The Data Link Layer Chapter 3

Position of the data-link layer

Data link layer duties

3.1 Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control

Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders

Functions of the Data Link Layer (2) Relationship between packets and frames.

3.1.1 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication.

3.1.1 Services Provided to Network Layer The data link layer can be designed to offer different services which is vary from system to system. Commonly provided services: -Unacknowledged connectionless service -Most LANs use this service -Acknowledged connectionless service -As in wireless systems -Acknowledged connection-oriented service -WAN subnet

3.1.2 Framing Possible errors: More bits, Less bits, Different bits Using gaps to frame bits, based on time based. Gaps might be squeezed or new gaps might be inserted in frames. Framing methods: Character spacing Flag bytes with byte stuffing Starting and ending flags, with a bit stuffing. Physical layer coding violation

Framing (2) A character stream. (a) Without errors. (b) With one error.

Framing (3) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

Framing (4) Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing.

3.2 Error Detection and Correction Error-Correcting Codes Error-Detecting Codes

Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected. Note:

Types of Error Single-Bit Error Burst Error

In a single-bit error, only one bit in the data unit has changed. Note:

10.1 Single-bit error

A burst error means that 2 or more bits in the data unit have changed. Note:

Burst error of length 5

Detection Detection Redundancy Parity Check Cyclic Redundancy Check (CRC) Checksum

Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination. Note:

Redundancy

Detection methods

Even-parity concept

In parity check, a parity bit is added to every data unit so that the total number of 1s is even (or odd for odd-parity). Note:

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 are 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.

Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd. Note:

Two-dimensional parity

Example 4 Suppose the following block is sent: However, it is hit by a burst noise of length 8, and some bits are corrupted When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded

In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block. Note:

CRC generator and checker

Binary division in a CRC generator

Binary division in CRC checker

A polynomial representing a divisor

A polynomial representing a divisor

Table 10.1 Standard polynomials NamePolynomialApplication CRC-8x 8 + x 2 + x + 1ATM header CRC-10x 10 + x 9 + x 5 + x 4 + x 2 + 1ATM AAL ITU-16x 16 + x 12 + x 5 + 1HDLC ITU-32 x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11 + x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1 LANs

Example 6 The CRC-12 x 12 + x 11 + x 3 + x + 1 which has a degree of 12, will detect all burst errors affecting an odd number of bits, will detect all burst errors with a length less than or equal to 12, and will detect, percent of the time, burst errors with a length of 12 or more.

Checksum The sender follows these steps: The unit is divided into k sections, each of n bits.The unit is divided into k sections, each of n bits. All sections are added using one’s complement to get the sum.All sections are added using one’s complement to get the sum. The sum is complemented and becomes the checksum.The sum is complemented and becomes the checksum. The checksum is sent with the data.The checksum is sent with the data. Note:

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

Example 7 Suppose the following block of 16 bits is to be sent using a checksum of 8 bits The numbers are added using one’s complement Sum Checksum The pattern sent is

Example 8 Now suppose the receiver receives the pattern sent in Example 7 and there is no error When the receiver adds the three sections, it will get all 1s, which, after complementing, is all 0s and shows that there is no error Sum Complement means that the pattern is OK.

Example 9 Now suppose there is a burst error of length 5 that affects 4 bits When the receiver adds the three sections, it gets Partial Sum Carry 1 Sum Complement the pattern is corrupted.

11.1 Flow and Error Control Flow Control Error Control

Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Note:

Error control in the data link layer is based on automatic repeat request, which is the retransmission of data. Note:

11.2 Stop-and-Wait ARQ Operation Bidirectional Transmission

11.1 Normal operation

11.2 Stop-and-Wait ARQ, lost frame

11.3 Stop-and-Wait ARQ, lost ACK frame

In Stop-and-Wait ARQ, numbering frames prevents the holding of duplicate frames. Note:

11.4 Stop-and-Wait ARQ, delayed ACK

10.3 Correction Retransmission Forward Error Correction Burst Error Correction

3.2.1 Error-Correcting Codes Use of a Hamming code to correct burst errors.

3.2.2 Error-Detecting Codes Calculation of the polynomial code checksum.

3.4 Sliding Window Protocols A One-Bit Sliding Window Protocol A Protocol Using Go Back N A Protocol Using Selective Repeat

Go-Back-N ARQ Go-Back-N ARQ Sequence Number Sender and Receiver Sliding Window Control Variables and Timers Acknowledgment Resending Frames Operation

Sender sliding window

Receiver sliding window

Control variables

Go-Back-N ARQ, normal operation

Go-Back-N ARQ, lost frame

In Go-Back-N ARQ, the size of the sender window must be less than 2m; the size of the receiver window is always 1. Note:

Selective-Repeat ARQ Selective-Repeat ARQ Sender and Receiver Windows Operation Sender Window Size Bidirectional Transmission Pipelining

Selective Repeat ARQ, sender and receiver windows

Selective Repeat ARQ, lost frame

In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2 m. Note:

3.6 Example Data Link Protocols HDLC – High-Level Data Link Control The Data Link Layer in the Internet

A home personal computer acting as an internet host.

PPP – Point to Point Protocol (2) A simplified phase diagram for bring a line up and down.