Voice Over IP


                 PROJECT ON




                          “ Voice Over Internet Protocol Version-4.”







  1. 1.     Akash Khadke


                            M.G.M College Of Engineering & Technology .

               Kamothe , Navi Mumbai.

PIN- 410209.

1                            Introduction to VOIP


Applications involving voice over internet protocol technology:


Ä   Internet Voice Telephony.

Ä   Intranet & Enterprise network voice telephony.

Ä   Internet fax service.

Ä   Multimedia internet collaboration.

Ä   Internet call centers

Ä   PBX intercommunications.

Approaches to deploy Voice Over internet Protocol Network:


Ä   Desktop Approach.

Ä   Shared Approach.

Desktop Approach:

Each individual purchases VOIP enabled terminals used to support remote communications.

Shared Approach:

VOIP capabilities are developed in an industrial strength mode using shared , network-resident servers.





General of VOIP:

Voice signaling protocols have evolved, keeping with the prevalent move from circuit to packet switched networks.

In past, voice analysis & synthesis using a vocoder technology produced a robotic sounding voice, but has changed dramatically during 1990’s.a lot of work is gone into all objective testing of the voice to determine how good the proposed algorithms are. The most frequently used test in  International Telecommunications Union (ITU-T) SG12 is the “Absolute Category Rating”(ACR)test.

Subjects listen to about 8 to 10’s of speech material & are asked to rate the quality of what they heard. Usually 5 point scale is used to represent the quality rating.


5= Excellent,

1= Bad.

By assigning the corresponding numerical values to each rating, a Mean Opinion Score(MOS) can be computed by each coder by averaging these scores.

Developers & planners are looking at VIOP for intranet & Enterprise Network Applications & enterprise network applications and at VOIP for geographically dispersed applications.

Products  for VOIP are emerging because organizations have significant investments in private data facilities that have the capacity available to carry additional on net traffic what is perceived to be little initial incremental expense.


Requirements of VOIP over data applications:

Ä   Compression.

Ä   Silence suppression.

Ä   Quality Of Service.

Ä   Signaling for voice traffic.

Ä   Echo control.

Ä   Voice switching.



 Compression significantly reduces the amount of bandwidth used by a voice conversation, while maintaining high quality.

Silence suppression:

The ability to recover bandwidth  during periods of silence in a conversation makes the bandwidth available for other user of the network.

Quality Of Service : 

Assuring  priority for voice transmission is critical..

Signaling for voice traffic :

Support of traditional PBX’s and the associated signaling is critical.

Echo Control :

Echo in annoying & descriptive control is key

Voice Switching:

 Data network equipment can generally support on net applications off net is also critical. At the very least the adjunct equipment must

 decide whether to route a call over the internal data network or route it to PSTN. 








Advantages of VOIP:


Ä   Long Distance cost savings :

By integrating voice, data & fax over an Internet Protocol Enterprise network, a company can reduce long distance charges for intra-company calls. By reducing the number of access lines, the organization can also reduce the fee charges.

Ä   Reduced Equipment Investment :

Companies generally lease/ purchase separate equipments and facilities for voice support. With VOIP the cost of securing & securing equipments is reduced, because all the intra-company traffic or voice and data is delivered over the  same network.

The different types Protocols used for Voice over IP are as follows :

Ä   MGCP and MEGACO/H.248

Ä   H.232

Ä   SIP (Session Initiation Protocol)











Introduction to H.323 standard




H.323 is a standard that specifies the components, protocols and procedures that provide multimedia communication services – real-time audio, video and data communications- over packet switched networks including Internet Protocol(IP)- based networks. H.323 is a part of a family of ITU-T recommendations called H.32X that provides multimedia communication services over a variety of networks.

                               Terminals on the packet Network

What is  H.323 ?

The H.323 standard is a cornerstone technology for the transmission of real-time audio, video and data communications over packet-based networks. It specifies the components, protocols and procedures providing multimedia communication over packet-based networks

H.323 can be applied in a variety of mechanisms

Ä   Audio only (IP telephony),

Ä   Audio and Video (Video telephony),

Ä   Audio, Video, Data.

