126-666: ALOHAnet , also known as the ALOHA System , or simply ALOHA , was a pioneering computer networking system developed at the University of Hawaii . ALOHAnet became operational in June 1971, providing the first public demonstration of a wireless packet data network . The ALOHAnet used a new method of medium access, called ALOHA random access , and experimental ultra high frequency (UHF) for its operation. In its simplest form, later known as Pure ALOHA, remote units communicated with
252-639: A Teletype Model 33 or a dumb CRT user terminal connected to the TCU using a standard RS-232 interface. Shortly after the original ALOHA network went into operation, the TCU was redesigned with one of the first Intel microprocessors, and the resulting upgrade was called a Programmable Control Unit (PCU). Additional basic functions performed by the TCUs and PCUs were generation of a cyclic-parity-check code vector and decoding of received packets for packet error detection purposes, and generation of packet retransmissions using
378-450: A captive portal webpage for access. Organizations, enthusiasts, authorities and businesses , such as airports, hotels, and restaurants, often provide free or paid-use hotspots to attract customers, to provide services to promote business in selected areas. Routers often incorporate a digital subscriber line modem or a cable modem and a Wi-Fi access point, are frequently set up in homes and other buildings, to provide Internet access for
504-793: A listen before send random-access protocol that can be used when all nodes send and receive on the same channel. CSMA in radio channels was extensively modeled. The AX.25 packet radio protocol is based on the CSMA approach with collision recovery, based on the experience gained from ALOHAnet. A variation of CSMA, CSMA/CD is used in early versions of Ethernet . ALOHA and the other random-access protocols have an inherent variability in their throughput and delay performance characteristics. For this reason, applications that need highly deterministic load behavior may use master/slave or token-passing schemes (such as Token Ring or ARCNET ) instead of contention systems . The central node communications processor
630-756: A protocol stack , often constructed per the OSI model, communications functions are divided up into protocol layers, where each layer leverages the services of the layer below it until the lowest layer controls the hardware that sends information across the media. The use of protocol layering is ubiquitous across the field of computer networking. An important example of a protocol stack is HTTP (the World Wide Web protocol) running over TCP over IP (the Internet protocols) over IEEE 802.11 (the Wi-Fi protocol). This stack
756-460: A 32-bit header and a 16-bit header parity check word, followed by up to 80 bytes of data and a 16-bit parity check word for the data. The header contained address information identifying a particular user so that when the Menehune broadcast a packet, only the intended user's node would accept it. In the 1970s ALOHA random access was employed in the nascent Ethernet cable based network and then in
882-497: A base station (Menehune) over two separate radio frequencies (for inbound and outbound respectively). Nodes did not wait for the channel to be clear before sending, but instead waited for acknowledgement of successful receipt of a message, and re-sent it if this was not received. Nodes would also stop and re-transmit data if they detected any other messages while transmitting. While simple to implement, this results in an efficiency of only 18.4%. A later advancement, Slotted ALOHA, improved
1008-492: A branch of computer science , computer engineering , and telecommunications , since it relies on the theoretical and practical application of the related disciplines. Computer networking was influenced by a wide array of technological developments and historical milestones. Computer networks enhance how users communicate with each other by using various electronic methods like email, instant messaging, online chat, voice and video calls, and video conferencing. Networks also enable
1134-456: A channel at a time. Wi-Fi's radio bands work best for line-of-sight use. Many common obstructions, such as walls, pillars, home appliances, etc., may greatly reduce range, but this also helps minimize interference between different networks in crowded environments. The range of an access point is about 20 m (66 ft) indoors, while some access points claim up to a 150 m (490 ft) range outdoors. Hotspot coverage can be as small as
1260-447: A city-wide campaign to convert old phone booths into digital kiosks in 2014. The project, titled LinkNYC , has created a network of kiosks that serve as public Wi-Fi hotspots, high-definition screens and landlines . Installation of the screens began in late 2015. The city government plans to implement more than seven thousand kiosks over time, eventually making LinkNYC the largest and fastest public, government-operated Wi-Fi network in
1386-460: A common Wi-Fi version. The versions differ between the radio wavebands they operate on, the radio bandwidth they occupy, the maximum data rates they can support and other details. Some versions permit the use of multiple antennas, which permits greater speeds as well as reduced interference. Historically, the equipment listed the versions of Wi-Fi supported using the name of the IEEE standards. In 2018,
SECTION 10
#17327904218451512-407: A computer and an interface controller is called a station . Stations are identified by one or more MAC addresses . Wi-Fi nodes often operate in infrastructure mode in which all communications go through a base station. Ad hoc mode refers to devices communicating directly with each other, without communicating with an access point. A service set is the set of all the devices associated with
1638-401: A computer network can include personal computers , servers , networking hardware , or other specialized or general-purpose hosts . They are identified by network addresses and may have hostnames . Hostnames serve as memorable labels for the nodes and are rarely changed after initial assignment. Network addresses serve for locating and identifying the nodes by communication protocols such as
1764-503: A diverse set of networking capabilities. The protocols have a flat addressing scheme. They operate mostly at layers 1 and 2 of the OSI model. For example, MAC bridging ( IEEE 802.1D ) deals with the routing of Ethernet packets using a Spanning Tree Protocol . IEEE 802.1Q describes VLANs , and IEEE 802.1X defines a port-based network access control protocol, which forms the basis for the authentication mechanisms used in VLANs (but it
1890-641: A greater degree by common building materials than the 2.4 GHz bands and usually give a shorter range. As 802.11 specifications evolved to support higher throughput, the protocols have become much more efficient in their bandwidth use. Additionally, they have gained the ability to aggregate channels together to gain still more throughput where the bandwidth for additional channels is available. 802.11n allows for double radio spectrum bandwidth (40 MHz) per channel compared to 802.11a or 802.11g (20 MHz). 802.11n can be set to limit itself to 20 MHz bandwidth to prevent interference in dense communities. In
