In
IEEE 802 LAN/MAN standards, the medium access control (MAC), also called media access control, is the layer that controls the hardware responsible for interaction with the
wired
Wired may refer to:
Arts, entertainment, and media Music
* ''Wired'' (Jeff Beck album), 1976
* ''Wired'' (Hugh Cornwell album), 1993
* ''Wired'' (Mallory Knox album), 2017
* "Wired", a song by Prism from their album '' Beat Street''
* "Wired ...
(electrical or optical) or wireless
transmission medium
A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modula ...
. The MAC sublayer and the
logical link control
In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface ...
(LLC) sublayer together make up the
data link layer
The data link layer, or layer 2, is the second layer of 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 p ...
. The LLC provides
flow control and
multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource� ...
for the logical link (i.e.
EtherType
EtherType is a two- octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of the frame and is used at the receiving end by the data link layer to determine how the payload is processed. The same ...
,
802.1Q VLAN tag etc), while the MAC provides flow control and multiplexing for the transmission medium.
These two sublayers together correspond to layer 2 of the
OSI model
The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems inter ...
. For compatibility reasons, LLC is optional for implementations of
IEEE 802.3 (the frames are then "raw"), but compulsory for implementations of other IEEE 802 physical layer standards. Within the hierarchy of the OSI model and IEEE 802 standards, the MAC sublayer provides a
control abstraction of the physical layer such that the complexities of physical link control are invisible to the LLC and upper layers of the network stack. Thus any LLC sublayer (and higher layers) may be used with any MAC. In turn, the medium access control block is formally connected to the
PHY via a
media-independent interface
The media-independent interface (MII) was originally defined as a standard interface to connect a Fast Ethernet (i.e., ) medium access control (MAC) block to a PHY#Ethernet physical transceiver, PHY chip. The MII is standardized by IEEE 802.3u ...
. Although the MAC block is today typically integrated with the PHY within
the same device package, historically any MAC could be used with any PHY, independent of the transmission medium.
When sending data to another device on the network, the MAC sublayer encapsulates higher-level frames into frames appropriate for the transmission medium (i.e. the MAC adds a
syncword
In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...
preamble and also padding if necessary), adds a
frame check sequence to identify transmission errors, and then forwards the data to the physical layer as soon as the appropriate
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 ...
permits it. For topologies with a
collision domain (bus, ring, mesh, point-to-multipoint topologies), controlling when data is sent and when to wait is necessary to avoid
collisions. Additionally, the MAC is also responsible for compensating for collisions by initiating retransmission if a
jam signal is detected. When receiving data from the physical layer, the MAC block ensures data integrity by verifying the sender's frame check sequences, and strips off the sender's preamble and padding before passing the data up to the higher layers.
Functions performed in the MAC sublayer
According to IEEE Std 802-2001 section 6.2.3 "MAC sublayer", the primary functions performed by the MAC layer are:
* Frame delimiting and recognition
* Addressing of destination stations (both as individual stations and as groups of stations)
* Conveyance of source-station addressing information
* Transparent data transfer of LLC PDUs, or of equivalent information in the Ethernet sublayer
* Protection against errors, generally by means of generating and checking frame check sequences
* Control of access to the physical transmission medium
In the case of
Ethernet
Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
, the functions required of a MAC are:
* receive/transmit normal frames
* half-duplex retransmission and backoff functions
* append/check FCS (
frame check sequence)
* interframe gap enforcement
* discard malformed frames
* prepend(tx)/remove(rx) preamble, SFD (
start frame delimiter), and padding
* half-duplex compatibility: append(tx)/remove(rx) MAC address
Addressing mechanism
The local network addresses used in
IEEE 802
IEEE 802 is a family of Institute of Electrical and Electronics Engineers (IEEE) standards for local area networks (LANs), personal area networks (PANs), and metropolitan area networks (MANs). The IEEE 802 LAN/MAN Standards Committee (LMSC) main ...
networks and
FDDI
Fiber Distributed Data Interface (FDDI) is a standard for data transmission in a local area network.
It uses optical fiber as its standard underlying physical medium.
It was also later specified to use copper cable, in which case it may be c ...
networks are called ''
MAC address
A MAC address (short for medium access control address or media access control address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use i ...
es''; they are based on the addressing scheme that was used in early
Ethernet
Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
implementations. A MAC address is intended as a unique serial number. MAC addresses are typically assigned to network interface hardware at the time of manufacture. The most significant part of the address identifies the manufacturer, who assigns the remainder of the address, thus providing a potentially unique address. This makes it possible for frames to be delivered on a network link that interconnects hosts by some combination of
repeater
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...
s,
hubs,
bridge
A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...
s and
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
es, but not by
network layer
In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate Router (computing), routers.