H.323 can also be applied to multipoint-multimedia communications. H.323 provides myriad services and therefore can be applied in a wide variety of areas : Consumer, Business and Entertainment.

H.323 Versions:

Version 1 of the H.323 standard does not provide guaranteed  Quality of Service(QoS). The emergence of voice over IP (VOIP) applications and IP telephony has paved the way for a revision of the H.323 specification. The absence of a standard for voice over IP resulted in products that were incompatible. With the development of VOIP, new requirements emerged such as :

Ä   providing communication between a PL-based phone on a traditional Switched Circuit Network (SCN).

Such requirements forced the need for a standard for IP-telephony.

Version 2 of H.323 – packet based multimedia communications system- was defined to accommodate these additional requirements and was accepted in Jan 1998.

New features are being added to the H.323 standard, which will evolve to Version 3 shortly.

The features being added include:

Ä   Fax- over-packet networks,

Ä   Gatekeeper-Gatekeeper communications and

Ä   Fast- connection mechanism.

The H.323 standard is part of the H.32X family of recommendations specified by ITU-T.

The H.323 Protocol Stack





 H.323 Components

H.323 Components :

The H.323 standard specifies 4 kinds of components, which when networked together, provide the point–to-point and point-to-point-to-multipoint multimedia – communication services :

Ä   Terminals

Ä   Gateways,

Ä   Gatekeepers,

Ä   Multipoint Control Units (MCU’s).


Used for real-time bidirectional multimedia communications, an H.323 terminal can either be a personal computer (PC) or a stand-alone device, running an H.323 & the multimedia applications. It supports audio communication and can optionally support video or data communications. As the basic service provided by an H.323 terminal is audio communications, an H.323 terminal plays a key role in IP-telephony services.

H.323 terminals are the client endpoints on the LAN that provide real-time, two-way communication. They can be realized either as SW-Clients running on a PC or workstation or as a dedicated HW-devices. All terminals must support voice communication; video and are optional.  

Function of the Terminal:

The terminal is the user or the end point.

Ä   Provides real-time 2 way communication.

Ä   Optional : Video & data streaming.

Ä   Mandatory voice streaming.

Protocols Supported by the Terminal

Ä   H.232

Ä   H.245 (Control channel usage & capabilities)

Ä   sQ.931 (Call setup & signaling )

Ä   RAS (for use with Gatekeepers/Registration/ Admission/Status)

Ä   RTC/RTCP (sequence Audio & status video packets)

Gateways :

A gateway connects two dissimilar networks. An H.323 gateway provides connectivity between an H.323 network and a non-H.323 network. For example, a gateway can connect and provide communication between an H.323 terminal and SCN networks (SCN networks includes all switched telephony networks, e.g. public switched telephone network [PSTN] ).

This connectivity of dissimilar networks is achieved by translating protocols for call setup and release, converting media between different networks, and transferring information between the networks connected by the gateway. A gateway is not required, however, for communication between two terminals on an H.323 network.

The Gateway also translates between audio and video codec’s and performs call setup and clearing on both the LAN side and the PSTN side.  

The Functions of the gateway can be stated as follows:

Ä   Task-translation.

Ä   Audio Codec

Ä   Video codec

Ä   H.245 à H.221 (ISDN Conference)

H.245 à H.242 (Audio-Visual terminals)

Gatekeepers :

A gatekeeper can be considered the brain of the H.323 network. It is the most important component of an H.323 enabled network.

It is the focal point for all calls within the H.323 network.

Although they are not required, gatekeepers provide important services such as addressing, authorization and authentication of terminals and gateways; bandwidth management; accounting; billing; and charging. Gatekeepers may also provide call-routing service.

The Functions of the Gatekeeper can be stated as follows :

Ä   Task: user information / “name server”

Ä   Gatekeeper is optional but essential.

Ä   Managing communications.

Ä   Address translation.

Ä   Call Control.

Ä   Routing services.

Ä   System management.

Ä   Security policies.

Multipoint Control Unit:

MCU’s  provide support for conferences of three or more H.323 terminals. All terminals participating in the conference establish a connection with the MCU.

