Core Telco Architecture: 2G to 5G in One Read

Introduction¶

Every mobile generation kept the same fundamental jobs — authenticate the SIM, track where the device is, set up bearers, route traffic, charge for it — and renamed the boxes that do them. Seeing the lineage makes the acronym soup tractable.

2G — GSM (circuit) + GPRS (packet)¶

Circuit-switched core (voice/SMS):

• MS (the phone) ↔ BTS ↔ BSC (radio) ↔ MSC (switch) / VLR (visitor register, cached subscriber data).
• HLR — home subscriber database (identity, services, current MSC/VLR).
• AuC — authentication centre (Ki, triplets).
• GMSC — gateway MSC to the PSTN.
• SMSC — store-and-forward SMS.

GPRS packet core (data), added later:

• SGSN — serving GPRS support node: mobility + session for data, the packet analogue of the MSC/VLR.
• GGSN — gateway GPRS support node: the anchor to external IP networks (the APN endpoint), assigns the device IP, does policy/charging.

3G — UMTS¶

Same core, new radio (Node B ↔ RNC). The MSC often becomes an MSC Server + Media Gateway split (control/bearer separation). SGSN/GGSN carry on for data at higher speeds. HSPA later boosts throughput.

4G — LTE / EPC (all-IP, no circuit switching)¶

The Evolved Packet Core is flat and all-IP. Voice isn't native — it's VoLTE (IMS) or CSFB (fall back to 2G/3G for a call).

• eNodeB — the radio, now with no separate controller (RNC functions absorbed).
• MME — Mobility Management Entity: signalling only (attach, auth, tracking area updates, bearer setup). The control-plane brain. No user traffic.
• S-GW (Serving Gateway) — user-plane anchor for intra-LTE mobility; routes packets between eNodeB and P-GW.
• P-GW (PDN Gateway) — anchor to external networks (the APN), IP allocation, policy enforcement, charging. = the GGSN's successor.
• HSS — Home Subscriber Server: HLR + AuC, now speaking Diameter (S6a).
• PCRF — Policy and Charging Rules Function: decides QoS/charging rules, talks Gx to the P-GW and Rx to IMS.
• PCEF — the enforcement point, inside the P-GW.

Interfaces you'll hear: S1-MME (eNodeB↔MME), S1-U (eNodeB↔S-GW), S11 (MME↔S-GW), S5/S8 (S-GW↔P-GW), SGi (P-GW↔internet).

5G — Service-Based Architecture (SBA)¶

The 5G Core (5GC) is a set of network functions exposing HTTP/2 + JSON REST APIs to each other (via the SBI, service-based interface), instead of point-to-point Diameter/GTP-C links. Control and user plane are fully split (CUPS).

5G NF	Does	~4G equivalent
AMF	access & mobility mgmt, signalling	MME (mobility part)
SMF	session management, IP allocation, controls the UPF	MME (session part) + S-GW-C + P-GW-C
UPF	the user-plane forwarder; can sit near the edge	S-GW-U + P-GW-U
AUSF	authentication server	part of HSS/AuC
UDM	unified data management (subscriber data)	HSS
UDR	the actual subscriber data repository	HLR DB
PCF	policy control	PCRF
NRF	NF repository — service discovery/registration	(new)
NSSF	network slice selection	(new)
NEF	exposes network capabilities to external apps	(new)

5G adds network slicing (multiple logical networks — eMBB, URLLC, mMTC — on shared infrastructure) and pushes the UPF toward the edge for low latency.

5G NSA (Non-Standalone) reuses the LTE EPC with a 5G radio (gNodeB) bolted on; 5G SA (Standalone) is the full 5GC above.

The throughlines¶

• Authenticate the SIM: AuC → HSS → AUSF/UDM.
• Know where the device is: VLR/HLR → MME/HSS → AMF/UDM.
• Give it connectivity: GGSN → P-GW → SMF+UPF.
• Apply policy & charge: (CAMEL) → PCRF/PCEF → PCF + CHF.
• Route between nodes: SS7/STP → Diameter/DRA → SBI/NRF (service mesh).

Verification and troubleshooting (operational touchpoints)¶

• Attach failures spike — MME/AMF overload, HSS/UDM slow or unreachable (S6a/SBI), or an authentication vector mismatch (AuC sync). Check the Diameter/HTTP2 error codes.
• Data sessions up, no throughput — S-GW/P-GW or UPF user-plane issue, GTP-U path broken, or APN/policy (PCRF/PCF) returned no rules.
• VoLTE calls fail on LTE — IMS (P-CSCF/S-CSCF) or the dedicated QCI-1 bearer isn't being set up (Rx→Gx), or the device fell back to CSFB and 2G/3G is congested.
• Roamers can't attach — S8/S6a (or N9/roaming SBI) to the home network via the IPX is down, or the roaming agreement's realm/APN isn't provisioned.
• 5G NF can't find a peer — NRF registration/discovery failed, or TLS between NFs (SBA mandates it) isn't established.

Related tools and reading¶

• On-site: Subnet Calculator.
• Related posts: SS7 vs Diameter, Charging systems: online vs offline.