Wireless Sensor Networks : Technology, Protocols, and Applications

176 MEDIUM ACCESS CONTROL PROTOCOLS FOR WIRELESS SENSOR NETWORKS
supports message integrity protection to prevent an adversary from tampering with
a data message from an authorized sender, while the message is in transit. The third
security service provides data confidentiality to keep the information carried by a
data message secret from unauthorized parties. This is achieved using the advanced
encryption standard (AES) as its core cryptographic algorithm. The encryption
scheme uses a 128-bit key to encrypt messages. The fourth security service deals
with sequential data freshness to prevent replay attacks. An adversary engages in a
replay attack by eavesdropping on legitimate messages sent between authorized
users and replaying them at a later time.
Using the basic security services, the MAC layer describes a variety of security
suites. Each suite offers a different set of security properties and guarantees. By
default, security is not enabled. The application must therefore explicitly set the appropriate
control parameters into the radio stack to enable the desired level of security.
The application layer consists of the application support sublayer (APS), the
ZigBee device object (ZDO), and the manufacturer-defined application objects.
The responsibilities of the APS sublayer include maintaining tables for binding
devices together, based on their services and their needs, and forwarding messages
between bound devices. The ZDO can be thought of as a special application object
that is resident on all nodes. It has its own profile, referred to as the ZigBee device
profile (ZDP), which user application endpoints and other ZigBee nodes can access.
The ZDO is responsible for overall device management and security keys and policies,
including defining the role of the device within the network, initiating and
responding to binding requests, and establishing a secure relationship between network
devices. The manufacturer-defined application objects implement the actual
applications according to the ZigBee-defined application descriptions.
In the following section we focus on physical- and MAC-layer design issues,
mechanisms, and protocols. First, the overall characteristics of the PHY layer are
highlighted. Following, a description of the different components and operations of
the MAC layer is provided.
5.6.1 PHY Layer
The design of the PHY layer is driven by the need for a low-cost power-effective
physical layer for cost-sensitive low-data-rate monitoring and control applications.
Under IEEE 802.15.4, wireless links can operate in three unlicensed frequency
bands: 858 MHz, 902 to 928 MHz, and 2.4 GHz. Based on these frequency
bands, the IEEE 802.15.4 standard defines three physical media:
1. Direct-sequence spread spectrum using BPSK operating in the 868-MHz band
at a data rate of 20 kbps
2. Direct-sequence spread spectrum using BPSK operating in the 915-MHz band
at a data rate of 40 kbps
3. Direct-sequence spread spectrum using O-QPSK operating in the 2.4-GHz
band at a data rate of 140 kbps