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Chapter 8: GPS and a Real- T ime Situational Displa y 211

Actual data packets are formed at the MAC, or layer 2, level where the data is prepended
with both the source and destination addresses. A sequence number is also assigned to allow
the receiver to determine the correct sequence of received packets. It is relatively easy to
receive out-of-sequence packets in this type of network. Frame control bytes are also
appended for error checking, which is the reason why ACK packets are required. ZigBee is
a type of connection network, similar to Ethernet, that has a very robust way of ensuring that
packets get where they need to go. ZigBee Layer 3 uses acknowledgement packet (ACK).
The receiver performs a 16-bit cyclic redundancy check (CRC) to verify that the packet was
not corrupted during transmission. If a good CRC is determined, the receiver will then
transmit an ACK; this action allows the transmitting XBee node to know that the data
were received properly. The packet is discarded if the CRC indicates the packet was
corrupted, and no ACK is transmitted. The network should be configured such that the
transmitting node will resend up to a predetermined number of times until either the packet
is successfully received or the resend limit is reached. The ZigBee protocol provides self-
healing capabilities if the path between the transmitter and receiver has become unreliable
or a complete network failure has happened. Alternate paths will be established if physically
possible.
Layers 1 and 2 support the following standards:

• Star, mesh, and cluster tree topologies
• Beaconed networks
• GTS for low latency
• Multiple power-saving modes (idle, doze, hibernate)

Layers 3 and 4 further refine the packets by identifying what the packet type is, where it
is going, and where it has been. They also set the data payload and support the following:

• Point-to-point and star network configurations
• Proprietary networks

Layer 4 sets up the routing, thus ensuring that the packets are sent along the correct
paths to reach the desired nodes. This layer also ensures that:

• ZigBee 1.0 specifications are met
• Support is provided for star, mesh, and tree networks

There are also three ZigBee standards that primarily involve Layers 3 and 4. These
standards are:

1. Routing—Defines how messages are sent through ZigBee nodes. Also referred to as
digi-peating.
2. Ad hoc network—Creates a network automatically without any operator
involvement.
3. Self-healing mesh—Determines automatically if a malfunctioning node exists and
reroutes messages, if physically possible.
Layer 5 is responsible for security, which is enforced by using the Advanced Encryption
Standard (AES) 128-bit security key.