Before
I can proceed to describe the wonders of VoIP, let me spend
some time describing the background of how it was and lay
the foundation for how it is and will be.
The traditional way of calling
Almost every one has a phone installed in their houses. Lets
suppose, Anil wants to place a call to Ankur, so he dials
a particular number, say 2286390. There is a beeping noise,
then follows a ringing tone and finally, someone on the other
side answers the phone. All this is just fine - but now lets
get a wee bit more technical on this.
When Anil picks up his phone, in Telephonyese (to coin a word),
its called going "off-hook". Anil's phone receives
a signal which we know as a dial-tone. Anil now dials 2286390
- this number is used by his local telephone exchange (shown
as LTE1) to decide where to direct this call to. This decision
is very similar to a network routing table - if the reader
is familiar with networking concepts. The concept is simple
- lets assume that there were only a couple of hundred telephone
numbers in the world. Then, its possible that each Exchange
can know the destination of each and every end user phone
number. However, as the number of telephone numbers increase,
it becomes impossible for each Exchange to know the destination
routes of every possible number. So instead what is usually
done is, that Exchanges keep a track of which other Exchanges
can handle routing for which range of numbers. For example,
LTE1 may be handling numbers beginning with 337* , LTE2 may
be handling numbers beginning with 444* and LTE3 handles numbers
with prefixes 523*. So now, when Anil dials 228… his
local exchange (that is, the exchange to which his phone is
connected to) realizes that this is outside its own domain,
but figures out that LTE2 is the correct Exchange for any
numbers beginning with 228* and so routes this call to LTE2.
LTE2 now receives the number 2286390 and realizes that this
is a number within its own domain and goes about contacting
Ankur's phone. In many phones, before ringing, you will hear
a beeping noise - this is mostly to tell the user that 'I'm
trying to reach your destination'. Once LTE2 manages to connect
to the terminal (phone) whose number is 2286390, it sends
a ringing tone to the caller - this is its way of saying 'I
think I've reached your destination, now I'm alerting the
destination of your call'. Once Ankur picks up his handset
(i.e. going off hook) the communication is established.
Note that the above description is very loose and is only
meant to be a basic description of what is going on - there
are more in-between signal etc. happening, which I haven't
outlined.
The important thing to note here is that once Anil and Ankur
are connected, a dedicated line is established between the
two points . What this means is that a physical connection
is established from Anil to LTE1 to LTE2 to Ankur for the
entire duration of the call - what this means is that while
Ankur and Anil are talking, this connection is reserved entirely
for them and nobody else can use this established connection
to talk.
Now lets suppose that you were to make an ISD call from India
to somewhere in the US. So what would happen is that a dedicated
line would have to be established from your exchange all the
way to the correct exchange in the US !!!! And this connection
could well traverse across multiple exchanges till an exchange
is reached which can directly connect to the destination number.
The above diagram showed only two exchanges - however, its
is very possible many such nodes might have to be traversed
to complete a call.
So its obvious then that since the end-end connection must
be dedicated (that is it cant be used for more than one connection
at a time), the expense of making calls will vary depending
on the distance (and some other factors too, which I will
not be expanding on). So now I guess, its quite obvious why,
making calls to your neighbor is so much cheaper than speaking
to your friend across the seven seas.
The above scenario is what happens when a regular phone call
is made - and this is what is known as the PSTN world (Public
Switched Telephone Network). This sort of a network is called
a Circuit Switched Network - this is so as a complete end-end
circuit needs to be established during connection from source
to destination.
The VoIP way of calling
Now what kind of a network is the Internet ? The internet
is based on a technology called TCP/IP (Transmission Control
Protocol / Internet Protocol) where the IP part is essentially
Packet Based. What this means is that multiple connections
can be transmitted over one connection at any instant of time,
unlike the traditional Circuit switched way. This is possible
by dividing each connection into small packets and multiplexing
them across the same connection. The following diagram compares
the two technologies:
PSTN World: When A1 & Z3 are speaking, no one else can
use the same connection and must wait till this connection
is released (or communicate using a new physical connection).
IP World: There is no dedicated connection - every one can
communicate using the same connection - this is achieved by
breaking each session into discrete packets and multiplexing
all the packets one by one across the same connection.
