Channel access method
Encyclopedia
In telecommunication
s and computer networks, a channel access method or multiple access method allows several terminal
s connected to the same multi-point transmission medium
to transmit over it and to share its capacity. Examples of shared physical media are wireless networks, bus network
s, ring network
s, hub networks and half-duplex point-to-point links.
A channel-access scheme is based on a multiplexing
method, that allows several data streams or signals to share the same communication channel or physical medium
. Multiplexing is in this context provided by the physical layer
. Note that multiplexing also may be used in full-duplex point-to-point communication between nodes in a switched network, which should not be considered as multiple access.
A channel-access scheme is also based on a multiple access protocol and control mechanism, also known as media access control
(MAC). This protocol deals with issues such as addressing, assigning multiplex channels to different users, and avoiding collisions. The MAC-layer is a sub-layer in Layer 2 (Data Link Layer
) of the OSI model
and a component of the Link Layer
of the TCP/IP model.
(TDMA) channel access scheme is based on the time division multiplex (TDM) scheme, which provides different time-slots to different data-streams (in the TDMA case to different transmitters) in a cyclically repetitive frame structure. For example, user 1 may use time slot 1, user 2 time slot 2, etc. until the last user. Then it starts all over again,but some times user 1 may use time slot 1 in first frame and use another time slot in next frame.
Packet mode
Packet mode multiple-access is typically also based on time-domain multiplexing, but not in a cyclically repetitive frame structure, and therefore it is not considered as TDM or TDMA. Due to its random character it can be categorised as statistical multiplexing
methods, making it possible to provide dynamic bandwidth allocation
.
(CDMA) scheme is based on spread spectrum
. An example is the 3G
cell phone system.
(SDMA).
s (LANs) and metropolitan area network
s (MANs), multiple access methods enable bus networks, ring networks, hubbed 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, or in switched networks (logical star topology). The most common multiple access method is CSMA/CD, which is used in Ethernet
. Although today's Ethernet installations typically are switched, CSMA/CD is utilized anyway to achieve compatibility with hubs.
to function as a portion of a communications link between more than one pair of satellite terminals concurrently. Three types of multiple access presently used with communications satellites are code-division, frequency-division
, and time-division multiple access.
to two or more switching centers by separate access
lines using a single message
routing indicator
or telephone number
.
, 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 the following MAC categories:
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s and computer networks, a channel access method or multiple access method allows several terminal
Terminal (telecommunication)
In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations....
s connected to the same multi-point transmission medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
to transmit over it and to share its capacity. Examples of shared physical media are wireless networks, bus network
Bus network
A bus network topology is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet...
s, ring 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...
s, hub networks and half-duplex point-to-point links.
A channel-access scheme is based on a multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
method, that allows several data streams or signals to share the same communication channel or physical medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
. Multiplexing is in this context provided by the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
. Note that multiplexing also may be used in full-duplex point-to-point communication between nodes in a switched network, which should not be considered as multiple access.
A channel-access scheme is also based on a multiple access protocol and control mechanism, also known as media access control
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC). This protocol deals with issues such as addressing, assigning multiplex channels to different users, and avoiding collisions. The MAC-layer is a sub-layer in Layer 2 (Data Link Layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
) of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
and a component of the Link Layer
Link Layer
In computer networking, the link layer is the lowest layer in the Internet Protocol Suite , the networking architecture of the Internet . It is the group of methods or protocols that only operate on a host's link...
of the TCP/IP model.
Fundamental types of channel access schemes
These are the four fundamental types of channel access schemes:Frequency Division Multiple Access (FDMA)
The frequency division multiple access (FDMA) channel-access scheme is based on the frequency-division multiplex (FDM) scheme, which provides different frequency bands to different data-streams. In the FDMA case, the data streams are allocated to different users or nodes. An example of FDMA systems were the first-generation (1G) cell-phone systems. A related technique is wave-length division multiple access (WDMA), based on wavelength division multiplex (WDM), where different users get different colors in fiber-optical communication.Time division multiple access (TDMA)
The time division multiple accessTime division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
(TDMA) channel access scheme is based on the time division multiplex (TDM) scheme, which provides different time-slots to different data-streams (in the TDMA case to different transmitters) in a cyclically repetitive frame structure. For example, user 1 may use time slot 1, user 2 time slot 2, etc. until the last user. Then it starts all over again,but some times user 1 may use time slot 1 in first frame and use another time slot in next frame.
