46-422: Frequency-division multiple access ( FDMA ) is a channel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a single communication channel , such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user. Users can send data through
92-458: A MAC layer in combination with a different LLC layer. In the ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring ( power lines , phone lines and coaxial cables ), the data link layer is divided into three sub-layers (application protocol convergence, logical link control and media access control). The data link layer
138-572: A function that computes the r (amount of redundant bits) corresponding to each string of N total number of bits. The simplest error detection code is the parity bit , which allows a receiver to detect transmission errors that have affected a single bit among the transmitted N + r bits. If there are multiple flipped bits then the checking method might not be able to detect this on the receiver side. More advanced methods than parity error detection do exist providing higher grades of quality and features. A simple example of how this works using metadata
184-454: A half-duplex system, communication only works in one direction at a time. A walkie-talkie is an example of a half-duplex system because both users can communicate with one another, but not at the same time, someone has to finish transmitting before the next person can begin. In a full-duplex system, both users can communicate at the same time. A telephone is the most common example of a full-duplex system because both users can speak and be heard at
230-474: A mobile-phone and a mobile phone base station ). Frequency-division multiplexing (FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel, like in a car radio . FDMA, on the other hand, is an access method in the data link layer . FDMA also supports demand assignment in addition to fixed assignment. Demand assignment allows all users apparently continuous access of
276-518: A network made up of people speaking, i.e. a conversation, they will each pause a random amount of time and then attempt to speak again, effectively establishing a long and elaborate game of saying "no, you first". The Media Access Control sublayer also performs frame synchronization , which determines the start and end of each frame of data in the transmission bitstream . It entails one of several methods: timing-based detection, character counting, byte stuffing, and bit stuffing. The services provided by
322-492: A subchannel by modulating it on a carrier wave at the subchannel's frequency. It is used in satellite communication systems and telephone trunklines. FDMA splits the total bandwidth into multiple channels. Each ground station on the earth is allocated a particular frequency group (or a range of frequencies). Within each group, the ground station can allocate different frequencies to individual channels, which are used by different stations connected to that ground station. Before
368-666: Is CSMA/CD, which is used in Ethernet . Although today's Ethernet installations use full-duplex connections directly to switches . CSMA/CD is still implemented to achieve compatibility with older repeater hubs . In satellite communications , multiple access is the capability of a communications satellite to function as a portion of a communications link between more than one pair of ground-based terminals concurrently. Three types of multiple access presently used with communications satellites are code-division , frequency-division , and time-division multiple access. In cellular networks
414-431: Is analogous to a neighborhood traffic cop; it endeavors to arbitrate between parties contending for access to a medium, without concern for their ultimate destination. When devices attempt to use a medium simultaneously, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them. Examples of data link protocols are Ethernet ,
460-491: Is based on spread spectrum , meaning that a wider radio channel bandwidth is used than the data rate of individual bit streams requires, and several message signals are transferred simultaneously over the same carrier frequency, utilizing different spreading codes. Per the Shannon–Hartley theorem , the wide bandwidth makes it possible to send with a signal-to-noise ratio of much less than 1 (less than 0 dB), meaning that
506-905: Is based on using variable transmission power between users in order to share the available power on the channel. Examples include multiple SCPC modems on a satellite transponder, where users get on demand a larger share of the power budget to transmit at higher data rates. Packet mode channel access methods select a single network transmitter for the duration of a packet transmission. Some methods are more suited to wired communication while others are more suited to wireless. Common statistical time-division multiplexing multiple access protocols for wired multi-drop networks include: Common multiple access protocols that may be used in packet radio wireless networks include: Where these methods are used for dividing forward and reverse communication channels, they are known as duplexing methods. A duplexing communication system can be either half-duplex or full duplex . In
