Data Communication and Networking

Data Communication and Networking

Data transmission

Data transmission, digital transmission, or digital communications is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, storage media and computer buses. The data are represented as an electromagnetic signal, such as an electrical voltage, radiowave, microwave, or infrared signal.

While analog transmission is the transfer of a continuously varying analog signal, digital communications is the transfer of discrete messages. The messages are either represented by a sequence of pulses by means of a line code (baseband transmission), or by a limited set of continuously varying wave forms (passband transmission), using a digital modulation method. The passband modulation and corresponding demodulation (also known as detection) is carried out by modem equipment. According to the most common definition of digital signal, both baseband and passband signals representing bit-streams are considered as digital transmission, while an alternative definition only considers the baseband signal as digital, and passband transmission of digital data as a form of digital-to-analog conversion.

Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard. It may also be an analog signal such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation (PCM) or more advanced source coding (analog-to-digital conversion and data compression) schemes. This source coding and decoding is carried out by codec equipment.

Distinction between related subjectsDigital transmission or data transmission traditionally belongs to telecommunications and electrical engineering. Basic principles of data transmission may also be covered within the computer science/computer engineering topic of data communications, which also includes computer networking or computer communication applications and networking protocols, for example routing, switching and inter-process communication. Although the Transmission control protocol (TCP) involves the term "transmission", TCP and other transport layer protocols are typically not discussed in a textbook or course about data transmission, but in computer networking.

The term tele transmission involves the analog as well as digital communication. In most textbooks, the term analog transmission only refers to the transmission of an analog message signal (without digitization) by means of an analog signal, either as a non-modulated baseband signal, or as a passband signal using an analog modulation method such as AM or FM. It may also include analog-over-analog pulse modulatated baseband signals such as pulse-width modulation. In a few books within the computer networking tradition, "analog transmission" also refers to passband transmission of bit-streams using digital modulation methods such as FSK, PSK and ASK. Note that these methods are covered in textbooks named digital transmission or data transmission, for example.[1]The theoretical aspects of data transmission are covered by information theory and coding theory.

Applications and historyData (mainly but not exclusively informational) has been sent via non-electronic (e.g. optical, acoustic, mechanical) means since the advent of communication. Analog signal data has been sent electronically since the advent of the telephone. However, the first data electromagnetic transmission applications in modern time were telegraphy (1809) and teletypewriters (1906), which are both digital signals. The fundamental theoretical work in data transmission and information theory by Harry Nyquist, Ralph Hartley, Claude Shannon and others during the early 20th century, was done with these applications in mind.

Data transmission is utilized in computers in computer buses and for communication with peripheral equipment via parallel ports and serial ports such as RS-232 (1969), Firewire (1995) and USB (1996). The principles of data transmission are also utilized in storage media for Error detection and correction since 1951.

Data transmission is utilized in computer networking equipment such as modems (1940), local area networks (LAN) adapters (1964), repeaters, hubs, microwave links, wireless network access points (1997), etc.

In telephone networks, digital communication is utilized for transferring many phone calls over the same copper cable or fiber cable by means of Pulse code modulation (PCM), i.e. sampling and digitization, in combination with Time division multiplexing (TDM) (1962). Telephone exchanges have become digital and software controlled, facilitating many value added services. For example the first AXE telephone exchange was presented in 1976. Since the late 1980s, digital communication to the end user has been possible using Integrated Services Digital Network (ISDN) services. Since the end of the 1990s, broadband access techniques such as ADSL, Cable modems, fiber-to-the-building (FTTB) and fiber-to-the-home (FTTH) have become wide spread to small offices and homes. The current tendency is to replace traditional telecommunication services by packet mode communication such as IP telephony and IPTV.

Transmitting analog signals digitally allows for greater signal processing capability. The ability to process a communications signal means that errors caused by random processes can be detected and corrected. Digital signals can also be sampled instead of continuously monitored. The multiplexing of multiple digital signals is much simpler to the multiplexing of analog signals.

Because of all these advantages, and because recent advances in wideband communication channels and solid-state electronics have allowed scientists to fully realize these advantages, digital communications has grown quickly. Digital communications is quickly edging out analog communication because of the vast demand to transmit computer data and the ability of digital communications to do so.

The digital revolution has also resulted in many digital telecommunication applications where the principles of data transmission are applied. Examples are second-generation (1991) and later cellular telephony, video conferencing, digital TV (1998), digital radio (1999), telemetry, etc.

