Data link layer

PART 2 DATA LINK LAYER

Data link layer is the second layer in the ISO reference model, it can transfer the data between the two network entities. In this layer it can perform the error check by using the Frame Check Sequence, if any errors are detected in the frame it will delete the frame. This layer provides the efficient and reliable communication between the adjacent machines and it also performs the error check, network topology, perform framing, physical addressing and flow control. By using the data link protocols the data will be delivered in order to transport the IP packets in various types of MAN's and LAN's to the particular host.

From the Ethernet links, wireless link and satellite link it can encapsulate the packet from on and off media is known as the Media Access Control(MAC) method.

Data link frames includes:

Data, Header, Trailer,

Four organizations which defining the data link layer

  • International Telecommunication Union( ITU)
  • Institute of Electrical and Electronic Engineering (IEEE)
  • International Organization for Standardization (ISO)
  • American National Standards Institute (ANSI)

Here IEEE divides the data link layer into two sublayers which are namely Logical Link Control(LLC) and Media Access Control(MAC), here the IEEE standard is IEEE 802.

In this data link layers the flow control will be control by the PPP protocol (point-to-point protocol). In this we used the LLC, SNAP, SMT, MAC, FDDI, TOKEN RING, ETHERNET.

Services in Data Link Layer:

It will appears in both ISO reference model and TCP/IP connection management, in this the protocol IEEE 802.2 at the logical link control which is the sublayer of the data link layer will provides the both connection oriented and connectionless services. In the session of connection oriented in data link layer protocol the entire data will sent on the same path in the entire communication system, here the protocols have no destination address and complete source.

Sub layers in Data Link Layer

The upper most layer in the data link layer is the Logical Link Control, it will mixes all the protocols which are passing over the data link layer additionally it can provides the error notification, flow control and acknowledge. It will provides the address and control to the data link layer. It will suggest that which mechanism have to be used for the addressing stations when it sends the data over the transmission media and it will controls the data exchange between the originators and machines.

Media access control sublayer in data link layer:

In this it has mainly two forms of media access control namely they are called as centralized and distributed, for the communication between the people it will be used and to compared. It will look after that when the network is made by the people in order to speak, it will check whether the conversation between the people is going on or not. It will also determines that where the one frame of data will ends and where the next one starts with frame synchronization. Totally there are four means of frame synchronizations namely they are bit stuffing, based, byte stuffing and character counting.

Interfaces:

The software that used for the implementing the data link layer is called as the "network card driver . The interface between the data link layer and network transport is defined by the operating system only. Interface is not only a layer itself , it is also an interfacing definition between any two layers.

TCP/IP Model relation

In both TCP/IP model , ISO reference model the lowest layer is data link layer only. By the RFC 3439 the layering in the TCP/IP is not a principle design because the it is generally considered as the harmful. TCP/IP is does not gives the hierarchical sequence of encapsulation requirements , why because means that it is attribute to the ISO reference model protocols. In networks the TCP/IP model is not the top to down design.

Example protocols for the data link layer:

ARCnet( attached resource computer network), ATM( asynchronous transfer mode), CDP( cisco discovery protocol), CAN(controller area network), Econet, Ethernet, EAPS(Ethernet automatic protection switching), FDDI(finer distributed data interface), Frame relay, HDLC(high-level data link control), IEEE 802.2, IEEE 802.11 wireless LAN, LAPD(link access procedures, D channel), LocalTalk, MPLS( multiprotocol label switching), PPP(point-to-point protocol), SLIP(serial line internet protocol), starLan, Token ring, serial communication, UDLD(unidirectional link detection).

Services provides to the network layer:

The main principle for the data link layer and network layer is that transmission of data from the source to destination of the network layer. The above service is established in totally three ways they are connection oriented service, acknowledge connectionless service, un acknowledge connectionless service. Un acknowledge connectionless service used in very very low chance of error transmission. In connection oriented the entire connection will be present for all the that transfer from source to destination. In acknowledge connectionless service it will sends the frames from source machine to destination machine independent only.

Framing:

In order to provide the service between the data link layer and network layer it must be uses the services from the physical layer. Her the transmission of the raw bit streams between the source bits received by the destination machine is less or more or equal. The general approach is that the bit stream should be breaks into discrete frames and for each frame the check sum should be compute. The methods used for the calculating the check sum are physical layer violation, character count, with the bit stuffing the starting and ending flags, with the character stuffing the starting and ending characters.

Error control:

Data link layer deals the delivering of the frames to the destination at the network layer. In general the common approach is that the destination machine give the acknowledge is negative or positive after the delivery of each and every frame. In any conditions the senders receives the positive acknowledge it know as that the frame is correctly receives to the destination , if it is negative acknowledge received by the sender it is know as that the frame not sent correctly to the destination it means that the frame has to send again. Some times the frame or acknowledgement may lost or miss due to some problems at this time the receiver have retransmit the frame, it will causes the potential problems for sending the same frame again and again. This can be prevent by giving the sequence number to each and every frame.

Flow control:

Here another potential problem is there it is that when the sender is must faster in transmitting the frames than the receiver to accept them , the general solution is that to introduce the flow control to the throttle the sender should not to send the frames mush faster that the receiver may handle the traffic. Here the throttle needs some feedback mechanism that the sender may aware of whether the receiver is handling the frame or not.

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