The Digital Control Channel (DCCH) in IS-136
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)
IS-136 Digital Control Channel
Q. What are the frequencies for the control channel in IS-136 and EDGE?
Mark van der Hoek (internal link):
"The control channels for IS-136 would be the same as for AMPS. In theory they could be any set of 21 channels, but in practice they are 334-354 for the B side carriers, and 333 to 313 for the A side. EDGE should follow the GSM control channels, although GSM is not my strong point. Some IS-136 handsets are dual mode compatible and so would seek only digital control channels."
"Professor Levine (internal link) explains more below. He describes what happens after an IS-136 mobile is booted up and finds that technology available in its area. My understanding is that the mobile (being AMPS compatible) will first scan the AMPS control channels. Once it locks onto and decodes the AMPS control channel, it looks to see if another technology is available. If it sees that, I forget the proper name right now, call it the Advanced Technology Bit, is set, then it goes into its TDMA scan mode. This will get clearer as you read . . ."
Professor Levine:
"Mark and Tom: In contrast to TIA-553 (analog) and IS-54 (dual mode digital- analog cellular), IS-136 standards have no pre assigned carrier frequency/ies for control/setup channels. The system operator/installer can choose any frequency in each cell for this purpose, and can even change that setup channel frequency from time to time if so desired. Since there is no pre-determined carrier frequency for the control channel, the base station transmits the 'carrier number' as a binary code value, contained in the last 12 bits of the TDMA time frame on EVERY downlink carrier frequency in that cell. (That corresponding bit field is all zeros in IS-54. Also, I may be wrong about 12 bits since I am quickly typing this from memory. There may be 11 bits and an extra bit for another purpose. If I recall properly, this is called the 'pointer' value.)"
"Therefore, when a power-on but non-conversation IS-136 mobile station enters a cell and is first scanning the various carrier frequencies in a cell, as soon as it receives a good and sufficiently strong carrier from a base station, it quickly finds the proper frequency for the setup/control channel of that particular cell, and does not need to exhaustively scan all the carrier frequencies in that cell to find the setup channel frequency."
"The IS-136 handset stores all the valid control frequencies in a FIFO (first in, first out) memory list, and on subsequent entries to a new cell while not in conversation, it tries just the frequencies in this FIFO list first. If they don't work (prove not to contain a setup time slot channel) the handset goes back to scanning and looking for the 'pointer' in every downlink carrier frequency. The standard does not call for a FIFO list or this process, but all handset manufacturers do this (copied from the similar process in GSM described in the next paragraph)."
"EDGE follows a method similar to GSM. The operator may assign 'setup' channels to any arbitrarily chosen carrier in the cell. Mobile stations scan all the carriers in 'carrier number' order (e.g., starting from carrier 1, then carrier 2 and so on up to 124 for 900 MHz band GSM/GPRS."
"When a new GSM handset with a brand new SIM chip (just out of the box) is first turned on, it scans all the carrier frequencies that it can receive. Note that due to less carrier frequencies (and more TDM channels per carrier) an exhaustive scan of all 124 carriers in 900 MHz band GSM takes much less time than scanning 416 carriers (from one of the two licensees in the 850 MHz band) in IS-136. The mobile recognizes a 'beacon' frequency that contains setup (broadcast, etc.) channels due to the distinctive presence of the 'frequency correction time burst' on such assigned carrier frequencies (that type of frequency correction burst does NOT occur on a carrier that is used only for traffic channels."
"In a city with multiple GSM service providers intermixed on the same radio band (like Paris or Frankfurt, on the 900 MHz band, in contrast to segregating different service providers into different subbands of the 1900 MHz band in North America) the mobile station also checks to ensure that the broadcast channel indicates a base station Mobile Network Identity (MNI) number that is in the list of those that the SIM chip indicates have a roaming agreement with the home service provider, and of course the home service provider itself. (There is more to this aspect of the process that I am not describing here for brevity.)"
"These carrier code numbers, as found, are stored in the SIM chip, so as the mobile station moves around the city, it automatically accumulates a list of 'acceptable MNI beacon frequencies' in a FIFO list stored in the SIM chip. In a base installation using a 7-frequency cell plan, this is typically only 7 beacon frequencies, although there are some exceptions to this. If you take your GSM handset (or your SIM chip) to another city, it will automatically update the FIFO list of beacon frequencies using actual scan data from the new city. Once a FIFO list is in place, each time you turn on your handset, it scans just the 7 or so frequencies in the FIFO list. This gets your handset up and ready for service very quickly. If none of these frequencies have an acceptable MNI in the broadcast setup channel, then it is likely that you have carried your SIM chip to another city and turned it on there, so it then goes through an exhaustive scan just as it did when it was first powered up (as explained in previous paragraphs)."
"Sorry to be so verbose, but it takes quite a few words to explain what's going on, but I hope this is clear. The most complete source on this is the GSM specifications, but I can't tell you which part from memory -- you may need to read several diverse sections and then put the pieces in order mentally to find out what you need. There is also a simpler but not highly detailed verbal explanation in the book GSM Superphones by Lawrence Harte and myself."
Regards, Richard Levine