Baseband or passband transmissionThe physically transmitted signal may be one of the following:

1. A baseband signal ("digital-over-digital" transmission): A sequence of electrical pulses or light pulses produced by means of a line coding scheme such as Manchester coding. This is typically used in serial cables, wired local area networks such as Ethernet, and in optical fiber communication. It results in a pulse amplitude modulated(PAM) signal, also known as a pulse train.

2. A passband signal ("digital-over-analog" transmission): A modulated sine wave signal representing a digital bit-stream. Note that this is in some textbooks considered as analog transmission, but in most books as digital transmission. The signal is produced by means of a digital modulation method such as PSK, QAM or FSK. The modulation and demodulation is carried out by modem equipment. This is used in wireless communication, and over telephone network local-loop and cable-TV networks.

Serial and parallel transmissionIn telecommunications, serial transmission is the sequential transmission of signal elements of a group representing a character or other entity of data. Digital serial transmissions are bits sent over a single wire, frequency or optical path sequentially. Because it requires less signal processing and less chances for error than parallel transmission, the transfer rate of each individual path may be faster. This can be used over longer distances as a check digit or parity bit can be sent along it easily.

In telecommunications, parallel transmission is the simultaneous transmission of the signal elements of a character or other entity of data. In digital communications, parallel transmission is the simultaneous transmission of related signal elements over two or more separate paths. Multiple electrical wires are used which can transmit multiple bits simultaneously, which allows for higher data transfer rates than can be achieved with serial transmission. This method is used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, The major issue with this is "skewing" because the wires in parallel data transmission have slightly different properties (not intentionally) so some bits may arrive before others, which may corrupt the message. A parity bit can help to reduce this. However, electrical wire parallel data transmission is therefore less reliable for long distances because corrupt transmissions are far more likely.

Types of communication channelsData transmission circuit

In telecommunication, data transmission circuit is the transmission media and the intervening equipment used for the data transfer between data terminal equipments (DTEs).Simplex communication

Simplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition. The ITU definition of simplex is termed "half duplex" in other contexts.

Duplex

A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions, simultaneously. An example of a duplex device is a telephone. The people at both ends of a telephone call can speak at the same time and simultaneously each be heard by the other at the same time. The earphone reproduces the speech of the other person as the microphone transmits the speech of the local person, because there is a two-way communication channel between them.

Half-duplexA half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying (antennas are of trans-receiver type in these devices, so as to transmit and receive the signal as well).

An example of a half-duplex system is a two-party system such as a walkie-talkie, wherein one must use "Over" or another previously designated command to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit and receive on the same frequency.

A good analogy for a half-duplex system would be a one-lane road with traffic controllers at each end, such as a two-lane bridge under re-construction. Traffic can flow in both directions, but only one direction at a time, regulated by the traffic controllers.

In automatically run communications systems, such as two-way data-links, the time allocations for communications in a half-duplex system can be firmly controlled by the hardware. Thus, there is no waste of the channel for switching. For example, station A on one end of the data link could be allowed to transmit for exactly one second, then station B on the other end could be allowed to transmit for exactly one second, and then the cycle repeats.

Full-duplexA full-duplex (FDX) system, or sometimes called double-duplex, allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time, with the transition from four to two wires being achieved by a hybrid coil in a telephone hybrid.

A good analogy for a full-duplex system would be a two-lane road with one lane for each direction. In full-duplex mode, transmitted data does not appear to be sentuntil it has been actually received and an acknowledgment was sent back by the other party.

Two-way radios can be designed as full-duplex systems, transmitting on one frequency and receiving on another. This is also called frequency-division duplex. Frequency-division duplex systems can be extended to farther distances using pairs of simple repeater stations, because the communications transmitted on any one frequency always travel in the same direction.

Full-duplex Ethernet connections work by making simultaneous use of two physical pairs of twisted cable (which are inside the jacket), where one pair is used for receiving packets and one pair is used for sending packets (two pairs per direction for some types of Ethernet), to a directly connected device. This effectively makes the cable itself a collision-free environment and doubles the maximum data capacity that can be supported by the connection.

There are several benefits to using full-duplex over half-duplex. Firstly, time is not wasted, since no frames need to be retransmitted, as there are no collisions. Secondly, the full data capacity is available in both directions because the send and receive functions are separated. Thirdly, stations (or nodes) do not have to wait until others complete their transmission, since there is only one transmitter for each twisted pair.

