Back In sync with asynchronous transfer
D. Murali
| Learn all about the ATM - not any time money or automated teller machine, but `asynchronous transfer mode' in the context of information sharing. |
THE fundamental function of a network is to transfer information from one entity to another, writes Sumit Kasera of Flextronics Software in ATM Networks: Concepts and Protocols, from Tata McGraw-Hill (www.tatamcgrawhill.com) .
The abbreviation doesn't stand for any time money or automated teller machine, but `asynchronous transfer mode'. For starters, "Transfer mode refers to the techniques used to transmit, switch and multiplex information." Or, simply, "means of packaging, sending and receiving information on the network."
Based on how the information is packaged, transfer mode can be classified as circuit switching and packet switching, explains the author.
In the former, "a dedicated circuit (or a channel) is established from source to destination and the information is sent as a bit stream."
In the latter, user information is carried in packets, which are "also called frames, cells or datagrams".
In between lies virtual circuit switching; this transfers information using virtual circuits. It works thus: Each packet carries a label called VCI (virtual circuit identifier); when this packet reaches the next hop, the VCI field of the packet header is overwritten with a new label.
"VCIs are locally unique and not globally," points out Kasera. "Cell switching is the latest form of virtual circuit switching," he adds.
Networks help by connecting; but difficulty arises when there are just too many networks, each one tailored to a specific service requirement. How useful, therefore, if a single platform handled all different needs! Which is how ATM came into being.
According to ITU-T (International Telecommunications Union-Telecommunication Standardisation), ATM is "a transfer mode in which the information is organised into cells; it is asynchronous in the sense that the recurrence of cells containing information is not periodic."
One learns that cell is the lowest unit of information in ATM, and that it is a fixed-size frame of 53 bytes (that is, 5 bytes of header and 48 bytes of payload). "The header carries information required to switch cells, while the payload contains the actual information to be exchanged."
Kasera elaborates that in asynchronous mode, "timing information is derived from the data itself", whereas in the opposite, that is, the synchronous mode, "the transmitter and receiver clocks are synchronised and frames are sent/received periodically".
He cites as example time division multiplexing, "where each time slot is reserved for a particular voice channel, and wherein frames recur at 125 microsecond interval."
Part 2 of the book is on ATM protocol reference model, where the author discusses layers such as physical and adaptation.
There are layers of abbreviations to adapt to, as for instance UNI (user network interface), GFC (generic flow control), PT (payload type), CLP (cell loss priority), HEC (header error control), TC (transmission convergence), PMD (physical medium dependent), POI (path overhead identifier), and UTOPIA (universal test and operations PHY interface for ATM).
Further on, there are: AESA (ATM end system address), PARIS (packetised automated routing integrated system), IISP (interim inter-switch protocol), DXI (data exchange interface), FUNI (frame-based UNI), FATE (frame-based ATM transport over Ethernet), and LANE (local area network emulation).
Despite the jungle of terms that is inevitable in any tome on network, the author adopts a simple style to make the book friendly to novices.
The core concepts of ATM, such as traffic and service parameterisation, addressing, signalling, routing, and security are dealt with in part 3. Internetworking with ATM, and applications are discussed in parts 4 and 5. At the end of each chapter, Kasera provides information on `further reading'.
A book to help sync with asynchronous transfer!
Is the Net masculine or feminine?
THE European Journal of Communications (EJC) celebrated its 20th anniversary in 2005. To mark the occasion, EJC put together `twenty outstanding articles' and brought out the collection as a book titled Communication Theory & Research, from Sage (www.sagepublications.com) .
The editors, Denis McQuail, Peter Golding, and Els de Bens, have organised the works, "which represent the best of European communication research", into five sections, viz. international communication, audience, policy and politics, journalism, and media culture.
Thus, one reads about international television flows in Preben Sepstrup's opening essay, which is followed by Marjorie Ferguson's debunking of the seven myths of globalisation.
"With 37 per cent of the overall broadcasting time, fiction is by far the most important programme category on European television," informs De Bens.
`Five traditions in search of the audience' are discussed by Klaus Bruhn Jensen; Daniel Biltereyst looks at `resisting American hegemony; and McQuail analyses the accountability of media to society.
`Who's afraid of infotainment?' asks Kees Brants, and Jay G. Blumier responds with a thesis on changes happening to political communication thus: "Conditions and tensions of everyday life in people's domestic, workplace, and neighbourhood milieux raise many issues that are suited to public discussion," and "it would be highly unfortunate if the sphere of official politics was left to the activities of highly motivated and better informed elites." That's the raison-d'etre of blogs, you'd agree.
The `journalism' section has articles on telling stories, code of ethics, `infosuasion', and so on. In the last section on `media culture', is Liesbet van Zoonen's piece titled, `Gendering the Internet: claims, controversies and cultures'.
She says, "Interpretations of the Internet as masculine, feminine, or even transgender are based on limited conceptualisations of both gender and technology."
Zoonen begins with a brief review of "the gender codes of the Internet's enabling technologies: the telephone and the computer."
One of the early stories about telephone is about how at a hearing of the Indiana Public Service Commission, one telephone company objected to women's uses of telephone because "women talked for long periods on the telephone about supposedly trivial matters and this was not what the medium was meant for."
It is not a far-fetched conclusion to say that women subscribers were largely responsible for the development of a culture of the telephone, writes Zoonen, citing other published works. She mentions how Ada, daughter of Byron, was an outspoken advocate of Charles Babbage's Difference Engine. "Babbage's Analytical Engine and Ada's work on it disappeared from the public eye until 1937, when his unpublished notebooks were discovered," notes Zoonen.
"We can consider the telephone and the computer as respectively the mother and the father of the Internet," she says. "The child is some 40 years old then but its gender is still undecided." Feminists, though, have claimed that the Net is a woman's medium.
"In a trend report conducted for the German women's magazine Freundin (translation: Girl Friend), it is argued that womanhood offers many opportunities nowadays and very few disadvantages, and that new technologies such as the Internet make life easier, and enhance the possibilities for communication."
The author's study, based on interviews of "24 young Dutch couples, between 20 and 30 years old, living together without children", finds that the Internet is "taken up as an extension of the male territory in the household".
There is no exclusion of women, because men "consciously leave the PC to their partners".
Insightful read.
Tailpiece
"Our MLA is moving to the other camp!"
"How do you know?"
"The radio tag!"
© Copyright 2000 - 2009 The Hindu Business Line