The OSI Model

A hub in an network


A Switch in an network


Ethernet Networking

• Ethernet is a contention media access method that allows all hosts on a network to share the same bandwidth of a link.
• Ethernet is popular because it’s readily scalable, meaning that it’s comparatively easy to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet, into an existing network infrastructure.
• It’s also relatively simple to implement in the first place, and with it, troubleshooting is reasonably straightforward.

Ethernet Collision Detection
CSMA/CD


Half and Full Duplex

Half-duplex Ethernet is defined in the original 802.3 Ethernet; Cisco says it uses only one wire pair with a digital signal running in both directions on the wire.

But full-duplex Ethernet uses two pairs of wires instead of one wire pair like half duplex. And full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device.

Full-duplex Ethernet can be used in three situations:

• With a connection from a switch to a host
• With a connection from a switch to a switch
• With a connection from a host to a host using a crossover cable

Internetworking Basics

How would you say the PC named Bob communicates with the PC named Sally?


Switches can replace the hub, breaking up collision domains.



Keep in mind that the hub used in the figure just extended the one collision domain from the switch port.


Here’s a list of some of the things that commonly cause LAN traffic congestion:

• Too many hosts in a broadcast domain
• Broadcast storms
• Multicasting
• Low bandwidth
• Adding hubs for connectivity to the network
• A bunch of ARP or IPX traffic (IPX is a Novell protocol that is like IP, but really, really chatty. Typically not used in today’s networks.)

Routers create an internetwork.


There are two advantages of using routers in your network:

• They don’t forward broadcasts by default.
• They can filter the network based on layer 3 (Network layer) information (e.g., IP address).

Four router functions in your network can be listed as follows:

• Packet switching
• Packet filtering
• Internetwork communication
• Path selection

Internetworking devices


Switched networks creating an internetwork

The Layered Approach

• A reference model is a conceptual blueprint of how communications should take place.
• It addresses all the processes required for effective communication and divides these processes into logical groupings called layers.
• When a communication system is designed in this manner, it’s known as layered architecture.

The OSI Model

The OSI isn’t a physical model. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network.

It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.


The upper layers



The lower layers



The Layer Functions



Connection-Oriented Communication



Windowing


Network Layer Routing Table used in a router



Router in an internetwork



Data Link Layer