IoT connectivity solutions: Media access control layer and network topology

161-Datalink-MAC

Media access control layer and network topology

For IoT applications, the main characteristics of the media access layer control (MAC) that need to be considered are multiple access, synchronization, and network topology.

Multiple Access. Looking back at decades of successful cellular system deployment, one can safely conclude that TDMA is a good fit for the IoT. TDMA is suited for low-power operation with a decent number of devices, as it allows for optimal scheduling of inactive periods. Hence, TDMA is selected for multiple access in the MAC layer.

Synchronization. In IoT applications, there are potentially a very large number of power-sensitive devices with moderate throughput requirements. In such a configuration, it is essential to maintain a reasonably consistent time base across the entire network and potentially across different networks. Given that throughput is not the most critical requirement, it is suitable to follow a beacon-enabled approach, with a flexible beacon period to accommodate different types of services.

Network topology. Mobile networks using a cellular topology have efficiently been servicing a large number of devices with a high level of security and reliability, e.g., 5,000+ per base station for LTE in urban areas. This typology is based on a star topology in each cell, while the cells are connected in a hierarchical tree in the network backhaul. This approach is regarded suitable for the IoT and is therefore selected.

The network layer and interface to applications

The network layer (NWK) and the interface to applications are less fundamental as far as power-efficiency and reliability is concerned. In addition, there is more variation in the field of IoT applications. Nevertheless, it is widely acknowledged that IoT applications need to support the Internet Protocol (IP), whether it is IPv4 or IPv6. In addition, the User Datagram Protocol (UDP) and Constrained Application Protocol (CoAP) could provide the relevant trade-off between flexibility and implementation-complexity on resource-constrained devices.

Furthermore, the IoT will represent an immense security challenge, and it is likely that state-of-the-art security features will become necessary. As of today, we can assume 128 bits Advanced Encryption Standard (AES) for encryption and Diffie-Hellman (DH), or the Elliptic Curve Diffie-Hellman (ECDH) variants, can become the baseline for securing communication.

Internet of Things Wireless Connectivity Option: Wi-Fi Pros and Cons

Wifi-Hotspots
Today one of the most common connectivity technologies for consumer products is Wi-Fi, whose 802.11b/g flavors are using the license-free 2.4 GHz frequency band. A Wi-Fi access point (or hotspot) has a range of about 20 meters (66 feet) indoors and a slightly greater range outdoors. Wi-Fi has the benefit of a large spectrum allowing high data rates, 54 Mbps and still increasing with 802.11n and 802.11ac, in a license-free frequency band that is almost harmonized worldwide1 [17]. Wi-Fi has been widely adopted and it is a great way to provide wireless broadband internet access. However, Wi-Fi is designed for high data rates needed for multimedia contents, as opposed to many IoT applications. There has been some effort to promote low-power Wi-Fi; however, it remains an order of magnitude hungrier than what battery-powered devices in IoT application can afford such as battery powered sensors. In short, Wi-Fi is not a suitable candidate for many IoT applications. It is overkill on the data rate for most applications and an absolute power guzzler.  Wi-Fi is likely to remain the major smartphone and Internet connectivity medium. One can envision that the IoT network gateway would be embedded in the Wi-Fi hub already present in most homes, commercial spaces, factories, and offices.

Lockitron smart lock – WiFi is clearly not the IoT solution. How about BLE?

bolt-mounted

Lockitron attempted using Wi-Fi as its way of communication for its “smart lock” product but could not deliver a consumer-friendly product due to its poor battery life. Its a good lesson all should learn from: WiFi just does not work for battery powered objects. It is just too power hungry. Now, Lockitron is using BLE.

BUT there is no doubt BLE has its downsides. It is operating in the crowded 2.4 GHz frequency band, along with Bluetooth Classic, Wi-Fi, ZigBee, and IEEE 802.15.4. This spectrum crowding will pose a severe reliability challenge to all 2.4 GHz devices, and the problem will only get worse when the number of connected object increases.

BLE is optimized for low-latency sporadic transmissions and therefore its efficiency degrades dramatically for larger data transfers. With its maximum of 20 bytes application payload size per packet, the gross 1 Mbps data rate of BLE translates into a theoretical maximum transfer rate of 250 kbps, and in practice the actual transfer rates drops below 100 kbps. This opposed to Bluetooth Classic v1.2 that achieves 700 kbps, and v2.1 + EDR reaches 2 Mbps actual transfer rate. An actual transfer rate of only 1/10 of the gross data rate is rather lackluster and translates into poor power-efficiency for such type of data traffic. Although many IoT applications may have a limited data amount to transfer, e.g., for switching off or changing the color of a light bulb, others would still require sizeable transfers. As a result, BLE is not suitable for IoT applications that require higher data transfers.

BLE has limited range and extending the network therefore requires a hybrid topology where some client nodes act as server nodes for other star networks. In Bluetooth-specific terminology, this is called scatternet, which yields high network complexity in real deployments. For instance, BLE is essentially asynchronous, such that this hybrid topology (mix of star and mesh) causes increased interference and increased power consumption, even inside a single network.

Finally, BLE suffers from interference from USB 3.0, and poses a challenge when operating with collocated LTE or WIMAX networks. This is reflected in Bluetooth SIG filtering recommendations. However, workarounds are developed as well.

BLE may be a viable short term solution. But we will see what unfolds if a future of 50 billion objects comes to fruition.