IEEE 802.15.4 Standard for Wireless Sensor Networks

Wireless Sensor Networks (WSNs) are a relatively new application of ad-hoc networks providing high quality monitoring for large geographical areas with relatively inexpensive equipment. The technology involves deploying (manually or from a plane) a large number (50-10,000) of small, inexpensive nodes over the area of interest (Figure 1). Each node is equipped with one or several sensors (suited to the application), a short range (10-100m) radio and a microcontroller. The nodes form an ad-hoc network capable of sending the sensed data to one ore more base stations that further forward the data using a long haul link to the monitoring center.

The first applications of WSNs were military (surveillance and command and control info), but there are also several civilian applications: structure health monitoring, habitat monitoring, border enforcement, etc.

The challenges in WSN usually stem from the limited resources (processing power, transmission bandwidth, memory, battery, etc.) available to each node (to be deployed in large numbers, they have to be inexpensive). Most solutions requiring centralized processing or global information are likely wasteful for large WSNs.

Figura 1 – Wireless Sensor Networks

The IEEE Std 802.15.4

IEEE Std 802.15.4 defines the physical layer (PHY) and medium access control (MAC) sublayer specifications for low-data-rate wireless connectivity with fixed, portable, and moving devices with very limited battery consumption requirement, typically operating in the personal operating space (POS) of 10 m. It is foreseen that, depending on the application, a longer range at a lower data rate may be an acceptable tradeoff.

LR-WPAN (Low-rate Wireless Personal Area Network)

An LR-WPAN is a simple, low-cost communication network that allows wireless connectivity in applications with limited power and relaxed throughput requirements. The main objectives of an LR-WPAN are ease of installation, reliable data transfer, short-range operation, extremely low cost, and a reasonable battery life, while maintaining a simple and flexible protocol.

Motivação

Criar uma tecnologia WPAN simples, de baixo custo e baixa potência, que pudesse atender a uma classe de aplicações wireless com demandas restritas de taxa de dados e QoS, inviáveis de se atingir com as outras tecnologias WPAN (ex: Bluetooth/IEEE 802.15.1) ou WLAN (WiFi/IEEE 802.11) (Figuras 2).

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Figura 2 – Padrões Wireless

Whereas IEEE 802.11 (WLAN) was concerned with features such as Ethernet matching speed, long-range (100m), complexity to handle seamless roaming, message forwarding, and data throughput of 2-11Mbps, WPANs are focused on a space around a person or object that typically extends up to 10m in all directions. The focus of WPANs is low-cost, low power, short range and very small size.

The IEEE 802.15 working group was formed to create WPAN standard. This group has defined three classes of WPANs that are differentiated by data rate, battery drain and quality of service (QoS). The high data rate WPAN (IEEE 802.15.3) is suitable for multimedia applications that require very high QoS. Medium rate WPANs (IEEE 802.15.1/Bluetooth) handle a variety of tasks, e.g. smartphones communications, and have QoS suitable for voice communications. The low rate

WPANs (IEEE 802.15.4/LR-WPAN) is intended to serve a set of industrial, residential and medical applications with very low power consumption and cost requirement not considered by the above WPANs and with relaxed needs for data rate and QoS. The low data rate enables the LR-WPAN to consume very little power.

High-rate WPAN was driven by applications requiring high data rates and/or wide spatial coverage, often involving complex solutions with non-trivial power requirements. However, not all applications have such demanding needs – some network applications involve the infrequent exchange of relatively small amount of data over restricted areas (for example, a home temperature monitoring and control network). Such applications are diverse in nature and represent considerable market potential. Bluetooth was not designed for multiple-node networks, therefore the IEEE devised a WPAN standard based on a new set of criteria:

Very low complexity Ultra low power consumption Low data rate Relatively short radio communication range Use of unlicensed radio bands Easy installation Low cost

A central feature of the IEEE 802.15.4 standard is the requirement for extremely low power consumption. The motivation for this strict power requirement is to enable the use of battery-powered network devices that are completely free of cabling (no network or power cables), allowing them to be installed easily and cheaply (no costly cable installation needed), possibly in locations where cables would be difficult or impossible to install. However, low power consumption necessitates short ranges. Some of the characteristics of an LR-WPAN are as follows:

