Understanding the difference between throughput and bandwidth are important for network troubleshooting. Although these terms are conflated at times, they are actually both unique. Bandwidth is the theoretical maximum speed a specific medium can deliver if all factors are perfect without any form of interference.

Throughput is the actual speed realized in a real-world scenario, given interference and other environmental factors such as configuration, contention, and congestion.

The difference between these terms is important when troubleshooting. If a PowerScale node supports 40 GbE, it does not necessarily mean the throughput is 40 Gb/s. The throughput between a client and a PowerScale node depends on all the factors between the two endpoints and can be measured with various tools.

During the design phase of a data center network, ensure that bandwidth is available throughout the hierarchy, eliminating bottlenecks and ensuring consistent bandwidth. The bandwidth from the access switches to the PowerScale nodes should be a ratio of what is available back to the distribution and core switches. For example, if a PowerScale cluster of 12 nodes has all 40 GbE connectivity to access switches, the link from the core to distribution to access should be able to handle the throughput from the access switches. Ideally, the link from the core to distribution to access should support roughly a bandwidth of 480 Gb (12 nodes * 40 GbE).

Bandwidth delay product

Bandwidth Delay Product (BDP) is calculated to find the amount of data a network link is capable of, in bytes, which can be transmitted on a network link at a given time. The keyword is transmitted, meaning the data is not yet acknowledged. BDP considers the bandwidth of the data link and the latency on that link, in terms of a round-trip delay.

The amount of data that can be transmitted across a link is vital to understanding Transmission Control Protocol (TCP) performance. Achieving maximum TCP throughput requires that data must be sent in quantities large enough before waiting for a confirmation message from the receiver, which acknowledges the successful receipt of data. The successful receipt of the data is part of the TCP connection flow. The following figure explains the steps of a TCP connection and where BDP is applicable:

Figure 3.   Transmission Control Protocol message flow

In the preceding figure, four states are highlighted during a TCP connection. The following summarizes each state:

1. TCP Handshake – Establishes the TCP connection through an SYN, SYN/ACK, ACK
2. Data transmitted to the server. BDP is the maximum amount of data that can be sent at this step.
3. Data acknowledged by Server
4. TCP Connection Close Sequence – Socket closure is initiated by either side

Once the BDP rate is calculated, the TCP stack is tuned for the maximum throughput, which is discussed in the next section. The BDP is calculated by multiplying the bandwidth of the network link (bits/second) by the round-trip time (seconds).

For example, a link with a bandwidth of 1 gigabit per second and a 1-millisecond round-trip time is calculated as:

Bandwidth * RTT = 1 Gigabit per second * 1 millisecond =

1,000,000,000 bits per second * 0.001 seconds = 1,000,000 bits = 0.125 MB

Thus, 0.125 MB may be sent per TCP message to the server.