Application Note: Analyzing Packet Loss and LatencyIssues of LTE Networks with Multi-Segment AnalysisIntroductionWhen troubleshooting LTE networks data throughput issues, the primary concerns are packet loss and latency. In mostinstances, a simultaneous capture will be needed from multiple segments along the network path to determine at which point,or at which interface, and device is dropping packets or having latency problems. It is possible to have multiple capture devicesat different points in the path of traffic, or collect captures from either side of a single device. NETSCOUT'S Network TimeMachine (NTM) is able to collect the captures from different points to generate a single flow representation of the packetsthrough the different segments/hops. This is called multisegment analysis.

LTE Considerations/ QCILTE is a packet-switched network from end-to-end that is designed primarily for high speed data services. To efficiently supportthe varying QoS requirements of different IP applications, LTE uses the concept of a "bearer" as the central element of QoScontrol. Each Evolved Packet System (EPS) bearer (bearer for short) is defined between the Packet Data Network Gateway(PDN-GW) and the User Equipment (UE), and maps to a specific set of QoS parameters such as data rate, latency and packeterror rate.

Packet Loss: One of the main issues with LTE troubleshooting is packet loss. Due to the high speed of traffic beingcaptured, the device capturing the traffic MUST be able to capture at line speed otherwise the integrity of the capture iscompromised. NTM is the first of its kind to be able to capture at up to 20 Gbps line speed with no packet drops. Thisenables one to trust the integrity of the data captured.Latency: Big network latency will affect the user’s experience. NTM is able to measure one-way latency as well as roundtrip latency.

CONFIGURING MULTI SEGMENT ANALYSIS

a. Configuring can be done by having NTMs at different segments collecting packet data. Tocoordinate the capture of multiple NTM to capture traffic and retrieve the relevant packets foranalysis, launch the Remote Agent Manager (RAM), then click on the multi-segment analysis(MSA) tab.

Traffic from each segment above is captured through a different interface on NTM. Multiple NTM devices, or multiple interfacesfrom each NTM, can be selected to collect packet data.

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b. Collect traces from NTM devices and merge them into a single capture file. This is mostcommonly used when the captures are collected from devices at different geographicallocations. Below is a capture that has two segments with traffic from either side of a PDNGateway. The capture is done simultaneously on both S5 and SGi interfaces after which thefile is merged.

c. On the NTM open the said trace file

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d. Once the capture loads, enable the Multi-Segment for all IP addresses and click the "Applyand Save Setting" button.

e. This will be the resulting output. Pay close attention to the packet loss and the issuescolumn.

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f. This is the ladder view of traffic moving from one segment and through one device to theother segment.

g. Scrolling through the conversation searching for issues. The NTM will highlight issues inthe color yellow. A scroll in this conversation reveals frame 639.

The frame is a retry

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h. To find the next issue, one can either scroll through the ladder view, or right click theconversation and find the next issue.

i. Among the issues, one is likely to see retries and out of order packets.

See below:Frame #3205 entered PGW, but didn’t pass through it. PGW is dropping packets, NTM cananalyze the packet loss and latency issues with this multi-segment correlation.

SummaryThe ability to extract and merge flows from multiple segments, provides a holistic view of the traffic from source through thenetwork infrastructure. The ability to stream traffic to disk from multiple segments at up to 20Gbps throughput ensures that theintegrity of the data is guaranteed, thus allowing the analyst to make correct deductions on throughputs and bottlenecks in thenetwork.

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© 2016 Fluke Corporation. Rev: 04/01/2016 11:55 am (Literature Id: 6002631)

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