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Faculty of Engineering, Computer and Mathematical Sciences, University of Adelaide, 2003
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The Data Link Layer
Our goals:
● Understand principles behind data
link layer services:
– Error detection, correction
– Sharing a broadcast channel:
multiple access
– Link layer addressing
– Reliable data transfer, flow control:
done!
● Instantiation and implementation of
various link layer technologies
Overview:
● Link layer services
● Error detection, correction
● Multiple access protocols and LANs
● Link layer addressing, ARP
● Specific link layer technologies:
– Ethernet
– hubs, bridges, switches
– IEEE 802.11 LANs
– PPP
– ATM
Faculty of Engineering, Computer and Mathematical Sciences, University of Adelaide, 2003
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DLL: Context
Faculty of Engineering, Computer and Mathematical Sciences, University of Adelaide, 2003
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One Link
● Two physically connected devices:
– host-router, router-router, host-host
● Unit of data: frame
application
transport
network
link
physical
network
link
physical
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frame
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protocol
adapter card
Faculty of Engineering, Computer and Mathematical Sciences, University of Adelaide, 2003
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Link Layer Services
● Framing, link access:
– Encapsulate datagram into frame, adding header, trailer
– Implement channel access if shared medium,
– ‘physical addresses’ used in frame headers to identify source, dest
● different from IP address!
● Reliable delivery between two physically connected devices:
– We learned how to do this already (Remember TCP?)
– Seldom used on low bit error link (fiber, some twisted pair)
– Wireless links: high error rates
● Q: why both link-level and end-end reliability?
Faculty of Engineering, Computer and Mathematical Sciences, University of Adelaide, 2003
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Link Layer Services (Cont.)
● Flow Control:
– Pacing between sender and receiver(s)
● Error Detection:
– Errors caused by signal attenuation, noise.
● Every link will have errors
– Receiver detects presence of errors:
● signals sender for retransmission or drops frame
● Error Correction:
– Receiver identifies and corrects bit error(s) without resorting to
retransmission
– Also called Forward Error Control
Faculty of Engineering, Computer and Mathematical Sciences, University of Adelaide, 2003
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Implementation
● Implemented in “adapter”
– e.g., PCMCIA card, Ethernet card
– Typically includes: RAM, DSP chips, host bus interface, and link interface
application
transport
network
link
physical
network
link
physical
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frame
phys. link
data link
protocol
adapter card
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