SIP and H.323 Protocols: Architecture, Components, and Functionality

Posted on Apr 8, 2025 in Computers

SIP: Session Initiation Protocol

SIP (Session Initiation Protocol) is a signaling protocol used at the application layer to create, modify, and terminate sessions with one or more participants. These sessions can include voice, video, data, and other forms of internet media.

IETF Architecture Protocols

The IETF architecture includes protocols such as:

  • RTP and RTCP: Provide real-time delivery of media.
  • RTSP: A real-time streaming protocol that provides a supply-demand mechanism in real time.
  • SDP: Session Description Protocol, which provides a standard format for describing media exchange capabilities.

SIP Attributes

Key attributes of SIP include:

  • Simplicity
  • Efficiency
  • Scalability
  • Modularity
  • Mobility support
  • User programming
  • Extensibility

System Components

The main system components are:

  • User Agents (UA): Endpoint applications that send and receive SIP requests.
  • Proxy Servers: Applications that receive requests from user agents and forward them towards the destination. They also maintain records to indicate routing directions.

Structure of SIP Messages

SIP messages consist of:

  • Start Line
  • Header
  • Body

Common SIP methods include:

  • INVITE: Starts a VoIP call.
  • ACK: Acknowledges a SIP response.
  • OPTIONS: Queries a server’s capabilities.
  • BYE: Signals the end of a call.

H.323 Protocol

H.323 is an ITU-T specification for transmitting audio, video, and data over IP networks. It supports signaling and call control, media transport and control, bandwidth management, and point-to-point and multipoint conferencing.

H Series Recommendations

H.323 is part of the H Series recommendations, including H.320 for ISDN and H.324 for POTS.

Components and Protocols

Key components and protocols include:

  • Signaling: H.225
  • Media Control: H.245
  • Audio Codecs: G.711, G.722, G.723, G.728, G.729
  • Video Codecs: H.261, H.263
  • Data Sharing: T.120
  • Media Transport: RTP/RTCP

H.323 Elements

H.323 networks consist of:

  • Terminals
  • Gateways: Provide interfaces with ISDN or PSTN for interworking.
  • Gatekeepers: Offer admission control and address translation services.
  • MCUs: Multipoint Control Units that enable audio and video conferencing between two or more terminals and gateways.

Main Control Areas

The main control areas are:

  • Signaling and Registration
  • Admission and RAS (Registration, Admission, and Status)
  • Call Control Signaling
  • Media Control and Transport

RAS Protocol

RAS provides pre-call control in H.323 networks with gatekeepers, establishing a zone between endpoints and gatekeepers over an IP network.

H.245 Control

H.245 procedures and messages enable:

  • Capability Exchange
  • Master-Slave Determination
  • Round-Trip Delay Measurement
  • Logical Channel Signaling

Fast Connect Procedure

The procedures available for establishing media channels between endpoints are H.245 and Fast Connect.

H.245 Tunneling

H.245 Tunneling improves call connection time and resource allocation and synchronizes signaling and call control.

Codecs

Waveform Codecs

Waveform codecs rebuild the input signal without modeling the process that created it.

Source Codecs

Source codecs attempt to replicate the physical process of sound creation.

Hybrid Codecs

Hybrid codecs combine modeling and waveform analysis, employing strategies like MPE, RPE, and CELP.

Conversation Quality Factors

Factors affecting conversation quality include:

  • Frame loss or codec errors
  • Background noise
  • Simultaneous speakers
  • Non-conversational speech
  • Multiple codec cycles

Audio and Signaling Paths

Audio Path

The audio path follows this structure: RTP, RTCP (12 bits) > UDP (8 bits) > IP (20 bits) > link > physical layer.

Signaling Path

The signaling path follows this structure: H.323 > RAS (admission, registration, status) > H.225 (setup Q.931) > H.245 (capability exchange, logical channel establishment, TCP channel establishment, A/V channel setup).

Common Codecs

Common codecs include:

  • G.711/PCM/64
  • G.726/ADPCM/32
  • G.728/LDCELP/16
  • G.729/CSACELP/8
  • G.729A/CSACELP/8
  • G.723.1/MPMLQ/6.3
  • G.723.1/ACELP/5.3

Codec Selection Criteria

Criteria for selecting codecs include:

  • Bit rate
  • Algorithm delay
  • Processing complexity
  • MOS (Mean Opinion Score)
  • Return signals that are not conversational

Voice over IP Calculations

Bytes voice/plot = (64000 * 0.020) / 8; +40 byte IP packets, +26 bytes eth frame; Bandwidth = bytes plot * ETHx8 / 0.020

VAD = Bandwidth