Atif's Blog

Pulse Code Modulation: Quantization

2011-05-26T21:04:00.000-07:00

The discrete signal i.e. sampled values produced as a result of sampling (discussed in the previous post) has to be quantized, hence the process called Quantization. In simple words, this is the process in which a value from a given set of values is assigned to each sample of the discrete signal. The number of values in the set actually is the number of quantization levels to which samples of discrete signals are assigned to. Digital communication is based on bits and bytes, the number of bits used identify the number of quantization levels, hence, in this case when the samples are binary encoded that means they are essentially being quantized in one of the fixed number of quantization levels.

In the figure below, the process has been explained where an input discrete signal s(t) has been quantized into a signal s_q(t). The input signal s(t) moves between low peak amplitude A_L to high peak amplitude A_H . This range from A_L to A_His divided into M intervals (an interval represent a quantization level, also referred as quantization interval) each of size L such that L = (A_H - A_L)/M. In the figure, eight intervals have been shown i.e. M=8. Let A_s be the peak-to-peak amplitude of s(t) i.e. total range between high and low peaks i.e. A_s = A_H - A_Lhence L = A_s /M. The value of each quantization interval is taken as the center of the interval, in figure, shown as a₀ , a₁ , a₂ , a₃ , a₄ , a₅ , a₆ , a₇ . The range of values for each interval has been shown as l₀, l₁, l₂ , l₃ , l₄ , l₅ , l₆ , l₇ . Moreover, the midpoints between consecutive quantization intervals are shown by A₀₁ , A₁₂ , A₂₃ , A₃₄ , A₄₅ , A₅₆ , A₆₇ . Each of these midpoints is at a distance of L/2 from its corresponding intervals values e.g. A₆₇is the midpoint between two quantization intervals whose values are a₆and a₇and is located at a distance of L/2 from a₆and a₇. The distance between two consecutive midpoints and also between two interval values is L. It is, in fact, these mid points which define when the value of the quantized signal s_q(t) changes. It should also be noted that the peak values i.e. A_H and A_L are at a distance of L/2 from their corresponding quantization levels. This results in affecting the peak-to-peak amplitude of s_q(t) shown as A_q and has a value of (M-1)L.

As shown in the figure, when the signal being quantized i.e. s(t) is in the range of l₇ , the quantized signal s_q(t) maintains a constant value of a₇ . As soon as s(t) gets in the range of l₆ , s_q(t) makes a jump to the value of a₆and maintains this value as long as s(t) assumes a value in the range of l₆ . The sudden jump is made when s(t) crosses the midpoint A₆₇. Similar changes occur to s_q(t) through the rest of the process of quantization. Therefore, at any time, the quantized signal s_q(t) holds a value from a₀ , a₁ , a₂ , a₃ , a₄ , a₅ , a₆ and a₇ . That means, at any moment, s_q(t) exits in one of the quantization intervals that is nearest to s(t).

As the above description and the figure indicate that s_q(t) is not same as s(t) rather it’s an approximation with a difference between the two. This difference is very important and is known as Quantization Error. Quantization Error’s magnitude is always less than or equal to L/2 (We’ll discuss Quantization Error later). The approximation can be improved by making the interval size smaller, hence increasing the number of intervals (quantization levels). In digital communication, high number of intervals requires high number of bits which itself, in turn, require high transmission bandwidth. On the other hand, if interval size is increased using lesser number of quantization levels, approximation is poor. Therefore, there has to be a tradeoff between approximation and bandwidth.

Pulse Code Modulation: Filtering and Sampling

2011-03-17T00:58:00.000-07:00

In Digital Telephony, analog signals have to be converted into digital signals before transmission because of several advantages they have over Analog signals, one of which is their strong immunity against noise. Pulse Code Modulation (PCM) is one of the techniques to achieve the objective. Generally, PCM is a four step process consisting of Filtering and Sampling, Quantization and Encoding. However, Quantization may be performed either of two ways: Linear Quantization or Non-Linear Quantization. For Non-Linear Quantization, sometimes an additional step of Companding is performed after Sampling and before Quantization to prepare the samples for non-linear Quantization.

Here, in this post, we’ll discuss Filtering and Sampling. Later, in the next posts, we’ll discuss Linear Quantization, Quantization Error, Non-Linear Quantization, Companding and Encoding. Our discussion is based on digital telephony.

Analog Low Pass Filtering:

Before an analog signal is converted into digital one, it is passed through an analog Low-Pass Filter (see here). Following figure shows the frequency responses of a Low Pass Filter.

This filtering comes handy later to avoid aliasing when the analog signal is sampled and then reconstructed from the digital signal (see here), hence also called Anti-Aliasing Filter. In simple words, this is performed to filter out the frequencies which are higher than a “certain frequency”. We are interested in the frequency band lower than the “certain frequency” and not the higher.

“Certain Frequency” is just a made up term for explanation and is not a technical term.

For Telephony Systems whose main objective is to transfer human voice, that “certain frequency” can be found from high school physics. We have read human voice consists of frequency range of 300Hz to 3000Hz. Due to some constraints in designing an ideal filter, we make this certain frequency 4000Hz instead of 3000Hz. So after filtering, for the next step of Sampling, we know that bandwidth of the signal is 4000Hz i.e. the greatest frequency analog signal consists of is 4000Hz.

Sampling

Sampling is the process of creation of a Discrete Signal by taking samples of an Analog Signal. These samples are taken at regular intervals; intervals may be time base (for time domain) or space based (for spatial domain) or any other independent variables. In our case, intervals are generally time based, these regular intervals define the sampling rate--the rate at which samples are taken, also called sampling frequency F_s.

