Version 5.2 391 September 2007
SIP User's Manual 10. Tunneling Applications
10 Tunneling Applications
10.1 TDM Tunneling
The gateway 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 (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 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.
10.1.1 Implementation
When TDM Tunneling is enabled (EnableTDMOverIP is set to 1 on the originating
gateway), the originating gateway automatically initiates SIP calls from all enabled B-
channels belonging to the E1/T1/J1 spans that are configured with the ‘Transparent’
protocol (for ISDN trunks), or ‘Raw 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 is set to 0 (By Phone Number).
Note: It's 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's 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's 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 'Configuring the Profile Definitions' on page
132) 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).
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