Physical Layer: Architectures, Devices and Circuits

1
Physical Layer Architectures, Devices and
Circuits

• After studying this Chapter you should
• Become familiar with different types of servers
  and clients
• Discuss the differences between host-based,
  client-based and client-server networks
• Understand the different types of network
  circuits
• Become familiar with the different media
• Understand front-end processors
• Understand how multiplexing works

2
Network Architectures

• Three components of a data communications network
• Client
• Server or Host
• Network circuits that connect them
• Host-Based Architectures
• Client-Based Architectures
• Client-Server Architectures

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 Networks

• Hosts (mainframes)
• Presentation
• Processing
• Data storage
• Clients (dumb terminals)
• Host does all the work

5
Client-Based Networks

• Hosts (microcomputers)
• data storage
• Clients (microcomputers)
• Presentation logic
• Data access logic
• Processing logic
• Clients perform most of the work

6
Client-Server Networks

• Hosts (micro-, mini-, mainframe computers)
• Processing
• Data access logic
• Data storage
• Clients (microcomputers)
• Presentation
• Processing
• Work is shared between hosts and clients

7
Client-Server Architectures
Server (microcomputer, minicomputer, or mainframe
computer)
Client (microcomputer)
Data Access Logic Data Storage Processing
Presentation Logic Application Logic Processing
8
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.

9
Middleware

• 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 applications
  data.

10
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

11
Three-tiered Architecture
Database server (microcomputer, minicomputer or
mainframe)
Application server (microcomputer or minicomputer)
Client (microcomputer)
Presentation Logic
Application Logic
Data Access Logic Data Storage
12
N-tier Architectures
Web server (microcomputer or minicomputer)
Client (microcomputer)
Presentation Logic
Application Logic
Database server (microcomputer, minicomputer or
mainframe)
Application server (microcomputer or minicomputer)
Data Access Logic Data Storage
Application Logic
13
Web Architecture
Client (microcomputer)
Web server (microcomputer, minicomputer, or
mainframe
Presentation Logic (web Browser)
Internet/Intranet
Circuits
Client (microcomputer)
Application Logic Data Access Logic Data Storage
Presentation Logic (web Browser)
14
Hosts or Servers

• Mainframe - suitable for WAN environments
• real-time data communications
• application processing
• transaction processing
• uses front-end processor
• Minicomputer - suitable to handle a specific
  number of facilities and terminals
• special purpose processing
• communications functions
• process control

15
Hosts or Servers - contd.

• Microcomputers - high powered desktop machines -
  suitable for a limited number of workstations
• LANs
• micro-to-mainframe configurations
• handles communications
• application sharing
• does not require long distance communications
  circuits

16
Clients

• The input/output hardware device at the users
  end of a communication circuit
• Terminals
• video display terminals (VDT)
• cathode ray tubes (CRT)
• dumb terminals - no processing
• asynchronous transmission
• transmission is which each information character
  is individually synchronized, usually by start
  and stop bits

17
Clients - contd.

• Microcomputers / Workstations
• suitable for application processing and
  specialized applications
• word processing, spreadsheets, simple graphics
• print sharing and telecommunications
• multi-tasking
• more memory and computer power

18
Clients - contd.

• A network computer is a new type of computer
  designed primarily as a thin client for use on
  the Internet.
• Some terminals are designed for special purposes
• Transaction terminals
• Automated teller machines.
• Point of sale terminals.

19
Fat/Thin Client or Server
20
Network Configurations

• The basic physical layout of the network
• point-to-point (two-point)- goes from one point
  to the next -- connects one client to one host
  (dedicated circuit)
• single user on the circuit
• multipoint (multidrop) - many clients are
  connected on the same circuit which in turn is
  connected to the host
• users must share the communication circuit
• less cable required

21
Point-to-point Configuration
Circuits
Client Computer
Host or Server Computer
Modem
Modem
22
Multipoint Configuration
Circuits
Host or Server Computer
Client Computer
Client Computer
Client Computer
Client Computer
23
Circuits

• Transmission circuits
• The path over which the voice, data, or image
  transmission travels. Circuits can be twisted
  wire pairs, coaxial cables, fiber optic cables,
  microwave transmission, etc. (Dial-up circuits)
• Voice grade leased circuits are normal telephone
  circuits that have been taken out of dial-up
  telephone service by the common carrier and
  dedicated to one organization. (private circuits,
  private lines, leased circuits, dedicated
  circuits)

