Data Link Layer - CH 4 (EXAM 1) Flashcards
What is the DL layer?
moves messages from one device to another on a binary level
controls the way messages are sent on device
What are the 3 major functions of the DL layer?
media access control
error control
message delineation (start and end)
What is media access control?
controls which device transmits and when
What are the 2 types of MAC?
contention - small networks (exponential)
controlled - large networks with high traffic (linear)
important on multipoint and half-duplex circuits
What is contention access?
transmits when the circuit is available
no centralized limit
Ethernet LAN
collision occurs when devices transmit at the same time
devices must be polite
What is controlled access?
wireless LAN
access request, polling (roll call and hub polling with tokens)
What are the 2 types of error control?
corrupted and lost data
What should networks be designed with?
error prevention
error detection
error correction
What are some sources of network errors?
electrical signal disruption
missing bits
flipped bits
noise and distortion
White noise
cause: electron movement
prevent: increase signal strength
Impulse noise
cause: sudden increase in electricity
prevent: shield/move wires
Cross-talk
cause: wires too close together
prevent: move the wires
Echo
cause: poor connection
prevent: fix connection
Attenuation
cause: gradual decrease in signal over distance
prevent: repeaters/amplifiers
Intermodulation noise
cause: circuit signals combine
prevent: move wires
How can you detect errors?
adding a check value to the message
through formula at source and destination to ensure a match
parity-check - based on the number of 1’s in the message
reduces 50% of errors
checksum - algorithm
detects 95% of errors
cyclic redundancy check - uses remainder as check value
more capacity, more data
How can you correct errors?
retransmission (backward error correction) - common
forward error correction - device corrects messages without retransmission
stop and wait (acknowledge/timeout)
continuous
What are the DL protocols?
async serial transmission
synchronous transmission - data sent in large blocks (frame)
SDLC (bit stuffing)
Ethernet (Ethernet II frame)
Point to point (PPP)
Formula for transmission efficiency
# of information (bits) / # of info + overhead bits
How can efficiency increase?
increase data bits
decrease overhead bits
What is throughput?
total # of information bits received per second
after overhead bit count and correcting errors
