Channel access method
In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to 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, that allows several data streams or signals to share the same communication channel or transmission medium. In this context, multiplexing is provided by the physical layer.
A channel access method is also based on a 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 the link layer of the TCP/IP model.
- 1 Fundamental types of channel access schemes
- 2 Common multiple access protocols
- 3 Hybrid channel access scheme application examples
- 4 Definition within certain application areas
- 5 Classifications in the literature
- 6 See also
- 7 References
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 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 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 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 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).
Time division multiple access (TDMA)
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 the last transmitter when it starts over. An advanced form is dynamic TDMA (DTDMA), where an assignment of transmitters to time slots vary one 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 signalling data.
Statistical time division multiplexing multiple access is typically also based on time-domain multiplexing, but not in a cyclically repetitive frame structure. Due to its random character, it can be categorised 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.
Code division multiple access (CDMA)/Spread spectrum multiple access (SSMA)
The code division multiple access (CDMA) scheme 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 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 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 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 VPAN. Frequency-hopping is used by Bluetooth to reduce the cross-talk and collision probability between nodes in different VPANs.
Space-division multiple access
Space-division multiple access (SDMA) transmits different information in different physical areas. Examples include simple cellular radio systems and more advanced cellular systems which use directional antennas and power modulation to refine spatial transmission patterns.
Power-division multiple access
Power-division multiple access (PDMA) scheme 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 methods
The following are examples of packet mode channel access methods:
- Contention based random multiple access methods
- Slotted Aloha
- Multiple Access with Collision Avoidance (MACA)
- Multiple Access with Collision Avoidance for Wireless (MACAW)
- Carrier sense multiple access (CSMA)
- Carrier sense multiple access with collision detection (CSMA/CD) - suitable for wired networks
- Carrier sense multiple access with collision avoidance (CSMA/CA) - suitable for wireless networks
- Carrier sense multiple access with collision avoidance and Resolution using Priorities (CSMA/CARP)
- Carrier Sense Multiple Access/Bitwise Arbitration (CSMA/BA) Based on constructive interference (CAN-bus)
- Token passing:
- Resource reservation (scheduled) packet-mode protocols
Where these methods are used for dividing forward and reverse communication channels, they are known as duplexing methods, such as:
Common multiple access protocols
Examples of common statistical time-division multiplexing multiple access protocols for wired multi-drop networks are:
- CSMA/CD (used in Ethernet and IEEE 802.3)
- Token bus (IEEE 802.4)
- Token ring (IEEE 802.5)
- Token passing (used in FDDI)
Examples of common multiple access protocols that may be used in packet radio wireless networks are:
- CSMA/CA (used in IEEE 802.11/WiFi WLANs)
- Slotted ALOHA
- Dynamic TDMA
- Reservation ALOHA (R-ALOHA)
- Mobile Slotted Aloha (MS-ALOHA)
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.
- Bluetooth 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.hn, an ITU-T standard for high-speed networking over home wiring (power lines, phone lines and coaxial cables) employs a combination of TDMA, Token passing and CSMA/CARP to allow multiple devices to share the medium.
Definition within certain application areas
Local and metropolitan area networks
In local area networks (LANs) and metropolitan area networks (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.
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 satellite terminals concurrently. Three types of multiple access presently used with communications satellites are code-division, frequency-division, and time-division multiple access.
In telecommunication switching centers, multiple access is the connection of a user to two or more switching centers by separate access lines using a single message routing indicator or telephone number.
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, 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 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
- Radio resource management for inter-base station interference control
- Statistical multiplexing
- Dynamic bandwidth allocation
- Diversity scheme
- Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN 1107143217.
- "Fundamentals of Communications Access Technologies: FDMA, TDMA, CDMA, OFDMA, AND SDMA". Electronic Design. 2013-01-22. Retrieved 2014-08-28.
- Halit Eren (Nov 16, 2005). Wireless Sensors and Instruments: Networks, Design, and Applications. CRC Press. p. 112. ISBN 9781420037401.
- Sadique, Abubaker. "Multiple Access Techniques in communication: FDMA, TDMA, CDMA". Retrieved 2018-08-14.
- Elinav, Doron; Rubin, Mati E.; Brener, Snir (Mar 6, 2014), Power Division Multiple Access, retrieved 2016-06-29
- Daniel Minoli (3 February 2009). Satellite Systems Engineering in an IPv6 Environment. CRC Press. pp. 136–. ISBN 978-1-4200-7868-8. Retrieved 1 June 2012.
- Rom, Raphael; Sidi, Moshe (1990). "Multiple Access Protocols: Performance and Analysis". Springer-Verlag/University of Michigan.
- Kornel Terplan (2000). The Telecommunications Handbook. CRC Press. pp. 266–. ISBN 978-0-8493-3137-4. Retrieved 1 June 2012.