Portability by pengxiang


									              A2 MODULE 5 (ICT5) TOPIC 14.10 PORTABILITY OF DATA

Protocols and standards (p237)
 Explain the need for portability of information e.g. ease of transferring numerical, graphical
   and textual data between applications.
 Describe the need for standards for interchanging numerical, graphical and textual data
   between different hardware and software platforms.

Communication standards (p235)
 Know of the existence, benefits and limitations of standards.
 Understand the protocols and addressing mechanisms used to support the World Wide Web.

Emergence of standards
 Recognize the existence of de facto standards based on historic precedent and sales success
  in comparison to formal standards.

Communications Software
Typical features you would expect from Communications Software include: -
Direction of data flow - Routing the signal to Dialling telephone numbers (dialup software)
and from modem etc
Speed of transmitting data                     Error correction
Programming macros                             Conducting multiple sessions
Setting up private functions such as e-mail,   Method of transmission - handling parameters,
BBS, conferencing                              protocols and handshaking
Type of parity used.

The rules and procedures used for allowing computers to communicate with each other is called
the communications protocol.

Speed of transmission
The range of frequencies that can be accommodated on a particular telecommunications medium
is called its bandwidth - the difference between the highest and lowest frequency that can be
accommodated on a single frequency. Digital TV. There are two common types of bandwith
 Baseband (see p.326)
 Broadband (see p.326)
The speed at which data is sent is expressed as the baud rate

           Medium                     Spee d
           twisted-pair               500bps to 10mps
           coaxial cable              56kps to 200mps
           fibre-optic                500 kpbs to 10gbps
           mic row ave                256 kbps to 100mps
           satellite                  256 kbps to 100mps
           trans mis sion rate is measured in bits per sec ond

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              A2 MODULE 5 (ICT5) TOPIC 14.10 PORTABILITY OF DATA

Asynchronous Transfer Mode (ATM) most corporations use intranets for voice, private-line
services and data each of which may be based on different technologies. ATM can seamlessly
and dynamically switch voice, data, images and video between users. Promises to tie LAN (lower
speed protocols) with WANs (higher speed). ATM parcels information - requires fibre-optics


A modem is necessary on non-digital networks because traditional telephone systems were
designed to carry analogue not digital signals. We can expect all telephone cabling to be digital in
the very near future.
 MODulation is the process of converting digital signals into analogue form.
 DEModulation is the process of converting analogue signals into digital form.
 Modem Speeds are measured in bits per second (bps) or baud rates - 2400, 9600, 14,400
 Physically modems can be inside or outside the computer.
 Modems often come with the following features
Error control - checking poor telephone lines at high speeds, request retransmission where
problems occur
Data compression e.g. V.42bis supports compression 4:1 text and bitmapped graphics
Compatibility with other modems e.g. Hayes AT command set

Direction of transmission p.327
       Simplex - one direc tion only
                                                         Handshaking is using procedures and
                                                         standards to establish communications
                                                         between two computers or between a
                                                         computer and a peripheral device.

     Half-Duplex - tw o way flow
     but only in one direc tion at a time

     Full -Duplex - both directions at
     the same time

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               A2 MODULE 5 (ICT5) TOPIC 14.10 PORTABILITY OF DATA

Serial and parallel transmission

Serial transmission
 Binary signals representing data are transmitted one after the other
 Normally used except for very short connections between a peripheral and a computer


                  Serial transmis sion
Parallel transmission

        Data bits are communicated as groups in parallel
        Quicker than sending serially
        only possible over short distances

                    Parallel Trans mis sion
        good for high speed devices (backing store)

Messages are divided into data packets, which are then directed through the network to their
destination under computer control.P.329

                                                     An analogu transmission techniques, but
 Much of the public switched telephone network (pstn) uses digital e s ignal is a continuous
+1                                                   sine w ave over a particular range of
some local connections to homes and businesses still use analogue signals.
                                                     frequencies . A pos itive voltage c harge
                                                     represents a “1” and a negative voltage
0                                                    represents a “0”. Analogue signals reflec t
                                                     variations in pitch.

 1            1
                              A digital s ignal contains discrete bursts
                              representing “on” and “off” electrical pulses .
                              Digital lines tend to be faster and more accurate
                              than analogue.

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              A2 MODULE 5 (ICT5) TOPIC 14.10 PORTABILITY OF DATA

 Arranging bits of data for transmission
Asynchronous transmission
 Bytes are transmitted one at a time, where each string is framed by control bits (start, parity
   and stop bits)
 When a sending device transmits characters at irregular intervals e.g keyboard device -
   transmitting asynchronously
Synchronous transmission
 Several characters at once in blocks, no start or stop bits
 Transmitting large volumes of data at high speeds

Telecommunications standards and protocols

The drive towards open systems
Historically Data Communications has always had to deal with problems of INCOMPATIBLE
standards. A solution to this i.e. to have systems that can communicate to any other systems
(open) is of prime importance.
The HUGE GROWTH in the use of data communications has been made possible by the use of
some common standards. Common standards lead to open systems which allows users to use
components from more than one manufacturer. (Remember Plug-and-Play, TWAIN). It is
unattractive to users to be restricted to particular systems.
There is a need for information to be PORTABLE - i.e. the ability of a program or system to be
used on different computer hardware. Commercial systems often remain in use far longer than
the original hardware and will be adapted or ported to new types of hardware e.g. UNIX,
COBOL. Software packages traditionally will only work on particular hardware platforms
(attempts HTML and Java)
Need to be able to IMPORT/EXPORT text, numbers, graphics data between applications - to
avoid any re-entry or editing of data - objects or pictures.