2016-405: A hub configuration, with the hub machine broadcasting packets to everyone on the outbound channel, and the various client machines sending data packets to the hub on the inbound channel. If data was received correctly at the hub, a short acknowledgment packet was sent to the client; if an acknowledgment was not received by a client machine after a short wait time, it would automatically retransmit
2142-440: A large, congested network into an aggregation of smaller, more efficient networks. A router is an internetworking device that forwards packets between networks by processing the addressing or routing information included in the packet. The routing information is often processed in conjunction with the routing table . A router uses its routing table to determine where to forward packets and does not require broadcasting packets which
2268-440: A multi-port bridge. Switches normally have numerous ports, facilitating a star topology for devices, and for cascading additional switches. Bridges and switches operate at the data link layer (layer 2) of the OSI model and bridge traffic between two or more network segments to form a single local network. Both are devices that forward frames of data between ports based on the destination MAC address in each frame. They learn
2394-432: A network, but the topology of interconnections of a network can significantly affect its throughput and reliability. With many technologies, such as bus or star networks, a single failure can cause the network to fail entirely. In general, the more interconnections there are, the more robust the network is; but the more expensive it is to install. Therefore, most network diagrams are arranged by their network topology which
2520-426: A particular Wi-Fi network. Devices in a service set need not be on the same wavebands or channels. A service set can be local, independent, extended, mesh, or a combination. Each service set has an associated identifier, a 32-byte service set identifier (SSID), which identifies the network. The SSID is configured within the devices that are part of the network. A basic service set (BSS) is a group of stations that share
2646-457: A repeater hub assists with collision detection and fault isolation for the network. Hubs and repeaters in LANs have been largely obsoleted by modern network switches. Network bridges and network switches are distinct from a hub in that they only forward frames to the ports involved in the communication whereas a hub forwards to all ports. Bridges only have two ports but a switch can be thought of as
SECTION 20
#17327904218452772-477: A second shared frequency and using an address in each packet to allow selective receipt at each client node. Separate frequencies were used for incoming and outgoing communications to the hub so that devices could receive acknowledgments regardless of transmissions. The original version of the protocol (now called Pure ALOHA, and the one implemented in ALOHAnet) was quite simple: Pure ALOHA does not check whether
2898-418: A simple random interval generator. If an acknowledgment was not received from the Menehune after the prescribed number of automatic retransmissions, a flashing light was used as an indicator to the human user. Also, since the TCUs and PCUs did not send acknowledgments to the Menehune, a steady warning light was displayed to the human user when an error was detected in a received packet. Considerable simplification
3024-410: A single carrier, whereas 802.11a, Wi-Fi 4, 5 and 6 use orthogonal frequency-division multiplexing . Channels are used half duplex and can be time-shared by multiple networks. Any packet sent by one computer is locally received by stations tuned to that channel, even if that information is intended for just one destination. Stations typically ignore information not addressed to them. The use of
3150-458: A single room with walls that block radio waves or as large as many square kilometers using many overlapping access points with roaming permitted between them. Over time, the speed and spectral efficiency of Wi-Fi have increased. As of 2019, some versions of Wi-Fi, running on suitable hardware at close range, can achieve speeds of 9.6 Gbit/s ( gigabit per second). A 1985 ruling by the U.S. Federal Communications Commission released parts of
3276-403: A standard voice telephone line. Modems are still commonly used for telephone lines, using a digital subscriber line technology and cable television systems using DOCSIS technology. A firewall is a network device or software for controlling network security and access rules. Firewalls are inserted in connections between secure internal networks and potentially insecure external networks such as
3402-877: A transmission medium. Power line communication uses a building's power cabling to transmit data. The following classes of wired technologies are used in computer networking. Network connections can be established wirelessly using radio or other electromagnetic means of communication. The last two cases have a large round-trip delay time , which gives slow two-way communication but does not prevent sending large amounts of information (they can have high throughput). Apart from any physical transmission media, networks are built from additional basic system building blocks, such as network interface controllers , repeaters , hubs , bridges , switches , routers , modems, and firewalls . Any particular piece of equipment will frequently contain multiple building blocks and so may perform multiple functions. A network interface controller (NIC)
3528-520: A transmission only at the beginning of a time slot, and thus collisions are reduced. In this case, only transmission-attempts within 1 frame-time and not 2 consecutive frame-times need to be considered, since collisions can only occur during each time slot. Thus, the probability of there being zero transmission attempts by other stations in a single time slot is: P r o b s l o t t e d = e − G {\displaystyle Prob_{slotted}=e^{-G}}
3654-560: A unit of time equal to T . Let G refer to the mean used in the Poisson distribution over transmission-attempt amounts. That is, on average, there are G transmission attempts per frame-time . Consider what needs to happen for a frame to be transmitted successfully. Let t refer to the time at which it is intended to send a frame. It is preferable to use the channel for one frame-time beginning at t , and all other stations to refrain from transmitting during this time. For any frame-time,
3780-426: A user who has data to send, using the acknowledgment and retransmission mechanism described earlier to deal with occasional access collisions. While the average channel loading must be kept below about 10% to maintain a low collision rate, this still results in better bandwidth efficiency than when fixed allocations are used in a bursty traffic context. Two 100 kHz channels in the experimental UHF band were used in
