Port 3452: SABP — The Protocol Behind Cell Broadcast

Port 3452 is registered with IANA under the name sabp-signal — the Service Area Broadcast Protocol (SABP), defined by 3GPP for use in UMTS (3G) mobile networks.¹

You probably know the end result of this protocol: the sudden screaming alert on your phone that doesn't ask permission, can't be silenced, and arrives on every device in the area at once. SABP is the machinery behind that.

What SABP Does

3G mobile networks have a subsystem called the Cell Broadcast Service (CBS) — a way to push messages to every phone in a geographic area without knowing who those phones are or addressing them individually. No unicast. No session. Pure broadcast.

SABP runs on the Iu-BC interface — the connection between two specific pieces of 3G infrastructure:²

• CBC — Cell Broadcast Centre. The server that holds the message to be sent.
• RNC — Radio Network Controller. The piece of infrastructure that actually commands the cell towers.

When an operator (or emergency service) wants to broadcast a message, the CBC sends a WRITE-REPLACE command over port 3452 to the RNC, specifying which cells should transmit, what message to carry, and how often to repeat it. The RNC replies with WRITE-REPLACE COMPLETE when the cells are transmitting. When it's time to stop, the CBC sends KILL. The RNC replies KILL COMPLETE.³

The protocol uses ASN.1 Basic Packed Encoding Rules (BASIC-PER) as its wire format — compact binary encoding suited to telecom infrastructure.

Why This Port Exists in the Registered Range

Port 3452 sits in the registered port range (1024–49151). These are ports that IANA assigns to specific services on request — not the famous well-known ports below 1024, but legitimate assignments with documented purposes. SABP earned its registration because it's purpose-built infrastructure with a defined spec (3GPP TS 25.419), not something improvised.⁴

You will not encounter this port on a general-purpose server or web application. It exists inside 3G operator networks, between specialized telecom equipment. If you see traffic on port 3452 on a machine that isn't 3G core infrastructure, something unusual is happening.

Checking What's Using This Port

# macOS / Linux — show what process is listening on port 3452
sudo lsof -i :3452

# Linux alternative
ss -tlnp | grep 3452

# Windows
netstat -ano | findstr :3452

On a typical machine, you'll find nothing. This port belongs to a narrow world of 3G telecom gear.

The Bigger Picture

SABP is obscure by design. It was built for mobile operators, runs inside their infrastructure, and never needs to face the public Internet. But it's a reminder that the port number space contains multitudes — thousands of registered assignments for protocols that power the world quietly, invisibly, in data centers and telecom equipment rooms you'll never visit.

The emergency alert that vibrates your phone? Somewhere in the chain, a Cell Broadcast Centre sent a WRITE-REPLACE message to a Radio Network Controller on port 3452, and the tower started broadcasting.