2. Q1: What Is a Computer Network?
• Network
– Collection of computers
– Communicate with one another over transmission
line
• Basic types of Network Topologies
– Local Area Network (LAN)
– Wide Area Network (WAN)
– Internet
6-2
3. LAN (Local Area Network)
• Computers connected on single company
site
• Usually located in small area, such as a
home, office building, or group of buildings
• Connects two to several hundred computers
6-3
4. WAN (Wide Area Network)
• Connects computers across metropolitan,
state, regional, national areas
• Uses communication networks from vendors
– Licensed by government
6-4
5. Internet
• Global network of networks
• Connects LANs, WANs, and other networks
• Public and private networks of networks
• Uses a variety of communication methods and
conventions
– Seamless flow of data provided by
standardized layered protocol
• Protocol: a set of standard rules for data
representation, signaling, authentication, and
error detection required to send information
6-5
7. Q2: What Are the Components of a
LAN? (cont’d)
• Switch/Router
– Special purpose computer that receives and
transmits messages
• Network interface card (NIC)
– Hardware that connects each device’s circuitry to
the cable
– Onboard NIC or card that fits into an expansion
slot
• MAC (media control address)
• UTP (unshielded twisted pair cable)
• Optical fiber cable
6-7
9. IEEE 802.3 Protocol
(Institute of Electrical and Electronics Engineers)
• World’s most popular protocol for LANs
• 10/100/1000 Mbps Ethernet
• Protocol specifies hardware characteristics,
cables, and how messages are packaged
and processed
6-9
10. Q3: What Are the Alternatives for a WAN?
• WANs connect computers at separate sites
– Use routers and public communications links between
sites
– Cable connections made through licensed public
telecommunications companies
• Internet service provider (ISP)
– A company that provides customers access to
the Internet
– Provides legitimate Internet address
– Serves as gateway to Internet
– Pay for Internet access
6-10
11. Q3: What Are the Alternatives for a
WAN? (cont’d)
1. Connect to an Internet service provider (ISP)
2. Network of Leased Lines
3. Public Switched Data Networks
4. Virtual Private Network (VPN)
6-11
12. Connecting the Personal Computer to
an ISP: Modems
1.Dial-up Modems
2.DSL Modems
3.Cable Modems
• Modem – a device to produce a signal that can be
transmitted easily and decoded to reproduce the
original digital data
• Narrowband – less than 56 kbps speed
• Broadband – more than 256 kbps speed
6-12
13. 1. Dial-up Modems
– Converts signals between analog and digital so computers
can communicate over a regular telephone line
– Connection made by dialing the ISP’s phone number
– Maximum transmission speed of 56 kbps (53 kbps really)
– Modulation standards – V.34, V.90, V.92 specify how digital
signals are converted to analog
– Point-to-Point Protocol (PPP) – governs how data
(messages) are packaged and handled between modem and
ISP
6-13
15. 2. DSL Modems
Digital Subscriber Line (DSL)
– Operates over telephone lines
– Data signals do not interfere with voice
telephone service
– Faster data transmission than dial-up
• 256 kbps to 768 kbps
– DSL modems use their own protocols
– Download and upload speeds differ
• Asymmetric digital subscriber lines (ADSL)
• Symmetrical digital subscriber lines (SDSL)
– 1.544 mbps both directions
6-15
16. 3. Cable Modems
• Provide high-speed data transmission
• Use cable television lines
– High-capacity optical fiber cable run to neighborhood
distribution substation
– Television cables connect at distribution substation and runs
to subscriber’s house
– Does not interfere with television transmission
– Up to 500 subscribers connected at each substation
• Performance can vary based on number connected
– Download speed up to 10Mbps, upload up to 256 kbps
6-16
17. Networks of Leased Lines
• WAN connects computers located at geographically
distributed sites
• Access devices connect sites to transmission
– Router or network switch– special-purpose computer to move
network traffic from one node to another
• Leased line alternatives
– T1 line (1.544 Mbps), T3 line (44.736 Mbps), OC-768 (40 Gbps)
– Set up as a point-to-point line
– Only predefined sites can use leased lines
– Requires highly trained specialists to set up
– Monthly connection and maintenance costs expensive
– Provide low cost per bit transmitted for high traffic networks
6-17
19. Public Switched Data Network (PSDN)
Public Switched Data Network
– Network of computers and leased lines
– Developed and maintained by vendor
– Time leased on network
• Utility that supplies networks for other companies
to lease
– Each site leases line to connect to PSDN network
at access point
• Point of presence (POP)
• Saves setup and maintenance activities
6-19
21. PSDN Protocols
• Frame relay
– Process traffic at 56 kbps to 40 Mbps
– Simplest and easiest to support
– Lower cost than ATM
• Asynchronous transfer mode (ATM)