3906-495: A variety of different sources, primarily to support circuit-switched digital telephony . However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames. Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing and encodes data into small, fixed-sized cells . This differs from other protocols such as
ALOHAnet - Misplaced Pages Continue
4032-742: A version of the Reservation ALOHA scheme first analyzed by a group at BBN Technologies . One of the early computer networking designs, development of the ALOHA network was begun in September 1968 at the University of Hawaii under the leadership of Norman Abramson and Franklin Kuo , along with Thomas Gaarder, Shu Lin, Wesley Peterson and Edward ("Ned") Weldon. The goal was to use low-cost commercial radio equipment to connect users on Oahu and
4158-657: A virtual system of links that run on top of the Internet . Overlay networks have been used since the early days of networking, back when computers were connected via telephone lines using modems, even before data networks were developed. The most striking example of an overlay network is the Internet itself. The Internet itself was initially built as an overlay on the telephone network . Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies. Address resolution and routing are
4284-678: A wider channel for higher throughput. Countries apply their own regulations to the allowable channels, allowed users and maximum power levels within these frequency ranges. 802.11b/g/n can use the 2.4 GHz band, operating in the United States under FCC Part 15 rules and regulations. In this frequency band, equipment may occasionally suffer interference from microwave ovens, cordless telephones , USB 3.0 hubs, Bluetooth and other devices. Spectrum assignments and operational limitations are not consistent worldwide: Australia and Europe allow for an additional two channels (12, 13) beyond
4410-468: A wire or satellite circuit, in ALOHAnet all client nodes communicated with the hub on the same frequency. This meant that some sort of mechanism was needed to control who could talk at what time. The ALOHAnet solution was to allow each client to send its data without controlling when it was sent, and implementing an acknowledgment/retransmission scheme to deal with collisions. This approach radically reduced
4536-482: Is 0.5/e frames per frame-time (reached when G = 0.5 {\displaystyle G=0.5} ), which is approximately 0.184 frames per frame-time. This means that, in Pure ALOHA, only about 18.4% of the time is used for successful transmissions. An improvement to the original ALOHA protocol was Slotted ALOHA, which introduced discrete time slots and increased the maximum throughput. A station can start
4662-412: Is 1/e frames per frame-time (reached when G = 1), which is approximately 0.368 frames per frame-time, or 36.8%. Slotted ALOHA is used in low-data-rate tactical satellite communications networks by military forces, in subscriber-based satellite communications networks, mobile telephony call setup, set-top box communications and in the contactless RFID technologies. Reservation ALOHA, or R-ALOHA,
4788-522: Is computer hardware that connects the computer to the network media and has the ability to process low-level network information. For example, the NIC may have a connector for plugging in a cable, or an aerial for wireless transmission and reception, and the associated circuitry. In Ethernet networks, each NIC has a unique Media Access Control (MAC) address —usually stored in the controller's permanent memory. To avoid address conflicts between network devices,
4914-470: Is a formatted unit of data carried by a packet-switched network . Packets consist of two types of data: control information and user data (payload). The control information provides data the network needs to deliver the user data, for example, source and destination network addresses , error detection codes, and sequencing information. Typically, control information is found in packet headers and trailers , with payload data in between. With packets,
5040-427: Is a set of computers sharing resources located on or provided by network nodes . Computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are made up of telecommunication network technologies based on physically wired, optical , and wireless radio-frequency methods that may be arranged in a variety of network topologies . The nodes of
5166-422: Is a virtual network that is built on top of another network. Nodes in the overlay network are connected by virtual or logical links. Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many peer-to-peer networks are overlay networks. They are organized as nodes of
ALOHAnet - Misplaced Pages Continue
5292-566: Is also found in WLANs ) – it is what the home user sees when the user has to enter a "wireless access key". Ethernet is a family of technologies used in wired LANs. It is described by a set of standards together called IEEE 802.3 published by the Institute of Electrical and Electronics Engineers. Wireless LAN based on the IEEE 802.11 standards, also widely known as WLAN or WiFi, is probably
5418-406: Is an effort to improve the efficiency of Slotted ALOHA. The improvements with Reservation ALOHA are markedly shorter delays and ability to efficiently support higher levels of utilization. As a contrast of efficiency, simulations have shown that Reservation ALOHA exhibits less delay at 80% utilization than Slotted ALOHA at 20–36% utilization. The chief difference between Slotted and Reservation ALOHA
5544-478: Is an electronic device that receives a network signal , cleans it of unnecessary noise and regenerates it. The signal is retransmitted at a higher power level, or to the other side of obstruction so that the signal can cover longer distances without degradation. In most twisted-pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart. Repeaters work on
5670-432: Is divided into a multitude of channels . In the standards, channels are numbered at 5 MHz spacing within a band (except in the 60 GHz band, where they are 2.16 GHz apart), and the number refers to the centre frequency of the channel. Although channels are numbered at 5 MHz spacing, transmitters generally occupy at least 20 MHz, and standards allow for neighbouring channels to be bonded together to form
5796-413: Is idle in one frame, and broadcasts that fact, which indicates that that slot will be a "free slot" in the next frame. Maximum channel efficiency for slotted ALOHA is 36%; PRMA improves maximum channel efficiency to 80%. Demand assigned multiple access (DAMA), also called reservation ALOHA, is an explicit reservation scheme often used in satellite communications. DAMA alternates between two phases: During
5922-412: Is inefficient for very big networks. Modems (modulator-demodulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more carrier signals are modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission. Early modems modulated audio signals sent over