Sampling is governed by Nyquist Sampling Theorem, according to which when a signal is sampled, the sampling rate (or sampling frequency) must be at least twice the highest frequency the signal contains, to avoid aliasing. In other words, sampling frequency must be at least twice the bandwidth of the signal. This means at least two samples are required per cycle of each frequency in the analog signal.

Therefore

F_s = 2 x 4000 = 8000 Samples/sec

Filtering and Sampling work in collaboration to overcome a possible artifact of aliasing which is a result of under sampling. Aliasing is an important concept to be understood and an important artifact to be overcome for correct transfer of voice data in telephony systems.

On the other hand, sampling rate, higher than minimum sampling rate of 8000 samples/sec, results in too much voice data to be transferred over the communication channel demanding a high bandwidth. Between the two extremes of under-sampling and over-sampling, above sampling frequency of 8K samples/sec is the threshold which is good enough for the reconstructed analog signal and is good enough for the usage of bandwidth.

A technique often used for sampling is Pulse Amplitude Modulation (PAM) in which a pulse is modulated with the filtered analog signal. This has been shown is the following figure created on Simulink Matlab, pink colored graph is the pulse, yellow colored graph is a sine wave to be sampled and aqua graph is the output of the PAM sampling.

Sampling creates discrete signals not the digitized signals; Quantization is the step to digitize the samples into binary encoded form.

Probability Density Function (pdf) of Sine Wave

2011-02-21T03:02:00.000-08:00

Probability Density Function, f(x), completely describes the probability that a Random Variable, X, assumes a value in a given range of possible values.

Mathematically, the probability that random variable X, assumes a value that lies in the interval [a,b].

The above equation represents the area under the curve f(x) from a to b. The limits of the above integral depend upon the domain of the f(x). However, the total area under the curve must not be greater 1 whatever the limits are. That is

Another jargon often used with Probability Density Function is Cumulative Distribution Function (cdf). As the name implies it is the cumulative probability that a random variable X takes on a value equal or less than some specific value. For given value a, F(a) is the Cumulative Distribution Function that shows the probability that the random variable X assumes a value equal to or less than a. Mathematically shown as below

P(X < a) = F(a) ………….(3)

Cumulative Distribution Function is based on probability, it has following properties:

0 <= F(a) <=1

F(a₁) <= F(a₂) if a₁< a₂

F(-∞) = 0

F(+∞) = 1

The relationship between Cumulative Distribution Function (cdf) and Probability Density Function (pdf) is shown below

That means the value of the F(a) is equal to the area under probability density function curve upto a.

From the above relationship, conversely, we have

Above two equations give us the relationship between cdf and pdf.

Following links provide a very simple understanding of pdf and cdf:

Engineered Software, Inc. (pdf)
Engineered Software, Inc. (cdf)
Wikipedia

Now for Probability Density Function of a sine wave. Let’s say we have

X = sin(Y) ………………… (6)

A sine wave has been shown in figure below:

For equation (6), we already know that Y is uniformly distributed over [- pi/2 , pi/2] so X takes on a value in the range [-1, 1]. In this case, X is our random variable for which probability density function is given by eq (1). We aim to find the function f(x) for the sine wave given in eq (6).

Generally the inverse of a function provides the distribution of the values of that function which can be converted into Cumulative Distribution Function (cdf) after a little manipulation. These manipulations are done so that it fits with the definition of cdf.

We have random variable X which can assume a value in the interval [-1, 1]. Therefore

P(X < a) = P (sin(Y) < a) = P(Y < arcsin(a)) where -1<=a<=1

Hence

Notice the modifications of dividing factor of and additive factor of ½ which have been done to make it compliant with the definition of Cumulative Distribution Function and its properties shown above.

Cumulative Distribution Function of a sine wave is shown below:

The possible values that a sine function can assume are shown along X-axis while probability is shown along Y-axis. The maximum probability can be 1.

Now making use of equation (5) above, we can calculate the Probability Density Function (pdf) of sine function (sine wave) by taking the derivative of equation (7). Hence pdf of sine wave is given below

This is quite intuitive that as the slope of a function increases at a value, the chances of occurring that value are higher. (Think of a straight line parallel to Y-axis which has infinite slope and has only one value occurring all the times along X-axis). The probability density function (pdf) of a sine wave, f(x), is shown below:

The pdf shows that probability is high near the extreme values of -1 and 1 as most of the possible values occur towards the extreme values. For example, sine function reaches at 0.5 (half of its max amplitude) just at pi/6 (i.e. 30 degrees).

All the above plots have been created from rechneronline.

Asterisk with Oracle

2011-01-08T07:43:00.000-08:00

...continuation

Now Asterisk:

Having installed unixODBC and unixODBC-devel, I download Asterisk and installed it.

./configure

make or make menuselect

make install

During the execution of ./configure script, on finding uinxODBC and unixODBC-devel installed, it prepares all ODBC related modules for installation, it can be verified it by using ‘make select’ instead of ‘make’. After ‘make install’ had finished, my Asterisk was ODBC enabled, all I needed to do was configure the conf files.

Before changing the conf files, I created a table called ‘sip_buddies’ in my database, schema can be found here, I had to make some changes in the schema according to my database. I was planning to test peer registration from database.

On Asterisk, I was interested in dynamic mode of Asterisk Realtime Arcitecture (ARA), based on that, I had to change the following conf files:

/etc/asterisk/modules.conf

/etc/asterisk/res_odbc.conf

/etc/asterisk/extconfig.conf

In modules.conf, I uncommented following two lines:

preload => res_odbc.so

preload => res_config_odbc.so

In res_odbc.conf, I added following context:

[asterisk_oracle]

enabled => yes

dsn => OracleODBC-11g ; the context added to odbc.ini

;username => testdb ;already provided in odbc.ini , may be provided here too

;password => testdb ;already provided in odbc.ini, may be provided here too

pooling => no

limit => 0

pre-connect => yes

Like username and password, there are other parameters in odbc.ini which may be overridden here in this file, as has been mentioned at the above link.

