Unit3-Data Link Layer-Logical Link Layer-CHAPTER 2.pdf

Unit3: Data Link Layer(10) Chapter 2: Logical Link Layer Logical Link Layer-Error Control & Flow Control; Error Control ; Types of Errors; Error Detection: Parity check method ; Checksum method; Cyclic redundancy check method (CRC)-Error correcting methods: Automatic repeat request method (ARQ), Forward error correction method (FEC), Flow Control Methods.

What Are the Sublayers of the Data Link Layer? The data link layer is divided into two sublayers: 1.Logical Link Control (LLC) Sublayer: Acts as an interface between the upper layers (network layer) and the lower layers (MAC sublayer). Manages flow control and error checking to ensure smooth communication. 2. Media Access Control (MAC) Sublayer: Handles the direct interaction with the physical layer. Manages how devices in a network gain access to the media and transmit data.

Logical Link Control (LLC) Sublayer Function: Facilitates communication between the upper layers (like the network layer) and the lower layers (like the MAC sublayer). Takes network protocol data and adds control information to help deliver packets to their destination. Implements flow control to manage data transmission rates and prevent data overflow. Process: Receives data from the network layer and encapsulates it into frames. Adds headers and trailers to the frames for error detection and control.

Logical Link Control (LLC) Protocol Data Unit Logical Link Control (LLC) is a sublayer that generally provides the logic for the data link as it controls the synchronization, multiplexing, flow control, and even error-checking functions of DLL (Data Link Layer). DLL is divided into two sublayers i.e. LLC sublayer and MAC (Medium Access Control) sublayer.

1. DSAP (Destination Service Access Point) ● 8 bits. ● Works like a receiver’s address at the network layer. ● It tells which network layer protocol (IP, ARP, etc.) should get the frame at the receiving side. 2. SSAP (Source Service Access Point) ● 8 bits. ● Works like a sender’s address at the network layer. ● Also indicates whether the PDU is a command or response. 3. Information Field ● Carries the actual user data from the higher layers (like IP packet). ● Present only in some types of PDUs (mainly Information PDUs).

○ a) Information (I) PDU ● Carries user data (payload). ● Also carries sequence numbers for error and flow control. ● Example: sending a file, message, or packet data. b) Supervisory (S) PDU ● Does not carry user data. ● Used only for error and flow control. ● Functions include: ○ ACK (Acknowledgement) → confirms correct receipt. ○ NAK (Negative ACK) → asks for retransmission. ○ Flow control (telling sender to slow down). c) Unnumbered (U) PDU ● Used for control, management, and special purposes. ● Does not always need sequence numbers. ● Examples: ○ Establish or terminate a connection. ○ Exchange special commands (like mode setting).

Types of Errors Single-bit error → Only one bit in the data unit is changed (0 → 1 or 1 → 0). Multiple-bit error→A multiple-bit error is an error type that arises when more than one bit in a data transmission is affected Burst error → Two or more bits in the data unit are changed. Lost frame → Frame never reaches the receiver. Duplicated frame → Same frame is received more than once.

Types of Errors in Data Transmission 1. Single-bit error ● Meaning: Only one bit in the data unit is altered (0 becomes 1 or 1 becomes 0). ● Cause: Usually caused by random noise in the communication channel. ● Example: Sent: 1011001 Received: 1011101 (only one bit changed → 0 became 1). ● Note: Very rare in real networks because signals are transmitted in groups, not single bits.

Multiple-Bit Error ● Meaning: More than one bit in the data unit is changed, but the errors are not necessarily consecutive. ● Cause: High noise levels, electrical interference, or hardware faults during transmission. ● Example: Sent: 1011001 Received: 1110001 (two or more bits changed, but not all in a sequence). ● Note: Less common than single-bit errors, but possible in noisy or faulty networks.

2. Burst error ● Meaning: Two or more bits in a sequence are changed. ● Cause: Line noise or signal interference that affects multiple bits at once. ● Example: Sent: 1011001 Received: 1110101 (several bits changed together). ● Note: More common than single-bit errors in real communication systems.

Difference from Burst Error: ● Burst error → affected bits are consecutive. ● Multiple-bit error → affected bits are scattered/non-consecutive.

Lost frame ● Meaning: A whole frame (block of data) fails to reach the receiver. ● Cause: Network congestion, buffer overflow, or severe transmission error. ● Example: Sender transmits Frame 1, Frame 2, Frame 3. Receiver gets only Frame 1 and Frame 3 (Frame 2 is lost). ● Effect: Breaks sequence, requires retransmission.

