Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
10 Cards in this Set
- Front
- Back
|
Data Link Layer Responsibilities
|
-Responsible for moving messages from one device to another
-Controls the way messages are sent on media -Organizes physical layer bit streams into coherent messages for the network layer |
|
|
Data Link Layer Major Functions
|
-Media Access Control
-Controlling when computers transmit -Error Control -Detecting and correcting transmission errors -Message Delineation -Identifying the beginning and end of a message |
|
|
Media Access Controll (MAC)
|
-Controlling when and what computer transmit
-Important when more than one computer wants to send data at the same time over the same, shared circuit -Point-to-point half duplex links computers take turns -Multipoint configurations Ensure that no two computers attempt to transmit data at the same time Two possible approaches -Controlled access -Contention based access |
|
|
Controlled Access
|
-Controlling access to shared resources
-Acts like a stop light -Commonly used by mainframes (or its front end processor) -Determines which circuits have access to mainframe at a given time -Also used by some LAN protocols; Token ring, FDDI -Major controlled access methods: X-ON/X-OFF and Polling |
|
|
Polling
|
-Process of transmitting to a client only if asked and/or permitted
-Client stores the information to be transmitted Server (periodically) polls the client if it has data to send -Client, if it has any, sends the data -If no data to send, client responds negatively, and server asks the next client -Types of polling: Roll call polling, Hub polling (also called token passing) |
|
|
Contention
|
-Transmit whenever the circuit is free
Collisions: Occurs when more than one computer transmitting at the same time -Need to determine which computer is allowed to transmit first after the collision -Used commonly in Ethernet LANs -Problematic in heavy usage networks |
|
|
Transmission Errors
|
-Line noise and distortion – major cause
-More likely on electrical media -Undesirable electrical signal Introduced by equipment and natural disturbances -Degrades performance of a circuit Manifestation: Extra bits, “flipped” bits,Missing bits |
|
|
Error Detection Methods
|
-Parity checks
-Checksum -Cyclic Redundancy Check (CRC) |
|
|
Parity Checking
|
-A single bit added to each character
-Even parity: number of 1’s remains even -Odd parity: number of 1’s remains odd -Receiving end recalculates parity bit -If one bit has been transmitted in error the received parity bit will differ from the recalculated one Simple, but doesn’t catch all errors If two (or an even number of) bits have been transmitted in error at the same time, the parity check appears to be correct -Detects about 50% of errors |
|
|
Cyclic Redundancy Check (CRC)
|
Formula: P / G = Q + R / G
P- G- Q- R- G- |
