1. Introduction
There are three fundamental hardware components in a data communications network in addition to the network operating software:
the servers or host computers,
the client computers,
the network circuits that connect them

2. Network Architectures
There are three fundamental network architectures:
Host-based networks: the host computer performs virtually all of the work
Client-based networks: the client computer performs virtually all of the work
Client-server networks: the work is shared between the hosts and clients

3. Network Architectures
The work done by any application program can be divided into four general functions:
data storage
data access logic
application logic
presentation logic

4. Host-Based Architectures
The very first data communications networks were host-based, with the host computer performing all four functions. The clients enabled users to send and receive messages to and from the host computer.
This very simple architecture often works very well. Application software is developed and stored on one computer and all data are on the same computer.

5. Host-Based Architectures

6. Host-Based Architectures
The fundamental problem with host-based networks is that the host must process all messages.
In the late 1970s and early 1980s, intelligent terminals were developed that could perform some of the presentation function.

7. Client-Based Architectures
In the late 1980s, there was an explosion in the use of microcomputers and microcomputer-based local area networks.
Part of this expansion was fueled by a number of low-cost, highly popular applications such as work processors, spreadsheets, and presentation graphics programs.

8. Client-Based Architectures
With client-based architectures, the clients are microcomputers on a local area network, and the host computer is a server on the same network.
This simple architecture often works well. However, as the demands for more and more network applications grow, the network circuits can become overloaded.

9. Client-Based Architectures

10. Client-Server Architectures
More organizations today are moving to client-server architectures.
Client-Server attempts to balance the processing between the client and the server by having both do some of the processing.

11. Client-Server Architectures

12. Costs and Benefits of Client-Server Architectures
Client-server architectures have some important benefits compared to host-based architectures.
Client-server architectures are scaleable
Client-server architectures can support many different types of clients and servers.
Because no single host computer supports all the applications, the network is generally more reliable.

13. Client-Server Architectures
Client-server architectures also have some critical limitations, the most important of which is their complexity.
Even updating the network with a new version of the software is more complicated too.
Much of the debate between host- and client-server networks has centered on cost. Microcomputer hardware is more than 1000 times cheaper than mainframe hardware for the same amount of computing power.

14. Client-Server Architectures
Client-server networks enable software and hardware from different vendors to be used together. Unfortunately, they have few standards. One solution is middleware, software that sits between the application software on both the client and the server.
There are dozens of standards for middleware, each of which is supported by different vendors, and each of which provides different functions.

15. Client-Server Architectures
Middleware does two things:
1. It provides a standard way of communicating that can translate between software from different vendors.
2. It manages the message transfer from clients to servers so that the clients need not know the specific server that contains the application’s data.

16. Two-tier, Three-tier, and N-tier Architectures
There are many ways in which the application logic can be partitioned between the client and the server.
Two-tiered
Three-tiered
N-tiered

17. Two-tier, Three-tier, and N-tier Architectures
Two-tiered client-server architecture

18. Two-tier, Three-tier, and N-tier Architectures
Three-tiered client-server architecture

19. N-tier Architectures
N-tiered client-server architecture

20. Two-tier, Three-tier, and N-tier Architectures
The primary advantage of n-tiered client-server architecture is that it separates out the processing that occurs to better balance the load on the different servers; it is more scaleable.”
There are two primary disadvantages to an n-tiered architecture.
1. IT puts a greater load on the network
2. It is much more difficult to program and test software for a n-tiered architectures.

21. Thin Clients versus Fat Clients
Another way of classifying client-server architectures is by examining how much of the application logic is placed on the client.
A “thin client” places little or no logic on the client, and are easier to manage.
A “fat client” places all or almost all of the application logic on the client.
There is no direct relationship between thin/fat clients and 2-/3-/n-tiered architectures.

