Data Link Layer and UDP Header

Topic 4: problem sheet
.Q5.The LLC is sometimes described as providing a multiplexed service to upper layer
protocols.
Describe what this statement means and try to give an example of this in action.
Ans. The LLC or the logical link control is the upper layers sub layer on the data link
layer. The logical link controls sub layer provides a multiplexing mechanism which
provides different services to the protocols of the data link layers upper layer.
Example of this action that is the logical link control provides a multiplexed
services to the upper layer protocols of the of the data link layer is when the logical link
control provide services to the network layer.
Q7. Describe why routers require at least 2 network cards.
Ans. The routers are required of at least two of the network cards so that if one network
card if fails then the other network card can give back up to the router so that the connectivity
does not hamper.
Q19. Describe when entries are added and removed from the ARP cache.
Ans. When an ARP command is successful then as a result the new entries are added.
The ARP entries are added as a local traffic.
After a certain time period the entries of the ARP aged out depending on the machines
operating system then the ARP entries are deleted.
Q20. As a Data Link frame moves through an internetwork the Ethernet frame header is
modified at each step.
Describe why this must happen at each router.
Ans. When the data link layer moves in the internetwork the frame header is modified at each
step because the frame header includes various variables like the start frame field, source address and
destination address, priority of the service field, type of the field, logical connection control field,
physical link control field, congestion control field and the flow control field. The mentioned field names
are not specific fields. Different type of the data link layer will use different types of fields. In the each
step the header is modified because each of the header has its own functions and purpose. And when
the data link layer is moving in the internetwork the functions may change like the source and the
destination address will change several times when the layer is moving in the internetwork.
This must happens at the each router for a stable connectivity and to recognize the devices
properly using their functions and the purposes. The source address and the destination address are one

of the main properties of the routers and the frame header consists of the addresses so it is necessary to
modify the frame header when the data link layer is moves in the internetworking in the each router.
Topic 6: problem sheet
Q 5. How many bits are used in a UDP header to store the destination port number?
Explain how you arrived at this number.
Ans.
8 bits used in a UDP header to store the port number of the destination.
The total size of the UDP header is 32bits and the total UDP header is divided into four parts
source port number, destination port number, length and the check sum. The size of the each is
8bits as the total 32 bits is divided in this four parts.