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#1732800904032552-406: Is concerned with local delivery of frames between nodes on the same level of the network. Data-link frames, as these protocol data units are called, do not cross the boundaries of a local area network. Inter-network routing and global addressing are higher-layer functions, allowing data-link protocols to focus on local delivery, addressing, and media arbitration. In this way, the data link layer
598-413: Is often divided into two sublayers: logical link control (LLC) and media access control (MAC). The uppermost sublayer, LLC, multiplexes protocols running at the top of the data link layer, and optionally provides flow control, acknowledgment, and error notification. The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over
644-461: Is provided by the physical layer . A channel access method may also be a part of the multiple access protocol and control mechanism, also known as medium access control (MAC). Medium access control deals with issues such as addressing, assigning multiplex channels to different users and avoiding collisions. Media access control is a sub-layer in the data link layer of the OSI model and a component of
690-646: Is the spreading code, and each message signal (for example each phone call) uses a different spreading code. Another form is frequency-hopping CDMA (FH-CDMA), based on frequency-hopping spread spectrum (FHSS), where the channel frequency is changed rapidly according to a sequence that constitutes the spreading code. As an example, the Bluetooth communication system is based on a combination of frequency-hopping and either CSMA/CA statistical time-division multiplexing communication (for data communication applications) or TDMA (for audio transmission). All nodes belonging to
736-488: Is transmitting the word "HELLO", by encoding each letter as its position in the alphabet. Thus, the letter A is coded as 1, B as 2, and so on as shown in the table on the right. Adding up the resulting numbers yields 8 + 5 + 12 + 12 + 15 = 52, and 5 + 2 = 7 calculates the metadata. Finally, the "8 5 12 12 15 7" numbers sequence is transmitted, which the receiver will see on its end if there are no transmission errors. The receiver knows that
782-577: Is typically also based on time-domain multiplexing, but not in a cyclically repetitive frame structure. Due to its random character, it can be categorized as statistical multiplexing methods and capable of dynamic bandwidth allocation . This requires a media access control (MAC) protocol, i.e. a principle for the nodes to take turns on the channel and to avoid collisions. Common examples are CSMA/CD , used in Ethernet bus networks and hub networks, and CSMA/CA , used in wireless networks such as IEEE 802.11 . The code-division multiple access (CDMA) scheme
828-534: The IEEE 802.11 WiFi protocols, ATM and Frame Relay . In the Internet Protocol Suite (TCP/IP), the data link layer functionality is contained within the link layer , the lowest layer of the descriptive model, which is assumed to be independent of physical infrastructure. The data link provides for the transfer of data frames between hosts connected to the physical link. Within the semantics of
874-564: The link layer of the TCP/IP model . Several ways of categorizing multiple-access schemes and protocols have been used in the literature. For example, Daniel Minoli (2009) identifies five principal types of multiple-access schemes: FDMA , TDMA , CDMA , SDMA , and random access . R. Rom and M. Sidi (1990) categorize the protocols into Conflict-free access protocols , Aloha protocols , and Carrier Sensing protocols . The Telecommunications Handbook (Terplan and Morreale, 2000) identifies
920-527: The radio spectrum by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by COMSAT for use on the Intelsat series IVA and V satellites. There are two main techniques: Channel access method In telecommunications and computer networks , a channel access method or multiple access method allows more than two terminals connected to
966-632: The OSI network architecture, the protocols of the data link layer respond to service requests from the network layer , and perform their function by issuing service requests to the physical layer . That transfer can be reliable or unreliable ; many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even perform any check for transmission errors. In those cases, higher-level protocols must provide flow control , error checking, acknowledgments, and retransmission. The frame header contains
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#17328009040321012-522: The WDMA case, different network nodes in a bus or hub network get a different color. An advanced form of FDMA is the orthogonal frequency-division multiple access (OFDMA) scheme, for example, used in 4G cellular communication systems. In OFDMA, each node may use several sub-carriers, making it possible to provide different quality of service (different data rates) to different users. The assignment of sub-carriers to users may be changed dynamically, based on
1058-407: The capacity of spectrum. Qualcomm knew that CDMA would greatly increase the efficiency and availability of wireless, but the wireless industry having already invested millions of dollars into TDMA was skeptical. Jacobs and Qualcomm spent several years improving infrastructure and performing tests and demonstrations of CDMA. In 1993, CDMA became accepted as the wireless industry standard. By 1995, CDMA
