Physical Layer
1. Wires
There are many types of communication media, usually divided into wired and wireless.
1.1 Twisted Pair
Twisted Pair Lines are a low cost medium produced by twisting a pair of wires together. This makes an unshielded twisted pair (UTP). The twisting helps reduce interference and cross-talk. Since it is cheap, it is commonly usd in telephone systems.
1.2 Patch Panel
A socket panel is where cables end up. These can lead to a network switch, or a private branch exchange (PBX), used to interlink phone systems. This makes up a patch panel.
1.3 Coaxial Cable
A coaxial cable reduces the problems associated with twisted air by placing the two conductors concentrically. This has shielding benefits, and supports a wider range of frequencies, meaning more bandwidth. However, there is a higher cost per metre.
1.4 Optical Fibre
Optical fibre does not suffer from or generate electromagnetic interference. It exploits refraction properties of light. A single fibre is
2. Signals & Waves
Wireless transmission is convenient when communication is required without wires. A signal is carried in the electormagnetic spectrum, with bidirectional communication.
2.1 WiFi
WiFi is the most popular wireless protocol. It uses bands officially assigned by ISM and U-NII. However, it can be prone to interference from other devices such as microwave ovens and baby monitors. We have:
| Name | IEEE | Freq |
|---|---|---|
| Wifi 0 | 802.11 | 2.4GHz |
| Wifi 1 | 802.11b | 2.4GHz |
| Wifi 2 | 802.11a | 5GHz |
| Wifi 3 | 802.11g | 2.4GHz |
| Wifi 4 | 802.11n | 2.4GHz/5GHz |
| Wifi 5 | 802.11ac | 5GHz |
| Wifi 6 | 802.11ax | 2.4GHz/5GHz |
| Wifi 6E | 802.11ax | 6GHz |
| Wifi 7 | 802.11be | 2.4GHz/5GHz/6GHz |
2.2 Information Representation
Signals can be represented as:
- Analogue - changes are represented in an analogous manner in a physical property of the channel.
- Digital - information is represented in a discrete set of astates, using a coding scheme. A binary digital channel is a digital channel which has two states,
and .
We can measure a channel by its baud rate (symbol rate per second, how many times per time unit the symbol changes). The bit rate is the number of bits per second.
A digital channel can by implemented by an analogue channel using a modem (modulator-demodulator), or vice verser with a codec (coder-decoder). These both require a digital to analogue converter (DAC) and an analogue to digital converter (ADC).
2.3 Signal Properties
- Waveform is the shape of the signal.
- Amplitude is the range of values the signal varies over. Maximum value or strength of the signal.
- Wavelength
is the distance between two peaks of a wave. The length of a signle cycle of the signal. - Frequency
is the number of repetitions per unit time.
2.4 Modulation
We can use a modulation scheme to encode digital information to be transmitted effectively using the bandwidth supported by the channel:
- Baseband modulation - transmit the information signal unmodified.
- Broadband modulation - use a physical carrier signal to encode information signal. We modify different features of the carrier signal.
We can modulate in amplitude with amplitude key shifting(ASK), or in frequency with frequency key shifting (FSK), or in phase with phase shift keying (PSK):
2.5 DSL
The digital subscriber line (DSL) is a technology that uses the existing telephone lines to transmit data. It uses a splitter to separate the voice and data signals. The data signal is then modulated and sent to the DSLAM (DSL Access Multiplexer) at the telephone exchange. An ADSL modem is used to perform modulation.
Extensions such as ADSL2 and VDSL are used to increase the bandwidth and bit rate.
DSL download rate depends on the cable length. In general, we find this by using