19.2.1.1. Motivation 
If two HFA IP phones are connected in a two-party call without the
Direct Media Connection DMC feature, a double IP/TDM conversion of the
payload would be performed. This causes a loss of quality of the voice
transmission which results from the multiple conversion of the payload
between IP and TDM and vice versa. To avoid this loss of quality the
IP payload is exchanged directly between the two subscribers if both
are in a two party connection.
IP Endpoints can be:
- HFA IP phones
- IP Trunking Gateways (z.B.: HG 3500)
- IPDA boards (z.B.: HG 3500)
The IP endpoints can be the real endpoints of the connection (i.e. there
is a Master Connection between two IP phones) or the IP endpoints are
only endpoints of a section within the Master Connection. E.g. an anate
or digite may also profit from a DMC if it is connected to its partner
via IPDA Access Point or IP Trunking Gateway.
The setup of a call is done within several steps.
Steps 1 and 2 are the normal call setup procedures as known in HiPath
4000 V1.0. The result is called Master Connection. The normal call setup
procedure is extended by steps 3 and 4 to build the optional Direct Media
connection.
- In a first step, a signaling connection between the users is established.
- Parallel to that signaling connection a so called Master Connection
(MC) for the payload is established. The payload path of the Master Connection
may be segmented into TDM and IP paths:
- within a TDM network this is a specific B channel
- within an IP network, this is a RTP connection.
The RTP payload connection is established and controlled by H.323 procedures (in particular by the use of the fast connect procedure). - After the Master Connection between the users is established and connected, i.e. both users are in the talk state, a DMC connection between the IP endpoints of the Master Connection can be established. The called IP endpoint initiates the setup of the DMC connection.
- When the DMC connection is established, the payload is transported using this H.323 connection. However, the Master Connection exists in parallel, but without conveying payload as long as the DMC connection exists (the Master Connection is on a "stand-by" mode).
-
If the talk-state is released (e.g. for the setup of a consultation call) the IP endpoint switches back immediately to the Master Connection and releases the DMC connection.