> For the complete documentation index, see [llms.txt](https://felicia232002.gitbook.io/notes-for-introduction-to-networking/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://felicia232002.gitbook.io/notes-for-introduction-to-networking/chapter-2.md).

# Chapter 2

Example of simple transmission is TV and Radio.

Example of Half-Duplex is Walkie-Talkie.

Example of Full-Duplex is Mobile phone.

An **analog signal** is any [continuous signal](https://en.wikipedia.org/wiki/Continuous_signal) for which the time-varying feature of the signal is a representation of some other time-varying quantity

A **digital signal** is a **signal** that is being used to represent data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values

![](https://2277444168-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Mc7aa7TpYiFT5jdyFYT%2F-McJcQGF2dPieVu66fLp%2F-McJirR6v_oWocSKkjyW%2Fdownload%20\(1\).png?alt=media\&token=77c43c25-f9f4-4227-a196-3d3b531080d2)

3 main factors of the signal are : magnitude , frequency , phase.

baud rate formula  : s(t) = m(t) cos(2\*phi\*f(t)+teta(t)).

To lower the probability of interruption , we have to be closer to the access point to be faster , because the smaller the difference, the easier it will be for interruptions to occur.

lightspeed = frequency x wavelength(lamda).

modulation : take original signal to a carrier.

demodulation : take original signal from carrier 

carrier is like a transportation and carrier is always an analog signal.

 **Amplitude modulation** (**AM**) is a [modulation](https://en.wikipedia.org/wiki/Modulation) technique used in electronic communication, most commonly for transmitting messages with a [radio](https://en.wikipedia.org/wiki/Radio) [carrier wave](https://en.wikipedia.org/wiki/Carrier_wave). In amplitude modulation, the [amplitude](https://en.wikipedia.org/wiki/Amplitude) (signal strength) of the carrier wave is varied in proportion to that of the message signal, such as an [audio signal](https://en.wikipedia.org/wiki/Audio_signal). This technique contrasts with [angle modulation](https://en.wikipedia.org/wiki/Angle_modulation), in which either the frequency of the [carrier wave](https://en.wikipedia.org/wiki/Carrier_wave) is varied as in [frequency modulation](https://en.wikipedia.org/wiki/Frequency_modulation), or its [phase](https://en.wikipedia.org/wiki/Phase_\(waves\)), as in [phase modulation](https://en.wikipedia.org/wiki/Phase_modulation).

 **Frequency modulation** (**FM**) is the encoding of [information](https://en.wikipedia.org/wiki/Information) in a [carrier wave](https://en.wikipedia.org/wiki/Carrier_wave) by varying the [instantaneous frequency](https://en.wikipedia.org/wiki/Instantaneous_frequency) of the wave. The technology is used in [telecommunications](https://en.wikipedia.org/wiki/Telecommunications), [radio broadcasting](https://en.wikipedia.org/wiki/Radio_broadcasting), [signal processing](https://en.wikipedia.org/wiki/Signal_processing), and [computing](https://en.wikipedia.org/wiki/Run-length_limited#FM:_.280.2C1.29_RLL).

 **Phase modulation** (**PM**) is a [modulation](https://en.wikipedia.org/wiki/Modulation) pattern for conditioning communication signals for [transmission](https://en.wikipedia.org/wiki/Transmission_\(telecommunications\)). It encodes a message signal as variations in the [instantaneous phase](https://en.wikipedia.org/wiki/Instantaneous_phase) of a [carrier wave](https://en.wikipedia.org/wiki/Carrier_wave). Phase modulation is one of the two principal forms of [angle modulation](https://en.wikipedia.org/wiki/Angle_modulation), together with [frequency modulation](https://en.wikipedia.org/wiki/Frequency_modulation).

All of those modulation are analog signals.