6048-585: Is simply referred to as WLAN , and it may or may not work with " Wi-Fi Certified " devices. As of 2017, the Wi-Fi Alliance consisted of more than 800 companies from around the world. As of 2019, over 3.05 billion Wi-Fi-enabled devices are shipped globally each year. Wi-Fi uses multiple parts of the IEEE 802 protocol family and is designed to work seamlessly with its wired sibling, Ethernet . Compatible devices can network through wireless access points with each other as well as with wired devices and
6174-641: Is submitted for certification. The lack of Wi-Fi certification does not necessarily imply that a device is incompatible with other Wi-Fi devices. The Wi-Fi Alliance may or may not sanction derivative terms, such as Super Wi-Fi , coined by the US Federal Communications Commission (FCC) to describe proposed networking in the UHF TV band in the US. Equipment frequently supports multiple versions of Wi-Fi. To communicate, devices must use
6300-460: Is that with Slotted ALOHA, any slot is available for utilization without regards to prior usage. Under Reservation ALOHA's contention-based reservation schema, the slot is temporarily considered "owned" by the station that successfully used it. Additionally, Reservation ALOHA simply stops sending data once the station has completed its transmission. As a rule, idle slots are considered available to all stations that may then implicitly reserve (utilize)
6426-578: Is the map of logical interconnections of network hosts. Common topologies are: The physical layout of the nodes in a network may not necessarily reflect the network topology. As an example, with FDDI , the network topology is a ring, but the physical topology is often a star, because all neighboring connections can be routed via a central physical location. Physical layout is not completely irrelevant, however, as common ducting and equipment locations can represent single points of failure due to issues like fires, power failures and flooding. An overlay network
SECTION 50
#17327904218456552-493: Is the process of selecting network paths to carry network traffic. Routing is performed for many kinds of networks, including circuit switching networks and packet switched networks. Wi-Fi Wi-Fi ( / ˈ w aɪ f aɪ / ) is a family of wireless network protocols based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio waves . These are
6678-459: Is used between the wireless router and the home user's personal computer when the user is surfing the web. There are many communication protocols, a few of which are described below. The Internet protocol suite , also called TCP/IP, is the foundation of all modern networking. It offers connection-less and connection-oriented services over an inherently unreliable network traversed by datagram transmission using Internet protocol (IP). At its core,
6804-417: Is used in applications such as motion detection and gesture recognition . Wi-Fi stations communicate by sending each other data packets , blocks of data individually sent and delivered over radio on various channels. As with all radio, this is done by the modulation and demodulation of carrier waves . Different versions of Wi-Fi use different techniques, 802.11b uses direct-sequence spread spectrum on
6930-551: The Eduroam international authentication infrastructure. In the early 2000s, many cities around the world announced plans to construct citywide Wi-Fi networks. There are many successful examples; in 2004, Mysore (Mysuru) became India's first Wi-Fi-enabled city. A company called WiFiyNet has set up hotspots in Mysore, covering the whole city and a few nearby villages. In 2005, St. Cloud, Florida and Sunnyvale, California , became
7056-652: The ISM bands for unlicensed use for communications. These frequency bands include the same 2.4 GHz bands used by equipment such as microwave ovens , and are thus subject to interference. In 1991 in Nieuwegein , the NCR Corporation and AT&T invented the precursor to 802.11, intended for use in cashier systems, under the name WaveLAN . NCR's Vic Hayes , who held the chair of IEEE 802.11 for ten years, along with Bell Labs engineer Bruce Tuch, approached
7182-462: The Institute of Electrical and Electronics Engineers (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six octets . The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce. A repeater
7308-530: The Institute of Electrical and Electronics Engineers (IEEE) to create a standard and were involved in designing the initial 802.11b and 802.11a specifications within the IEEE. They have both been subsequently inducted into the Wi-Fi NOW Hall of Fame. In 1989 in Australia, a team of scientists began working on wireless LAN technology. A prototype test bed for a wireless local area network (WLAN)
7434-736: The Internet Protocol . Computer networks may be classified by many criteria, including the transmission medium used to carry signals, bandwidth , communications protocols to organize network traffic , the network size, the topology, traffic control mechanisms, and organizational intent. Computer networks support many applications and services , such as access to the World Wide Web , digital video and audio , shared use of application and storage servers , printers and fax machines , and use of email and instant messaging applications. Computer networking may be considered
7560-454: The Marisat (now Inmarsat ) satellite network. In the early 1980s frequencies for mobile networks became available, and in 1985 frequencies suitable for what became known as Wi-Fi were allocated in the US. These regulatory developments made it possible to use the ALOHA random-access techniques in both Wi-Fi and in mobile telephone networks. Computer network A computer network
7686-464: The Wi-Fi Alliance formed as a trade association to hold the Wi-Fi trademark under which most IEEE 802.11 products are sold. The major commercial breakthrough came with Apple Inc. adopting Wi-Fi for their iBook series of laptops in 1999. It was the first mass consumer product to offer Wi-Fi network connectivity, which was then branded by Apple as AirPort . This was in collaboration with
SECTION 60
#17327904218457812-853: The Wi-Fi Alliance introduced simplified Wi-Fi generational numbering to indicate equipment that supports Wi-Fi 4 ( 802.11n ), Wi-Fi 5 ( 802.11ac ) and Wi-Fi 6 ( 802.11ax ). These generations have a high degree of backward compatibility with previous versions. The alliance has stated that the generational level 4, 5, or 6 can be indicated in the user interface when connected, along with the signal strength. The most important standards affecting Wi‑Fi are: 802.11a, 802.11b, 802.11g, 802.11n ( Wi-Fi 4 ), 802.11h, 802.11i, 802.11-2007, 802.11–2012, 802.11ac ( Wi-Fi 5 ), 802.11ad, 802.11af, 802.11-2016, 802.11ah, 802.11ai, 802.11aj, 802.11aq , 802.11ax ( Wi-Fi 6 ), 802.11ay . Wi-Fi technology may be used to provide local network and Internet access to devices that are within Wi-Fi range of one or more routers that are connected to
7938-416: The bandwidth of the transmission medium can be better shared among users than if the network were circuit switched . When one user is not sending packets, the link can be filled with packets from other users, and so the cost can be shared, with relatively little interference, provided the link is not overused. Often the route a packet needs to take through a network is not immediately available. In that case,