 The MCU manages conference resources, negotiates between terminals for the purpose of determining the audio or video coder/decoder (CODEC) to use, and may handle the media stream.

The gatekeepers, gateways and MCU’s are logically separate components of the H.323 standard  but can be implemented as a single physical device.

The functions of the MCU can be stated as follows:

Ä   Task : maintain all audio, video data & control streams mandatory for conferences. 

An MCU is usually splitted into 2 devices :

Ä   MC (Multipoint Controller)

Ä   MP (Multipoint Processor)

Usually they are located inside a Gateway or a Gatekeeper.

H.323 Zone:

An H.323 zone is a collection of all terminals, gateways and MCU’s managed by a single gatekeeper. A zone includes at least one terminal and may include gateways or MCU’s. A zone has only one gatekeeper.

 A zone may be independent of network topology and may be comprised of multiple network segments that are connected using routers or other devices.

           An H.323 Zone

Zone Management:

The gatekeeper provides the above functions :

Ä   Address translation.

Ä   Admission control

Ä   Bandwidth Control.

For terminals, gateways and MCU’s located within the Zone control.

Zone managed by Gatekeepers

Protocols Specified by H.323:

The protocols specified by h.323 are listed below.H.323 is independent of the packet network and the transport protocol over which it runs and does not specify them:

Ä   Audio Codec’s

Ä   Video Codec’s

Ä   H.225 RAS

Ä   H.225 Call Signaling

Ä   H.245 Control Signaling

Ä   Real-time Transport Protocol (RTP)

Ä   Real-time Transport Control Protocol (RTCP)


Terminal Side Protocol Stack

Audio CODEC:

An audio CODEC encodes the audio signal from the microphone for transmission on the transmitting h.323 terminal and decodes the received audio code that is sent to the speaker on the receiving h.323 terminal. As audio is the minimum service provided by the H.323 standard, all H.323 terminals must have at least one audio CODEC support.

Video CODEC:

A Video CODEC encodes video from the camera for transmission on the transmitting h.323 terminal and decodes the received video code that is send to the video code that is send to the video display on the receiving H.323 terminal. As H.323 specifies support of video is optional, the support of video Codec’s is optional as well. However , any H.323 terminal providing video communication must support video encoding and decoding as specified in the ITU-T  H.261 recommendation.

Registration Admission And Status:

Registration, Admission and Status(RAS), is the protocol between end points (Terminals & Gateways ) and gatekeepers. The RAS is used to perform registration, admission control, Bandwidth changes, status and disengaged procedures between end points and Gatekeepers.

An RAS channel is used to exchange RAS messages. This signaling channel is opened between an end point and a gatekeeper prior to the establishment of any other channels.

H.225 Call Signaling:

H.225 call signaling is used to establish a connection between 2, H.323 end points. This is achieved by exchanging H.225 protocol messages on the call signaling channel. The call signaling channel is opened bet 2, H.323 end points or between an end point and the gatekeeper.

H.245 Control Signaling:

H.245 control signaling is used to exchange end-to- end control messages governing the operation of the H.323 end point. These control messages carry information related to the following:

Ä   Capabilities Exchange.

Ä   Opening and closing of logical channels used to carry media streams.

Ä   Flow- Control messages.

Ä   General commands and indications.

Real-time Transport Protocol:

Real-time Transport Protocol (RTP) provides end-to-end delivery services of real-time audio and video. Whereas , H.323 is used to transport data over IP-based networks, RTP is typically used to transport data via the User Datagram Protocol (UDP). RTP,  together with UDP, provides transport-protocol functionality. RTP provides payload-type identification, sequence numbering, time-stamping and delivery monitoring. UDP provides multiplexing and checksum services. RTP can also be used with other transport protocols.

Real-time Transport Control Protocol:

Real-time Transport Control Protocol (RTCP) is the counter part of RTP that provides control services. The primary function of RTCP is to provide feedback on the quality of the data distribution. Other RTCP functions include carrying a transport-level identifier for an RTP  source, called a canonical name , which is used by receivers to synchronize audio and video.

