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4. Scenario Considerations and Parameters for 6LoWPAN Routing

IP-based LoWPAN technology is still in its early stage of development, but the range of conceivable usage scenarios is tremendous. The numerous possible applications of sensor networks make it obvious that mesh topologies will be prevalent in LoWPAN environments and robust routing will be a necessity for expedient communication. Research efforts in the area of sensor networking have put forth a large variety of multi-hop routing algorithms [Bulusu]. Most related work focuses on optimizing routing for specific application scenarios, which can be realized using several modes of communication, including the following [Watteyne]:

o Flooding (in very small networks)

o Hierarchical routing

o Geographic routing

o Self-organizing coordinate routing

Depending on the topology of a LoWPAN and the application(s) running over it, different types of routing may be used. However, this document abstracts from application-specific communication and describes general routing requirements valid for overall routing in LoWPANs.

The following parameters can be used to describe specific scenarios in which the candidate routing protocols could be evaluated.

a. Network Properties:

  • Number of Devices, Density, and Network Diameter: These parameters usually affect the routing state directly (e.g., the number of entries in a routing table or neighbor list). Especially in large and dense networks, policies must be applied for discarding "low-quality" and stale routing entries in order to prevent memory overflow.

  • Connectivity: Due to external factors or programmed disconnections, a LoWPAN can be in several states of connectivity -- anything in the range from "always connected" to "rarely connected". This poses great challenges to the dynamic discovery of routes across a LoWPAN.

  • Dynamicity (including mobility): Location changes can be induced by unpredictable external factors or by controlled motion, which may in turn cause route changes. Also, nodes may dynamically be introduced into a LoWPAN and removed from it later. The routing state and the volume of control messages may heavily depend on the number of moving nodes in a LoWPAN and their speed, as well as how quickly and frequently environmental characteristics influencing radio propagation change.

  • Deployment: In a LoWPAN, it is possible for nodes to be scattered randomly or to be deployed in an organized manner. The deployment can occur at once, or as an iterative process, which may also affect the routing state.

  • Spatial Distribution of Nodes and Gateways: Network connectivity depends on the spatial distribution of the nodes and on other factors, such as device number, density, and transmission range. For instance, nodes can be placed on a grid, or randomly located in an area (as can be modeled by a two-dimensional Poisson distribution), etc. Assuming a random spatial distribution, an average of 7 neighbors per node are required for approximately 95% network connectivity (10 neighbors per node are needed for 99% connectivity) [Kuhn]. In addition, if the LoWPAN is connected to other networks through infrastructure nodes called gateways, the number and spatial distribution of these gateways affect network congestion and available data rate, among other things.

  • Traffic Patterns, Topology, and Applications: The design of a LoWPAN and the requirements for its application have a big impact on the network topology and the most efficient routing type to be used. For different traffic patterns (point-to-point, multipoint-to-point, point-to-multipoint) and network architectures, various routing mechanisms have been developed, such as data-centric, event-driven, address-centric, and geographic routing.

  • Classes of Service: For mixing applications of different criticality on one LoWPAN, support of multiple classes of service may be required in resource-constrained LoWPANs and may require a new routing protocol functionality.

  • Security: LoWPANs may carry sensitive information and require a high level of security support where the availability, integrity, and confidentiality of data are of prime relevance. Secured messages cause overhead and affect the power consumption of LoWPAN routing protocols.

b. Node Parameters:

  • Processing Speed and Memory Size: These basic parameters define the maximum size of the routing state and the maximum complexity of its processing. LoWPAN nodes may have different performance characteristics, queuing strategies, and queue buffer sizes.

  • Power Consumption and Power Source: The number of battery- and mains-powered nodes and their positions in the topology created by them in a LoWPAN affect routing protocols in their selection of paths that optimize network lifetime.

  • Transmission Range: This parameter affects routing. For example, a high transmission range may cause a dense network, which in turn results in more direct neighbors of a node, higher connectivity, and a larger routing state.

  • Traffic Pattern: This parameter affects routing, since highly loaded nodes (either because they are the source of packets to be transmitted or due to forwarding) may contribute to higher delivery delays and may consume more energy than lightly loaded nodes. This applies to both data packets and routing control messages.

c. Link Parameters: This section discusses link parameters that apply to IEEE 802.15.4 legacy mode (i.e., not making use of improved modulation schemes).

  • Throughput: The maximum user data throughput of a bulk data transmission between a single sender and a single receiver through an unslotted IEEE 802.15.4 2.4 GHz channel in ideal conditions is as follows [Latre]:

    • 16-bit MAC addresses, unreliable mode: 151.6 kbit/s
    • 16-bit MAC addresses, reliable mode: 139.0 kbit/s
    • 64-bit MAC addresses, unreliable mode: 135.6 kbit/s
    • 64-bit MAC addresses, reliable mode: 124.4 kbit/s

    Throughput for the 915 MHz band is as follows:

    • 16-bit MAC addresses, unreliable mode: 31.1 kbit/s
    • 16-bit MAC addresses, reliable mode: 28.6 kbit/s
    • 64-bit MAC addresses, unreliable mode: 27.8 kbit/s
    • 64-bit MAC addresses, reliable mode: 25.6 kbit/s

    Throughput for the 868 MHz band is as follows:

    • 16-bit MAC addresses, unreliable mode: 15.5 kbit/s
    • 16-bit MAC addresses, reliable mode: 14.3 kbit/s
    • 64-bit MAC addresses, unreliable mode: 13.9 kbit/s
    • 64-bit MAC addresses, reliable mode: 12.8 kbit/s

  • Latency: Latency ranges -- depending on payload size -- of a frame transmission between a single sender and a single receiver through an unslotted IEEE 802.15.4 2.4 GHz channel in ideal conditions are as shown below [Latre]. For unreliable mode, the actual latency is provided. For reliable mode, the round-trip time, including transmission of a Layer-2 acknowledgment, is provided:

    • 16-bit MAC addresses, unreliable mode: [1.92 ms, 6.02 ms]
    • 16-bit MAC addresses, reliable mode: [2.46 ms, 6.56 ms]
    • 64-bit MAC addresses, unreliable mode: [2.75 ms, 6.02 ms]
    • 64-bit MAC addresses, reliable mode: [3.30 ms, 6.56 ms]

    Latency ranges for the 915 MHz band are as follows:

    • 16-bit MAC addresses, unreliable mode: [5.85 ms, 29.35 ms]
    • 16-bit MAC addresses, reliable mode: [8.35 ms, 31.85 ms]
    • 64-bit MAC addresses, unreliable mode: [8.95 ms, 29.35 ms]
    • 64-bit MAC addresses, reliable mode: [11.45 ms, 31.82 ms]

    Latency ranges for the 868 MHz band are as follows:

    • 16-bit MAC addresses, unreliable mode: [11.7 ms, 58.7 ms]
    • 16-bit MAC addresses, reliable mode: [16.7 ms, 63.7 ms]
    • 64-bit MAC addresses, unreliable mode: [17.9 ms, 58.7 ms]
    • 64-bit MAC addresses, reliable mode: [22.9 ms, 63.7 ms]

Note that some of the parameters presented in this section may be used as link or node evaluation metrics. However, multi-criteria routing may be too expensive for 6LoWPAN nodes. Rather, various single-criteria metrics are available and can be selected to suit the environment or application.

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