22. Web Architecture

23. Servers
A computer’s suitability to serve as the server or host for an online, real-time data communication network depends on both its own capabilities and the capabilities of other attached hardware.
There are three typical types of hosts:
Mainframe computers
Minicomputers
Microcomputers

24. Servers
A mainframe is a very large general purpose computer usually costing millions of dollars.
A microcomputer is a large general purpose computer usually costing hundreds of thousands of dollars that is used both for data communications as well as application processing.
A microcomputer can range from a small desktop system to a workstation costing $50,000 or more.

25. Clients
The client is the input/output hardware device at the user’s end of the communications circuit.
There are four major categories of clients:
Terminals
Microcomputers / workstations
Network computers
Special purpose terminals

26. Clients
When online transaction processing system became common in the 1970s, so did the use of terminals. The very first terminals were known as dumb terminals.
Intelligent terminals were developed to reduce processing demands.
Microcomputers are more powerful computers that are designed for use in more technical applications.

27. Clients
A network computer is a new type of computer designed primarily as a thin client for use on the Internet (or an intranet or extranet).
Some terminals are designed for special purposes:
Transaction terminals
Automated teller machines.
Point of sale terminals.

28. Network Configuration
Network configuration is the basic physical layout of the network.
There are two fundamental network configurations:
Point-to-point configuration (or two-point) - sometimes called dedicated circuits.
Multipoint configuration (or multidrop).
Most complex computer networks have many circuits, some of each type.

29. Network Configuration

30. Multipoint Configuration

31. Data Flow
Circuits can be designed to permit data to flow in one or both directions. There are three ways to transmit:
Simplex - One way transmission
Half-duplex -Two way communications link, but only one system can talk at a time.
Full duplex -Transmit in both directions simultaneously.

32. Data Flow

33. Communication Media
The medium is the matter or substance that carries the voice or data transmission.
There are two basic types of media:
Guided media - those in which the message flows through a physical media.
Radiated media (unguided) - Those in which the message is broadcast through space.
Circuits sold by the common carriers are called communications services.

34. Guided Media
Twisted Pair Wire - insulated pairs of wire, twisted to minimize electromagnetic interference between wires.
Coaxial Cable - wire with a copper core and an outer cylindrical shell for insulation.
Fiber Optic Cable - high speed streams of light pulses from lasers or LEDs carry information inside hair-thin strands of glass or plastic called optic fibers.

35. Guided Media
Insert Figure 3-10

36. Guided Media

37. Guided Media

38. Guided Media
The earliest fiber optic systems were multimode, (light could reflect inside the cable at many different angles).
Single mode fiber optic cables transmit a single direct beam of light through a cable that ensures the light only reflects in one pattern.
Fiber optic technology is a revolutionary departure from the traditional message-carrying systems of copper wires.

39. New Brunswick Fiber

40. Radiated Media
Radio (wireless) data transmission uses the same basic principles as standard radio transmission.
Infrared Transmission uses low frequency light waves to carry data through the air on a direct line-of-sight path between two points.

42. Radiated Media
A microwave is an extremely high frequency radio communication beam that is transmitted over a direct line-of-sight path between two points.
Transmission via satellite is similar to transmission via microwave except, instead of transmitting to another nearby microwave dish antenna, it transmits to a satellite 22,300 miles in space.

43. Radiated Media

44. Radiated Media
One disadvantage of satellite transmission is the delay that occurs because the signal has to travel out into space and back to Earth (propagation delay).
One problem associated with some types of satellite transmission is raindrop attenuation (some waves at the high end of the spectrum are so short they can be absorbed by raindrops).

45. Radiated Media
Ku-band satellites use waves that are so short they can be caught and concentrated in much smaller dish antennas, called very small aperture terminals (VSAT).
The larger Earth dish hubs can cost as much as several hundred thousand dollars.
In late 1994, RCA introduced a direct broadcast satellite (DBS) system that uses 18-inch KU-band VSATs to receive satellite broadcasts for about $500.