8064-455: The 11 permitted in the United States for the 2.4 GHz band, while Japan has three more (12–14). 802.11a/h/j/n/ac/ax can use the 5 GHz U-NII band , which, for much of the world, offers at least 23 non-overlapping 20 MHz channels. This is in contrast to the 2.4 GHz frequency band where the channels are only 5 MHz wide. In general, lower frequencies have longer range but have less capacity. The 5 GHz bands are absorbed to
8190-402: The 360 were converted into packets by the Menehune, which were queued and broadcast to the remote users at a data rate of 9600 bit/s. Unlike the half-duplex radios at the user TCUs, the Menehune was interfaced to the radio channels with full-duplex radio equipment. The original user interface developed for the system was an all-hardware unit called an ALOHAnet Terminal Control Unit (TCU) and
8316-408: The 5 GHz band, 20 MHz, 40 MHz, 80 MHz, and 160 MHz channels are permitted with some restrictions, giving much faster connections. Wi-Fi is part of the IEEE 802 protocol family. The data is organized into 802.11 frames that are very similar to Ethernet frames at the data link layer, but with extra address fields. MAC addresses are used as network addresses for routing over
8442-447: The ALOHAnet design were the two-channel star configuration of the network and the use of random access for user transmissions. The two-channel configuration was primarily chosen to allow for efficient transmission of the relatively dense total traffic stream being returned to users by the central time-sharing computer. An additional reason for the star configuration was the desire to centralize as many communication functions as possible at
8568-440: The Internet protocol suite or Ethernet that use variable-sized packets or frames . ATM has similarities with both circuit and packet switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before
8694-540: The Internet. Different versions of Wi-Fi are specified by various IEEE 802.11 protocol standards, with different radio technologies determining radio bands, maximum ranges, and speeds that may be achieved. Wi-Fi most commonly uses the 2.4 gigahertz (120 mm) UHF and 5 gigahertz (60 mm) SHF radio bands, with the 6 gigahertz SHF band used in newer generations of the standard; these bands are subdivided into multiple channels. Channels can be shared between networks, but, within range, only one transmitter can transmit on
8820-574: The Internet. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in cyber attacks . A communication protocol is a set of rules for exchanging information over a network. Communication protocols have various characteristics. They may be connection-oriented or connectionless , they may use circuit mode or packet switching, and they may use hierarchical addressing or flat addressing. In
8946-615: The Internet. The coverage of one or more interconnected access points can extend from an area as small as a few rooms to as large as many square kilometres. Coverage in the larger area may require a group of access points with overlapping coverage. For example, public outdoor Wi-Fi technology has been used successfully in wireless mesh networks in London. An international example is Fon . Wi-Fi provides services in private homes, businesses, as well as in public spaces. Wi-Fi hotspots may be set up either free of charge or commercially, often using
9072-594: The LAN. Wi-Fi's MAC and physical layer (PHY) specifications are defined by IEEE 802.11 for modulating and receiving one or more carrier waves to transmit the data in the infrared, and 2.4, 3.6 , 5, 6, or 60 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee ( IEEE 802 ). The base version of the standard was released in 1997 and has had many subsequent amendments. The standard and amendments provide
9198-581: The Netherlands simultaneously claim the invention of Wi-Fi, and a consensus has not been reached globally. In 2009, the Australian CSIRO was awarded $ 200 million after a patent settlement with 14 technology companies, with a further $ 220 million awarded in 2012 after legal proceedings with 23 companies. In 2016, the CSIRO's WLAN prototype test bed was chosen as Australia's contribution to
9324-493: The TDM phase of a frame, the ground stations obey the reservation list and each one only transmits during the long TDM slot(s) reserved for it. Collisions may occur during the reservation phase, but not during the TDM phase. Maximum channel efficiency for slotted ALOHA is 36%; DAMA improves maximum channel efficiency to 80%. The use of a random-access channel in ALOHAnet led to the development of carrier-sense multiple access (CSMA),
9450-543: The Wi-Fi Alliance did use the advertising slogan "The Standard for Wireless Fidelity" for a short time after the brand name was created, and the Wi-Fi Alliance was also called the "Wireless Fidelity Alliance Inc." in some publications. IEEE is a separate, but related, organization and their website has stated "WiFi is a short name for Wireless Fidelity". The name Wi-Fi was partly chosen because it sounds similar to Hi-Fi , which consumers take to mean high fidelity or high quality . Interbrand hoped consumers would find
9576-413: The Wi-Fi Alliance, the term Wi-Fi was chosen from a list of ten names that Interbrand proposed. Interbrand also created the Wi-Fi logo . The yin-yang Wi-Fi logo indicates the certification of a product for interoperability . The name is often written as WiFi , Wifi , or wifi , but these are not approved by the Wi-Fi Alliance. The name Wi-Fi is not short-form for 'Wireless Fidelity', although
9702-627: The actual data exchange begins. ATM still plays a role in the last mile , which is the connection between an Internet service provider and the home user. There are a number of different digital cellular standards, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne , CDMA2000 , Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). Routing
9828-468: The association of physical ports to MAC addresses by examining the source addresses of received frames and only forward the frame when necessary. If an unknown destination MAC is targeted, the device broadcasts the request to all ports except the source, and discovers the location from the reply. Bridges and switches divide the network's collision domain but maintain a single broadcast domain. Network segmentation through bridging and switching helps break down
9954-411: The backoff scheme chosen significantly influences the efficiency of the protocol, the ultimate channel capacity, and the predictability of its behavior. To assess Pure ALOHA, there is a need to predict its throughput, the rate of (successful) transmission of frames. First make a few simplifying assumptions: Let T refer to the time needed to transmit one frame on the channel, and define frame-time as
