The tiers of a three-tiered system consist of the client
(user interface) tier, business logic tier, and data storage
tier.

program logic,business logic and database

in a three tiered architecture the three tier are 
presentation, application and database servers that r 
installed on a singel computer

1.Presentation tier :this is used by clients 4 making 
interative presenation usin graphical tool.
2.middle tier : this is used 4 tarnsfering data between 
prenation tier and database.
3.database management tier: used for web application

Hi if your asking about storage Tiered archiecture, it is 
First tier: Primary storage (NAS, SAN and DAS)
Second Tier: Backup and Recovery appliance(Disk Based)
Third Tier: Archival / Offline Storage Appliance

Reengineering is the organizational process required to 
align people, processes and technology with strategies to 
achieve business integration. It can also be thought of as 
taking a business in its current state and forming 
an organizational and operational blueprint to redirect 
skills, policies, information (data), cultural values, 
organizational structures, processing and incentives 
towards targeted improvements.

Distributed Application Environment is  designed to allow 
businesses to use IBM's RISC System/6000 and PS/2 computers 
in a client/server network (or standalone) for their 
business applications. 
DAE consists (1) MESSAGING services; (2) DATABASE 
services ; (3)  PRESENTATION services; (4)  DEVICE services 
to capture data and to control a broad category of 
controllers, devices, equipment and sensors.  

DOMS provide a way of pulling diverse components together 
and managing their communications. They are aimed at 
addressing the following: 
 - A single interface to manage the complexities of a 
heterogeneous environment 
 - A uniform framework, based on standards and 
extensibility, to build,integrate, and deploy open 
distributed-computing applications
 - A method for creating location independence for client 
applications.

DCE is the Distributed Computing Environment, from the Open 
Software Foundation. (It is called "the DCE" by sticklers 
for grammatical consistency.)

DCE consists of multiple components which have been 
integrated to work closely together.  They are the Remote 
Procedure Call (RPC), the Cell and Global Directory 
Services (CDS and GDS), the Security Service, DCE
Threads, Distributed Time Service (DTS),and Distributed 
File Service (DFS).  The Threads, RPC, CDS, Security, and 
DTS components are commonly referred to as the "secure 
core" and are the required components of any DCE 
installation.  DFS is an optional component.

DCE is called "middleware" or "enabling technology."  It is 
not intended to exist alone, but instead should be 
integrated or bundled into a vendor's operating system 
offering.  DCE's security and distributed filesystem, for 
example, can completely replace their current, non-network, 
analogs.

NAS is a software suite that enables applications to be 
integrated across a distributed network of multivendor 
systems: NAS services are divided into four main groups:
 - Application Access Services which support user interface 
(forms based,graphical or character terminal based)
 - Communication and Central services which support 
communications using electronic mail and EDI
 - Information and resource sharing, which supports 
database access and compound documents (collection of 
images, graphics and conventional text)
 - System Access services which provides a standard 
operating system interface.

Open Network Computing (ONC) is the Sun architecture, with 
third party alliances providing the missing pieces.  Sun's 
RPC is licensed by more than 300 companies, including DEC, 
IBM, and Apple Computer.  It is a lean, efficient  RPC that 
is particularly effective for installations not requiring 
WAN support.  Transport Independent Remote Procedure
Calls (TI-RPC) were co-developed by Sun and AT&T.  TI-RPC 
runs independent of the underlying network transport, 
allowing applications to run unmodified over multiple 
transport protocols including NetWare,PCNFS, AIX/6000, 
Solaris, UNIX/SVR4, AIX/ESA.  NetWise developed the
TI-RPC Application Toolkit source code generation 
technology.

VITAL provides a way of building information systems 
constructed from generalized modules that can be reused and 
shared.It specifies how to construct and provide interfaces 
for these modules. A cornerstone of Apple's client/server 
architecture is Data Access Language (DAL).  DAL 
consists of software components for the client and the 
server (Macintosh System 7 includes the client component, 
DOS and Windows clients are sold separately).  DAL sits 
between the client and server components.  
This approach allows varied clients to communicate with 
many different servers.  With DAL, a change in the DBMS 
does not affect the front-end; 
it simply requires the appropriate DAL link.