Terminal Characteristics:

H.323 terminals must support the following :

Ä   H.245 for exchanging terminals capabilities and creation of media channels.

Ä   H.225 for call signaling and call setup.

Ä   RAS for registration and other admission control with a Gatekeeper.

Ä   RTP/RTCP for sequencing audio and video packets.

H.323 terminals must also support the G.711 Audio CODEC. Optional components in an H.323 terminal are Video Codec’s, T.210 data-conferencing  protocols and MCU capabilities.

Gateway and Gatekeeper Characteristics:

Gateway characteristics:

A gateway provides translation of protocols for call-setup and release, conversion of media formats between different networks, and the transfer of informati0on between H.323 and non-H.323 networks. An application of the H.323 Gateway is in IP- telephony, where the H.323 gateway connects an IP- network and SCN network(example : ISDN network).

On the H.323 side, a Gateway runs H.245 control signaling for exchanging capabilities, H.225 call signaling for call set-up and release, and H.225 registration, admission and status (RAS) for registration with the Gatekeeper. On the SCN side, a Gateway runs SCN-specific protocol(e.g.: ISDN and Signaling System number 7 (SS7) protocols).

                                                 Gateway Protocol Stack

Terminals communicate with Gateways using the H.245 control-signaling protocol and H.225 call-signaling protocol. The Gateway translates these protocols in the transparent fashion to the respective counter parts on the non-H.323 network and vice versa. The Gateway also performs call-set-up & clearing on both the H.323-network side and the non-H.323-network side. Translation between audio, video and data formats may also be performed by the Gateway. Audio and video translation may not be required if both terminal types find a common communication mode.

Gatekeepers are aware of which end points are Gateways because this is indicated when the terminals and Gateways register with the Gatekeeper. A Gateway may be able to support several simultaneous calls between the H.323 and non-H.323 networks.

Gatekeeper Characteristics:     

Gatekeepers provide call-control services for H.323 end points such as address translation and bandwidth management  as defined within RAS. Gatekeepers in H.323 networks are optional. If they are present in a network, however, terminals and Gateways must use their services. The H.323 standard both define mandatory services that the gatekeeper must provide and specify other optional functionality that it can provide.

An optional feature of a Gatekeeper  is called signaling routine. End points send call-signaling  messages to the Gatekeeper, which the Gatekeeper routes to the destination end points. Routing calls through Gatekeepers provides better performance in the network, as the Gatekeeper can make routing decisions based on a variety of factors, for example, load balancing among Gateways.

The services offered by a Gatekeeper are defined by RAS and include address translation, admissions control bandwidth control and zone management. H.323 networks that do not have Gatekeepers may not have these capabilities, but H.323 networks that contain IP-telephony, Gateways should also contain a Gatekeeper to translate incoming E.164 telephone addresses into transport address. A gatekeeper is a logical component of H.323, but can be implemented as a part of a Gateway or MCU.

Gatekeeper Components

Mandatory Gatekeeper functions :

Address Translation:

Calls originating within an H.323 network may use an alias to address the destination terminal. Calls originating outside the H.323 network and received by a Gateway may use an E.164 telephone Number (e.g. 310-442-9222) to address the destination terminal. The gatekeeper translates this E.164 telephone number or the alias into the network address(e.g. 204.252.32:456 for an IP-based network) for the destination terminal. The destination end point can be reached using the network address on the H.323 network.

Admission control:

The Gatekeeper can control the admission of the endpoints into the H.323 network. It uses RAS messages, admission request(ARQ), confirm(AFC) and reject(ARJ) to achieve this.

Bandwidth Control:

The gatekeeper provides the support for bandwidth control by using the RAS message, bandwidth request(BRQ) , confirm(BCF) and reject (BRJ). The result is to limit the total allocated bandwidth to some fraction of the total available, remaining bandwidth for data applications. Bandwidth control may also be a null function that accepts all requests for bandwidth changes.

