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Putting Voice into Packets

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PCQ Bureau
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Conventional telephony 

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When you make a telephone call, the telephone exchange establishes an exclusive connection to the number you dial. While you're having the conversation, anybody else trying to dial either party will get an 'engaged' tone. That's

essentially what a circuit-switched network does. It establishes an exclusive and continuous physical connection between two parties. Circuit-switched technology itself has evolved quite a bit. It started with cordboard switches where an operator manually connected two parties through a cord. From there, it moved on to SxS systems, also called the Strowger exchanges, named after its inventor. These were electromechanical in nature. After this came the crossbar exchanges, which are still being used in many countries. Crossbar used electromagnetic principles. Today, electronic telephone exchanges are fairly common, which are more compact and powerful, and convert voice into digital signals for transmission.

Why is Internet telephony illegal?

If one were to get philosophical, there are two types of illegalities: those of the moral kind and those of the economic kind. While murder and stealing fall in the first category, Internet telephony clearly does not. So, it could be an economic offence at best. Offence against whom? Is the country losing anything because you use Internet telephony? Is some other country benefiting at our cost if you switch over to Internet telephony? Clearly not.

The only loser is VSNL/BSNL whose revenues would be affected, as the bulk of their revenue still comes from voice traffic, and they seem to be in no hurry to attempt a switch over to a different way of life. No wonder then that Internet telephony (and VoIP) is treated as illegal.

But clearly, the day is not far off when the ban can no longer hold. Any private player with a voice and an ISP license could open the doors for VoIP and Net telephony, and change the way we use the telephone or the PC, and the way we pay for it, once and for all.

Digitizing sound is an interesting process. When we make a telephone call, our voice is first converted into analog electrical signals. This signal is then encoded into digital format using a technique called PCM (Pulse Code Modulation). This technique takes samples of the analog signals at a rate of 8,000 samples per second. Each sample therefore represents 125 microseconds of a voice stream, and is eight bits, or one byte long. This signal is then carried over high-speed digital lines and again decoded into an analog electrical signal at the receiving end. The analog signal is finally converted into the original sound. 

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Speaking of sound, any conversation consists of two components-sound and silence. When the digital sound signals are transmitted over a circuit switched network, both components have to be sent. Not only that, but the order of transmitting signals also has to be retained, else quality of transmission suffers. That's why all equipment in a circuit-switched network must be highly synchronized using expensive TDM (Time Division Multiplexing) equipment. Since sound and silence are both transmitted, a lot of bandwidth gets wasted in circuit-switched networks. In fact, one voice conversation requires a 64 kbps channel, which is quite a lot of bandwidth. 

How VoIP works

In VoIP, analog voice signal is digitized using PCM. These digital voice samples are then buffered on an IP gateway. This device converts the PCM data stream into a compressed IP packet stream using DSPs (Digital Signal Processors). DSPs are responsible for converting from analog to digital as well as compression. The set of PCM samples are analyzed as a discrete set of binary data. It checks the speech for all the moments of silence, which are a lot. Even when we speak, there are pauses in between that go unnoticed to the human ear, but are quite discernible to the sampling device. The length and beginning of these pauses is noted, while the remaining 'silence' is removed from the data set. Similarly, redundant data is also removed, making the data set more compact. Finally, an IP header is attached to this compressed data, which is then sent out on the network as discrete data packets. 

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Once the voice packet is sent out, it finds its way to the destination just like any other data packet. It passes through various routers and switches to reach the destination gateway. Here, it gets decompressed, meaning all the periods of silence and redundant data are reinserted, and is finally decoded to produce an approximation of the original sound. The compression algorithms used in this process can compress the voice signals and can even carry voice over as little as 5.3 kbps bandwidth.

muLinux

A fully-configurable, application-centric and tiny

distribution

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muLinux installs on the Windows or DOS partition and requires

a minimum of a 386 processor with 8 MB of RAM. It bundles many packages

including those for console, networking, and X-Window.