– Process speeds from 1 to 156 Mbps
– Support voice and data communication
• Ethernet
– Operates at speeds of 10 to 40 Gbps
6-21
22. Q4: How Does Encryption Work?
• Encryption
– Process of transforming clear text into coded text
– Used for secure storage or communication
– Uses algorithms
• DES (Data Encryption Standard)
• 3DES (Triple DES)
• AES (Advanced Encryption Standard)
• Key
– Used to encrypt data
– Algorithm applies key to produce coded message
• Symmetric encryption – same key encrypts and decrypts
• Asymmetric encryption – different keys are used
– Key used to decrypt message
6-22
23. Q4: How Does Encryption Work?
(cont’d)
• Public key/private key cryptography
– Public key used to encrypt messages
– Public key sent to other party to encode messages to be sent
back
– Decrypted with private key
– Complex and slow
• Asymmetric public key encryption — message encrypted using
recipient's public key. Recipient decrypts with own public key.
• HTTPS uses secure socket layer (SSL, a.k.a. TLS) protocol to
encrypt data
– Fast, secure
– See next slide for how it works
6-23
24. Q4: How Does Encryption Work?
(cont’d)
Figure 6-14
6-24
25. Q5: What Is the Purpose of a
Firewall?
• Computer device that prevents unauthorized network access
• May be special-purpose computer or program on a general-purpose
computer
• Organizations may have multiple firewalls
– Perimeter firewalls sit outside organizational network
– Internal firewalls are inside network
– Packet-filtering firewalls examine each part of a message (source
address, destination address, and other bytes) before allowing message
to pass
– May filter both incoming and outgoing messages
• Access control list (ACL)
encodes rules stating which
IP addresses are allowed into
or prohibited from the network
6-25
26. Q6: What Is a VPN, and Why Is It
Important?
Virtual private network
– Uses Internet or private network to create
appearance of point-to-point connections
– Uses public Internet to create appearance of
private network
– Client and server have point-to-point connection
called tunnel
• Private pathway over shared network
– Secure, encrypted communications
• VPN client software encrypts data and receiving VPN
server decrypts it
• Users and their keys must be registered with VPN server
6-26
28. How Does the Knowledge in This Chapter
Help Dee and You?
• Dee and salespeople use thin client computers
– Dee creates blog entries on Web pages using Movable
Type
– Salespeople use pages served by Movable Type
• Client computers contain VPN client software
– Interact with VPN server via Internet
– Secure, private connections
– Firewalls stop traffic not addressed to VPN server
– Salespeople know how to use VPN
6-28
29. How Does the Knowledge in This Chapter
Help Dee and You? (cont’d)
• IT department cannot schedule resources to setup
blog server in timely fashion
– Will not allow outside person to do setup
• Would then have access to system
• Consultant can create server on unprotected test
server
– Can be copied onto network server after acceptance
– Requires some internal IT labor
– Consultant could include Trojan horse virus or malware
• Install only software from known sources
• Code should not reside on production server
6-29
