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IEM 5743
INFORMATION SYSTEMS AND TECHNOLOGY
Lecture Notes
Communication Architectures
Week #7, 2/23/00
Prepared by Dr. M. Kamath
Slide 1 is a quick review of the client/server concepts that we covered in Week 6. This slide shows
the relationships among the three different classification schemes that we used for the client/server
implementations. A brief description of the functional units is as follows:
 Presentation Logic: Manages the GUI and the takes care of the data formatting.
 I/O Processing Logic: Responsible for data entry validation and basic error checking.
 Business Logic: Executes business rules.
 Data Management Logic: Determines what data are needed.
 Data Manipulator: Deals with the actual data storage and retrieval.
Slide 2 splits the middleware into two levels. I gave the example of a customer using the telephone
to access an automated banking system. The telephone system takes care of the communication
between the customer of the bank’s computer. The software running on the server (bank) side
translates the voice and push-button commands into a form suitable for the computer program and
also translates the computer’s response into a form that the customer can understand. This type of
software is an example of the logical level of the middleware.
Hence, the middleware insulates the client from details of network protocols and server process
protocols. The front-end does not have to contain code that is specific to each server process. For
example, the programmer can use generic SQL syntax and the database middleware converts it
into database server’s expected syntax – an SQL dialect or non-SQL commands. An example of a
database middleware is ODBC (Open DataBase Connectivity) from Microsoft.
One other issue that I commented on is the hardware and software independence of client/server
software. Hardware independence says that client, server, and communications middleware
processes should run on multiple hardware platforms (IBM, Apple, Sun, etc.) without any
functional difference. Software independence says the client, server, and communications
middleware processes should support multiple operating systems (Windows, Unix, OS/2, etc.),
multiple network protocols (IPX, TCP/IP) and multiple application types (spreadsheets, e-mail,
DBMS, etc.). The above software and hardware independence is a desirable characteristic of
client/server software. In reality, we have various degrees of independence. Software written in
JAVA is perhaps the closest to the ideal situation. In other cases we have client (server) versions
that provide the same functionality, but are tailored to specific platforms.
In slide 4, we talked about LAN covering short distances. This limitation is primarily due to signal
degradation over long cables. With coaxial cables, a segment length can go up to 500 m. With the
twisted pair, which is the most popular, the segment length is limited to 100 m. With fiber optic
cables we can go long distances (few kilometers), but installation and cost are the main barriers.
Examples of network operating systems are NetWare, Unix, and Windows NT. Each has server
and client side components that need to be installed depending on how a machine is configured.
Message transfer is managed by protocols, such as TCP/IP or IPX (Internetwork Packet
Exchange), which is a NetWare communication protocol.
With respect to slide 5, I mentioned that we had already talked about many of these benefits when
we discussed the distributed computing architecture. Regarding the administration of software, I
gave the example of network versions of software packages, such as MS Word and Excel. Instead
of storing separate copies on PCs, the LAN can deliver a temporary copy to the PC client. This
reduces the number of copies or licenses that need to be purchased, for example, 10 instead of 25,
and simplifies the update and maintenance process.
The bullet “Ethernet uses a bus topology” in slide 8 generated some discussion. Invented at Xerox
in the seventies, Ethernet is currently the most widely used LAN access methodology. The older
generations of Ethernet, ThickNet and ThinNet, used coaxial cables and a linear bus topology.
The current twisted pair installations require a hub and the wiring is in a star configuration with all
computers connected to a hub. There was some confusion between this star topology for Ethernet
wiring and the star topology mentioned in slides 10 and 11. In the star-shaped topology mentioned
in slides 10 and 11, one computer is designated as the central hub and all other stations are directly
connected to it. The “hub” computer is part of the communication link between any two stations
connected to it. Whereas in the case of the 10BaseT (using twisted pair wiring) Ethernet the server
and all other computers, which are part of the LAN, are directly connected to the hub, which just
directs the traffic like a traffic cop. Bandwidth for standard Ethernet is 10 Mbps (million bits per
second) and is shared by all the nodes. In a switched Ethernet, a virtual connection is established
between two nodes and entire bandwidth is available for transmission between these two.
Ethernet is a contention-based protocol standard. Two or more computers can send data at once,
which causes a collision. Techniques to detect collision and retransmit are part of the Ethernet
protocol. In contrast, a toke ring network is more structured. A token is like a container being
passed around the network. There is only one token and a station can transmit data only if it gets a
token, i.e., an empty container. Token ring networks were made popular by IBM in the mid
eighties. Token ring networks are more reliable under heavy loads (more collision in Ethernet),
but are more expensive.
In the baseband transmission (slide 13) data, voice, and video signals are transmitted using a single
digitally coded signal by giving each type of signal a fixed unit of time for its transmission. This
method of interleaving digital signals is called Time Division Multiplexing (TDM). In the
broadband transmission, we have true simultaneous transmission by using different carrier
frequencies or the Frequency Division Multiplexing (FDM) method.
A bridge device (slide 14) operates at the OSI layer 2.
A router (slide 15) reads the network address of the destination and determines the best route based
on traffic load, line costs, speed, etc. Router works at OSI layer 3.
A gateway (slide 16) functions as an entry/exit point to a network and does complete conversion of
one protocol to another when two or more dissimilar networks are connected. It is operates at the
OSI layer 4.
VANs (slides 19 and 20) provide a fully outsourced network solution, including such value-added
services as network management, service level maintenance, and integration with other EDI
services. The connectivity is essentially that of a private WAN between businesses and their
trading partners.
OSI Reference Model (Slides 21-25)
7. Application Layer
6. Presentation Layer
Encryption, data conversion
5. Session Layer
Start, stop session
4. Transport Layer
Ensures delivery of file or message
3. Network Layer
Routing to different LANs, WANs,...
2. Data Link Layer
Transmission of packets from node to node
1. Physical Layer
Electrical signals and cabling
User B
Or Host B
User A
Or Host A
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Flow of Data