3Com VCX V7122, User Manual, Page: 317 / 488

V7122 Gateway User Guide
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PPLICATIONS
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 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.
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 ‘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).
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 V7122
gateways reestablish the tunneling connections as soon as the network restores.
It is recommended to use the keep-alive mechanism for each connection by
activating ‘session expires’ timeout, and using Reinvite messages.