24
Data Flow

• one or two directional
• Simplex - one-way transmission (radio, TV)
• Half duplex - two-way transmission
• one-way at a time
• control signals negotiate sending and receiving
• turnaround time, retrain time, reclocking time
• Full duplex - bi-directional
• simultaneous transmission
• no turnaround time

25
Terminology

• Medium - the substance or material that carries a
  message from one point to another, may be copper
  wire, fiber optics or air(signals)
• Circuit - the path over which data move, uses the
  above media
• Line - a physical connection between two points
• Link - an unbroken circuit or path
• Channel - the subdivision of a circuit

26
Communication Media

• Guided Media
• Twisted Pair Wire
• Coaxial Cable - copper core
• Fiber Optic Cable - hair-thin strands of glass
• Radiated Media
• Infrared Transmission - light waves
• Microwave Transmission - high frequency radio
  wave
• Satellite Transmission

27
Guided Media

• Twisted Pair Wire - insulated pairs of wire,
  twisted to minimize electromagnetic interference
  between wires.

28
Guided Media

• Coaxial Cable - wire with a copper core and an
  outer cylindrical shell for insulation.

29
Guided Media

• 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.

30
Radiated Media

• Infrared Transmission uses low frequency light
  waves to carry data through the air on a direct
  line-of-sight path between two points.

31
Radiated Media

• Radio (wireless) data transmission uses the same
  basic principles as standard radio transmission.

32
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.

33
Radiated Media

• 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.

Geosynchronous Communication Satellites (GEOs)
C-band 6/4 Ku-band 14/12 Ka-band 30/20
34
Media Selection
Guided Media Radiated Media
Network Transmission Error Media Type Cost Di
stance 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.Hig
h
Network Transmission Error Media Type Cost Di
stance 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
35
Special Purpose Communication Devices

• Front-end Processors (FEP)
• Multiplexers
• Protocol Converters

36
Front End Processors

• performs the processing and control functions
  required for the network to operate
• master controller for the entire network
• programmable - can handle some or all input and
  output activity
• non-programmable - hardwired communication
  control unit designed to adapt specific line and
  terminal characteristics to the computer

37
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.

38
Multiplexing

• To place two or more simultaneous transmissions
  on a single communication circuit
• done in multiples of 4, 8, 16, and 32
• transparent - does not interrupt the flow of data
• saves money

39
Multiplexing

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

40
Multiplexers

• Frequency Division Multiplexing (FDM)
• dividing the circuit horizontally
• series of separate channels transmitting at
  different frequencies
• Separated by guardbands

41
Multiplexers - contd.

• Time Division Multiplexing (TDM)
• dividing the circuit vertically
• time assigned to each terminal
• wastes time when terminals are idle

42
Multiplexers - contd.

• Statistical Time Division Multiplexing (STDM)
• allows more terminals to be connected to the
  circuit
• selecting transmission speed is based on
  statistical analysis of the usage requirements
• more efficient use of the circuit
• saves money over the other two types of
  multiplexing
• data must be identified by an address
• can cause time delays if all devices are
  transmitting

43
Multiplexers - contd.

• Fast Packet Multiplexing (FPM)
• variant of STDM
• high data transmission rates
• combines voice, video and data transmissions
• Inverse Multiplexing (IMUX)
• divides transmission over two smaller circuits
• previous term (biplexer)

44
Multiplexers - contd.

• Wavelength Division Multiplexing (WDM)
• allows multiplexing on fiber optics
• up to 10 simultaneous circuits
• variant of FDM
• Concentrators
• special form of statistical multiplexers
• no end user clients attached

45
Protocol Converters

• Protocol - the rules that allow two machines to
  communicate
• Protocol Converters - hardware and software that
  interconnect two dissimilar computer systems or
  terminals so they can communicate with each other
• Hardware protocol converter boxes convert the
  communication protocol used by one computer
  vendor to that required for another computer
  vendors equipment

46
Protocol Converters - contd.

• Add-on circuit boards convert microcomputers
  protocols to the protocol of the host computer
• Software protocol conversion packages support
  most terminal or microcomputer transmission
• LAN gateways as protocol converters
• gateway - hardware devices that connect local
  area networks to other dissimilar networks
• translate one network protocol into another