Internet of Things Prof. M. Cesana. Exam June 26, Family Name Given Name Student ID 3030 Course of studies 3030 Total Available time: 2 hours

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1 Internet of Things Prof. M. Cesana Exam June 26, 2011 Family Name Given Name John Doe Student ID 3030 Course of studies 3030 Total Available time: 2 hours E1 E2 E3 Questions Questions OS 1 Exercise (8 points) A personal area network (PAN) is composed of 4 motes and a PAN Coordinator. The PAN works in beacon-enabled mode. Ø Mote 1 and Mote 2 require a deterministic communication channel to the PAN Coordinator of 200 [bit/s] Ø Mote 3 requires a deterministic communication channel to the PAN Coordinator of 600 [bit/s] Ø Mote 4 has statistical (non-deterministic) traffic towards the PAN coordinator characterized by the following probability distribution: P(required rate=0)= 0.2 P(required rate=100 [bit/s])= 0.2 P(required rate=400 [bit/s])= 0.6 Assuming that: Ø the active part of the Beacon Interval (BI) is composed of Collision Free Part only Ø the motes use b=100 [byte] packets to communicate with the PANC which fit exactly one slot in the CFP Ø the nominal rate is 250 [kb/s] Ø Mote 1 and Mote 2 are 5[m] away from the PANC Ø Mote 3 and 4 are 10[m] away from the PANC Find 1. The duration of the single slot, the duration of Beacon Interval (BI), 2. A consistent slot assignment for the four motes 3. The duration of the CFP and the duration of the inactive part. 4. The duty cycle 5. The average energy consumption in a BI for a mote of Type 3 if the energy required to operate the TX/RX circuitry is E c =50 [nj/bit], the energy required to support sufficient transmission output power E tx (d)= k d 2 [nj/bit], being k=1 [nj/bit/m2], the energy of being idle in a slot is E idle = 40 [uj] and the energy for sleeping is E sleep = 1.5 [uj] 6. Assuming that the energy budget for the four motes is E budget =1 [J], what is the PAN lifetime (number of beacon interval after which the first sensor runs out of energy)?

2 Solution To find the Beacon Interval duration we have to consider the minimal rate required by the motes. Mote 4 requires (with probability 0.2) a rate of 100 [bit/s], which is the minimum among the other motes. We can thus write: The duration of the single slot is: Assigning one slot ina beacon interval is equivalent to a channel of 100 [bit/s]. A consistent slot assignment is thus given by: Mote 1: 2 slots Mote 2: 2 slots Mote 3: 6 slots Mote 4: 4 slots The duration of the CFP is given by: T cfp = T slot x 14 = 44.8 [ms] The duration of the inactive part is by difference: T inactive = BI - T cfp = [s] (it is correct also considering the duration of the beacon slot, that is, T inactive = BI - T cfp - T_ slots ) The duty cycle is: DC= : T cfp / BI = 5.6x10-3 (or alternatively DC= (T cfp + T slot )/BI) The equivalent number of slots in the inactive part is: N = [s] / 3.2 [ms] = 2486 The energy consumed by all the four motes in the inactive part is: E s = 2486 E sleep = [mj] As for the average energy consumed by the four motes, there are two possible cases: - Case 1: each mote does not overhear the transmissions of the other three motes - Case 2: each mote overhears the transmissions of the other three motes Case 1 which leads to: E 1 =E 2 = [mj]

3 E 3 = [mj] E 4 =4.537 [mj] The calculations above have been carried out assuming also the energy for receiving the beacon. Solutions neglecting the energy for receiving the beacon have been considered correct anyways. Case 2 which leads to: E 1 =E 2 = [mj] E 3 = [mj] E 4 =4.537 [mj] Note that the average energy consumed by the four motes is the same in the two cases since the energy consumed for being idle and for overhearing is the same. The mote that will die out first is mote 3. That will happen after K=E budget / E 3 = 207 beacon intervals, which is equal to (more or less) 27 minutes. 2 Exercise (5 points) The PAN in the figure runs the SPARE MAC protocol. Assuming that: Ø The frame is composed of 7 slot in the signalling subframe and 5 slots in the data subframe Ø Each slot (data and signaling) is used to send packets of exactly 100 [byte] Ø The channel corresponding to one slot per mote per signaling subframe is 400 [bit/s] Tell: 1. how many additional motes could be added to the network. 2. what is the duration of the frame 3. what is slot duration (all the slots are equal) B C D E Solution The data subframe has 5 slots. Since two nodes in the same one-hop cluster cannot have the same slot in the data subframe, only one additional node can be added to the network without conflicts (and without modifying the network topology).

