Chapter 4 Flashcards
What is the Data Link Layer?
responsible for moving a message from one computer or network device to the next computer or network device in the overall path from the sender or receiver. It controls the way messages are sent on the physical media.
What is a Data Link Protocol?
determines who can transmit at what time, where a message begins and ends, and how a receiver recognizes and corrects a transmission error.
What are the two main functions or sublayers of the Data Link?
- logical link control [LLC] sublayer
2. media access control [MAC] sublayer)
What does the Logical Link Control do?
The data link layer’s connection to the network layer above it. At the sending computer, the LLC sublayer software is responsible for communicating with the network layer software (e.g., Internet Protocol (IP)) and for taking the network layer Protocol Data Unit (PDU)—usually an IP packet—and surrounding it with a data link layer PDU—often an Ethernet frame. At the receiving computer, the LLC sublayer software removes the data link layer PDU and passes the message it contains (usually an IP packet) to the network layer software.
What does the Media Access Control (MAC) sublayer do?
controls the physical hardware. The MAC sublayer software at the sending computer controls how and when the physical layer converts bits into the physical symbols that are sent down the circuit. At the receiving computer, the MAC sublayer software takes the data link layer PDU from the LLC sublayer, converts it into a stream of bits, and controls when the physical layer actually transmits the bits over the circuit. At the receiving computer, the MAC sublayer receives a stream of bits from the physical layer and translates it into a coherent PDU, ensures that no errors have occurred in transmission, and passes the data link layer PDU to the LLC sublayer.
What are the three functions of a Data Link Protocol?
Controls when computers transmit (media access control)
Detects and corrects transmission errors (error control)
Identifies the start and end of a message by using a PDU (message delineation)
What are two fundamental approaches to media access control?
contention and controlled access
What is contention?
computers wait until the circuit is free (i.e., no other computers are transmitting) and then transmit whenever they have data to send. Contention is commonly used in Ethernet—Local Area Networks (LANs).
What is Controlled Access?
one device controls the circuit and determines which clients can transmit at what time. There are two commonly used controlled access techniques: access requests and polling.
What is Access Request?
client computers that want to transmit, send a request to transmit to the device that is controlling the circuit (e.g., the wireless access point). The controlling device grants permission for one computer at a time to transmit. When one computer has permission to transmit, all other computers wait until that computer has finished, and then, if they have something to transmit, they use a contention technique to send an access request.
What is Polling?
the process of sending a signal to a client computer that permits it to transmit. With polling, the clients store all messages that need to be transmitted. Periodically, the controlling device (e.g., a wireless access point) polls the client to see if it has data to send. If the client has data to send, it does so. If the client has no data to send, it responds negatively, and the controller asks another client if it has data to send.
What are some of the main sources of error?
White noise, Impulse noise, cross-talk, echoes, attenuation, intermodulation noise
What are three methods for error detection?
Parity Checking, Check sum, Cyclic Redundancy Check