 The **ISM radio bands** are [portions](https://en.wikipedia.org/wiki/Radio_band) of the [radio spectrum](https://en.wikipedia.org/wiki/Radio_spectrum) reserved internationally for industrial, scientific and medical (ISM) purposes, excluding applications in [telecommunications](https://en.wikipedia.org/wiki/Telecommunications).[\[1\]](https://en.wikipedia.org/wiki/ISM_radio_band#cite_note-1) Examples of applications for the use of [radio frequency](https://en.wikipedia.org/wiki/Radio_frequency) (RF) energy in these bands include [radio-frequency process heating](https://en.wikipedia.org/wiki/Radio_frequency_heating), [microwave ovens](https://en.wikipedia.org/wiki/Microwave_oven), and medical [diathermy](https://en.wikipedia.org/wiki/Diathermy) machines. The powerful emissions of these devices can create [electromagnetic interference](https://en.wikipedia.org/wiki/Electromagnetic_interference) and disrupt [radio communication](https://en.wikipedia.org/wiki/Radio_communication) using the same [frequency](https://en.wikipedia.org/wiki/Frequency), so these devices are limited to certain bands of frequencies. In general, communications equipment operating in ISM bands must tolerate any interference generated by ISM applications, and users have no regulatory protection from ISM device operation in these bands.

4 steps for digital signal to anaolog signal 

1. Sampling
2. Quantization 
3. Magnitude
4. PCM Encoding

Encoding 

Self Clocking: to put clocking information into signal , so we don't have to match the clocking from the sender and receiver.

{% file src="/files/-Mcf7rMTB45DBtfvt\_QU" %}

 **Power over Ethernet**, or **PoE**, describes any of several [standards](https://en.wikipedia.org/wiki/Technical_standard) or [ad hoc](https://en.wikipedia.org/wiki/Ad_hoc) systems that pass [electric power](https://en.wikipedia.org/wiki/Electric_power) along with data on [twisted-pair Ethernet](https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair) cabling. This allows a single cable to provide both data connection and electric power to devices such as [wireless access points](https://en.wikipedia.org/wiki/Wireless_access_point) (WAPs), [Internet Protocol (IP) cameras](https://en.wikipedia.org/wiki/IP_camera), and [voice over Internet Protocol (VoIP) phones](https://en.wikipedia.org/wiki/VoIP_phone).

 A **modulator-demodulator**, or simply a **modem**, is a [hardware](https://en.wikipedia.org/wiki/Computer_hardware) device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a [transmission medium](https://en.wikipedia.org/wiki/Transmission_medium) such as telephone lines or radio. A modem [modulates](https://en.wikipedia.org/wiki/Modulation#Digital_modulation_methods) one or more [carrier wave](https://en.wikipedia.org/wiki/Carrier_wave) signals to encode [digital information](https://en.wikipedia.org/wiki/Digital_information) for transmission, and [demodulates](https://en.wikipedia.org/wiki/Demodulation) signals to decode the transmitted information. The goal is to produce a [signal](https://en.wikipedia.org/wiki/Signal_\(electronics\)) that can be transmitted easily and decoded reliably to reproduce the original digital data.

 In [telecommunications](https://en.wikipedia.org/wiki/Telecommunications) and [computer networks](https://en.wikipedia.org/wiki/Computer_networks), **multiplexing** (sometimes contracted to **muxing**) is a method by which multiple analog or digital signals are combined into one signal over a [shared medium](https://en.wikipedia.org/wiki/Shared_medium). The aim is to share a scarce resource.

#### Frequency-division multiplexing

[![](https://upload.wikimedia.org/wikipedia/commons/thumb/f/f1/Frequenzmultiplex001.svg/400px-Frequenzmultiplex001.svg.png)](https://en.wikipedia.org/wiki/File:Frequenzmultiplex001.svg)Frequency-division multiplexing (FDM): The spectrum of each input signal is shifted to a distinct frequency range.

[Frequency-division multiplexing](https://en.wikipedia.org/wiki/Frequency-division_multiplexing) (FDM) is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. **In FDM the signals are electrical signals.** One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches a customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the appropriate frequency (channel) to access the desired signal.

