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XIII The Digital Control Channel (DCCH) in IS-136: Page 1 (You are here) Page 2

We just looked at the digital traffic channel in IS-54, now IS-136. Now let's look at the digital control channel in IS-136, which, again, is the most prominent TDMA based cellular system in America. At least for now, with AT&T saying they will convert their networks to another TDMA technology, GSM, in the years ahead. IS-136's most important feature is the digital control channel.

The DCCH handles only signaling but it is not the only routine in IS-136 handling signaling. Does that make sense? Other parts handle other signaling tasks. The digital traffic channel in IS-136, for example, uses sub-channels to signal things associated with it. Like messages needed to hand over an active call from one cell to the next. The digital control channel, on the other hand, uses signals for administrative work and providing services. Such as sending cell system information to mobiles or relaying text messages.

The digital control channel builds on IS-54 practices, to some extent, but includes many new things. Among the possibilities:

Caller ID

E-mail

Sleep mode

Voicemail message waiting indicator

Text paging (2-way short messaging)

Normal paging

Advanced fraud protection

International mobile station identification

 

Blah, blah, blah, blah!

The DCCH also permits properly equipped IS-136 mobiles to act as extended cordless phones in private systems, small wireless networks for in-building and on campus use. How are all these new features achieved? A different kind of modulation.

Click here for wonderful information on IS-136. It's from IS-136 TDMA Technology, Economics, and Services, by Harte, Smith, and Jacobs (1.2mb, 62 pages in .pdf)

Book description and ordering information (external link to Amazon.com)

Modulation

A different modulation scheme provides more capability. Modulation means putting information on a telephone wire or a radio wave. (Here's more on modulation) How that's done has a big impact. AMPS uses frequency shift keying or FSK to send control information. FSK sends data by slightly shifting frequencies. Frequency shift keying uses the existing carrier wave, say, 879.990 MHz. The data rides 8kHz above and below that frequency. Just like early modems. 0's and 1's. 0's go on one frequency and 1's go on another. They alternate back and forth in rapid succession. FSK gives you only two states to send information.

The DCCH transmits data not with frequency shift keying, but rather with the awesomely titled differential quadrature phase shift keying or DQPSK. This scheme, used by most high speed modems, allows quicker data transfer than FSK. It gives you four states to send information.

Differential quadrature phase shift keying changes a sine wave's normal pattern. It shifts or alters a wave's natural fall to rest or 0 degrees. By forcing changes in a sine wave you can convey information. You don't stop or abbreviate the sine wave, you change its shape or angle of attack. Ever watch Star Trek? And seen someone who is supposed to be out of phase? They appear ghostly, with much of their body set off at an angle. That's out of phase.

With the digital control channel we're discussing a fully digital system. That means bits, 0's and 1's, on and off pulses of electrical energy. This staccato beat of electrical pulses pulses gets sent through the atmosphere on radio waves. What might not be clear is how or why we need an analog like looking wave to send digital information. We form the wave to carry digital information. A carrier wave. The original signal, which are electrical pulses, doesn't have anything to do with the way we shape the carrier wave which actually transports the signal. Get the difference?

Remember the digital basics page? A normal landline digital phone call after sampling takes up 64,000 bits. And how better techniques for wireless exist, which reduce bandwidth to 7,500 bits. That's efficient. Similarly, differential quadrature phase shift keying is more efficient than FSK, with at least four possible states to carry information in every wave.

A continuous wave produced to transmit analog or digital information. The many phases or angles of a sine permit different ways to modulate

To review, and to quote someone I cannot now remember, three modulations schemes exist:

"Three methods of digital signal modulation. A digital signal, representing the binary digits 0 and 1 by a series of on and off amplitudes, is impressed onto an analog carrier wave of constant amplitude and frequency."

"1) In amplitude-shift keying (ASK), the modulated wave represents the series of bits by shifting abruptly between high and low amplitude."

"2) In frequency-shift keying (FSK), the bit stream is represented by shifts between two frequencies."

"3) In phase-shift keying (PSK), amplitude and frequency remain constant; the bit stream is represented by shifts in the phase of the modulated signal."

Don't be put off by the many abbreviations and strange concepts; PCS and GSM use related techniques so what you learn here will definitely help later. These modulation types work in either the 800 MHz cellular or the 1900 MHz PCS band. They are not frequency dependent. IS-136, though, is backward compatible with analog AMPS service. You can buy a dual mode phone, dual band phone, for example, that hunts for an IS-136 signal at 1900 Mhz, moves to 800 Mhz if not found, and then uses analog service as a last resort. Coverage gets improved, even if you don't have all features in every territory. It's what AT&T's "nationwide" Digital One Rate Service is based on.

Maintaining backward compatibility with existing services while adding new ones was a major task. But IS-136 lets TDMA cellular carriers offer advanced wireless services to compete against rival and incompatible PCS systems. GSM uses similarly elaborate data structures to provide its features.

We've looked at how frames, slots and channels make up what goes in a bit stream. In IS-136 frames are organized into hyperframes, an extended collection of frames, all working together to provide the extra information IS-136 needs. Don't worry about the complexity. I'll cover the highlights and you can go further elsewhere (external link). The example below depicts a hyperframe and its time slots. Two so called superframes make it up.

IS-136 hyperframe and super frame structure

To repeat our previous discussion, one slot happens every 6.67 seconds. Six slots make up a frame. A frame happens every 40 milliseconds.

Complex, eh? It gets more complicated. Sorry. What makes up the individual digital control channel within a time slot is amazingly complex. Sub-channel upon sub-channel run together, like a layer cake with swirls. To describe this data structure engineers use an artificial construct, a framework of ideas called a layered model. What's known as the OSI model. (OSI discussion at the bottom of this page.) While layers and how they work are beyond the scope of this article, we can first look at what these sub-channels do. And then in the call processing article we'll see how they work.

The diagram below is based on one from a PCS article at the Web Proforum, the best wireless writing on the web:http://www.iec.org/online/tutorials/ (external link)

Click here for wonderful information on IS-136. It's from a chapter in IS-136 TDMA Technology, Economics, and Services, by Harte, Smith, and Jacobs (1.2mb, 62 pages in .pdf)

 

 IS-136 Digital Control Channel

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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

XIII The Digital Control Channel (DCCH) in IS-136: Page 1 (You are here) Page 2

 

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