Access method (network)

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The media access in networks is determined via the access method . In the OSI model , this is understood to mean communication between the physical layer (layer 1 = bit transmission layer) and the MAC layer (layer 2a = part of the security layer).

The access procedure regulates which station, ie which data terminal is allowed to transmit which amount of data to whom at which point in time.

In the LAN, it is important to determine how the stations and network components involved access the network cable. Controlling access and the associated transmission of suitable data within a defined framework are among the main tasks of the MAC layer. In connection with the MAC layer, one does not usually speak of network cables, but of transmission media.

Known access methods

CSMA / CD (Carrier Sense Multiple Access / Collision Detection)

This is the best-known and also the oldest access method for shared media operations. It covers the procedures for

CSMA / CD was standardized by the IEEE working group, as were the further developments:

  • IEEE 802.3 for the Ethernet ,
  • IEEE 802.3u for Fast Ethernet,
  • IEEE 802.3z for Gigabit Ethernet.

Further access methods based on CSMA are: CSMA / CA and CSMA / CR .

Token Passing

While chance plays a decisive role in the CSMA / CD method, the class of token-passing methods is a controlled access method. By assigning a transmission permit, it is ensured that exactly one data terminal device gets access to the network and is allowed to transmit at a certain time. The right to send is passed on from data terminal to data terminal with a token circulating in the network (a token for the transmission authorization).

Token passing has been standardized by the IEEE working group in two variants:


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