10080-605: The basis for wireless network products using the Wi-Fi brand. While each amendment is officially revoked when incorporated in the latest version of the standard, the corporate world tends to market to the revisions because they concisely denote capabilities of their products. As a result, in the market place, each revision tends to become its own standard. In addition to 802.11, the IEEE 802 protocol family has specific provisions for Wi-Fi. These are required because Ethernet's cable-based media are not usually shared, whereas with wireless all transmissions are received by all stations within
10206-481: The capability. Some laptops that have a cellular modem card can also act as mobile Internet Wi-Fi access points. Many traditional university campuses in the developed world provide at least partial Wi-Fi coverage. Carnegie Mellon University built the first campus-wide wireless Internet network, called Wireless Andrew , at its Pittsburgh campus in 1993 before Wi-Fi branding existed. Many universities collaborate in providing Wi-Fi access to students and staff through
10332-543: The central network node (the Menehune) to minimize the cost of the original all-hardware terminal control unit (TCU) at each user node. The random-access channel for communication between users and the Menehune was designed specifically for the traffic characteristics of interactive computing. In a conventional communication system, a user might be assigned a portion of the channel on either a frequency-division multiple access or time-division multiple access basis. Since it
10458-431: The channel is busy before transmitting. Since collisions can occur and data may have to be sent again, ALOHA cannot efficiently use 100% of the capacity of the communications channel. How long a station waits until it retransmits, and the likelihood a collision occurs are interrelated, and both affect how efficiently the channel can be used. This means that the concept of retransmit later is a critical aspect; The quality of
10584-569: The channel to be idle at the same time and thus begin transmission simultaneously. A collision happens when a station receives signals from multiple stations on a channel at the same time. This corrupts the transmitted data and can require stations to re-transmit. The lost data and re-transmission reduces throughput, in some cases severely. The 802.11 standard provides several distinct radio frequency ranges for use in Wi-Fi communications: 900 MHz , 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz, 6 GHz and 60 GHz bands . Each range
10710-511: The complexity of the protocol and the networking hardware, since nodes do not need to negotiate who is allowed to speak. This solution became known as a pure ALOHA, or random-access channel, and was the basis for subsequent Ethernet development and later Wi-Fi networks. Various versions of the ALOHA protocol (such as Slotted ALOHA) also appeared later in satellite communications , and were used in wireless data networks such as ARDIS , Mobitex , CDPD , and GSM . The Aloha network introduced
10836-458: The data packet after waiting a randomly selected time interval. This acknowledgment mechanism was used to detect and correct for collisions created when two client machines both attempted to send a packet at the same time. ALOHAnet's primary importance was its use of a shared medium for client transmissions. Unlike the ARPANET where each node could only talk to a single node at the other end of
10962-519: The database. Signal fluctuations can cause errors, which can be reduced with noise-filtering techniques. For low precision, integrating Wi-Fi data with geographical and time information has been proposed. The Wi-Fi RTT capability introduced in IEEE 802.11mc allows for positioning based on round trip time measurement, an improvement over the RSSI method. The IEEE 802.11az standard promises further improvements in geolocation accuracy. Wi-Fi sensing
11088-525: The efficiency of the protocol by reducing the chance of collision, improving throughput to 36.8%. ALOHA was subsequently employed in the Ethernet cable based network in the 1970s, and following regulatory developments in the early 1980s it became possible to use the ALOHA random-access techniques in both Wi-Fi and in mobile telephone networks. ALOHA channels were used in a limited way in the 1980s in 1G mobile phones for signaling and control purposes. In
11214-516: The exhibition A History of the World in 100 Objects held in the National Museum of Australia . The name Wi-Fi , commercially used at least as early as August 1999, was coined by the brand-consulting firm Interbrand. The Wi-Fi Alliance had hired Interbrand to create a name that was "a little catchier than 'IEEE 802.11b Direct Sequence'." According to Phil Belanger, a founding member of
11340-414: The first cities in the United States to offer citywide free Wi-Fi (from MetroFi ). Minneapolis has generated $ 1.2 million in profit annually for its provider . In May 2010, the then London mayor Boris Johnson pledged to have London-wide Wi-Fi by 2012. Several boroughs including Westminster and Islington already had extensive outdoor Wi-Fi coverage at that point. New York City announced
11466-418: The frame, and that ground station can continue transmitting with a guaranteed data rate during that slot of the frame; other ground stations are careful *not* to transmit during that slot of the frame, so there are no collisions during reserved slots. When a ground station with a reserved slot has nothing to send, it simply stops transmitting, which gives up its reservation; the satellite notices its reserved slot
11592-497: The implemented system, one for the user-to-computer random-access channel and one for the computer-to-user broadcast channel. The system was configured as a star network, allowing only the central node to receive transmissions in the random-access channel. All user TCUs received each transmission made by the central node in the broadcast channel. All transmissions were made in bursts at 9600 bit/s , with data and control information encapsulated in packets. Each packet consisted of
11718-566: The late 1980s, the European standardization group GSM who worked on the Pan-European Digital mobile communication system GSM greatly expanded the use of ALOHA channels for access to radio channels in mobile telephony. In the early 2000s additional ALOHA channels were added to 2.5G and 3G mobile phones with the widespread introduction of General Packet Radio Service (GPRS), using a slotted-ALOHA random-access channel combined with
11844-668: The literature as the physical medium ) used to link devices to form a computer network include electrical cable , optical fiber , and free space. In the OSI model , the software to handle the media is defined at layers 1 and 2 — the physical layer and the data link layer. A widely adopted family that uses copper and fiber media in local area network (LAN) technology are collectively known as Ethernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined by IEEE 802.3 . Wireless LAN standards use radio waves , others use infrared signals as
