Chapter 3 Networks and Telecommunications

I. Why Networking1. 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.

• 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.

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

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.

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.

– 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.

• 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.

• Transmission Media

– Types of cablea) 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 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): 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 PC’s 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.

• 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.

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.

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.

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 Internet1. Internet• Internet: an international network of networks.

Computer – ISP – NSP – ISP - Computer (p.102)

• Email address

INDIVIDUAL OR INDIVIDUAL OR ORGANIZATION NAMEORGANIZATION NAME

CountryCountry

caoq@winthrop.edu

HOST HOST COMPUTERCOMPUTER

FunctionFunction

DomainDomain

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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)

Parts of a Uniform Resource Locator (

URL)

http://www.winthrop.edu/ oncampus/academics/default.htm

hypertext transfer protocol

Host name andcomputer name

path (directories and file name on the web server)

hypertext markup language

organization type

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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.

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.

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.

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.

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.

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.