Chapter 9

Communications and Networks

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By: M.Hazazi

Communications

It is the process of sharing data, programs, and information between two or more computers.

Numerous applications depend on communication systems:

E-mail

Instant messaging (IM)

Internet telephone

Electronic commerce

Connectivity

- Connectivity is a concept related to using computer networks to link people and resources.

- Example: Microcomputers, minicomputers, and mainframes can all be connected

Wireless revolution

- Use of mobile or wireless devices

- The revolution is the support of more than just the wireless telephone

- Many devices can and will connect to one another without any physical connection

Communication Systems

- Electronic systems that transmit data from one location to another.

- Communication systems can be wired or wireless.

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Communication Systems have 4 basic elements

- Sending and receiving devices \Computer or a specialized communication device

- Communication channel \ Carries the message

- Connection devices \ Act as an interface between sending and receiving devices; convert

outgoing messages into packets that can travel across the communication channel.

- Data transmission specifications \ Rules and procedures that coordinate the sending and

receiving devices

Communication Channels

Channels carry data from one computer to another

Two categories of communication channels

1. -Physical connection

Use a solid medium to connect sending and receiving devices.

Include:

a) twisted-pair cable:

a. Telephone lines

b. Ethernet cables

Used in networks and to connect a variety of components to the system unit.

Pairs of copper wire twisted together of twisted pair cable; slowest; being phased out

by more advanced and reliable media

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b) Coaxial cable

a. High frequency transmission

b. Single solid-copper core ; 80 times transmission of twisted pair; television and

computer networks.

c) Fiber-optic cable

a. Transmit data as pulses of light through glass

b. Higher, faster and more reliable

c. Fiber optic – 26,000 times capacity of twisted pair cable; more secure and reliable; best

over limited distances; lighter, more reliable, and less expensive than coaxial cable

2. Wireless Connections

Wireless connections do not use a solid substance to connect; uses the air itself

Types of wireless connections are:

Radio frequency (RF):

uses radio signals

Wi-Fi (wireless fidelity) 802.11

Bluetooth

WiMax (Worldwide Interoperability for Microwave Access)

Microwave:

uses high frequency radio waves

Line of sight

Good medium for sending data between buildings.

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Satellite:

uses satellites orbiting about 35,ooo km above the earth as microwave relay stations.

Uplink refers to send and receive data to satellite

Downlink refers to receiving data from a Satellite

Important application: GPS (Global positioning system) use a network of 24 satellites

owned and managed by the Defense Department which continuously sends location

information to earth

Infrared:

Light waves used over Short distances

line of sight communication

Sending and receiving devices must be in clear view of one another

Broadcast radio:

Uses towers called transceivers

Communication Channels: Summary

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Connection Devices

Types of signals: Analog & Digital

Sent analog – telephone signals; continuous electronic wave

Sent digital – computer signals; presence or absence of an electronic pulse; on/off

Modem: a short word for modulator-demodulator

Modulation: converting signal from digital to analog

Demodulation: converting signal from analog to digital

The speed of transmitting data measured in thousands of bits (kilobits) per second (Kbps).

The higher the speed, the faster you can send and receive information.

4 types of Modems:

Telephone modem:

- Can be either internal or external; connects computer directly to a telephone line.

DSL (digital subscriber line):

- Uses standard phone lines; external and uses either USB or Ethernet ports.

Cable modem:

- Uses the same coaxial cable as your television; uses either USB or Ethernet

Wireless modem:

- Small plug-in USB or Express Card provides high speed connectivity.

- Also known as WWAN (wireless wide are network modem)

- Signals are sent through the air

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Connection Services

Dial-Up services

Leased lines – T1

T3 and DS3 have higher capacity

These lines have replaced by Optical Carrier (OC) which is much faster.

Digital subscriber line (DSL) service:

Provided by telephone companies

Uses existing telephone lines to provide high-speed connections.

Uses existing phone lines

One type widely used is ADSL .

Cable service:

Provided by cable television companies.

Uses existing TV cable

faster than DSL

Satellite connection services:

Use satellite to provide wireless connection

Available almost anywhere using a satellite receiving disk.

Slower than DSL & cable modem

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Cellular Services:

Use 3G and 4G cellular networks

Provide wireless connectivity to the internet

Lower speed

Data Transmission:

Bandwidth

Bandwidth: is a measurement of the capacity of the channel (how much information can

move across the communication channel)

Categories:

1) Voice band also known as low bandwidth, used for telephone line.

