Here are some comparisons between the various features of the ZigBee and DigiMesh protocols including:
Topology Comparison Starting a Network Addressing Nodes Sleep Differences Security Differences Large Network Support Comparative Advantages
Needs a structured join sequence to get started Multiple node types and roles More complex deployment 2.4 GHz only Two mile maximum range Interoperable with others Basic sleep modes available Only end-devices can sleep Very low sleep currents By default, nodes send data to the Coordinator Very easy to get started Single node type Simple deployment strategy 2.4 GHz, 900/868/865 MHz options Long range options Proprietary - only works with Digi Intl devices Basic sleep modes available Sleeping router support - all devices can sleep Low sleep currents By default, nodes send data in broadcast mode For more detail about the ZigBee protocol, see ZigBee protocol overview.
If your application has non-routing nodes that may join and leave the network arbitrarily, DigiMesh is a good solution. With DigiMesh, you can create a backbone of routers as a base network, then accept “visitor” nodes easily at any time. Visitors can serve as routers while they are joined to the network, however routing can be turned off if desired to improve network stability
With Zigbee, it is not possible to turn off routing. However DigiMesh provides the option to turn off routing services. Here's a few application examples that illustrate such moving modules:
Military training where soldiers wear biometric sensors that capture heart rate and other data, such as GPS for location. However data should not be routed through other soldiers' devices because they are constantly moving. DigiMesh allows creating a network of static routers in fixed locations to backhaul the data from the mobile soldiers. An assembly line, where bins travel down a conveyor. The workers perform a process on the bins, and the factory needs these actions tracked without routing data between the bins since managing their constant rearrangement would create a heavy network load. A tire company runs cars around a test track, and wants to collect data without routing it through the other nodes moving around the track. DigiMesh allows sending point-to-multipoint or mesh packets on the fly. In API mode, developers have the option to send data via multipoint or mesh. In the case of multipoint, the message won't be routed via other nodes, while choosing mesh will send the data through intermediary hops as needed. There are applications that require switching from one mode to another quickly. For example talking directly from a backpack to a locally handheld device in point-to-point mode is faster and more reliable. Communications can then be switched to mesh mode to route data from the backpack over the larger network, allowing communication over a wide area.
Starting a Network There are important process differences between initiating a Zigbee and DigiMesh network on Digi XBee radio modules.
Starting a Zigbee network. These are the steps in initializing a Zigbee network:
Coordinator selects the channel in within the mask Coordinator selects a (random) 16-bit PAN ID Routers and end devices join the network Routers can be added to extend network coverage
Important considerations: Set the SC command (bitmask – channels to scan). Set the ID command (extended PAN ID to select). By default, routers and end devices scan the channels allowed by the SC bitmask and join any PAN ID. Ensure a coordinator (or joined router) is running on a valid PAN ID and on a channel included in the SC bitmask. Read the actual operating PAN ID and operating channel using the OI and CH commands. Join failures can be diagnosed using the AI command.
Starting a DigiMesh network All nodes must be pre-configured with the same Preamble ID (HP), Channel Mask (CM) and Network ID (ID). Nodes can be added later to extend network coverage. Selectively, routing can be enabled or disabled on individual nodes (CE). Consider using the Preamble ID to split networks in nearby locations.
Addressing Nodes ZigBee devices support both 64-bit and 16-bit addressing, and a node can be identified using either type of addressing. DigiMesh only supports 64-bit addressing.
Supports 64-bit address
Unique per device, the MAC address Fixed – best way to absolutely address a device X X Supports 16-bit address
Assigned to devices when joining Used in routing tables Volatile – can change under certain conditions X Supports broadcast addressing
Note: Performance is slow with Zigbee.
X X
Sleep Differences Zigbee Zigbee coordinator and router devices never sleep, because they need to be powered on to route any messages sent by other nodes through the mesh. Zigbee end devices can sleep cyclically, polling their parent node for incoming messages at each wakeup.
DigiMesh Synchronous sleep, or sleeping routers, are only available in the DigiMesh protocol, where every node can route data. All DigiMesh nodes can sleep and all nodes on a network will synchronize their sleep times with each other. Because all DigiMesh routers wake up simultaneously, there is no need for parent devices and no need for devices to stay powered on continuously as messages will only be exchanged during the synchronized wake-ups.
Security Differences Zigbee Zigbee protocol uses two encryption keys:
Link Key – shared by all devices Network Key: Created by the Coordinator Shared with other nodes encrypted by the link key Note: for added security, joining can be disabled (use the NJ command in Digi Bee radios).
DigiMesh DigiMesh is simpler and uses one pre-shared 128-bit AES encryption key. This makes the network very easy to set up and manage in realtime.
Enhanced security for DigiMesh
Because of the single encryption key, gaining physical access to a DigiMesh node could allow sending unauthorized remote AT commands. This is possible even though the network is encrypted. In upcoming firmware, AT commands will be password-protected to eliminate this potential risk.
Large Network Support Both ZigBee and DigiMesh unicast messages are limited by the module’s routing table size, and they each have unique features to overcome these limitations:
ZigBee Many-to-one routing Many-to-one routing is optimized for routing data messages from a large number of remote nodes to one central location. It is frequently used for sensor networks. The central location broadcasts a single routing configuration message out to the network, allowing all remote devices to set up and save a reverse path back toward the central destination node. After this path is created, no more discoveries are needed for information to be properly delivered to the central base station. The AR command is used to enable many-to-one broadcasting on Digi XBee devices.
Source routing Source routing allows a central location to discover and store individual outbound routes to remote nodes. It is typically used for networks that send commands out from a central base station to a large number of remote devices. These routes are not stored on the central location’s radio, but obtained by that device or computer controlling it. On Digi International devices, this is accomplished by using the XBee’s Route Record API frame. When the time comes to send a message to one particular remote node, the central location specifies a route to the remote node with a Create Source Route API frame. It then includes the data and the destination address in an API Transmit Request. Routes include the address of each intermediate hop that messages need to pass through to reach their destination.
DigiMesh Directed broadcasting Unicast messages are sent out via broadcast, but with a specific destination address so that only the receiver node passes the message to its serial port. This mode removes the need for large network routing tables, and is most appropriate for large networks with very low traffic.
Aggregator support For networks like sensor networks where a large number of remote nodes send data to an aggregator base station, issuing the AG command for the desired aggregator node will cause all other nodes in the network to build routes to that node. All XBee devices that receive an AG broadcast update their routing table information to create a route to the sending device, regardless of whether their DH/DL address is updated. This routing information is used for future unicast transmissions.
Comparative Advantages ZigBee advantages: Open standard with interoperability between vendors. Large community of skilled developers Third party integration. For example, Home Automation or Smart Energy profiles work seamlessly between manufacturers Enhanced security is available DigiMesh advantages: Network setup is simpler. Multiple frequency bands are available. More flexibility to expand the network; no end device to router ratios to worry about. Quieter network allows larger packets and higher throughput. Ideal for broadcast intensive programs such as Modbus.