Finally, I changed the extconfig.conf and added the following line:

sippeers => odbc, asterisk_oracle, sip_buddies

‘asterisk_oracle’ is the database as I created in res_odbc.conf. sip_buddies was the table in my database where peers resided.

After running asterisk, I executed following command on CLI

odbc show

The output showed that I was connected to OracleODBC-11g.

Then I added some peers in my peers table while Asterisk was running and tried to register them on Asterisk through Soft Phones.

Note: I had a little problem here while registering the peer with following output:

chan_sip.c:20603 handle_request_register: Registration from…….. failed for…….. - No
matching peer found.

It was fixed by populating the ‘defaultip’ field of my table sip_buddies, a null value in this field was creating the problem. This problem has been discussed with a patch that is available here. I hope the future release of Asterisk would have it fixed or you can do it yourself from above link by changing the source.

It’s all worth it. I was successfully able to register my peers without a reload. Everything I did was working fine especially about unixODBC-devel which seemed like a nightmare!!!!!!

You can do more things with this, I hope this long post (splited into four parts) is helpful to all you guys who dealing with the same issue. Let me know, if it is useful for you.

Asterisk with Oracle

2011-01-08T04:38:00.000-08:00

...continuation

Put it all Together:

As a summary of above discussion, l used following components:

unixODBC 2.3.0 (available here)
Oracle Instant Client 11g 11.2.0.2-both Basic and ODBC Supplement Packages (available here)
unixODBC-devel 2.2.11 (Initially unixODBC 2.2.11 is also needed to identify the inter-dependency between both)

As said before, I’ll be using absolute paths for referring directories and file in this discussion. Also I executed all the commands as super user—root.

Following command installed unixODBC 2.2.11 and unixODBC -devel 2.2.11 from repository (you may install them by downloading rpm packages from internet).

yum –install unixodbc unixodbc-devel

Files related to both of these packages resided in different directories after installation which can be check using the commands ‘rpm -ql unixodbc-devel’ and rpm ‘rpm -ql unixodbc’. Here I was merely interested in identifying the dependency of unixODBC-devel on unixODBC. Apart from other files, both packages had their Static and Shared library files in /usr/lib/ directory. Some of the Shared library files of unixODBC-devel were actually soft links to the Shared library files of unixODBC which formed the dependency on unixODBC. I was interested only in these shared library files of unixODBC, others had no importance. My plan was to provide unixODBC-devel with these shared library files from unixODBC 2.3.0. That was the only purpose of installing unixODBC 2.2.11 and was supposed to be uninstalled after it had served its purpose. But before uninstalling it I decided to make a copy of all of its files especially shared library files, so that I could use them if I didn’t find any of the shared libraries from unixODBC 2.3.0. I used the following Shell script to copy all files of unixODBC 2.2.11 in a directory /usr/backup/ unixODBC/.

#!/bin/sh

for file in `rpm -ql unixODBC`

do

     cp "$file"  /usr/backup/unixODBC/

done

Now I could uninstall unixODBC 2.2.11 using command (yum –e unixodbc) but as unixODBC-devel had a dependency on unixODBC, Package Manager asked to remove unixODBC-devel too which I needed for Asterisk. I used the following command to remove unixODBC package forcing the package not to check the dependencies.

rpm –e --no-deps unixodbc

This removed the unixODBC package successfully but this made following soft linked shared library files of unixODBC-devel unstable/corrupted having no source file in the directory /usr/lib/ (try ls -lrt). As mentioned before, I had planned to provide these library files from unixODBC 2.3.0.

UnixODBC-devel 2.2.11	UnixODBC 2.2.11
libtemplate.so	libtemplate.so.1.0.0
libtdsS.so	libtdsS.so.1.0.0
libsapdbS.so	libsapdbS.so.1.0.0
liboraodbcS.so	liboraodbcS.so.1.0.0
liboplodbcS.so	liboplodbcS.so.1.0.0
libodbctxtS.so	libodbctxtS.so.1.0.0
libodbcnnS.so	libodbcnnS.so.1.0.0
libodbcminiS.so	libodbcminiS.so.1.0.0
libodbcinst.so	libodbcinst.so.1.0.0
libodbcdrvcfg2S.so	libodbcdrvcfg2S.so.1.0.0
libodbcdrvcfg1S.so	libodbcdrvcfg1S.so.1.0.0
libodbccr.so	libodbccr.so.1.0.0
libnn.so	libnn.so.1.0.0
libmimerS.so	libmimerS.so.1.0.0
libgtrtst.so	libgtrtst.so.1.0.0
libesoobS.so	libesoobS.so.1.0.0
libboundparam.so	libboundparam.so.1.0.0

Next I downloaded and put unixODBC-2.3.0.tar.gz in the directory /usr/local/src/ and installed it by executing the following set of commands:

tar zxvf unixODBC-2.3.0.tar.gz (This created a directory named unixODBC-2.3.0)

cd unixODBC-2.3.0

./configure --prefix=/usr/

make

make install

This installed the unixODBC-2.3.0’s binary files in /usr/bin/, header files in /usr/include, ini files (odbc.ini, odbcinst.ini) in /usr/etc/ and shared library files in /usr/lib/. The prefix (--prefix=/usr/) passed to the ‘configure’ script purposely so that shared libray files of unixODBC 2.3.0 are put in /usr/lib/ directory to resolve the unstable linked shared library files of unixODBC-devel 2.2.11.