Historically, some computer-based systems of the 1960s and 1970s required full-duplex facilities even for half-duplex operation, because their poll-and-response schemes could not tolerate the slight delays in reversing the direction of transmission in a half-duplex line.

Full-duplex emulationWhere channel access methods are used in point-to-multipoint networks (such as cellular networks) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods, such as time-division duplexing and frequency-division duplexing.Time-division duplexingTime-division duplexing (TDD) is the application of time-division multiplexing to separate outward and return signals. It emulates full duplex communication over a half duplex communication link.

Time-division duplexing has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. As the amount of uplink data increases, more communication capacity can be dynamically allocated, and as the traffic load becomes lighter, capacity can be taken away. The same applies in the downlink direction.

For radio systems that aren't moving quickly, another advantage is that the uplink and downlink radio paths are likely to be very similar. This means that techniques such as beamforming work well with TDD systems.

Examples of time-division duplexing systems are:

UMTS 3G supplementary air interfaces TD-CDMA for indoor mobile telecommunications.

The Chinese TD-LTE 4-G, TD-SCDMA 3-G mobile communications air interface.

DECT wireless telephony

Half-duplex packet mode networks based on carrier sense multiple access, for example 2-wire or hubbed Ethernet, Wireless local area networks and Bluetooth, can be considered as Time Division Duplexing systems, albeit not TDMA with fixed frame-lengths.

IEEE 802.16 WiMAX PACTOR ISDN BRI U interface, variants using the Time Compression Multiplex (TCM) line system

G.fast, a digital subscriber line (DSL) standard under development by the ITU-TFrequency-division duplexingFrequency-division duplexing (FDD) means that the transmitter and receiver operate at different carrier frequencies. The term is frequently used in ham radio operation, where an operator is attempting to contact a repeater station. The station must be able to send and receive a transmission at the same time, and does so by slightly altering the frequency at which it sends and receives. This mode of operation is referred to as duplex mode or offset mode.

Uplink and downlink sub-bands are said to be separated by the frequency offset. Frequency-division duplexing can be efficient in the case of symmetric traffic. In this case time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry.

Another advantage of frequency-division duplexing is that it makes radio planning easier and more efficient, since base stations do not "hear" each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere with each other. On the converse, with time-division duplexing systems, care must be taken to keep guard times between neighboring base stations (which decreases spectral efficiency) or to synchronize base stations, so that they will transmit and receive at the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio)

Examples of Frequency Division Duplexing systems are:

ADSL and VDSL Most cellular systems, including the UMTS/WCDMA use Frequency Division Duplexing mode and the cdma2000 system.

IEEE 802.16 WiMax also uses Frequency Division Duplexing mode

Summary Simplex - Communication in one direction only, e.g. TV or radio broadcasts.

Half-duplex - Communication in both directions, one direction at a time, e.g. Two-way radio.

Full-duplex - Communication in both directions simultaneously, e.g. telephone calls.

Point-to-point (telecommunications)

In telecommunications, a point-to-point connection refers to a communications connection between two nodes or endpoints. An example is a telephone call, in which one telephone is connected with one other, and what is said by one caller can only be heard by the other. This is contrasted with a point-to-multipoint or broadcast communication topology, in which many nodes can receive information transmitted by one node. Other examples of point-to-point communications links are leased lines, microwave relay links, and two way radio. Examples of point-to-multipoint communications systems are radio and television broadcasting.

The term is also used in computer networking and computer architecture to refer to a wire or other connection that links only two computers or circuits, as opposed to other network topologies such as buses or crossbar switches which can connect many communications devices.

Point-to-point is sometimes abbreviated as P2P, Pt2Pt.[citation needed] This usage of P2P is distinct from P2P referring to peer-to-peer file sharing networks.

Multidrop bus

A multidrop bus (MDB) is a computer bus in which all components are connected to the electrical circuit. A process of arbitration determines which device sends information at any point. The other devices listen for the data they are intended to receive.

Multidrop buses have the advantage of simplicity and extensibility. However, modern SDRAM chips exemplify the problem of electrical impedance discontinuity.[clarification needed] Fully Buffered DIMM is an alternative approach to connecting multiple DRAM modules to a memory controller. Since 2000, multidrop standards such as PCI and Parallel ATA are increasingly being replaced by point-to-point systems such as PCI Express and SATA.

Bus network

A bus network is a network topology in which nodes are connected in a daisy chain by a linear sequence of buses.

How it worksThe bus is the data link in a bus network. The bus can only transmit data in one direction, and if any network segment is severed, all network transmission ceases.