A variant technology, called [wavelength-division multiplexing](https://en.wikipedia.org/wiki/Wavelength-division_multiplexing) (WDM) is used in [optical communications](https://en.wikipedia.org/wiki/Optical_communication).

#### Time-division multiplexing

[![](https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Telephony_multiplexer_system.gif/400px-Telephony_multiplexer_system.gif)](https://en.wikipedia.org/wiki/File:Telephony_multiplexer_system.gif)Time-division multiplexing (TDM).

[Time-division multiplexing](https://en.wikipedia.org/wiki/Time-division_multiplexing) (TDM) is a digital (or in rare cases, analog) technology which uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.

Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400 [baud](https://en.wikipedia.org/wiki/Baud), so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.[\[3\]](https://en.wikipedia.org/wiki/Multiplexing#cite_note-Bates-3) Some [web proxy servers](https://en.wikipedia.org/wiki/Proxy_server#Web_proxy_servers) (e.g. [polipo](https://en.wikipedia.org/wiki/Polipo)) use TDM in [HTTP pipelining](https://en.wikipedia.org/wiki/HTTP_pipelining) of multiple [HTTP](https://en.wikipedia.org/wiki/HTTP) transactions onto the same [TCP/IP connection](https://en.wikipedia.org/wiki/Transmission_Control_Protocol).

[Carrier sense multiple access](https://en.wikipedia.org/wiki/Carrier_sense_multiple_access) and [multidrop](https://en.wikipedia.org/wiki/Multidrop) communication methods are similar to time-division multiplexing in that multiple data streams are separated by time on the same medium, but because the signals have separate origins instead of being combined into a single signal, are best viewed as [channel access methods](https://en.wikipedia.org/wiki/Channel_access_method), rather than a form of multiplexing.

TD is a legacy multiplexing technology still providing the backbone of most National fixed line Telephony networks in Europe, providing the 2m/bit voice and signalling ports on Narrow band Telephone exchanges such as the DMS100. Each E1 or 2m/bit TDM port provides either 30 or 31 speech timeslots in the case of CCITT7 signalling systems and 30 voice channels for customer connected Q931, DASS2, DPNSS, V5 and CASS signalling systems.

####

#### Code-division multiplexing

[Code division multiplexing](https://en.wikipedia.org/wiki/Code_division_multiplexing) (CDM), [Code division multiple access](https://en.wikipedia.org/wiki/Code_division_multiple_access) (CDMA) or [spread spectrum](https://en.wikipedia.org/wiki/Spread_spectrum) is a class of techniques where several channels simultaneously share the same [frequency spectrum](https://en.wikipedia.org/wiki/Frequency_spectrum), and this spectral bandwidth is much higher than the bit rate or [symbol rate](https://en.wikipedia.org/wiki/Symbol_rate). One form is frequency hopping, another is direct sequence spread spectrum. In the latter case, each channel transmits its bits as a coded channel-specific sequence of pulses called chips. Number of chips per bit, or chips per symbol, is the [spreading factor](https://en.wikipedia.org/wiki/Spreading_factor). This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber or radio channel or other medium, and asynchronously demultiplexed. Advantages over conventional techniques are that variable bandwidth is possible (just as in [statistical multiplexing](https://en.wikipedia.org/wiki/Statistical_multiplexing)), that the wide bandwidth allows poor signal-to-noise ratio according to [Shannon-Hartley theorem](https://en.wikipedia.org/wiki/Shannon-Hartley_theorem), and that multi-path propagation in wireless communication can be combated by [rake receivers](https://en.wikipedia.org/wiki/Rake_receiver).

A significant application of CDMA is the [Global Positioning System](https://en.wikipedia.org/wiki/Global_Positioning_System) (GPS).

For the brief explanation :

FDM : example for TV Station , it can be divided into sub-channel and can be used by different people.