11970-665: The means that allow mapping of a fully connected IP overlay network to its underlying network. Another example of an overlay network is a distributed hash table , which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a map ) indexed by keys. Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees achieve higher-quality streaming media . Previous proposals such as IntServ , DiffServ , and IP multicast have not seen wide acceptance largely because they require modification of all routers in
12096-418: The mechanism of randomized multiple access, which resolved device transmission collisions by transmitting a packet immediately if no acknowledgement is present, and if no acknowledgment was received, the transmission was repeated after a random waiting time. The probability distribution of this random waiting time for retransmission of a packet that has not been acknowledged as received is critically important for
12222-415: The most well-known member of the IEEE 802 protocol family for home users today. IEEE 802.11 shares many properties with wired Ethernet. Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from
12348-476: The most widely used computer networks, used globally in home and small office networks to link devices and to provide Internet access with wireless routers and wireless access points in public places such as coffee shops, hotels, libraries, and airports. Wi-Fi is a trademark of the Wi-Fi Alliance , which restricts the use of the term " Wi-Fi Certified " to products that successfully complete interoperability certification testing. Non-compliant hardware
12474-452: The name catchy, and that they would assume this wireless protocol has high fidelity because of its name. Other technologies intended for fixed points, including Motorola Canopy , are usually called fixed wireless . Alternative wireless technologies include Zigbee , Z-Wave , Bluetooth and mobile phone standards . To connect to a Wi-Fi LAN, a computer must be equipped with a wireless network interface controller . The combination of
12600-729: The network. On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers . The overlay network has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes that a message traverses before it reaches its destination . For example, Akamai Technologies manages an overlay network that provides reliable, efficient content delivery (a kind of multicast ). Academic research includes end system multicast, resilient routing and quality of service studies, among others. The transmission media (often referred to in
12726-475: The other Hawaiian islands with a central time-sharing computer on the main Oahu campus. The first packet broadcasting unit went into operation in June 1971. Terminals were connected to a special purpose terminal connection unit using RS-232 at 9600 bit/s. ALOHA was originally a contrived acronym standing for Additive Links On-line Hawaii Area. The original version of ALOHA used two distinct frequencies in
12852-444: The packet is queued and waits until a link is free. The physical link technologies of packet networks typically limit the size of packets to a certain maximum transmission unit (MTU). A longer message may be fragmented before it is transferred and once the packets arrive, they are reassembled to construct the original message. The physical or geographic locations of network nodes and links generally have relatively little effect on
12978-520: The physical layer of the OSI model but still require a small amount of time to regenerate the signal. This can cause a propagation delay that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters used in a network, e.g., the Ethernet 5-4-3 rule . An Ethernet repeater with multiple ports is known as an Ethernet hub . In addition to reconditioning and distributing network signals,
13104-430: The previous frame, or there was a collision when multiple ground stations transmitted in that slot of the previous frame). If exactly one ground station happens to transmit during a "free slot", that ground station succeeds in reserving that slot of a frame -- the corresponding slot is implicitly reserved in all future frames. From then on, the satellite broadcasts that that particular ground station has reserved that slot of
13230-503: The probability ( P r o b p u r e {\displaystyle Prob_{pure}} ) of there being zero transmission-attempts between t-T and t+T (and thus of a successful transmission for us) is: The throughput can be calculated as the rate of transmission attempts multiplied by the probability of success, and it can be concluded that the throughput ( S p u r e {\displaystyle S_{pure}} ) is: The maximum throughput
13356-567: The probability of a transmission requiring exactly k attempts is (k-1 collisions and 1 success): P r o b s l o t t e d k = e − G ( 1 − e − G ) k − 1 {\displaystyle Prob_{slotted}k=e^{-G}(1-e^{-G})^{k-1}} The throughput is: S s l o t t e d = G e − G {\displaystyle S_{slotted}=Ge^{-G}} The maximum throughput
13482-463: The probability of there being k transmission-attempts during that frame-time is: G k e − G k ! {\displaystyle {\frac {G^{k}e^{-G}}{k!}}} The average number of transmission-attempts for two consecutive frame-times is 2G . Hence, for any pair of consecutive frame-times, the probability of there being k transmission attempts during those two frame-times is: Therefore,
13608-566: The project, which was to be completed in 2015. Wi-Fi positioning systems use known positions of Wi-Fi hotspots to identify a device's location. It is used when GPS isn't suitable due to issues like signal interference or slow satellite acquisition. This includes assisted GPS , urban hotspot databases, and indoor positioning systems. Wi-Fi positioning relies on measuring signal strength ( RSSI ) and fingerprinting. Parameters like SSID and MAC address are crucial for identifying access points. The accuracy depends on nearby access points in
13734-456: The protocol suite defines the addressing, identification, and routing specifications for Internet Protocol Version 4 (IPv4) and for IPv6 , the next generation of the protocol with a much enlarged addressing capability. The Internet protocol suite is the defining set of protocols for the Internet. IEEE 802 is a family of IEEE standards dealing with local area networks and metropolitan area networks. The complete IEEE 802 protocol suite provides
13860-551: The range that employ that radio channel. While Ethernet has essentially negligible error rates, wireless communication media are subject to significant interference. Therefore, the accurate transmission is not guaranteed so delivery is, therefore, a best-effort delivery mechanism. Because of this, for Wi-Fi, the Logical Link Control (LLC) specified by IEEE 802.2 employs Wi-Fi's media access control (MAC) protocols to manage retries without relying on higher levels of
13986-504: The receiver uses the destination address to determine whether the transmission is relevant to the station or should be ignored. A scheme known as carrier-sense multiple access with collision avoidance (CSMA/CA) governs the way stations share channels. With CSMA/CA stations attempt to avoid collisions by beginning transmission only after the channel is sensed to be idle, but then transmit their packet data in its entirety. CSMA/CA cannot completely prevent collisions, as two stations may sense
