TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
23
3. The VoWiFi Handset as
In figure 5 the media descriptor identifies four payload types that a particular handset
supports. The first payload type “8” is the codec (G711 ALaw) preferred by this handset.
Figure 5. SDP Media Description:
In this example, the handset initiating the invite is indicating to the called party handset
that the preferred codec for the media session is G.711 ALaw. If the called party handset
does not support this codec, try G.723 as the next preference, G.729 the third preference
and G.711MuLaw being the least preferred. In the unlikely event of the other handset not
supporting any of the codecs suggested by the call initiator, a SIP unsupported media type
message will be returned in response to the initial invitation to setup the call and the RTP
media session will not be established.
Media Attribute
An example of an SDP media attribute is
as
follows:
Media attribute (a): fmtp:18 annexa=yes
It shows how the G.729 codec protocol is specified with it’s payload type annexa set to
“yes”, that is, silence suppression. Silence suppression is thus set if the SIP endpoints
negotiate this codec protocol for use in a connection.
An understanding of how the properties of a connection are expressed through SDP media
attributes can sometimes be of help in troubleshooting and resolving issues.
3.8.2 SIP Call Setup
An attempt to establish a SIP sess
io
n begins when a calling party, say user1, makes an
INVITE request to a called party, say user2. Because the call will be using IP, user1 ultimately
needs to discover the IP address of user2. User1 thus forwards the INVITE request to its SIP
proxy servera.com but ServerA realizes by the domain name part of the AOR (serverb.com)
that user2 is not registered with its own domain servera.com. ServerA therefore needs to
initiate a search for the proxy server in the network that is able to associate the AOR with an
IP address.
The first hop in this simplified example is to pass the INVITE request t
o
serverB. ServerB
realizes by the domain name in the AOR (serverb.com) that user2 is connected to it and it
has no further need to forward the INVITE to any other servers in the network. Instead, it can
To Next Page
Loading...
Need help?
General
