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4.1.3. Burst structure
There are four different types of bursts used for transmission in GSM [16]. The normal burst is used to carry data and most signalling. It has a total length of 156.25 bits, made up of two 57 bit information bits, a 26 bit training sequence used for equalization, 1 stealing bit for each information block (used for FACCH), 3 tail bits at each end, and an 8.25 bit guard sequence, as shown in Figure 2. The 156.25 bits are transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.
The F burst, used on the FCCH, and the S burst, used on the SCH, have the same length as a normal burst, but a different internal structure, which differentiates them from normal bursts (thus allowing synchronization). The access burst is shorter than the normal burst, and is used only on the RACH.
Whoa, whoa, whoa! Too much information too quickly. Let's go slow. Four bursts exist:
1) The normal burst
2) The "F" or frequency control burst
3) The "S" or synchronous control burst
4) The access control burst.
There are many references below to quarter bits, which is really an impossibility. They are instead an effective quarter bit. All bits have fixed sizes save the guard bits. As you'll see we need a total rate of 148 bits for a burst. But we can't come up with an even 148 bits without some "slop" or adjusting. That's where the guard bits come in. The time rate for those is equivalent to 8.25 bits. Don't let this put you off, you will see what I mean as you look over the diagrams.
Remember, too, that you don't need to commit this all to memory; bookmark this page or make it a favorite so you can come back for reference.
Now, let's take a look at the most common burst first, the normal burst.
1) The Normal Burst
Pictured above is a burst of bits. A poetic name, eh? One can also call it a data packet. This normal burst is just one of four possible within a single GSM TDMA time slot. We've already seen how this burst fits within the data stream in GSM. Now we look at the burst itself. Let's see, what did John say about this burst?:
The normal burst is used to carry data and most signaling. It has a total length of 156.25 bits, made up of two 57 bit information bits, a 26 bit training sequence used for equalization, 1 stealing bit for each information block (used for FACCH), 3 tail bits at each end, and an 8.25 bit guard sequence, as shown in Figure 2. The 156.25 bits are transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.
This burst carries our conversation in digital form. That's what the two 57 information, message, or data bits are for. The normal burst also carries signaling information needed to manage call processing, that is, data for setting up, maintaining, and then ending a call. What then are training, tail, stealing, and guard bits? Once again we go step by step.
a.) Training sequence bits. Used for equalization. Bits which get the base station and mobile in "tune" with each other. You need some background. As John will write later on,
At the 900 MHz [and 1900 Mhz] range, radio waves bounce off everything -- buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections.
So while traffic is being transmitted, equalization bits in every time slot work to keep that traffic in phase with the base station and the mobile. It is a continuous, automatic, ongoing operation, as the equalizers try to compensate for the problems found in any radio path.
b.) Stealing bits. Whereby a bit is stolen from message bits, just temporarily, to make way for the Fast Associated Channel. It runs in a blank and burst mode. It transmits during handovers or when the slow associated channel can't send information quickly enough.. Like when entering a tunnel or possibly when a large truck gets in front of you. At that point the data link might be broken so the FACCH acts quickly. As an engineer puts it, "The FACCH overrides the voice payload, degrading speech quality to convey control information." This keeps Mr. Mobile linked to the base station.
c.) Tail bits: It's my understanding that tail bits clear the code that has gone before, setting everything back to 0 or a null state.
d.) Guard bits: Empty time spaces separating data packets to make sure one burst does not run into another. Scourias is more specific. He says the guard period allows "the sender some freedom to shift transmission timing to allow the receiver to receive aligned bursts." Guard bits, in other words, permit some leeway or slack.
2) The "F" or Frequency Control Burst
Significant for its lack of significance. 142 "O" bits, essentially an empty frame. But it is so distinctive that it acts as an important marker in call processing.
3) The "S" or Synchronous Control Burst
Welcome to the synchronization burst. What the base station transmits to a mobile to get in order with the rest of the digital traffic. It exists, not surprisingly, on a channel called the Synchronization Channel or SCH. More on this in call processing.
4) The Access Control Burst.
Another distinctive digital signature in the data stream from the handset to the base station. The access control burst is only broadcast on the random access channel or RACH. Macario says a mobile uses it to request for a "subsequent operation, e.g., to establish a call or perform a location update." This channel occurs only on the uplink, that is, from the mobile to base station.
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