Data link Layer and Mac

GROUP MEMBERS

MD. SARWAR ZAHAN(MATRIX:14614192)

SAZDOV(MATRIX: 1560657)

Agenda

• Introduction - Zahan

• Data link layer overview - Zahan

• Data link layer working example - Zahan

• Introduction of Mac- Sazdov

• Brief overview of Mac – Sazdov

• Conclusion- Sazdov

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Why data link layer ?

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• Reliable transmission of data frames between two nodes connected by a physical layer.

• The Data-Link layer ensures that an initial connection has been set up.

• Divides output data into data frames.

• Handles the acknowledgements from a receiver that the data arrived successfully.

How its work?

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Data-link layer takes packets from Network Layer

and encapsulates them into Frames. Then, it sends

each frame bit-by-bit on the hardware. At receiver’

end, data link layer picks up signals from hardware

and assembles them into frames.

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Data link layer taskAddressingData-link layer provides hardware addressing mechanism. Hardware address is assumed to be unique on the link. It is encoded into hardware at the time of manufacturing.SynchronizationWhen data frames are sent on the link, both machines must be synchronized in order to transfer to take place.Error ControlThese errors are detected and attempted to recover actual data bits. It also provides error reporting mechanism to the sender.Flow ControlStations on same link may have different speed or capacity. Data-link layer ensures flow control that enables both machine to exchange data on same speed.Multi-AccessData-link layer provides mechanism such as CSMA/CD to equip capability of accessing a shared media among multiple Systems.

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Formatting Data for Transmission

When data travels on the media, it is converted into a stream of bits. If a node is receiving long streams of bits, how does it determine where a frame starts and stops or which bits represent the address?

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Framing breaks the stream into decipherable groupings, with control information inserted in the header and trailer as values in different fields. This format gives the physical signals a structure that can be received by nodes and decoded into packets at the destination.

Farming and Example

Example:

(NIC), which inserts into the system bus of a computer and makes the connection between running software processes on the computer and physical media. Software associated with the NIC enables the NIC to perform its intermediary functions of preparing data for transmission and encoding the data as signals to be sent on the associated media.

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Media Access Control(MAC)

The medium access control (MAC) layer is the lower sub layer of the data link layer of the seven-layer OSI model.

The MAC sub layer provides addressing and channel access control mechanisms that make it possible for

several terminals or network nodes to communicate within a multiple access network that incorporates a shared

medium in network

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Media Access Control Method

MAC protocols are responsible for managing access to the shared communication medium

Which are:1. When a node accesses the shared medium2. Resolving conflicts of competing nodes

MAC ChallengesMedium access in wireless networks is difficult

1.Impossible to send and to receive at the same time (with a single transducer)

2.Interference situation at receiver is what counts for transmission success, but can be very different to what sender can observe

3.High error rates make establishment of well coordinated communication links difficult

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MAC Protocol Classification

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Contention‐Based Contention‐Based

ALOHA CSMAMACA

MACAW

MAC

FDMATDMACDMA

POLLING

Example of Mac protocol (ALOHA)

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Aloha method, refers to a simple communications scheme. in which each source (transmitter) in a network sends data when ever there is a frame to send. If the frame successfully reaches the destination the next frame is sent. If the frame fails to be received at the destination, it is sent again.

Problem in Aloha:

If several nodes transmit during the same period, the messages interfere and are corrupted (“collision”).

Slotted Aloha

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Time is divided into time slots of equal length.

Whenever a node has a message to transmit, it transmits in the next time slot.

If a destination receives a message properly, it transmits an ACK in the next time slot.

Conclusions

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Wireless platforms are becoming less expensive and more powerful, enabling the promise of

widespread use for everything from health monitoring to military use.

As a result various MAC protocols with different objectives have been proposed for access the

medium. However low power MAC protocols have received a lot of consideration in wireless

sensor network for example: S MAC, B MAC etc. MAC protocol will continue update since

wireless technology is a leader of the today's communication.