LoRaWAN

LoRa vs. LoRaWAN

• LoRa: physical layer protocol
• LoRaWAN: MAC and network layer In practice, LoRa means both.

MAC

Uplink is done via ALOHA, so transmission in done whenever. Packets are randomly spread across uplink channels to reduce odds of collision. Devices using different spreading factors in the same channel do not collide. However, packets are very long.

There are three classes:

• Class A device: listen-after-send, two windows for RX on different channels
• Class B device: periodic listening, 2 RX windows after TX, and is synchronized with periodic beacons
• Class C device: continuous listening, always on receivers

Packet Format

Physical Layer
+----------+----------------------+-----------------------------+-------------+
| Preamble | Header + Header CRC  |         PHY Payload         | Payload CRC |
|          |      (20 bits)       |           P bytes           |  (16 bits)  |
+----------+----------------------+-----------------------------+-------------+
                               /                                 \
                          /                                       \
MAC Layer           /                                              \
                  +------------+-----------------------+------------+
                  | MAC Header |      MAC Payload      |    MIC     |
                  |   1 byte   |        M bytes        |  4 bytes   |
                  +------------+-----------------------+------------+
                               /                        \
                        /                                  \
Application     /                                            \
Layer          +--------------+------------+------------------+
               | Frame Header | Frame Port |  Frame Payload   |
               | 7 ~ 22 byte  |  1 bytes   |     N bytes      |
               +--------------+------------+------------------+

Frame header includes device address. The MAC payload maximum size depends on data rate.

Data Rate Index	MAC Payload Size
0	19 bytes
1	61 bytes
2	133 bytes
3	250 bytes
4	250 bytes
LoRaWAN uses a star-of-stars [[Network Topologies#star-and-tree-topologies	topology]]. Abstractly, we can think of a LoRaWAN service as

air quality monitor {} -> LoRaWAN network -> application server []

Joining a LoRaWAN Network

Activation By Personalization (ABP) is a legacy approach with pre-shared security keys.

Over The Air Activation (OTAA) is a dynamic join procedure that sets up security keys. A device knows three things:

1. AppEUI: identifier for the application the device uses on the application server
2. DevEUI: device ID
3. AppKey: AES-128 bit secret registered with the network server

sequenceDiagram
    participant ED as End Device
    participant NS as Network Server
    participant AS as Application Server

    Note over ED, NS: AppKey
    ED->>NS: 1 Join-request
    Note right of NS: 2 Processes the Join-request,<br/>session key generation<br/>(AppSKey, NwkSKey)
    NS->>ED: 3 Join-accept
    Note right of NS: 4 Keeps NwkSKey
    NS->>AS: AppSKey distribution
    Note left of ED: 5 Session key generation<br/>(AppSKey, NwkSKey)

• The device sends $$(\text{AppEUI, DevEUI, Nonce})+\text{MIC}$$ using the AppKey.
• The network server responds with AppNonce encrypted with AppKey
• The device and network server compute the same AppSKey to encrypt all future payloads. This establishes the session.