Footnotes and an extended discussion
IS-136 migrating to GSM
A major change in the United States cellular radio landscape began on Thursday, July 19th, 2001 in Seattle, Washington. AT&T began a transition from the technology they invented, IS-136, to GSM, a technique originally European that has now gone global.
Both IS-136 and GSM are digital or second generation cellular systems. Both are TDMA based. But AT&T has progressed beyond second generation to 2.5G, since their newest offering includes GPRS or Global Packet Radio Service. GPRS is an advanced packet switched data network that promises more services and higher data transfer rates than the original Cellular Data Packet Data or CDPD technology common across America.
The official name then for AT&T's new service is GSM/GPRS. In a confusing press release short on facts, AT&T left many questions unanswered. I want to know how the GSM/GPRS system will co-exist with the existing IS-136/CDPD service which AT&T will continue to support. One good GPRS report is here: http://www.cisco.com/warp/public/cc/so/neso/gprs/gprs_wp.htm (external link)
Is the OSI model important to understanding cellular radio?
OSI stands for for Open System Interconnection, a standard defining rules communication networks should follow. Seven levels or layers make it up. It was first thought system designers following the OSI model could make their different communication systems more compatible. But for many reasons the OSI model was never fully implemented in every network scheme. Computer networks use it most, radio systems least. Here's an excellent link if you want to know more, a funny, stylish web page: http://routergod.com/ccnabootcamp/osi.html
Graphic from http://www.lightreading.com/ (external link)
The OSI model reminds me of Esperanto, that failed universal language. It promises a way for all Western people to communicate but its promise cannot overcome its impracticality and lack of appeal. (As an aside, a more difficult but far more applicable language has emerged as the world's universal tongue: broken English. ) Similiary, text books do not realistically describe the OSI model's actual, limited use. They stress its universality, its possibilities. Not its problems. Beginning students think that if mastered a knowledge of the OSI model will help them understand dissimilar communication networks by considering them through a common, uniform framework. Each will relate to the other since the OSI model applies to them all. Which is, of course, not the case. Learning is about not only picking certain subjects up, but leaving others down.
Professor Richard Levine (internal link) responds to a recent question from a reader:
"The OSI model is a theoretical structure used for description and documentation of certain communication protocols. Some protocols, particularly those that were developed before the original papers on the OSI model were published (in the 1970s) do not 'fit' or agree with the OSI layers, or there have been several alternative ways to describe what some protocols do in which different authors choose to place different parts of the same protocol in different 'layers.'"
"There are also several instances in which the original authors of the descriptive articles on OSI made the wrong assignment of layers for various purposes, probably due to lack of knowledge of how some specific systems work. For example, many systems have no explicit presentation layer. Some authors place encryption, if used, in the presentation layer."
"But most military systems (and also GSM air Um interface) actually puts encryption at a lower level (like level 2 or 3) which does not correspond to a unique layer (that is, in the Um GSM air interface, all the bits except for those that establish frame synch (the training bits) and time slot boundaries are encrypted in most (not all) types of GSM logical air interface channels."
"It is not always possible or meaningful to try to analyze real systems, such as cellular base station processes, in accordance with the OSI seven layer model. Don't be worried or concerned about it. Sometimes the OSI model is not the best or the appropriate way to describe some communication protocols. "
Regards, Richard Levine