Data Link Layer

Position of the data-link layer

Data link layer duties

Data Link Layer Design Issues Network layer services Framing Error control Flow control

Data Link Layer Design Issues Physical layer delivers bits of information to and from data link layer. The functions of Data Link Layer are: 1.Providing a well-defined service interface to the network layer. 2.Dealing with transmission errors. 3.Regulating the flow of data so that slow receivers are not swamped by fast senders. Data Link layer –Takes the packets from Network layer, and –Encapsulates them into frames Each frame has a –frame header – a field for holding the packet, and –frame trailer. Frame Management is what Data Link Layer does.

Packets and Frames Relationship between packets and frames.

Services Provided to the Network Layer Principal Service Function of the data link layer is to transfer the data from the network layer on the source machine to the network layer on the destination machine. –Process in the network layer that hands some bits to the data link layer for transmission. –Job of data link layer is to transmit the bits to the destination machine so they can be handed over to the network layer there (see figure in the next slide).

Network Layer Services (a) Virtual communication. (b) Actual communication.

Possible Services Offered 1.Unacknowledged connectionless service. 2.Acknowledged connectionless service. 3.Acknowledged connection-oriented service.

Unacknowledged Connectionless Service It consists of having the source machine send independent frames to the destination machine without having the destination machine acknowledge them. Example: Ethernet, Voice over IP, etc. in all the communication channel were real time operation is more important that quality of transmission.

Acknowledged Connectionless Service Each frame send by the Data Link layer is acknowledged and the sender knows if a specific frame has been received or lost. Typically the protocol uses a specific time period that if the given time has passed without getting acknowledgment it will re-send the frame. This service is useful when an unreliable channel is being utilized (e.g., WiFi).

Acknowledged Connection Oriented Service Source and Destination establish a connection first. Each frame sent is numbered –Data link layer guarantees that each frame sent is indeed received. –It guarantees that each frame is received only once and that all frames are received in the correct order. Examples: –Satellite channel communication –Long-distance telephone communication, etc.

Framing To provide service to the network layer the data link layer must use the service provided to it by physical layer. Stream of data bits provided to data link layer is not guaranteed to be without errors. It is up to data link layer to correct the errors if necessary.

Framing Transmission of the data link layer starts with breaking up the bit stream –into discrete frames –Computation of a checksum for each frame, and –Include the checksum into the frame before it is transmitted. Receiver computes its checksum error for a receiving frame and if it is different from the checksum that is being transmitted will have to deal with the error. Framing is more difficult than one could think!

Framing Methods 1.Byte count. 2.Flag bytes with byte stuffing. 3.Flag bits with bit stuffing.

Byte Count Framing Method It uses a field in the header to specify the number of bytes in the frame. Once the header information is being received it will be used to determine end of the frame. Trouble with this algorithm is that when the count is incorrectly received the destination will get out of synch with transmission. –Destination may be able to detect that the frame is in error but it does not have a means (in this algorithm) how to correct it.

Framing (1) A byte stream. (a) Without errors. (b) With one error.

Flag Bytes with Byte Stuffing Framing Method This methods gets around the boundary detection of the frame by having each appended by the frame start and frame end special bytes. If they are the same (beginning and ending byte in the frame) they are called flag byte. If the actual data contains a byte that is identical to the FLAG byte (e.g., picture, data stream, etc.) the convention that can be used is to have escape character inserted just before the “FLAG” character.

Framing (2) A frame delimited by flag bytes. Four examples of byte sequences before and after byte stuffing.

Flag Bits with Bit Stuffing Framing Method This methods achieves the same thing as Byte Stuffing method by using Bits (1) instead of Bytes (8 Bits). Each frames begins and ends with a special bit pattern: –Whenever the sender’s data link layer encounters five consecutive 1s in the data it automatically stuffs a 0 bit into the outgoing bit stream.

Framing (3) Bit stuffing. (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in the receiver’s memory after de stuffing.

Error Detection and Correction Types of Errors Detection Correction

Basic concepts  Networks must be able to transfer data from one device to another with complete accuracy.  Data can be corrupted during transmission.  For reliable communication, errors must be detected and corrected.  Error detection and correction are implemented either at the data link layer or the transport layer of the OSI model.

Types of Errors

Single-bit error Single bit errors are the least likely type of errors in serial data transmission because the noise must have a very short duration which is very rare. However this kind of errors can happen in parallel transmission.

Burst error burst error The term burst error means that two or more bits in the data unit have changed from 1 to 0 or from 0 to 1. Burst errors does not necessarily mean that the errors occur in consecutive bits, the length of the burst is measured from the first corrupted bit to the last corrupted bit. Some bits in between may not have been corrupted.

Error detection Error detection means to decide whether the received data is correct or not without having a copy of the original message. Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

Redundancy

Detection methods

Parity Check A parity bit is added to every data unit so that the total number of 1s(including the parity bit) becomes even for even-parity check or odd for odd-parity check

Detection -examples Example 1 Suppose the sender wants to send the word world. In ASCII the five characters are coded as The following shows the actual bits sent

Example 2 Now suppose the word world in Example 1 is received by the receiver without being corrupted in transmission The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data is accepted.

Example 3 Now suppose the word world in Example 1 is corrupted during transmission The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

Vertical Redundancy Check VRC

Longitudinal Redundancy Check LRC

Two –Dimensional Parity Check

Cyclic Redundancy Check CRC

Cyclic Redundancy Check Given a k-bit frame or message, the transmitter generates an n-bit sequence, known as a frame check sequence (FCS), so that the resulting frame, consisting of (k+n) bits, is exactly divisible by some predetermined number. The receiver then divides the incoming frame by the same number and, if there is no remainder, assumes that there was no error.

Binary Division

Polynomial

Polynomial and Divisor

Standard Polynomials

Checksum

At the sender  The unit is divided into k sections, each of n bits.  All sections are added together using one’s complement to get the sum.  The sum is complemented and becomes the checksum.  The checksum is sent with the data

At the receiver  The unit is divided into k sections, each of n bits.  All sections are added together using one’s complement to get the sum.  The sum is complemented.  If the result is zero, the data are accepted: otherwise, they are rejected.

Performance  The checksum detects all errors involving an odd number of bits.  It detects most errors involving an even number of bits.  If one or more bits of a segment are damaged and the corresponding bit or bits of opposite value in a second segment are also damaged, the sums of those columns will not change and the receiver will not detect a problem.

Error Correction It can be handled in two ways: 1)receiver can have the sender retransmit the entire data unit. 2)The receiver can use an error-correcting code, which automatically corrects certain errors.

Single-bit error correction To correct an error, the receiver reverses the value of the altered bit. To do so, it must know which bit is in error. Number of redundancy bits needed Let data bits = m Redundancy bits = r  Total message sent = m+r The value of r must satisfy the following relation: 2 r ≥ m+r+1

Error Correction

Hamming Code

Example of Hamming Code

Single-bit error

Error Detection