So what all does this amount to? Supposing we utilize this
existing packet based network as the backbone of our calls
instead of the expensive dedicate lines that PSTN offers us,
cant we reduce calling charges? Of course ! Look at the following
diagram, with the same old example.
So lets describe the above. Anil calls 2286390. There is a
device called a 'Gateway' that receives this request, converts
it into the appropriate packet based format and forwards this
request to some node in the internet, which looks at the number,
decides which destination Gateway can handle this number and
forwards the request to the correct gateway, which in turn
contacts the destination phone (Ankur's phone) and the call
is set up.
Some important points:
What is this gateway and why is it required?
Well, as I mentioned, the phones we have communicate using
a protocol that is suited for the PSTN world - their signaling
format is different from what is used in the internet and
hence there needs to be some conversion to and from the PSTN
to IP world in order for them to inter operate. Compare this
to two people. one who speaks Chinese and the other Hindi
- left to their own, they will not be able to communicate
to each other - we need a translator who understands both
languages and can communicate with both. This is exactly what
a Gateway is - it is capable of translating or Gatewaying
between different protocols. Shown above is a PSTN-IP gateway.
So what have we achieved?
Notice that now there is no need for a dedicated connection
between Ankur and Anil. The main backbone of communication
is now through the internet and as mentioned before, it is
a Packet Based network where dedicated connections are not
used. "So what ?" you ask ?
Hmm… some more explanation to convince you, oh skeptic
!
Lets assume you in India connect to the internet (using VSNL
as your gateway). Now you launch up your browser and hit http://www.timesofindia.com.
A little later, you hit http://www.cs.columbia.edu/sip
. You get both the sites on your browser. But do you realize
where the information on your screen is coming from? In the
former case, the information on your screen is coming from
a site somewhere in India (maybe even close to where you stay)
and in the latter case, this information is being transmitted
to you from some where in the US . But does that mean when
you are connecting to http://www.cs.columbia.edu/sip,
your phone bills show an ISD call ? No ! what you see is a
charge for connecting to your local Gateway (VSNL) which is
a local call and that's all (plus some fixed charge for using
this service).
Now do you see the possibilities? Is it then, not possible
to extend this concept and use the existing internet services
and backbone to talk across the internet just like you browse
the internet (and hence all over the world) at the cost of
a local call?
Of course! And that is a part of what VoIP is all about.
Transmit Voice across the internet just like you transmit
data - and use the already present backbone to talk to your
friend across the world and forget all about STD and ISD rates
.
But of-course, not everything is so simple. There are legal
and technical constraints to this. Many countries do not yet
allow you to use this technology (since this means the existing
PSTN providers stand to lose a lot of money J amongst many
other reasons). Also, passing voice across the Internet, which
is essentially an unreliable network bring in all the complexities
of voice getting lost somewhere in the way, delay of transmission
and other such factors form technical challenges which must
be met to make this a truly commercially viable technology.
But all this forms the part of another page of my tutorial
and not this one.
So finally, what is VoIP ?
Simply put VoIP is a technology that aims to provide real-time
transmission of voice across the internet. Actually, the 'V'
in VoIP is very misleading - VoIP does not necessarily talk
only about speech transmission - it also deals with Video
transmission - in general any Multimedia content across the
internet in real-time.
Please note that the above diagram illustrated a case of
PSTN-IP-PSTN communication. It is also possible to have pure
IP-IP communication (where you don't need a Gateway), IP-PSTN
communication or PSTN-IP communication.
I have illustrated some of the above scenarios below. I will
not describe each of them, as they are pretty self descriptive.
Conclusion
I have tried to describe What is VoIP and part of why VoIP
in this tutorial. While trying to explain the technology,
I have tried to keep the technicalities as simple as possible.It
is evident that the advantage of VoIP is not only reduced
cost, but much more than that. But since reduced cost is what
most customers, at least prima facie are worried about as
comparing it to current PSTN rates, I've stressed only on
that point. In my following tutorials, I plan to introduce
the various VoIP protocols that exist in the market as of
today, merits and demerits as well as illustrate the added
benefits that VoIP unveils to the customer.
Before
I can proceed to describe the wonders of VoIP, let me spend
some time describing the background of how it was and lay
the foundation for how it is and will be.