1104-500: The current radio channel conditions and traffic load. Single-carrier FDMA (SC-FDMA), a.k.a. linearly-precoded OFDMA (LP-OFDMA), is based on single-carrier frequency-domain-equalization (SC-FDE). The time-division multiple access (TDMA) channel access scheme is based on the time-division multiplexing (TDM) scheme. TDMA provides different time slots to different transmitters in a cyclically repetitive frame structure. For example, node 1 may use time slot 1, node 2 time slot 2, etc. until
1150-436: The data link layer are: In addition to framing, the data link layer may also detect and recover from transmission errors. For a receiver to detect transmission errors, the sender must add redundant information as an error detection code to the frame sent. When the receiver obtains a frame it verifies whether the received error detection code matches a recomputed error detection code. An error detection code can be defined as
1196-556: The data link layer is described in more detail with media access control (MAC) and logical link control (LLC) sublayers; this means that the IEEE 802.2 LLC protocol can be used with all of the IEEE 802 MAC layers, such as Ethernet, Token Ring , IEEE 802.11 , etc., as well as with some non-802 MAC layers such as FDDI . Other data-link-layer protocols, such as HDLC , are specified to include both sublayers, although some other protocols, such as Cisco HDLC , use HDLC's low-level framing as
1242-401: The data link layer. In the Internet Protocol Suite (TCP/IP), OSI's data link layer functionality is contained within its lowest layer, the link layer . The TCP/IP link layer has the operating scope of the link a host is connected to, and only concerns itself with hardware issues to the point of obtaining hardware (MAC) addresses for locating hosts on the link and transmitting data frames onto
1288-454: The first-generation 1G cell-phone systems, where each phone call was assigned to a specific uplink frequency channel, and another downlink frequency channel. Each message signal (each phone call) is modulated on a specific carrier frequency . A related technique is wavelength division multiple access (WDMA), based on wavelength-division multiplexing (WDM), where different data streams get different colors in fiber-optical communications. In
1334-507: The following MAC categories: Channel access schemes generally fall into the following categories. The frequency-division multiple access (FDMA) channel-access scheme is the most standard analog system, based on the frequency-division multiplexing (FDM) scheme, which provides different frequency bands to different data streams. In the FDMA case, the frequency bands are allocated to different nodes or devices. An example of FDMA systems were
1380-472: The last number received is the error-detecting metadata and that all data before is the message, so the receiver can recalculate the above math and if the metadata matches it can be concluded that the data has been received error-free. Though, if the receiver sees something like a "7 5 12 12 15 7" sequence (first element altered by some error), it can run the check by calculating 7 + 5 + 12 + 12 + 15 = 51 and 5 + 1 = 6, and discard
1426-535: The last transmitter when it starts over. An advanced form is dynamic TDMA (DTDMA), where an assignment of transmitters to time slots varies on each frame. Multi-frequency time-division multiple access (MF-TDMA) combines time and frequency multiple access. As an example, 2G cellular systems are based on a combination of TDMA and FDMA. Each frequency channel is divided into eight time slots, of which seven are used for seven phone calls, and one for signaling data. Statistical time-division multiplexing multiple access
Frequency-division multiple access - Misplaced Pages Continue
1472-492: The link. The link-layer functionality was described in RFC 1122 and is defined differently than the data link layer of OSI, and encompasses all methods that affect the local link. The TCP/IP model is not a top-down comprehensive design reference for networks. It was formulated for the purpose of illustrating the logical groups and scopes of functions needed in the design of the suite of internetworking protocols of TCP/IP, as needed for
1518-402: The received data as defective since 6 does not equal 7. More sophisticated error detection and correction algorithms are designed to reduce the risk that multiple transmission errors in the data would cancel each other out and go undetected. An algorithm that can even detect if the correct bytes are received but out of order is the cyclic redundancy check or CRC. This algorithm is often used in
1564-426: The same transmission medium to transmit over it and to share its capacity. Examples of shared physical media are wireless networks , bus networks , ring networks and point-to-point links operating in half-duplex mode. A channel access method is based on multiplexing , which allows several data streams or signals to share the same communication channel or transmission medium. In this context, multiplexing
