Ережепова А.К. - магистрант, Костанайский Государственный
Университет имени А. Байтурсынова, г.
Костанай.
Иванова И.В. – кандидат педагогических наук, старший
преподаватель кафедры программного обеспечения, Костанайский Государственный
Университет имени А.Байтурсынова, г.Костанай.
IP - telephony:
principle of operation and its features
Under packet calls, you know the
voice communication that takes place via the data transmission network, in
particular through digital channels or the Internet. For this purpose, use
special devices — voice gateways. In very short summary, IP network looks like
this:
1 - the Central part is the server. It is
associated with the telephone lines (which allows you to connect to any
telephone handset), and with the "world wide web" (so it is possible
to connect computer, tablet computer, netbook, notebook);
2 - incoming signal on the server is digitized,
compressed, converted into data packets and sent to the recipient;
3 - reaching the recipient, the message is
decoded and returned in original condition;
4 -
signal almost simultaneously enters the network and leaves her. This conversation has
no pauses and breaks.
Unlike conventional analog method of
communication, Internet telephony uses the maximum capacity of lines and
requires no external PBX. And Internet access have now all the subscribers who
need it, and it allows to reduce the costs of connection to the IP network.
Thus, packet telephony is beneficial for the providers of communication
services, and users. In addition, this technology allows to significantly
improve the quality of communication. For such approval there are at least 3
reasons:
- telephone servers from year to year improving;
- owners of private networks (ISPs) control the
network load and can change its parameters, diminishing -
- delay time when transmitting data;
- designed and implemented new, more relevant
protocols.
How does it work?
As mentioned
above, the technology is completely different than in standard telephony, the
principle of data transmission in which the analog signal (speech) is converted
into a digital signal, is packaged into data packets, sent across the network
and re-translated into sound.
Feel
the difference? Around the world traveling is not an analog signal, which
requires a dedicated line, served by a large number of employees, and compact
files, almost instantly moving around the world wide web.
The
lack of the need for cooperation with Telecom operators another region or
country substantially reduces costs, allowing the companies offering IP
connectivity, to attract customers very affordable prices.
The structure of
the IP-network: physical and data link layers
See how IP telephony. Describing the system, it
is necessary to consider all its levels, namely:
1. - physical;
2. channel;
3. network;
4. transport;
5. - level data.
On the physical
level is the transfer of information units (bits). To do this use existing
infrastructure: twisted copper wire (standard telephony), coaxial cables, fiber
optics.
Data link layer
plays the role of mediator between the physical and the network layers. There
are switches to create the perfect collection of nodes in a computer network.
At this level unfolds VLAN technology needed for building virtual networks.
Voice VLAN has two useful qualities:
- it reduces the risk of interception and
decoding of voice packets and thereby improves the security of the network;
technology improves the quality of communication
because it allows you to assign voice packets heightened priority during
transmission.
How is packet
telephony: network and transport levels
At this level
there is routing, that is, selects the path that data will travel to a
destination with a specific IP address. This is a network Protocol that gave
its name to the technology.
Routers have built-in voice gateways that allow
adapting analog phones to packet telephony. In addition, the gateway performs
the following functions:
- provide user authorization;
- distribute IP address;
- ensure the safety of the compound;
- support voice mail and some other additional
services;
- keep records of the traffic;
- support protocols N.323 and SIP.
The transport layer is implemented using three
main protocols:
1. TCP. Provides guaranteed delivery of data;
2. UDP. Provides best-effort delivery (no
confirmation prompts);
3. RTP. Provides the procedure for receiving
packages. The Protocol recognizes the data type and assigns packets a timestamp
that determine the order of admission.
The data layer: protocols
In packet telephony
uses two protocols: N.323 and SIP. The first of them is older, stronger
associated with traditional telephony and describes standards for coding analog
data into any existing networks.
SIP more relevant and universal Protocol. It
initiates, monitors and terminates the communications, the type of the data
themselves does not matter. SIP connections occur through remote servers and
work on the principle "request-response".
The main types of requests in SIP is:
- "invitation" — request for the
establishment of a new communication session;
- "confirmation" — agreement for the
session initiation or confirmation of receipt of data;
- "the end" — the end of the session.
- cancel — the termination of the processing of
the request;
- registration — the transfer to the remote
server information on the location of the subscriber;
- "options" — exchange of information
on the characteristics of terminal equipment (SIP phones).
The algorithm is very simple communication
between two subscribers can be described as follows:
1. the first user dials the number to the SIP
phone;
2. the remote server gets the request
"invitation";
3. the second subscriber picks up the phone. The
server receives the command "confirm";
4. starts a communication session;
5. one of the interviewees puts it up on the
server gets the command "end".
The data layer: codecs
Codec is the program
that "compresses" or "expands" the audio and video. In
Internet telephony are used by many. For example:
1. 729. Codec for transmission of audio
information, which is not with the data themselves, but only with the
characteristics of the signal. Saves the basic parameters of the voice (tone,
amplitude, etc.);
2. 711. Standard codec for voice transmission;
3. G. 723.1. The codec is used for audio transmissions
at low flow. Greatly distorts speech;
4. H. 264. Codec for transmission of video data.
From application to
finish — less than 5 minutes
We briefly looked at how organized IP telephony. Furnishing the
telephone number of the analog network for Russians for a long time and forever
turned into a real headache. Long queue. The long wait of installation.
Questionable quality of services. The telephony online compared to all these
simple utterly.
Often enough only to choose the provider to
contact the consultant online to choose a tariff and then pay for services and
rent rooms, to obtain access to the program with which your communication
device will be connected to VoIP.
Since the filing of the application to access
the communication services will be only a few minutes. Deep knowledge of PC's
don't need that – most providers have prepared for users simple and convenient
interface of management of services.
The most popular method of organization of
IP-telephony is the use of PC, mobile devices, tablets, laptops – any equipment
that has Internet access and an audio headset (PC – headset-MIC).