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Communication infrastructure is the heart of wireless sensor networks. Sensor nodes wirelessly communicate to each other to transfer data to the user or between nodes. There are different protocols used in a wireless sensor network. The most broadly used interface is a radio transceiver that operates at the ISM band. We also use IEEE 802 family in deploying WSNs. WSNs adapt existing wireless communication technology like Bluetooth, ZigBee, and wireless LAN, to name a few.
Radio Standard
The IEEE 802.11 protocol was the first standard wireless local area network in 1997. IEEE 802.11b upgraded the IEEE 802.11 protocol to increase data rate suitable for media access control (MAC). Two frequency bands for WSNs: A 2.4-GHz band for IEEE 802.11b and IEEE 802.11g, and a 5-GHz band for IEEE 802.11a are available for usage. The early wireless sensor network installed WLAN in efforts for WSNs. The early adaptation of IEEE 802.11 in WSNs recommends the protocol is not suitable for low-power WSNs. It has high energy demand and data rate.
The IEEE 802.15.4 protocol with ZigBee offers an efficient option for WSNs. The ZigBee/IEEE 802.15.4 is a short-range communication protocol for low-power WSNs. It has a transmission rate of up to 250 kbps within 200 km. It works with other technology like Wi-Fi and Bluetooth. It has low energy demand and data rate as to IEEE 802.11, which is design for wireless LAN. Commercial sensor nodes support 802.15.4 adaptation to WSNs. The IEEE 802.15.4 protocol is design for wireless PAN.
WSNs use two frequency bands: the ISM band and the U-NII band. WSNs operate mainly on 2.4-GHz and 5-GHz spectrum. We broadly implement IEEE technologies in commercial WSNs, which operate at the 2.4-GHz ISM band. Bluetooth is a short-range communication protocol, which uses a 2.4-GHz ISM band and 1 to 3 Mbps bandwidth. IEEE 802.11a/b/g/n is a collection of technology operating at the 2.4-GHz ISM band. It transmits data up to 54 Mbps, which is twice the IEEE 802.11n protocol).
Bluetooth (IEEE 802.15.1) uses omnidirectional radio waves to transfer data through nonmetal barriers in the unlicensed, 2.4-GHz ISM band. It supports frequency hoping where signal hops among 79 frequencies ar 1-MHz interval. Bluetooth version 1.2 allows a maximum transfer rate of 1.2 Mbps, and in 2004, it reaches up to 3 Mbps. WSNs shy out in adapting Bluetooth due to performance restrictions. Zigbee is a better option in the same environment.
ZigBee addresses the demand for low-cost, low-power WSNs. The wireless system demands high data rates at the expense of power consumption. In 2004, ZigBee/IEEE 802.15.4 opens a new horizon for WSNs. ZigBee protocol is power efficient, cost-effective, and high data rate. IEEE 802.11b/11g allowed services for Internet access and VoIP applications. WiMax (IEEE 802.16) connectivity delivers metro-wide Internet and VoIP services.
The IEEE 802.16 is a point-to-multipoint broadband wireless access. It operates in the frequency range of 10 to 66 GHz. It needs a line of sight (LOS) at a higher frequency, typically greater than 10 MHz. The IEEE 802 MAC accommodates different physical layers (PHY) and services, which address the need for different metropolitan environments.
The medium access control (MAC) support layers of transport protocols like ATM, Ethernet, and IP. The MAC has high bit rates up to 268 Mbps that operate in the broadband physical layer. Its frame structure allows dynamic assignment of uplink and downlink profiles for their link condition.
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Medium Access Control (MAC) Protocol
Wireless sensor networks have a large number of wireless devices deployed over a geographic area in ad hoc. Sensor nodes are resource-scarce and therefore have limited processing and communication. WSNs need to self-organize into a multi-hop wireless network to support the underlying application.
Unlike a wired network, WSNs require a multi-hop wireless infrastructure to set the communication link to neighboring sensor nodes. The transmission medium shares the communication medium to all sensor nodes fairly. We achieve it using the media access control (MAC) protocol. WSNs relies heavily on the MAC protocol to effectively perform the task.
Wireless sensor nodes communicate using a uniques channel at any given time. The MAC protocol enables shared wireless communication between nodes to fulfill the application task. Data Link Layer (DLL) allocates two sublayers for MAC protocol, which manages access for shared access communication medium. The LLC sublayer supports several MAC options but depending on network topology, communication channel, and service quality.
The physical layer (PHY) sets the specification of the transmission medium and network topology. It coordinates the procedures and functions for physical devices and communication interfaces, including streaming bits over a wireless communication medium. The MAC sublayer is on top of the physical layer.
The LCC sublayer in DLL assembles data into frames by appending header fields to contain addressing information. It also adds trailer fields for error detection in the frame. Aside from assembly, LLC sublayer extracts addressing and error control information from the received frame. Lastly, LCC regulates access to a shared transmission medium.
The choice of a MAC protocol dictates the performance of WSNs. There are several approaches to solve the shared medium access problem. Fixed-assignment protocols assign each node a fixed amount of channel resources. The allocation for each node gives it exclusivity of usage without competing with other nodes. Frequency-division multiple access (FDMA), time-division multiple access (TDMA), and coded-division multiple access (CDA) belong to this protocol. Random assignment protocol assigns time slots in TDMA. Random assignment hopes to eliminate the shortcoming of bandwidth preallocation.
Energy conservation is a critical issue for wireless sensor networks. MAC protocols should be scalable and stable for WSNs. The protocol must regulate access to the media to prevent inherent problems in WSNs like excessive overhead and idle listening. There are two protocols that MAC-layer protocols address these problems: Schedule-based and content-based protocol.
A schedule-based protocol is a class of MAC-layer protocol in which each node has a schedule in accessing the communication channel. It limits one sensor node to access the media at any given time. The contention-based MAC-layer protocol does not preallocate bandwidth resources to individual sensor nodes but makes them available on demand.
The IEEE 802.15.4 is standard for wireless medium control (MAC) and physical layer (PHY) for low-rate wireless personal area networks (LR-WPANs). It supports periodic, intermittent, and repetitive traffics. The protocol operates at 20 to 250kbps for fixed, portable, and moving devices. For communication lines over 30 ft, the protocol allows a self-configuring multi-hop network. It complements other wireless technology like IEEE 802.11 and 802.15.1.
Conclusion
Wireless sensor network relies heavily on the shared communication medium. WSNs can maximize existing wireless technology rather than specifically developing technology for it. The IEEE 802 protocols offer wireless technology for WSNs like Bluetooth, Zigbee, WLAN, and WiMax. MAC-layer protocols for WSNs enables wireless sensor nodes to perform its task effectively. An efficient design of MAC-layer protocol must be scalable to accommodate network changes. The MAC-layer protocol should be able to organize the sensor node into a multi-hop wireless network.