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engineering
telecommunication engineering

Questions and Answers of Telecommunication Engineering

List four techniques used by firewalls to control access and enforce a security policy.
What information is used by a typical packet-filtering router?
The ESP header in IPsec includes an Integrity Check Value (ICV), which is calculated over all of the fields of the IPsec packet except IP header fields that either do not change in transit
In the context of an IDS, we define a false positive to be an alarm generated by an IDS in which the IDS alerts to a condition that is actually benign. A false negative occurs when an IDS fails to
The overlapping area of the two probability density functions of Figure 19.6 represents the region in which there is the potential for false positives and false negatives. Further, Figure 19.6 is an
An example of a host-based intrusion detection tool is the tripwire program. This is a file integrity checking tool that scans files and directories on the system on a regular basis and notifies the
Test the vulnerability of a machine at the following site: grc.com/default.htm. Follow the ShieldsUP! link for a series of free tests listed midway down the page.
The tiny fragment attack is a form of firewall attack. The intruder uses the IP fragmentation option to create extremely small fragments and force the TCP header information into a separate packet
In an IPv4 packet, the size of the payload in the first fragment, in octets, is equal to Total Length - (4 × IHL). If this value is less than the required minimum (8 octets for TCP), then this
Check out the strategies of different network operators while migrating towards third generation systems. Why is a single common system not in sight?
What are the means to mitigate narrowband interference? What is the complexity of the different solutions?
Why, typically, is digital modulation not enough for radio transmission? What are general goals for digital modulation? What are typical schemes?
Think of a phase diagram and the points representing bit patterns for a PSK scheme (see Figure 2.29). How can a receiver decide which bit pattern was originally sent when a received 'point' lies
What are the main benefits of a spread spectrum system? How can spreading be achieved? What replaces the guard space in Figure 2.33 when compared to Figure 2.34? How can DSSS systems benefit from
What are the main reasons for using cellular systems? How is SDM typically realized and combined with FDM? How does DCA influence the frequencies available in other cells?
What limits the number of simultaneous users in a TDM/FDM system compared to a CDM system? What happens to the transmission quality of connections if the load gets higher in a cell, i.e., how does an
Why can waves with a very low frequency follow the earth's surface? Why are they not used for data transmission in computer networks?
Why does the ITU-R only regulate 'lower' frequencies (up to some hundred GHz) and not higher frequencies (in the THz range)?
What are the two different approaches in regulation regarding mobile phone systems in Europe and the US? What are the consequences?
Why is the international availability of the same ISM bands important?
Is it possible to transmit a digital signal, e.g., coded as square wave as used inside a computer, using radio transmission without any loss? Why?
Is a directional antenna useful for mobile phones? Why? How can the gain of an antenna be improved?
What are the main problems of signal propagation? Why do radio waves not always follow a straight line? Why is reflection both useful and harmful?
Name several methods for ISI mitigation. How does ISI depend on the carrier frequency, symbol rate, and movement of sender/receiver? What are the influences of ISI on TDM schemes?
What is the main physical reason for the failure of many MAC schemes known from wired networks? What is done in wired networks to avoid this effect?
What are benefits of reservation schemes? How are collisions avoided during data transmission, why is the probability of collisions lower compared to classical Aloha? What are disadvantages of
How can MACA still fail in case of hidden/exposed terminals? Think of mobile stations and changing transmission characteristics.
Which of the MAC schemes can give hard guarantees related to bandwidth and access delay?
Redo the simple CDMA example of section 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
Recall the problem of hidden and exposed terminals. What happens in the case of such terminals if Aloha, slotted Aloha, reservation Aloha, or MACA is used?
How does the near/far effect influence TDMA systems? What happens in CDMA systems? What are countermeasures in TDMA systems, what about CDMA systems?
Who performs the MAC algorithm for SDMA? What could be possible roles of mobile stations, base stations, and planning from the network provider?
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
Considering duplex channels, what are alternatives for implementation in wireless networks? What about typical wired networks?
What are the advantages of a fixed TDM pattern 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
Explain the term interference in the space, time, frequency, and code domain. What are countermeasures in SDMA, TDMA, FDMA, and CDMA systems?
Assume all stations can hear all other stations. One station wants to transmit and senses the carrier idle. Why can a collision still occur after the start of transmission?
Name some key features of the GSM, DECT, TETRA, and UMTS systems. Which features do the systems have in common? Why have the three older different systems been specified? In what scenarios could one
What multiplexing schemes are used in GSM and for what purpose? Think of other layers apart from the physical layer.
How is synchronization achieved in GSM? Who is responsible for synchronization and why is it so important?
What are the reasons for the delays in a GSM system for packet data traffic? Distinguish between circuit-switched and packet-oriented transmission.
Where and when can collisions occur while accessing the GSM system? Compare possible collisions caused by data transmission in standard GSM, HSCSD, and GPRS.
Why and when are different signaling channels needed? What are the differences?
How is localization, location update, roaming, etc. done in GSM and reflected in the data bases? What are typical roaming scenarios?
Why are so many different identifiers/addresses (e.g., MSISDN, TMSI, IMSI) needed in GSM? Give reasons and distinguish between user-related and system-related identifiers.
Give reasons for a handover in GSM and the problems associated with it. What are the typical steps for handover, what types of handover can occur? Which resources need to be allocated during handover
What are the functions of authentication and encryption in GSM? How is system security maintained?
How can higher data rates be achieved in standard GSM, how is this possible with the additional schemes HSCSD, GPRS, EDGE? What are the main differences of the approaches, also in terms of
