BANDWIDTH

Bandwidth is the amount of information (bits) that can pass through a network in a given period of time.

The bandwidth of a network depends on the technology used in the network. And, the technology is dependent on the laws of physics. Therefore, bandwidth is dependent on technology and physics.

IMPORTANCE OF UNDERSTANDING BANDWIDTH

The understanding of bandwidth is important because:

For all networks, bandwidth is limited.
Bandwidth costs money.
The demand for bandwidth will increase for the foreseeable future.
Understanding bandwidth allows for network optimization (maximum efficiency)

ANALOGY

The concept of bandwidth can be compared to water pipes.

A network of pipes brings fresh water to homes and businesses and carries waste water away. This water network is made up of pipes of different diameters. The main water pipes of a city may be two meters in diameter, while the pipe to a kitchen faucet may have a diameter of only two centimeters. The width of the pipe determines the water-carrying capacity of the pipe. Therefore, the water is like the data, and the pipe width is like the bandwidth. Many networking experts say that they need to put in bigger pipes when they wish to add more information-carrying capacity. (Source: Cisco Systems)

UNIT OF MEASURE

Bandwitdh is measured in bits per second (bps).

As bandwidth values have increased over time, we have added Greek prefixes to the measurement.

Bits per second	bps	1 bps
Kilobits per second	Kbps	1000 bps
Megabits per second	Mbps	1000000 bps (one million)
Gigabits per second	Gbps	1000000000 bps (one billion)
Terrabits per second	Tbps	1000000000000 bps (one trillion)

It is important not to get confused between bit and byte. To get the number of bytes transferred in a second, we must divide by 8. Note that the abbreviation for byte is an upper case B.

BANDWIDTH VS SPEED

We often use speed and bandwidth interchangeably. This is false because it suggests that bits in a high bandwidth network move faster than they do in a lower bandwidth network. In reality, only the overall amount of data necessarily moves faster.

THROUGHPUT

While the bandwidth transfer rate is possible, it usually only occurs in optimum conditions (few active users). The actual measure of transfer is called throughput.

Factors that affect throughput are:

Internetworking devices
Type of data being transferred
Network topology
Number of users on the network
User computer
Server computer

FORMULAS

We can calculate the bandwidth by using:

BW = bits transferred in optimal conditions / time in seconds

We can calculate the throughput by using:

TP = bits transferred on typical conditions / time in seconds

HOW TO DECIDE HOW MUCH BANDWIDTH YOU NEED

This video goes through the details needed to make a bandwidth decision.

Link: https://www.youtube.com/watch?v=2LOkI3Xyd_E

WORK

1. What is the throughput for the following situations?

a) 2000 bits are transferred over a 1 second period (slow)

b) 500 Kilobits are transferred over a 2 second period

c) 1 byte is transferred over 2 seconds (very slow)

d) 5 Megabytes are transferred over 4 seconds