Yes, link layer encapsulate the diagram received from the network layer into a frame. As we know that frame consists of a data field, in which the network layer datagram is introduced, and a number of header fields. I other word I can say that a frame may also comprise trailer fields and we will mention to both header and trailer fields as header fields. A data link protocol requires the structure of the frame, as well as a channel access protocol that specifies the rules by which a frame is transmitted onto the link. For point to point relations that have a single sender on one end of the link and a single receiver at the other end of the link, the link entrance protocol is simple the sender can send a frame whenever the link is shiftless. The more stimulating case is when multiple nodes share a single broadcast link the so known as multiple access problem.
1. Is the link layer incapable of detecting an error in a received frame?
No, link layer is capable of detecting an error in a received frame. Here we can see that how Data link layer find out the error. Data link layer uses some error control mechanism to ensure that frames are conveyed with certain level of accurateness. But to understand how errors is controlled, it is vital to know what types of errors may occur.
There may be three types of errors;
· Single bit error
· Multiple bits error
· Burst error
There are many reasons for error such as noise, cross talk and many more which may help data to get corrupted during transmission. The higher layers work on some sweeping view of system architecture and are not aware of actual hardware data processing. So, higher layers expect error-free communication between systems. Applications such as voice and video may not be that affected and with some errors they may still purpose well.
Moreover, Error control device may involve two possible ways which is mention following:
· Error detection
· Error correction
Errors in the received frames are detected by two ways means of Parity Check and Cyclic Redundancy Check. We have to check in both situations, few extra bits are sent along with actual data to confirm that bits received at other end are same as they were sent. If the checks at receivers end fail, the bits are corrupted.
· Parity Check
One extra bit is directed along with the original bits to make number of 1s either even, in case of even parity or odd, in case of odd parity.
The dispatcher while creating a frame counts the number of 1s in it, for example, if even equality is used and number of 1s is even then one bit with value 0 is added. This way number of 1s remainders even. Or if the number of 1s is odd, to make it even a bit with value 1 is added.
The receiver simply counts the number of 1s in a frame. If the count of 1s is even and even parity is used, the frame is considered to be not degraded and is accepted. If the count of 1s is odd and odd parity is used, the frame is still not corrupted.
If a single bit flips in transport, the receiver can detect it by counting the number of 1s. But when more than one bit is in error it is very hard for the receiver to detect the error. So this way we can check Error detection in Parity check.
· Cyclic Redundancy Check
CRC is a different approach to detect if the frame received contains legal data. This technique involves binary division of the data bits being sent. The divisor is generated using polynomials. The sender performs a division operation on the bits being sent and calculates the remainder. Before transfer the actual bits, the sender adds the remainder at the end of the actual bits. Actual data bits plus the remainder is called a code word. The sender transmits data bits as code words.
In the digital world, error correction can be done in two ways:
· Backward Error Alteration: When the headset identifies an error in the data received, it requirements back the sender to retransmit the data unit.
· Forward Error Alteration: When the receiver identifies some error in the data received, it executes error modifying code, which helps it to auto recover and to correct some kinds of errors.
So, this is way link layer find detecting an error in a received frame