4 From the text, a channel equivalent to 1 slot per SPARE MAC frame conveys a bit rate of 400 [bit/s]. We can write: T frame = 100 [byte] / 400 [bit/s] = 2 [s]. The duration of a single slot can be calculated as: T slot = T frame / 13 = 153 [ms], where 13 is the number of slots in a SPARE MAC frame (signalling+data+wake up slot). 3 Exercise (5 points) A personal area network (PAN) is composed of 8 motes and a PAN coordinator. Each mote is assigned a slot in the GTS within the Beacon Interval. 4 motes of type 1 are characterized by a traffic distributed as Poisson point process with parameter λ 1 = 1 [packet/beacon interval] whereas the remaining 4 motes are characterized by a traffic distributed according to Poisson with parameter λ 2 = 0.5 [packet/beacon interval]. The superframe is composed only of GTS (no CAP). The overall TX/RX energy (circuitry + transmission/reception) is E tx =50 [uj]. The energy for being idle is E idle =20 [uj]. The sleeping energy is negligible. 1. Write the expression of the average energy consumed by the PAN coordinator within one Beacon Interval 2. Write the expression of the average energy consumed by a mote of type 1 within one Beacon Interval Solution The probability P i that a given mote of type i has packet to send in a beacon interval is the probability that at least one packet is generate by the mote itself in the previous beacon interval. Thus, we can write for mote of type one and 2: The average energy consumed by the PAN coordinator can be written as: The first term accounts for the beacon transmission. The average energy consumed by a mote of type 1 depends on wheter the mote is in range of all the other motes or not. Case 1. Mote not in range Case 2. Mote in range

5 Note that in case 2, the energy consumed by any mote is equivalent to energy consumed by the PAN coordinator. 3 Questions (10 points) 1. A wireless link is characterized by a bit error rate (probability) of What is the average ETX assuming that the packets used over the link are 100 [byte] long. A. The packet success probability can be defined as: P=( ) 800 = The average ETX is the average number of transmissions required to send a packet correctly. Thus, Solutions considering P==( ) 800 ( ) 800 have been considered correct. 2. What is the frame length of frame aloha that maximizes the single-frame efficiency if 5 tags are to be resolved? What is the corresponding maximum efficiency? A. The optimal frame length is 5. The optimal efficiency is A ZigBee network topology runs the tree-based routing protocol. The tree is composed of 4 full levels. Each level of the tree has 2 ZigBee routers and 3 ZigBee simple devices. How many nodes are in the network? How many nodes are in the subtree rooted at a ZigBee router in level 1? A. The number of addresses assigned to a router node at depth d is given by: Therefore we have: A(3) = 1+2+3=6 A(2) = x6 = 16 A(1) = x16 = 36 A(0 = x36 = 76 The PAN has 76 nodes in total. The router node at level 1 has 35 nodes in its own subtree. 4. Given the network topology in the figure, tell if the following slot assignments are feasible under SPARE MAC. If not, briefly explain why? A B C D A: signaling slot: 1, data slot:1 B: signaling slot: 2, data slot: 1 C: signaling slot: 3, data slot: 2 D: signaling slot: 1, data slot: 1 Assignment 1

6 A: signaling slot: 1, data slot:1 B: signaling slot: 2, data slot: 2 C: signaling slot: 3, data slot: 2 D: signaling slot: 1, data slot: 1 Assignment 2 A. Neither A1 nor A2 are feasible. There is a collision in the data subframe. 5. The figure represents a routing pattern of a PAN. Assuming that all the links have a nominal capacity of 100 [kb/s] and that all the four nodes B, C, D, E generate the same amount of traffic (R [bit/s]) to the gateway (GW), find the maximum rate R that each one of the four nodes can generate. GW# C B D A. The maximum rate is 25 [kb/s] E Questions OS (5 points) 1. Explain the concept of platform independent data types in TinyOS. When and why their use is necessary? A1: Platform-independent data types are used when defining message formats. The basic problem is that in TinyOS message formats are defined using structs, and the memory layout of such structs depends on the particular chip we're compiling for. Since wireless sensor nodes may have different chips and data encoding (i.e., endianness) this can cause several access problem to structs (especially when those structs are transferred from one sensor to another, as it is the case of radio messages). To solve this problem TinyOS allows to declare platformindependent data types using the prefix nx_ (e.g., nx_uint8_t) which are are either big-endian or little-endian, independently of the underlying chip hardware. The same applies for structs (e.g. nx_struct) and any hardware architecture that compiles such structure uses the same memory layout and the same endianness for all of the fields. [see p.138 of TinyOS programming guide] 2. Considering the frame header in Figure 1, write the MoteRunner code that prepares for transmission a packet with the following attributes: frame type: DATA short destination addressing short source addressing source and destination PAN address = 0x22 destination address = 0x01 payload length 10 bytes

7 field /2 0/2/8 0/2 0/2/8 0/5/6/10/14... # of bytes per FCF FCA SEQNO DSTPAN DSTADDR SRCPAN SRCADDR aux.security payload field name < addressing fields > A. // Prepare data frame xmit = new byte[21]; //11 header + 10 payload xmit[0] = Radio.FCF_DATA; xmit[1] = Radio.FCA_SRC_SADDR Radio.FCA_DST_SADDR; Util.set16le(xmit, 3, 0x22); Util.set16le(xmit, 5, 0x01); Util.set16le(xmit, 7, 0x22); Util.set16le(xmit, 9, Radio.getShortAddr());