Packet mode
Packet mode multiple-access is typically also based on time-domain multiplexing, but not in a cyclically repetitive frame structure, and therefore it is not considered as TDM or TDMA. Due to its random character it can be categorised as statistical multiplexing
Statistical multiplexing
Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation . In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams. The link sharing is adapted to the...
methods, making it possible to provide dynamic bandwidth allocation
Dynamic bandwidth allocation
Dynamic bandwidth allocation is a technique by which traffic bandwidth in a shared telecommunications medium can be allocated on demand and fairly between different users of that bandwidth. This is a form of bandwidth management, and is essentially the same thing as statistical multiplexing...
.
Code division multiple access (CDMA)
The code division multiple accessCode division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
(CDMA) scheme is based on spread spectrum
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
. An example is the 3G
3G
3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
cell phone system.
Space division multiple access (SDMA)
Space division multiple accessSpace-division multiple access
Space-Division Multiple Access is a channel access method based on creating parallel spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which it is able to offer superior performance in radio multiple access communication systems...
(SDMA).
Circuit mode and channelization methods
The following are common circuit mode and channelization channel access methods:- Frequency division multiple access (FDMA), based on frequency-division multiplex (FDM)
- Wavelength division multiple access (WDMA)
- Orthogonal frequency division multiple access (OFDMA), based on Orthogonal frequency-division multiplexingOrthogonal frequency-division multiplexingOrthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...
(OFDM) - Single-carrier FDMASingle-carrier FDMASingle-carrier FDMA is a frequency-division multiple access scheme. Like other multiple access schemes , it deals with the assignment of multiple users to a shared communication resource...
(SC-FDMA), a.k.a. linearly-precoded OFDMA (LP-OFDMA), based on single-carrier frequency-domain-equalization (SC-FDE).
- Time-division multiple access (TDMA), based on time-division multiplex (TDM)
- Multi-Frequency Time Division Multiple AccessMulti-frequency time division multiple accessMF-TDMA is the leading technology for dynamically sharing bandwidth resources in an over-the-air, two-way communications network...
(MF-TDMA)
- Multi-Frequency Time Division Multiple Access
- Code division multiple accessCode division multiple accessCode division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
(CDMA), a.k.a. Spread spectrum multiple accessSpread spectrumSpread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
(SSMA)- Direct-sequence CDMA (DS-CDMA), based on Direct-sequence spread spectrumDirect-sequence spread spectrumIn telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...
(DSSS) - Frequency-hopping CDMA (FH-CDMA), based on Frequency-hopping spread spectrumFrequency-hopping spread spectrumFrequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver...
(FHSS) - Orthogonal frequency-hopping multiple access (OFHMA)
- Multi-carrier code division multiple accessMulti-carrier code division multiple accessMulti-Carrier Code Division Multiple Access is a multiple access scheme used in OFDM-based telecommunication systems, allowing the system to support multiple users at the same time.MC-CDMA spreads each user symbol in the frequency domain...
(MC-CDMA)
- Direct-sequence CDMA (DS-CDMA), based on Direct-sequence spread spectrum
- Space division multiple access (SDMA)
Packet mode methods
The following are examples of packet mode channel access methods:- ContentionContention (telecommunications)In packet mode communication networks, contention is a media access method that is used to share a broadcast medium.-Collision detection and recovery:...
based random multiple access methods- Aloha
- Slotted Aloha
- Multiple Access with Collision AvoidanceMultiple Access with Collision AvoidanceMultiple Access with Collision Avoidance is a slotted media access control protocol used in wireless LAN data transmission to avoid collisions caused by the hidden station problem and to simplify exposed station problem....