Optional gatekeeper functions:

Call-control Signaling:

 The gatekeeper can route call signal messages between H.323 end points. In a point-to-point conference, the gatekeeper may process H.225 call- signaling messages. Alternatively, the gatekeeper may allow the end points to send H.225 call-signaling messages directly to each other.

Call Authorization:

When an end point sends call-signaling messages to the gatekeeper, the gatekeeper may accept or reject the call, according to the H.225 specification. The reasons for rejection may include access-based or time-based restrictions, to and from particular terminals or gateways.

Call Management:

The gatekeeper may maintain information about all active H.323 calls so that it can control its zone by providing the maintained information to the bandwidth-management functions or by re-routing the calls to different endpoints to achieve load balancing.

Registration, Admission and Status

The H.225 RAS is used between H.323 endpoints ( terminals and gateways) and gatekeepers for the following:

Ä   Gatekeeper Discovery (GRQ)

Ä   End point registration

Ä   End point location

Ä   Access tokens

The RAS messages are carried on a RAS channel that is unreliable. Hence, RAS message exchange may be associated with timeouts and retry counts.

Gatekeeper discovery:

The gatekeeper discovery process is used by the H.323 endpoints to determine the gatekeepers which the endpoint must register. The gatekeeper discovery can be done statically or dynamically.

In static discovery, the end point knows the transport address of its gatekeeper a priori.

In the dynamic method of gatekeeper discovery, the send point multicasts a GRQ message on a gatekeepers discovery multicast address: “Who is my Gatekeeper?” one or more Gatekeepers may respond with a GCF message: “ I can be your Gatekeeper.”

End point registration:

Registration is a process used by the endpoints to join a zone and inform the gatekeepers of the zones transport and alias addresses. All end points register with a gatekeeper as part of their configuration.

Endpoint location:

Endpoint location is a process by which the transport address of an endpoint is determined and given its alias name or E.164 address.

Other Control:

The RAS channel is used for other kinds of control mechanisms, such as admission control, to restrict the entry of an endpoints into a zone, bandwidth control, and disengagement control, where an endpoint is disassociated from a gatekeeper and its zone.

H.225 Call Signaling and H.245 Control Signaling

H.225 Call Signaling:

H.225 Call signaling is used to set up connections between H.323 endpoints (terminals and Gateways), over which the real-time data can be transported. Call signaling involves the exchange of H.225 Protocol messages over a reliable call-signaling channel.

H.225 messages are exchanged between the endpoints if there is no gatekeeper in H.323 network. When a gatekeeper exists in the network, the H.225 messages are exchanged either directly between the end points or between the end-points after being routed through the gatekeeper. The method chosen is decided by the gatekeeper during RAS-admission message exchange.

Gatekeeper-Routed Call Signaling:

 The admission messages are exchanges between endpoints and the gatekeeper on RAS channels. The gatekeeper receives the Call-signaling messages on the call-signaling channel from 1 endpoint and routes them to the other endpoint on the call-signaling channel of the other endpoint.

Direct Call-Signaling:

During the admission confirmation, the gatekeeper indicates that the endpoints can exchange call-signaling messages directly. The endpoints exchange the call-signaling on the call-signaling channel.

H.245 Control Signaling:

The H.245 control Channel is the logical channel 0 and is permanently open, unlike the media channels.

Connection Procedures:

The Following example describes the steps involved in creating an H.323 call, establishing media communication and releasing the call. The example network contains 2, H.323 terminals (T1 and T2) connected to a gatekeeper. Direct call signaling is assumed. It is also assumed that the media stream uses RTP encapsulation. The below diagram illustrates the Call- establishment:

                                                            H.323 Call Signaling


The steps can be listed as:

  1. T1 sends the RAS ARQ message on the RAS channel to the gatekeeper for registration. T1 requests the use of direct call signaling.
  2. The gatekeeper confirms the admission of T1 by sending ACF to T1. The gatekeeper indicates in ACF that T1 can use direct call signaling.
  3. T1 sends an H.225 call signaling setup message to T2 requesting a connection.
  4. T2 responds with an H.225 cal proceeding message to T1.
  5. Now T2 has to register with the gatekeeper. It sends an RAS ARQ message to the gatekeeper on the RAS channel.
  6. The gatekeeper confirms the registration by sending an RAS ACF message to T2.
  7. T2 alerts T1 of the connection establishment by sending an H.225 alerting message.
  8. Then T2 confirms the connection establishment by sending an H.255 connect message to T1, and the call is established. The below figure denotes the H.323


