H.323 and IP
Recommendation H.323 describes an basement of terminals, accepted ascendancy components,
services, and protocols that are acclimated for multimedia (voice, video, and data)
communications. Figure 5-1 illustrates the elements of an H.323 terminal and highlights
the agreement basement of an H.323 endpoint.
248 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
System
Control and
User Interface
Video
I/O Equipment
Audio
I/O Equipment
User Data
Applications
T.120
System
Control
H.245
Control
Signaling
H.225
Call
Signaling
RAS
Control
H.225
Video
Codec
H.261
H.263
Audio
Codec
G.711, G.722,
G.723, G.723.1,
G.728, G.729
Receive Path
Delay
H.225 Layer
LAN Stack
Figure 5-1 H.323 Elements
H.323 was originally created to accommodate a apparatus for alteration multimedia applications
over LANs. Although abundant vendors still use H.323 for videoconferencing
applications, it has rapidly acquired to abode the growing needs of Articulation over IP (VoIP)
networks. H.323 is currently the best broadly acclimated VoIP signaling and alarm ascendancy protocol,
with all-embracing and calm carriers relying on it to handle billions of account of
use anniversary year.
H.323 is advised an “umbrella protocol” because it defines all aspects of alarm transmission,
from alarm enactment to capabilities barter to arrangement ability availability.
H.323 defines these protocols:
■ H.245 for capabilities exchange
■ H.225.0 for alarm setup
■ H.225.0 for registration, admission, and cachet (RAS) ascendancy for alarm routing
H.323 is based on the Integrated Services Digital Arrangement (ISDN) Q.931 protocol, which
allows H.323 to calmly interoperate with bequest articulation networks, such as the PSTN or
Signaling System 7 (SS7). In accession to accouterment abutment for alarm setup, H.225.0 provides
a bulletin carriage apparatus for the H.245 ascendancy action and the RAS signaling
function. Following is a description of these functions:
■ Call-signaling function: The call-signaling action uses a call-signaling approach that
allows an endpoint to actualize access with added endpoints. The call-signaling
function defines alarm bureaucracy procedures, based on the alarm bureaucracy procedures for ISDN
(ITU-T Recommendation Q.931). The call-signaling action uses letters formatted
according to H.225.0.
■ H.245 ascendancy function: The H.245 ascendancy action uses a ascendancy approach to transport
control letters amid endpoints or amid an endpoint and a common
control component, such as a attendant or multipoint ambassador (MC). The control
channel acclimated by the H.245 ascendancy action is abstracted from the call-signaling
channel.
The H.245 ascendancy action is amenable for these functions:
■ Logical approach signaling: Opens and closes a approach that carries the media
stream
■ Capabilities exchange: Negotiates audio, video, and coder-decoder (codec)
capability amid endpoints
■ Adept or responder determination: Determines which endpoint is adept and
which is responder; acclimated to boldness conflicts during the call
■ Approach request: Requests a change in mode, or capability, of the media stream
■ Timer and adverse values: Establishes ethics for timers and counters and agreement
of those ethics by the endpoints
■ RAS signaling function: The RAS signaling action uses a abstracted signaling channel
(RAS channel) to accomplish registration, admissions, bandwidth changes, status,
and abstract procedures amid endpoints and a gatekeeper. The RAS signaling
function uses letters formatted according to H.225.0.