Transcript Lecture 4

H. 323
Chapter 4
Definition
• The H.323 standard provides a foundation for
audio, video, and data communications across
IP-based networks, including the Internet.
• H.323 is an umbrella recommendation from
the ITU that sets standards for multimedia
communications over LAN’s that do not
provide a guaranteed Quality of Service (QoS).
Why is it used
• H.323 standards are important building blocks
for a broad new range of collaborative, LANbased applications for multimedia
communications.
• It includes parts of H.225.0 - RAS, Q.931,
H.245 RTP/RTCP and audio/video codecs, such
as the audio codec's (G.711, G.723.1, G.728,
etc.) and video codec's (H.261, H.263) that
compress and decompress media streams.
H323 is an umbrella
• When dealing with H.323, it is good to realize
that it is not a single protocol but rather an
entire group of protocols.
• The individual protocols used under the
umbrella of H.323 include:
• H.225.0 for call signalling;
• Q.931, a protocol borrowed from ISDN, also
used for call signalling;
• H.245 for negotiating audio/video channel
parameters;
• H.235 for security and authentication;
• RTP, the Real Time Protocol defined by IETF,
used to transmit audio/video streams;
• H.450.x for additional services like call
transfer, call diversion, etc.
What Layer in OSI
• Media streams are transported on RTP/RTCP.
RTP carries the actual media and RTCP carries
status and control information.
• The signalling is transported reliably over TCP.
• The following protocols deal with signalling:
H.323 Zone
• H.323 defines several entity types that work
together to form a “Zone”, all under the
control of a single gatekeeper.
• A Zone is the collection of all Terminals (Tx),
Gateways (GW), and Multipoint Control Units
(MCUs) managed by a single Gatekeeper (GK).
Zone
• A Zone includes at least one terminal, and
may or may not include Gateways or MCUs. A
• Zone must have exactly one Gatekeeper.
Otherwise a collection of entities is not
considered a valid Zone, although that is not a
requirement for operability.
Zone ....
• A Zone may be independent of network
topology and may be comprised of multiple
network segments, which are connected using
routers (R) or other devices.
• See Figure for a sample Zone configuration.
H. 323 Zone
Terminals and Gateways
Terminal
• Terminals are single endpoints used to communicate
through the transmission of information streams, as
specified in H.323.
Gateway
• In general, the purpose of the Gateway is to interface
between a packet-based network endpoint to a traditional
switched circuit network (SCN) endpoint, and the reverse,
in a transparent fashion.
• The Gateway provides the appropriate translation between
transmission formats and between communications
procedures
Gatekeeper
• The Gatekeeper provides call control services
to the H.323 endpoints.
• More than one Gatekeeper may be present
and communicate with each other in an
unspecified fashion.
• The Gatekeeper is logically separate from the
endpoints; however, its physical
implementation may coexist with a terminal,
MCU Gateway, or other device.
Gatekeeper functions
• Address translation - The Gatekeeper translates
between alias address to transport address.
• Admissions control - The Gatekeeper authorizes
network access.
• Bandwidth control - The Gatekeeper performs
bandwidth control for bandwidth requests.
• Zone management - The Gatekeeper provides the
above functions for terminals,
• MCUs, and Gateways, which have registered with
it.
Other Gatekeeper services
• Call control signalling - The Gatekeeper may choose to
complete the call signalling with the endpoints, and may
process the call signalling itself.
• Alternatively, the Gatekeeper may direct the endpoints to
connect the Call Signalling Channel directly to each other
• Call authorization - The Gatekeeper may reject calls due to
authorization failure.
• Bandwidth management - The Gatekeeper may reject calls
from a terminal due to bandwidth management criteria.
• Call management - The Gatekeeper may maintain call
status for endpoints.
• Directory services - The Gatekeeper may provide some
directory information to route calls.
Multipoint Control Unit (MCU)
• The MCU is an endpoint, which provides
support for multipoint conferences
• If several users are on stream the MCU
controls conferencing between them
MCU
121 wants to call 122
1. The endpoint that initiates the call knows the
called number (122) but it does not know the
IP address associated with that number. At the
same time, since it is registered with the
gatekeeper, it must ask the gatekeeper for a
permission to place the call. It does so by
sending the Admission Request message to
the gatekeeper. The Admission Request (ARQ)
will contain the called number (122),
indicating to the gatekeeper that the endpoint
needs to have the number resolved to an IP
address.
2. The gatekeeper will check it's database of
registered endpoints whether it contains the
number 122. If so, the gatekeeper will check if
121 is allowed to call 122 and if it is possible
to place the call — for example, if there is
enough bandwidth (a bandwidth limit could
be configured on the gatekeeper for a subnet
that's connected via some WAN link). After
that, the gatekeeper will form an answer —
the message Admission Confirm (ACF) with an
IP address and send the ACF to the calling
endpoint.
Next step – TCP!
3. The endpoint 121 will now open a call
signalling channel to the address provided by
the gatekeeper in the ACF message. The call
signalling messages are sent over TCP and the
protocol is H.225.0, embedded in Q.931 (we
will denote this as Q.931/H.225.0). With the
gatekeeper-routed call model, the endpoint
121 will open a TCP channel to the gatekeeper
and send the Q.931/H.225.0 message Setup.
The gatekeeper will in turn open a second TCP
channel to the endpoint 122 and forward
the Setup message.
4. The endpoint 122 will first respond with the
Q.931/H.225.0 message Call Proceeding to
indicate it has started working on setting up
the call and the gatekeeper will forward the
message to the calling endpoint. After that,
122 will ask the gatekeeper for a call
permission (Admission Request, ARQ) and the
gatekeeper will respond with Admission
Confirm (ACF).
5. The called telephone (122) starts ringing and
this is signalled to the other party with the
Q.931/H.225.0 message Alerting.
6. The called party picks up the handset and the
endpoint can signal the call has been
accepted. This is done by sending the
Q.931/H.225.0 message Connect. At this
point, the parties will need to negotiate
parameters for audio (and optionally video)
channels. The protocol H.245 is used for this
negotiation.
7. The calling endpoint opens a TCP channel to
the H.245 address it has received in
the Connect message, and the gatekeeper will
establish the second "half" of the H.245
signalling channel. The endpoints can start
exchanging H.245 messages. The H.245
negotiation has three parts:
Deciding which endpoint is the "master" and
which is the "slave".
Exchanging information about the capability set
of each party.
Deciding what codec’s will be used
8. Finally, the two endpoints can start sending
the RTP streams and the two people will hear
one another. Note that each of the two
directions can be encoded with a
different codec.
When the call is over ...
• The two endpoints stop sending the RTP
streams.
• They announce the closing of logical channels
(H.245 RequestCloseLogicalChannel).
• The H.245 signalling channel is closed (H.245
command
message EndSessionCommand followed by
closing of the TCP connection).
• The main signalling connection is also closed
— the endpoints exchange Q.931/H.225.0
messages ReleaseComplete and the TCP
connection is closed.Each of the two
endpoints informs the gatekeeper about the
completed call with the H.225.0-RAS
message Disengage Request (DRQ) and the
gatekeeper confirms with Disengage
Confirm (DCF).