Chapter 2 Answers

Answers to Exercises

1. d. Traditional Ethernet does not support any quality of service metrics. That is one of the issues with traditional Ethernet. Choice c is wrong, since traditional Ethernet was not designed to meet the requirements needed for guaranteed and real-time communications. In other words, traditional Ethernet supports only the best-effort service.
2.  c. preemption allows high priority packets to interrupt low priority ones. This is exactly opposite of choice a. It does not change the throughput of the system. It just reduces the transmission latency for high priority frames at the expense of increasing the latency for low priority frames.
3. a. Ethernet TSN guarantees the latency for real time critical data, but not for non-real time data. It is not based on best effort and therefore b and c are not correct.
4. d. Terabit Ethernet (TbE) is very fast and therefore is not widely used for IoT applications anywhere. It is usually used for Internet service provider’s (ISP) core routing. So, both b and c are correct.
5. a. Narrowband PLC operates at frequencies between 3 KHz and 500 kHz. The broadband PLC works in frequencies between 1.8 MHz to 250 MHz. Narrowband PLC can be used for data rates of up to 100s of kbps.
6. c. Narrowband PLC is a better IoT connectivity scheme as compared to wireless schemes in metropolitan areas and dense urban regions, since in these regions attenuation of wireless signals is high.
7. b. The BLE on IoT gateway plays the role of a central node (master) and sends the Connect Request packet. The BLE node on the IoT device plays the role of advertiser and after connection is established becomes a peripheral BLE node.
8. b. The value of connection interval (CI) is equal to Interval × 1.25ms. Therefore, Interval needs to be 6.
9. d. If the Bluetooth module is set to PHY coded s=8, the maximum range is four times larger than when PHY 1Mbps is used. This is explained in Table 2.2. So, the range would be 200× 4= 800 m.
10. a.  Bluetooth 5 can send advertisement packets of up to 256 bytes in length. The size has been increased from maximum 32 bytes in BLE 4.2. The data can be sent during advertisement and there is no need for connection. However, the possibility of collision exists.
11. b. Zigbee operates on 2.4 GHz globally.
12. a.  It is true that in-building WiFi coverage is almost ubiquitous. So, answer a is correct. WiFi does not have the lowest power consumption among other short-range wireless technologies in the market. Actually, BLE and Zigbee draw less current as compared to WiFi. Answer c is wrong, since many smart home applications do not need very high-speed data transfer. It is true that WiFi can provide higher data rate as compared to other short-range wireless technologies. WiFi can provide substantially higher data rates as compared to BLE or Zigbee.

Answers to Review Questions

1. Time synchronization, traffic scheduling based on QoS, and frame preemption.
   1. Monitoring charge on the computer or phone, receiving notifications when charge is complete, over-the-air firmware updates, remote service if necessary, taking advantage of offpeak or low-rate charging time, negotiating for charging slots that does not overload the system.
   2. Technologies such as Homeplug Green
2. The number of connection events the peripheral BLE node can skip is defined as latency parameter. If the peripheral node does not have any data to send, it has the option to skip several connection events. This can provide some reduction in power consumption for the the peripheral device.
3. Yes, in many IoT applications smartphones play the role of IoT gateway. Since most smartphones, tablets and computers do not have Zigbee module, they cannot be used as IoT gateway.
4. In a smart building use case, where the building has thick concrete walls, or specific thermal insulation material, the implementation of wireless technology may introduce many challenging issues and if possible the wired connectivity solution is preferable.