Communication channel facts for kids

A communication channel is like a pathway for sending information. It can be a physical thing, like a wire, or a wireless connection, like a radio signal. It helps send messages from one or more senders to one or more receivers. Think of it as a road for data!

Different types of physical transmission media supporting communication channels

To send information over a distance, you need a pathway or medium. These pathways are called communication channels. They use two main types of media:

• Wired connections: These include cables like twisted-pair (used in old phone lines), coaxial cable (used for TV), and fiber-optic cable (which sends data using light, making it super fast).
• Wireless connections: These use signals that travel through the air, like microwave signals, satellite signals, radio waves, and infrared (like your TV remote).

In information theory, which is a field of study about how information works, a channel can also be a way to store information. For example, a storage device like a hard drive is a communication channel because you "send" information to it (save a file) and "receive" information from it (open the file later). This allows information to be communicated over time.

What are some examples of communication channels?

Here are some common examples of communication channels:

• A connection between two points in a telecommunication system, like during a phone call.
• A single path within a transmission medium, such as one pair of wires in a multi-wire cable. Or, it can be a path where many signals share one connection by using different frequencies or taking turns (time-division multiplexing).
• A path for carrying electrical or radio signals, usually separate from other similar paths.
• A data storage device that can send a message over time, like a track on a hard drive.
• A buffer (a temporary storage area) where messages can be placed and then retrieved.
• In a communications system, the physical or logical link that connects where data comes from to where it goes.
• A specific radio frequency or band of frequencies, often given a name or number. For example:
  • Marine VHF radio uses about 88 channels for two-way voice communication. Channel 16, for instance, is at 156.800 MHz.
  • Television channels like North American TV Channel 2 (at 55.25 MHz) or Channel 13 (at 211.25 MHz). Each channel used to be 6 MHz wide for analog television. Now, with digital television, these "physical channels" can carry multiple "virtual channels" because digital signals are more efficient.
  • Original Wi-Fi uses 13 channels in the ISM bands, from 2412 MHz to 2484 MHz.
  • The radio channel between an amateur radio repeater and an amateur radio operator often uses two frequencies that are 600 kHz apart. For example, a repeater might transmit on 146.94 MHz and listen for a ham operator on 146.34 MHz.

All these communication channels transfer information using a signal.

How do we model communication channels?

Scientists and engineers create mathematical models to describe how a signal changes when it travels through a channel. These models help them understand how the input signal (what's sent) becomes the output signal (what's received).

• Physical models try to calculate how physical things, like reflections from objects in a room for wireless signals, change the signal. They might also add random numbers to simulate outside interference or electronic noise.
• Statistical models look at the probability of errors. For example, if you send a symbol i, what's the probability that the symbol o is received?
• Often, both types of modeling are combined. For instance, in wireless communication, the signal might be modeled as getting weaker (fading) and then having noise added to it. The fading represents how the signal power changes, and the noise represents interference or electronic issues.

Channel models can be either digital or analog.

Digital channel models

In a digital channel model, the message is seen as a digital signal. These models focus on things like:

• bit rate (how many bits per second)
• bit errors (how many bits are wrong)
• latency/delay (how long it takes for the signal to arrive)
• delay jitter (how much the delay varies)

Examples of digital channel models include:

• Binary symmetric channel (BSC): A simple model where each bit has a certain chance of being flipped (0 becomes 1, or 1 becomes 0).
• Binary erasure channel (BEC): A model where bits can be "erased" (meaning they are detected but cannot be read).
• Packet erasure channel: Where entire packets of data are lost.

Analog channel models

In an analog channel model, the message is seen as an analog signal. These models can be simple or complex, and they look at issues like:

• Noise: Such as Additive white Gaussian noise (AWGN), which is random noise spread evenly across frequencies.
• Interference: Like crosstalk (signals from one channel affecting another) or intersymbol interference (where one symbol's signal bleeds into the next).
• Distortion: When the signal changes its shape in an unwanted way.
• Frequency response: How the channel affects different frequencies, including attenuation (signal weakening) and phase-shift.
• Radio frequency propagation models: These describe how radio waves travel, including:
  • Fading: When the signal strength changes over time or distance.
  • Doppler shift: When the frequency of the signal changes because the sender or receiver is moving.

Different types of channels

Channels can be classified in many ways:

• Digital or analog channels.
• Based on the Transmission medium, like a fibre channel.
• Multiplexed channel: Where multiple signals share one physical channel.
• Virtual channel: A logical connection over a network.
• Simplex communication (one-way), duplex communication (two-way at the same time), or half duplex communication (two-way, but only one direction at a time).
• Return channel: A channel for sending information back.
• Uplink (sending data up to a satellite or base station) or downlink (receiving data from a satellite or base station).
• Broadcast channel (sending to everyone), unicast channel (sending to one specific person), or multicast channel (sending to a specific group).

How do we measure channel performance?

Here are some ways we measure how well a communication channel works:

• Spectral bandwidth: How wide the frequency range is, measured in Hz.
• Symbol rate: How many symbols (or changes in the signal) are sent per second.
• Digital bandwidth: How many bits per second can be sent. This includes gross bit rate (total bits), net bit rate (actual information bits), channel capacity (maximum possible rate), and maximum throughput (actual rate achieved).
• Channel utilization: How much of the channel's capacity is being used.
• Link spectral efficiency: How many bits per second can be sent per Hertz of bandwidth.
• Signal-to-noise ratio: How strong the signal is compared to the noise, often measured in decibels.
• Bit-error rate (BER): The percentage of bits that are received incorrectly.
• Latency: The time it takes for a signal to travel from sender to receiver, measured in seconds. This includes propagation time (travel time) and transmission time (time to send the data).
• Delay jitter: How much the delay varies over time.

Channels in networks (Multi-terminal channels)

In networks, many devices (called terminals) share the communication medium. Depending on how they communicate, they can either help each other or cause interference. Complex networks can be seen as combinations of simpler multi-terminal channels. Here are some main types:

• Point-to-multipoint channel: One sender sends multiple messages to different receivers. Most wireless channels can act this way. For example, the downlink (sending data from a cell tower to phones) in a cellular system can be seen as this type of channel if we only look at one cell.
• Multiple access channel: Multiple senders send different messages over a shared physical medium to one or more receivers. This needs rules (channel access scheme) for how devices share the channel. This model applies to the uplink (sending data from phones to a cell tower) in cellular networks.
• Relay channel: One or more intermediate devices (called relays or repeaters) help a sender send a message to its final destination.
• Interference channel: Two different senders send data to different receivers, and their signals might interfere with each other. This happens with co-channel interference between different cell towers in cellular networks.
• A unicast channel sends data to one specific user, like a phone call.
• A broadcasting channel sends data to all users in a network, like a paging service or emergency alerts.
• A multicast channel sends data to a specific group of users who have subscribed.

Images for kids

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  A friendly robot, representing communication.

See also

• Channel capacity
• Channel access method
• Traffic generation model