NetSim Network Emulator
Last updated: September 2026
NetSim Network Emulator connects the simulator to real hardware running live applications such as video, voice, file transfer, and email. Also termed System-in-the-Loop (SITL), it lets physical hardware and a simulation interact as a single system. Real traffic flows through a virtual network where it experiences the delay, loss, and error you configure, and the impact is seen directly on the live application.
It is a cost-effective alternative to hardware emulators and full physical test labs, which carry high cost, complicated configuration, and limited scale. Build a network of up to a few hundred virtual nodes on a single PC and map them to any number of real devices.
Real, simulated, real
A real PC running the NetSim Emulation Client sends live traffic to the PC running the NetSim Emulation Server. The simulation runs in sync with the wall clock and acts as a transit network in which datagrams experience simulated network effects.
- A real packet arrives at the emulation server interface.
- NetSim converts it into a simulation packet at ingress.
- It crosses the virtual network, picking up delay, loss, and error.
- At egress it is re-converted and re-injected to the real destination.
Why use the emulator
Test real software and hardware against network conditions you control.
Application performance
Measure how real applications behave subject to network effects such as error, delay, and loss, before deployment.
Impact you can see
The effect is immediately apparent on the live video, voice, or transfer, with no need to interpret it from simulation statistics.
Protocol R&D
Develop new protocols and test their performance with real applications instead of synthetic load generators.
Design-stage testing
Validate a design before the real devices or software are ready, by emulating the network they will eventually run over.
Cyber test beds
Integrate into cyber ranges and test beds to study how attacks and defences behave over a realistic transit network.
No expensive lab
Replace racks of hardware impairment boxes with one PC, and scale to topologies that would be impractical to build physically.
Connect real applications and devices
NetSim is an IP-based, data-plane, flow-through emulator. Any IP application can be driven through it.
Live video
Stream an RTP / MPEG transport stream from VLC and watch the quality change as the virtual network adds loss and jitter.
Voice calls
Place a two-way Skype or VoIP call between real systems and study call quality under controlled delay and jitter.
File transfer
Move files with FileZilla over FTP and compare transfer speed with and without the emulated link constraints in place.
Throughput tests
Run JPerf or iPerf and observe goodput drop to the virtual link capacity, for example 120 Kbps over a constrained radio link.
Databases
Send PostgreSQL / pgAdmin queries across the emulator and time the response as network conditions vary between runs.
GIS and map servers
Drive a GeoServer map client through the emulator and measure how the map load time responds to the transit network.
Example use cases
From tactical radios to enterprise WANs, run real applications over a modelled link.
Military radios (MANET)
- Soldiers communicate as a Mobile Ad-hoc Network using UHF / VHF radios modelled in the NetSim emulation server.
- Each soldier is a virtual machine on a physical server, with the MANET covering mobility, RF propagation, and L3 / MAC / PHY attributes.
- Real-time data, voice, or video flows between the VMs, soldier to soldier.
- Change a parameter such as PHY rate, then watch real iPerf throughput follow the new link capacity. Measure loss, delay, error, throughput, and quality of service.
5G networks
- A typical 5G network with cameras uploading video and phones (UEs) uploading and downloading files.
- Change network parameters such as bandwidth, pathloss, transmit power, antenna count, and mobility.
- Run what-if scenarios by modifying the file transfer and video rates.
- Study application performance and analyse loss, delay, error, throughput, and quality of service.
WAN application testing
- Model a branch to data centre WAN with the bandwidth, latency, and queuing of the real circuit.
- Run the application your users run: file transfer, database queries, video conferencing, or a web client.
- Change the circuit speed, add congestion from other sites, or take a link down, and watch the effect on the application.
- Record response time and throughput on each run, and compare circuit designs before committing to the bandwidth.
Satellite and NTN links
- Run real applications over a LEO, MEO, or GEO link, with the propagation delay and path loss of the orbit you choose.
- Satellites move on their orbits, so the link budget, the delay, and the visibility window change as the pass progresses.
- Change the constellation, altitude, or beam layout, and repeat the same application test against each design.
- See how a long round trip affects file transfer, video, and interactive sessions, and measure loss, delay, throughput, and quality of service.
Configuration and measurement
Choose which traffic enters the simulation, then capture every packet for analysis.
Traffic filtering
Map real flows into the simulation by source and destination IP and port. Use 0.0.0.0 as a wildcard to admit all matching traffic.
Device or application scope
Device-specific emulation maps all traffic between two hosts; application-specific emulation narrows it to a single port pair.
PCAP capture
Wireshark-compatible logs record all network packets, those dispatched to the emulator, those re-injected, and those not dispatched.
Jitter control
Introduce jitter with Poisson background traffic. More background packets induce greater delay variance in the competing flow.
PCAP file input
Replay a recorded pcap file into the emulator, mapping its real source and destination addresses onto the virtual topology.
Round-trip delay
Two-way applications such as ping need a forward and a reverse emulation application so both request and reply traverse the virtual network.
Scheduled outages
Fail a link at a chosen time and bring it back, or degrade it while the session runs. The application stays live throughout, so you see how it behaves during the outage and how quickly it recovers.
Features and specifications
How the emulator is built, set up, and bounded.
Flow-through, data plane
An IP-based, data-plane, flow-through emulator that processes actual data between clients and servers through a virtual network. It models the data plane, not the control plane.
Three-system lab
A dedicated system runs NetSim on Windows 10 or 11, connected by an L2 switch to the servers or PCs running your applications, with the NetSim system set as the gateway.
Convert and re-inject
Real packets are converted to virtual NetSim packets at ingress, subjected to the configured delay, loss, and error, then converted back to real packets at egress.
Both ends real or virtual
Source and destination must both be in the real world or both virtual. A virtual node cannot be the sink for real traffic, and vice versa.
Generic devices, COTS
The emulator models generic network devices rather than vendor-specific equipment and offers socket-based connectivity. This describes the COTS build; custom development extends it.
Source code in C
NetSim ships with protocol source code in C. Modify the stack and develop custom protocols, waveforms, and conversion logic for requirements beyond the standard build.
Frequently asked questions
Short answers to the questions test teams ask most often.
How do I set a specific packet loss rate?
Loss is set through the bit error rate on the link, and the resulting packet loss depends on your packet size. Tell us the loss percentage and the packet size you are testing at, and we will give you the value to enter.
How do I control jitter?
Add background traffic with an exponential inter-arrival time so that it shares a link with your flow. Raising its rate raises the delay variance your application sees. A lightly loaded path gives near-constant delay, so the same scenario also serves low-jitter testing.
Can I emulate packet duplication?
Yes, by pairing NetSim with a host-side tool. On Linux, the standard netem facility duplicates a set percentage of packets on the sending machine. On Windows, the free tool clumsy does the same, with a duplication chance and a copy count. Duplicates created at the sender still travel the emulated path, so each copy picks up its own delay and can be lost independently. NetSim supplies the delay, bandwidth, loss and congestion, and the host tool supplies the duplication.
How many machines do I need?
Three: the real source, the machine running NetSim, and the real destination. Virtual machines and containers work in place of physical hosts.
Put your hardware in the loop
See your real applications and devices run over a network you control. Request an evaluation, or talk to us about a custom emulation set-up for your test bed.