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(Wireless) Medium Access Control [MAC]

MEDIA ACCESS CONTROL

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Media Access Control(MAC):

  1. 1) MAC is abbreviated as Media Access Control (or) Medium Access Control
  2. 2) we can say MAC is a sublayer of data link layer
  3. 3) The MAC is an hardware that must be run, so the hardware that implements MAC is Media Access Controller
  4. 4) It acts an interface between logical link layer(LLC) and network physical layer
  5. 5)MAC is a full duplex communication, because at a time it deals with two way communication so that you can send & receive at the same time
  6. 6)MAC supports the services like UNICAST, MULTICAST, and BROADCAST. www.media access control diagrams

UNICAST:

Unicast is one-one transmission from one point in network and another point that is one sender and one receiver.

Example: Transmission would be a phone call between two people. www.unicast diagrams

MULTICAST:

Multicast is a group of communications where data transmission is addressed to a group of destination computers.

Multicast can be done one-to-many (or) many-to-many

Example: Suppose if there are five[5] users from five[5] different countries requested access to the same stream, branches would be created close the orginal source. www.multicast diagrams

BROADCAST:

Broadcast means sending data packet to multiple recipients all at ones

Example: We can say a radio station broadcasts a single to many listeners. www.broadcast diagrams

MOTIVATION FOR A SPECIALIZED MAC:

One of the most commonly used ‘MAC’ schemes for wired network is carrier sense multiple access with collision detection (CSMA/CD)

Whenever we are talking about wired network the mostly commonly used “MAC” scheme is [CSMA/CD]

In this scheme a sender senses the medium (A wire of coaxial cable) to see if it is free.

So the concept here is : They are two nodes one is sender and another end is receiver these two[2] nodes are connected to the wired network, so here we are using CSMA/CD.

Suppose:

1) If the medium is busy, sender waits until its free

2) If the medium is free, sender starts transmitting data and continues to listen into the medium

3) If the sender detects a collision while sending it stop at once and sends a jamming.

Note:-This scheme does not work with wireless network

REASONS:

  • Signal strength decreases proportional to the square of distribution.
  • The sender would apply CS & CD (Collision sense & Collision detection) but collisions happen at receiver side.
  • It might be a case that a sender cannot “HEAR” the collision is CD does not work in wireless.
  • Further more, CS may not work
    • Example:- A terminal is hidden, if suppose CD & CS are wireless and they are not working in sender side the is the reason because they are in HIDDEN TERMINALS

HIDDEN AND EXPOSED TERMINALS: www.hidden and exposed terminals diagrams

Consider the scenario with three[3] mobile phones.

  • The transmission range of ‘A’ reaches ‘B’, but not ‘C’.
  • The transmission range of ‘c’ reaches ‘B’, but not ‘A’.
  • Finally, transmission range of ‘B’ reaches ‘A’ and ‘C’ that is’ A’ cannot detect ‘C’ and vice versa.

HIDDEN TERMINAL:

  • ‘A’ sends to ‘B’, ‘C’ cannot hear ‘A’.
  • ‘C’ wants to send ‘B’, ‘C’ senses a “Free” medium and starts transmitting.
  • Collision at ‘B’ occurs, ‘A’ cannot detects this collision (CD fails) and continues with its transmission ‘B’.
  • ‘A’ is “hidden” from ‘c’ & vice versa

EXPOSED TERMINALS:

  • ‘B’ sends ‘A’, ‘C’ wants to send to another terminal outside the range
  • ‘C’ senses the carrier and detects that the carrier is busy
  • ‘C’ postpones the transmission until it detects the medium as being idle again
  • But ‘A’ is outside radio range of ‘C’ waiting is not necessary
  • ‘C’ is exposed to ‘B’.

NOTE:- Hidden terminal cause collisions, where as exposed terminals cause unnecessary delay

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