CPS-1848 User Manual
93
June 2, 2014
Formal Status
This document is confidential and is subject to an NDA.
Integrated Device Technology
5. Performance
This chapter discusses the packet switching performance characteristics of the CPS-1848. Topics discussed include the
following:
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Port-to-Port Performance Characteristics
5.1
Overview
Performance for packet switching is characterized by three measurements: throughput, latency, and latency jitter. Performance
is specified for error-free transmission and reception of packets, and for end-to-end transfers through the CPS-1848.
Performance is specified for a single switch. Performance for larger systems can be computed from this data.
5.1.1
Throughput
Throughput for packets is a measurement of the amount of packet data that can be transferred in a given amount of time. It can
be presented in different forms:
• Percentage of a link’s bandwidth (for example, 56% of a 1x @ 6.25 Gbaud)
• Number of packets of a given size per unit time (for example, 3000 44-byte packets per second)
• Bit transfer rate (for example, 300 Mbps)
Throughput measurements include only successfully transferred packets. Measured throughput does not include control
symbols, retried packets, or other non-packet data transmitted/received on a link (/K/ and /R/ characters).
5.1.2
Latency
Latency is the amount of time between when a packet is received and when it is transmitted. The specific time at which a
packet is received and transmitted are deemed to have started, however, must still be defined. Latency is measured as the
time interval between the first bit of the Start-of-Packet arriving at the ingress of the CPS-1848 and that same bit leaving the
device.
displays the path a packet flows through the CPS-1848. For CPS-1848 latency performance, packet reception time
begins with the time the first bit of a packet is seen on the input pins. Packet transmission begins when the first bit of a packet
is transmitted on the output pins.
As part of the resolution of resource contention, higher priority packets can pass packets of lower priority. Latencies should
therefore increase as the priority of a packet decreases. The latency of higher priority packets are consistently low.
A specific time for packet latency can be specified only when there are no conditions that create resource contention between
packets. For example, if a single stream of packets passing from an ingress port to an egress port is the only traffic handled by
the CPS-1848, it is possible to specify the latency for the packets in this stream.