Port 3598: A15 (AN-to-AN) — A CDMA Relic

What This Port Does

Port 3598 is assigned to the A15 interface in CDMA2000 mobile networks. A15 handles AN-to-AN communication — Access Node to Access Node — specifically for inter-AN paging.

Here's the scenario it solves: a mobile device is registered with one CDMA Access Node (a cell tower's network controller), but the device has moved into range of a different Access Node's coverage area. When an incoming call arrives, the home AN needs to tell the neighboring AN: "Page this device on your air interface." That coordination happens over A15, using TCP and UDP, on port 3598.

Qualcomm registered this port with IANA in September 2002. CDMA2000 was the dominant 3G mobile standard in North America at the time — Verizon and Sprint both ran it. The A15 interface was part of the 3GPP2 specification for how the radio access network's internal nodes communicated.¹

The Range This Port Lives In

Port 3598 sits in the registered ports range (1024–49151). These ports are assigned by IANA to specific applications that have requested a registration. Unlike well-known ports (0–1023), registered ports don't require root privileges to bind on most systems, but they carry an official assignment in the IANA registry.

Why You Won't See It

CDMA2000 networks in the United States have been shut down. Verizon decommissioned its CDMA network at the end of 2022. Sprint's network was folded into T-Mobile's infrastructure, which is LTE and 5G. The A15 interface this port was designed for exists now primarily in 3GPP2 specification documents and in the IANA registry.²

If you see traffic on port 3598 on a modern network, it's not CDMA infrastructure — it's something else running on an unoccupied port.

Checking What's Listening

To see if anything is listening on port 3598 on your system:

macOS/Linux:

# Check for listeners on port 3598
ss -tlnp | grep 3598

# Or with lsof
lsof -i :3598

# Or with netstat
netstat -an | grep 3598

Windows:

netstat -ano | findstr :3598

If you see something listening here, find out what it is. A known application deliberately choosing an obscure registered port is unusual behavior worth investigating.