Over-the-air data rates of 250 kb/s, 100kb/s, 40 kb/s, and 20 kb/s Star or peer-to-peer operation Allocated 16-bit short or 64-bit extended addresses Optional allocation of guaranteed time slots (GTSs) Carrier sense multiple access with collision avoidance (CSMA-CA) channel access Fully acknowledged protocol for transfer reliability Low power consumption Energy detection (ED) Link quality indication (LQI) 16 channels in the 2450 MHz band, 30 channels in the 915 MHz band, and 3 channels in the

868 MHz band.

IEEE 802.15.4 Applications

The applications of IEEE 802.15.4 based networks are wide ranging, covering both industrial and domestic use. Essentially, for IEEE 802.15.4 to be used in a networking solution, the required data rate must be low (≤ 250 kbps) and the maximum range for communicating devices must be short. In addition, a device with an autonomous power supply (no power cables) must have an extremely low power consumption.

Home Automation and Security: HVAC, lighting, curtains/blinds, doors, locks, home entertainment systems; intruder and fire detection.

Consumer products: remote control for the various components of a home entertainment system; computer systems and toys, in which a wireless radio link may be used to replace a point-to-point cable link .

Healthcare: sensors and diagnostic devices that can be networked by means of a wireless PAN .

Vehicle Monitoring: Vehicles usually contain many sensors and diagnostic devices, and provide ideal applications for wireless PANs. A prime example is the use of pressure sensors in tyres, which cannot be connected by cables.

Agriculture: Wireless PANs can help farmers monitor land and environmental conditions in order to optimise their crop yields. Such networks can operate at very low data rates and latencies, but require wide geographical coverage – the latter issue is addressed by using network topologies that allow the relaying of messages across the network.

Figure 3 presents some of the application domains for ZigBee LR-WPAN networks.

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Figura 3 – LR-WPAN (ZigBee) Application Domains

IEEE 802.15.4 versus IEEE 802.15.1 (Bluetooth)

Obs.:

Embora tenha sido projetado para redes de pequeno alcance, pode-se encontrar hoje no mercado módulos ZigBee/IEEE 802.15.4 com alcances que variam de 30m a 1600m (Digi International). Outra característica relevante do ZigBee/IEEE 802.15.4 é o tempo bastante curto que os dispositivos gastam para se associar a uma rede e o tempo em que um dispositivo que se encontrava em standby leva para voltar ao estado ativo. Para aplicações industriais onde o tempo é um fator crítico (sistemas de tempo real), esta característica é de grande importância. As Figuras 4a, 4b e 4c apresentam uma comparação entre os padrões wireless ZigBee (802.15.4), Bluetooth (802.15.1) e WiFi (802.11).

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(c)Figura 4 – Comparação entre os padrões ZigBee (802.15.4) x Bluetooth (802.15.1) x WiFi (802.11)

Tipos de Nós

A camada MAC define dois tipos de nós/dispositivos: FFD (Full Function Device) ou “Dispositivos de Funções Completas”, e RFD (Reduced Function Device) ou §“Dispositivos de Funções Reduzidas”. An FFD is a device that provides the full set of IEEE 802.15.4 MAC services, allowing it to act as a personal area network coordinator (PAN Coordinator), a coordinator/router or a end device (Figura 5).

Figura 5 – Tipos de Dispositivos (Nós) 802.15.4

An FFD can talk to RFDs or other FFDs, while an RFD can talk only to an FFD. When acting as a network coordinator, FFDs provide synchronisation, communication and network join services. An RFD is a device that provides a reduced set of IEEE 802.15.4 MAC services, with restricted processing and memory resources, so it cannot act as a coordinator. RFDs can only act as end-devices and are equipped with sensors/actuators like transducers, light switches, lamps, etc. An RFD is intended for applications that are extremely simple, such as a light switch or a passive infrared sensor; they do not have the need to send large amounts of data and may only associate with a single FFD at a time. Consequently, the RFD can be implemented using minimal resources and memory capacity.

