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EL873 / 2020 
(2 hours) 
There are FOUR questions. Candidates must answer THREE questions. 
turn over 
Q1 (i) In your own words, explain why synchronisation and ranging are 
required for upstream data transmission using (co-axial) cable modems. 
Describe the synchronisation and ranging procedures used in the Data 
Over Cable Service Interface Specification (DOCSIS) standard. 
[7 marks] 
(ii) Describe, in your own words, the principles involved in the scheduling 
for upstream transmissions in DOCSIS. 
[3 marks] 
(iii) A 5G (5th Generation) Fixed Wireless Access (FWA) system operates 
with the parameters shown in Table 1.1. 
Table 1.1 
System nominal bandwidth BW = 100 MHz 
FFT size 4096 
Oversampling factor n = 1.2288 
Guard period ratio G = 1/14 
Data subcarriers Ndata = 3300 
Pilot subcarriers Npilots= 256 
(a) What is the subcarrier spacing? 
(b) How many guard and null subcarriers are there? 
(c) What is the useful symbol time? 
(d) What is the total symbol duration? 
(e) What is the raw data rate in this system for 64 Quadrature 
Amplitude Modulation (QAM)? 
[6 marks] 
(iv) Referring to your answers for (iii), discuss the resilience of the system to 
multipath and Doppler effects. 
[4 marks] 
2 EL873 / 2020 
Q2 (i) With reference to Synchronous Digital Hierarchy (SDH) networks, 
describe, in your own words, the operation of: 
(a) add-drop multiplexers 
(b) cross-connects. 
[6 marks] 
(ii) In your own words, describe the reasons why the Optical Transport 
Network (OTN) standard specifications were developed, instead of 
simply extending SDH to higher rates. Your discussion should include 
the use of forward error correction (FEC) and tandem connection 
monitoring (TCM). 
[9 marks] 
(iii) Sketch the wavelength (virtual) topology for the unidirectional optical 
ring network shown in Figure 2.1, where the nodes are optical 
(wavelength) add-drop multiplexers at which the labelled wavelengths 
are added and dropped. 
[5 marks] 
Figure 2.1 
2, 3, 4 
1, 4, 5 
2, 5, 6 
1, 3, 6 
3 EL873 / 2020 
tun over 
Q3 (i) Contrast the Expedited Forwarding and Assured Forwarding standard 
Differentiated Services (DS) approaches. 
[3 marks] 
(ii) In your own words, discuss how different switch/router queuing 
disciplines, such as weighted fair queuing and strict priority queuing, 
can help provide for different quality of service for different traffic flows 
or classes. 
[4 marks] 
(iii) Describe, in your own words, why Content Distribution Networks 
(CDNs) are needed by content providers, such as video-on-demand 
service providers. What are the trade-offs involved in distributing 
content to greater numbers of local data centres? 
How can IP Anycast addresses be used in server cluster selection? 
[7 marks] 
(iv) In Multi-Protocol Label Switching (MPLS), state two ways in which 
label-switched paths can be set up. 
[2 marks] 
(v) In Multi-Protocol Lambda Switching (MPS), describe the equivalence of 
the functions in an optical cross-connect (OXC) with those provided in a 
label switch router (LSR). Discuss the two levels of operation that 
should be provided in a MPS OXC. 
[4 marks] 
4 EL873 / 2020 
Q4 (i) Describe, in your own words, the processes involved in call setup and 
information transfer in a voice/video over IP call using Session Initiation 
Protocol (SIP). How is a user permitted to move to different locations? 
[4 marks] 
(ii) State the advantages and disadvantages of using Hypertext Transfer 
Protocol (HTTP) for video streaming instead of Real-Time Transport 
Protocol/Real-Time Transport Streaming Protocol (RTP/RTSP). 
Refer specifically to the use of Dynamic Adaptive Streaming over HTTP 
[5 marks] 
(iii) Describe, in your own words, the upstream channel access in current 
Passive Optical Networks (PONs). What is the key difference between 
downstream and upstream transmission and channel access in IEEE 
and ITU-T PONs? 
[6 marks] 
(iv) How can wavelength be used to provide a point-to-point logical overlay 
in a (point-to-multipoint) PON? Describe, in your own words, how 
different generations of PON technologies can thereby co-exist. 
[5 marks] 
IEEE: Institute of Electrical and Electronics Engineers 
ITU-T: International Telecommunications Union – Telecommunications 
Standardisation Sector 
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