5G New Calls: SIP Protocol Explained and Hands‑On Implementation

Why SIP Matters for 5G

5G delivers higher data rates, lower latency, and broader coverage, enabling IoT, remote health, and autonomous driving. To support these services, a flexible session‑control protocol is required; SIP fulfills this role by creating, modifying, and terminating multimedia sessions over IP networks.

SIP in the OSI Stack

SIP operates at the application layer (layer 7) of the OSI model. The article lists all seven layers to show where SIP fits relative to physical, data link, network, transport, session, presentation, and application layers.

Core Features of SIP

SIP is an open, extensible, text‑based protocol that works with transport protocols such as UDP, TCP, and TLS, and is usually paired with RTP for media transport. Its advantages include vendor‑agnostic interoperability and suitability for VoIP, video conferencing, instant messaging, and IMS‑based services (VoLTE, VoNR).

SIP Message Structure

Messages are UTF‑8 encoded and divided into requests and responses. Each message consists of a start line, a series of header fields, and an optional body, following the RFC 2822 format. Required headers include Via, From, To, Call‑ID, CSeq, and Contact; missing any of these results in a 400 Bad Request.

Common SIP Methods and Use Cases

INVITE – initiates a session

BYE – terminates a session

REGISTER – registers a user agent with a server

Typical deployment scenarios are VoIP (e.g., Skype, Google Voice, carrier VoLTE), video conferencing (Zoom, Cisco Webex), enterprise PBX systems (Asterisk, FreeSWITCH), carrier IMS (5G VoNR, 4G VoLTE), and SIP‑enabled access control or robots.

Development Frameworks: eXosip and pjsip

eXosip is a high‑level wrapper around oSIP, offering session management, RTP/RTCP support, and proxy handling. pjsip is a cross‑platform SIP stack with built‑in RTP, NAT traversal (ICE, STUN), and support for Windows, Linux, and Android.

Environment Setup

Windows

# Clone eXosip source
git clone https://github.com/linphone/exosip.git
# Configure with Visual Studio
cmake -G "Visual Studio 15 2015" .
# Build
msbuild exosip.sln

Linux

# Install dependencies
sudo apt-get install build-essential libssl-dev
# Clone pjsip source
git clone https://github.com/pjsip/pjsip.git
# Configure and build
cd pjsip
./configure
make

Key API Examples

eXosip – Initiating an INVITE

#include <stdio.h>
#include <exosip2/exosip.h>
int main() {
    eXosip_t *exosip;
    eXosip_init(&exosip);
    eXosip_set_user_agent(exosip, "eXosip Client");
    eXosip_call_build_initial_invite(exosip, "sip:[email protected]", NULL, NULL);
    eXosip_exit(exosip);
    return 0;
}

pjsip – Creating an Account and Handling Calls

#include <pjsua-lib/pjsua.h>
static void on_incoming_call(pjsua_acc_id acc_id, pjsua_call_id call_id, pjsip_rx_data *rdata) {
    pjsua_call_answer(call_id, 200, NULL, NULL);
}
int main() {
    pjsua_config cfg;
    pjsua_acc_config acc_cfg;
    pjsua_config_default(&cfg);
    cfg.cb.on_incoming_call = on_incoming_call;
    pjsua_init(&cfg, NULL, NULL);
    pjsua_acc_config_default(&acc_cfg);
    acc_cfg.id = pj_str("sip:[email protected]");
    pjsua_acc_add(&acc_cfg, PJ_TRUE, NULL);
    pjsua_start();
    return 0;
}

Practical Hands‑On Projects

The article provides complete client code using eXosip and a simple server using pjsip, demonstrating registration, INVITE handling, and call answering.

Further Learning

Read RFC 3261 for the SIP specification.

Use Wireshark to capture and analyze SIP traffic.

Experiment with eXosip or pjsip to build your own SIP applications.

As 5G and cloud‑based communications evolve, SIP will continue to be a cornerstone for voice and video services.