4. Duplicated frame ● Meaning: A frame is delivered more than once to the receiver. ● Cause: Acknowledgment problems or retransmission errors. ● Example: Sender transmits Frame 1, Frame 2, Frame 3. Receiver gets Frame 1, Frame 2, Frame 2, Frame 3. ● Effect: Receiver must discard duplicate frame to avoid confusion.

Error Detection Techniques Used to check whether errors occurred during transmission. a) Parity Check Method ● Simplest method of error detection. ● A parity bit (extra bit) is added to data. ● Two types: ○ Even parity → Number of 1’s in the data + parity bit is even. ○ Odd parity → Number of 1’s in the data + parity bit is odd.

Example: Data = 1010 (two 1’s → even) ● Even parity → Add 0 → 10100 ● Odd parity → Add 1 → 10101 Limitation: ● Detects only single-bit errors. ● Cannot detect burst errors or if two bits are changed.

Parity Check Method Parity check is an error detection method used to detect errors in data transmission. It is simple and widely used in networking. How it Works 1. Data Bits: Data is divided into bytes (usually 8 bits). 2. Parity Bit: An extra bit, called the parity bit, is added to each byte to make the number of 1s even or odd. ○ Even Parity: The total number of 1s (including parity bit) is even. ○ Odd Parity: The total number of 1s (including parity bit) is odd. 3. Transmission: The sender sends the data along with the parity bit. 4. Checking at Receiver: ○ Receiver counts the number of 1s in the received data (including parity bit). ○ If it matches the expected parity (even/odd), the data is assumed correct. ○ If it does not match, an error is detected.

Receivers End On receiving a frame, the receiver counts the number of 1s in it. In case of even parity check, if the count of 1s is even, the frame is accepted, otherwise, it is rejected. In case of odd parity check, if the count of 1s is odd, the frame is accepted, otherwise, it is rejected. Error Detection by Parity Check Senders End While creating a frame, the sender counts the number of 1s in it and adds the parity bit the value of which is determined as follows - In the case of even parity: If a number of 1s is even, the parity bit value is 0. If a number of 1s is odd, the parity bit value is 1. In case of odd parity: If a number of 1s is odd, the parity bit value is 0. If a number of 1s is even, the parity bit value is 1.

Advantages of Parity Check 1. Simple and Easy – Very easy to implement in hardware/software. 2. Low Cost – Requires only one extra bit (parity bit). 3. Quick Error Detection – Detects single-bit errors easily. 4. Low Overhead – Minimal extra storage or transmission cost. Disadvantages of Parity Check 1. Cannot Correct Errors – It only detects, not corrects. 2. Limited Detection – Detects only single-bit errors; fails for two-bit or even-numbered errors. 3. Not Reliable for Large Data – As data size grows, chances of undetected errors increase. 4. Weak Security – Provides no protection against intentional data modification.

Error Control in Data Link Layer

Ensures accurate delivery of frames from sender to receiver. • Detects and retransmits lost or corrupted frames. • Uses protocols to identify errors and resend affected frames. Main technique: ARQ (Automatic Repeat Request) – retransmission when errors are detected.

Error Detection : Error detection, as the name suggests, simply means detection or identification of errors. These errors may occur due to noise or any other impairments during transmission from transmitter to the receiver, in communication system. It is a class of techniques for detecting garbled i.e. unclear and distorted data or messages.There are three main techniques for detecting errors in frames: Parity Check, Checksum and Cyclic Redundancy Check (CRC). Error Correction : Error correction, as the name suggests, simply means correction or solving or fixing of errors. It simply means reconstruction and rehabilitation of original data that is error-free. But error correction method is very costly and very hard.

Error Control Techniques in Data Link Layer

1. Stop-and-Wait ARQ • Also called Alternating Bit Protocol. • Sender sends one frame, waits for ACK. • If ACK not received within time → resend frame. • Simple but slow (one frame at a time).

2. Sliding Window ARQ • Allows continuous transmission of multiple frames. • Uses window size to control number of frames sent. a) Go-Back-N ARQ • Sender keeps sending frames within window. • If error occurs → resend entire window from error frame onwards. • Simple but causes many retransmissions. b) Selective Repeat ARQ • Only the erroneous/lost frames are retransmitted. • More efficient (fewer retransmissions). • But more complex → each frame must be acknowledged separately.

● This protocol is based on the method of using the Sliding Window Protocol as the basis of data exchange, where the ’N’ in the protocol represents the window size. ● The term ARQ represents the Automatic Repeat Request, which refers to sending multiple frames simultaneously from the sender to the receiver end.

