Question E2D11:
How do APRS stations relay data?  

Choices:  
A: By packet ACK/NAK relay  
B: By C4FM repeaters  
C: By DMR repeaters  
D: By packet digipeaters    

Correct Answer is  D: By packet digipeaters  

Correct answer: D — By packet digipeaters

1) Why D is correct
- APRS uses AX.25 Unnumbered Information (UI) frames on VHF (typically 144.39 MHz in North America) and is relayed over-the-air by packet digipeaters.
- Digipeaters listen for APRS packets and re-transmit them according to the path in the header (e.g., WIDE1-1, WIDE2-1) to extend range.

2) Why the others are incorrect
- A: By packet ACK/NAK relay
  - APRS position beacons use unconnected UI frames, which are not ACK/NAK–based. While APRS text messages may use application-level acks, relaying across the network is done by digipeaters, not via link-layer ACK/NAK relays.
- B: By C4FM repeaters
  - C4FM (Yaesu System Fusion) is a digital voice/data mode. Standard APRS on VHF packet is not relayed by C4FM voice repeaters.
- C: By DMR repeaters
  - DMR is a digital voice/data trunking system. It does not provide the standard AX.25 packet digipeating used by APRS.

3) Useful rules of thumb
- APRS pathing uses WIDEn-N aliases. Each digipeat reduces the N by 1:
  $$\text{WIDE}n\text{-}N \rightarrow \text{WIDE}n\text{-}(N-1) \rightarrow \dots \rightarrow \text{WIDE}n\text{-}0 \ (\text{stop})$$
- Recommended paths (to limit congestion):
  - Mobile: WIDE1-1,WIDE2-1 (about 2 hops total)
  - Fixed: WIDE2-2 (no more than 2 hops)
- APRS frames are UI (unconnected), so there is no link-layer ACK/NAK for beacons.
- I-Gates move APRS packets between RF and the Internet (APRS-IS) but do not RF-repeat; digipeaters do the on-air relaying.