User`s manual

SIP User's Manual 12. Special Applications
Version 5.0 267 October 2006
12 Special Applications
12.1 TDM Tunneling
The gateway's TDM Tunneling feature allows you to tunnel groups of digital trunk spans or
timeslots (B-channels) over the IP network. TDM Tunneling utilizes the internal routing
capabilities of the gateway (working without Proxy control) to receive voice and data
streams from TDM (1 to 16 E1/T1/J1) spans or individual timeslots, convert them into
packets and transmit them automatically over the IP network (using point-to-point or point-
to-multipoint gateway distributions). A gateway opposite it (or several AudioCodes
gateways, when point-to-multipoint distribution is used) converts the IP packets back into
TDM traffic. Each timeslot can be targeted to any other timeslot within a trunk in the
opposite gateway.
12.1.1 Implementation
When TDM Tunneling is enabled (‘EnableTDMOverIP’ is set to 1 on the originating
gateway), the originating gateway automatically initiates H.323 calls from all enabled B-
channels belonging to the E1/T1/J1 spans that are configured with the ‘Transparent’
protocol (for ISDN trunks), or ‘Ras CAS’ (for CAS trunks). The called number of each call is
the internal phone number of the B-channel that the call originates from. The IP to Trunk
Group routing table is used to define the destination IP address of the terminating gateway.
The terminating gateway automatically answers these calls if its E1/T1 protocol is set to
‘Transparent’ (ProtocolType = 5), or ‘Raw CAS’ (ProtocolType = 3 for T1 and 9 for E1) and
the parameter ‘ChannelSelectMode = 0’ (By Phone Number).
Note: It is possible to configure both gateways to also operate in symmetric
mode. To do so, set ‘EnableTDMOverIP’ to 1 and configure the Tel to IP
Routing tables in both gateways. In this mode, each gateway (after it is
reset) initiates calls to the second gateway. The first call for each B-
channel is answered by the second gateway.
The gateway monitors the established connections continuously, if for some reason one or
more calls are released, the gateway automatically reestablishes these ‘broken’
connections. In addition, when a failure in a physical trunk or in the IP network occurs, the
gateways reestablish the tunneling connections as soon as the network restores.
Note: It is recommended to use the keep-alive mechanism for each connection
by activating ‘session expires’ timeout, and using Reinvite messages.
By utilizing the ‘Profiles’ mechanism (refer to Section
5.5.6 on page 77) you can configure
the TDM Tunneling feature to choose different settings, based on a timeslot or groups of
timeslots. For example, you can use low-bit-rate vocoders to transport voice, and
‘Transparent’ coder to transport data (e.g., for D-channel). You can also use Profiles to
assign ToS (for DiffServ) per source, a time-slot carrying data or signaling gets a higher
priority value than a time-slot carrying voice.
For tunneling of E1/T1 CAS trunks set the protocol type to Raw CAS (ProtocolType = 3 / 9)
and enable RFC 2833 CAS relay mode (CASTransportType = 1).
Figure
12-1 and Figure 12-2 show an example of ini files for two Mediant 2000 gateways
implementing TDM Tunneling for four E1 spans. Note that in this example both gateways
are dedicated to TDM tunneling.