Ch 5. Link layer and Local Area Networks Myungchul Kim

2 o 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. o Error detection, retransmission, flow control, and random access o A single link in the path o 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

3 o Adaptors: network interface cards (NICs) o Fig 5.3 o The link interface is responsible for implementing the link-layer protocol

4 o Point-to-point link: PPP, HDLC o Broadcast link: multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. o Fig 5.9 Multiple access protocol

5 o Packet collisions: channel partitioning protocols, random access protocols, and taking-turns protocols. o Channel partitioning protocols – TDM, FDM – Fig 5.10

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

7 o Fig 5.11

8 o Random access protocols: slotted ALOHA, ALOHA, CSMA o Slotted ALOHA – Page 440. – Fig 5.13 – At best only 37 percent of the slots do useful work.

9 o CSMA – Listen before speaking: carrier sensing – If someone else begins talking at the same time, stop talking: collision detection. – CSMA vs CSMA/CD – The longer this propagation delay, the larger the chance that a carrier-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.

10 o Fig 5.15

11 o Fig 5.16

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