TDM : Who get it first who used it first.

CDM : the orthogonal signal encoded with different ways so they don't disturb each other.

 **Telephony** is the field of technology involving the development, application, and deployment of [telecommunication](https://en.wikipedia.org/wiki/Telecommunication) services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the invention and development of the [telephone](https://en.wikipedia.org/wiki/Telephone).

The **public switched telephone network** (**PSTN**) provides [infrastructure](https://en.wikipedia.org/wiki/Communications_infrastructure) and services for public [telecommunication](https://en.wikipedia.org/wiki/Telecommunication). The PSTN is the aggregate of the world's circuit-switched [telephone](https://en.wikipedia.org/wiki/Telephone) networks that are operated by national, regional, or local [telephony](https://en.wikipedia.org/wiki/Telephony) operators. These consist of [telephone lines](https://en.wikipedia.org/wiki/Telephone_line), [fiber optic cables](https://en.wikipedia.org/wiki/Fiber_optic_cable), [microwave transmission](https://en.wikipedia.org/wiki/Microwave_transmission) links, [cellular networks](https://en.wikipedia.org/wiki/Routing_in_cellular_networks), [communications satellites](https://en.wikipedia.org/wiki/Communications_satellite), and [undersea telephone cables](https://en.wikipedia.org/wiki/Undersea_cable), all interconnected by [switching centers](https://en.wikipedia.org/wiki/Switching_center) which allow most telephones to communicate with each other.

Originally a network of fixed-line [analog](https://en.wikipedia.org/wiki/Analog_signal_processing) telephone systems, the PSTN is now almost entirely digital in its [core network](https://en.wikipedia.org/wiki/Core_network) and includes [mobile](https://en.wikipedia.org/wiki/Mobile_phone)[\[1\]](https://en.wikipedia.org/wiki/Public_switched_telephone_network#cite_note-huffpost-1) and other networks, as well as fixed telephones.

 **Circuit switching** is a method of implementing a [telecommunications network](https://en.wikipedia.org/wiki/Telecommunications_network) in which two [network nodes](https://en.wikipedia.org/wiki/Network_nodes) establish a dedicated [communications channel](https://en.wikipedia.org/wiki/Communications_channel) ([circuit](https://en.wikipedia.org/wiki/Telecommunication_circuit)) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the [communication session](https://en.wikipedia.org/wiki/Communication_session). The circuit functions as if the nodes were physically connected as with an [electrical circuit](https://en.wikipedia.org/wiki/Electrical_circuit). Circuit switching originated in analog [telephone networks](https://en.wikipedia.org/wiki/Telephone_network) where the network created a dedicated circuit between two telephones for the duration of a [telephone call](https://en.wikipedia.org/wiki/Telephone_call).[\[1\]](https://en.wikipedia.org/wiki/Circuit_switching#cite_note-1) It contrasts with [message switching](https://en.wikipedia.org/wiki/Message_switching) and [packet switching](https://en.wikipedia.org/wiki/Packet_switching) used in modern digital networks in which the [trunklines](https://en.wikipedia.org/wiki/Trunkline) between switching centers carry data between many different nodes in the form of [data packets](https://en.wikipedia.org/wiki/Data_packet) without dedicated circuits.

 In [telecommunications](https://en.wikipedia.org/wiki/Telecommunication), **packet switching** is a method of grouping [data](https://en.wikipedia.org/wiki/Data_\(computing\)) that is transmitted over a digital [network](https://en.wikipedia.org/wiki/Telecommunications_network) into [packets](https://en.wikipedia.org/wiki/Network_packet). Packets are made of a [header](https://en.wikipedia.org/wiki/Header_\(computing\)) and a [payload](https://en.wikipedia.org/wiki/Payload_\(computing\)). Data in the header is used by networking hardware to direct the packet to its destination, where the payload is extracted and used by [application software](https://en.wikipedia.org/wiki/Application_software). Packet switching is the primary basis for data communications in [computer networks](https://en.wikipedia.org/wiki/Computer_networks) worldwide.

