SBI — Service-based interface — HTTP/2 and JSON between network functions

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The core network

Where it sits

Sits atLevel 3 of the hierarchy · The core network · explained
Also writtenservice-based interface
Learn firstService-based architecture — 3 concepts in the full chain, see the paths
UnlocksNF service
Primary clauseTS 29.500 §5.1
Used inref-core 16 · index 5 · ref-session 2
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. The interface every control-plane network function speaks. TS 29.500 §5.1 gives its protocol stack, and it is worth quoting because of how ordinary it is:

Application · HTTP/2 · TLS · TCP · IP · L2

HTTP/2 as specified in RFC 9113 (TS 29.500 §5.2.1), JSON as the serialisation (§5.4), and TLS for transport security — which "shall be used within a PLMN if network security is not provided by other means", per §5.1.

That is the whole novelty, and it is a real one. The signalling protocol of the mobile core used to be Diameter, and before that SS7 — protocol families invented by and for telecoms, with their own transport, their own encodings and their own tooling. The 5G core signals over the same stack a web API does. Every ordinary property follows: a request has a method and a path, a response has a status code, bodies are JSON that a person can read, and the whole thing can be inspected with tools nobody had to write for it.

Why HTTP/2 and not HTTP/1.1. Core signalling is many small requests between the same pair of functions, often concurrently, and frequently in both directions. HTTP/2 multiplexes them onto one TCP connection without head-of-line blocking at the HTTP layer, and compresses the repeated headers. Over HTTP/1.1 the same load needs a connection pool per peer.

Custom headers are a real part of the specification. TS 29.500 §5.2.3 defines a set of mandatory-to-support custom headers — routing and binding information that a general-purpose HTTP proxy has no reason to have. So it is genuinely HTTP, and genuinely not just HTTP.

The consequence for the rest of the core. Because the interface is uniform, the only thing a consumer needs in order to call a producer is its address — and it gets that from the NRF rather than from configuration. That is what makes NF service discovery sufficient, and it is why the SBA is more than a redrawing of the LTE core.

Read on

This concept is read as part of one argument in ref-core, alongside the rest of its group.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand SBI (level 3) you first need 3 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 5G Core
1 Network function
2 Service-based architecture
Immediately before SBI: Service-based architecture.
Keep going — where this sits on the route
◀Resource elementstep 23 · level 3
Step 24 of 118level 3 · The core network
UPFstep 25 · level 3▶
Level 3 → 4 · this unlocks
NF service
↳ all 15 concepts at level 4 · level 3 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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