Communications protocols are a standard set of rules describing the transfer of data between
devices, typically to
Control - identify each device in the communication path
Secure the attention of the other device
Verify the correct receipt of a transmitted message
Verify a message requires retransmission because it cannot be correctly interpreted
Perform recovery when errors occur

DE Jure standards
De Facto standards - these are established by the fact that a certain manufacturer dominates the
market e.g. Intel processors and Microsoft software.
Examples of De Facto standards:
 QWERTY keyboards (why don't we adopt a different layout?)

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 Light switches (down is on in Britain but off in America)
 Windows and DOS (see case study p.328-329)
ISO (International Standards Organisation) and Open System Interconnect (OSI) (p.114)
In 1978 the ISO issued a model of network protocols called Open System Interconnect (OSI)
allowing computers of different origins regardless of manufacture to be linked
Is a UN body setting telecommunication standards, with headquarters in Geneva. Standards
apply to modems, fax and networks.
V series of standards relate to equipment covered on telephone lines
X series of standards cover digital packet transmission

V.32bis is the CCITT protocol for 14400 bps modem
V.34 is the CCITT protocol for 28800 bps modem
V.42bis data compression protocol
The X.25 is the CCITT protocol for packet switching networks
Divides message into packets
 Error checking and re-transmission if necessary
 Addressing format allowing international transmission
 Control of transmission.

The growth of de facto standards (P.242)
   Standards of protocols did not arise naturally, because as with much of IT, things just grew
    out of sales - so were in fact driven by commercial and political needs- UNIX, Apple, IBM
    clones etc. Standards Authorities have been set up after the event.
   Examples of this include Kodak Photo CD formats for representing graphics data and the
    PostScript language developed by the Adobe Corporation for laser printers. The commercial
    success of these has led to other companies using the same standard for similar equipment.
    This is sometimes called a de facto standard since it is accepted simply because it is in wide
   An alternative to copying data is to embed an object created by one application in another.
    This too requires an agreed standard between the two applications and again, as standard for
    this is imposed on Windows applications by the operating system which is responsible for
    Object Linking and Embedding (OLE ).

Advantages and Disadvantages of Standards
+ There is a wider marketplace for computers that comply with standards.
+ Standards allow the development of "open systems".
- Some major manufacturers have the power to dictate "de facto" standards.
- Standards can slow down technological advancement because standards have to be agreed
before changes can be made.

Internet protocol p.239
On the Internet, there is every kind of computer, even WAP phones and cable TVs. The Internet
would not work without protocols.
TCP/IP (Transmission Control Protocol/Internet Protocol)
Transmission Control Protocol/Internet Protocol (TCP/IP) was launched in 1972 by the US
Department of Defence. Divides telecommunications into five layers and contains special error-
checking software for unreliable networks.
This suite of protocols provide the rules governing:

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              A2 MODULE 5 (ICT5) TOPIC 14.10 PORTABILITY OF DATA

    Addressing so that each network node has a unique address
 Routing so that data can flow through the network from the source to destination
 Data format so that data packets can be recognised and reassembled at their destination
 Error control so that errors in transmission can be detected and corrected.
Software and hardware that complies with the TCP/IP protocols will work on the Internet.
HTTP is the standard for requesting and receiving HTML pages
FTP allows files to be transferred over the Internet
POP3 allows e-mail to be transferred.

Addressing Mechanisms on the
IP Addresses - every host has a unique IP address of 4x3 numbers e.g. 123.456.789.987
Domain - because the IP address is difficult to remember, they are translated into a domain name,
which identifies the organisation at which the computer is located - e.g. bbc.co.uk
URL (Uniform Resource Locator) - is a web page address e.g. http://www.bbc.co.uk
The first part of the URL specifies the protocol to be used for connection to the server. Protocols
 http:// - Hypertext Transfer Protocol
 https:// - HTTP for web sites with secure servers (used for e-commerce sites)
 ftp:// - File Transfer Protocol (allows access to FTP sites, where files can be
 www - indicates the part of the Internet to be used, in this case the World Wide Web
 bbc - this is the first part of the domain (called the sub-domain)
 co - this is the top level domain. There are many top-level domains and these are being added
    to, to cope with the increasing demand for domain names:
 .co.uk - British commercial
 .com - commercial (mainly US or international)
 .org (non-profit organisation)
 .edu (American University)
 .ac.uk (British University
 .sch.uk (British school)
 uk - this is the country identifier. All countries (except the US) have one. For example:
 .uk - UK
 .jp - Japan
 .de - Germany
 .fr - France


Notes from connected - the Intenet encyclopedia about the OSI seven layer model.


HTML slideshow about protocols and standards. Starts off well but review it yourself - 399


Brief online dictionary definition for "de facto standards." May be useful for revision?

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