The Network Cores
Last updated 13 Oct 2026
At a glance
There are two network cores: the mobile core and the IP core. The CS and PS cores are the two halves of the mobile core; the fixed / broadband core is the IP core’s subscriber edge; optical transport is the layer beneath it; and IMS is a service layer over both. Which of them know the subscriber decides what BSS integrates with.
The picture
Two network cores, what belongs to each, and the service layer over both. Select any slab for the detail.
BSS integrates with the slabs that know the subscriber: mobile core, broadband edge, IMS.
OSS manages every slab as resources; the IP core and optical layer are OSS’s alone.
How it works
Two network cores
“Core” is used loosely in telecoms. There are two network cores. The mobile core looks after mobile subscribers: authentication, mobility, sessions, policy and charging. The IP core is the IP/MPLS backbone that carries every service’s traffic between sites, regions and other networks; it routes packets and knows nothing about subscribers. Everything else called a core is a part of one of them, an extension, a layer beneath, or a service layer on top.
The mobile core: CS and PS
The CS core (circuit-switched) is the 2G/3G voice side: the MSC for call control and mobility, the MGW for the voice itself, the HLR and VLR for subscriber data, and the SMSC for SMS. The PS core (packet-switched) is the data side in every generation: SGSN and GGSN in 2G/3G, the EPC (MME, SGW, PGW, PCRF, HSS) in 4G, the 5GC (AMF, SMF, UPF, PCF, UDM) in 5G. The jobs stay the same; the functions are renamed and split. When a job advert asks for “CS and PS core” experience, it means the whole mobile core.
From 4G there is no CS core in the target design. Voice runs as IMS over the PS core (VoLTE, then VoNR), which is why the CS core is being switched off with 2G and 3G. The subscriber database spans the line throughout: the HLR serves CS and PS, the HSS also serves IMS, and the UDM is its 5G form.
The IP core and what belongs to it
The fixed / broadband core is an extension of the IP core: its subscriber edge. The BNG, usually running on the IP core’s edge routers, terminates each household’s session, applies its speed profile and gives it an address, with RADIUS / AAA authenticating the line. Optical transport (DWDM, ROADM, OTN) is the layer beneath: the fibre and wavelengths the routers run over. With 5G convergence, fixed access can instead end on the 5G core.
IMS over both, and the PSTN it replaces
IMS is not a network core. It provides voice, video and messaging over IP and reaches customers through the mobile PS core or the fixed broadband edge. It replaced the CS core for mobile voice and is replacing the legacy PSTN switches for fixed voice.
Who owns what
| Entity | System of Record | System of Engagement | System of Reference | Notes |
|---|---|---|---|---|
| Mobile subscriber profile (IMSI, MSISDN, services, data profile) | HLR / HSS / UDM | BSS (provisioning) | BSS (product inventory) | BSS decides what was sold; the core holds what the network enforces. |
| Broadband line profile and speed tier | AAA (RADIUS) | BSS (provisioning) | OSS (service inventory) | The BNG applies it per session. |
| Voice numbers and features | IMS (HSS, TAS) or the legacy switch | BSS | — | Number portability databases sit outside both. |
| Usage and charging records | The core that produced them (PGW / SMF, MSC, IMS, AAA) | — | BSS (mediation, charging, billing) | Real-time for prepaid; records for postpaid. |
| IP core and optical topology | The network | — | OSS (resource inventory) | No subscriber state; OSS only. |
In practice
Most operators run more than one generation of core at once: a 5G core beside the EPC, and often a CS core kept alive for 2G roaming, machine connections or emergency calls. BSS provisions into each of them, so a single mobile plan can mean writes to the HSS, the UDM and the IMS for the same subscriber.
The trade-off
What it solves
It tells you which parts of the network BSS has to integrate with (the ones that know the subscriber) and which OSS alone manages as resources.
What it does not solve
- How each core is configured and orchestrated. The sections on the IP/MPLS domain, the mobile core and fixed access cover that.
- Which vendor’s core to buy, or how many generations to run at once.
What it adds
- Running two or three generations of mobile core in parallel, each with its own provisioning and charging interfaces.
- Fixed-mobile convergence moves the broadband edge onto the 5G core, changing which systems BSS provisions.
Architecture decision
When a legacy core (CS or PSTN) is switched off, where does the migration start?
Network first
Choose when the customer base is small and products map cleanly to the new IP voice service, so the network team can move lines in bulk.
Product and customer first
Choose when many legacy products have no clean equivalent, so customers must be moved to new offers in BSS before their lines can be cut over.
Breaks if lines are moved in the network while BSS still bills the old product. Billing truth and network truth diverge, and the reconciliation lands on the billing team.
Takeaways
Key Takeaways
- There are two network cores: the mobile core and the IP core
- CS (voice, 2G/3G) and PS (data, every generation) are the two halves of the mobile core
- The fixed / broadband core is an extension of the IP core: its subscriber edge
- IMS is a service layer over both cores, not a network core
- BSS integrates with the cores that know the subscriber; OSS manages them all as resources
Check yourself
A job advert asks for five years’ experience with “CS and PS core”. What is it asking for?Show answerHide
Hands-on work across the whole mobile core: the circuit-switched voice side (MSC, MGW, HLR) and the packet-switched data side (SGSN/GGSN, EPC, 5GC).
Why does BSS integrate with the broadband core but not with the IP core?Show answerHide
The broadband core knows the subscriber: the AAA holds each line’s profile and the BNG reports its sessions. The IP core only routes traffic and holds no subscriber state, so OSS manages it as a resource.