Topic 13 Networks and telecommunications

connected in various ways in order to communicate and share

resources
Usually, the connections between computers in a network are made using physical wires or cables
However, some connections are wireless, using radio waves or infrared signals

on a network
Data transfer rate The speed with which

data is moved from one place on a network to another
Data transfer rate is a key issue in computer networks

world of computing called the client/server model
Figure 15.1 Client/Server interaction

multiple users on a network
Web server A computer dedicated to

responding to requests (from the browser client) for web pages

a relatively small number of machines in a relatively close

geographical area

administer LANs
Ring topology A configuration that connects all nodes

in a closed loop on which messages travel in one direction
Star topology A configuration that centers around one node to which all others are connected and through which all messages are sent
Bus topology All nodes are connected to a single communication line that carries messages in both directions

industry standard for local-area networks
Figure 15.2 Various network topologies
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two or more local-area networks over a potentially large geographic

distance
Often one particular node on a LAN is set up to serve as a gateway to handle all communication going between that LAN and other networks
Communication between networks is called internetworking
The Internet, as we know it today, is essentially the ultimate wide-area network, spanning the entire globe

been developed in and around large cities

or company owns the Internet or even controls it entirely.

As a wide-area network, it is made up of many smaller networks. These smaller networks are often owned and managed by a person or organization. The Internet, then, is really defined by how connections can be made between these networks.

to create a wide-area network

Internet traffic
These networks are provided by companies such as AT&T,

GTE, and IBM
Internet service provider (ISP) A company that provides other companies or individuals with access to the Internet

to connect a home computer to the Internet
A phone modem

converts computer data into an analog audio signal for transfer over a telephone line, and then a modem at the destination converts it back again into data
A digital subscriber line (DSL) uses regular copper phone lines to transfer digital data to and from the phone company’s central office
A cable modem uses the same line that your cable TV signals come in on to transfer the data back and forth

than 128 bits per second
DSL connections and cable modems are

broadband connections
The speed for downloads (getting data from the Internet to your home computer) may not be the same as uploads (sending data from your home computer to the Internet)

shared communication line, messages are divided into fixed-sized, numbered packets
Network

devices called routers are used to direct packets between networks

Figure 15.4 Messages sent by packet switching

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by a particular commercial vendor
One system couldn’t communicate with another,

leading to the need for
Interoperability The ability of software and hardware on multiple machines and from multiple commercial vendors to communicate
Leading to
Open systems Systems based on a common model of network architecture and a suite of protocols used in its implementation

Systems Interconnection (OSI) Reference Model
Each layer deals with a particular

aspect of network communication

Figure 15.5 The layers of the OSI Reference Model

that govern the communication between communicating peers
set of valid messages
meaning

of each message
Network protocols are layered such that each one relies on the protocols that underlie it
Sometimes referred to as a protocol stack

Figure 15.6 Layering of key network protocols

packets, hands them off to the IP software for delivery,

and then orders and reassembles the packets at their destination
IP stands for Internet Protocol
IP software deals with the routing of packets through the maze of interconnected networks to their final destination

to TCP
The main difference is that TCP is highly reliable,

at the cost of decreased performance, while UDP is less reliable, but generally faster

TCP/IP protocol suite
Simple Mail Transfer Protocol (SMTP)
File Transfer Protocol (FTP)
Telnet
Hyper

Text Transfer Protocol (http)

is the concept of a file’s MIME type
MIME stands for

Multipurpose Internet Mail Extension
Based on a document’s MIME type, an application program can decide how to deal with the data it is given

special gateway to a network, protecting it from inappropriate access
Filters

the network traffic that comes in, checking the validity of the messages as much as possible and perhaps denying some messages altogether
Enforces an organization’s access control policy

on the Internet
For example
matisse.csc.villanova.edu
condor.develocorp.com

address
For example
205.39.145.18

specifies a specific network
host number, which specifies a particular machine

in that network

Figure 15.9 An IP address is stored in four bytes

by the domain name
csc.villanova.edu is the domain name
A domain name

is separated into two or more sections that specify the organization, and possibly a subset of an organization, of which the computer is a part
Two organizations can have a computer named the same thing because the domain name makes it clear which one is being referred to

called its top-level domain (TLD) name
Figure 15.10 Top-level domains, including

some relatively new ones

States use a top-level domain that corresponds to their two-letter

country codes

Figure 15.11 Some of the top-level domain names based on country codes

to translate hostnames into numeric IP addresses
DNS is an example

of a distributed database
If that server can resolve the hostname, it does so
If not, that server asks another domain name server