Chapter2_Communication

Posted on 2022-08-01 Edited on 2023-02-04 In Notes , ASCS Views: Disqus:

Networks Including the Internet

Networking Devices

Benefits of networking devices:

Devices, such as printers, can be shared.
Licences to run software on networks are often far cheaper than buying licences for an equivalent number of stand-alone computers.
Users can share files and data
Data and files can be backed up centrally at the end of each day
User can communicate using email and instant messaging
A network manager can oversee the network and, for example, apply access rights to certain files, or restrict access to external networks, such as the internet.
The employee’s computers are the clients
The server hosts the (shared) files
An employee can request a file from the server from any of the client computers
Several employee can access the same file at the same time.

LAN & WAN

LAN(Local area network): network limited to one building or site.

Device connected over a small geographical area
Uses a dedicated infrastructure// company-owned infrastructure

WAN(Wide area network): covers a wider distance.

A number of LANs are joined together using a modem or router.
Eg: ATMs
Make use of public communication networks: telephone lines, satellite.

Models

	Client-server	P2P
Properties:	· At least one computer used to be the “server” · The other computers are the client · Server provides services · Which may be requested by clients · Examples: Email server, print server, File server, Web server, Proxy server	· Each nodes provides and receive service · “look up computer” · Peers to participate as equals
Benefits:	1. Different servers provide different services 2. Central storage of all files 3. Administration of critical info is centralized, more secure Internet monitoring 4. Intranet capability 5. Clients can be less powerful machine and less expensive to buy 6. Saving resources on server 7. reduces the burden on clients 8. Creation of security 9. Users need user name and password to access network.	1. No server required 2. Additional computer can join easily(no requirement for authenticate user) 3. Each computer can provide and receive resources
Drawbacks:	1. Set-up cost 2. Require specific OS 3. All user depend on server	1. De-centralized, hard to administrator 2. Easily attacked
Reasons:	1. Large user-base 2. Access to network resource needs to be properly controlled 3. Need for network security 4. Data needs to be backed up	1. The network of user is fairly small 2. No need for robust security 3. Workstation-based application rather than server-based

Thin&Thick Clients

	Thin-client	Thick-client
Benefits:	1. Require few local resources 2. Server can offer protection 3. Easy to expand	1. Job can also be done without connection 2. Works under slow network 3. More customization 4. Store data on local resources such as HDD or SSD 5. More robust 6. Client have more control
Drawbacks:	1. Highly rely on server 2. Highly rely on good network 3. Start-up cost	1. Rely on local resources 2. Easily attacked 3. Synchronization 4. Data integrity issues 5. Need to update data and software individually 6. Less secure
Differences:	· Relies on a connection to a remote server or computer for it to work · Requires very few local resources · Relies on good, stable, and fast network connection for it to work · Data is stored on a remote server	· Can run some of the features of the software even when not connected to the server · Relies heavily on local resources (SSD, HDD, RAM, processing time) · More tolerant of a slow network connection ·Data is stored on local resources such as HDD and SSD.

Topologies

Star🌟

uses a central hub/switch and each computer/devices in connected to the hub/switch.

Hub: Packets will be send to every device
Switch: Packets will only be send to the node where the recipient address matches.

Packets has address of the recipient
Sender transmits packets directly to the server
Server reads the address and identifies where recipient is
Server transmit packets directly to the recipient
Server transmit packets only to the recipient

Pros

Less data collision
More secure
Node doesn’t effect others

Cons

Set-up cost
Central hub effects all

Bus 🚌

uses a sigle central cable to which all computers and devices are connected.

Packets pass through every node
Each node looks at each packet and determines whether or not the address of recipient in the package match the node address
If so, the node accept the package.
If not, the package is ignored.

Suitable for a small company or an office environment. (light traffic occurring)

Pros

Node doesn’t effect others
Easy to connect new nodes

Cons

Less secure
Heavy traffic
Main cable effects all

Mesh

Routing: by giving the nodes routing logic, so that data/packets is directed to its destination by the shortest route.
Flooding: simple send to all the nodes

Pros

Failed link doesn’t affect other parts
Secure

Cons

Hard to maintain
Need lots of cable

Hybrid

Pro

Could have all the pros above

Cons

Try to minimize the cost above

Cloud

a method of data storage where data is stored on offsite servers - the physical storage covers hundreds of servers in many locations.

Public cloud: a storage environment where the customer/client and cloud storage provider are different companies

Private cloud: a storage provided by a dedicated environment behind a company firewall. Customer/client and cloud storage provider are integrated and operate as a single entity.

Pros:

No need to carry an external storage device
Recovers data when hard disk failure happens
Almost unlimited storage capacity
Access data any time and place with internet connection

Cons:

Accessing problem under slow or unstable internet connection
Costs can be high
Limits with internet service provider (ISP)
Potential failure of the cloud storage company

Wired and Wireless networking

Wireless: Wi-Fi and Bluetooth.
- Spread spectrum frequency hopping: If a channel is already being used, it randomly picks another channel.
- Wi-Fi offers much faster data transfer rates, better range and better security.
Use of satellite: the curvature of the earth prevents the use of microwaves/radio waves transmitting data globally. To overcome this problem, we need to adopt satellite technology.

