wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

51-4 Industrial Communication Systems
PAN coordinator
(FFD)
Coordinator (FFD)
End device (RFD)
Star topology
Peer-to-peer topology
FIGURE 51.2
IEEE 802.15.4 network topologies.
data is pending to be received by the specific RFD and the time slot reserved for the data transmission.
Thus, RFDs only need to wake up and activate power-consuming radio for receiving of beacons and data
addressed directly to them.
An IEEE 802.15.4 network requires at least one FFD as a network coordinator; all other devices can
be RFDs. Because transmission range of LR WPAN is very limited, by means of integrating additional
routers, meshed networks can be built. These routers are FFDs. They forward data hop wise and enlarge
the dimension of the network.
All devices shall have unique 64 bit IEEE end-system unique identifier (EUI) addresses [EUI64], typically
hard coded by manufacturers. This 64 bit address may be used for direct communication in the
network. To elide protocol and addressing overhead, alternatively a PAN coordinator may allocate a short
16 bit address to devices that allows theoretically 65,535 end devices to be included in one network. Short
addresses are unique for one subset of the network and the same short address can be used in different
subset networks. In such a case, the additional 16 bit PAN-ID is used to differentiate between the networks.
The IEEE 802.15.4 standard defines two PHY layers: the 2.4.GHz and 868/915.MHz band PHY layers.
The PHY layer depends on local regulation and user preference. The unlicensed 2.4.GHz is valid worldwide
and has a higher data rate (240.kbps) compared to 915.MHz band with 40.kbps (United States) and
868.MHz (EU) band with 20.kbps data rate. The radio can operate on 16 channels in the 2.4.GHz band,
10 channels in the 915.MHz band, and 1 channel in the 868.MHz band.
The IEEE 802.15.4 MAC defines four frame structures:
1. A data frame, used for all transfers of data and all later-described 6LoWPAN transmissions
(see Figure 51.3)
2. An acknowledgment frame, used for confirming successful frame reception
3. A MAC command frame, used to manage and control network
4. A beacon frame, used by a coordinator to transmit beacons
Data frames of PHY layer start with 4 bytes preamble and 1 byte Start-of-Frame (SOF) delimiter and
both are used for synchronization. The following 7 bit field specifies the frame length of following data
frame (up to 127), a last bit in that octet is reserved.
MAC frame format is composed of MAC Header (MHR), Payload, and MAC Footer (MFR). MHR
consists of 2 byte frame control field, 1 byte sequence number, address information, and security-related
information (4–20 byte). The frame control field specifies the type of above-mentioned frame types and
address mode. The payload has a variable data length and provides information specific to individual
frame types. The frames end with 2 byte checksum.
© 2011 by Taylor and Francis Group, LLC