2) Medium band used for special leased line to connect minicomputers and mainframes

and transmit data over long distance.

3) Broadband used for DSL , cable and satellite connections to the internet

4) Baseband used to connect local computers near to each other.

Protocols

Rules for exchanging data between computers

Standard for Internet: TCP/IP (Transmission Control Protocol / Internet Protocol)

Essential features of this protocol:

1- Identifying sending and receiving devices

2- Breaking information into small parts for transmission across the internet.

(IP address) Internet protocol address.

(DNS) domain name server \ converts text-based addresses to IP addresses

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Networks

A computer network is a communication system that connects two or more computers so

that they can exchange information and share resources

Node – any device connected to a network

Client – a node that requests and uses resources available from other nodes.

Server – a node that shares resources with other nodes; dedicated servers specialize in

performing specific tasks—could be an application server, communication server,

database server, file server, printer server, or Web server.

Directory server – a specialized server that manages resources such as user accounts for

an entire network.

Host – large centralized computer.

Switch – the center or central node for other nodes; previously done by something called a

hub.

Network interface cards (NIC) – connects the computer to a network

Network operating system (NOS) – software to control and coordinate activities

between computers on a network

Network administrator – a computer specialist; network administrator

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Network Types

Communications networks differ in geographical size. Three important types are:

LANs ( local area networks )

- Computers and devices linked in close proximity to each other

- Linked by cable

- Typically use a bus form of organization

- With gateways, LANs can be connected to other LANs or any other type of network. Ethernet is

one standard for connecting network nodes together.

Home Networks

- LANs are now being commonly used by individuals in home/apartments; allow different

computers to share resources including a common Internet connection.

- Home networking includes the following:

Radio frequency, telephone lines, electric wiring, cables, NIC/adapter cards.

Wireless LAN (WLAN) wireless local area network :

- All communications pass through the network’s centrally located wireless access point or base

station

- Access points are known as hotspot and use Wi-Fi technology.

Personal Area networks (PAN)

- works within a very small area around 30 feet

- Connects cell phones to headsets, PDAs to other PDAs, keyboards to cell phones, etc.

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Metropolitan area network (MANs)

- also known as a regional network; span distances up to 100 miles

- Network linking nodes and resources within the geographical bounds of a city

- Cellular phones can extend the reach of a MAN

wide area networks (WANs)

- Countrywide and worldwide networks

- Use microwave relays and satellites to reach users

- Internet is the widest WAN

Network Architecture

Describes how a network is arranged and how resources are coordinated and shared

Arrangement is called Topology

Types of network topology

1) Star: each device connected directly to a central network switch. Most common type

today.

2) Tree (hierarchical): a central node connected to subordinate nodes and so forth, forming

a treelike structure often used to share corporate wide date.

3) Mesh: newest, each node has two or more connecting nodes, data can be routed around

disrupted path. Used in wireless technology.

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Strategies:

Every network has a strategy or way of sharing information and resources.

Common strategies are:

1) Client/server (hierarchical) network:

- Central computer coordinate and supply services to other nodes; based on

specialization of nodes.

- Widely used in internet

- Handle very large networks

- Has Powerful management software

2) Peer-to-Peer network:

- Nodes have equal authority and act as both client and server

- Widely used to share movie , games and music over the internet

- Easy to setup and use

- Has low security control

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Organizational Networks

Internet Technology

Most organizations face the challenge of making different networks work together

effectively and securely.

Intranet:

- Private network within an organization that resembles the internet

- Uses browser ,web sites and web pages

- Provides information to employees (email addresses, telephone directories ...etc)

Extranet:

- Private network that connects organizations

- Used to allow suppliers and others access

- Purpose is to increase efficiency and reduce costs

Network Security

Firewalls

- Consist of hardware & software

- Control access to a company’s intranet

- Protects against external threats

- Use a special computer or software called proxy server

- Proxy server: Decides whether it is safe to let a file or a message pass into or out of the

organization’s network.

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Intrusion detection systems (NIDS)

- Work with firewalls to protect an organization network

- Use sophisticated statistical techniques to recognize and disable network attack

- Uses advanced pattern matching and heuristics

- Can recognize signs of a network attack and disable access before an intruder can do damage

Virtual private network (VPN)

- Secure private connection between a remote user and an organizations’ internal network.

- Dedicated line between a user’s home or laptop computer and a company server.

- The connection is encrypted