A host on a bus network is called a station or workstation. In a bus network, every station receives all network traffic, and the traffic generated by each station has equal transmission priority.[1] Each network segment is, therefore, a collision domain. In order for nodes to transmit on the same cable simultaneously, they use a media access control technology such as carrier sense multiple access (CSMA) or a bus master.

Advantages and disadvantagesAdvantages

Easy to connect a computer or peripheral to a linear bus

Requires less cable length than a star topology

It works well for small networks.

Disadvantages

Entire network shuts down if there is a break in the main cable

Terminators are required at both ends of the backbone cable

Difficult to identify the problem if the entire network shuts down

Not meant to be used as a stand-alone solution in a large building

It is slow when more devices are added into the network.

BUS NETWORK

Ring network

A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travel from node to node, with each node along the way handling every packet.

Because a ring topology provides only one pathway between any two nodes, ring networks may be disrupted by the failure of a single link.[1] A node failure or cable break might isolate every node attached to the ring. In response, some ring networks add a "counter-rotating ring" (C-Ring) to form a redundant topology: in the event of a break, data are wrapped back onto the complementary ring before reaching the end of the cable, maintaining a path to every node along the resulting C-Ring. Such "dual ring" networks include Spatial Reuse Protocol, Fiber Distributed Data Interface (FDDI), and Resilient Packet Ring. 802.5 networks - also known as IBM token ring networks - avoid the weakness of a ring topology altogether: they actually use a star topology at the physical layer and a media access unit (MAU) to imitate a ring at the datalink layer.

Advantages Very orderly network where every device has access to the token and the opportunity to transmit

Performs better than a bus topology under heavy network load

Does not require a central node to manage the connectivity between the computers

Due to the point to point line configuration of devices with a device on either side (each device is connected to its immediate neighbor), it is quite easy to install and reconfigure since adding or removing a device requires moving just two connections.

Point to point line configuration makes it easy to identify and isolate faults.

Disadvantages One malfunctioning workstation can create problems for the entire network. This can be solved by using a dual ring or a switch that closes off the break.

Moving, adding and changing the devices can affect the network

Communication delay is directly proportional to number of nodes in the network

Bandwidth is shared on all links between devices

More difficult to configure than a Star: node adjunction = Ring shutdown and reconfiguration

Ring Network Topology

Star network

Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which act as a conduit to transmit messages. This consists of a central node, to which all other nodes are connected; this central node provides a common connection point for all nodes through a hub. In star topology, every node (computer workstation or any other peripheral) is connected to a central node called a hub or switch. The switch is the server and the peripherals are the clients.[1] Thus, the hub and leaf nodes, and the transmission lines between them, form a graph with the topology of a star. If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

The star topology reduces the damage caused by line failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.[2]It is also designed with each node (file servers, workstations, and peripherals) connected directly to a central network hub, switch, or concentrator.

Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable. However, it can also be used with coaxial cable or optical fibre cable.

Advantages Better performance: star topology prevents the passing of data packets through an excessive number of nodes. At most, 3 devices and 2 links are involved in any communication between any two devices. Although this topology places a huge overhead on the central hub, with adequate capacity, the hub very high utilization by one device without affecting others.

Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of individual devices straightforward and amounts to disconnecting each device from the others. This isolation also prevents any non-centralized failure from affecting the network.

Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Centralization also allows the inspection of traffic through the network. This facilitates analysis of the traffic and detection of suspicious behavior.

Easy to detect faults and to remove parts.

No disruptions to the network when connecting or removing devices.

Installation and configuration is easy since every one device only requires a link and one input/output port to connect it to any other device(s).

Disadvantages Failure of the central hub renders the network inoperable

There is central server dependency.

Expensive to purchase.

Requires a large amount of cable to be connected.

Star Network Topology

Mesh networking

A mesh network is a network topology in which each node (called a mesh node) relays data for the network. All nodes cooperate in the distribution of data in the network.

A mesh network can be designed using a flooding technique or a routing technique. When using a routing technique, the message is propagated along a path, by hopping from node to node until the destination is reached. To ensure all its paths' availability, a routing network must allow for continuous connections and reconfiguration around broken or blocked paths, using self-healing algorithms. A mesh network whose nodes are all connected to each other is a fully connected network. Mesh networks can be seen as one type of ad hoc network. Mobile ad hoc networks (MANET) and mesh networks are therefore closely related, but MANET also have to deal with the problems introduced by the mobility of the nodes.

