Visvesvaraya Technological University (VTU) 2007 B.E Computer Science on mobile communications - Question Paper

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3. Medium Access Control
3.1 What is the main physical cause for the failure of many MAC schemes known from
wired networks? What is done in wired networks to avoid this effect?
3.2 Recall the issue of hidden and exposed terminals. What happens in the case of
such terminals if Aloha, slotted Aloha, reservation Aloha, or MACA is used?
3.3 How does the near/far effect influence TDMA systems? What happens in CDMA
systems? elaborate countermeasures in TDMA systems, what about CDMA systems?
3.4 Who performs the MAC algorithm for SDMA? What could be possible roles of mobile
stations, base stations, and planning from the network provider?
3.5 What is the basic prerequisite for applying FDMA? How does this factor increase
complexity compared to TDMA systems? How is MAC distributed if we consider the
whole frequency space as presented in chapter 1?
3.6 Considering duplex channels, elaborate options for implementation in wireless
networks? What about typical wired networks?
3.7 elaborate the advantages of a fixed TDM trend compared to random, demand
driven TDM? Compare the efficiency in the case of several connections with fixed
data rates or in the case of varying data rates. Now discuss why traditional mobile
phone systems use fixed patterns, while computer networks generally use random
patterns. In the future, the main data being transmitted will be computer-generated
data. How will this fact change mobile phone systems?
3.8 discuss the term interference in the space, time, frequency, and code domain. What
are countermeasures in SDMA, TDMA, FDMA, and CDMA systems?
3.9 presume all stations can hear all other stations. 1 station wants to transmit and
senses the carrier idle. Why can a collision still occur after the begin of transmission?
3.10 elaborate benefits of reservation schemes? How are collisions avoided during data
transmission, why is the probability of collisions lower compared to classical Aloha?
elaborate disadvantages of reservation schemes?
3.11 How can MACA still fail in case of hidden/exposed terminals? Think of mobile stations
and changing transmission characteristics.
3.12 Which of the MAC schemes can provide hard guarantees related to bandwidth and
access delay?
3.13 How are guard spaces realised ranging from users in CDMA?
3.14 Redo the simple CDMA example of part 3.5, but now add random ‘noise’ to the
transmitted signal (–2,0,0,–2,+2,0). Add, for example, (1,–1,0,1,0,–1). In this case,
what can the receiver detect for sender A and B respectively? Now include the

Earning:   Approval pending.