I noticed there were only two files (libodbcinst.so.1.0.0 and libodbccr.so.1.0.0) from unixODBC 2.3.0 which matched two of unstable/corrupted shared library files of unixODBC-devel (libodbcinst.so and libodbccr.so); highlighted in yellow in above table. For the rest of the files, I copied the original shared library files of unixODBC 2.2.11 of which I had made a backup in /usr/backup/ unixODBC/ previously. After copying these shared library files, all unstable files of devel were stable now. (Again try ls -lrt).

unixODBC can be checked by through following command:

odbcinst –j

Next I downloaded Oracle Instant Client 11g, both Basic and ODBC Supplement packages and unzipped both of them in same directory /opt/instantclient_11/.

/opt/instantclient_11/ directory contains a script called odbc_update_ini.sh. This script takes four arguments, three of which are optional. I passed only the compulsory argument and leave the optional arguments. The only argument I sent (the one which is mandatory) is the path to Driver Manager that simply means the prefix of etc/odbc.ini and etc/odbcinst.ini.

As I mentioned before when I installed unixODBC 2.3.0, it put ini files in /usr/etc/ so I executed the above script as below:

cd /opt/instantclient_11/

./odbc_update_ini.sh ‘/usr/’

This script automatically configures the Oracle Driver with unixODBC 2.3.0 by updating two files; /usr/etc/odbcinst.ini and /root/.odbc.ini. (If non-super user is logged in while executing this script, then /home/username/.odbc.ini is update)

In /usr/etc/odbcinst.ini, I had an entry like the one below:

[Oracle 11g ODBC driver]

Description = Oracle ODBC driver for Oracle 11g

Driver = /opt/instantclient_11_2/libsqora.so.11.1

Setup =

…………….

In /root/.odbc.ini, I had an entry like the one below:

[OracleODBC-11g]

Application Attributes = T

…………….

Driver = Oracle 11g ODBC driver

DSN = OracleODBC-11g

…………….

……………

I edited the above entry in /root/.odbc.ini, I added some new fields, commented some existing fields (using ;) and changed the values of some of the fields. After these changes its looks like the below:

[OracleODBC-11g]

Application Attributes = T

Attributes = W

BatchAutocommitMode = IfAllSuccessful

BindAsFLOAT = F

CloseCursor = F

DisableDPM = F

DisableMTS = T

Driver = Oracle 11g ODBC driver ; the entry from odbcinst.ini

DSN = OracleODBC-11g

EXECSchemaOpt =

EXECSyntax = T

Failover = T

FailoverDelay = 10

FailoverRetryCount = 10

FetchBufferSize = 64000

ForceWCHAR = F

Lobs = T

Longs = T

MaxLargeData = 0

MetadataIdDefault = F

QueryTimeout = T

ResultSets = T

Database = XE ;db name

ServerName = xxx.xxx.xxx.xxx ; db ip

Port = 1521 ;db port

;ServerName = 192.168.0.105:1521/xe ; has been commented, may be used as well

SQLGetData extensions = F

Translation DLL =

Translation Option = 0

DisableRULEHint = T

;UserID = testdb ;has been commented

UserName = testdb ;db user

Password = testdb ;db password

StatementCache=F

CacheBufferSize=20

UseOCIDescribeAny=F

I copied this entry i.e. [OracleODBC-11g] in /root/.odbc.ini to /usr/etc/odbc.ini, so both ini files have the entry [OracleODBC-11g] with same fields and their values.

.odbc.ini (with a dot) is read when db connection is being made by the user himself or herself while odbc.ini (without a preceding dot) is read when a db connection is being made by an application such as Asterisk. (Try odbcinst –q –d to see the driver’s details)

I executed the following command of isql, which is a utility application shipped with unixODBC, to check my Oracle DB connection.

isql –v OracleODBC-11g

I got the following output showing that connection has been successful with the DB.

+---------------------------------------+

| Connected! |

| |

| sql-statement |

| help [tablename] |

| quit |

| |

+---------------------------------------+

This successful connection does not mean that what I did for unixODBC-devel is okay, that could only be confirmed if Asterisk was successful in communicating with the databse. I could make a successful connection with database using isql from unixODBC 2.3.0 without even installing unixODBC-devel. However, unixODBC 2.2.11 wasn’t successful in making a connection with database through isql because that doesn’t work with Oracle Instant Client.

to be continued

Asterisk with Oracle

2011-01-08T03:18:00.000-08:00

....continuation

Note: As I mentioned previously, this is not a step by step guide of integration of Asterisk with Oracle, I am just writing down everything which I did to make this integration work, which means some of the things I did may not be necessary i.e. there might be some extra steps I did. I didn’t have the opportunity and the time to identify which steps are exactly required. Also in this discussion, I am using absolute paths to referring to directories and files which I had on my system. You can adjust your paths accordingly or just use the ones I did, they are standard places.

Problems and Solutions:

Asterisk works with any RDBMS through ODBC. I was working on Linux CentOS 5, so I used unixODBC—an ODBC implementation for UNIX based Operating Systems. For more details see here. unixODBC (or any implementation of ODBC) needs a driver to communicate with the RDBMS e.g. in my case, I needed Oracle Driver for unixODBC that allows it to communicate with Oracle. There is one Oracle Driver available from easysoft but for this you have to purchase the license- A Problem!!!!!!

So I decided to use Oracle Instant Client with two of its packages—Basic and ODBC Supplement (see here). These two together solve my first problem of a free Oracle Driver. However, somewhere on the web (I can’t remember the link), I read that Oracle Instant Client 10g works only with specific distros of Linux and CentOS wasn’t there in the list-Another Problem!!!!

For which I opted to use Oracle Instant Client 11g—ver:11.2.0.2.0 (available here), this solves both of my problems.

