MG-IP TDM Over IP Gateway Reference Manual
104
T1 Framing
Both the Extended Super Frame (ESF) mode and the D4 mode are valid for T1 in structured mode. The
T1 data rate is split into 24 time slots, each being allocated 8 bits in turn, for a total of 192 bits. The
selected protocol defines a bit pattern in the 193rd bit across a predetermined number of frames. When
the port has a channel data rate of 64K, all eight bits of the channel are dedicated; no signal information
is carried. However, when the port has a channel rate of 56k, only seven bits of the channel are
dedicated, and the eighth bit is reserved for signaling information, contained in the “robbed bit” every
sixth frame.
T1 data at a 56K data rate, whether ESF or D4, is sent from the PBX (in our example) to the MG-IP.
There the eighth bit is stripped from the channel. The remaining 7 bits of channel information are
packetized and sent out to the Packet Switched Network. When the module detects a change in signal
for any channel forwarded in a pseudowire, an out-of-stream signal message is sent out to the PSN to
inform the destination that a signal status change has occurred.
Figure 36: T1 Framing, 56K Data Channel
E1 Framing
The E1 data, in pcm30 format, is split into 32 time slots, each being allocated 8 bits in turn. Each time
slot sends and receives an 8-bit sample 8000 times per second.
One timeslot (TS0) is reserved for framing purposes, and alternately transmits a fixed pattern. This
allows the receiver to lock onto the start of each frame and match up each channel in turn. The
standards allow for a full Cyclic Redundancy Check to be performed across all bits transmitted in each
frame, to detect if the circuit is losing bits (information).
Another timeslot (TS16) is reserved for signaling purposes, to control call setup and teardown
according to one of several standard telecommunications protocols.
Figure 37: E1 pcm30 Format