Let’s get straight into setting it up. Copy the directory

mulinux from the /os directory on the CD to C:\. Unzip ‘DOS TOOLS.ZIP’ in

the same directory. Now restart the computer in full DOS mode. Change to the

directory C:\MULINUX and type ‘INSTALL’. In the first screen, select the

second option, which would install muLinux in the directory C:\LINUX. The

compressed archive is uncompressed and then the system reboots. After the

reboot, go to full-DOS by using the F8 key. Change to the directory C:\LINUX and

type ‘LINUX’. This will start up an interactive setup where you’ll go

through a massive (but very helpful because the configuration files need not be

edited later) question and answer session that covers the entire range from

setting up your keyboard to setting up networking.

You are asked to create a swap space, which can be skipped if

you have enough RAM say more than 128 MB. Otherwise leave swap file name as

/SWAP/LINUX.SWP. The swap file size can be selected to 64 MB.

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You are asked to configure keyboard and also some add-ons

including X-Windows. Select Skip when you are asked to upgrade, as these add-ons

have already been setup. Specify the serial and parallel ports to which your

mouse, modem, and printer are connected. Remember ttys0 is COM1 and lp0 is LPT1.

Let mouse protocol be the default.

For networking, you are asked for the model of your Ethernet

card and only six network card models of EtherLink, Intel, RealTek, and AMD are

supported. Then though a series of questions, which are well explained, you can

configure your network, NFS services for Linux–Linux sharing and SAMBA for

Windows-Linux sharing. If you choose to have PPP support, you can configure your

dial-up connection. Subsequently you can configure Fetchmail for retrieving mail

from a POP3 account. muLinux auto detects your IDE CD-ROM drive. Finally you can

decide what daemons or background processes you need to run at startup.

You are now given the login prompt. Login, start up X-Windows

using the startx command and you are ready to swing!

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FreeDOS BETA5 ("LARA")

Full distributionIf you are a DOS lover, you’ll love this

This is a free and fully MS-DOS compatible OS. System

requirements are minimal and it installs on a x486 with as little as 8 MB RAM.

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The distribution on the CD has zipped files and you need ‘UNZIP’

(included) to extract the installation disks. Keep a pack of formatted 3½"

floppies at hand. Through the DOS prompt, Unzip ‘BASE1. ZIP’ using the

command UNZIP BASE1. ZIP —d A: (note the lowercase -d). This is the only

floppy you need for the Mini FreeDOS distribution. For the full version, repeat

the unzip process with rest of the install disks. Now you need to install the

Boot floppy image. Use ‘RAWRITE’ (included in the package). Run RAWRITE from

DOS prompt, read the .BIN image file (MINI.BIN for Mini version and FULL.BIN for

Full version) and write to a preformatted floppy in A: drive. Now you are ready

to install FreeDOS.

Reboot and start your PC with the boot floppy you created.

You are left at the DOS prompt. You can install FreeDOS in a folder in the

existing partition or work purely on FreeDOS. Use FDISK and FOR MAT at the

prompt in the latter case. Then run INSTALL at the prompt. Give the location

from where you’ll install as A: and the destination can be C:\FDOS. Pop in the

install floppies you created one after the other. The process is a bit slow due

to slow FDD access. Alternatively you can install from the hard disk directly.

Note, however, that in this case you’ll have to unzip all the install floppies

in one single directory say C:\FDINSTALL and give the source path accordingly

after booting up.

The package also includes the Seal GUI for FreeDOS.

MINIX

A small, free UNIX clone for those who want to peep into Unix

Unlike Linux which requires at least a 386 processor to run,

Minix can be run even on an 8088 or 80286. Basic memory requirements are as low

as 3 MB.

On our CD we have the 386 precompiled binaries (i386.tar).

The procedure for Minix installation is outlined in readme.txt and install.txt.

Unzip the i386.tar using Winzip to get the three files Root, Usr, and Usr.taz.

Then you create the Minix bootable floppies using FDVOL utility. This is also

included on the CD. These floppies are then used to boot the machine and install

the OS. We have also included zipped files for the compiler, debugger,

networking utilities, etc, in the package. DOSUTILS come in handy to prepare

your hard disk for the installation.

Shekhar Govindarajan and Ashish Sharma

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