What are the main problems when transmitting data using wireless systems that were made for voice transmission? What are the possible steps to mitigate the problems and to raise efficiency? How can
What limits the data rates that can be achieved with GPRS and HSCSD using real devices (compared to the theoretical limit in a GSM system)?
Using the best delay class in GPRS and a data rate of 115.2 kbit/s - how many bytes are in transit before a first acknowledgement from the receiver could reach the sender (neglect further delays in
How much of the original GSM network does GPRS need? Which elements of the network perform the data transfer?
What are typical data rates in DECT? How are they achieved considering the TDMA frames? What multiplexing schemes are applied in DECT and for what purposes? Compare the complexity of DECT with that
Who would be the typical users of a trunked radio system? What makes trunked radio systems particularly attractive for these user groups? What are the main differences to existing systems for that
Summarize the main features of third generation mobile phone systems. How do they achieve higher capacities and higher data rates? How does UMTS implement asymmetrical communication and different
Compare the current situation of mobile phone networks in Europe, Japan, China, and North America. What are the main differences, what are efforts to find a common system or at least interoperable
What disadvantage does OVSF have with respect to flexible data rates? How does UMTS offer different data rates (distinguish between FDD and TDD mode)?
How are different DPDCHs from different UEs within one cell distinguished in UTRA FDD?
Which components can perform combining/splitting at what handover situation? What is the role of the interface Iur? Why can CDMA systems offer soft handover?
Which types of different services does GSM offer? Give some examples and reasons why these services have been separated.
How does UTRA-FDD counteract the near-far effect? Why is this not a problem in GSM?
Name the main elements of the GSM system architecture and describe their functions. What are the advantages of specifying not only the radio interface but also all internal interfaces of the GSM
Describe the functions of the MS and SIM. Why does GSM separate the MS and SIM? How and where is user-related data represented/stored in the GSM system? How is user data protected from unauthorized
Looking at the HLR/VLR database approach used in GSM - how does this architecture limit the scalability in terms of users, especially moving users?
Why is a new infrastructure needed for GPRS, but not for HSCSD? Which components are new and what is their purpose?
What are the limitations of a GSM cell in terms of diameter and capacity (voice, data) for the traditional GSM, HSCSD, GPRS? How can the capacity be increased?
Name basic applications for satellite communication and describe the trends.
Why are GEO systems for telecommunications currently being replaced by fiber optics?
How do inclination and elevation determine the use of a satellite?
What characteristics do the different orbits have? What are their pros and cons?
What are the general problems of satellite signals travelling from a satellite to a receiver?
Considered as an interworking unit in a communication network, what function can a satellite have?
What special problems do customers of a satellite system with mobile phones face if they are using it in big cities? Think of in-building use and skyscrapers.
Why is there hardly any space in space for GEOs?
2G and 3G systems can both transfer data. Compare these approaches with DAB/DVB and list reasons for and against the use of DAB/DVB.
Which web pages would be appropriate for distribution via DAB or DVB?
How could location based services and broadcast systems work together?
How is mobility restricted using WLANs? What additional elements are needed for roaming between networks, how and where can WLANs support roaming? In your answer, think of the capabilities of layer 2
How are fairness problems regarding channel access solved in IEEE 802.11, HiperLAN2, and Bluetooth respectively? How is the waiting time of a packet ready to transmit reflected?
In what situations can collisions occur in all three networks? Distinguish between collisions on PHY and MAC layer. How do the three wireless networks try to solve the collisions or minimise the
Compare the overhead introduced by the three medium access schemes and the resulting performance at zero load, light load, high load of the medium. How does the number of collisions increase with the
How is roaming on layer 2 achieved, and how are changes in topology reflected? What are the differences between infrastructure based and ad-hoc networks regarding roaming?
What are advantages and problems of forwarding mechanisms in Bluetooth networks regarding security, power saving, and network stability?
Name reasons for the development of wireless ATM. What is one of the main differences to Internet technologies from this point of view? Why did WATM not succeed as stand-alone technology, what parts
What are the basic differences between wireless WANs and WLANs, and what are the common features? Consider mode of operation, administration, frequencies, capabilities of nodes, services,
With a focus on security, what are the problems of WLANs? What level of security can WLANs provide, what is needed additionally and how far do the standards go?
Compare IEEE 802.11, HiperLAN2, and Bluetooth with regard to their ad-hoc capabilities. Where is the focus of these technologies?
If Bluetooth is a commercial success, what are remaining reasons for the use of infra red transmission for WLANs?
Why is the PHY layer in IEEE 802.11 subdivided? What about HiperLAN2 and Bluetooth?
Compare the power saving mechanisms in all three LANs introduced in this chapter. What are the negative effects of the power saving mechanisms, what are the trade-offs between power consumption and
Compare the QoS offered in all three LANs in ad-hoc mode. What advantages does an additional infrastructure offer? How is QoS provided in Bluetooth? Can one of the LAN technologies offer hard QoS
How do IEEE 802.11, HiperLAN2 and Bluetooth, respectively, solve the hidden terminal problem?
Recall routing in fixed IP networks (Kurose, 2003). Name the consequences and problems of using IP together with the standard routing protocols for mobile communications.
What advantages does the use of IPv6 offer for mobility? Where are the entities of mobile IP now?
What are general problems of mobile IP regarding security and support of quality of service?
What is the basic purpose of DHCP? Name the entities of DHCP.
How can DHCP be used for mobility and support of mobile IP?
Name the main differences between multi-hop ad-hoc networks and other networks. What advantages do these ad-hoc networks offer?
Recall the distance vector and link state routing algorithms for fixed networks. Why are both difficult to use in multi-hop ad-hoc networks?
How does dynamic source routing handle routing? What is the motivation behind dynamic source routing compared to other routing algorithms from fixed networks?

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