Term
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Definition
| responsible for moving messages from one device to another, controls the way messages are on media, Organizes the bits from physical layer into coherent messages |
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Term
| Major functions of data link layer protocol |
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Definition
*Media Access Control
*Error Control
*message delineation |
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Term
| What is Media Access Control? |
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Definition
| controlling when and what a computer transmit. Important for half duplex links and multipoint configuration when they're on a shared circuit |
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Term
| Media Access Control Approaches |
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Definition
| Controlled Access and contention based access |
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Term
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Definition
A Media Access Control in Data Link Layer. Controlling access to shared resources and acting like a stop light. Used by mainframes and determines which circuits have access to the mainframe at a given time. Ex. Polling
*BEST FOR LARGE NETWORKS, OVER 20 COMPUTERS |
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Term
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Definition
| A Controlled Access method. Only process transmission only if asked and/or permitted. So client has to say yes to send data. There's Roll Call polling and Hub (Token) Polling |
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Term
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Definition
A Media Access Control in the Data Link Layer. Transmit whenever the circuit is free. Can have collisions when more than 1 computer is trying to transmit at the same time.
*Used By Ethernet LANs
*Problematic in Heavy usage Networks
*BEST FOR SMALL NETWORK, less than 20 computers |
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Term
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Definition
Done in Data Link Layer (layer 2). Handles network errors caused by problems in transmission.
* Can be network errors or human errors.
*Can be corrupted (data changed)or lost (data can't be found) |
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Term
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Definition
*Error rate is 1 bit n n bits transmitted (1 in 500k) while
*Burst is how many bits corrupted at the same time and errors are not uniformly distributed. |
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Term
| Sources of Errors: line outages |
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Definition
| *Line Outages - faulty equipment, storms, accidents |
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Term
| Sources of Error: White Noise |
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Definition
Movement of electrons/heat
Prevent by increasing signal strength |
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Term
| Sources of Error:Impulse Noise |
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Definition
electricity surges
Prevent by shielding or moving the wires |
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Term
| Sources of Error: Cross talk |
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Definition
wires too close together, guard bands are too small
Prevent by moving wires or increasing guard bands |
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Term
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Definition
poor connection
Prevent by fixing connection, tuning equipment |
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Term
| Sources of Error:Attenuation |
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Definition
signal fades over distance
prevent by using repeaters or amplifiers |
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Term
| Error Detection Techniques |
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Definition
| Parity Checks, checksum and Cyclic Redundancy Check |
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Term
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Definition
| adding a bit to each character and catches error if things don't add up, only detects 50% |
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Term
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Definition
a checksum (usually a byte) is added at the end of a message,
*95% effecitve
*Add decimal values of each character message, divide sum by 255 and the remainder is the checksum value |
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Term
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Definition
*P/G = Q + R/G
*P = message as a long binary #
*G = a fixed number
*Q = quotient (whole #)
Most powerful and most common, detects ~100% of errors
*MOST POWERFUL AND MOST COMMON |
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Term
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Definition
*Can transmitted - Automatic Repeat Request (ARQ)
*Or Forward Error Correction: correcting without retransmission |
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Term
| Data Link Protocols Classification |
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Definition
*Asynchronous Transmission
*Synchronous Transmission |
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Term
| Data Link Protocols Differ By? |
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Definition
| Message Delineation, Frame Length, Frame Field Structure |
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Term
| Asynchronous Transmission |
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Definition
Called Start/Stop Transmission
*Used on point-to-point asynchronous circuits
*for file transfer protocols |
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Term
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Definition
Data sent in large block = called a frame or packet
*includes addressing information
*includes a series of Synchronization character
*Main Problem is transparency (misinterpretating the ends of messages ) which an be solved by bit stuffing, adding zeros |
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Term
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Definition
Most widely used LAN protocol,
*data link layer
*uses contention based media access control
*byte count data link layer protocol
*no transparency problem
*error correction is optional |
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Term
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Definition
*objective of network: to move as many bits as possible with min errors aka higher efficiency and lower costs
= Total # of info bits to be transmitted/ total # of bits transmitted
*the bigger the message length, the better the efficiency |
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Term
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Definition
| More accurate than Transmission but difficult to calculate |
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Term
| Transmission Rate of Information Bits |
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Definition
= Number of info bits accepted / total time required to get the bits
or
* K(M-C)(1-P)/ (M/R) + T
k= bit/character
M= Packet length in Characters
C= avg # of non-info characters/block
P = probability that block will require retransmission
R = Data transmission rate in Char/second
T = time between blocks in sec (propagation time + turnaround time, aka reclocking time) |
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Term
| Transmission Rate of Information Bits ex. K=7 bits/character, M = 400 char/block, R= 4.8 Kb/s, C = 10 char/block, P = 1%, T = 25 ms |
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Definition
TRIB = K(M-C)(1-P)/(M/R) + T
= 7(400-10)(1-.01)/(400/600)+.025
=3.908 KB/S |
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