Todos os dispositivos operando na rede, em qualquer topologia, terão, cada um, um único endereço estendido IEEE-64, de 64 bits. Entretanto, um dispositivo RFD pode obter endereços de apenas 16 bits do coordenador da rede (FFD).

Obs.: These roles exist at the network level – a ZigBee/IEEE 802.15.4 node may also be performing tasks at the application level independent of the role it plays in the network. For instance, a network of ZigBee/IEEE 802.15.4 devices measuring temperature may have a temperature sensor application in each node, irrespective of whether they are End Devices, Routers or the Co-ordinator.

Nó Roteador (Router)

O Roteador só pode ser implementado por intermédio de um dispositivo FFD. Tem as características de um nó normal na rede, mas com poderes extras de também exercer a função de

roteador intermediário entre nós sem precisar do Coordenador. Por meio de um roteador, uma rede 802.15.4 pode ser expandida e assim ter mais alcance (rede multi-hop). A network which uses tree or mesh topology requires the presence of at least one router.

The main tasks of a Router are: Relays messages from one node to another Allows child nodes to connect to it

In ZigBee/IEEE 802.15.4 standard the router cannot be put in the sleep mode, as router need to be always mains powered. The only device in the ZigBee network can be in the slepp mode is an end device, which means to be working as battery powered devices.

Dispositivo Final (End Device)

O Dispositivo Final (End Device) é onde os atuadores ou sensores serão hospedados. Pode ser implementado utilizando-se um dos dispositivos FFD ou RFD. This is a node which has an input/output function but no co-ordinating functionality. The term “End Device” is not used in the IEEE 802.15.4 standard, but is commonly used in the field.

The main tasks of an End Device at the network level are sending and receiving messages. An End Device can often be battery-powered and, when not transmitting or receiving, can sleep in order to conserve power. Note that End Devices cannot relay messages and cannot allow other nodes to connect to the network through them.

Nó Coordenador (Co-ordinator)

All IEEE 802.15.4 networks must have one (and only one) Co-ordinator (PAN Co-ordinator) irrespective of the network topology. The PAN coordinator is the primary controller of the PAN. The PAN coordinator might often be mains powered, while the devices will most likely be battery powered. O Coordenador da PAN é responsável pela inicialização, distribuição de endereços, manutenção da rede e reconhecimento de todos os nós, entre outras funções, podendo servir como ponte entre várias outras redes 802.15.4.

The tasks of the Co-ordinator at the network layer are: Selects the frequency channel to be used by the network i.e., finding a suitable radio

frequency for network operation (usually the one with the least detected activity) Starts the network Assigning a PAN ID to the network Distributes short addresses to the devices Handling requests from other devices to join the network (e.g., allows child nodes to

connect to it) Provides message routing, security management and other services.

A Tree network can have one or more local Co-ordinators (as well as a PAN Co-ordinator). Each of these Co-ordinators serves its own children and its roles include:

Handling requests from other devices to join the network Relaying messages from one node to another

A Figura 6 resume as principais funções dos dispositivos 802.15.4.

Figura 6 – Funções dos Dispositivos (Nós) 802.15.4

Topologias

Uma rede IEEE 802.15.4 suporta três topologias de rede: estrela (star), malha (mesh) e árvore (cluster-tree). A Figura 7 ilustra estas topologias. Observa-se que o padrão define, na verdade, apenas duas topologias: star e peer-to-peer. As topologias mesh e cluster-tree são exemplos de topologias peer-to-peer.

Figura 7 – Topologias de uma rede 802.15.4

a) Star

The network has a central node, which is linked to all other nodes in the network (Figure 8). All messages travel via the central node. In the star topology, the communication is established between devices and a single central controller, called the PAN coordinator. The PAN coordinator may be mains power ed while the devices will most likely be battery powered. Applications that benefit from this topology include home automation, personal computer (PC) peripherals, toys and games.

After an FFD is activated for the first time, it may establish its own network and become the PAN coordinator. Each start network chooses a PAN identifier, which is not currently used by any other network within the radio sphere of influence. This allows each star network to operate independently.

