Computer (Transport) Layer

1
Computer (Transport) Layer

• Manages communication between two computers
• Platform independence one computer does not
  know even what platform the other is (mainframe,
  PC, etc.)
• TCP/IPs most popular transport standard is TCP

2
Terminal-Host Platform

• Processing done at a central host computer
• Mainframe (very large)
• Minicomputer (mid-size)

Programs Execute on the Host
Host
3
PC Network

• Client PC sits on desks of ordinary users
• Receives services from servers
• Stand-alone PC with hardware, software added

Client PC
Client PC
4
PC Network

• Servers
• Provide services to client PCs
• Usually PCs but not always

Server
Service
Service
Service
Server
Server
5
PC Network

• Servers are Specialized
• Optimized for their services for cost,
  performance, and reliability

File Server
File Service
Database Server
Internet Access Gateway Server
6
File Server Program Access

• Programs executed on the client PC
• Program and data files downloaded over the network

Program Executed On Client PC
Downloaded Over Network
Client PC
Server
Network
7
Client Workstation

• Workstations are more powerful than PCs
• Come in client and server versions

21 display Powerful RISC microprocessor Large,
fast RAM, hard disk Usually UNIX OS, can
be Windows NT
Client Workstation
8
Networked PC

• The Dominant Computer Platform Today is the
  Networked PC
• High penetration of PCs on desktops
• Most are already networked
• Legacy Systems Must be Maintained
• Mainframe terminal-host systems
• Minicomputer terminal-host systems
• Many are being downsized to PC networking

9
Managed PCs

• Designed to be Managed Remotely
• Slashes maintenance costs
• Slashes software upgrade costs
• Users have less control over their machines
• Company can search for illegal games
• Management standards are still immature
• Eventually, all networked PCs will be managed

10
NetPC

• Standard Pushed by Microsoft, Intel
• Closed box with limited upgradability
• Should ease maintenance by reducing
  incompatibilities and add-in problems
• Will users be content?
• Remote management built in

11
Network Computers (NCs)

• Designed for Use on the Internet (Java)
• Little or no local disk drive storage
• Programs downloaded over Internet when needed (or
  from corporate servers using Internet standards)
• User always gets the latest version of the
  software
• Downloading can take time small applications
  would load faster but might lack functionality
• When the network is down, so are you

The Internet
12
Low-Cost PCs for the Home

• WebTV (Microsoft)
• Uses TV for its display
• User only needs 500 box
• Problem TV display resolution is far lower than
  that of a TV screen webpages look bad
• Problem Need special Internet service provider
  who cannot work with your regular PC
• Problem No printing
• Problem Limited to WWW and email
• Sub 1,000 PCs

13
Networks Mix Traffic of Multiple Platforms
Network (LAN, WAN, internet)
PC network traffic
Terminal-host traffic
14
F2-7 Program Interactions on a Stand-Alone PC
Application Program
Request
Response
Operating System
Commands related to files
Files or error messages
Hardware
15
Client/Server Processing
Client Program
Server Program
Platform independent
TWO programs on TWO machines
Client Machine (PC)
Server Machine (UNIX Minicomputer)
16
Client/Server Processing

• Application Layer Protocols
• Control the requests and responses
• Application independence any browser can work
  with any webserver program
• Transport Layer Protocols
• Allow two computers with different architectures
  to work together
• Brings platform independence

17
Internet Access
To connect to the Internet, you must have an ISP
Webserver
Internet Service Provider (ISP)
The Internet Backbone
Internet Service Provider (ISP)
PC with browser
18
Internet Service Provider (ISP)

• Basic Service Links You to the Internet
• Provides a connection
• Pays for your use. (The Internet is NOT Free.)
• Most Provide Additional Services
• E-mail
• World Wide Web Homepage Hosting
• Premium Providers have Specialized
  Services(America Online, CompuServ, MSN)

19
Internet Access with PPP
ISP
Modem, TCP/IP software PPP software
PPP link over telephone line
20
Internet Access with PPP routers
Router
Router
Router
The Internet 1. a network of routers connected
by transmission lines
ISP Router
2. Subnets connected by routers
ISP
21
Layered Interactions in Internet Access
User PC
Router
Webserver
Browser
Webserver Software
TCP Software
TCP Software
Subnet Layer (PPP)
IP Software
IP Software
IP Software
CSLIP/PPP Software
PPP Software
Subnet
Point-to-Point Connection
22
Point-to-Point Connection

