3GPP Release 4: Core Network Split
Release 4 arrived in 2001, just as the first Rel-99 UMTS networks were going live. Rather than redefining the radio, it turned its attention to the core: breaking apart the monolithic Mobile Switching Centre into two separate network functions, laying the groundwork for an all-IP backbone and introducing the world to multimedia messaging.
What Changed in Rel-4
Rel-99 defined 3G — UMTS, UTRAN, W-CDMA, the dual CS/PS core. But the Circuit-Switched domain it inherited was still built on 1970s telephony thinking: a single monolithic node, the MSC (Mobile Switching Centre), that handled both call signalling and the actual transport of voice simultaneously, tightly coupled together.
In the early 2000s, IP transport was becoming dramatically cheaper than traditional TDM (Time-Division Multiplexing) circuits for carrying voice traffic between core nodes. Rel-4's central contribution was a clean architectural break: it separated call control from voice transport, allowing voice to travel over IP in the network backbone while the MSC retained the sole job of managing call state. This is the softswitch model — a design pattern that the rest of the industry (including fixed-line carriers) adopted in parallel.
The MSC Split — MSC Server and Media Gateway
The monolithic MSC was split into two distinct functional entities, each with a precisely defined role:
The MSC Server handles everything that is pure signalling: setting up calls, managing call state, routing, mobility management, and interfacing with the HLR for subscriber data. It speaks BICC (Bearer Independent Call Control) or SIP-T toward other MSC Servers over the Nc interface. Critically, it never touches actual voice samples — it only tells the MGW what to do with them.
The Media Gateway (MGW) carries the actual voice bearer. It terminates the RTP/IP streams coming from the network backbone on one side and the TDM circuits going to the PSTN or to the radio access network on the other. The MGW does whatever the MSC Server tells it via the Mc interface: connect this bearer, transcode this codec, release that connection.
The Mc interface (MSC Server to MGW) uses the H.248/MEGACO protocol — a straightforward command-response protocol where the MSC Server is the "master" and the MGW is the "slave." The Nc interface (MSC Server to MSC Server) carries BICC signalling for calls that traverse multiple switches — analogous to ISUP in the old TDM world, but now running over IP.
The key benefit: voice bearer traffic no longer needs to follow the same physical path as signalling. An operator can route IP voice traffic cheaply across its IP backbone between any two MGWs, while the MSC Servers coordinate the call over a separate signalling network. TDM circuits remain only at the edges — between the MGW and the PSTN, or between the MGW and the radio access network.
BICC — Bearer Independent Call Control
BICC (Bearer Independent Call Control) is the protocol that made the MSC split possible. Traditional telephony protocols like ISUP assumed that signalling and bearer always travelled together — if you set up a call from switch A to switch B, both the control messages and the voice circuit went through the same path. BICC breaks that assumption.
In a BICC-based call:
- The MSC Servers exchange call control messages (call setup, ringing, answer, release) over the IP signalling network using BICC
- Separately, the two MGWs negotiate and establish a direct RTP/IP media path between themselves — the voice bearer can take a completely different route through the network
- The MSC Server passes the bearer parameters (codec, IP address, UDP port of the MGW) to its peer MSC Server inside the BICC signalling, which then configures its own MGW accordingly
BICC is functionally similar to SIP but was designed to carry the full ISUP call model — it was familiar territory for vendors building on their existing ISUP expertise. In practice, some vendors implemented both BICC and SIP-T on the same MSC Server, giving operators a choice.
MMS — Multimedia Messaging Service
Alongside the core network changes, Rel-4 standardised MMS (Multimedia Messaging Service) — the first time mobile devices could send pictures, audio clips, and short video in a structured message format, beyond the 160-character text limit of SMS.
MMS uses a store-and-forward model rather than the real-time circuit of a voice call. When a user sends an MMS message:
- The handset encodes the message (image, text, audio) and uploads it to an MMSC (MMS Centre) over the PS domain using HTTP
- The MMSC stores the message and sends an SMS notification to the recipient
- The recipient's handset connects to the MMSC and downloads the message, also over HTTP via the PS domain
- Inter-carrier MMS delivery (sending to a subscriber on a different network) uses MM4 — an SMTP-based relay protocol between MMSCs
MMS was commercially deployed from 2002 onward. Its reach eventually became near-universal across handsets, and it set the expectation that mobile messaging should support rich media — an expectation that WhatsApp, iMessage, and every modern messaging platform inherited directly.
TD-CDMA — The TDD Radio Formalised
Rel-99 had defined two duplex modes for UMTS: FDD (Frequency Division Duplex, using paired spectrum) and TDD (Time Division Duplex, using unpaired spectrum). The TDD mode in Rel-99 was based on TD-CDMA (Time Division CDMA), which Rel-4 fully formalised with a complete set of specifications.
TD-CDMA combines time-division multiplexing within each frame — dividing the channel into time slots — with CDMA spreading within each slot, so multiple users share both the time and code dimensions. This flexibility makes it well-suited to asymmetric traffic patterns, since the ratio of uplink to downlink time slots can be configured by the operator.
China's government declined to adopt W-CDMA FDD for its 3G rollout, instead commissioning its own domestic variant — TD-SCDMA — which is built on the same TD-CDMA principles standardised in Rel-4. China Mobile deployed TD-SCDMA nationally from 2009 onward, making the Rel-4 TDD specification the foundation for one of the largest 3G rollouts in history.
Why Rel-4 Mattered
- First step toward an all-IP core — separating signalling from the voice bearer meant IP transport could replace expensive TDM trunks between core nodes, dramatically cutting inter-switch connectivity costs for operators
- The softswitch model became the industry template — the MSC Server / MGW split that Rel-4 defined is the same architecture that IMS (Rel-5) and VoLTE are built upon; Rel-4 is why those systems look the way they do
- BICC enabled voice-over-IP in the backbone without touching the edges — operators could upgrade their core networks to IP transport while leaving existing TDM connections to the PSTN and the radio access network completely unchanged
- MMS launched the mobile multimedia era — before smartphone apps and mobile internet became mass-market, MMS was the first way hundreds of millions of people sent photos from their phone; it established the commercial and technical baseline for rich mobile messaging
- TD-CDMA provided the specification base for China's 3G — TD-SCDMA served hundreds of millions of subscribers in China and demonstrated that the TDD variant of the Rel-4 spec was viable at massive scale