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									                        Week 2



            Application Layers



ITEC 3210    Applied Data Communications and Networks
            Learning objectives
• Application architecture
   – Overview
   – Operations
   – Best practices

• Application exercises

• Warning:
   – Application architecture ≠ network architecture
   – Cost of development is LOW for any architecture that uses off-the-
     shelf application software package (see slide 16)
   – Use of quotation marks when using “server” for non-C/S architectures
     (not in figures though!)
   – Other: see slides!

ITEC 3210     Applied Data Communications and Networks
            Course overview




ITEC 3210   Applied Data Communications and Networks
                Application architectures
• Typical network application programs carry out four functions:
    –   Data storage: Storage and retrieval of data
    –   Data access logic: Query required to access a subset of data
    –   Application logic: Work performed by application
    –   Presentation logic: User interface

• Three basic application architectures determine how these functions are split
  between clients and servers:
    – Host-based architecture
    – Client-based architecture
    – Client-server architecture

• Application architecture:
    – Describes how application layer functions are spread among computers to deliver
      service to users
    – Not the same as network architecture which refers to a network physical and
      logical topology (hardware layer, see chapter 4)

    ITEC 3210         Applied Data Communications and Networks
                Host-based architecture
• Very first application architecture to be developed
   – The host, usually a mainframe, performs all 4 application program functions
   – The “client”, usually a terminal or a PC running a terminal emulation
     program sends and receives messages to and from host (See figure 2.1)


• Main limitation:
   – Upgrades come in large increments and are expensive
   Warning: Dennis writes that overload on host side can increase response
     time. This is true but it applies to ANY application architecture!


• Typical applications:
   – Support main line of business in large “paper-based factories” such as banks,
     insurance companies, government agencies, hospitals, universities, etc.
   – Why?

    ITEC 3210       Applied Data Communications and Networks
            Host-based architecture




ITEC 3210     Applied Data Communications and Networks
               Client-based architecture
• AKA peer-to-peer architecture

• Became important in the late 1980s with the widespread use of
  PCs and LANs
   – “Server” is a microcomputer responsible for data storage
   – “Client” is a microcomputer responsible for the data access, application
     and presentation logic (See figure 2-2)


• Limitations:
   – No access logic on “server” side: All data must travel to client for
     processing since queries are performed by client  Overload the network
     circuits  poor network performance
   – “Server” is a microcomputer: Limited processing power on “server” side
      poor network performance


   ITEC 3210      Applied Data Communications and Networks
       Client-based architecture




ITEC 3210   Applied Data Communications and Networks
            Client/Server architecture
• Most common application architecture used in today’s networks
   – Server handles data storage and data access logic
   – Client takes care of the presentation logic
   – Application logic may reside on the client, server or be split up between the
     two (see next four slides)


• Benefits:
   – Network performance:
       • Typically, servers have higher processing power
       • Processing is distributed between client and server
   – Lower required capacity (No unnecessary traffic)

• Comes in different versions: See next slides


   ITEC 3210       Applied Data Communications and Networks
Two-tier, three-tier and N-tier architectures
Refers to ways the application logic is partitioned in C/S architecture

• 2-tier architecture (see Figure 2-3):
    – 1 client, 1 server, application logic is “partitioned” between the 2
    – Most common architecture when number of users is less than 50

• 3-tier architecture (see Figure 2-4):
    – The client handles the presentation logic
    – The application server handles the application logic
    – The database server handles the data storage and data access logic

• N-tier architecture (see Figure 2-5):
    – More than three types of computers are used
    – Typically, a web server is added  Easy access from anywhere
    – Major limitation: complexity
    ITEC 3210        Applied Data Communications and Networks
       Client/Server architecture
            (2-tier C/S architecture)




ITEC 3210   Applied Data Communications and Networks
            Three-tier architecture




ITEC 3210     Applied Data Communications and Networks
            N-tier architecture




ITEC 3210   Applied Data Communications and Networks
           Thin versus thick clients
• Refers to how much of the application logic resides on
  the client
   – Little or no application logic resides on a thin client
   – All or most of the application logic resides on a fat client


• Thin clients are becoming popular because:
   – Systems are easier to manage: only the server application logic
     generally needs to be updated
   – Thin clients are cheaper



   ITEC 3210     Applied Data Communications and Networks
      More on clients and servers


   – http://www.serverwatch.com/

   – Technical focus 2.1 p.34

   – Assignment#1




ITEC 3210   Applied Data Communications and Networks
            Best practice architecture
• Often, the application architecture is given. So no decision has to be made

• When it is not given, 3 criteria help select an architecture:

    – Cost of Infrastructure: mainframes are expensive, which is why client-based and
      C/S architectures have become popular

    – Scalability refers to the ability to increase (or decrease) computing capacity as
      network demand changes. C/S architectures are the most scalable (both clients and
      servers can be added (reduced) to fit network requirements)

    – Cost of Development: Lower for ANY architecture that uses off-the-shelf
      software


 “For most projects, a C/S architecture, usually a three-tier thin or thick client,
   is recommended” p.41

    ITEC 3210        Applied Data Communications and Networks
          Application exercise:
         More on C/S architecture
Rate the C/S architecture on the criteria below:
  –   Centralized data storage
  –   Prevention of unauthorized access to file/applications
  –   Reliability (built-in reliability)
  –   Network administration
  –   Performance (faster processing)
  –   Number of users
  –   Relative cost
   ITEC 3210   Applied Data Communications and Networks
             Application exercise

Read minicase 2 p.60 –61:
  –    Which C/S architecture would you suggest?
  –    Why?


Modular design:
  –    Definition
  –    Application in the context of minicase 2


 ITEC 3210    Applied Data Communications and Networks
                 Thank you




ITEC 3210   Applied Data Communications and Networks

								
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