**Message switching** is a network switching technique in which data is routed in its entirety from the source node to the destination node, one hope at a time. During message routing, every intermediate switch in the network stores the whole message. If the entire network's resources are engaged or the network becomes blocked, the message-switched network stores and delays the message until ample resources become available for effective transmission of the message.

**DSL** stands for **Digital Subscriber Line**. Users get a high speed bandwidth connection from a phone wall jack on an existing telephone network. DSL works within the frequencies that the telephone doesn’t so you can use the Internet while making phone calls.

 **Asymmetric digital subscriber line** (**ADSL**) is a type of [digital subscriber line](https://en.wikipedia.org/wiki/Digital_subscriber_line) (DSL) technology, a data communications technology that enables faster data transmission over [copper](https://en.wikipedia.org/wiki/Copper) [telephone lines](https://en.wikipedia.org/wiki/Telephone_line) than a conventional [voiceband](https://en.wikipedia.org/wiki/Voiceband) [modem](https://en.wikipedia.org/wiki/Modem) can provide. ADSL differs from the less common [symmetric digital subscriber line](https://en.wikipedia.org/wiki/Symmetric_digital_subscriber_line) (SDSL). In ADSL, [bandwidth](https://en.wikipedia.org/wiki/Bandwidth_\(computing\)) and [bit rate](https://en.wikipedia.org/wiki/Bit_rate) are said to be asymmetric, meaning greater toward the customer premises ([downstream](https://en.wikipedia.org/wiki/Downstream_\(networking\))) than the reverse ([upstream](https://en.wikipedia.org/wiki/Upstream_\(networking\))). Providers usually market ADSL as an [Internet access](https://en.wikipedia.org/wiki/Internet_access) service primarily for [downloading](https://en.wikipedia.org/wiki/Download) content from the Internet, but not for serving content accessed by others.

 The term **Base station** is used in the context of [mobile telephony](https://en.wikipedia.org/wiki/Mobile_telephony), [wireless computer networking](https://en.wikipedia.org/wiki/Wireless_computer_networking) and other [wireless communications](https://en.wikipedia.org/wiki/Wireless_communications) and in [land surveying](https://en.wikipedia.org/wiki/Land_surveying). In surveying, it is a [GPS receiver](https://en.wikipedia.org/wiki/GPS_receiver) at a known position, while in wireless communications it is a [transceiver](https://en.wikipedia.org/wiki/Transceiver) connecting a number of other devices to one another and/or to a wider area. In mobile telephony, it provides the connection between [mobile phones](https://en.wikipedia.org/wiki/Mobile_phone) and the wider [telephone network](https://en.wikipedia.org/wiki/Telephone_network). In a [computer network](https://en.wikipedia.org/wiki/Computer_network), it is a transceiver acting as a [switch](https://en.wikipedia.org/wiki/Switch_\(computing\)) for [computers](https://en.wikipedia.org/wiki/Computer) in the network, possibly connecting them to a/another [local area network](https://en.wikipedia.org/wiki/Local_area_network) and/or the [Internet](https://en.wikipedia.org/wiki/Internet). In traditional wireless communications, it can refer to the hub of a [dispatch](https://en.wikipedia.org/wiki/Dispatch_\(logistics\)) fleet such as a [taxi](https://en.wikipedia.org/wiki/Taxi) or [delivery](https://en.wikipedia.org/wiki/Delivery_\(commerce\)) fleet, the base of a [TETRA](https://en.wikipedia.org/wiki/TETRA) network as used by [government](https://en.wikipedia.org/wiki/Government) and [emergency services](https://en.wikipedia.org/wiki/Emergency_services) or a [CB](https://en.wikipedia.org/wiki/Citizen%E2%80%99s_band_radio) [shack](https://en.wikipedia.org/wiki/Radio_shack).