Chapter 3 Networks and Telecommunications

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• Chapter 3 Networks and Telecommunications

• I. Why Networking
• 1. Purpose of networking Connect computers for
  the purpose of sharing information and resources.
  
• Data sharing (p.81)
• Master copies of data files reside on a computer
  elsewhere on the network and users access the
  master copy to do their work.
• It allows multiple users access the same file
  simultaneously and it is able to merge multiple
  updates to keep a single master copy consistent
  and correct.

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• Hardware sharing (p.85) Printers, scanners,
  storage spaces, special processors, and other
  devices can be attached to a network. For many
  businesses, this capability alone justifies the
  costs involved in networking.
• Software sharing (p.86) A network version of
  software is stored in one computer (file server),
  and users load the software package into the RAM
  of their computers when they want to use it.
• It saves storage spaces and time for upgrading
  since only one copy has to be installed.
• The number of users may exceed the number of
  licenses if they do not use the software at the
  same time.

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• 2. Business reasons for data sharing
• Managers can see data immediately as it is
  collected or updated.
• POS (point of sales)
• Decision support
• speed of decision process
• error prevention
• updated information
• Information to circulate among users E-mail,
  Bulletin Board, Newsgroup and Chat Room.
• Calendar and appointment scheduling
• Teamwork and Groupware (ICQ)
• Data backup by system administrator

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• II. Components of a network
• 1. LAN and WAN
• Local Area Network (LAN) a small network that
  encompasses a limited distance (normally no more
  than 1,000 feet and one or two buildings).
• Wide Area Network (WAN) A large network that
  connects multiple groups of users in multiple
  locations, e.g., enterprise networks. It spans
  distance measured in miles and involves links
  that are controlled by public carriers.

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• 2. Four basic components of a network (p.88-p.96)
• Computers two basic roles in networks
• Server a computer that responds to request by
  providing the requested information and shares
  its resource across the network.
• Client A computer that requests information and
  accesses shared resources.

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• Client/server network Certain computers function
  more or less exclusively as servers while users
  computers function more or less exclusively as
  clients. Servers have more CPU power and storage
  capacity. The message traffic between servers and
  clients is heavy.
• Peer-to-peer network Computers function as
  either servers or clients and at more or less the
  same level of capacity. Older machines may slow
  down too much while working as servers.

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• Transmission Media
• (cabling and wireless networking)
• Transmission capacity is typically measured in
  Mbps (megabits per second).
• Example
• Voice channel 300 Hz 3400 Hz its bandwidth
  is 3100 Hz. Suppose 1 bit per hertz of bandwidth,
  the bit rate is 3100 bps.

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• Transmission Media
• Types of cable
• a) Coaxial cable (10 Mbps)
• b) Twisted-pair cable (10 Mbps)
• c) Fiber-optic cable (100 Mbps - 1 Gbps)
• Wireless networking (Radio, Micro, and Infrared
  Waves)
• a) Temporary connections into existing wired
  networks.
• b) Contingency connections for existing wired
  networks.
• c) Extend span beyond wired networks.
• d) Travel with computers within certain limits.

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Exercise Sample Calculations

• Suppose you wanted to download the movie Titanic.
  In compressed form it would contain about 4109
  bytes or 32109 bits. If you had a typical 28.8
  Kbps modem, how long would it take to download
  the movie?
• Answer 32109 bits/ 28.8103 bits/sec
• 1.11106 sec or
• 1.11106 sec/3.6 103 sec/hour
• 308 hours or
• 308 hours/ 24 hours/day
• 12.8 days

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• Connection devices
• NIC (Network Interface Card) A physical
  interface between your computer and the data
  network. It plugs into an adapter slot inside the
  computer case. (Modem if telephone voice channel
  is used.)
• Driver software A device driver for NIC must be
  installed on your computer.
• Hub connect PCs to network.
• Bridge Switch connect two network segments,
  filter and forward packets.
• Router connect multiple network segments or
  connect multiple networks to form a complicated
  network such as Internet.
• Recognize node address and network address
• Repack data and select the best path
• Convert signals to interface different
  transmission media and networks.

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• Software
• Server network software NOS (Network Operating
  System) installed on servers. It is a multi-user
  operating system.
• Client network software the portion of NOS
  installed on client computers for sending and
  receiving information on network. It also can
  convert data format for incoming and outgoing
  messages.
• Application software network version.

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• III. Networking standards (p.98)
• Ensure that computers from different vendors can
  effectively work together.
• Meet up-to-date standards.
• Many standards and variations on standards.

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• History of Network Standards
• 1965 IBM research for LAN only 20 information
  flow goes outside.
• Protocols for LAN Netware, Token Ring.
• 1969 ARPANET (now Internet) was a host-to-host
  network between four universities. Protocols were
  E-mail, Telenet, and later FTP (File Transfer
  Protocol).
• 1980 UNIX NOS
• 1980 TCP/IP (p.99) (Transport Control Protocol /
  Internet protocol) became the standard of
  Internet protocols.
• 1991 new language HTML (Hypertext Markup
  Language) for pages and protocol HTTP (Hypertext
  Transfer Protocol) for the transfer of data
  formats between the server and the client.
• Protocol a set of rules that govern functional
  units to achieve communication IEEE dictionary.

