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VII GSM Call Processing: (1) Introduction / (2) The Radio elements/ (3) The Network or Switching elements / (4) Main discussion

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7. More background: BCCH, FCB, and other ugly abbreviations

"We continue our call processing discussion, started two pages ago in the introduction. I hope you read it, as well as the radio and switching elements pages. If you haven't, well, don't complain if you can't follow what comes next. :-)

We had just made a connection with the cellular network. A bare connection, but enough to send preliminary data. Enough to start getting our mobile recognized and then later assigned a channel. Our mobile had detected a "burst of bits" in the Broadcast Control Channel sent from a nearby base station.

(To review and define quickly, Weick calls a burst "a sequence of signals counted as a unit in accordance with some specific criterion or measure." A burst of bits is a marker, an indicator, a signal within a signal. It's what the mobile is looking for and is contained within an individual frame in the digital stream flowing from the base station.)

Burst explanation is here

Now that the BCCH has been located, it's time to look for the synchronizing bits. But there's one thing left to do before that, and that's to find the frequency correction channel. And its attendant frequency control burst or FCB.

PLMN. Public land mobile network. A cellular wireless carrier. Ugly and confusing phrase but used throughout the GSM community. All cellular systems are land mobile networks but I'm not sure that all PLMNs are cellular. Info on private land mobile is here. BCCH. As discussed here. The Broadcast Control Channel. Operates on the downlink, that is from the base station to the mobile. FCB: Frequency control bits. BSIC: Base station identity code. Identifies the cellular carrier and the base station the mobile is connecting to.

The call processing diagram above is reprinted with permission of Clint Smith. His books are a necessity for wireless professionals, those whose jobs touch on celluar radio, and for anyone working in telecom.

Burst explanation is here

The frequency correction channel or FCCH is tied closely to the BCCH and the synchronization channel. A distinctive burst marks the FCCH. This burst is called the FCB or frequency control burst. Don't confuse the two terms. One is the channel, the FCCH. The other is the burst on that channel, the FCB. Clear? See what a big role it plays in call processing in the diagram above. The Frequency control burst helps sets the radio frequency and then synchronization bits get everything into the proper time.