Wednesday, 6 July 2011

Access schemes

In telecommunications and computer networks, a channel access method or multiple access method allows several terminals 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 networks, ring networks, 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
Fundamental types of channel access schemes

These are the four fundamental types of channel access schemes:

1 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 access (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.
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
Code division multiple access (CDMA)

The code division multiple access (CDMA) scheme is based on
spread spectrum. An example is the 3G cell phone system.
Space division multiple access (SDMA)
List of channel access methods

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 multiplexing (OFDM)
Single-carrier FDMA (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 Access (MF-TDMA)
Code division multiple access (CDMA), a.k.a. Spread spectrum multiple access (SSMA)
Direct-sequence CDMA (DS-CDMA), based on Direct-sequence spread spectrum (DSSS)
Frequency-hopping CDMA (FH-CDMA), based on Frequency-hopping spread spectrum (FHSS)
Orthogonal frequency-hopping multiple access (OFHMA)
Multi-carrier code division multiple access (MC-CDMA)
Space division multiple access (SDMA)
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
            Distributed Coordination Function (DCF)
            Point Coordination Function (PCF)
        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:
        Token ring
        Token bus
    Resource reservation (scheduled) packet-mode protocols
        Dynamic Time Division Multiple Access (Dynamic TDMA)
        Packet reservation multiple access (PRMA)
        Reservation ALOHA (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 duplex (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.
    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., 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.
Satellite communications

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.
Switching centers

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)[1] identifies five principal types of multiple-access schemes: FDMA, TDMA, CDMA, SDMA, and Random access. R. Rom and M. Sidi (1990)[2] categorize the protocols into Conflict-free access protocols, Aloha protocols, and Carrier Sensing protocols.

The Telecommunications Handbook (Terplan and Morreale, 2000)[3] 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

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