As I said earlier, Asterisk needs unixODBC (an implementation of ODBC), it also needs unixODBC-devel to communicate with the RDBMS. You have to have both these packages installed on your Linux box before you make/install Asterisk if you intend to make Asterisk communicate with a RDBMS. On executing ‘yum’ command to install both these packages (yum install unixODBC unixODBC-devel), I was able to install unixODBC 2.2.11 and unixODBC-devel 2.2.11 packages. The worst thing I came to know: Oracle Instant Client does not work with unixODBC 2.2.11, it needs unixODBC > 2.2.11 and the latest available on CentOS’s repository was 2.2.11. (I also checked at Ubuntu’s Repository on another machine, it was same as CentOS)-Yet Another Problem!!!!

Okay fine!!! I decided to use unixODBC 2.3.0 available here, this fixes my problem but I didn’t find the corresponding devel package anywhere on the web (I was assuming it would be something like unixODBC -devel 2.3.0)-Problem after Problem!!!

After doing quite a lot search for some work around, I decided to go with unixODBC -devel 2.2.11 and unixODBC 2.3.0. I tried to install unixODBC -devel 2.2.11 using ‘yum’ command (yum install unixODBC -devel). This also tries to install unixODBC 2.2.11 showing there is a dependency of unixODBC -devel upon unixODBC -PROBLEMS!!!!!!!!!

Now the new task I had was to identify the dependencies of unixODBC -devel 2.2.11 upon unixODBC 2.2.11 and map them to unixODBC 2.3.0 instead of unixODBC 2.2.11. That means I was going to use unixODBC 2.3.0 and satisfy unixODBC -devel 2.2.11 from unixODBC 2.3.0 for what it needed from unixODBC 2.2.11. If there still are dependencies unresolved from unixODBC 2.3.0, I resolved them through unixODBC 2.2.11 as you see later.

Luckily, this proved to be the last problem that I had to resolve. Following discussion describes how I actually did all the above things.

to be continued ...

Asterisk with Oracle

2011-01-06T05:43:00.000-08:00

Asterisk works with its conf files where it finds the information it needs. At start up, Asterisk loads all the information from those files, now if you make a change in any of the conf, you have to either restart Asterisk or reload the module the file belongs to. This is our first situation, let’s call it Scenario A.

Now think about a practical deployment of Asterisk where you want do some sort of load sharing having multiple instances of Asterisk. However, you want to have same configuration for all instances of Asterisk e.g. dial plans, peers, users etc. You will have to do the tedious job of updating the conf files for each instance of Asterisk; the situation worsens if you have to make a change in any of the conf files. This is our second situation, let’s call it Scenario B.

Both of these scenarios are not a big problem, if they don’t have to be performed frequently but there are some files which might be required to be updated frequently e.g. changes in dial plans, adding/removing/updating peers or users, these files includes extensions.conf, sip.conf, iax.conf, users.conf etc.

Well scenario A may be resolved by having someone reloading the conf files after the changes, or you may write an external script (e.g. Shell Script) using a command something like asterisk reload and have that script running on regular basis in a scheduler e.g. crontab. However, that doesn’t sound like a reasonable solution, moreover, it doesn’t fit with our Scenario B.

How about putting this configuration information in an external database and having Asterisk to load that information from the database rather than files. That’s solves our problem in scenario B, put the configuration in one database and configure all instances of Asterisk to load the configuration from a single place--Database. Asterisk is equipped with the capability/feature to read the configuration from database through a feature called Asterisk Real-time Architecture (ARA). You can find more information about ARA here. Or you can find this information in ‘doc’ directory of untarred Asterisk directory. ARA mainly works in two modes: Static Real-time and Dynamic Real-time. Static Mode works like the plain conf files i.e. configuration is loaded at the startup and if you make changes, you have to do a reload; asterisk loads the configuration from the database. On the other hand, Dynamic Real-time works in dynamic way, it does load the information at the startup from database but if you make a change in database, it automatically reflects on Asterisk, that means NO RELOAD. Isn’t it great? For example, you add a sip user to database through a front end interface; it gets provisioned automatically on Asterisk!!!!! That fixes our problem for scenario A.

You can find ample information on ARA and its configuration with MySQL and PostgreSQL on the internet and Asterisk’s free book available here. I’m going to discuss Asterisk’s integration with Oracle. My discussion is more a kind of itinerary rather than a guide towards the issue, it describes what problems I faced with and what did I do to resolve them.

to be continued...

FreeRADIUS with Oracle

2011-01-05T07:09:00.000-08:00

Recently working on FreeRADIUS Sever v2.1.10, I found that there are some problems in making FreeRADIUS to work with Oracle. I had the same problem described at here. Following is the problem statement put by Alexandre, at the above link: (Great work Alexandre!!)

Unless I missunderstood the compil process it seems to me that todays source tree is stuck to Oracle 10g. Indeed the configure.in file for the rlm_sql_oracle module has a library name harcoded: libnnz10.
If you run Oracle instantclient 11g (I guess it's the same with a full installation of Oracle 11g) the library is named libnnz11. As a result, oracle linking test wiil always fail even with a correct --with-oracle-lib-dir set.
To be able to compile with both version of Oracle (10g and 11g)
I suggest a new option can be set that would define the version during ./configure.
If it sounds ok for everyone and can be tested by more people (I only tested it on Debian with instantclient_11.1), here is "works for me" patch that introduce the --with-oracle-version switch.

As Alexandre rightly guessed, there exists the same problem with full installation of Oracle 11g. However I reckon there shouldn’t be any problem with Oracle Instant Client 10g or Full Oracle installation of 10g when being configured with FreeRADIUS v2.1.10 and v2.1.9.

