Baseband transmission and broadband transmission

A digital signal carries information using only two levels, written as 1 and 0. Every text message, photo and video stored on a phone or computer is, underneath, a long string of these ones and zeros. To reach another device, the string has to travel through a channel. The channel may be a copper cable, an optical fibre or open air.

There are two ways to put that string into a channel. It can be sent exactly as it is, as a pattern of high and low pulses. Or it can first be turned into an analogue wave, a smooth wave that rises and falls, and sent in that form. The first way is called baseband transmission. The second is called broadband transmission. Which one is used depends mainly on how far the signal has to travel and how much data has to move.

Think of two computers in a school lab joined by a network cable. When one computer sends a file, its data, such as 101011, first goes to an encoder. The encoder turns the ones and zeros into a digital signal: a train of electrical pulses that jump between a high level and a low level. These pulses travel along the cable as square pulses. At the far end, a decoder reads the high and low levels and hands exactly the same data, 101011, to the receiving computer. Nowhere on this journey does the signal become a smooth wave.

In baseband transmission, the process of sending and receiving signals is done in the same channel. The one digital signal takes up the whole cable, so the cable does not carry several signals side by side at different frequencies.

Baseband is used for short-distance transmission. As square pulses travel along a long wire, their sharp edges get rounded off and weakened. After a long enough distance, the receiver can no longer tell a 1 from a 0 with confidence. Inside a room or a building the distance is small, so this does not matter. Sending data from one computer to another by connecting them with cables, as in an office network or a school computer lab, is baseband transmission.

In ordinary conversation, broadband internet means high-speed internet. The name comes from the method behind it. Broadband internet is the broadband transmission of data through various devices, such as the provider's equipment, optical fibre lines, mobile towers and the router in a home.

Changing a digital signal into an analogue one is done by modulation, so broadband transmission requires modulation. Once the data is riding on an analogue wave, it can be sent as optical waves, which means light travelling inside an optical fibre, or as electromagnetic waves, such as radio waves from a tower. Different signals can be given different transmission frequencies. This lets one medium carry a large amount of data at once.

A broadband connection has to send and receive at the same time. A video streams in while your messages go out. To manage this, the transmission medium or channel is divided into two separate channels: one for sending and one for receiving. As an alternative, two separate cables can be used, one for each direction.

Broadband transmission is used for long-distance transmission. The internet that reaches a home through an optical fibre from a provider's office several kilometres away, and the 4G data a phone receives from a mobile tower, both travel this way.

Modulation is the step that makes broadband possible, so it helps to see what happens inside it.

Three things are involved. The modulating signal is the information to be sent. In broadband transmission it is the digital signal. The carrier signal is a steady high-frequency wave that can travel a long way. The modulator is the device that combines the two. What comes out of the modulator is called the modulated signal, and this is what goes into the channel. At the receiving end, a demodulator does the opposite job and takes the information back off the carrier.

A digital signal has only two values, 1 and 0. So modulating it means making the carrier look one way for a 1 and another way for a 0. There are three common ways of doing this:

• Amplitude shift keying (ASK): the height, or amplitude, of the carrier wave changes. The wave is tall for a 1 and small or flat for a 0.
• Frequency shift keying (FSK): the frequency of the wave changes. The waves are packed closely together for a 1 and spread out for a 0.
• Phase shift keying (PSK): the height and spacing of the wave stay the same, but the wave for a 1 starts in the opposite direction to the wave for a 0.

Placed side by side, the two methods differ on every point:

The speed of an internet connection tells how much data it can move each second. It is measured in megabits per second, written Mbps. A 20 Mbps connection can move about 20 million bits every second.

A speed test website measures this by sending test data to your device and receiving data back from it. It usually reports three things: download speed, which is how fast data comes to you; upload speed, which is how fast data goes out from you; and ping, which is how long a tiny signal takes to go and come back. Ping is measured in milliseconds (ms).

The result depends on the provider, the type of connection (optical fibre, 4G wireless or cable) and even the time of day, because a connection shared by many users at once slows down. Measuring the speed of two different providers, or of one provider at two different times, and writing the numbers in a table is a simple way to compare them.

Telephone networks, television and the internet have all moved to digital transmission because it has clear advantages over analogue transmission. The main advantages are these:

• Deterioration and noise have very little effect on digital signals. Noise is unwanted disturbance picked up along the way. Since the receiver only has to decide between 1 and 0, a little noise does not change the message.
• The circuits used for digital transmission are much more reliable.
• Digital circuits are cheaper and easier to design than analogue circuits.
• There is less chance of signals overlapping and of cross-talk. Cross-talk is when the signal on one line leaks into a neighbouring line, like faintly hearing someone else's conversation during a phone call.
• The properties of digital signals do not change under normal conditions.
• The security of information can be maintained through encoding and compression. Encoded data cannot be read by someone who does not know how to decode it.
• Codes for finding and correcting errors are used, so there is less chance of error during transmission.

These are the reasons a video call today can sound clear across the world, while an old analogue telephone line crackled over just a few kilometres.