Wired

More reliable and stable network
Data transfer rate tend to be faster
Tends to be cheaper overall
Devices are not mobile
Lots of wires can lead to tripping hazards.

Wireless

It is easier to expand networks and is not necessary to connect devices using cables
Devices have increased mobility
Increased chance of interference from external sources
Data is less secure than with wired systems
Data transmission rate is slower than wired networks
Signals can be stopped by thick walls.
Different types of device can be connected at the same time

Twisted pair

Usage: Telephone, LANs
Capacity: Lower bandwidth than fibre optic
Larger and heavier cables than FO for similar data capacity

Fibre optic:

Transmits data as light; uses a glass/plastic thread to transmit data.

High-speed network Asynchronous Transfer mode (ATM) and long cable runs.
Faster data transmission
More stable
Greater bandwidth
Need less signal boosting/ can travel over longer distances
Greater security / more difficult to hack
Lighter in weight
Less interference in signal
Trained personnel needed to install
Expensive
Difficult to terminate
Break when bent
Only transmits data in one direction
If a fibre-optic cable connection fails, more services can be affected.

Copper cable: use of existing telephone network

Carries data as electrical signals and can consist of a twisted pair.

Less expensive overall
Easier to install because it is more flexible
Easier to make terminations using copper cabling
The expertise in use of copper cabling is more extensive

Radio waves

Carries data wirelessly, often known as Wi-Fi.
Carries data in the form of electromagnetic wave

Satellite

A communication device in Earth’s orbit that receives and transmits data.
Due to curvature of the earth.
Antenna to Satellite
Signal boosted by the satellite

Hardware for LAN

Network interface card(NIC):

Unique physical address – MAC address

Needed to allow a device to connect to a network

Wireless Access points(WAPs)

Allows a device to access LAN without a wired connection

Forms WLAN

Allows Wi-Fi devices to connect to a wired network

Wireless network interface card

Allowing devices to connect to the LAN via radio communication

… instead of using cable

… easy to move a device to different location

Hubs:

Centre of a star topology

Broadcast to all

One hub connected to another hub

Built in broad band modem - connection to the internet

Switch:

Function as a hub

Connect a number of devices together to form an LAN

MAC address

More intelligent - store addresses of connected devices

Unicast

Reduces network traffic

Gateway

Capable of connecting two dissimilar networks. Connect a LAN to a WAN.

A device used between two dissimilar LANs. The device is required to convert data packets from one protocol to another

Can connect two networks with different protocols

Gateway can also act as firewall or server.

Router

Most Intelligent - takes decision to forward the received transmission

May function as a gateway or NAT box.

Forward packets of data from one network to another; routers read each incoming packet of data and decide where to forward the packet.

Can route traffic from one network to another network

Can be used to join LANs to form a WAN and also connect a number of LANs to the internet

Offer additional features such as dynamic routing.

Connects two networks with the same protocol.

Transmit data between the servers and the internet

Connect devices/servers to the internet

Modem

Converts between digital data and analogue data

To connect the servers to the internet over a telephone line

Repeater: boost the signal so it can travel greater distance.

Bridge: connect one LAN to another LAN that uses the same protocol.

Server: a device or software that provides a specific function for computers using a network. The most common examples handle printing, file storage, and the delivery of web pages.

	Router	Gateway
Differences	· Forward packets of data from one network to another; read each incoming packets of data and decide (using IP address) · Route traffic from one network to another network · Can be used to join LANs together to form a WAN; connect a number of LANs to the internet · Offers additional features such as dynamic routing · Connects two or more networks · Assign private IP address · Can be used to segment a network	· Convert one protocol to another protocol used in a different network · Convert data packets from one protocol to another · Translate from one protocol to another · Do not support dynamic routing · Can connect a network to WAN // act as single access point · Assigns private IP address
Similarities	Both regulate network traffic between two network	Both receives packets from one network and forward it to another.

Ethernet

A protocol used by many wired LANs.
Adopted by IEEE
Made up of:
- A node
- A medium
- Frame
Collisions
- Two messages using the same data channel could be sent at the same time, leading to a collision.
- Carrier sense multiple access with collision detection (CSMA/CD): define the random time period for a device to wait before trying again.

When a frame is sent it causes a voltage change on the Ethernet cable. When a collision is detected, a node stops transmitting a frame and transmits a ‘jam’ signal and then waits for a random time interval before trying to resend the frame.

CSMA/CD

Workstations listen to the communication channel
If no data is being transmitted, the computer can send its data
Collision caused when 2 devices transmit at the same time
If collision occurs, each workstation waits a random time
… before retransmitting
Each time a collision occurs, random time is increased.
--
Carrier sense multiple access with collision detection
Before transmitting a device checks if the channel is busy
If it is busy the device waits // if channel is free data is sent
When transmission begins the device listens for other devices also beginning transmission
If there’s a collision, transmission is aborted
Both devices wait a random time, then try again

Bit streaming

Sequence of digital signals
Over a communication path / internet
Transfer data at high speed
Requires fast broadband connection
Requires some form of buffering
Bits arrive in the same order as sent
--
The data is compressed before transmission
Data is transmitted continuously as a series of bits
Video hosted on a media server
On download, the server sends data to the buffer on the client computer
The recipient receives bit streams from the buffer.

