Networks, Cables, OSI, and TCP Models

Today you will learn about the following:

• Network devices and diagrams
• The OSI and TCP models
• Cables and media
• Connecting to a router

This module maps to the following ICND1 syllabus requirements:

• Recognise the purpose and functions of various network devices, such as routers,
• switches, bridges, and hubs
• Select the components required to meet a given network specification
• Identify common applications and their impact on the network
• Describe the purpose and basic operation of the protocols in the OSI and TCP/IP models
• Predict the data flow between two hosts across a network
• Identify the appropriate media, cables, ports, and connectors to connect Cisco network
• devices to other network devices and hosts in a LAN

 

Network Devices

As a network engineer, you will be using a range of network cables and other media. You need
to know which cables will work with which devices and interfaces for WAN, LAN, and
management ports. Much of the information will serve as a review if you have studied the
CompTIA Network+ before the CCNA (which I recommend).

Common Network Devices

Network Switches

Only a few years ago, networks were still pretty small. This meant that you could simply plug all
devices into a hub or a number of hubs. The hub’s job was to boost the signal on the network
cable, if required, and then pass out the data on the wire to every other device plugged in. The
problem with this, of course, is that the message was intended for only one network host, but
it would be sent to tens or hundreds of other hosts connected to other hubs on the network.
(Hubs and switching technology will be covered in more detail in the next module.)
Network switches are a more intelligent version of hubs. Switches use Content Addressable
Memory (CAM) and therefore have the ability to remember which device is plugged into which
port. Cisco manufactures switch models which are designed to work in small offices and all the
way up to large enterprise networks consisting of thousands of devices. We will explore this in
more detail later, but, basically, switches operate by using the device’s MAC addresses (known
as Layer 2) and IP addresses (known as Layer 3), or they can perform more complex tasks, such
as processing lists of permit/deny traffic or protocols and port numbers (known as Layer 4), or a
combination of all these layers and more. We will cover what comprises these layers and their
functions later in this module.
Early versions of switches were referred to as network bridges. Bridges examined the source
ports and MAC addresses of frames in order to build a table and make forwarding decisions.
The tables were typically accessed via software, whereas switches used hardware (i.e.,
Application Specific Integreated Chips, or ASICs) to access a CAM table (more on this later).
Therefore, a switch can be thought of as a multiport bridge.
Using a switch (see Figure 1.1) allows you to divide your network into smaller, more
manageable sections (known as segments). This in turn allows the teams who work inside your
company, such as human resources, finance, legal, etc., to work on the same section of the
network at the same time, which is useful because the devices will spend most of their time
communicating with each other.

 

Each device will connect to an interface on the switch, which is referred to as a port. Common
network port speeds are 100Mbps and 1000Mbps (usually referred to as 1Gbps). There are
often fibre ports you can use to connect a switch to another switch. Each switch features
management ports, which you can connect to in order to perform an initial configuration and
gain general access for maintenance over the network.

Figure 1.2 below shows a close-up of a Cisco 2960 switch. Several models of the 2960 are
available to meet the needs of a small- to medium-sized business. 

You can also use IP telephones with the switches and, even better, the switch ports can provide
power to these telephones (using power over Ethernet (PoE) interfaces). The basic network
switch will be used to:

• Connect network devices such as printers and PCs
•  Give access to network servers and routers
• Segment the network with VLANs