H.323 Control Signaling Flows



  1. The H.245 control channels is established between T1 and T2. T1 sends an H.245 TerminalCapabilitySet message to T2 to exchange its capabilities.
  2. T2 acknowledges T1’s capabilities by sending an H.245 TerminalCapabilitySetAck message.
  3. T2 exchanges its capabilities with T1 by sending an H.245 TerminalCapabilitySet message.
  4. T1 acknowledges T2’s capabilities by sending an H.245 TerminalCapabilitySetAck message.
  5. T1 opens a media channel with T2 by sending an H.245 OpenLogicalChannel message. The transport address of the RTCP channel is included in the message.
  6. T2 acknowledges the establishment of the unidirectional logical channel from T1 to T2 by sending an H.245 OpenLogicalChannelAck message. Included in the acknowledge message are the RTP transport address allocated by T2 to be used by the T1 for sending the RTP media stream and the RTCP address received from T1 earlier.
  7. Then, T2 opens a media channel with T1 by sending an H.245 OpenLogicalChannel message. The transport address of the RTCP channel is included in the message.
  8. T1 acknowledges the establishment of the unidirectional logical channel from T2 to T1 by sending an H.245 OpenLogicalChannelAck message. Included in the acknowledging message are the RTP transport address allocated by T1 to be used by T2 for sending the RTP media stream and the RTCP address received from T2 earlier. Now the bidirectional media stream communication is established.



                H.323 Media Stream & media Control Flows

  1. T1 sends the RTP encapsulated media stream to T2.
  2. T2 sends the encapsulated media stream to T1.
  3. T1 sends the RTCP message to T2.
  4. T2 sends the RTCP message to T1.


  1. T2 initiates the call release. It sends an H.245 EndSessionCommand message toT1.
  2. T1 releases the call endpoint and confirms the release by sending an H.245 EndSessionCommand message to T2.
  3. T2 completes the call release by sending an H.225 release complete message to T1.
  4. T1 and T2 disengage with the gatekeeper by sending an RAS DRQ message to the Gatekeeper.
  5. The gatekeeper disengages T1 and T2 and confirms by sending DCF messages to T1 and T2.     





                                                 H.323 Call Release

      An example for H.323 call signaling.

An Example network can be represented as follows:


An Introduction to Session Initiation Protocol (SIP)


      Ancestors of SIP

Ä   HTTP (basic request/ response format, status codes, etc)

Ä   Email/SMTP (Address style)

Ä   URL (addresses)

Ä     Transfer control protocol / user datagram protocol (TCP/UDP) (message transport)

Ä   Real Time Protocol(RTP) (voice session)

         The clients for SIP:

Ä   Terminal act as UserAgentServer(UAS)  or UserAgentClient (UAC) could be soft phones or IP-phones. 

Ä   Gateways  primary task: translation. 

        The Server for SIP:

Ä    Proxy server  receives SIP messages and forward them to the next server in the network.

Ä   Redirect Server provides information against the next hop. 

Ä   Registrar Server requests current location for UAC registration; they are often co-located with a redirect or proxy server. 

An example of the SIP network


An example of SIP messages :

The commands the SIP uses are called “methods”

SIP method            Description.

INVITE                 Invites a user to call.

ACK                      Used to facilitate reliable message exchange for INVITE’s

BYE                      Terminates the connection between users/ declines a call

CANCEL              Terminates a request or search for a user

OPTIONS              Requests Information about the server capabilities.

REGISTER           Registers the users current location.

INFO                     Used for mid session signaling.

The Server Responses

1xx             Information

2xx                        Successful

3xx                        Redirection

4xx                        Request failure

5xx                        Server failure

6xx                        Global failure.

A SIP call example

                              Comparison between SIP and  H.323







































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