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