Functions
The network layer provides t ...
routers. Thus, for example, when an
IP packet reaches its destination (sub)network, the destination IP address (a layer 3 or network layer concept) is resolved with the
Address Resolution Protocol
The Address Resolution Protocol (ARP) is a communication protocol for discovering the link layer address, such as a MAC address, associated with a internet layer address, typically an IPv4 address. The protocol, part of the Internet protocol s ...
for
IPv4
Internet Protocol version 4 (IPv4) is the first version of the Internet Protocol (IP) as a standalone specification. It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. ...
, or by
Neighbor Discovery Protocol
The Neighbor Discovery Protocol (NDP), or simply Neighbor Discovery (ND), is a protocol of the Internet protocol suite used with Internet Protocol Version 6 (IPv6). It operates at the internet layer of the Internet model, and is responsible for ...
(IPv6) into the MAC address (a layer 2 concept) of the destination host.
Examples of physical networks are
Ethernet
Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
networks and
Wi-Fi
Wi-Fi () is a family of wireless network protocols based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital devices to exchange data by ...
networks, both of which are IEEE 802 networks and use IEEE 802 48-bit MAC addresses.
A MAC layer is not required in
full-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
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 method. This makes it possible for several stations connected to the same
physical medium to share it. Examples of shared physical media are
bus network
A bus network is a network topology in which Node (networking), nodes are directly connected to a common half-duplex link called a bus (computing), bus.
A Host (network), host on a bus network is called a ''station''. In a bus network, every ...
s,
ring networks, hub networks,
wireless networks
A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking allows homes, telecommunications networks, and business installations to avoid the costly process of introducing cables in ...
and
half-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
point-to-point links. The multiple access method may detect or avoid data packet
collisions if a packet mode
contention based
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 ...
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
Multiplex may refer to:
Science and technology
* Multiplex communication, combining many signals into one transmission circuit or channel
** Multiplex (television), a group of digital television or radio channels that are combined for broadcast
* ...
scheme.
The most widespread multiple access method is the contention-based
CSMA/CD used in Ethernet networks. This mechanism is only utilized within a network collision domain, for example, an Ethernet bus network or a hub-based star topology network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches.
A multiple access method is not required in a switched
full-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
network, such as today's switched Ethernet networks, but is often available in the equipment for compatibility reasons.
Channel access control mechanism for concurrent transmission
Use of directional antennas and
millimeter-wave communication in a
wireless personal area network increases the probability of concurrent scheduling of non‐interfering transmissions in a localized area, which results in an immense increase in network throughput. However, the optimum scheduling of concurrent transmission is an
NP-hard problem.
Cellular networks
Cellular network
A cellular network or mobile network is a telecommunications network where the link to and from end nodes is wireless network, wireless and the network is distributed over land areas called ''cells'', each served by at least one fixed-locatio ...
s, such as
GSM
The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...
,
UMTS
The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses Wideband Code Division Multiple Access, wideband code-division multiple access (W-CDMA) radio access technolog ...
or
LTE networks, also use a MAC layer. The MAC protocol in cellular networks is designed to maximize the utilization of the expensive licensed spectrum.
The
air interface
The air interface, or access mode, is the communication link between the two stations in mobile or wireless communication. The air interface involves both the physical and data link layers (layer 1 and 2) of the OSI model for a connection.
Phys ...
of a cellular network is at layers 1 and 2 of the OSI model; at layer 2, it is divided into multiple protocol layers. In UMTS and LTE, those protocols are the
Packet Data Convergence Protocol Packet Data Convergence Protocol (PDCP) is specified by 3GPP in TS 25.323 for UMTS
The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses Wideband Code Division Mult ...
(PDCP), the
Radio Link Control Radio Link Control (RLC) is a layer 2 Radio Link Protocol used in UMTS, LTE and 5G on the Air interface. This protocol is specified by 3GPP
The 3rd Generation Partnership Project (3GPP) is an umbrella term for a number of standards organizatio ...
(RLC) protocol, and the MAC protocol. The base station has absolute control over the air interface and schedules the downlink access as well as the uplink access of all devices. The MAC protocol is specified by
3GPP
The 3rd Generation Partnership Project (3GPP) is an umbrella term for a number of standards organizations which develop protocols for mobile telecommunications. Its best known work is the development and maintenance of:
* GSM and related 2G and ...
in TS 25.321 for UMTS, TS 36.321 for LTE and TS 38.321
3GPP TS 38.321 NR; Medium Access Control (MAC) protocol specification
/ref> for 5G.
See also
* Isochronous media access controller
* List of channel access methods
* MAC-Forced Forwarding
* MACsec (IEEE 802.1AE)
References
{{Channel access methods
Computer network technology
Link protocols