MEDIUM ACCESS CONTROL ( MAC ) SUBLAYER

MEDIUM ACCESS CONTROL SUBLAYER

Introduction

As discussed earlier , networks are divided into two categories: Broadcast network and print – to – point network.

In broadcast network , several stations share a single communication channel. The major issue in these networks is, which station should transmit data at a given time interval. This process of deciding the turn of different stations is known as channel allocation.

In broadcast network channel is also known as Multi access control or random access channel . therefore, to coordinate the access to the channel, multiple access protocols are required. All these protocols belong to the MAC sublayer.

Network layer

Logical link control ( LLC )

Medium Access control (MAC)

Physical layer

                                                                              Data link layer

Data link layer is divided into two sublayers as the above figure.

1.      Logical  Link Control

2.      Medium Access Control

LCC is responsible for error control and flow control.

MAC is responsible for multiple access resolution.

In this computer, we will be dealing with broadcast networks & their protocols.

CHANNEL ALLOCATION PROBLEM

In broadcast or multipoint networks, single channel is shared by several stations. This channel can be allotted only to one transmitting user at a time.

There  are two different methods of channel allocation:

1.      Static channel allocation

·        In static channel allocation method, a single channel is divided amongst various users either on the basis of frequency or on the basic of time.

·        The static channel allocation either uses FDM ( Frequency Division Multiplexing) or TDM (Time Division Multiplexing).

·        In FDM, fixed frequency is statically assigned to each user, whereas in TDM , a fixed time slot or slice is allocated to each user.

The static allocation methods can suffer from some problems:

i.       In case of FDM , if there are n users, it means there will be n frequency bands. If at a particular time, just two or three users are transmitting data, majority of the channel bandwidth is wasted. Also, in TDM, if user does not transmit data during its time slot , it goes empty leading to wastage of channel time.

ii.      If more than n users want to use channel, some of the users will be denied the access because of lack of bandwidth .

Therefore , dividing the channel into static sub channel is inefficient .

2.      Dynamic channel allocation in LANs & MANs

In dynamic channel allocation method, none of the users in assigned fixed frequency or fixed time slot. Following assumptions are made in order to implement this method:

1.      Station model:  The model consists of N independent stations that may be a computer, telephone or personal communicator etc. Each station has a program or a user that generates frames for transmission. Once the frame has been generated, the station is blocked and does nothing until the frame has been successfully transmitted.

2.      Single channel: A single channel is available for all communication. All stations can transmit on it and all can receive from it.

3.      Collision: If two stations transmit frames simultaneously , then these frames will overlap and the resulting signal is garbled. This is known as collision. All stations can detect collision. A collision frame must be transmitted again later.

4.      Continuous time: It means that frame transmission can begin at anytime. There is no master clock that divides the time into discrete intervals.

5.      Slotted time:  It means the time is divided into slots. A slot may contain 0,1 or more frames. If there is no frame, it indicates idle slot. If there is one frame it indicates successful transmission. If there is more than one frame in a slot, it indicates a collision.

6.      Carrier sense: it means the stations are able to sense the channel before transmission. Station can tell if the channel is in use before trying to use it.

7.      No carrier sense: it means stations cannot sense the channel before trying to use it. They simply transmit without checking the channel. They can determine whether the transmission is successful or not only at later stage.