1610-463: The same frequency, allowing more conversations to be transmitted over the same amount of spectrum; this is one reason why CDMA eventually became the most widely adopted channel access method in the wireless industry. The origins of CDMA can be traced back to the 1940s where it was patented by the United States government and used throughout World War II to transmit messages. However, following
1656-583: The same time on each end. Some types of full-duplexing methods are: Note that hybrids of these techniques are frequently used. Some examples: Different channel access constraints and schemes apply to different applications. In local area networks (LANs) and metropolitan area networks (MANs), multiple access methods enable bus networks, ring networks, star networks, wireless networks and half-duplex point-to-point communication, but are not required in full-duplex point-to-point serial lines between network switches and routers. The most common multiple access method
1702-791: The same user (to the same piconet ) use the same frequency hopping sequence synchronously, meaning that they send on the same frequency channel, but CDMA/CA or TDMA is used to avoid collisions within the virtual personal area network (VPAN). Frequency-hopping is used by Bluetooth to reduce the cross-talk and collision probability between nodes in different VPANs. Other techniques include OFDMA and multi-carrier code-division multiple access (MC-CDMA). Space-division multiple access (SDMA) transmits different information in different physical areas. Examples include simple cellular radio systems and more advanced cellular systems that use directional antennas and power modulation to refine spatial transmission patterns. Power-division multiple access ( PDMA ) scheme
1748-411: The seven-layer OSI model of computer networking . This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer . The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer. The data link layer
1794-426: The source and destination addresses that indicate which device originated the frame and which device is expected to receive and process it. In contrast to the hierarchical and routable addresses of the network layer, layer 2 addresses are flat, meaning that no part of the address can be used to identify the logical or physical group to which the address belongs. In some networks, such as IEEE 802 local area networks,
1840-406: The theoretical OSI model . Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission. FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple users simultaneous access to a transmission system, FDD refers to how the radio channel is shared between the uplink and downlink (for instance, the traffic going back and forth between
1886-438: The transmission begins, the transmitting ground station looks for an empty channel within the frequency range that is allocated to it and once it finds an empty channel, it allocates it to the particular transmitting station. Alternatives include time-division multiple access (TDMA), code-division multiple access (CDMA), or space-division multiple access (SDMA). These protocols are utilized differently, at different levels of
Frequency-division multiple access - Misplaced Pages Continue
1932-467: The transmission medium and for controlling the data exchanged between the originator and recipient machines. MAC may refer to the sublayer that determines who is allowed to access the media at any one time (e.g. CSMA/CD ). Other times it refers to a frame structure delivered based on MAC addresses inside. There are generally two forms of media access control: distributed and centralized. Both of these may be compared to communication between people. In
1978-435: The transmission power can be reduced to a level below the level of the noise and co-channel interference from other message signals sharing the same frequency range. One form is direct-sequence CDMA (DS-CDMA), based on direct-sequence spread spectrum (DSSS), used for example in 3G cell phone systems. Each information bit (or each symbol) is represented by a long code sequence of several pulses, called chips. The sequence
2024-409: The two most widely adopted technologies are CDMA and TDMA. TDMA technology works by identifying natural breaks in speech and utilizing one radio wave to support multiple transmissions in turn. In CDMA technology, each individual packet receives a unique code that is broken up over a wide frequency spectrum and is then reassembled on the other end. CDMA allows multiple people to speak at the same time over
2070-404: The war the patent expired and the use of CDMA diminished and was widely replaced by TDMA. That was until Irwin M. Jacobs an MIT engineer, and fellow employees from the company Linkabit founded the telecommunications company Qualcomm . At the time Qualcomm was founded, Jacobs had already been working on addressing telecommunications problems for the military using digital technology to increase
2116-402: Was being used commercially in the wireless industry as the foundation of 2G . [REDACTED] This article incorporates public domain material from Federal Standard 1037C . General Services Administration . Archived from the original on 2022-01-22. (in support of MIL-STD-188 ). Data link layer The data link layer , or layer 2 , is the second layer of
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