Computer Networking

Transport Layer

Medium Access Protocols

The Data Link Layer is responsible for transmission of data between two nodes. So Data Link Layer use some protocols to access the medium.

As we know Data Link layer has to sub layers

1. Logical Link control (LLC) : The main responsibility of Logical link control is flow control.

Keep in mind: If there is a dedicated link between the sender and the receiver (i.e. Mesh Topology) then there is no need of MAC, only the logical link control layer is sufficient.

2. Multiple Access Control (MAC): The main responsibilities of Medium Access control (MAC) are

  • Framing 
  • Physical Addressing
  • Error Control 
  • Access Control

Keep in mind: if there is no dedicated link present (i.e. BUS Topology) then multiple stations can access the same medium simultaneously. So MAC protocols are required to avoid from collision and crosstalk.

Real Life Example, if a classroom is full of students, when a teacher asks a question and all the students start answering at same time, then crosstalk and collisions factors comes to occur. Then it is the job of the teacher (MAC protocols) to manage the all students, by one answer one at a time.

There are three major Types of Medium Access Control Protocols. Following figure shows types

Short introduction of each category, explained below

1. Random Access Protocols

In random access protocols, all stations are independent to access the medium. It just depending on the channel’s state (idle or busy). If medium is idle then transmission is possible otherwise wait for its transmission. However, if more than one station sends the data over a common medium, then collision is possible.

Further types of Random Access Protocols are

  • Aloha 
  • CSMA

2. Control Access Protocols

All stations in control access protocols seek information from one another to find which station has the right to data transmission. It allows only one station (device) to send data at a time, to avoid collision of frames on shared medium.
Further types of controlled-access methods are:

  1. Reservation
  2. Polling
  3. Token Passing

3. Channelization Protocols

In channelization protocols, the available bandwidth of the transmission medium is shared with the prospect of time, frequency or code for multiple stations to access channel simultaneously.

Methods for channelization are

  • Frequency Division Multiple Access (FDMA) 
  • Time Division Multiple Access (TDMA) 
  • Code Division Multiple Access (CDMA) 
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