April 2012 - Telecommunications Blog

A blog for mobile communications systems GSM , UMTS and LTE

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Wednesday, April 18, 2012

VoIP protocols

4/18/2012 07:03:00 PM 2
VoIP protocols

Real-Time Transport Protocol (RTP)

RTP is used for transmitting audio and video packets between communicating computers. RTP is defined in RFC 3550 and "profiles", which specify how to packetize various audio and video codecs used in an RTP session, are defined in RFC 3551.

 RTP also addresses issues like packet order and provides mechanisms to help address delay and jitter. RTP was improved upon with the result being called Secure RTP which provides for encryption, authentication, and integrity of the audio and video packets transmitted between communicating devices.

H.323 

Developed by Study Group 16 of the ITU-T, H.323 is an umbrella standard encompassing
many subcomponent standards and annexes for transmitting multimedia (voice, video and data) across packet based networks. The two main subcomponent protocols are H.225 (call control), & H.245 (bearer control and capabilities exchange). H.225 consists of two main parts Q.931 (basic call control as used in ISDN networks) and RAS (Registration, Admission & Status).

H.323 also references other standards such as Real Time Protocol (RTP), G.xxx audio codecs, H.26x video codecs and T.120 real time data conferencing protocol.

SIP 

Developed by IETF, SIP is a mechanism to initiate, terminate & modify sessions in an IP
network. It uses a client / server architecture and the protocol is request-response based. It
enables personal mobility by tracking down users and delivering calls to an endpoint. It is a
lightweight, text-based protocol and reuses much of the construct of other internet protocols
such as HTTP and SMTP. SIP does not know about the underlying details of a session and relies on IETF protocol Session Description Protocol (SDP) to describe the session. It also interworks with other IETF protocols such as Megacop, RTP, RTSP, RSVP and SAP.


Both H.323 and SIP can be referred to as "intelligent endpoint protocols". What this means is that all of the intelligence required to locate the remote endpoint and to establish media streams between the local and remote device is an integral part of the protocol.

MGCP & Megacop/H.248 

There is another class of protocols which is complementary to H.323 and SIP referred to as "device control protocols". Those protocols are H.248 and MGCP. 

MGCP (Media Gateway Control Protocol) & Megacop/H.248 are relatively low level, master/slave protocols used between Call Agents and MGs. MGCP is not officially a standard but does exist as an informational RFC (RFC 3435). Megacop (IETF RFC 3015) & H.248 (ITU Rec.) are the same protocol developed by an IETF and ITU collaboration. It is derived from, and draws heavily from MGCP but adds several new enhancements.

Some service providers provide users with devices that implement H.248 or MGCP (or comparable protocols). In the core of the network, some device serving as the MGC provides the H.323 or SIP logic necessary to properly terminate VoIP calls around the world.

VoIP OSI Protocol Stack

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I will explain QOS at next article .

Tuesday, April 10, 2012

What is VoIP ? - Introduction

4/10/2012 06:37:00 PM 4
What is VoIP ? - Introduction
What is VoIP

Around 1995 there was a revolutionary advancement in the way traditional voice traffic was
carried when voice, video and data were transmitted over a single ‘converged’ network. The
terms Voice Over IP (VoIP) and IP Telephony (IPT) were introduced to describe this method of
transport of voice signals as well as fax, DTMF tones, signalling traffic, and network management
traffic.


There are a number of protocols that may be employed in order to provide for VoIP communication services like :

  • SIP (Session Initiation Protocol) / RTP  (Real-Time Protocol)
  • IMS
  • H.323
  • H.248 / MGCP   ( comparable protocols )
  • Skype
VoIP can deliver voice, video, and data communications capabilities across a number of separate devices that work together, such as a mobile phone, a PC, and even an HD TV!.

You can enjoy VoIP sevice by using:
  1. An analog Telephone Adapter (ATA). It connected existing telephone jack, so you no need to change your old telephone hardware (system).
  2. Dedicated VoIP Phones. It is a Phones that create specially for IP/Packet Switch connection. Just plug this phone to your IP/internet network then you can enjoy VoIP service.
  3. A Softphone. It is a piece of software that installed at PC/Laptop to make you can enjoy VoIP service without dedicated hardware.
I will explain the protocols at the next article.

Wednesday, April 4, 2012

wimax network planning part 1

4/04/2012 01:00:00 PM 0
wimax network planning part 1
WiMAX (Worldwide Interoperability for Microwave Access) ( 802.16 - 802.16e ).WIMAX was developed to promote conformance and interoperability of the IEEE 802.16 standard (Wireless MAN).


we will learn how we can planning a wimax network but first we should know some important things .

 
Frequency Planning :

         • The frequency re-use is extremely important in reducing the interference and therefore increasing the capacity
         • Off-the-shelf frequency assignment do not minimize the deployment cost for your system.
         • An optimum frequency assignments can be applied by considering
         • Site locations
         • Power levels
         • User distribution
         • Spectrum availability
         • Geography and building characteristics.


Site Features Optimization

         • Number of antennas
         • Antenna directions
         • Antenna heights
         • Antenna powers
         • Antenna tilts
         • Number of channels
         • Frequency of channels
         • OFDM Parameters
         • Cyclic prefix
         • Number of subchannels
         • Subchannelization rates
         • Available modulations
         • Handover parameters
         • MIMO settings
         • Uplink / Downlink traffic ratios
         • Voice over IP related issues
         • Backhaul planning: fiber, minilink, HDSL…. capacity issues