Data Link Layer.

Data link layer is divided into two sub layer:

1)      DLC

2)      MAC

DLC: it stands for data link control. It deals with the procedure for communication between node to node communications. It includes flow control, error control and framing.

1)      Framing: Framing in the data link layer separate the message from source to destination by adding a sender address. The destination address defines where the packet has to go.

Frame size:  frame can be of fixed or variable size.  In fixed size frame there is no need to defining the boundary of frame, the size itself can be used as delimiter. In variable size there is need of a way for defining the end of one frame and the beginning of next frame. For this [purpose two approaches are used:

1)      Character oriented    2) Bit oriented.

Character oriented:  in character oriented framing data to be carried are 8-bit character forming a code system such as ASCII.  It has two parts: header and trailer. Header contains the source and destination address and other control information. The trailer part contains error detection redundant bits. To separate frame flag is used which is also of 8-bit. It is useful only fro sending character.  Byte stuffing technique is used in this. In byte stuffing a special byte is added to the frame .the data section is stuffed with an extra byte. This byte is usually called the Escape sequence and has a predefined pattern. Whenever the receiver encounters the ESC character, it removes it from the data section and treats the next character as data not as delimiting flag.

Bit Oriented:

In character oriented, flag pattern is used in order to tell the receiver that frame Is not ended here. For this problem bit stuffing is used. In this single bit is stuffed instead of byte.

2)      Flow and error control: Sender sends the data in the form of frame and receiver receive the data. The rate of sender and receiver is not same. May be the rater sender is greater than receiver. Due to that reason data item may lost at the receiver end. So it is necessary to control the flow.

A)     Buffer:  Flow control can be implemented with the use of two buffers; one at the sending data link layer and the other at receiver end. When the buffer of the receiving data link layer is full, it informs the sending data link layer to stop pushing frames.

Error control:  error control at the data link layer is normally very simple and implemented using one of the following two methods. In both methods a CRC is added to the header by the sender and checked by the receiver.

a)      In first method if the frame is corrupted it is silently discarded. If it is not corrupted it is delivered to the network layer.

b)      In second method if frame is not corrupted it is silently discarded, if it corrupted than an acknowledgement is send to the sender.

Connection oriented protocol and connectionless protocol:

In DLC protocol may be connection oriented or connectionless. In connectionless protocols, frames are sending to the next without any relationship between the frames; each frame is independent. Connectionless does not means that there is no physical connection between the nodes; it means there is no connection between frames.

In connection oriented protocol a logical connection should first be established between the two nodes. In this the frames are numbered and send in order. If they are not received in that order the receiver wait until all frames belonging to the sender set are received and then delivered to the network layer.