LUNDS TEKNISKA HÖGSKOLA

Dept of Electrical and Information Technology

Exam in AVANCERAD TELECOMMUNICATIONS, 2008-03-14

Time: 14.00 – 19.00

Write clear!

Write your name an all the sheets you hand in!

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Questions from the book:

  1. 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?(2.8) (3 cr)
  1. Assume that 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? (3.9) (3 cr)
  1. Why are GEO systems for telecommunications currently being replaced by fiber optics? (5.2). How do inclination and elevation determine the use of a satellite? (5.3) (3 cr)
  1. Compared to the TCHs offered, standard GSM should provide much higher data rate (33.8 kbps)when looking at the air interface. What lowers the data rates available to a user? (4.4) (3 cr)
  1. Compare the QoS offered the three LANs in ad-hoc mode. (IEEE802.11, HiperlanII and Bluetooth respectively).What advantages does as additional infrastructure offer? How is QoS provided in Bluetooth? Can one of the LAN technologies offer hard QoS (i.e. not only statistical guarantees regarding a QoS papameter)? (7.8) (3 cr)

Other questions:

  1. a) Give the main reason why spread spectrum techniques are used. (3 cr)

b) WLANs may operate either in an infrastructure mode or in an ad-hoc mode. Describe the two modes. (3 cr)

c) Describe the “Hidden Terminal Problem”. (4 cr)

  1. a) Which spread spectrum technique is used in Bluetooth? (3 cr)

b) In Bluetooth you find two different types of links, the Synchronous connection oriented link (SCO) and the Asynchronous connectionless link (ACL). Describe the two links. (3 cr)

c) Describe the MAC-protocol used for the ACL-links. (4 cr)

  1. In GSM there are several databases and registers. Describe the function of the HLR, the VLR, the AuC and the EIR, respectively. (10 cr)
  1. a) In UMTS (in UTRA) there are two modes, the UTRA-FDD and the UTRA-TDD. Describe the two modes. (3 cr)

b) When a mobile sends to the RNS (the Radio Network System), i.e. uplink, two different codes are used, the Spreading code and the Scrambling code (see Figure 1). Describe the use of these two types of codes on the uplink. (4 cr)

c) In GPRS, two new types of backbone elements are introduced, the SGSN and the GGSN. Describe the two. (3 cr)

  1. a) For an IEEE802.11 WLAN all the participating nodes need to be synchronized in time. Figure 2 shows the beacon transmission in a busy 802.11 infrastructure network. Describe how the synchronization is achieved. (4 cr)

b) How is the synchronization handled in ad-hoc mode? (6 cr)

c) Figure 3 shows an example of the Power management procedure in 802.11 working in infrastructure mode. Describe the function with comments related to the figure! Note that you should also mention about the TIM interval as well as the DTIM intervals. (5 cr)

  1. a) In IEEE802.11 networks, three different MAC algorithms are defined. There is just one that is mandatory. The three MAC-protocols are: “Basic DFWMAC-DCF”, “DFWMAC-DCF with RTS/CTS extension” and DFWMAC-PCF with polling. (DFWMAC: Distributed Foundation Wireless Medium Access Control; DCF: distributed coordination function; PCF: point coordination function) Describe the three! (10 cr)

b) Describe the exponential back-off algorithm. (5 cr)

Good luck!

You need at least 43 cr in order to pass the exam.

54-64 cr => Give you a 4 on the exam

65 or more cr give you a 5 on the exam

Figure 1

Figure 2

Figure 3