Medium access control

Source: Wikipedia, the free encyclopedia.

In

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

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

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:[2]

  • 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, the functions required of a MAC are:[3]

  • 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

(IPv6) into the MAC address (a layer 2 concept) of the destination host.

Examples of physical networks are Ethernet networks and Wi-Fi 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 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

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 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
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

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.[4]

Cellular networks

Cellular networks, such as GSM, UMTS 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.[5] The air interface 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 (PDCP), the Radio Link Control (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 in TS 25.321[6] for UMTS, TS 36.321[7] for LTE and TS 38.321[8] for 5G.

See also

References

  1. ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
  2. ^ "IEEE 802-2001 (R2007) IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture" (PDF). IEEE.
  3. ^ "4.1.4", IEEE 802.3-2002, IEEE
  4. .
  5. .
  6. ^ 3GPP TS 25.321 Medium Access Control (MAC) protocol specification
  7. ^ 3GPP TS 36.321 Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification
  8. ^ 3GPP TS 38.321 NR; Medium Access Control (MAC) protocol specification