Media Access Control
Encyclopedia
The media access control (MAC) 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
(layer 2), and in the four-layer TCP/IP model (layer 1). It provides addressing and channel access control mechanisms that make it possible for several terminals
or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. Ethernet
. The hardware that implements the MAC is referred to as a medium access controller.
The MAC sub-layer acts as an interface between the logical link control
(LLC) sublayer and the network's physical layer
. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast
, multicast
or broadcast
communication service.
In 100Mbps and faster MACs, the MAC address
is not actually handled in the MAC layer. Doing so would make it impossible to
implement IP because the ARP layer of IP-Ethernet needs access to the MAC address.
MACs, there is no required addressing mechanism. However, the MAC address
inherited from the original MAC layer specification is used in many higher level protocols such as Internet Protocol
(IP) over Ethernet.
The local network address used in IP-Ethernet is called MAC address
because it historically was part of the MAC layer in early Ethernets. The MAC layer's addressing mechanism is called physical address or MAC address
. A MAC address is a unique serial number. Once a MAC address has been assigned to a particular network interface (typically at time of manufacture), that device should be uniquely identifiable amongst all other network devices in the world. This guarantees that each device in a network will have a different MAC address (analogous to a street address). This makes it possible for data packets to be delivered to a destination within a subnetwork
, i.e. host
s interconnected by some combination of repeater
s, hubs, bridges and switch
es, but not by IP routers. Thus, when an IP packet reaches its destination (sub)network, the destination IP address (a layer-3, network layer
, construct) is resolved
into the MAC address (a layer-2 construct) of the destination host.
An example of a physical network is an Ethernet network, perhaps extended by wireless local area network (WLAN) access points and WLAN network adapters, since these share the same 48-bit MAC address hierarchy as Ethernet.
A MAC layer is not required in full-duplex point-to-point communication, but address fields are included in some point-to-point protocols for compatibility reasons.
to share it. Examples of shared physical media are bus network
s, ring network
s, hub networks, wireless networks and half-duplex point-to-point links. The multiple access protocol may detect or avoid data packet collisions if a
packet mode contention
based channel access method
is used, or reserve resources to establish a logical channel if a circuit switched or channelization based channel access method is used. The channel access control mechanism relies on a physical layer multiplex
scheme.
The most widespread multiple access protocol is the contention based CSMA/CD protocol used in Ethernet networks. This mechanism is only utilized within a network collision domain, for example an Ethernet bus network or a hub network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches.
A multiple access protocol is not required in a switched full-duplex network, such as today's switched Ethernet networks, but is often available in the equipment for compatibility reasons.
Examples of common multiple access protocols that may be used in packet radio wireless networks are:
For a more extensive list, see List of channel access methods.
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....
specified in the seven-layer 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...
(layer 2), and in the four-layer TCP/IP model (layer 1). It provides addressing and channel access control mechanisms that make it possible for several terminals
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....
or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. 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....
. The hardware that implements the MAC is referred to as a medium access controller.
The MAC sub-layer acts as an interface between the logical link control
Logical Link Control
The logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
(LLC) sublayer and the network's 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....
. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
, multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
or broadcast
Broadcast
Broadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
communication service.
Functions performed in the MAC layer
According to 802.3-2002 section 4.1.4, the functions required of a MAC are:- receive/transmit normal frames
- half-duplex retransmission and backoff functions
- append/check FCS (frame check sequenceFrame Check SequenceA frame check sequence refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction. Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message...
) - interframe gap enforcement
- discard malformed frames
- append(tx)/remove(rx) preamble, SFD, and padding
- half-duplex compatibility: append(tx)/remove(rx) MAC address
In 100Mbps and faster MACs, the MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
is not actually handled in the MAC layer. Doing so would make it impossible to
implement IP because the ARP layer of IP-Ethernet needs access to the MAC address.
Addressing mechanism
In 100Mbps and faster EthernetEthernet
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....
MACs, there is no required addressing mechanism. However, the MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
inherited from the original MAC layer specification is used in many higher level protocols such as Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) over Ethernet.
The local network address used in IP-Ethernet is called MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
because it historically was part of the MAC layer in early Ethernets. The MAC layer's addressing mechanism is called physical address or MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
. A MAC address is a unique serial number. Once a MAC address has been assigned to a particular network interface (typically at time of manufacture), that device should be uniquely identifiable amongst all other network devices in the world. This guarantees that each device in a network will have a different MAC address (analogous to a street address). This makes it possible for data packets to be delivered to a destination within a subnetwork
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....
, i.e. host
Host (network)
A network host is a computer connected to a computer network. A network host may offer information resources, services, and applications to users or other nodes on the network. A network host is a network node that is assigned a network layer host address....
s interconnected by some combination of repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
s, hubs, bridges and switch
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
es, but not by IP routers. Thus, when an IP packet reaches its destination (sub)network, the destination IP address (a layer-3, network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
, construct) is resolved
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
into the MAC address (a layer-2 construct) of the destination host.
An example of a physical network is an Ethernet network, perhaps extended by wireless local area network (WLAN) access points and WLAN network adapters, since these share the same 48-bit MAC address hierarchy as Ethernet.
A MAC layer is not required in full-duplex point-to-point communication, but address fields are included in some point-to-point protocols for compatibility reasons.
Channel access control mechanism
The channel access control mechanisms provided by the MAC layer are also known as a multiple access protocol. This makes it possible for several stations connected to the same physical mediumPhysical medium
Physical medium may refer to:* For a medium used in the propagation of energy waves, see Transmission medium* For the practice of physical mediumship and the conducting of seance, see Physical mediumship...
to share it. Examples of shared physical media are 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, wireless networks and half-duplex point-to-point links. The multiple access protocol may detect or avoid data packet collisions if a
packet mode contention
Contention (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 channel access method
Channel access method
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...
is used, or reserve resources to establish a logical channel if a circuit switched or channelization based channel access method is used. The channel access control mechanism relies on a physical layer multiplex
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...
scheme.
The most widespread multiple access protocol is the contention based CSMA/CD protocol used in Ethernet networks. This mechanism is only utilized within a network collision domain, for example an Ethernet bus network or a hub network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches.
A multiple access protocol is not required in a switched full-duplex network, such as today's switched Ethernet networks, but is often available in the equipment for compatibility reasons.
Common multiple access protocols
Examples of common packet mode multiple access protocols for wired multi-drop networks are:- CSMA/CD (used in EthernetEthernetEthernet 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....
and IEEE 802.3IEEE 802.3IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...
) - 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...
(IEEE 802.4) - Token ring (IEEE 802.5IBM 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 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....
(used in FDDI)
Examples of common multiple access protocols that may be used in packet radio wireless networks are:
- CSMA/CA (used in IEEE 802.11IEEE 802.11IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
/WiFiWIFIWIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...
WLANWireless LANA wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
s) - Slotted ALOHA
- Dynamic TDMA
- 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) - Mobile Slotted Aloha (MS-ALOHA)
- CDMA
- OFDMAOFDMAOrthogonal Frequency-Division Multiple Access is a multi-user version of the popular Orthogonal frequency-division multiplexing digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the illustration below...
For a more extensive list, see List of channel access methods.