Network Topologies

Star and Tree Topologies

In each, there is a central node that is called the PAN Coordinator.

• receives/relays all messages
• most capable and power-intensive

The tree topology also has coordinators (or routers) which

• controls clusters
• receives and relays to its children
• communicates up to parent coordinator

The end devices only communicate with a single parent coordinator. These are the least capable and least power intensive.

Star Topology:

graph TD
    PC((PAN Co-ordinator))
    
    PC --- ED1((End Device))
    PC --- ED2((End Device))
    PC --- ED3((End Device))
    PC --- ED4((End Device))
    PC --- ED5((End Device))
    PC --- ED6((End Device))
    PC --- ED7((End Device))
    
    style PC fill:#008080,stroke:#333,stroke-width:2px,color:#fff
    style ED1 fill:#add8e6,stroke:#333
    style ED2 fill:#add8e6,stroke:#333
    style ED3 fill:#add8e6,stroke:#333
    style ED4 fill:#add8e6,stroke:#333
    style ED5 fill:#add8e6,stroke:#333
    style ED6 fill:#add8e6,stroke:#333
    style ED7 fill:#add8e6,stroke:#333

Tree Topology:

graph TD
    Root((PAN Co-ordinator))
    
    C1((Co-ordinator))
    C2((Co-ordinator))
    C3((Co-ordinator))
    
    ED1((End Device))
    ED2((End Device))
    ED3((End Device))
    ED4((End Device))
    ED5((End Device))
    ED6((End Device))
    ED7((End Device))
    ED8((End Device))

    Root --- ED1
    Root --- C1
    Root --- C2
    
    C1 --- ED2
    C1 --- ED3
    C1 --- C3
    
    C2 --- ED4
    C2 --- ED5
    C2 --- ED6
    
    C3 --- ED7
    C3 --- ED8

    style Root fill:#008080,stroke:#333,stroke-width:2px,color:#fff
    style C1 fill:#ff0000,stroke:#333,color:#fff
    style C2 fill:#ff0000,stroke:#333,color:#fff
    style C3 fill:#ff0000,stroke:#333,color:#fff
    style ED1 fill:#add8e6,stroke:#333
    style ED2 fill:#add8e6,stroke:#333
    style ED3 fill:#add8e6,stroke:#333
    style ED4 fill:#add8e6,stroke:#333
    style ED5 fill:#add8e6,stroke:#333
    style ED6 fill:#add8e6,stroke:#333
    style ED7 fill:#add8e6,stroke:#333
    style ED8 fill:#add8e6,stroke:#333

Break and Mesh Networks

Most devices are capable of communicating with multiple neighbors.

• PRO:
  • devices can communicate over longer distances
  • device failures less likely to collapse the entire network
• CON:
  • some nodes have to spend more energy communicating
  • network protocol is more complicated to manage routing

$"\\begin{document}\n\\begin{tikzpicture}[\n pan/.style={circle, draw, fill=teal, text=white, minimum size=0.9cm, font=\\bfseries\\tiny},\n coord/.style={circle, draw, fill=red, minimum size=0.6cm},\n enddev/.style={circle, draw, fill=cyan!30, minimum size=0.6cm, font=\\tiny}\n]\n\n$

source code

Modes of Operation

• Beacon-enabled PAN
  • Slotted CSMA/CA
    • Have data to send
    • Wait for next backoff slot (synchronized from beacon)
    • Listen for two empty slots. If idle, then transmit. Otherwise wait for 0-15 backoff slots and repeat.
  • Structured communication patterns
  • Optionally with some TDMA scheduled slots
  • Beacons occur every 15 ms to 245 seconds.
  • Energy savings are determined by when the radios are off
  • There is some communication latency.

The idea here is that the PAN Coordinator keeps everyone on schedule via beacon and coordinated “slots” where every peripheral can talk. This is also used to synchronize everyone’s clocks. For very important data, the optional TDMA (guaranteed time slots GTS) can be used.

• Non-beacon-enabled PAN
  • Unslotted CA
    • Have data to send
    • Wait for 0-7 backoff slots
    • Listen, but for any empty slot.
  • No particular structure for communication
    • Could be defined by other specifications, like Thread or Zigbee
  • All devices can be identical (no coordinator needed)
  • Requires a custom communication scheme

Devices will listen before talking. If a device has data, it will listen to the channel. If it’s quiet, it sends immediately. Devices do not synchronize.

Receiving Messages

The following are two strategies an End Device can use depending on the network mode.

1. Direct Listening
   1. Listen during entire contention period
   2. Can immediately respond
2. Polling/Indirect
   1. Request messages from coordinator
   2. Coordinator can include list of devices with pending data in beacon

There are more complicated listening algorithms available.

Clear Channel Assessment (CCA)

This is the listen part of CA. There are many implementations, such as:

1. Energy above threshold? RSSI
2. Carrier present? Valid 802.15.4 carrier signal
3. Energy AND/OR Carrier