data transmission protocol which are very effective for the purely data transmission. And moreover no synchronization is required in it.

But In Slotted Aloha the following points are there:

* synchronous system: time divided into slots

* slot size equals fixed packet transmission time

* when Packet ready for transmission,waituntil start ofnextslot

* Packets overlap completely or not at all.

So it is an improved version of aloha protocols.

3.When bit stuffing is used, is it possible for loss, insertion or modification of a single bit to cause an error not detected by checksum? If not why not? If so how? Does checksum play a role there?

Ans: It is possible. If data is 01111110 -> 011111010 if the second 0 is lost due to transmission error, 01111110 will receive, which can be interpreted as end of frame.
The bits before the end of the frame will then be interpreted as the check-sum. If the check-sum is 4 bits, there is 1 chance in 24 that these random bits will be interpreted as a valid check-sum. The longer the check-sum, the less likely that this mistake will happen.

4. Give two reasons why network might use an error correcting code instead of error detection and retransmission?

Ans: In error correction method there is redundant information along with each block of data sent so that it is known that what actually the transmitted data have been but in error detection only the detection is don and that also we don't know where the data has been detected.

So there is preference to error correction like

* Checkparity

* Check sum

* Hamming codes

Q5. Wireless transmission and wired transmission use different set of multiple channel allocation strategies. Why there was a need of avoidance when detection was already available?

Ans: We just get a LAN cable which is usually ships with wireless routers. Connect one end of the cable in wireless router port which is called wan or internet. Another end of the cable connects to your modem. Then restart the modem and the router. Sometimes you might need to change settings for internet connection and DHCP server. But usually it's not required.

6. Bluetooth supports two types of links between a slave and master .What are they and why is each one used for?

Ans: The basic unit of Bluetooth is piconet which consists of master node and seven active slave nodes within a distancve of 10 m. multiple piconets can exist in the same room and can even be connected by bridge node.

The reason for the master and slave design is that the designers intended to facilliatte the implementation of complete Bluetooth chips for under $5. the consequence of this decision is that the slaves are fairly dumb. Basically just doing the master tells them to do.

All communication is between master and slave,direct slave-slave is not possible.

7. Using 5-bit sequence numbers, what is the maximum size of the send and receives windows for each of the following protocols?



1.Stop and wait ARQ

2.Go-back -n ARQ

3.selective repeat ARQ

Send window size




Receive window size




8. If an Ethernet destination address is 07:01:02:03:04:05, what is the type of the address (unicast, multicast, or broadcast)?

Ans: If an Ethernet destination address is 07:01:02:03:04:05, then the type of the address is multicast.

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