Although, required changes have been mentioned at the above link, I am going to provide the complete file here, so you don’t have to make the changes yourself and everything is in place before you start installing FreeRADIUS. This will work for 10g and 11g both instant client and full installation. By the way, I have tried this on Red Hat and CentOS 5. While trying on CentOS, I was trying to install FreeRADIUS Sever v2.1.9 with full Oracle 11g Installation and was struck with the same problem.

Once you have downloaded and untarred the FreeRADIUS tar file. (My untarred FreeRADIUS directory is in /usr/local/src i.e. /usr/local/src/freeradius-server-2.1.10). In this directory, go to ./src/modules/rlm_sql/drivers/rlm_sql_oracle directory. So the complete path becomes /usr/local/src/freeradius-server-2.1.10/src/modules/rlm_sql/drivers/rlm_sql_oracle. Replace the contents of ‘configure.in’ file with the contents I have provided here. (I can upload this file to some location if you need it and if you let me know some place)

AC_INIT(sql_oracle.c) AC_REVISION($Revision: 1.10 $) AC_DEFUN(modname,[rlm_sql_oracle]) fail= sql_oracle_ldflags= sql_oracle_cflags= if test x$with_[]modname != xno; then AC_MSG_CHECKING([for oci.h]) dnl # dnl # See if the user passed in the oracle_home_dir option and dnl # use that first. If not, use $ORACLE_HOME. If that's dnl # not defined, give up. There's no point in blindly dnl # hunting around for Oracle - there's no standard place dnl # for it. Any sane Oracle user/developer should have $ORACLE_HOME dnl # defined anyways. dnl # dnl ############################################################ dnl # Check for command line options dnl ############################################################ dnl extra argument: --with-oracle-version=VER oracle_version= AC_ARG_WITH(oracle-version, [AS_HELP_STRING([--with-oracle-version=VER], [Version of Oracle to search for. Should be 10 for Oracle 10g and 11 for 11g])], [case "$withval" in 11) oracle_version="$withval" ;; 10) oracle_version="$withval" ;; *) AC_MSG_ERROR(Need oracle-version) ;; esac]) dnl extra argument: --with-oracle-include-dir=DIR oracle_include_dir= AC_ARG_WITH(oracle-include-dir, [AS_HELP_STRING([--with-oracle-include-dir=DIR], [Directory where the Oracle includes may be found. It should be located under ORACLE_HOME if you have client SDK installed. Use --with-oracle-include-dir=yes if you are sure that your compiler include path includes Oracle include dir. Hint: you might try to do: locate oci.h])], [case "$withval" in no) AC_MSG_ERROR(Need oracle-include-dir) ;; yes) ;; *) oracle_include_dir="$withval" ;; esac]) dnl extra argument: --with-oracle-lib-dir=DIR oracle_lib_dir= AC_ARG_WITH(oracle-lib-dir, [AS_HELP_STRING([--with-oracle-lib-dir=DIR], [Directory where the oracle libraries may be found. It should be located under ORACLE_HOME. iUse --with-oracle-lib-dir=yes if you are sure that your linker will find the necessary Oracle client libs. Hint: you might try to do: locate libclntsh.so])], [case "$withval" in no) AC_MSG_ERROR(Need oracle-lib-dir) ;; yes) ;; *) oracle_lib_dir="$withval" ;; esac]) AC_TRY_COMPILE([#include ], [ int a = 1;], ORACLE_INCLUDE=" ", ORACLE_INCLUDE= ) RLM_SQL_ORA_WORKING_CFLAGS_FOR_LINKING_TEST= if test "x$ORACLE_INCLUDE" = "x" ; then old_CFLAGS="$CFLAGS" CFLAGS="$old_CFLAGS -I$oracle_include_dir" AC_TRY_COMPILE([#include ], [ int a = 1;], ORACLE_INCLUDE="-I$oracle_include_dir", ORACLE_INCLUDE= ) RLM_SQL_ORA_WORKING_CFLAGS_FOR_LINKING_TEST="$CFLAGS" CFLAGS="$old_CFLAGS" fi # Proceed to linking makes only sense if include dir is OK. if test "x$ORACLE_INCLUDE" != "x" ; then old_LIBS="$LIBS" old_CFLAGS="$CFLAGS" CFLAGS="$RLM_SQL_ORA_WORKING_CFLAGS_FOR_LINKING_TEST" ORACLE_LIBDIR_SWITCH= if test "x$oracle_lib_dir" != "x" ; then ORACLE_LIBDIR_SWITCH="-L${oracle_lib_dir} " fi if test "x$oracle_version" = "x" ; then AC_MSG_RESULT(no) AC_MSG_WARN([oracle version not found. Use --with-oracle-version={10|11}.]) fail="$fail Oracle version" else LIBS="$old_LIBS $ORACLE_LIBDIR_SWITCH -lclntsh -lnnz${oracle_version}" AC_TRY_LINK([#include static OCIEnv *p_env; static OCIError *p_err; static OCISvcCtx *p_svc; static OCIStmt *p_sql; static OCIDefine *p_dfn = (OCIDefine *) 0; static OCIBind *p_bnd = (OCIBind *) 0; ], [ int p_bvi; char p_sli[20]; int rc; char errbuf[100]; int errcode; rc = OCIInitialize((ub4) OCI_DEFAULT, (dvoid *)0, /* Initialize OCI */ (dvoid * (*)(dvoid *, size_t)) 0, (dvoid * (*)(dvoid *, dvoid *, size_t))0, (void (*)(dvoid *, dvoid *)) 0 ); ], ORACLE_LIBS="$ORACLE_LIBDIR_SWITCH -lclntsh -lnnz${oracle_version}", ORACLE_LIBS= ) LIBS="$old_LIBS" CFLAGS="$old_CFLAGS" fi fi if test "x$ORACLE_INCLUDE" = "x"; then AC_MSG_RESULT(no) AC_MSG_WARN([oracle headers not found. Use --with-oracle-include-dir=.]) fail="$fail oci.h" else sql_oracle_cflags="${sql_oracle_cflags} ${ORACLE_INCLUDE}" AC_MSG_RESULT(yes) if test "x$ORACLE_LIBS" = "x"; then AC_MSG_WARN([oracle libraries not found. Use --with-oracle-lib-dir=.]) fail="$fail libclntsh libnnz${oracle_version}" else sql_oracle_ldflags="${sql_oracle_ldflags} $ORACLE_LIBS" AC_MSG_RESULT(yes) fi fi targetname=modname else targetname= echo \*\*\* module modname is disabled. fi dnl Don't change this section. if test "x$fail" != x; then if test "x${enable_strict_dependencies}" = xyes; then AC_MSG_ERROR([set --without-]modname[ to disable it explicitly.]) else AC_MSG_WARN([silently not building ]modname[.]) AC_MSG_WARN([FAILURE: ]modname[ requires:$fail.]); targetname= fi fi AC_SUBST(sql_oracle_ldflags) AC_SUBST(sql_oracle_cflags) AC_SUBST(targetname) AC_OUTPUT(Makefile)