Figura 8 – Toplogia Estrela

b) Peer-to-Peer

In peer-to-peer topology, there is also one PAN coordinator. In contrast to star topology, any device can communicate with any other device as long as they are in range of one another. A peer-to-peer network can be ad hoc, self-organizing and self-healing. Applications such as industrial control and monitoring, wireless sensor networks, asset and inventory tracking would benefit from such a topology. It also allows multiple hops to route messages from any device to any other device in the network. It can provide reliability by multipath routing.

Figura 9 – Toplogia Peer-to-Peer

Uma rede peer-to-peer em malha (mesh) (Figure 9) é uma rede ad hoc que provê alta confiabilidade. The network has a tree-like structure in which some leaves are directly linked. Um roteador em uma rede em malha, ao contrário de uma rede cluster-árvore, pode se comunicar com qualquer outro diretamente, i.e., messages can travel across the tree when a suitable route is available. Nessa topologia, a rede pode se ajustar automaticamente, tanto na sua inicialização como na entrada ou saída de dispositivos na rede. A rede se auto-organiza para otimizar o tráfego de

dados. Com vários caminhos possíveis para a comunicação entre os nós, esse tipo de rede pode abranger, em extensão, uma ampla área geográfica. In the Mesh topology, a “route discovery” feature is provided which allows the network to find the best available route for a message. Note that message propagation is handled by the network layer software and is transparent to application programs.

c) Cluster-Tree Cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs andan RFD may connect to a cluster-tree network as a leave node at the end of a branch. Any of the FFD can act as a coordinator and provide synchronization services to other devices and coordinators. Only one of these coordinators however is the PAN coordinator.

The simplest form of a cluster tree network is a single cluster network (Figure 10), but larger networks are possible by forming a mesh of multiple neighboring clusters. Once predetermined application or network requirements are met, the first PAN coordinator instructs a device to become the PAN coordinator of a new cluster adjacent to the first one. Other devices gradually connect and form a multicluster network structure, such as the one seen in Figure 11. The advantage of a multicluster structure is increased coverage area, while the disadvantage is an increase in message latency.

Figura 10 – Topologia Single Cluster-tree

In a multicluster structure (Figure 11), the PAN coordinator forms the first cluster by establishing itself as the cluster head (CLH) with a cluster identifier (CID) of zero, choosing an unused PAN identifier, and broadcasting beacon frames to neighboring devices. A candidate device receiving a beacon frame may request to join the network at the CLH. If the PAN coordinator permits the device to join, it will add this new device as a child device in its neighbor list. The newly joined device will add the CLH as its parent in its neighbor list and begin transmitting periodic beacons such that other candidate devices may then join the network at that device. Once application or network requirements are met, the PAN coordinator may instruct a device to become the CLH of a new cluster adjacent to the first one. The advantage of this clustered structure is the increased coverage area at the cost of increased messagelatency.

Figura 11 – Topologia Multi Cluster-tree

Addressing

In a ZigBee/IEEE 802.15.4 network, each node must have unique identification (Figure 12). This is achieved by means of two addresses:

IEEE (MAC) address: This is a 64-bit address, allocated by the IEEE, which uniquely identifies the device – no two devices in the world can have the same IEEE address. In a ZigBee network, it is also sometimes called the extended address.

Short network address: This 16-bit address identifies the node in the network and is local to that network (thus, two nodes on separate networks may have the same network address). Network addresses are allocated by the parent node (Router or Co-ordinator) when a node joins a network.

Figura 12 – IEEE 802.15.4 Addressing

Network Configuration

The effective size and configuration of a Zigbee/IEEE 802.15.4 network is defined at initialisation by the Co-ordinator. This defines the number of nodes that can be connected to a Router, the maximum number of Routers that can be children of a Router (and hence the maximum number of Routers in the network), and the number of hops from the most distant point in the network to its effective centre (the Co-ordinator). Therefore, this defines the maximum number of nodes that the network can support.

The following concepts are important in network configuration: Depth: The depth of a device is the number of nodes from the root of the network tree (the

Co-ordinator) to the device. The maximum network depth is then the maximum number of hops to the most distant device in the network from the Co-ordinator. This is set at configuration time and determines the overall diameter for the network. Note that a Star network has a network depth of 1.

