Vowifi; Voip Signalling Protocols; Supplementary Services For H.323; Session Initiation Protocol (Sip) - ASCOM VoWiFi System System Description

System Description
Ascom VoWiFi System
3

VoWiFi

Voice over Wireless Fidelity (VoWiFi) is a wireless-based VoIP service. Where VoIP consists of
the hardware and software that enables people to use the Internet Protocol as the
transmission medium for telephone calls, VoWiFi is the wireless version of this technology
which is designed to work on wireless devices.
The voice signal is digitized, compressed and converted to IP packets and then transmitted
over the network. Since voice is a real-time application, a VoWiFi system must ensure that
the data streams are reconstructed accurately and that sufficient mechanisms for network
delay detection have been implemented.
3.1

VoIP Signalling Protocols

Two of the protocols used for voice over IP (VoIP) signalling are H.323 and the Session
Initiation Protocol (SIP). H.323 was the first standard and is in fact a set of protocols
designed to enable multimedia traffic in single LANs. One protocol of many in the set of
protocols defined in H.323 is H.450, which is a series of protocols that defines

Supplementary Services for H.323.

Like H.323, SIP can be used for VoIP, but while H.323 is ISDN-based (Q. 931 and earlier H
series) SIP is text-based. As opposed to H.323 which uses Abstract Syntax Notation number
One (ASN.1), SIP encodes its messages as text, similar to HTTP and SMTP.
3.1.1 H.323
H.323 was developed by the International Telecommunications Union (ITU) and was
designed from a telecommunications perspective. Ratified in 1996 it has become a defacto
choice for interoperability among VoIP equipment. It is a standard that provides
specification for computers, equipment, and services for multimedia communication over
networks that do not provide a guaranteed QoS.
H.323 equipment can carry real-time video, audio, and data, or any combination of these
elements. Included in the H.323 standard are H.225, H.245 and the IETF protocols RTP and
RTCP, with additional protocols for call signalling, data and audiovisual communications.
H.323 products and services offer the following benefits to users:
• Products and services developed by multiple manufacturers under the H.323 standard
can interoperate without platform limitations. H.323 conferencing clients, bridges,
servers, and gateways support this interoperability.
• H.323 provides multiple audio and video codecs that format data according to the
requirements of various networks, using different bit rates, delays, and quality options.
Users can choose the codecs that best support their computer and network selections.
3.1.2 H.450 Supplementary Services for H.323
H.450 is a series of protocols which are used to exchange signalling information to control
the supplementary services such as, Call Transfer, Call Diversion, Call Waiting etc. over a LAN.
3.1.3

Session Initiation Protocol (SIP)

SIP is an application layer control (signalling) protocol for creating, modifying, and
terminating sessions with one or more participants. These sessions include Internet
multimedia conferences, Internet telephone calls and multimedia distribution. SIP is
designed as part of the IETF standards.
7 September 2011 / Ver. G
TD 92313EN
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