Session 6: NetworkingTeaching Computing to GCSE Level with Python

Topics todayNetworking

Topologies

The Internet & Packet switching

IP addresses

Protocols

Specification LinksOCR

(a) explain the advantages of networking stand-alone computers into a local area network

(b) describe the hardware needed to connect stand-alone computers into a local area network, including hub/switches, wireless access points

(c) explain the different roles of computers in a client-server and a peer-to-peer network

(d) describe, using diagrams or otherwise, the ring, bus and star network topologies

(e) describe the differences between a local area network and a wide area network such as the Internet

(f) explain the terms IP addressing, MAC addressing, packet and protocols

(g) explain the need for security measures in networks, such as user access levels, suitable passwords and encryption techniques

(h) describe and justify network policies such as acceptable use, disaster recovery, failover, back up, archiving.

AQAa) understand what a computer

network is b) be able to discuss the advantages

and disadvantages of using a computer network

c) be able to describe and explain the bus, ring and star networking topologies

d) be able to discuss the advantages and disadvantages of each of these topologies

e) understand the client-server model f) be able to explain, in simple terms,

the handshake process used in most modern networking protocols

g) be able to explain how coding for a client-server model is different from

coding for a stand-alone application

Networked ComputersComputers and peripherals connected together

May be in one building or site (LAN)

Or across large geographical area (WAN)

LAN

A LAN is a Local Area Network.

It is a connected set of computers and other devices.

Each device is called a node.

A LAN is installed on one site.• It does not cover a wide geographical area.• All the infrastructure is owned by the

organisation or individual who owns the LAN.

Stand-alone ComputersNeed own software installed

Backups - own responsibility

Need printer

No access to centralised resources

Advantages of NetworksShare resources

Share information

Centralised access to data

Mail/messaging systems

Access to own files from anywhere on network

Disadvantages of NetworksUsers’ dependence on network

Busy network ---> delays

Needs to be well-designed for optimum performance

Threat of viruses/hacking

Wide Area NetworkSpread over large geographical area

May be public or private

Communication may be by: telephone network satellite link dedicated cables microwave

Network TopologiesBus

Star

Ring

Network topologies: Bus

Bus Network Topology

One common or “omnibus” cable connects all nodes

Network topologies: BusAll workstations are connected by a single

cable (a bus)

Max length of cable 500m

Only suitable for LANs (Ethernet)

One PC used as a file server/print server

Advantages of BusCheaper to install than ring

Requires least amount of cable

Easy to install and extend

Failure of one workstation does not affect network

Disadvantages of BusNetwork performance degrades under heavy

load

Dependent on reliable main cable

Cable failure difficult to isolate

Network topologies: StarSuitable for LAN or WAN

If one link or node (other than the central one) fails, the rest of the network can still function

All data passes through the central node, so if it fails the whole network “goes down”

Formerly commonly used formulti-user systems(terminals plus “mainframe”)

Network topologies: StarSuitable for central computer (hub) and dumb terminals

Very popular for WAN where organisation relies on central computer for bulk of processing tasks, limiting the nodes to their local processing needs and the validation of data, prior to

transmission to central computer

Advantages of Star If one node or cable faulty the network can still function

Centralised control of message switching allows high degree of security control

Methods of data transmission and speeds used can vary from one spoke to anothere.g. public phone line or leased line

No problems with collisions of data

Disadvantages of StarNeeds a lot of cable (costly)

Dependent on central node functioning

Control of communication requires expensive technology at the hub

Network topologies: Ring

Ring Network Topology

Each node connected to two neighbouring nodes

Network topologies: RingNo central server

Not suitable for a WAN

None of the nodes have overall control

Messages flow in one direction only

Advantages of RingNo dependence on central computer

High transmission rates are possible

More predictable than bus

Transmission of messages relatively simple

Disadvantages of RingDifficult to extend as new cable must preserve

the ring topology

If one node breaks down, transmission between the other nodes is disrupted

Bus NetworkThe type of network topology in which all of the nodes of the network are connected by a backbone which has two endpoints. All data that is transmitted between nodes in the network is transmitted via the backbone.

Star NetworkThe type of network topology in which each of the nodes of the network is connected to a central node. All data that is transmitted between nodes in the network is transmitted to the central node, which is usually a switch or a hub that then retransmits the data to some or all of the other nodes in the network.

Network Topologies

Star Network with hub

File server

Hub

Client Workstation

Data Packet

Printer

AQA Computing A Level © Nelson Thornes 2008

Network Topologies

Star Network with switch

File server

Printer

Switch

Client Workstation

Data Packet

LAN hardware NIC – network interface card

Each node needs a NIC to connect to the LAN. Produces the electrical signals that carry data.

Hub A device that connects nodes together. Makes them act as a single segment. Not smart – all data packets are transmitted. Cheap devices – USB hubs useful in home

computer installations.

OCR Computing for GCSE © Hodder Education 2011

LAN hardware• Wireless access point

Wireless technology has many advantages over wired networks.

• No building work needed

• Quick to add new workstations

• Flexible deployment of nodes

• Visitors can access network / internet

• But there are drawbacks Slower performance Security issues

Peer-to-peer networksAll computers have the same status.

