Calculating Overhead and Wireless Data Throughput Rates
Throughput vs. Data Rates
A last word regarding wireless connectivity is appropriate. earlier
I listed the
theoretical download times of a Tom Clancy novel using various connection types.
These are theoretical because real network transmission involves some
"overhead," or additional information sent to aid in delivery of the actual
data.
On a network, data is divided into pieces and packaged for
transmission over the network. Called packets, these pieces all have
additional information attached to them. Think of the attached information as an
"address label" for the packet. At minimum, the "label" has the address of its
destination on the network and that of its sending computer. It also has a
sequence number so that the packets can be reassembled in proper order. (There
are other items included as well, but these will do for our purposes
here.)
Because of this added information, the data transfer rate
associated with any medium refers to the maximum amount of total data
transmitted per second, including "address labels." The actual content
transmitted is less.
In wireless networking, there is even more overhead than
encountered in cabled connectivity. Because a radio is used, a small slice of
time is used to switch from transmit to receive mode. Other internal functions
required to receive data signals from the bridge and alter them to work over a
radio connection consume more slices of time. Since time lost equals data
throughput lost, a radio connection generally is not as efficient as a
direct-cabled connection.
Two specifications are normally provided for wireless bridges.
The term data rate is normally used to specify the theoretical bit
transfer rate of a particular implementation of radio frequency transmission.
Throughput specifies the maximum amount of data that can be pushed across
the link. Some spreading technologies are more effective than others, so the
throughput will vary.
As a rule of thumb, you can take the data rate and divide it in
half to obtain an estimate of actual throughput. In my estimates of throughput
in Table 2, I included a
range of throughputs for each performance level of wireless bridge: the low end
is the one-half-data-rate rule, and the high end is the average of vendor claims
for throughput.
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