Videoconferencing or videotelephony is an application of communications technologies that allows to perform videocalls where voice and image are share simultaneously.
Videoconferencing Concepts
A videoconference terminal is a device that is used by end users to perform video conferences.
Transcoding refers to the conversion of call media from one voice encoding algorithm (also known as a codec or a vocoder) to another.
Jitter Buffer
The jitter buffer is a buffer of temporary memory for streaming to avoid the jitter effect.
The size of a jitter buffer is typically measured in time (milliseconds) rather than the number of packets. It defines the amount of time audio or video packets can be delayed before being played out to compensate for network jitter.
Increasing the jitter buffer size reduces the likelihood of packet loss.
Increasing the size of the jitter buffer increases overall latency but on the other hand if the jitter buffer is too small, call quality issues may arise.
Videoconferencing Codecs
A codec (a portmanteau of coder/decoder) is hardware or software that is able to code or decode a data stream. They usually imply compression and decompression of information.
Processing time may produce a delay.
Groups of coders:
- Source coder
- Channel coder
- Line coder
A source coder is used to reduce the volume of data to be transmitted.
A channel coder is used to detect and correct errors during a transmission.
A line coder adapts the signal to the transmission medium.
Types of compression codecs:
- Depending on loss
- Lossless
- Lossy
- Depending on compression/decompression speed difference
- Symmetric
- Assymmetric
As a general rule, the codec that applies more compression require a higher compression ratio.
Videoconferencing Codecs
There are different types of videoconferencing codes:
- Videoconferencing audio codecs
- Videoconferencing video codecs
Videoconferencing Audio Codecs
Popular signal digitization methods for audio used in videoconferencing are PCM, DPCM and ADPCM.
As a general rule, variable flow audio coding schemes suit better to networks that do not have a constant flow reserve, such as DiffServ (IPv4) or UBR and ABR in ATM.
On the other hand, fixed flow coding schemes suit better to connection-oriented services, such as IntServ (RVSP) or CBR services in ATM.
You can read this post about audio coding formats.
Videoconferencing Video Coding Formats
Popular signal digitization methods for video used in videoconferencing are PCM, DPCM and ADPCM.
The codecs most used in videoconferencing protocols are most usually based on standards belonging to ITU-T’s H.26x family.
The main organizations that publishes codecs are ITU-T’s Video Coding Expert Group and ISO/IEC Moving Pictures Expert Group (MPEG).
You can read this post about video coding formats.
Videoconferencing video coding formats:
- H.261
- MPEG-1 Layer 2
- H.262 / MPEG-2 Part 2
- H.263
- H.264 / MPEG-4 part 10
- H.265
- H.266
Videoconferencing Protocols
Videoconferencing protocols featured on this post:
- H.310
- T.120 family
- H.320 family
- SIP
The main videoconferencing protocols as of 2025 are H.323, SIP and H.320.
Before the 1990s proprietary protocols were used instead.
H.245
H.245 is a control channel protocol. Its purpose is to negociate the terminal capabilities and set and control media channels.
H.310
H.310 is thought for videoconferencing over ATM networks.
H.320-H.324 Family
The H.320-H.324 family, probably incorrectly called H.32x, is a group of ITU-T recommendations related to videoconferencing.
- H.320
- H.321
- H.322
- H.323
- H.324
- H.324M
The most important is H.323. Historically H.320 was very used and it may still used, as of 2016.
H.320 family recommendations may be combined with H.233, H.234 or H.235 to provide security.
H.320
H.320 was designed to transfer multimedia over ISDN networks.
H.320 modes:
- H.221 Px64
- H.221 H0
- BONDING.
Bandwith ON Demand INteroperability Group (BONDING) or ISO 13781 is a standard developed in 1992.
H.321
H.321 adapts H.320 terminals to wide band-ISDN environments.
H.322
H.322 was designed to transfer over isochronous Ethernet, such as IEEE 802.9a, Iso-Ethernet and ISLAN.
H.323
H.323 is a set of standards to provide audio-visual communication sessions on any packet network.
It was designed to transfer over LAN (internet) without QoS.
You can read this post about H.323.
H.324
H.324 was designed to transfer over public switched telephone network (PSTN) or plain old telephone systems (POTS).
H.324M
H.324M (mobile network).
Nowadays videoconferencing mobile applications are mostly compatible with H.323.
V.70
V.70 is a ITU-T for simultaneous voice and data transmission.
T.120 Family
The T.120 family is a set of protocols for point-to-point and point-to-multipoint data collaboration in videoconferences.
It consists of many recommendations.
It may complement H.323 to allow data collaboration.
H.239
H.239 defines how to transmit dual-stream video or data and video during a video conference.
It is compatible with H.323 and H.320.
SIP
Session Initialization Protocol (SIP) for VoIP establishes the communication link between endpoints.
It is newer than H.323, and SIP-based technologies has mainly replaced H.323.
You can read this post about SIP.
IMS
IP Multimedia Subsystem (IMS) is a protocol to send multimedia data over IP. It uses SIP for protocol.
It was initially developed by 3GPP.
IMS layers:
- Service/application
- Control
- Transport
Videoconferencing Security
Videoconferencing security:
- H.235
H.235
H.235 is a ITU-T standard with the title “Security and encryption for H-series”.
Firewall and NAT Bypassing on Videoconferencing
There is a general issue on videoconferencing related to getting connections blocked by firewalls and NAT.
Each technology provides its own solution, but there are some solutions that are common to all of them:
- Using a VPN
ITU-T standards H.460.x address this issue.
Webconferencing
Webconferencing implies videoconferencing over the web instead on other traditional protocols.
Web Real-Time Communications (WebRTC) is a standard API developed by the WWW Consortium (W3C).
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External References
- Wikipedia community; “Videotelephony“; Wikipedia
- Petri Vuorimaa; “Video conferencing“; SlideShare