"Exercise Sheet 5 UMTSandIEEE 802.11 Wireless LANs"
Wintersemester 2009/2010 Mobilkommunikation Universität Paderborn Exercise Sheet 5: Fachgebiet Rechnernetze UMTS and IEEE 802.11 Wireless LANs 03.02.2010 1. Universal Mobile Telecommunications System (UMTS): UMTS Terrestrial Radio Access (UTRA) and Handover (a) Calculate the maximum and minimum total data rate (including payload and control data) on the UTRA-FDD and UTRA-TDD interface. Assume a chip rate of RC = 3.84 MChip/s and QPSK modulation. (b) Explain the fundamental difference between hard and soft handover. When are these methods used? How do we prevent to lose data during hard handovers? What changes are required in the UTRAN to support soft handovers? 2. IEEE 802.11 in General (a) Summarize and describe the basic functions in the IEEE 802.11 Physical layer (PHY) and Data Link Control layer (DLC). (b) How do IEEE 802.11 stations test for a free channel? (c) Explain the purpose of the Medium Access Control (MAC) timing parameters Short Inter- Frame-Space (SIFS), Point (coordination function) Interframe Space (PIFS), and Distributed (coordination function) Interframe Space (DIFS). Explain why they should be carefully cho- sen. Why has a station at least to wait one DIFS prior to transmission? (d) Explain the function of the Network Allocation Vector (NAV)? (e) Compare the IEEE 802.11 infrastructure and ad-hoc mode according to their architecture, services, and MAC functions. (f) Discuss the suitability of using CSMA/CD (collision detection) and CSMA/CA (collision avoidance) in Wireless Local Area Networks (WLANs). Provide examples where these meth- ods are inefﬁcient. Mobilkommunikation WS 2009/10 Exercise Sheet 5 1 3. IEEE 802.11 Medium Access Control (MAC): Distributed Foundation Wireless MAC (DFWMAC) with RTS/CTS (a) CSMA/CA provides reliable transmission and avoids collisions. Explain the beneﬁts of ad- ditionally using the RTS/CTS technique in this case. How can we decrease the delays and overhead introduced by this technique? (b) Assume the topology below consisting of the nodes A, B, C, D, and E. A C E B D This following list describes which nodes can reach each other. Starting with a free channel, provide one timing diagram for each of the 3 possible connections. (a) B → D and E → D (b) C → B and E → D (c) A → B and C (c) Assume that in scenario 3.b(a) station B wins the contention phase. For this case, calculate the overhead and effective data rate (so-called goodput) for the transmission of a single DLC data frame. Assume standard IEEE 802.11a parameters, in particular, a transmission rate of 6 Mbit/s for control frames and 12 Mbit/s for data frames of 1500 Bytes. Control frames are 20 Bytes (RTS), 14 Bytes (CTS), and 14 Bytes (ACK) in size. With IEEE 802.11a an SIFS is 16 µs and a DIFS is 34 µs. Mobilkommunikation WS 2009/10 Exercise Sheet 5 2