FreeRADIUS needs some library and header files to be integrated with Oracle, these files are shipped with Oracle Instant Client and Oracle Full Package both. You can tell FreeRADIUS where to find these files either by setting some system variables before executing the configure script or by passing some arguments to the configure script. We will stick to the second option here.

On my system, I had Oracle Instant Client (both Basic and SDK Packages) unzipped in /opt/instantclient/ so all of my references to Oracle are based on this path.

Now go back to FreeRADIUS’s untarred directory i.e. /usr/local/src/freeradius-server-2.1.10. Now you can run configure script from here by executing following command.

./configure --with-oracle-version=VER --with-oracle-include-dir=DIR --with-oracle-lib-dir=DIR

On my system this command takes the following form having instant client 11g on the path /opt/instantclient/.

./configure --with-oracle-version=11 --with-oracle-include-dir=/opt/instantclient/sdk/include/ --with-oracle-lib-dir=/opt/instantclient/

On a system with full Oracle 11g Installation, I had to execute the following command.

./configure --with-oracle-version=11 --with-oracle-include-dir=/u01/app/oracle/product/11.2.0/dbhome_1/rdbms/public/ --with-oracle-lib-dir=/u01/app/oracle/product/11.2.0/dbhome_1/lib/

Note: If you are using Instant Client or full Oracle 10g, you don’t have to replace the above ini file nor do you need to send the argument --with-oracle-version=VER to the configure script.

Rest of the commands for FreeRADIUS is quite common as below:

make

make install

I hope this post is helpful to those who are facing this problem.

Image Processing: Contrast and Brightness

2010-03-14T01:17:00.000-08:00

In Computer Graphics, RGB color model is often used to describe an image which means that the appearance of a pixel (often referred as Picture Element) is characterized by three primary bands i.e. Red, Green and Blue. Different kinds of effects with respect to the appearance of the image are created by manipulating the RGB values of pixels of the image. These effects include image blurring, rotation, edge detection, sharpening and different kinds of transformations etc. Image's contrast and brightness are two of those effects related to the appearance the image. There exist other models including HSV, HSI, YCbCr etc which can also be used to create these types of visual effects.
A pixel of a color image is represented by 24 bits (8 bits for each primary band which may assume any value between 0 to 255) in contrast to binary images, grey-scale images etc where the bits to represent the pixel may be less. However, another 8 bits are sometimes added (as is the case with Java) to represent the pixel which stands for alpha value representing transparency (i.e. opacity) of the pixel hence making a total of 32 bits to represent the pixel. There is plenty of information available on the web about different color models and different ways for their representation and manipulation. Here we want describe about an image's contrast and brightness.
The contrast of an image is defined by the distribution of the color values of the pixels of the image i.e. in a high contrast image; the color values of pixels exist over a wide distribution. On the other hand, a low contrast image, they exist in a narrow distribution. Hence color values are grouped together tightly over a short range of value for low contrast and are grouped loosely over a wide range of values for high contrast images. This grouping of color values also has an effect on the appearance of the image called Brightness.

In case of High Contrast Image, image appearance is much detailed while most of the details disappear in case of low contrast image. That's why often contrast enhancement is performed on images to make details visible which cannot be seen in low contrast. Coming back to the location of grouping of color values, the image may appear in some form grey color depending upon the location of grouping. That is if the location of the grouping of the values is (or close to) the maximum extreme (e.g. for RGB model, bands have a value of or near to 255), the image appears to be whitish while if location of grouping is (or close to) the minimum extreme (e.g. for RGB model, bands have a value of or near to 0), the image appears to be blackish. Therefore, this location of grouping of values make image look bright or dark.

Contrast of an image can be increased (or decreased) by increasing (or decreasing) the width of the color distribution. Brightness of an image can be increased by shifting the location of the grouping of values towards the maximum extreme value or can be decreased by shifting the location of grouping towards the minimum extreme value.

Histogram Analysis of an image is an analytic approach for studying images which is graphical representation of the color values occurring in the image, hence a representation of image contrast and brightness. The histogram shows the number of occurrences in an image of all possible values. A technique based on Histogram Analysis is called Histogram Equalization that is used controlling image contrast and brightness. Histogram Equalization can be performed in ways either Global Histogram Equalization (GHE) on the whole of image or Local Histogram Equalization in which case equalization is performed on a section or sections of an image. Generally, equalization is performed on grey-scale images, for color images equalization is performed on each primary band if color model is RGB. However, it is recommended to convert the image into some other color model before applying Histogram Equalization. For further details see following links:
Wikipedia Histogram Equalization
Ruye Wang Histogram Equalization
HIPR2 Histogram Equalization

Getting Started with BlackBerry J2ME Development

2009-07-08T12:20:00.000-07:00

Check out my article on java.net on blackberry application development with J2ME.