• Physical Layer
• Serial Port
• Modem
• Telephone Transmission
• Data Link Layer
• Packaging of data for delivery over line
• Error detection and correction
• CSLIP or PPP standard

23
Layered Interactions in Internet Access
User PC
Webserver
Routers
Browser
Webserver Software
Internet Layer Governs Routing Across Multiple
Routers On the Internet
TCP Software
TCP Software
IP
IP
IP Software
IP Software
IP Software
PPP Software
Subnet
Subnet Software
24
Routing Across the Internet

• Routing
• Message (packet) must travel over a network of
  routers
• The user PC tells the first router the
  destination address of the webserver (e.g.,
  voyager.cba.hawaii.edu)
• Working together, routers route the packet to its
  destination host

25
Layered Interactions in Internet Access
User PC
Webserver
Routers
Browser
Webserver Software
Transport Layer (TCP)
TCP Software
TCP Software
IP Software
IP Software
IP Software
PPP Software
Subnet
Subnet Software
26
Transport Layer

• Connects the Two Computers
• Establishes a temporary connection
• Breaks connection when transmission ends
• Asks for the retransmission of damaged packets
• Asks for the retransmission of lost packets
• Flow control tell the other computer to pause
• Platform independence connects computers from
  different vendors

27
Layered Interactions in Internet Access
User PC
Routers
Webserver
Browser
Webserver Software
Application Layer (HTTP, HTML)
TCP Software
TCP Software
IP Software
IP Software
IP Software
PPP Software
Subnet
Subnet
28
Protocol Data Units (PDUs)

• At each layer, messages are strings of 1s and 0s
• These messages are organized
• Called protocol data units (PDUs)
• Each layer (except Physical) has a PDU
• E.g., Application PDU or APDU
• E.g., Computer (Transport) PDU or CPDU

PDU
29
Organization of PDU

• Header (delivery information)
• Trailer (sometimes present for error handling)
• Data Field
• Often PDU of next higher layer

PDU at Layer N1
PDU at Layer N
Trailer
Data Field
Header
30
PDU Transmissions from the PC
User PC
Application Layer
Browser
Application PDU
APDU
Computer (Transport) Layer
TCP Software
APDU
CH
(CPDU) TCP PDU
Internet Layer
IP Software
CPDU
IH
Internet PDU (IPDU)
Subnet Layer
CSLIP/PPP Or Other Subnet Software
SnT
IPDU
SnH
Subnet PDU (SnPDU) CSLIP or PPP
31
At Each Router
2. Subnet process removes IP PDU from SnPDU,
delivers it to the IP process.
3. IP process considers best route
Router
4. IP process delivers the IP PDU to the subnet
process
IP Software
IP PDU
IP PDU
Subnet Process
SnPDU for Subnet A
SnPDU on Subnet B
SnA
SnB
1. Router subnet process accepts the SnPDU
Subnet A
Subnet B
32
At the Webserver
Webserver
4. TCP Software delivers APDU to Webserver Program
Webserver Software
APDU
3. IP Software delivers CPDU to Transport Process
TCP Software
CPDU
IP Software
2. delivers IPDU to IP Software Process
IPDU
Subnet Software
1. accepts SnPDU
SnPDU
Subnet
33
Intranets

• Use of Internet Technology within Firms
• Standards are mature
• Standards for many applications
• Security is good for internal use
• Platform independent No need to create separate
  applications for PCs, Macs, Unix
• Browser runs software no need to install
  software on PCs
• Major concern of corporations today

34
Intranets

• Must keep out hackers
• Yet users need access to Internet services
  outside the Internet
• Solution create a firewall to limit access from
  the outside

Firewall
OK
Intranet
No
Hacker
35
Extranets

• Give Buyers or Suppliers Access
• Limited to certain resources, such as inventory
  and pricing databases
• Uses the Internet for transmission

Extranet Resource
OK
Customer
No
Any Other Resource
36
Extranets

• Tunneling
• Extranet transmission uses the unsecure Internet
• Add secure transmission through the Internet by
  means of encryption, other measures
• Creates a secure tunnel through the Internet

Tunnel