== Networks ==

LAN

LAN stands for Local Area Network.  Local means that all the hardware is on one site - e.g. in one building or a group of buildings.  In a LAN, you will find :

• Two kinds of computers :  servers and clients
• Connections between computers provided by:  
  -    cables and switches
       OR
  -    wireless
•  Peripheral devices such as :  printers, FAX machines, video surveillance cameras, etc.

WAN

WAN stands for Wide Area Network.  This is different than a LAN because it :

• covers a larger geographical area, for example offices in many different cities
• uses connections that are not owned by the organization, e.g. public telephone lines
• probably connects several LANs together

Topology

The topology of a network refers to the basic structure.

• A star topology consists of servers in the middle connected by cables to many switches,
  which are connected by cables to many nodes (clients) - there are LOTS of cables
• A bus topology has just one single cable that runs from one node to the next -
  this is slow and inefficient for large networks, but simple and cheap for small networks
• a hybrid topology has both star and bus portions, all connected

Protocol

Protocol refers to a system or rules for communicating.  This includes some or all of the following:

•  connect (starting a connection by exchanging ID , IP address , password, etc)
•  transmission (agreeing on a transmission speed, packet type)
•  error handling (parity checking, start/stop bits)
•  security (id verification, encryption)
•  disconnect (what to do if signal disappears, how to sign off, accounting)

Networking Software

A variety of software technologies are used in various networking system.  Hardware - network cards, modems and routers - deal automatically with things like error checking and steering packets to the right place..  Software controls higher-level processing like choosing and transmitting whole files, DNS lookups, hyperlinks, etc.  Some common software includes:

• web-browsers and web-servers (software) using the http: protocol
• network operating systems supporting TCP/IP connections
• terminal software like Telnet and FTP (command line interface for typing commands)
• many other proprietary software systems

Security

Security prevents loss of data as well as misuse (stealing).

• data integrity refers to ensuring the data is not corrupted/destroyed by errors or erasure
• data security refers to keeping data private, so unauthorized people cannot steal it
• data integrity is preserved by parity checks, checksums, and retransmission
• data security can be protected by passwords, restricting physical access, assigning access rights, and encryption

Speed

The performance (speed) of a network depends on many factors:

•  connection signaling speed (Mbits per sec)
•  bandwidth (total amount of data carried per sec for ALL machines)
•  quantity of data needing to be transmitted (can be reduced using compression)
•  error rate - noisy cable (electrical interference) can cause errors and retransmission, or can cause the nodes to reduce their signaling rates (common in wireless networks)

Sharing

Why is networking useful?  Usually it involves SHARING:

•  data (files)
•  information (e-mail, web-sites, entertainment)
•  peripherals (printers, scanners)
•  software (calendar program, word-processor, etc)

Applications

Some standard application areas include:

•  messaging (LAN)
•  e-mail (LAN and WAN)
•  e-commerce (WAN)
•  virtual conferencing (LAN and WAN)
•  distributed processing (more LAN, some WAN)
•  software distribution and updates (both)

The Web

The World-Wide-Web is a Wide Area Network.  It uses :

•  web-servers containing HTML pages
•  clients running web-browsers that display HTML pages
•  the Internet Backbone built of optical fiber and routers
•  ISPs (Internet Service Providers) connecting clients to the backbone

HTML

HTML stands for Hyper Text Markup Language.  It consists of text-codes like the following:

   Click <a href="www.ipod.com">here</a> to download music files.

The browser must:

•  retrieve a web-page (HTML) from a web-server
•  parse HTML, separating <markup codes> and text
•  display text
•  fetch and display images
•  display links and respond to clicks by jumping to (retrieving) another page

New Technologies

Some emerging technologies include:

•  peer-to-peer networking (file sharing)
•  voice-over-IP (free phone calls)
•  WAP (Wireless Application Protocol) for web-surfing on cell phones and other small devices
•  VPN (Virtual-Private-Networks) using encryption and IP-tunneling to make a "private" WAN

Why use a WAN?

An international corporation can use a WAN to connect offices is various countries.  This consists of :

• a LAN at each office
• gateways connecting each LAN to the WAN
• appropriate security procedures to prevent unauthorized access (by hackers or criminals)

Ethernet

LANs generally use Ethernet to make connections.  Ethernet protocol uses:

• a network adapter card in each node (PC, printer, server)
• cables connecting nodes
• hubs and/or switches
• half-duplex transmission (listen or talk separately) in bus networks without switches
• full-duplex transmission (listen and talk simultaneously) in star networks with switches

Connections to the Phone System

The telephone system provides communication paths reaching around the world.  There are several possible ways to connect a computer or a LAN to the phone system:

• modem (MOdulator/DEModulator) - changes digital computer signals to analog phone signals up to 56 Kilobits per sec
• ISDN (Integrated Services Digital Network) - up to 64 Kilobit or 128 Kilobit
• ADSL (Asynchronous Digital Subscriber Line) - 1 Megabit or more downstream (downloads), but only 500 Kilobits upstream (uploading)

Cables or Not

Nodes (devices) must send signals over some sort of "carrier", for example:

• cables (copper) - cheap, fast, and reliable
• wireless (radio) - easy to install (no drilling), cheap, rather slow, not so reliable
• optical fibre (glass) - very expensive, very fast, very reliable, needs special connectors

Packets

In the o..o..o..l..l..l..d..d..d days, you could dial directly to a Bulletin Board Service, and your modem was connected directly to theirs.  A direct line could use any kind of protocol for communication, including an uninterrupted flow of data transferring an entire file.  Now almost all network traffic depends on packet switching.  

• messages are chopped up into small packets (e.g. 16 Kilobytes each)
• each packet is labeled with a destination address and sender address
• the packets are passed around by routers and switches until they arrive
• packets might arrive in a different order than they were sent - they must be reassembled at the receiving end - so there needs to be a code in each packet showing the order for reassembly
• packets contain error-detection codes (check-sum)
• packets also contain a time code so packets can die if they never arrive

Routing

Routing is the process that carries a packet from source to destination.  This can be done by a variety of devices:

• hubs are the simplest, cheapest connectors - they accept an incoming signal and send it back out on all the attached cables - the correct receive will accept it and the others ignore it - this creates lots of useless extra traffic
• switches are smarter than hubs - if they know which cable leads to the destination, they send the packet only to that cable - they must keep track of successful transmissions and change these "routes" in case computers disappear or new computers appear
• routers are the cleverest and most sophisticated - they make choices of where to send packets to other routers (if there are multiple possibilities) and try spread traffic out on the least-busy lines - good routers can also do some packet filtering, blocking "bad" packets
• in the Internet, routers are large machines transferring many millions of packets per second

Unauthorized Access

Since networks share everything and provide distributed access, there are many possible dangers. Common network security measures include:

• physical security (locked doors) prevents intruders from using clients in a LAN
• log-in user ID and password prevent unauthorized users from accessing servers
• access rights limit users to accessing only appropriate files
• read-only access prevents data from being changed or erased either accidentally or maliciously
• firewalls prevent unauthorized packets and messages from penetrating a LAN from outside
• in WANs, most security methods depend on encryption, as the methods above are unmanageable - this does not protect against data-loss, but it does protect privacy (e.g. credit card numbers)