CANOGA PERKINS 9145E NID Software Version 4.10 - Manual (Page 67)

Diagnostics 9145E NID Software User’s Manual
Network Performance
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ICMP ECHO REQUEST/REPLY packets with special fields designated in the ICMP Message
Body. Therefore, the resultant Test Packet protocol stack is of the format ICMP over IP over
Ethernet.
This Synthetic Test Packet is sent by the near (or local) NID (also known as the Master NID). The
test packet contains test control information and fields for time stamps. Four time stamp fields
are contained in the payload of the Test Packet: Originating Departure Time, Destination Arrival
Time, Destination Departure Time and Originating Arrival Time. This is a similar scheme to that
used in ITU-T Y.1731 [2].
Time Stamps are inserted by the Field Programmable Gate Array (FPGA) contained in the NID
(also known as “hardware time stamping”). Time stamping by the FPGA eliminates variable
processing times associated with the Management Processor IP Stack and reflects the true time
it took for the test packet to transverse the network.
Time Stamps allow Round Trip Latency and One-Way Jitter parameters to be calculated and
reported. Control information tracks the packets and their sequence and accounts for Lost
Packets and Out of Sequence Packets.
Figure 4-12 Performance Measurement Model
4.6.1.1 Round-Trip Latency
Round-Trip Latency is the time it takes a packet to propagate from one NID to another and back
again. Over a high-speed fiber optic network, Latency is some small, non-zero value, since a
packet cannot travel between Network Elements instantaneously. Latency is affected by packet
size, physical transmission medium, Network Elements (routers, etc.) and network congestion.
Round-Trip Latency is calculated as follows: Round Trip Latency = (Origination Departure –
Origination Arrival) – (Destination Departure-Destination Arrival).
4.6.1.2 One-Way Jitter
One-Way Jitter is the difference in one-way latencies between two subsequent received packets.
The four Time Stamps allow Jitter to be measured independently in each direction (origin-
destination, destination-origin). Typical causes of Jitter include network queuing and congestion,
variations in Network Element forwarding times, path rerouting, etc.
Jitter can be measured as negative or positive. Negative Jitter occurs when packets arrive
sooner (or closer together) compared to previous intervals being considered. Positive Jitter