● The frames shared in the protocol are sequenced for better efficiency, to avoid any retransmission of shared data, and differentiate between the frames. ● The protocol is designed to share multiple frames at a time, with the receiver end, before expecting any acknowledgment from it. This simultaneous exchange of data is termed protocol pipelining. ● If the acknowledgment is not shared to the sender side within a certain time frame, all the frames after the non-acknowledged frame are to be retransmitted to the receiver side.

Advantages ● Multiple frames can be simultaneous to the receiver side. ● Increase the efficiency of the data transfer and has more control over the flow of frames. ● Time delay is less for sharing data frames. Disadvantages ● The storage of data frames at the receiver side. ● Retransmission of frames, when the acknowledgement is not received by the sender end.

Selective Repeat ARQ The overall working of the selective repeat ARQ is simple. Initially, the sender sends several frames according to the window size. The receiver on the other end receives the frames and sends the ACK for the correct frames and NACK for lost or damaged frames. The sender re-transmits the frames for which the NACK was sent by the receiver. After receiving all the frames, the receiver sorts the frame according to the sequence number for further usage.

The steps of data transmission can be: 1. The sender sends frames 0 and 1. 2. The receiver receives the frames and sends back ACK 0 and ACK 1. 3. Again the sender sends the frames 2 and 3. 4. The receiver only receives the frame 3. So it sends back NACK 2 which means that the 2nd frame is lost and needs to be re-transmitted. 5. So, the sender sends back frame 2 and this process is continued till all the frames have been received by the receiver.

Key Difference: • Go-Back-N: Retransmit all frames after error. • Selective Repeat: Retransmit only the erroneous frame.

Checksums A checksum is a value calculated from data before sending it. It is attached to the data. ● At the receiver’s side, the same calculation is done again. ● If the new checksum matches the sent checksum → data is correct. ● If it does not match → an error occurred. Error Detection using Checksums At the Sender: ○ Data is divided into equal-sized blocks (segments). ○ All blocks are added using 1’s complement arithmetic. ○ The result is complemented (inverted) → this is the checksum. ○ Data + checksum are sent together. At the Receiver: ○ All received data blocks + checksum are added using 1’s complement arithmetic. ○ The result is complemented. ○ If the final value = 0 → no error. ○ If not = error detected.

Detecting and preventing errors is crucial, and Cyclic Redundancy Check (CRC) is an error detection technique.

Examples for Better Understanding Let us consider the following example to understand how error checking is performed using Cyclic Redundancy Check.Let the data be 1001101 and the binary equivalent of the generic polynomial is 1011. On the sender side, the remainder can be found using binary division as follows,

The remainder of the above division is 101. This remainder is appended to the data to create the signal which will be sent to the receiver. The signal is 1001101101. Then error checking is performed on the receiver side as follows,

Examples: Case 1: No error in transmission

Forward Error Correction (FEC) Forward error correction (FEC) is an error correction technique to detect and correct a limited number of errors in transmitted data without the need for retransmission. In this method, the sender sends a redundant error-correcting code along with the data frame. The receiver performs necessary checks based upon the additional redundant bits. If it finds that the data is free from errors, it executes error-correcting code that generates the actual frame. It then removes the redundant bits before passing the message to the upper layers.

Hamming Codes − It is a block code that is capable of detecting up to two simultaneous bit errors and correcting single-bit errors. Binary Convolution Code − Here, an encoder processes an input sequence of bits of arbitrary length and generates a sequence of output bits. Reed - Solomon Code − They are block codes that are capable of correcting burst errors in the received data block. Low-Density Parity Check Code − It is a block code specified by a parity-check matrix containing a low density of 1s. They are suitable for large block sizes in very noisy channels.

Flow control is a technique that allows two stations working at different speeds to communicate with each other. It is a set of measures taken to regulate the amount of data that a sender sends so that a fast sender does not overwhelm a slow receiver. In data link layer, flow control restricts the number of frames the sender can send before it waits for an acknowledgment from the receiver. Feedback based Flow Control In these protocols, the sender sends frames after it has received acknowledgments from the user. This is used in the data link layer. Rate based Flow Control These protocols have built in mechanisms to restrict the rate of transmission of data without requiring acknowledgment from the receiver. This is used in the network layer and the transport layer.

Flow Control Techniques in Data Link Layer Data link layer uses feedback based flow control mechanisms. There are two main techniques

Unit3-Data Link Layer-Logical Link Layer-CHAPTER 2.pdf

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Unit3-Data Link Layer-Logical Link Layer-CHAPTER 2.pdf