CNA Practice
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Link Layer

24 questions · 12 topics

2013 Q4(a)Link LayerCSMA/CD· 2 marks

If clients listen for an empty link before transmitting, how is it possible that collisions can occur?

Ethernetcollision
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2013 Q4(b)Link LayerError Detection· 4 marks

“All links are error-prone and no link with errors can be made error-free.” Carefully explain whether it is true or false based on what you have learned regarding the link layer, the physical layer and strategies employed for error correction.

EDCFECreliability
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2013 Q4(c)(i)Link LayerCRC· 1 marks

The generator function G for your Layer 2 Protocol’s Cyclic Redundancy Check is 1001. You have received the data D, 101000011. Answer the following questions with reference to G and D. You may assume that any error detection bits are found after the data bits. What is the largest burst error that we can detect with G?

EDCburst error
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2013 Q4(c)(ii)Link LayerCRC· 2 marks

The generator function G for your Layer 2 Protocol’s Cyclic Redundancy Check is 1001. You have received the data D, 101000011. Answer the following questions with reference to G and D. You may assume that any error detection bits are found after the data bits. Which of the bits in D are the error detection bits and what is their value?

EDC
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2013 Q4(c)(iii)Link LayerCRC· 5 marks

The generator function G for your Layer 2 Protocol’s Cyclic Redundancy Check is 1001. You have received the data D, 101000011. Answer the following questions with reference to G and D. You may assume that any error detection bits are found after the data bits. Has this data been transmitted correctly? Provide all working required to justify your answer.

EDCXOR division
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2013 Q4(d)Link LayerMPLS· 6 marks

Your corporate network incorporates an MPLS network purchased from an ISP to link your US and Australian offices. Explain how a packet in your offices in Sydney is routed through the MPLS network to arrive in San Francisco.

label switchingtraffic engineering
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2013 Q4(e)Link Layer802.11· 4 marks

Why do we need to send an acknowledgement in 802.11 protocols when a frame is successfully received?

wirelessCSMA/CAhidden terminal
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2013 Q4(f)Link LayerSpanning Tree Protocol· 3 marks

What is the role of the Spanning Tree Protocol in Layer 2 networks?

switchingloops
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2013 Q4(g)Link LayerToken Ring· 3 marks

Why would token ring protocol be inefficient if a LAN had a very large perimeter? Explain your answer in terms of the average length of frame length, L (bytes), and the transmission rate of the links, R (bytes per second) and propagation delay.

token passingefficiency
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2013 Q4(h)Link LayerARP· 2 marks

Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within frame with a specific destination MAC address?

Ethernetaddress resolution
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2014 Q4(a)Link LayerMPLS· 6 marks

Explain how a packet enters a Multi-Protocol Label Switching (MPLS) network, how it is routed through the network and how it is ultimately delivered. Consider using a simple diagram to help you explain your answer.

label switchingtraffic engineering
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2014 Q4(b)Link LayerSpanning Tree Protocol· 4 marks

Briefly explain why we need to use Spanning Tree Protocol to shut down specific ports when we have redundant connections in a Layer 2 network.

switchingloops
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2014 Q4(c)Link LayerCSMA/CD· 6 marks

Explain how CSMA/CD is implemented in Ethernet. You may express your answer as an algorithm if you wish.

Ethernetmedium accesscollision
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2014 Q4(d)Link LayerPPP· 3 marks

How is the address field used in PPP? Justify your answer.

PPPframing
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2014 Q4(e)Link LayerError Detection· 2 marks

Why do we carry out error detection and/or correction at Layer 2, given that it is carried out at Layers 3 and 4 as well?

EDCreliability
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2014 Q4(f)Link LayerCSMA/CD· 2 marks

Explain how the use of full-duplex mode in a LAN can reduce collisions to zero.

Ethernetswitching
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2014 Q4(g)Link LayerALOHA· 8 marks

Given that lim_{N→∞} (1 − 1/N)^N = 1/e, derive the maximum efficiency of slotted ALOHA.

medium accessefficiency
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2015 Q5(a)Link LayerALOHA· 4 marks

Pure Aloha (sender may start transmission during any time) has a maximum efficiency that is half that of Slotted Aloha (sender can only start transmission at synchronised time slots). Explain in detail why Pure Aloha is only able to achieve half the efficiency.

medium accesscollision
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2015 Q5(b)Link LayerMultiple Access· 8 marks

Suppose 3 hosts share a 2 Mbps data link. Each host transmits at 1 Mbps when it has access to the link. Also suppose each host transmits 20 percent of the time. You are told to propose an access control mechanism for sharing the 2 Mbps broadcast channel. Clearly explain the data rates achievable with each of the following schemes in this scenario: TDMA, FDMA, slotted ALOHA, pure ALOHA),

medium accessTDMAFDMAALOHA
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2015 Q5(c)Link LayerCSMA/CD· 4 marks

CSMA/CD efficiency is governed by the equation shown. There are two ways to improve the efficiency of a CSMA/CD network 1) increase the transmission delay or 2) decrease the propagation delay. Explain how each of these could be achieved in an actual network without changing the medium of the link (ie you can’t change from optical fibre to copper). efficiency = 1/1 + 5tprop/ttrans

Ethernetefficiency
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2015 Q5(d)Link LayerMPLS· 4 marks

Consider the Multi-Protocol Label Switching (MPLS) network shown in Figure 3, and suppose that routers R5 and R6 are now MPLS enabled. Suppose that we want to perform traffic engineering so that packets from R6 destined for A are switched to A via R6-R4-R3-R1, and packets from R5 destined for A are switched via R5-R4-R2-R1. Show the MPLS tables in R5 and R6, as well as the modified table in R4, that would make this possible. (Clearly indicate the name of the router and clearly draw the routing table where required to show additions or changes to routing tables. You do not need to reproduce the network)

label switchingtraffic engineering
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2015 Q5(e)Link LayerARP· 4 marks

Why is an ARP query sent within a broadcast frame? Is an ARP response sent within a broadcast frame and why or why not?

Ethernetaddress resolution
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2015 Q5(f)Link LayerParity· 4 marks

Suppose the information content of a packet is the bit pattern 1110 0110 1001 1101 and an even parity scheme is being used. What would the value of the field containing the parity bits be for the case of a two-dimensional parity scheme? Your answer should be such that a minimum-length EDC (error dectection and correction) field is used.

EDCparity
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2015 Q5(g)Link LayerCSMA/CD· 3 marks

Given that in a CSMA/CD protocol using binary exponential back off, the adapter waits K × 512 bit times after a collision, where K is drawn randomly. What is the maximum length of time the adapter will wait until the next transmission attempt given 7 collision has occured so far. Assume that you have a 10 Mbps broadcast channel?

Ethernetbackoff
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