46. Media Selection Guided Media Radiated Media Network Transmission Error
Media Type Cost Distance Security Rates Speed
Twisted Pair LAN Low Short Good Low Low-high
Coaxial Cable LAN Mod. Short Good Low Low-high
Fiber Optics any High Mod.-long V. Good V.Low High-V.High
Network Transmission Error
Media Type Cost Distance Security Rates Speed
Radio LAN Low Short Poor Mod Low
Infrared LAN, BN Low Short Poor Mod Low
Microwave WAN Mod Long Poor Low-Mod Mod
Satellite WAN Mod Long Poor Low-Mod Mod

47. SPECIAL PURPOSE COMMUNICATION DEVICES

48. Special Purpose Communication Devices
Front end processors perform certain network functions for a host computer so that the host does need not spend as much time managing the network.
Multiplexers break one high speed communication circuit into several lower speed circuits so that many different devices can simultaneously use it.

49. Front End Processors (FEP)
An FEP can take two forms:
1. A non-programmable, hardware, communication control unit designed by the computer manufacturer to perform specific communications services for one specific type of mainframe.
2. A general purpose microcomputer or minicomputer that can handle some or all the communication activity for many types of computers.

50. Front End Processors (FEP)
The primary application of the FEP is to serve as the interface between the host computer and the data communications network with its terminals or microcomputers.
Many of the more powerful front end processors can do message processing.

51. Front End Processors (FEP)
Intelligent controllers are scaled-down FEPs that perform the same function for the FEP that the FEP does for the host.
Remote intelligent controllers (or intelligent terminal controllers) reside at the far end of a communications circuit and control 4 to 32 terminals.

52. Front End Processors (FEP)

53. Multiplexers
A multiplexer puts two or more simultaneous transmissions on a single communications circuit.
Multiplexing usually is done in multiples of 4, 8, 16, or 32.
Generally speaking, the multiplexed circuit must have the same capacity as the sum of the circuits it combines.
The primary benefit of multiplexing is to save money.

54. Multiplexed Circuit

55. Multiplexing There are three major types of multiplexers
Frequency division multiplexers (FDM)
Time division multiplexers (TDM)
Statistical time division multiplexers (STDM)

56. Frequency Division Multiplexing (FDM)
Frequency division multiplexers can be described as dividing the circuit “horizontally” so that many signals can travel a single communication circuit simultaneously.
The circuit is divided into a series of separate channels, each transmitting on a different frequency.

57. Frequency Division Multiplexing (FDM)

58. Frequency Division Multiplexing (FDM)
Frequency division multiplexers are somewhat inflexible because once you determine how many channels are required, it may be difficult to add more channels without purchasing an entirely new multiplexer.
Wavelength division multiplexing (WDM) is a version of FDM used in fiber optic cables. WDM should reach 25 terabits per second within five years.

59. Time Division Multiplexing (TDM)
Time division multiplexing shares a circuit among two or more terminals by having them take turns, dividing the circuit “vertically.”
Time on the circuit is allocated even when data are not transmitted, so that some capacity is wasted when a terminal is idle.
Time division multiplexing is generally more efficient and less expensive to maintain than frequency division multiplexing, because it does not need guardbands.

60. Time Division Multiplexing (TDM)

61. Statistical Time Division Multiplexing (STDM)
Statistical time division multiplexing is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines.
STDM is called statistical because selecting the transmission speed for the multiplexed circuit is based on statistical analysis of the usage requirements of the circuits to be multiplexed.

62. Statistical Time Division Multiplexing (STDM)
STDM provides more efficient use of the circuit and saves money. However, STDM introduces two complexities:
1. STDM can cause time delays, if all terminals decide to transmit simultaneously.
2. All data must be identified by an address that specifies the device to which terminal it belongs.
Most STDM multipexers do not send one character at a time from each terminal, but send groups of characters at one time.

63. (STDM)

64. NASA’s Ground Network

65. Inverse Multiplexing
Inverse multiplexing (IMUX) combines several low speed circuits to appear as one high speed circuit.
One of the most common uses is to provide T-1 (1.544 Mbps) circuits for wide area networks, by combining 24 slower (64 Kbps) circuits.
Until recently, there were no standards for inverse multiplexing.

66. Inverse Multiplexing

67. Bank Networks