This article describes the “local breakout” function in YateBTS/OpenBTS GPRS cells, where user IP traffic originates directly from the cell site instead of from a centralized GGSN in the core network.
Standard GPRS Architecture
The standard GPRS data network has these parts:
- BTS – the actual radio basestation, which handles user traffic up to layer 2
- BSC – the basestation controller, which handles layer 3 for multiple BTS units
- BSS – the BSC and the BTS units it controls, together as the “basestation subsystem”
- SGSN – “serving GPRS support node”, handles authentication and mobility procedures for data connections
- GGSN – “gateway GPRS support node”, connects data sessions to the internet and provides the public IP address
Role of the GGSN
A data session originates in the mobile station (the phone, modem, etc.) and passes through the BSS and SGSN using a protocol called GTP (“GPRS tunneling protocol”). The GTP tunnel terminates at the GGSN, where the data session connects to the internet. Because the data session public IP address is provided by the GGSN, the mobile station can move around in the network, across different BSSs and different SGSNs, while still keeping the same public IP address. So from the point of view of services in the internet, the mobile station has a fixed address, a so-called “anchor” for the mobile data session. (Note that the public IP address is usually bound to a session, not to the mobile station itself, and each new session is still likely to get a different public IP address, unless specifically configured to have a static IP.)
Legacy of GPRS
The GPRS data network architecture continued almost unchanged in 3G UMTS. It also influenced the design of the 4G LTE “enhanced packet core” (EPC), where the MME/SGW and PGW have functions nearly identical to the SGSN and GGSN. And even in 5G NR, we still use GTP to carry user traffic from the radio network through the core network.
Local Breakout (“LBO”)
Local breakout is when the data session is connected to the internet at the cell site, instead of tunneling back into the core network. Local breakout requires that all of the BSS, SGSN, and GGSN functions are, effectively, inside the cell site, making the cell site a completely self-contained GPRS network. Legba’s GSM products support this mode of operation as part of the “Network in a PC” feature. This mode of operation can be used along with authentication bypassing (covered in a previous article) to completely eliminate the need for a core network in a GPRS-only installation.
Because LBO does not use a centralized GGSN, the mobile station will change its IP address when it moves from one cell to another, which will disrupt in-progress data sessions. But for applications that use only a single cell site, or where the mobile station is not likely to move between cells during a data session, LBO can simplify the network to a collection of independent, functionally identical nodes.