SAA is a collection of selected software interfaces, 
conventions, and 
protocols that are used as a framework for developing 
consistent, 
integrated applications across the major IBM computing 
environments.

Four major components of this architecture are:
 - Common User Access (CUA) defines conventions for GUI 
look and feel.
 - Common Programming Interface (CPI) provides languages, 
tools, and  APIs that give applications greater portability 
and more consistent user interfaces across multiple 
platforms. 
 - Common Communication Support (CCS) supports existing 
   communications standards, such as LU 6.2. 
 - Common Applications, written by IBM, will serve as 
demonstrations of SAA concepts and make it easy for users 
to migrate between systems. 

Here the split occurs in the application functionality, one 
part going to the client, other to the server. Distributed 
function applications are the most complex of the three 
typologies  since two separately compiled application 
programs must be developed.  Developers must analyze where 
each function should reside and what type of dialog must 
occur between the two programs. The underlying 
communications facilities may implement either a message-
based or remote procedure call (RPC) mechanism for 
transfer of dialog and data.  However, there are many 
variants of this typology. One variant of the distributed 
function style is where data management and application 
function occur at both the client and server. 
In this instance, data management at the client would 
include referential, read-only data. Data frequently 
updated or accessed by many users would 
reside on the server.

In remote data management, the entire application resides 
on the client and the data managment is located on a remote 
server/host.   Remote Data Management is relatively easily 
to program for because there is just one application 
program. The client communicates with the server using SQL, 
the server then responds with data that satisfies the 
query.  RDBMS products that offer remote data management 
provide a layer of software on the client to handle the 
communication with the DBMS server.This style represents a 
more traditional LAN database server or file server 
approach. 
Workstations support the presentation and function logic 
and interface with the data server through the data 
manipulation language.Distributed data management is an 
extension of remote data management and uses the 
distributed facilities of the DBMS to access distributed 
data in a manner transparent to users. This is most 
relevant for architectures having data spread  across  
several servers and when access to a DBMS on another 
server is required.

The simplest form of servers are disk servers and file 
servers. With a file server, the client passes requests for 
files or file records over a network to the file server. 
This form of data service requires large bandwidth and can 
slow a network with many users down considerably. 
Traditional LAN computing allows users to share resources, 
such as data files and peripheral devices, by moving them 
from standalone PCUs onto a Networked File Server (NFS).

The more advanced form of servers are database servers, 
transaction server and application servers (Orfali and 
Harkey 1992). In database servers, clients passes SQL 
(Structured Query Language) requests as messages to the 
server and the results of the query are returned over the 
network. The code that processes the SQL request and the 
data resides on the server allowing it to use its own 
processing power to find the requested data, rather than 
pass all the records back to a client and let it find its 
own data as was the case for the file server. In 
transaction servers, clients invoke remote procedures that 
reside on servers which also contains an SQL database 
engine. There are procedural statements on the server to 
execute a group of SQL statements (transactions) which 
either all succeed or fail as a unit. The applications 
based on transaction servers are called On-line Transaction 
Processing (OLTP) and tend to be mission-critical 
applications which require 1-3 second response time, 100% 
of the time and require tight controls over the security 
and integrity of the database.  
The communication overhead in this approach is kept to a 
minimum as the  exchange typically consists of a single 
request/reply (as opposed to multiple SQL statements in 
database servers). Application servers are not necessarily 
database centered but are used to server user needs, such 
as. download capabilities from Dow Jones or regulating a  
electronic mail process. Basing resources on a server 
allows users to share data, while security and management 
services, which are also based in the server, ensure data 
integrity and security. 

1.File server
2.Database Serevr
3.Groupware server
4.Object server
5.Transaction server
6.Web server
7.Appilication server

Servers are computer that provides the services. As for 
Example - 
1. DNS Server
2. WINS Server
3. DHCP Server
4. RAS Server
5. VPN Server 
6. File Server
7. IIS server
etc