Latency is the time it takes to establish a connection. Latency is
influenced by hardware i.e. which connection device is used (e.g.,
wireless connections take longer than wired) and also other factors
such as the length of the route or latency bottlenecks in the route.
Latencies are easiest measured with the Linux tool ping.
Ping takes a hostname or IP address as a parameter and sends a small
test packet to the node that is echoed by it. Once the echo is
received time is measured. If the host does not send an echo back in
a reasonable time, a timeout is signaled indicating the host is
offline. This is repeated until the user presses CTRL+C.
Under Linux ping is sometimes accessible as /usr/sbin/ping
and on Mac it's sometimes /sbin/ping.
Ping can also be used on entire
networks. This is called broadcast ping, whereby all online hosts on
the Ethernet bus will respond. In order to ping your network you
need to find out the IP address of your own host and derive the
network IP. There are several ways to determine your own IP but the
easiest is way (when you are not behind a router) is to open your
browser to www.google.com
and search for "my IP". Then replace the last digits of the IP
(say XXX.YYY.ZZZ.001) address with 255. So if your IP address is
192.150.186.72, your replacement would be 192.150.186.255.
Then you execute:
ping -b XXX.YYY.ZZZ.255
Go ahead and try it!
Note that this may not always work as some networks forbid the
broadcast ping or are configured differently. Nevertheless, for a
system administrator it's an easy way to test all the hosts along
the bus at once.
Throughput
Throughput is a term which describes how much data
is being transferred at a time. You may also be familiar with another term,
bandwidth, which means the same thing. It is usually measured in
terms of "bps" or bits per second. For example, you may have an internet
connection capable of 5Mbps. This means, that (in theory!) each
second you computer could download 5 megabits of data from another location.
Of course, in reality, throughput varies widely, and it's quite difficult
to obtain the maximum speeds of any connection.
Throughput is much harder to measure. It also massively depends on
what other processes (applications) running on your computer take bandwidth,
as well as any other devices connected through the same Ethernet port or
wireless access device. A simple way but not necessarily very accurate way
to measure bandwidth is to use a website
designed to test your connection. One example is provided by the website
http://www.speedtest.net.
Give it a quick try and answer the following questions:
Run the speed test three times. How much did the throughput
vary?
Is upload speed different than download speed? If so, why?
Synchronize with your peers and measure the speed
concurrently. How much did it drop?