The self-healing capability enables a routing based network to operate when one node breaks down or a connection goes bad. As a result, the network is typically quite reliable, as there is often more than one path between a source and a destination in the network. Although mostly used in wireless situations, this concept is also applicable to wired networks and software interaction.

Advantages Point-to-point line configuration makes identification and isolation of faults easy.

Messages travel through a dedicated line, directly to the intended recipient; privacy and security are thus enhanced.

Should a fault occur in a given link, only those communications between that specific pair of devices sharing the link will be affected.

Disadvantages The more extensive the network, in terms of scope or of physical area, the greater the investment necessary to build it will be, due, among other considerations, to the amount of cabling and the number of hardware ports it will require. For this reason, such networks are uncommon.

Because every device must be connected to every other device, installation and reconnection are difficult.

The huge bulk of the wiring can often be greater than the available space in the ceiling or under floors can accommodate.

Wireless mesh networksWireless mesh networks were originally developed for military applications. Mesh networks are typically wireless. Over the past decade, the size, cost, and power requirements of radios has declined, enabling multiple radios to be contained within a single device, i.e., mesh node, thus allowing for greater modularity; each can handle multiple frequency bands and support a variety of functions as neededsuch as client access, backhaul service, and scanning (required for high-speed handoff in mobile applications)even customized sets of them.

Work in this field has been aided by the use of game theory methods to analyze strategies for the allocation of resources and routing of packets.

Mesh Topology

Wireless network

A wireless network is any type of computer network that uses wireless data connections for connecting network nodes.

Wireless networking is a method by which homes, telecommunications networks and enterprise (business) installations avoid the costly process of introducing cables into a building, or as a connection between various equipment locations.[1] Wireless telecommunications networks are generally implemented and administered using radio communication. This implementation takes place at the physical level (layer) of the OSI model network structure.[2]Examples of wireless networks include cell phone networks, Wi-Fi local networks and terrestrial microwave networks.

Wireless links Terrestrial microwave Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low-gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately 48km (30mi) apart.

Communications satellites Satellites communicate via microwave radio waves, which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically in geosynchronous orbit 35,400km (22,000mi) above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.

Cellular and PCS systems use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.

Radio and spread spectrum technologies Wireless local area networks use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. IEEE 802.11 defines a common flavor of open-standards wireless radio-wave technology known as Wifi.

Free-space optical communication uses visible or invisible light for communications. In most cases, line-of-sight propagation is used, which limits the physical positioning of communicating devices.

Types of wireless networksWireless PANWireless personal area networks (WPANs) interconnect devices within a relatively small area, that is generally within a person's reach. For example, both Bluetooth radio and invisible infrared light provides a WPAN for interconnecting a headset to a laptop. ZigBee also supports WPAN applications.[4] Wi-Fi PANs are becoming commonplace (2010) as equipment designers start to integrate Wi-Fi into a variety of consumer electronic devices. Intel "My WiFi" and Windows 7 "virtual Wi-Fi" capabilities have made Wi-Fi PANs simpler and easier to set up and configure.Wireless LANA wireless local area network (WLAN) links two or more devices over a short distance using a wireless distribution method, usually providing a connection through an access point for Internet access. The use of spread-spectrum or OFDM technologies may allow users to move around within a local coverage area, and still remain connected to the network.

Products using the IEEE 802.11 WLAN standards are marketed under the Wi-Fi brand name. Fixed wireless technology implements point-to-point links between computers or networks at two distant locations, often using dedicated microwave or modulated laser light beams over line of sight paths. It is often used in cities to connect networks in two or more buildings without installing a wired link.

Wireless mesh networkA wireless mesh network is a wireless network made up of radio nodes organized in a mesh topology. Each node forwards messages on behalf of the other nodes. Mesh networks can "self heal", automatically re-routing around a node that has lost power.

Wireless MANWireless metropolitan area networks are a type of wireless network that connects several wireless LANs.

WiMAX is a type of Wireless MAN and is described by the IEEE 802.16 standard.[Wireless WANWireless wide area networks are wireless networks that typically cover large areas, such as between neighboring towns and cities, or city and suburb. These networks can be used to connect branch offices of business or as a public internet access system. The wireless connections between access points are usually point to point microwave links using parabolic dishes on the 2.4GHz band, rather than omnidirectional antennas used with smaller networks. A typical system contains base station gateways, access points and wireless bridging relays. Other configurations are mesh systems where each access point acts as a relay also. When combined with renewable energy systems such as photo-voltaic solar panels or wind systems they can be stand alone systems.