May be very slow because of• data collisions• shared processor power

Only suitable for small low traffic networks.

Client–server networksOne or more computer is

designated as a server.• A high specification machine.• It offers services to client machines,

such as centrally held software, data, file storage and peripheral access.

• Servers control network traffic.• Log usage of resources.• Provide security.• Clients have to log in – this identifies

users and assigns rights.• Big networks may have hundreds of

servers.

OCR Computing for GCSE © Hodder Education 2011

Protocols Protocols are the rules for

communication between devices.

Standard protocols encourage network development.

Network protocols are constructed in layers.

This allows independent development of small parts of a protocol.

Internet Protocol is a group of protocols that underlies the internet.

TCP/IP now the most important set of protocols.

Includes many special protocols.OCR Computing for GCSE © Hodder Education

2011

Protocol Meaning Application

DNS Domain Name System

Translates domain names such as ocr.org.uk into IP addresses.

TLS/SSL Transport Layer Security / Secure Sockets Layer

Cryptographic protocols designed for secure communications.

FTP File Transfer Protocol

For copying files from one host to another.

Gopher An early means of searching for files on the Internet.

HTTP Hypertext Transfer Protocol

For distributing hypermedia files – essentially web pages.

IMAP Internet Message Access Protocol

One method for accessing emails.

POP3 Post Office Protocol (version 3)

Another method for accessing emails, used by most webmail services.

Telnet Allows bidirectional text communications on a network.

IP Addresses Every Web site has a ‘Home page’ with a unique address

known as its IP address 4 numbers, each in range 0 to 255, separated by full-

stops. e.g. 177.234.243.186

IP addresses are difficult to remember, so can use Domain Name instead

Domain name system maps the domain names onto the IP addresses

Ethernet AddressingEach Ethernet card that is manufactured is

assigned a unique Ethernet hardware address.

Ethernet header contains two addresses:- 48-bit source address that uniquely

identifies the source of the frame- 48-bit destination address that uniquely

identifies the destination of the frame

The Ethernet address enables the frames to be routed between two computers that are in communication. The frame just looks for a matching hardware address in each network card it encounters over the Ethernet cable.

IP addresses are different from Ethernet addresses. They may be moved from computer to computer.

Sending a file over a networkUser sends file using FTP

File is passed to another layer of software – the TCP/IP protocol stack

(A protocol stack has multiple layers)

TCP/IP software breaks file up into packets

What is a packet? “A packet is a discrete collection of information

that includes user data plus protocol information, such as destination and source addresses, and controls for error detection/correction, sequencing, flow control etc.”

Packet Switching

Messages are broken down into smaller blocks (packets)

Packets routed individually through the network

Packets from a particular message may travel along different routes through network

At the receiver, packets are reassembled to construct complete message.

Datagram Packet Switching Network

E D C B A

E

D

C

B

A

E C B E D C B A

Original message

Re-assembled message

Packet switch node

EDCBA

EDCBA

Message packets

Network

E

C A D

A

D

1

2

3

4

5

6

2

Computer X

Computer Y

Packet switching demoPacket switching demo here

Client/Server Model of the InternetA server provides a service requested by a

client

A client that requests and uses services provided by a server

In the client-server model of the internet the client and server are software processes that fulfil these roles

HTTP is a client/server protocol

ProtocolsA protocol is a set of rules that is used to ensure successful communication between different computers

FTPFile Transfer Protocol

An application-layer protocol

Enables on one computer to be copied to another computer

Uses port 20 for data and port 21 for commands

A port is an address for a software process

HTTPHypertext Transfer Protocol

An application-layer protocol

Used to enable web pages to be viewed

A client machine sends a request message to the server and the server responds with a message, usually in the form of a web page.

Uses port 80 and 8080

POP3Post Office protocol 3

Application layer protocol

Used to retrieve stored mail

Uses port 110

SMTPSimple Mail Transfer Protocol

Application layer protocol

Used by email clients to send email.

Recipients of the message are specified and the message text is transferred to a mail server listening on port 25

HTTPSHyperText Transfer Protocol over Secure

Sockets layer

Application-layer web protocol

Encrypts and decrypts user web page requests

Uses port 443

Security

Transmitted data is vulnerable.

Users on a network do not need to see all the data.

Access levels• Users granted privileges.• Associated with login names.

Passwords• Must be changed regularly.• Passwords should be strong.

Encryption• Especially important on wireless networks.• WEP or WPA should be used.

OCR Computing for GCSE © Hodder Education 2011

Policies

Network owners should have policies to protect their assets.

Users have responsibilities.

Acceptable use policies are common.

Policy ensures that users understand what is expected of them.

Sanctions used if users misuse resources.

OCR Computing for GCSE © Hodder Education 2011

Disasters

Disasters do happen.

Need to plan for worst case scenario.

Example strategies:• mirrors of systems • use of disk protection technology such as RAID • surge protectors — to minimise the effect of power surges on delicate

electronic equipment• uninterruptible power supply (UPS) • back-up generator in case of a power failure• fire preventions — alarms, fire extinguishers • anti-virus software and other security measures• failover systems

OCR Computing for GCSE © Hodder Education 2011