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Data link layer
Chapter 5: 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

5: DataLink Layer

5-1

Link Layer
❒
❒
❒
❒
❒

5.1 Introduction and services 5.2 Error detection and correction
5.3Multiple access protocols 5.4 Link-Layer
Addressing
5.5 Ethernet

5.6 Hubs and switches
❒ 5.7 PPP
❒ 5.8 Link Virtualization:
ATM and MPLS
❒

5: DataLink Layer

5-2

Link Layer: Introduction
Some terminology:

“link”

❒ hosts and routers are nodes
❒ communication channels that

connect adjacent nodes along communication path are links
❍
❍
❍

wired links wireless links
LANs

❒ layer-2 packet is a frame,

encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5: DataLink Layer

5-3

Link layer: context
❒

Datagram transferred by different link protocols over different links:
❍

❒

e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link

Each link protocol provides different services e.g., may lights,
Traffic or may not provide Airportrdt over link control, Platform scheduling,
❍

transportation analogy
❒ trip from Princeton to

Lausanne
❍ limo: Princeton to JFK
❍ plane: JFK to Geneva
❍ train: Geneva to Lausanne

tourist = datagram
❒ transport segment = communication link
❒ transportation access = link layer protocol
❒ travel agent = routing algorithm ❒

5: DataLink Layer

5-4

Link Layer Services
❒

Framing, link access:
❍
❍
❍

❒

encapsulate datagram into frame, adding header, trailer channel access if shared medium
“MAC” addresses used in frame headers to identify source, dest
• different from IP
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