Learning Module C: Network and Telecommunications I. Why

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Transcript Learning Module C: Network and Telecommunications I. Why

Learning Module C:
Network and Telecommunications
I. Why Networking
II. Components of Network
III. Types of Network
IV. Protocols of Network
V. Network Security (supplement)
I. Why Networking
1. Purpose of networking: Connect computers for
the purpose of sharing information and resources.
• Data sharing
– 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.
• Hardware sharing: 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
• Software sharing (Why?): 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.
2. Business reasons for data sharing
• Managers can see data immediately as it is
collected or updated.
• POS (point of sales – collect data at origin)
• Decision support (OLAP)
• 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
II. Four Basic Components of Network
• Computers: two basic roles in networks (p.171)
– Client: A computer that requests information
and accesses shared resources (e.g., files,
– Server: a computer that responses to request
by providing the requested information and
shares its resource across the network.
• Transmission Media
(cabling and wireless networking)
– Bandwidth: the range of frequencies available on
a communication medium (like multiple-lane
highway). The greater the bandwidth, the greater
the transmission capacity. The capacity is typically
measured in Mbps (megabits per second).
– Cable and Wireless
a) Twisted-pair cable (low speed)
b) Coaxial cable (low speed)
c) Optic cable (high speed)
d) Wireless networking
(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.
Exercise: Sample Calculations
• Suppose you wanted to download the movie Titanic. In
compressed form it would contain about 4*109 bytes or
32*109 bits. If you had a typical 28.8 Kbps modem,
how long would it take to download the movie?
• Answer: 32*109 bits/ 28.8*103 bits/sec
= 1.11*106 sec or
= 1.11*106 sec/3.6* 103 sec/hour
= 308 hours or
= 308 hours/ 24 hours/day
=12.8 days
• Connection devices
– NIC (Network Interface Card) (p.168): A physical
interface between your computer and the network.
It plugs into an adapter slot inside the computer
– Driver software: A device driver (software) for NIC
must be installed on your computer.
– Bridge (p.185): connect networks of the same kind.
– Router and gateway (p.185): connect dissimilar
networks to form a complicated network such as
-Recognize node address and network address
-Repack data and select the best path
-Convert signals to interface different transmission
media and networks.
• Software (supplement)
– 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.
III. Types of 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
• 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.
2. Client/Server and Peer-to-peer
– 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.
– Peer-to-peer network: Computers function as
either servers or clients and at more or less the
same level of capacity.
– Compare Client/server network and peer-to-peer
 Message traffic.
Additional cost such as network administrator.
Control shared resource.
The number of users (10 for peer-to-peer).
Central control over security and back up.
Many passwords?
Centralized organizational scheme to locate
Performance of user’s computer.
3. Virtual Private Network (p.179)
• It is a public network that provides services
to many companies.
• Your privacy is not protected by dedicated
• Your privacy is protected by encryption
services provided by VPN provider.
IV. Network Protocols (p.186)
1. 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
• 1980 TCP/IP (Transmission Control
Protocol/Internet Protocol) became the standard
of Internet protocols.
• 1991 new language HTML and protocol HTTP for
Web pages.
2. Asynchronous Transfer Mode (ATM)
• Conventional telephone network is synchronous.
Both communicating sides must be online.
• Why ATM?
Chopped up message into packets and
Packets can be stored and forwarded.
Therefore, if part of network is down, packets
can be stored and forwarded later.
Therefore, if error occurs, only re-send the
packet with error rather than whole
 Therefore, ATM can handle a lot more users.
V. Network Security
1. What security services should network systems
provide? (p.217 but not only for e-commerce)
Access Control
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.