Chapter 4

Question 1

3 functions of data link layer?

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

Contention

Answer 2

Computers wait until the circuit is free (no other computers are transmitting) and then transmit whenever they have data to send

Commonly used in Ethernet

Question 3

Controlled access

Answer 3

One device controls the circuit and determines which clients can transmit at what time.

Question 4

Access Request

Answer 4

Controlled access technique where the 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 5

Polling

Answer 5

The process of sending a signal to a client computer that gives it permission to transmit. Periodically the controlling device (ex a wireless access point) polls the client to see if it has data to send.

Several types: roll-call polling, hub polling (token passing)

Question 6

Roll-call polling

Answer 6

The controller works consecutively through a list of clients, first polling client1, then client 2, and so on, until all are polled

Question 7

Hub polling (token passing)

Answer 7

One device starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next.

Question 8

Error prevention

Answer 8

Shielding (protecting wires by covering them with an insulating coating)

Moving cables away from sources of noise

Using repeaters or amplifiers

Question 9

Error detection

Answer 9

Parity checking, checksum, cyclic redundancy checking

Question 10

Error correction

Answer 10

Retransmission

Forward error correction (satellites)

Question 11

Asynchronous transmission

Answer 11

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.

Each character is transmitted independently of all other characters. To separate the characters and synchronize transmission, a start bit and stop bit are put on the front and back of each individual character.

Question 12

Synchronous Transmission

Answer 12

All the letters or data in one group of data are transmitted at one time as a block of data called a frame. The start and end of each frame sometimes are established by adding synchronization characters (SYN) to the start of the frame.