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Digital Wireless Basics Series

Introduction

Standards

Basic Radio Principles

Cellular defined
Frequency reuse
Cell splitting
Cellular and PCS frequencies
Transmitting digital signals
 
Introducing wireless systems
The network elements
The main wireless categories
 
Basic digital principles
Turning speech into digital
You are here
Frames, slots and channels
IS-54: D or Digital AMPS
IS-136: TDMA based cellular
 
Download R.C. Levine's comprehensive, somewhat easy to read work on cellular and PCS by clicking here. It's a 300K download in .pdf format. 100 pages.
 

 
Introduction to GSM
 



 
<-- Last topic: Frames and Layers Next topic: IS-136 Channel -->
 
VII GSM Call Processing: (1) Introduction / (2) The Radio elements/ (3) The Network or Switching elements / (4) Main discussion / Next page--->

 

XIII. GSM Call Processing

(1) Introduction
(2) The Radio elements
(3) The Network or Switching elements
(4) Main discussion

Introduction to GSM call processing

Call processing means all steps which set up, maintain, and then end a call. According to the Telecom Glossary put out by the American National Standard for Telecommunications, call processing means:

1. The sequence of operations performed by a switching system from the acceptance of an incoming call through the final disposition of the call. 2. The end-to-end sequence of operations performed by a network from the instant a call attempt is initiated until the instant the call release is completed. . . .

GSM is a European derived all digital cellular system. Perhaps 70% of digital cellular customers world-wide use it. The abbreviation first stood for Groupe Speciale Mobile (external link), after the study group that created it.. It's now known as Global System for Mobile Communications, although the "C" isn't included in the abbreviation. Read more about its history by clicking here.

GSM is a TDMA or time division multiple access system. GSM will eventually replace conventional cellular, IS-136, another TDMA based system. GSM will in turn be replaced by a radio system based on CDMA, or code division multiple access. But these two events are years away. Right now GSM is the fastest growing cellular radio scheme and it's worth trying to understand. I can only cover main details since the GSM standard covers more than 5,000 pages. Let's start analyzing call processing by turning on the phone.

Initialization

The first part to mobile call processing is initialization. It's what happens when you first turn on your phone. You get a connection to a nearby cell site, then the cellular network checks your account. If you have a valid telephone number and your account is good then your call proceeds. Let's take this step by step.

You turn on your phone. Let's say you're in your home territory. And let's say, too, that you're using the wireless carrier you signed up for service with. Things should go smoothly. But if you're out of your home area or you want to use a different carrier, well, there may be some problems. We'll learn why later. But first let's make a wireless connection. You can't make a call unless your mobile has a link to a cell site.

Okay, we need a connection to the cellular system. Which means we need a frequency to transmit on. Which one? The mobile checks a frequency list contained in its SIM card, you know, that removeable memory chip in the handset. It checks the bit stream carried by these frequencies, looking for a Broadcast Control Channel or BCCH within one of them. Each BCCH transmits a unique data marker, so the mobile knows when it has found this channel. This is a big difference between AMPS and GSM. With AMPS a dedicated radio frequency is in each cell, just for setting up calls. With GSM any frequency can carry set up information. It's the channel within the data stream that's important to find, not a specific radio frequency. How does this work? And what's contained in the BCCH?

A base station's Broadcast Control Channel continuously sends out identifying information about its cell site. Things like its network identity, that is, which wireless carrier owns it, the area code for the current location, whether frequency hopping is used, and information on surrounding cells. All important information to let the base station know a mobile is activated and wants service. Again, the BCCH is not a dedicated radio frequency. It is rather a channel within the bit stream carried by any of the frequencies in a cell.

How does the radio check for a broadcast control channel? Easy. It listens. The mobile becomes a receiver first, checking for a signal from any base station within range. The mobile acts like a scanning radio, in other words, going through each BCCH frequency on its list, one by one, testing reception as it goes. It measures the received level for each channel. The GSM system, not the handset, decides after this test which cell site should take the call. That's usually the cell site delivering the highest signal strength to the mobile. Now, what's next?

Once homed in on the Broadcast Control Channel the mobile monitors the ongoing data stream from the base station. It's looking for what's called a frequency control burst or frequency control channel burst. FCCB for short, if not for understanding. A burst of bits, 142 of them actually, along with 3 tailing bits in front and behind. See the diagram below. This distinctive burst says that sychronization bits will soon follow. Those bits will then let the mobile synch up with the cellular system to make a wireless connection. And once that is done, moble and base station can communicate and everything can start working.

 

 

Pictured above is a burst of bits. A poetic name, eh? One burst of many within a single GSM TDMA frame. This burst is the digital signature the mobile is looking for in the BCCH. Bits are single pulses of electrical energy. Much like the single dash of a Morse Code key. With Morse code we use long and short pulses of energy to stand for letters. Although of uniform length, the pulses we use in digital radio do the same thing. Bits grouped in patterns represent voice and data. We also use bits, as shown here, for signaling. More on this in the main discussion.

Unsure about bits and bytes? This won't take long to read: http://www.privateline.com/bitsandbytes/bitsandbytes.htm

I mentioned synchronizing bits, that these would follow the normal burst of bits above. To review a little, GSM is a time based multiplexed system. Many calls on the same radio frequency, each part of every call divided by time. Like a long set of fast moving box cars. Our new call must fit on the digital train somewhere. Like in each and every third boxcar. And like soldiers synchronizing their watches, the synchronization bits provide the mobile and base station with exact timing details for the coming conversation. Once our mobile is assigned a place in this digital freight train it can take and send information. But can we make a call? Not yet.

More on frames, slots, and channels here

 

After connecting, a call must be validated or authenticated. At the least we must know if the customer's account is good before going further. So, we need to check some databases. And that means accessing your account, which could be on a computer half a country away. Notice how we are now talking about two different things in the same discussion? The first is the radio part and the second is the network part. Let's break this down.

A) The radio subsystem. Sometimes called the air interface. How we set up, maintain, then later tear down a radio connection from the mobile to the cell site.

AND

B) The network subsystem, sometimes called the switching element, which decides who gets on the system, how the call is set up and terminated, and what services and resources the customer can use.

We have two parts, each working to help the other out. Without these two parts working in tandem a call would not go through. Let's leave our call processing story for now, and turn to identifying all the components that enable that call processing. On the next page we will look at the radio part, and on the page after that the network part, before returning to call processing. Patience, grasshopper.

Next page--->

Network resources depicted below, along with the radio components in the lower right corner. Intimidating looking, isn't it? That's why I want to describe these parts first, before describing how it all works together to process a call.

Guide to the more obscure abbreviations. AXE: Automatic Exchange Electric: Ericsson's digital switch. They operate as either a landline or wireless switch. OSS: Operations support system EET: Ericsson engineering tool, network planning software. SOG: service order gateway BGW: billing gateway. MIN: Mobile intelligent network. SCP: service control point.

More details on all of the above at this page: Structure. Click here to go there

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

Next page--->

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