Mobile Communications Exercise: Satellite Systems and Wireless LANs. Georg von Zengen, IBR, TU Braunschweig,

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1 Mobile Communications Exercise: Satellite Systems and Wireless LANs

2 N 1 Please define the terms inclination and elevation using the following two figures. How do these parameters influence the usefulness of a satellite?

3 Inclination Angle between plane of satellite orbit and equatorial plane

4 Elevation Angle between earth surface and center of satellite beam

5 N 2 a) Please name the typical orbits of satellite systems and fill in the respective distance to the earth surface. b) Please calculate the minimum propagation delay (earth satellite) through satellite systems of the three orbit types. Please assume a wave propagation speed of m. s c) Please name advantages and disadvantages of the orbits as well as examples of techniques using such orbits.

6 N 2 a) Please name the typical orbits of satellite systems and fill in the respective distance to the earth surface. Orbit GEO: Geostationary Orbit Distance to the earth surface 35,768 km MEO: Medium Earth Orbit 5,000 20,000km LEO: Low Earth Orbit 500 1,500 km

7 N 2 Exercise: Satellite Systems and Wireless LANs b) Please calculate the minimum propagation delay (earth satellite) through satellite systems of the three orbit types. Please assume a wave propagation speed of m s. Orbit GEO MEO LEO Distance to the earth surface km km km Propagation Delay t = m m s = s= 0.119s=119ms t = m = m s= 0.017s=17ms s t = m = m s= s=1.7ms s

8 N 2 Exercise: Satellite Systems and Wireless LANs c) Please name advantages and disadvantages of the orbits as well as examples of techniques using such orbits. GEO (TV Broadcast services, telephony, internet access) + fixed antenna positions, large footprint - high transmit power, high latency - bad elevation in areas above 60 latitude MEO (GPS, telephony) medium latency, medium footprint, medium visibility handover may be necessary, moving satellites LEO (Telephony) + low latency, small footprint: better frequency reuse - short visibility, handovers necessary - moving satellites

9 N 3 a) How are priorities of different frame types modeled in IEEE ? How many priority level are defined in the standard? b) How is fairness among stations implemented in IEEE ?

10 N 3 Exercise: Satellite Systems and Wireless LANs a) How are priorities of different frame types modeled in IEEE b? How many priority level are defined in the standard? Different Inter Frame Spaces (IFS) model priority. Shorter IFS mean higher priority. Short IFS: Before ACK, CTS (10 μs) Point Coordination Function IFS (30 μs) Distributed Coordination Function IFS (50 μs) Extended IFS: After receiving erroneous frame Reduced IFS: Introduced in n; before ACK, CTS (2 μs)

11 N 3 b) How is fairness among stations implemented in IEEE ? Fairness means, that all stations will eventually be able to access the medium. IEEE models fairness by suspending and resuming the backoff timer.

12 N 4 Please explain the Hidden- and Exposed Terminal Problem and outline how IEEE (WLAN) and IEEE (Bluetooth) solve these problems.

13 Hidden Terminal In wireless networking, collisions occur at the receiver DATA DATA A B C

14 Exposed Terminal Station B cannot transmit, since the medium is sensed busy DATA DATA A B C

15 Hidden Terminal with RTS/CTS RTS A to B CTS A CTS A DATA RTS C to B CTS C CTS C DATA A B C

16 Exposed Terminal with RTS/CTS RTS B to A RTS C to D RTS C to D CTS B CTS C DATA DATA A B C

17 N 4 Please explain the Hidden- and Exposed Terminal Problem and outline how IEEE (WLAN) and IEEE (Bluetooth) solve these problems. Bluetooth is a master/slave system and does not suffer from the Hidden- and Exposed Terminal Problem. All nodes that can see the master may be part of the network; all others are not.

18 N 5 Assuming a wireless IEEE b-compliant network with 4 stations that are within the same area, please outline how media access using DFWMAC-DCF CSMA/CA is handled. Please ignore Acknowledgement frames assume that the MAC layer of each node receives a frame from the upper layer at the points in time designated by the arrows. The medium has been busy before t 0 and nodes chose their respective backoff timer to be their station number plus 3 in slots.

19 Suspend Backoff Timer Suspend Backoff Timer Suspend Backoff Timer Station 1 Station 2 Station 3 Station 4 t 0 Time [Slots] t SIFS (1 Slot) PIFS (2 Slots) DIFS (3 Slots) Data Frame (5 Slots) Back off (3 + n Slots)

20 N 6 The throughput of IEEE primarily limited by MAC mechanisms and not by the physical layer capabilities. Please calculate the maximum real-world upper-layer throughput of IEEE b at 11 Mbit/s for a frame payload size of 500, 1500 and 2312 bytes. Please assume that each packet uses the backoff timer and also requires a subsequent acknowledgement frame.

21 Backoff Timer PLCP Header Payload Data PLCP Header ACK DIFS MAC Header SIFS Element Data Bitrate Time [μs] t DIFS 50 Backoff Timer 310 PLCP Header 192 bit 1 Mbit/s MAC Header 272 bit 11 Mbit/s Payload Data 500 / 1500 / 2312 byte 11 Mbit/s 192bit 10 6 bit s SIFS bit bit s 500 8bit bit s = s=192ms = s= 24.7ms = s= 363.6ms ACK Frame 112 bit 1 Mbit/s 112bit 10 6 bit s = s=112ms

22 Payload length Time per packet [μs] Maximum Throughput 500 byte byte byte Goodput = bit time 500 byte ms = 3.189MBit / s 1500 byte ms = 6.056MBit / s 2312 byte ms = 7.191MBit / s G = = = 3.189MBit / s

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