Number of Children: Each Router in the network can have a number of child devices attached to it. These may be either Routers or End Devices. The Co-ordinator specifies the number of child devices allowed per Router, and how many of these child devices can be Routers.

Network Address Allocation: Network addresses are allocated during initialisation of the network. The Co-ordinator allocates each child Router a block of consecutive addresses to allocate to its children. These address blocks provide each Router with enough addresses to allocate to all of its potential children, as defined by the maximum routers, maximum children and maximum depth parameters. In turn, the block of addresses given to a Router will be subdivided amongst its child Routers and so on down the tree. Since the topology of a deployed network is not necessarily uniform (as many nodes may be needed in one geographic area and relatively fewer in another), it is possible for one Router to have all of its allocated addresses in use while those on another may be unused.

Starting a Network

The ZigBee/IEEE 802.15.4 Co-ordinator is responsible for starting a network. It first searches for a suitable radio channel (usually the one which has least activity). This can be limited to those known to be usable; for example, avoiding frequencies where it is known a wireless LAN is operating. After choosing a channel, the Co-ordinator starts the network, identifying the network with a PAN ID; and is then ready to respond to queries from other nodes that want to join.

Star Network Formation After an FFD is activated, it can establish its own network and become the PAN coordinator Choose a PAN Identifier different from surrounding networks (within RF sphere of

influence) The PAN coordinator allows other devices, potentially both FFDs and RFDs, to join its

network.

Peer-to-Peer Network Formation Each device is capable of communicating with any other device within its radio sphere of

influence One Device is nominated as the PAN coordinator Form first cluster by choosing an unused PAN identifier and broadcasting beacon frames to

neighboring devices. A candidate device receiving a beacon frame may request to join the network at the PAN

coordinator. If the PAN coordinator permits the device to join, it adds the new device as a child device in

its neighbor list. Newly joined device adds the PAN coordinator as its parent in its neighbor list and begins

transmitting periodic beacons Other candidate devices may then join the network at that device. Once predetermined application or network requirements are met, the first PAN coordinator

may instruct a device to become the PAN coordinator of a new cluster adjacent to the first one.

Other devices gradually connect and form a multi-cluster network structure

Joining a Network

Routers and End Devices can join an existing network already created by a Co-ordinator. Both Routers and the Co-ordinator have the capability to allow other nodes to join the network. The join process is as follows:

1. The new node first scans the available channels to find operating networks and identifies which one it should join. Multiple networks may operate in the same channel and are differentiated by their PAN IDs.

2. The node may be able to ‘see’ multiple Routers and a Co-ordinator from the same network, in which case it selects which one it should connect to. Usually, this is the one with the best signal.

3. The node then sends a message to the Router asking to join the network. 4. The Router decides whether the node is a permitted device, whether the Router is currently

allowing devices to join and whether it has address space available. If all these criteria are satisfied, the Router will then allow the device to join and allocate it an address.

Typically, a Router or Co-ordinator can be configured to have a time-period during which joins are allowed. The join period may be initiated by a user action, such as pressing a button. An infinite join period can be set, so that child nodes can join the parent node at any time.

Low Duty Cycle

A typical battery-powered network device presents significant technical challenges for battery usage. Since these devices are generally small, they use low-capacity batteries. Infrequent device maintenance is often another requirement, meaning long periods between battery replacement and the need for long-life batteries. Battery use must therefore be carefully managed to make optimum use of very limited power resources over an extended period of time.

Most of the power consumption of a wireless network device corresponds to the times when the device is transmitting. The transmission time as a proportion of the time interval between transmissions is called the duty cycle. Battery use is optimised in IEEE 802.15.4 devices by using extremely low duty cycles, so that the device is transmitting for a very small fraction of the time. This is helped by making the transmission times short and the time interval between transmissions long. In all cases, when not transmitting, the device should revert to a low-power sleep mode to minimise power consumption.

Note: In practice, not all devices in a network can be battery-powered, particularly those that need to be switched on all the time (and cannot sleep), such as Coordinators. Such devices can often be installed in a mains-powered appliance that is permanently connected to the mains supply (even if not switched on); for example, a ceiling lamp or an electric radiator. This avoids the need to install a dedicated mains power connection for the network device.