UDP Stream Test: Measuring latency and timely data delivery with uni-directional traffic

Measuring data latency in mobile networks

Demanding real-time applications, services, content, and the 5G rollout have increased the need to test latency and transmission reliability in mobile networks. Protocols and traffic patterns typical for real-time services need to be used in the network optimization process.

The Rohde & Schwarz MNT portfolio can already measure round-trip latency for UDP data packets in mobile networks with the Interactivity Test. Several stories were published about this setup. The Interactivity Test sends a set of packets with pre-defined packet sizes and frequencies from a client to a server, which ‘reflects’ the packets and sends them back to the client. The packets in the uplink and downlink directions have a one-to-one relationship.

However, many applications are not interactive by design and a continuous stream of packets is sent either from server to client or vice versa, without any or an asynchronous exchange of packets in the other direction. Examples include uplink or downlink video transmissions, network traffic from regular status updates and different types of packet streams that travel between a client and a server in industrial applications.

UDP Stream Test

We have developed a new test to better emulate these situations for testing and measurement. The UDP Stream Test sends a pre-defined stream of packets downlink or uplink, as with an Iperf test. However, the parametrization of our UDP Stream Test is tailored to real-time mobile services. Bi-directional test setups, with simultaneous but non-synchronized packet streams in the uplink and downlink, are also possible.

The UDP Stream Test is based on Open Broadband – UDP Speed Test (UDPST) software from the Broadband Forum and AT&T Communications in line with ITU-T recommendation Y.1540. Rohde & Schwarz extended the open source UDPST client/server software utility to derive detailed per-packet round-trip latency results and reproduce pre-defined load patterns with bitrate changes from segment to segment.

In contrast to the Interactivity Test, the UDP Stream Test does not use reflecting packets or apply loads in both directions. Since there is no dependency on the packet stream from the opposite direction, a weak uplink channel has no impact on the throughput or the presence of transmission gaps in the downlink channel. Compared to Iperf, the UDP Stream provides detailed per-packet results, and allows the emulation of any type of traffic pattern in uplink, downlink and both channels simultaneously.

Test principle

The UDP Stream Test works is as follows. A test request comes from the client and load packets are sent from the sender to the receiver with a certain packet frequency and size to emulate the desired traffic pattern. At the same time, control packets travel in the opposite direction at a lower frequency to update the sender about the network. The round-trip latency at the application level is then computed for each received packet from the packet stream using the transmission times for load and control packets. Note that one-way latency cannot be computed in this setup because the clock synchronization in the server and client cannot be guaranteed.

The UDP Stream Test can be configured to emulate any packet stream characterized by packet size and frequency. It is also possible to use pre-defined traffic patterns where the stream characteristics change over time.

The current UDP Stream Test implementation in the Rohde & Schwarz MNT portfolio allows the selection of constant patterns in a broad set of data rates, from a few kbit/s to several hundred Mbit/s in uplink and downlink. There are also pre-defined load patterns available including the ‘eGaming’ and ‘Stepwise’ patterns from the Interactivity Test with bitrate changes by segment. The new ‘Industry 4.0 watchdog’ pattern simulates underlying monitoring processes with smaller and less frequent packets.

UDP Stream Test results

Figure 2 shows how UDP Stream Test results reflect cell handovers and technology changes. Here, round-trip latency is computed using small packets sent from the client to server (uplink) at 50 ms intervals with the ‘Industry 4.0 watchdog’ pattern. Handovers can cause latency peaks (see the yellow marker), and the latency values and corresponding jitter (latency spreads) are clearly higher for LTE than 5G. The map visualization allows easy identification of locations with poor latency.

Other KPIs can be derived from the UDP Stream Test in addition to per-packet round-trip latency. Examples include latency jitter (or packet delay variation), inter-packet arrival time, the throughput based on selected data rates, the percentage of lost, duplicated and reordered packets, and the number and duration of transmission gaps. Our tools report these values for the overall test and at each one-second interval within a test.

Suitability for industrial applications

The UDP Stream Test can be used in industrial networks to emulate any type of inter-machine UDP based packet streams. The detailed results from this test can be used to study network reliability for latency, lost packets, and interruptions for dedicated use cases.

The overall performance of Industry 4.0 mobile networks can be assessed by combining the UDP Stream test and other tests in our portfolio, such as the Interactivity Test and the UDP and HTTP capacity tests.