Ch 5. Link layer and Local Area Networks

Myungchul Kim

mckim@icu.ac.kr

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A transmitting node encapsulates the datagram in a link-layer frame and transmits the frame into the link; and a receiving node receives the frame and extracts the datagram.

Error detection, retransmission, flow control, and random access A single link in the path A link-layer protocol includes

– Framing– Link access: multiple access problem– Reliable delivery– Flow control: frame buffering capacity– Error detection– Error correction– Half-duplex and full-duplex

Data link layer

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Adaptors: network interface cards (NICs) Fig 5.3

The link interface is responsible for implementing the link-layer protocol

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Point-to-point link: PPP, HDLC Broadcast link: multiple sending and receiving nodes all connected

to the same, single, shared broadcast channel. Fig 5.9

Multiple access protocol

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Packet collisions: channel partitioning protocols, random access protocols, and taking-turns protocols.

Channel partitioning protocols– TDM, FDM– Fig 5.10

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Code Division Multiple Access (CDMA)– Assigns a different code to each node– Allows different nodes to transmit simultaneously and yet have t

heir respective receivers correctly receive a sender’s encoded data bits in spite of interfering transmissions by other node.

– Partitions the codespace– Issues: 1. codes must be carefully chosen, 2. the received signal

strengths from various senders at a receiver are the same.

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Fig 5.11

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Random access protocols: slotted ALOHA, ALOHA, CSMA

Slotted ALOHA– Page 440.– Fig 5.13

– At best only 37 percent of the slots do useful work.

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CSMA– Listen before speaking: carrier sensing– If someone else begins talking at the same time, stop talking: col

lision detection.– CSMA vs CSMA/CD– The longer this propagation delay, the larger the chance that a c

arrier-sensing node is not yet able to sense a transmission that has already begun at another node in the network.

– When a node performs collision detection, it will cease transmission as soon as it detects a collision.

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Fig 5.15

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Fig 5.16

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Taking-turns protocol– Polling protocol– Token-passing protocol