On-demand bit streaming

Digital video tape/ digital files are converted to bit streaming format for broadcast on the net; this is known as encoding, these encoded streaming video files are then uploaded to a dedicated server.
A link for the encoded video is placed on the website
A user clicks on the link to download the encoded streaming video
Then broadcast to the user when they require it
Can be paused/ go back and rewatch/ fast-forward.

How is it possible for on-demand to not pause?

User needs high speed broadband
Data is streamed to a buffer
Buffering stops video pausing as bits streamed
As buffer is emptied, it fills up again so that viewing is continuous

Real-time bit streaming

An event is captured live with a video camera
The video camera is connected to a computer
The video signal is converted to streaming media files on the computer
Encoded feed is then uploaded from the computer to a dedicated streaming server.
The server sends the encoded live videos to all users requesting it as real-time video.
Cannot be paused.

Issue:

Bandwidth
Security
Copyright

Benefits

No need to wait for the whole film to be downloaded
No need to store large files on user’s computer
Allows on demand play back
No specialist software is required for playback

Problems

Video stop if very slow internet
Video stop if inadequate buffering capacity
Loss of internet means can’t access films
May require specific software to run the films
Viruses can be downloaded from the website

Bit streaming to send video:

The data is compressed before transmitting.
The video is transmitted continuously as a series of bits.
The video is hosted on a media server.
The server sends the data to a buffer on the client computers.
The user’s software receives the bit stream from the buffer.

Difference between WWW and internet

Internet	World wide web
· A massive network of networks · Interconnected network · Makes use of transmission control protocol(TCP)/ internet protocol(IP)	· Collection of multimedia web pages and other documents · … which are stored on websites · Https protocols are written using HTML(hypertext mark-up language) · Uniform Resource Locators(URLs) specify the location of all web pages · Web resources are accessed by web browsers · The WWW uses the internet to access information from servers and other computers.

Hardware for the internet

Modem

A device that converts digital data to analogue data;
Converts analogue data to digital data

Public switched telephone network(PSTN)

Consists of many different communication lines.
Data is transmitted at both direction at the same time
The communication passes through different switching centres
Connect computers/devices and LANs between towns and cities.

Dedicated Lines

Connection that is only use for that business/organization

Benefits

Faster data transmission
Improved security
Consistent transmission speed

Drawbacks

Expensive set up
Disruption to the dedicated lines would leave no alternative

Cell phone network

Send data to cell towers over mobile connection

Satellite

Send data to satellites in orbit

IP

IP address:

Gives each device on a network an identifier // used to locate a device on a network
Each address is unique within the network
Allows a device/gateway/node to send data to the correct destination

IP format:

Composed of four integers
Each in the range 0-255
Each stored in 8 bits; 32 bits in total
Separated into network ID and host ID.

IPV4:

Four numbers separated with .
Each number range from 0 to 255
32 bits long

Why the router has both a private and public IP address:

The router needs a public IP address so it can be identified on the internet.
The router needs a private IP address so that it can be identified on the home network
The router has a public IP and a private IP address so that it can route data between the two networks.

Why no public IP address:

Improved security because the IP address is not visible/accessible outside the network.
An internet presence is not required for each employee computer.
Only the router needs a public Ip address.
Reduces the number of public IP address needed.
All data from the internet must be transmitted via the router.

Why do laptop has IP address:

To identify the laptop on the home network.
To allow the router to send data to the laptop from the internet or other devices on the home network.

Public IP address	Private IP address
the ones allocated by a user’s ISP to identify the location of their device	are reserved for internal use behind a router or other NAT device

· Public address can be reached across the internet · Public addresses are provided by ISP(internet service provider/ assigned by interNIC) · Public addresses are unique (to the internet)	· Private address can only be reached internally through the LAN/intranet · NAT(network address translation) is necessary for a private IP address to access the internet directly. · A private address is more secure than a public IP address · A private address is assigned by the router of the network connected · Private addresses are only unique within their network, can be duplicated within other networks.
Static IP address:	Dynamic IP address:
· When a computer disconnects and re-joins a network · … the address does not change · Assigned by ISP · The DNS does not need updating · Which might be delayed causing ‘address not found’ errors. · The web server may be accessed directly just using IP address.	· Each time the computer re-joins a network · … the address changes · Address is assigned by the network OS

DNS

DNS(Domain Name System): - DNS is a hierarchical distributed database installed on domain name servers that is responsible for mapping a domain name to an IP address.

Domain - This is a section of the internet operated by an authority.

Uniform Resources Locator (URL): or website address is an address to reference resources on the Internet.

HTTP(Hypertext Transfer Protocol): - protocol for data communication of the web. It determines how message are formatted , transmitted, and the actions web servers and browsers should take in response to commands.

The browser requests the web page
The server access the web page
The server processes/executes the code
The server produce an html
The server returns the html to the client
The browser display the webpage.