Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-05-06
2003-07-22
Marcelo, Melvin (Department: 2666)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S465000, C704S270000
Reexamination Certificate
active
06597702
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a packet-based multimedia communications system and, more particularly, to a system having a fast connect option for enforcing symmetric codec capabilities in call setup procedures such as the fast connect setup in the International Telecommunication Union Standardization Sector (ITU-T) Recommendation H.323, incorporated herein by reference.
2. Description of the Prior Art
The International Telecommunications Union (ITU) H.323 standard describes terminal and other entities that provide multimedia communication services over Packet Switched Networks (PSN). H.323 devices provide real-time audio, video, and/or data communications. H.323 devices must support audio, while data and video are optional. If data or video is supported, the device must use a specified common mode of operation, so that all terminals supporting that media type can interface properly.
FIG. 1
is a functional block diagram of a packet-based multimedia communications system
10
. The system
10
includes a PSN
12
connected to terminals
14
,
16
, and
18
, gatekeeper
20
, gateway
22
, and multi-point control unit
24
. Gateway
22
provides Voice Over Internet Protocol (VoIP) access between the PSN
12
and a Public Switched Telephone Network (PSTN)
26
and an Integrated Services Digital Network (ISDN)
28
. PSTN
26
is connected to several PSTN endpoints, such as PSTN endpoints
30
and
32
. PSTN endpoints
30
and
32
are standard circuit switched telephones. Phones
30
and
32
access one another through PSTN
26
. Phones
30
and
32
access ISDN endpoints
34
and
36
through gateway
22
. Similarly, ISDN
28
is connected to ISDN endpoints, such as ISDN endpoints
34
and
36
. ISDN endpoints
34
and
36
are, for example, digital visual telephone terminals and equipment. ISDN endpoints
34
and
36
access one another through the ISDN
28
and access phones
30
and
32
through gateway
22
.
VoIP services are accessed from the phones
30
and
32
via PSTN
26
, more directly through the PSN
12
by digital phones
34
and
36
via ISDN
28
, or from an H.323 endpoint to another through the PSN
12
. In the first two cases, a phone connection involves dialing into an incoming gateway and connecting to a terminating gateway that eventually connects to a destination telephone.
The H.323 standard covers the technical requirements for multimedia communications systems where the underlying transport is a PSN
12
. The PSN
12
may include Local Area Networks (LAN), Enterprise Area Networks, Metropolitan Area Networks, Intra-Networks, and Inter-Networks (including the Internet). The PSN
12
also includes dial up connections or point-to-point connections over the PSTN
26
or the ISDN
28
that use an underlying packet based transport such as a point-to-point protocol. These networks may consist of a single network segment, or they may have complex topologies that incorporate many network segments interconnected by other communications links.
The terminals
14
,
16
, and
18
provide audio and optionally video and data communications capability in point-to-point or multi-point conferences. Interworking with other terminals, such as PSTN endpoints
30
and
32
or ISDN endpoints
34
and
36
, is accomplished through gateways like gateway
22
if a PSTN
26
or an ISDN
28
, respectively, is involved. Gatekeeper
20
provides admission control and address translation services. Multi-point control unit
24
provides support for multi-point conferences. The scope of H.323 is defined by the dotted lines
38
and does not include the network interface, the physical network, or the transport protocol used on the network. Examples of these networks that could comprise part of PSN
12
include but are not limited to the Ethernet (Institute of Electrical and Electronic Engineers (IEEE) 802.3), Fast Ethernet (IEEE 802.3&mgr;), Fiber Distributed Data Interface (FDDI), Token Ring (IEEE 802.5), and Asynchronous Transfer Mode (ATM).
H.323 endpoints, such as terminals
14
,
16
, or
18
, multi-point control unit
24
, or gateway
22
, may establish media channels in a call using either the procedures defined in ITU-T Recommendation H.245 (H.245) or the Fast Connect procedure described in the H.323 standard although the latter is preferred. The Fast Connect procedure allows the endpoints to establish a basic point-to-point call with as few as one round-trip message exchange, enabling immediate media stream delivery upon call connection. The H.245 requires a set of message exchanges to setup the call. For example, the H.245 requires determining a master-slave exchanging a capability, opening logical channels on both endpoints and acknowledging the logical channels on both endpoints, closing and re-opening the logical channels, and the like. The Fast Connect feature eliminates the need to establish a separate H.245 Transfer Control Protocol (TCP) connection by using the existing H.225 channel for a simplified capability exchange between two endpoints. Another advantage of a Fast Connect call is that the audio, data, and/or video paths, if supported, are available much sooner than would be the case using the H.245 procedures. In essence, Fast Connect uses the resources more efficiently, reduces the overhead of establishing media channel(s) for a call, and establishes the media paths sooner than using the H.245 procedures.
FIG. 2
is a functional block diagram describing a drawback with the communications system
10
shown in
FIG. 1. A
calling endpoint
40
, such as gateway
22
(FIG.
1
), initiates a call by sending a message containing certain predetermined elements to a called endpoint
42
, such as terminal
18
. In H.323, the Fast Connect call procedure is initiated by sending a SETUP message containing a faststart element. The faststart element consists of a sequence of OpenLogicalChannel structures
43
describing a particular media channel proposal—audio, video, or data—and a certain direction—send or receive—. Where the media channel proposal involves audio, the OpenLogicalChannel structure
43
describes, among other things, media channel capabilities associated with respective audio coder/decoders—commonly known as codecs—that the calling endpoint
40
can use to encode and decode audio signals. In other words, the calling endpoint
40
advertises its send and receive codec capabilities to the called endpoint
42
. Here, the send direction—the term “forward” is used in the H.323 standard—refers to signals traveling from the calling endpoint
40
to the called endpoint
42
. Conversely, the receive direction—“reverse” in the H.323 standard—refers to signals traveling from the called endpoint
42
to the calling endpoint
40
.
For example, the calling endpoint
40
sends notification
43
to the called endpoint
42
that it has audio codec
1
, codec
2
, . . . , codec N (
44
) available for encoding signals to packets (send direction) and audio codec
1
, codec
2
, . . . , codec M (
46
) available for decoding the packets back into audio or video signals (receive direction). In the H.323, the plurality of codec options is included in the OpenLogicalChannel structure
43
sent in the call setup message. The codec options are, for example, audio G.729, G.711, G.723, G.726, and the like.
The called endpoint
42
chooses one of the codec capabilities
44
proposed by the calling endpoint
40
for the send direction and one codec
46
for the receive direction. For example, endpoint
42
selects codec
2
for encoding packets in the send direction
48
and selects codec
1
for decoding packets in the receive direction
50
. The choice of which codec the called endpoint
42
will choose depends on several factors including which codecs the called endpoint
42
supports and is configured to use. More specifically, the choice of codecs depends on the codecs supported by digital signal processing software or firmware included in the called endpoint
42
.
Often, the calling endpoint
40
does not support using different codecs f
Cisco Technology Inc.
Marcelo Melvin
Marger Johnson & McCollom
Mehra Inder Pal
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