148 Networking Explained, Second Edition
should not consider baud rate and data rate synonymous. Unless otherwise noted, we will
always express bandwidth in terms of bps or bits per second or as a multiple of bits per
second such as: kilobits per second (kbps), which is 1000 bps; megabits per second
(Mbps), which is 1 million bps; and gigabits per second (Gbps), which is 1 billion bps.
27. While we’re at it, is there a difference between bandwidth and throughput? I
often hear these two terms used interchangeably.
When applied to computer communications and networking, bandwidth represents the
theoretical capacity of a communications channel expressed in bits per second. To under-
stand the difference between bandwidth and throughput, let’s assume the network we use
is a Fast Ethernet LAN. (Note: We discuss Fast Ethernet in Chapter 8.) Fast Ethernet has a
maximum transfer rate of 100 Mbps. Does this mean we can expect all data transfer rates
to be at 100 Mbps? No. Extraneous factors such as a node’s processing capability, input/
output processor speed, operating system overhead, communications software overhead,
and amount of trafﬁc on the network at a given time all serve to reduce the actual data rate.
Consequently, there is a difference between the maximum theoretical capacity of a
communications channel and the actual data transmission rate realized. This “reality rate”
is known as throughput, which refers to the amount of data transmitted between two nodes
in a given period. It is a function of hardware or software speed, CPU power, overhead,
and many other items. Summarizing, bandwidth is a measure of a channel’s theoretical
capacity; it describes the amount of data a channel is capable of supporting. Throughput,
on the other hand, informs us of what the channel really achieves. Just because a medium
or LAN architecture is speciﬁed to operate at a certain data rate, it is not valid to assume
that this rate will be the actual throughput achieved on any given node or group of nodes.
28. What else do I need to know about what bandwidth and throughput?
Well, a related issue that is tied to bandwidth and throughput is noise. We have men-
tioned it several times so far but have yet to discuss it formally.
29. Noise? You mean like loud, senseless shouting, or a crying baby in a restaurant?
OK, we’ll be precise. We mean electrical noise. (You’re not that far off, though.) In the
context of computer communications and networking, noise is any undesirable, extraneous
signal in a transmission medium. It occurs in two forms—ambient noise and impulse noise.
Ambient noise, also called thermal noise, is always present and is generated primarily by
transmission equipment like transmitters, receivers, and repeaters. Ambient noise also can
be induced by external sources such as ﬂuorescent light transformers, electrical facilities,
heat, and, in fact, the background radiation from the Big Bang. If ambient noise is present,
receiving equipment can have problems in distinguishing between incoming signals.
Impulse noise consists of intermittent, undesirable signals induced by external sources
such as lightning, switching equipment, and heavy electrically operated machinery such as
elevator motors and photocopying machines. Impulse noise increases or decreases a cir-
cuit’s signal level; this causes the receiving equipment to misinterpret the signal. Which-
ever the type or source, noise degrades the quality and performance of a communications
channel and is one of the most common causes of transmission errors in computer net-