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Chapter 9��Introduction to Data Link Layer

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Objective

defining the concept of links and nodes

the services provided by the data-link layer
point-to-point and broadcast links
two sub-layers at the data-link layer

link-layer addressing

rationale behind the existence of an addressing mechanism at the data-link layer
three types of link-layer addresses
Address Resolution Protocol (ARP)

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9.1 Introduction

Fig. 9.1 Communication at the data-link layer

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9.1.1 Nodes and Link

Communication at the data-link layer is node-to-node.
LANs and WANs are connected by routers.
It is customary to refer to the two end hosts and the routers as nodes and the networks in between as links.

Fig. 9.2 Nodes and Links

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9.1.2 Services

The data-link layer provides services to the network layer; it receives services from the physical layer.

Framing
Flow Control
Error control
Congestion Control

Fig. 9.3 A communication with only three nodes

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9.1.3 Two Categories of Links

a point-to-point link.
a broadcast link.

9.1.4 Two Sublayer

data link control (DLC)
media access control (MAC).

Fig. 9.4 Dividing the data-link layer into two sublayers

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9.2 Link-Layer Addressing

Link layer address

Link address
Physical address
MAC address

Fig. 9.5 IP addresses and link-layer addresses in a small internet

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9.2.1 Three Types of Addresses

unicast,
multicast,
and broadcast

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9.2.2 Address Resolution Protocol (ARP)

is helpful in moving a frame through a link; we need the link-layer address of the next node

Fig. 9.6 Position of ARP in TCP/IP protocol suite

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Fig. 9.7 ARP operation

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Caching

The storing of information in a small, fast memory

Packet Format

Fig. 9.8 ARP Packet

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Ex. 9.4

Fig. 9.9 Example 9.4

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Ex. 9.4

Fig. 9.9 Example 9.4

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9.2.3 An Example of Communication

Fig. 9.10 The internet for our example

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Activities at Alice’s Site

Fig. 9.11 Flow of packets at Alice’s computer

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Activities at Router R1

Fig. 9.12 Flow of activities at router R1

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Activities at Router R2

Fig. 9.13 Flow of activities at router R2

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Activities at Bob’s Site

Fig. 9.14 Flow of activities at Bob’s site