(MACA) - Multiple Access with Collision Avoidance for WirelessMultiple Access with Collision Avoidance for WirelessMultiple Access with Collision Avoidance for Wireless is a slotted Medium Access Control protocol widely used in Ad-hoc networks. Furthermore, it is foundation of many other MAC protocols used in Wireless Sensor Networks . The IEEE 802.11 RTS/CTS mechanism is adopted from this protocol...
(MACAW) - Carrier sense multiple accessCarrier Sense Multiple AccessCarrier Sense Multiple Access is a probabilistic Media Access Control protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum."Carrier Sense" describes the fact that a...
(CSMA) - Carrier sense multiple access with collision detectionCarrier sense multiple access with collision detectionCarrier sense multiple access with collision detection is a Media Access Control method in which:*a carrier sensing scheme is used....
(CSMA/CD) - suitable for wired networks - Carrier sense multiple access with collision avoidanceCarrier sense multiple access with collision avoidanceCarrier sense multiple access with collision avoidance , in computer networking, is a wireless network multiple access method in which:*a carrier sensing scheme is used....
(CSMA/CA) - suitable for wireless networks- Distributed Coordination FunctionDistributed Coordination FunctionDistributed coordination function is the fundamental MAC technique of the IEEE 802.11 based WLAN standard. DCF employs a CSMA/CA with binary exponential backoff algorithm....
(DCF) - Point Coordination FunctionPoint Coordination FunctionPoint coordination function is a Media Access Control technique used in IEEE 802.11 based WLANs. It resides in a point coordinator also known as Access Point , to coordinate the communication within the network. The AP waits for PIFS duration rather than DIFS duration to grasp the channel...
(PCF)
- Distributed Coordination Function
- Carrier sense multiple access with collision avoidance and Resolution using Priorities (CSMA/CARPCSMA/CARPIn computer networking, Carrier Sense Multiple Access With Collision Avoidance and Resolution using Priorities is Channel access method. CSMA/CARP is similar in nature to the CSMA/CD Channel access method used in Ethernet networks, but CSMA/CARP provides no detection of network collisions...
) - Carrier Sense Multiple Access/Bitwise Arbitration (CSMA/BA) Based on constructive interference (CAN-bus)
- Token passingToken passingIn telecommunication, token passing is a channel access method where a signal called a token is passed between nodes that authorizes the node to communicate. The most well-known examples are token ring and ARCNET....
:- Token ringIBM token ringthumb|Two examples of token ring networks: a) Using a single [[Media Access Unit|MAU]] b) Using several MAUs connected to each otherthumb|Token ring networkthumb|IBM hermaphroditic connector with locking clipthumb|An IBM 8228 MAU...
- Token busToken busToken bus is a network implementing the token ring protocol over a "virtual ring" on a coaxial cable. A token is passed around the network nodes and only the node possessing the token may transmit. If a node doesn't have anything to send, the token is passed on to the next node on the virtual ring...
- Token ring
- PollingPolling (computer science)Polling, or polled operation, in computer science, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output , and is also referred to as polled or software driven .Polling is sometimes used...
- Resource reservation (scheduled) packet-mode protocols
- Dynamic Time Division Multiple Access (Dynamic TDMA)
- Packet reservation multiple access (PRMA)
- Reservation ALOHAReservation ALOHAReservation ALOHA, or R-ALOHA, is a channel access method for wireless transmission that allows uncoordinated users to share a common transmission resource. Reservation ALOHA is a schema or rule set for the division of transmission resources over fixed time increments, also known as slots...
(R-ALOHA)
Duplexing methods
Where these methods are used for dividing forward and reverse communication channels, they are known as duplexing methods, such as:- Time division duplexDuplex (telecommunications)A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
(TDD) - Frequency division duplex (FDD)
Hybrid channel access scheme application examples
Note that hybrids of these techniques can be - and frequently are - used. Some examples:- The GSM cellular system combines the use of frequency division duplex (FDD) to prevent interference between outward and return signals, with FDMA and TDMA to allow multiple handsets to work in a single cell.
- GSM with the GPRS packet switched service combines FDD and FDMA with slotted Aloha for reservation inquiries, and a Dynamic TDMA scheme for transferring the actual data.
- BluetoothBluetoothBluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
packet mode communication combines frequency hopping (for shared channel access among several private area networks in the same room) with CSMA/CA (for shared channel access inside a medium). - IEEE 802.11b wireless local area networks (WLANs) are based on FDMA and DS-CDMA for avoiding interference among adjacent WLAN cells or access points. This is combined with CSMA/CA for multiple access within the cell.
- HIPERLAN/2 wireless networks combine FDMA with dynamic TDMA, meaning that resource reservation is achieved by packet scheduling.
- G.hnG.hnG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
, an ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
standard for high-speed networking over home wiring (power lines, phone lines and coaxial cables) employs a combination of TDMATime division multiple accessTime division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, Token passingToken passingIn telecommunication, token passing is a channel access method where a signal called a token is passed between nodes that authorizes the node to communicate. The most well-known examples are token ring and ARCNET....
and CSMA/CARPCSMA/CARPIn computer networking, Carrier Sense Multiple Access With Collision Avoidance and Resolution using Priorities is Channel access method. CSMA/CARP is similar in nature to the CSMA/CD Channel access method used in Ethernet networks, but CSMA/CARP provides no detection of network collisions...
to allow multiple devices to share the medium.
Local and metropolitan area networks
In local area networkLocal area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LANs) and metropolitan area network
Metropolitan area network
A metropolitan area network is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the...
s (MANs), multiple access methods enable bus networks, ring networks, hubbed 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, or in switched networks (logical star topology). The most common multiple access method is CSMA/CD, which is used in Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
. Although today's Ethernet installations typically are switched, CSMA/CD is utilized anyway to achieve compatibility with hubs.
Satellite communications
In satellite communications, multiple access is the capability of a communications satelliteCommunications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
to function as a portion of a communications link between more than one pair of satellite terminals concurrently. Three types of multiple access presently used with communications satellites are code-division, frequency-division
Frequency-division multiple access
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like...
, and time-division multiple access.
Switching centers
In telecommunication switching centers, multiple access is the connection of a userUser (telecommunications)
In telecommunications, a user is a person, organization, or other entity that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information....
to two or more switching centers by separate access
Access network
An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...
lines using a single message
Message
A message in its most general meaning is an object of communication. It is a vessel which provides information. Yet, it can also be this information. Therefore, its meaning is dependent upon the context in which it is used; the term may apply to both the information and its form...
routing indicator
Routing indicator
In telecommunication, the term routing indicator has the following meanings:#In a message header, an address, i.e., group of characters, that specifies routing instructions for the transmission of the message to its final destination....
or telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
.
Classifications in the literature
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, TDMATime division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, CDMA, SDMA
Space-division multiple access
Space-Division Multiple Access is a channel access method based on creating parallel spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which it is able to offer superior performance in radio multiple access communication systems...
, and Random access
Random access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "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 the following MAC categories:
- Fixed assigned: TDMA, FDMA+WDMA, CDMA, SDMA
- Demand assigned (DA)
- Reservation: DA/TDMA, DA/FDMA+DA/WDMA, DA/CDMA, DA/SDMA
- Polling: Generalized polling, Distributed polling, Token Passing, Implicit polling, Slotted access
- Random access (RA): Pure RA (ALOHA, GRA), Adaptive RA (TRA), CSMA, CSMA/CD, CSMA/CA
See also
- Radio resource managementRadio resource managementRadio resource management is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems, for example cellular networks, wireless networks and broadcasting systems...
for inter-base station interference control - Statistical multiplexingStatistical multiplexingStatistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation . In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams. The link sharing is adapted to the...
- Dynamic bandwidth allocationDynamic bandwidth allocationDynamic bandwidth allocation is a technique by which traffic bandwidth in a shared telecommunications medium can be allocated on demand and fairly between different users of that bandwidth. This is a form of bandwidth management, and is essentially the same thing as statistical multiplexing...
- Diversity schemeDiversity schemeIn telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity plays an important role in combatting fading and co-channel interference and avoiding error bursts...