J2ME: Browsing File System on Mobile Device using FileConnection APIs

2009-02-20T17:30:00.000-08:00

There can be application where we need to browse through different files on our mobile phone, File Viewer, File Uploader are few to name. Here I’m going to provide a complete program that helps you browse through your file on mobile device and therefore helps you develop more sophisticated applications using it. Its based on J2ME FileConnection APIs (JSR 75) which is a optional package within MIDP. Though the midlet code provided is quite simple and straightforward and needn’t much explanation, however, please consult JavaDoc for FileConnection API for details which is available here. The midlet allows the user to browse through file system on the Java Enabled devices.
There are some other ways to open file connection but they open connection to specific folders e.g. System.getProperty("fileconn.dir.photos") which retruns the path to ‘photos’ folder to which connection can be opened. Moreover this method might not be supported by the some of the devices. In my case, I tried this approach for SE K800i and Nokia 6630 where it worked but not for Motorola V3. In that case, the approach outlined by the code snippet here is more suitable.

Note: FileConnection APIs (JSR 75) being an optional package of MIDP might not be available on the all the devices. So you’ll have to check if the particular device has support for it using system property System.getProperty("microedition.io.file.FileConnection.version").

/**
*
* @author Atifs
*/
import javax.microedition.lcdui.*;
import javax.microedition.midlet.MIDlet;

public class BrowserLauncher extends MIDlet implements CommandListener {

private Display display;
private final Command CMD_EXIT = new Command("Exit", Command.EXIT, 1);
FileSysBrowser fsb;

public BrowserLauncher() {
display = Display.getDisplay(this);
}

protected void startApp() {
fsb = new FileSysBrowser(this);
fsb.showFiles();
}

public Display getDisplay() {
return display;
}

protected void destroyApp(boolean unconditional) {
}

protected void exitApp() {
destroyApp(false);
notifyDestroyed();
}

protected void pauseApp() {
}

public void commandAction(Command c, Displayable d) {
if (c == CMD_EXIT) {
exitApp();
}
}
}

/**
*
* @author Atifs
*/
import java.io.IOException;
import javax.microedition.lcdui.*;
import javax.microedition.io.*;
import javax.microedition.io.file.*;
import java.util.*;

public class FileSysBrowser extends List implements CommandListener {
// 'DATHHEAD' holds the root, please see javadoc for details
// This may vary for different devices.
String DATHHEAD = "file:///";
private Command CMD_SUBMIT = new Command("Open", Command.ITEM, 0);
private Command CMD_EXIT = new Command("Exit", Command.EXIT, 1);
private Command CMD_BACK = new Command("Back", Command.BACK, 1);
private BrowserLauncher fl;
private String path = null;
private String selectedItem = null;
FileConnection srcconn = null;

public FileSysBrowser(BrowserLauncher fl) {
super("File List", List.IMPLICIT);
addCommand(CMD_SUBMIT);
addCommand(CMD_EXIT);
addCommand(CMD_BACK);
setCommandListener(this);
this.fl = fl;
}

protected void showFiles() {
if (path == null) {
Enumeration e = FileSystemRegistry.listRoots();
path = DATHHEAD;
setTitle(path);
while (e.hasMoreElements()) {
String root = (String) e.nextElement();
// you can pass an 'image' object instead of 'null' to
// append method
append(root, null);
}
fl.getDisplay().setCurrent(this);
} else {
if (selectedItem != null) {
setTitle(path + selectedItem);
} else {
setTitle(path);
}
try {
// works when users opens a directory
if (selectedItem != null) {
srcconn = (FileConnection) Connector.open(path + selectedItem, Connector.READ);
} else // works when presses 'Back' to go one level above/up
{
srcconn = (FileConnection) Connector.open(path, Connector.READ);
}
// Check if the selected item is a directory
if (srcconn.isDirectory()) {
if (selectedItem != null) {
path = path + selectedItem;
selectedItem = null;
}
Enumeration files = srcconn.list();
while (files.hasMoreElements()) {
String file = (String) files.nextElement();
append(file, null);
}
//
fl.getDisplay().setCurrent(this);
try {
if (srcconn != null) {
srcconn.close();
srcconn = null;
}
} catch (IOException ex) {
ex.printStackTrace();
}
}//if (srcconn.isDirectory())
else {
// 'else' is unnecessary here, i just out here to signify the
// fact that you handle the selected item if it is a 'file'
// instead of a directory. For example you can display a file
}
} catch (Exception e) {
e.printStackTrace();
}
}
}

protected String getDirectoryName() {
int select = getSelectedIndex();
return getString(select);
}

public void commandAction(Command c, Displayable d) {
if (c == CMD_SUBMIT) {
selectedItem = getDirectoryName();
deleteAll();
showFiles();
}
if (c == CMD_EXIT) {
fl.exitApp();
}
if (c == CMD_BACK) {
if (!path.equals(DATHHEAD)) {
int slashIndex = path.lastIndexOf('/', path.length() - 2);
path = path.substring(0, slashIndex + 1);
if (path.equals(DATHHEAD)) {
path = null;
}
deleteAll();
showFiles();
}
}
}
}

Disclaimer: The code provided has been written for exploratory purposes, not following the official guideline of Software Engineering and therefore has not been tested thoroughly. However, you are FREE to use the code for any purpose and be aware to test it before using for production uses!!!