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• What are basic functions of a protocol?
• Segmentation and reassembly Break up the data
  message into packets and put packets back into
  their original order.
• Encapsulation add header and control
  information to packets.
• Connection control connection establishment,
  data transfer, and connection termination.
• Ordered delivery assign sequence numbers to
  packets for reassembly on the receiving side.
• Flow control manage the data flow such that
  buffer memories do not overflow, but maintain
  full capacity.
• Error control recovery of lost or errored
  packets.
• Adapted from Practical Data Communications,
  R.L. Freeman

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• What is OSI Networking Model?
• The Open Systems Interconnection (OSI) reference
  model was proposed by the International Standards
  Organization (ISO).
• Networking can be broken into a series of tasks
  (layers)
• 7 Application (provide interface between
  application software and network)
• 6 Presentation (standardized data format,
  encryption/decryption, compression)
• 5 Session (setup, hold, end a communication
  session)
• 4 Transport (data segmentation, flow control,
  error control, resequencing packets)
• 3 Network (handle addressing messages decide
  how to route transmissions quality, cost, and
  priority router is working in this layer)
• 2 Data Link (package and send data frames
  basic unit for network traffic on the wire.
  Bridge and switch are working in this layer.)
• 1 Physical (covert bits into signals for
  outgoing messages and signals to bits for
  incoming messages hub is working in this
  layer)
• Each layer can be handled separately and its
  issues solved independently.

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IV. The Internet

• 1. Internet
• Internet an international network of networks.
• Computer ISP NSP ISP - Computer (p.102)
• Email address

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• URL (Uniform Resource Locator)
• A URL is a location that indicates where a
  resource on the Web can be found. In some sense
  it is like the call number of a book.
• A URL contains three pieces of information
• the protocol used to access the resource
• the names of the host and the computer where
  the resource is located (domain) and the
  organization type
• the path name of the resource itself
• Example (next slide)

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Parts of a Uniform Resource Locator (URL)
hypertext transfer protocol
hypertext markup language
organization type
http//www.winthrop.edu/ oncampus/academics/defaul
t.htm
path (directories and file
name on the web server)
Host name and computer name
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• Intranets
• networks within an organization that use the
  infrastructure and standards of the Internet and
  WWW technology but have firewalls (hardware and
  software) to prevent outsiders from invading
  private networks.
• Employees can go out but unauthorized users
  cannot come in.

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• 2. Three Approaches to Data Switching
• Circuit Switching end-to-end connection during
  the duration of a message.
• Message Switching store and forward message.
• Packet Switching
• Break down the message into short packages
  packets
• Add header and tail so packets can be put back
  into the original message on the receiving side.
• Packets can be stored, forwarded, and sent on
  diverse routes.
• What are advantages of packet switching?
• Efficient use of transmission links diverse
  routes, store and forward
• Near real time connectivity
• Highly reliable - if error occurs, only re-send
  the packet with error rather than whole
  message.
• Highly survivable - if part of network is down,
  packets can be stored and forwarded later.

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• 3. Virtual Private Network
• It is a public network that provides services to
  many companies.
• Your privacy is not protected by dedicated line.
• Your privacy is protected by encryption services
  provided by VPN provider.

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• V. Network Security (Supplement)
• 1. What security services should network systems
  provide?
• Confidentiality
• Access Control
• Integrity
• Non-repudiation
• Authentication
• Availability
• Confidentiality and EavesdroppingEavesdropping
  packet sniffing on net, in which attackers read
  transmitted information, including logon
  information and database contents.

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• History of Encryption and Decryption
• 1586 VIGENERE - paper and pencil, polyalphabetic
  substitution cipher.
• 1920s - 1970s ENIGMA substitution rings
  (rotors).
• 1975 US National Bureau of Standard (NBS) Data
  Encryption Standard (DES) a 56-bit key is no
  longer considered to be very secure.
• 1990 Xuejia Lai and James Massey IDEA with a
  128-bit key, approximately twice as fast as DES
  and considerably more secure.
• 1977 Rivest, Shamir and Adleman RSA public key
  algorithm a 2048-bit key is considered to be
  very secure in the foreseeable future but is
  about 1000 times slower than DES.
• And more.

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• What are two major cryptographic methods?
• Conventional encryption Message sender and
  recipient share single secret key for encryption
  and decryption. There are three basic
  operations
• - Substitution replace bits with other bits.
• - Transposition (permutation) arrange bits in a
  different order
• - XOR 10110010 ? 01110110 11000100
• Public-key encryption Key owner generates a
  pair of keys. One key, called public key (e), is
  made available for anyone to get. Another key,
  called private key (d), is kept by the owner.
  Message encrypted with one key can be decrypted
  with another. The RSA algorithm is one
  implementation of public key cryptography.

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• How do you choose an encryption algorithm?
• No inherent mathematical weakness Algorithm
  survived extensive public review and assume
  that the brute force approach is the only
  efficient attack.
• Key length A 128-bit key makes a brute force
  attack impractical with current technology.
• Key is easy to change and to manage Frequent
  key change makes encryption more secure.
• Cost Many algorithms are royalty-free.
• Permission for export Strong cryptography
  products may not have permission to export.