Ch 4: Data Link Layer

Question 1

Three Functions of the Data Link Protocol

Answer 1

1. Controls when computers transmit (media access control)
2. Detects and corrects transmission errors (error control)
3. Identifies the start and end of a message by using a PDU (message
   delineation)

Question 2

Media Access Control

Answer 2

• Media access control refers to the need to control when computers
  transmit
• With point-to-point full-duplex configurations, media access control is unnecessary
• Media access control becomes important when several computers share the same communication circuit
• Here, it is critical to ensure that no two computers attempt to transmit data at the same time

Question 3

Two fundamental approaches to media access control

Answer 3

1. Contention
2. Controlled access

Question 4

Contention

Answer 4

• With contention, computers wait until the circuit is free and then transmit whenever they have data to send
• Contention is commonly used in Ethernet LANs
• There must be some technique to continue after a collision

Question 5

Controlled Access

Answer 5

• With controlled access, one device controls the circuit and determines which clients can transmit at what time
• There are two commonly used controlled access techniques
  1. Access requests
  2. Polling
• With the access request technique, client computers that want to transmit send a request to transmit to the device that is controlling the circuit
• The controlling device grants permission for one computer at a time to transmit

Question 6

Polling

Answer 6

• Polling is the process of sending a signal to a client computer that permits it to transmit
• Periodically, the controlling device polls the client to see if it has data to send
• There are several types of polling:
  o With roll-call polling, the controller works consecutively through a list of clients
  o With hub polling (often called token passing), one device starts the poll and passes it to the next computer on the multipoint circuit, and so on

Question 7

Relative Performance

Answer 7

• Contention approaches work better than controlled
  approaches for small networks that have low usage
• For large networks with high usage controlled access works
  better
• The key to selecting the best access control technique is to
  find the crossover point

Question 8

Error Control

Answer 8

• There are human errors and network errors
  o Human errors are controlled through the application program
  o Network errors are controlled by the network hardware and software

Question 9

There are two categories of network errors:

Answer 9

1. Corrupted data (data that have been changed)
2. Lost data
   * Networks should be designed to (1) prevent, (2) detect, and (3) correct both

Question 10

Burst Error

Answer 10

• Normally, errors appear in bursts
• In a burst error, more than 1 data bit is changed by the error-causing condition
• In other words, errors are not uniformly distributed

Question 11

Why is it both good and bad that errors tend to occur in bursts?

Answer 11

o If the errors were not clustered, character-checking schemes would be effective at detecting errors
o With burst errors, more reliance must be placed on error detection and correction methods

Question 12

Error Detection

Answer 12

1. Parity Checking
2. Checksum
3. Cyclic Redundancy Check

Question 13

Parity Checking

Answer 13

o With this technique, one additional bit is added to each byte in the message
o The value of this additional parity bit is based on the number of 1s in each byte transmitted

Question 14

Checksum

Answer 14

o With the checksum technique, a checksum (typically 1 byte) is added to the end of the message
o The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum

Question 15

Cyclic Redundancy Check(CRC)

Answer 15

o With CRC, a message is treated as one long binary number, which is divided by a preset number, and the remainder is used as the CRC code

Question 16

Forward Error Correction

Answer 16

• Forward error correction uses codes containing sufficient redundancy to prevent errors by detecting and correcting them at the receiving end without retransmission of the original message
• The redundancy, or extra bits required, varies with different schemes
  o It ranges from a small percentage of extra bits to 100% redundancy
• Forward error correction is commonly used in satellite transmission

Question 17

Error Control in Practice

Answer 17

• Most network cables—especially LAN cables—are very reliable, and errors are far less common than they were in the 1980s
• Most data link layer software used in LANs is configured to detect errors, but not correct them
• Any time a packet with an error is discovered, it is simply discarded

Question 18

Asynchronous Transmission

Answer 18

• Asynchronous transmission is often
  referred to as start–stop transmission because the transmitting computer can transmit a character whenever it is convenient, and the receiving computer will accept that character
• With asynchronous transmission, each character is transmitted independently of all other characters

Question 19

Asynchronous Transmission Continued

Answer 19

• To separate the characters and synchronize transmission, a start bit and a stop bit are put on the front and back of each individual character
• The recognition of the start and stop of each message (called synchronization) takes place for each individual character

Question 20

Synchronous Transmission

Answer 20

• With synchronous transmission, all the letters or data in one
  group of data are transmitted at one time as a block of data
• This block of data is called a frame
• The start and end of the entire frame must be marked, not the
  start and end of each letter

Question 21

Four Commonly used Synchronous Data Link Protocols

Answer 21

1. Synchronous data link control (SDLC)
2. High-level data link control (HDLC)
3. Ethernet
4. Point-to-point protocol (PPP)

Question 22

Synchronous Data Link Control (SDLC)

Answer 22

• This is a mainframe protocol developed by IBM in 1972 that is
  still in use today
• Each SDLC frame begins and ends with a special bit pattern
  (01111110), known as the flag
• The address field identifies the destination
• The control field identifies the kind of frame that is being
  transmitted

Question 23

Synchronous Data Link Control (SDLC) Continued

Answer 23

• An information frame is used for the transfer and reception of
  messages, frame numbering of contiguous frames, and the like
• A supervisory frame is used to transmit acknowledgments and
  negative acknowledgment
• The message field is of variable length and is the user’s message
• The frame check sequence field is a 32-bit CRC code

Question 24

High-Level Data Link Control (HDLC)

Answer 24

• This is a formal standard developed by the ISO often used in WANs
  o HDLC is essentially the same as SDLC, except that the address and control fields can be longer
• HDLC also has several additional benefits:
  o A larger sliding window for continuous ARQ
  o It uses a controlled-access media access protocol

Question 25

Ethernet

Answer 25

* It uses a contention media access protocol * The frame starts with a 7-byte preamble * This is followed by a start of frame delimiter

Question 26

Ethernet (Continued)

Answer 26

* The destination address specifies the receiver * The source address specifies the sender * The VLAN tag field is an optional 4-byte address field used by virtual LANs * The length indicates the length in 8-bit bytes of the message portion * The DSAP and SSAP are used to pass control information between the sender and the receiver

Question 27

Point-to-Point Protocol (PPP)

Answer 27

* This was developed in the early 1990s and is often used in WANs * It is designed to transfer data over a point-to-point circuit but provides an address so that it can be used on multipoint circuits * The frame starts with a flag and has a 1-byte address * The control field is typically not used

Question 28

Point-to-Point Protocol (PPP) Continued

Answer 28

* The protocol field indicates what type of data packet the frame contains * The data field is variable in length and may be up to 1,500 bytes * The frame check sequence is usually a CRC-16 but can be a CRC-32 * The frame ends with a flag

Question 29

Transmission Efficiency

Answer 29

* One objective of a data communication network is to move the highest possible volume of accurate information through the network * Transmission efficiency is defined as the total number of information bits divided by the total bits in transmission * We can improve efficiency by reducing the number of overhead bits in each message or by increasing the number of information bits

Question 30

Transmission Efficiency (Continued)

Answer 30

* The general rule is that the larger the message field, the more efficient the protocol * In designing a protocol, there is a trade-off between large and small frames * Efficiency versus retransmission when there are errors * Throughput is the total number of information bits received per second, taking into account the overhead bits and the need to retransmit frames containing errors