VoIP Architecture: VoIP Call Components

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Last time we went over the VoIP essentials including the benefits of VoIP and the fundamental VoIP terminology as well as the VoIP Protocols

Now is a good time to introduce all the components that make Voice over IP a reality. Without them, IP telephony as we know it today wouldn't exist.

In this article I will mainly focus on introducing the major VoIP components as well as describing the functional characteristics of each one of them.

Different standards provide their own unique specification for each VoIP component. To make things simpler, I will stick to the most important components that are globally acceptable and recognized by everyone. I hope that by the end of your reading, you will have a clear picture of what builds up VoIP.

VoIP Components

The four most important VoIP components are:

  • Signaling Gateway Controller
  • Media Gateway
  • Media Server
  • Application Server

  •   Signaling Gateway Controller

As you might remember from my VoIP Essentials article, the Signaling Gateway Controller (SGC) is known as a "called agent" because of its call control function and is also commonly referred to as a "Media Gateway Controller" because of its Media Gateway control function.

The SGC entity has multiple roles. It is the heart of VoIP platform; its main role is to connect the PSTN (public switched telephone network) world with the IP world. To simplify, the main characteristics of the SGC component are:

  • Support of Signaling System 7 (SS7) protocol stack which is the PSTN world's main Signaling protocol suite (sometimes a separate entity called Signaling Gateway is used for this exact purpose).

  • Full support of voice call control protocols such as H.323 or SIP which are purely IP signaling protocols.

  • Full support of media control protocols such as MGCP or Megaco (H.248) which are used for controlling Media Gateway session connections and parameters.

  • Generate Call Detailed Records (CDRs) for billing purposes.

  • Provide bandwidth management control through admission control mechanisms, in other words, new sessions are admitted only if the system is able in terms of bandwidth to provide acceptable service to them.

  • Support of bandwidth policing mechanisms -- with the use of media flow profiles, the Signaling Gateway Controller instructs the Media Gateway to monitor the RTP media flow and apply rate limit policies to aggressive flows. This mechanism also preserves appropriate Quality of Service levels.

  • Provisioning media connection -- allocating media connection characteristics such as coding and packetization to Media Gateways as well as specific DS0 allocation for the reservation of Media resources.

As you can see, the SGC is the most important component of the whole VoIP structure, therefore, it has to be redundant. Hardware or software malfunction of this component is not tolerant. Also due to it's multitask and multiprocess behavior, it must be powerful in terms of CPU and memory.

  •   Media Gateway

The Media Gateway's main role is the transmission of voice packets using the RTP transmission protocol. When the media gateway is used in a converged PSTN/IP network it has extra functions to perform such as packetization, since it uses TDM trunks from the one side and IP trunks on the other.

Let's examine the Media Gateway's main functions:

  • Support of MGCP or MEGACO for call control under the administration of the Media Gateway Controller.

  • Transmission of Voice data using RTP --packetization of data is also applied when TDM trunks are interfacing the Media Gateway.

  • Support of T1/E1 trunks for transferring voice in SS7 networks.

  • Support of different Compression algorithms for fulfilling the requirements of the call as instructed by the SGC.

  • Manage Digital Signal Processing (DSP) resources for ideal service offering.

Some sort of high availability can also be achieved by maintaining redundant IP links. The capabilities of the Media Gateway in terms of concurrent call support, mainly depends on the capacity of onboard DSPs and also the selection of codecs since different codecs have different processing requirements.

  •   Media Server

A Media Server is used where added features are needed such as voicemail or video conferencing. Moreover, a media server is used when special tones or announcements need to be transmitted. Therefore, the media server has an important role within the VoIP architecture.

The main functions of the Media Server are:

  • Transmission of call progress tones and special service announcements.

  • Voicemail functionality.

  • Voice activated dialing.

  • Voicemail to email transmission --voicemail can be transmitted as attachment to an email address.

  • Support for Interactive Voice Response (IVR) -- call routing or even service activation can be performed based on dialed DTMF digits. The caller according to voice menus selects the appropriate DTMF digit that triggers the required service.

The Media Server is mainly controlled by an Application Server using SIP or pure XML. For the proper transmission of IVR, tone and announcement media proper IP routing towards the Media Gateway should exist.

  •   Application Server

The major responsibility of an Application Server is to provide value-added services to the IP network. Global and customer specific services are provisioned here. Call characteristics and session specifications are influenced by the application server.

The main functions of the Application Server component are:

  • Support of customized private dialing plans.

  • Basic service offering --basic services such as call forward always, call forward on busy, call waiting, call transfer, call park and voicemail are offered though the Application Server.

  • Advance service offering -- advanced features such as call authorization using PIN, remote office, "follow me" plans can be offered by this component.

  • Generation of Call Detailed Records (CDRs).

  • FreePhone Service -- support of 800 number service where charging is applied to the called party.

The Application Server is the brain of the VoIP architecture. It communicates with the Signaling Gateway Controller through protocols such as H.323 or SIP. Services are implemented here and allocated to customers.

It is very important to have high availability configuration; you can not tolerate service interruption in any way.

More VoIP Information to Come

I tried grouping the most important functions of each VoIP call component and present them to you in the simplest possible way. I hope you found this information useful.

Stay tuned; more VoIP articles are yet to come.

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Stelios Antoniou

Stelios Antoniou holds a BSc in Electronic Engineering and an MSc in Communication Networks. He has over three years of experience in teaching MS Office applications, networking courses and GCE courses in Information Technology. Stelios is currently working as a VoIP Engineer in a Telecom company, where he uses his knowledge in practice. He has successfully completed training on CCNP topics, Linux and IMS. His enthusiasm, ambition and knowledge motivate him to offer his best. Stelios has written many articles covering Cisco CCENT, CCNA, and CCNP.