Cellular networkA cellular network or mobile network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver, known as a cell site or base station. In a cellular network, each cell characteristically uses a different set of radio frequencies from all their immediate neighbouring cells to avoid any interference.

When joined together these cells provide radio coverage over a wide geographic area. This enables a large number of portable transceivers (e.g., mobile phones, pagers, etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission.

Although originally intended for cell phones, with the development of smartphones, cellular telephone networks routinely carry data in addition to telephone conversations:

Global System for Mobile Communications (GSM): The GSM network is divided into three major systems: the switching system, the base station system, and the operation and support system. The cell phone connects to the base system station which then connects to the operation and support station; it then connects to the switching station where the call is transferred to where it needs to go. GSM is the most common standard and is used for a majority of cell phones.[7] Personal Communications Service (PCS): PCS is a radio band that can be used by mobile phones in North America and South Asia. Sprint happened to be the first service to set up a PCS.

D-AMPS: Digital Advanced Mobile Phone Service, an upgraded version of AMPS, is being phased out due to advancement in technology. The newer GSM networks are replacing the older system.

Global area networkA global area network (GAN) is a network used for supporting mobile across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial wireless LANs.

Space networkSpace networks are networks used for communication between spacecraft, usually in the vicinity of the Earth. The example of this is NASA's Space Network.

Different usesSome examples of usage include cellular phones which are part of everyday wireless networks, allowing easy personal communications. Another example, Inter-continental network systems, use radio satellites to communicate across the world. Emergency services such as the police utilize wireless networks to communicate effectively as well. Individuals and businesses use wireless networks to send and share data rapidly, whether it be in a small office building or across the world.

Wireless Network ElementsThe telecommunications network at the physical layer also consists of many interconnected wireline Network Elements (NEs). These NEs can be stand-alone systems or products that are either supplied by a single manufacturer, or are assembled by the service provider (user) or system integrator with parts from several different manufacturers.

Wireless NEs are products and devices used by a wireless carrier to provide support for the backhaul network as well as a Mobile Switching Center (MSC).

Reliable wireless service depends on the network elements at the physical layer to be protected against all operational environments and applications (see GR-3171, Generic Requirements for Network Elements Used in Wireless Networks - Physical Layer Criteria).[12]What are especially important are the NEs that are located on the cell tower to the Base Station (BS) cabinet. The attachment hardware and the positioning of the antenna and associated closures/cables are required to have adequate strength, robustness, corrosion resistance, and rain/solar resistance for expected wind, storm, ice, and other weather conditions. Requirements for individual components, such as hardware, cables, connectors, and closures, shall take into consideration the structure to which they are attached.

DifficultiesInterference

Compared to wired systems, wireless networks are frequently subject to electromagnetic interference. This can be caused by other networks or other types of equipment that generate radio waves that are within, or close, to the radio bands used for communication. Interference can degrade the signal or cause the system to fail.

Absorption and reflection

Some materials cause absorption of electromagnetic waves, preventing it from reaching the receiver, in other cases, particularly with metallic or conductive materials reflection occurs. This can cause dead zones where no reception is available.

Multipath fading

In multipath fading two or more different routes taken by the signal, due to reflections, can cause the signal to cancel out at certain locations, and to be stronger in other places (upfade).

Hidden node problem

The hidden node problem occurs in some types of network when a node is visible from a wireless access point (AP), but not from other nodes communicating with that AP. This leads to difficulties in media access control.

Shared resource problem

The wireless spectrum is a limited resource and shared by all nodes in the range of its transmitters. Bandwidth allocation becomes complex with multiple participating users. Often users are not aware that advertised numbers (e.g., for IEEE 802.11 equipment or LTE networks) are not their capacity, but shared with all other users and thus the individual user rate is far lower. With increasing demand, the capacity crunch is more and more likely to happen. User-in-the-loop (UIL) may be an alternative solution to ever upgrading to newer technologies for over-provisioning.

Asynchronous and synchronous data transmissionAsynchronous transmission uses start and stop bits to signify the beginning bit ASCII character would actually be transmitted using 10 bits. For example, "0100 0001" would become "1 0100 0001 0". The extra one (or zero, depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data are sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.

Synchronous transmission uses no start and stop bits, but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signal(s) built into each component.[vague] A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some bytes could become corrupted (by losing bits).Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the byte is correctly interpreted and received

Prepared by Sir Matt (