14112-427: The reservation phase of a frame, DAMA acts like slotted ALOHA for some fixed number of short slots, except instead of ground stations sending complete packets, ground stations only send short requests for later transmission. The satellite collects all the successful requests (i.e., the ones not destroyed by collision) and sends them back as a reservation list assigning specific ground stations to specific TDM slots. During
14238-400: The same channel also means that the data bandwidth is shared, so for example, available throughput to each device is halved when two stations are actively transmitting. As with other IEEE 802 LANs, stations come programmed with a globally unique 48-bit MAC address. The MAC addresses are used to specify both the destination and the source of each data packet. On the reception of a transmission,
14364-411: The same group that helped create the standard: Vic Hayes , Bruce Tuch, Cees Links , Rich McGinn, and others from Lucent . In the year 2000, Radiata, a group of Australian scientists connected to the CSIRO, were the first to use the 802.11a standard on chips connected to a Wi-Fi network. Wi-Fi uses a large number of patents held by many different organizations. Australia, the United States and
14490-515: The same wireless channel, SSID, and other settings that have wirelessly connected, usually to the same access point. Each BSS is identified by a MAC address called the BSSID . The IEEE does not test equipment for compliance with their standards. The Wi-Fi Alliance was formed in 1999 to establish and enforce standards for interoperability and backward compatibility , and to promote wireless local-area-network technology. The Wi-Fi Alliance enforces
14616-482: The sharing of computing resources. For example, a user can print a document on a shared printer or use shared storage devices. Additionally, networks allow for the sharing of files and information, giving authorized users access to data stored on other computers. Distributed computing leverages resources from multiple computers across a network to perform tasks collaboratively. Most modern computer networks use protocols based on packet-mode transmission. A network packet
14742-494: The slot on a contention basis. Packet reservation multiple access (PRMA) is an implicit reservation scheme. Some fixed number of slots form a frame. After each frame, the satellite broadcasts the status of each slot from the previous frame, which indicates the reservation status of the corresponding slots of the next frame. All ground stations wishing to transmit compete exactly like slotted ALOHA during any "free slot" of that next frame (i.e., either no one transmitted in that slot of
14868-444: The stability of Aloha-type communication systems. The average waiting time for retransmission is typically shorter than the average time for generation of a new packet from the same client node, but it should not be allowed to be so short as to compromise the stability of the network, causing a collapse in its overall throughput. Also important was ALOHAnet's use of the outgoing hub channel to broadcast packets directly to all clients on
14994-445: The structure. Similarly, battery-powered routers may include a mobile broadband modem and a Wi-Fi access point. When subscribed to a cellular data carrier, they allow nearby Wi-Fi stations to access the Internet. Many smartphones have a built-in mobile hotspot capability of this sort, though carriers often disable the feature, or charge a separate fee to enable it. Standalone devices such as MiFi - and WiBro -branded devices provide
15120-762: The use of the Wi-Fi brand to technologies based on the IEEE 802.11 standards from the IEEE. Manufacturers with membership in the Wi-Fi Alliance, whose products pass the certification process, gain the right to mark those products with the Wi-Fi logo. Specifically, the certification process requires conformance to the IEEE 802.11 radio standards, the WPA and WPA2 security standards, and the EAP authentication standard. Certification may optionally include tests of IEEE 802.11 draft standards, interaction with cellular-phone technology in converged devices, and features relating to security set-up, multimedia, and power-saving. Not every Wi-Fi device
15246-639: The world. The UK has planned a similar project across major cities of the country, with the project's first implementation in the London Borough of Camden . Officials in South Korea's capital Seoul were moving to provide free Internet access at more than 10,000 locations around the city, including outdoor public spaces, major streets, and densely populated residential areas. Seoul was planning to grant leases to KT, LG Telecom, and SK Telecom. The companies were supposed to invest $ 44 million in
15372-526: Was an HP 2100 minicomputer called the Menehune, which is the Hawaiian language word for dwarf people, and was named for its similar role to the original ARPANET Interface Message Processor (IMP) which was being deployed at about the same time. In the original system, the Menehune forwarded correctly received user data to the UH central computer, an IBM System 360 /65 time-sharing system. Outgoing messages from
15498-573: Was developed in 1992 by a team of researchers from the Radiophysics Division of the CSIRO (Commonwealth Scientific and Industrial Research Organisation) in Australia, led by John O'Sullivan . A patent for Wi Fi was lodged by the CSIRO in 1992 The first version of the 802.11 protocol was released in 1997, and provided up to 2 Mbit/s link speeds. This was updated in 1999 with 802.11b to permit 11 Mbit/s link speeds. In 1999,
15624-591: Was incorporated into the initial design of the TCU as well as the PCU for interfacing a human user into the network. In later versions of the system, simple radio relays were placed in operation to connect the main network on the island of Oahu to other islands in Hawaii, and Menehune routing capabilities were expanded to allow user nodes to exchange packets with other user nodes, the ARPANET , and an experimental satellite network. Two fundamental choices which dictated much of
15750-399: Was the sole piece of equipment necessary to connect a terminal into the ALOHA channel. The TCU was composed of a UHF antenna, transceiver, modem, buffer and control unit. The buffer was designed for a full line length of 80 characters, which allowed handling of both the 40- and 80-character fixed-length packets defined for the system. The typical user terminal in the original system consisted of
15876-483: Was well known that in time-sharing systems (circa 1970), computer and user data are bursty, such fixed assignments are generally wasteful of bandwidth because of the high peak-to-average data rates that characterize the traffic. To achieve a more efficient use of bandwidth for bursty traffic, ALOHAnet developed the random-access packet switching method that has come to be known as a pure ALOHA channel. This approach effectively dynamically allocates bandwidth immediately to
#844155