TTSI2K32T Datasheet, PDF (22/66 Pages) Agere Systems – 2048-Channel, 32-Highway Time-Slot Interchanger

TTSI2K32T
2048-Channel, 32-Highway Time-Slot Interchanger
Preliminary Data Sheet
February 1999
TDM Highway Interface Timing
Virtual and Physical Frames
Figure 10 below shows a virtual frame offset from the physical frame. The FSYNC pulse marks the beginning of the
physical frame, but the TSI can be programmed to interpret the location of time slot 0 at any point in a frame. Sev-
eral parameters are available to make up the offset for a virtual frame with various levels of granularity. There is
XTSOFF/RTSOFF for transmit/receive time-slot offsets. This offset can be up to 31 time slots for a 2.048 Mbits/s
highway, 63 time slots for a 4.096 Mbits/s highway, or 127 time slots for an 8.192 Mbits/s highway. XBITOFF/
RBITOFF allow the setting of up to a 7-bit offset for transmit/receive frames. XFBOFF/RFBOFF allow fractional bit
offsets of 0, 1/4, 1/2, or 3/4 bits. All of these offsets mentioned above can be independently programmed for each
one of the transmit and receive highways. The maximum offset that can be introduced on an 8.192 Mbits/s highway
is 127 time slots, 7 3/4 bits. The maximum offset on a 4.096 Mbits/s highway is 63 time slots and 7 3/4 bits. The
maximum offset on a 2.048 Mbits/s highway is 31 time slots, 7 3/4 bits.
The following examples indicate how virtual offsets can be used to simplify system designs. For example, data that
is being sent to the TSI on a particular Rx highway may have incurred a several time-slot delay due to processing
by HDLC formatters, echo cancellers, communication protocol processors, etc. Rather than adding an external
buffer to realign all the highway data to the next FSYNC, an offset to create a virtual frame on that Rx highway can
be used instead. On a transmit highway, for example, there may be a device downstream that has a processing
latency of N time slots. An offset of (32 – N) time slots can be added beforehand on a 2.048 Mbits/s highway so
that after processing, the TDM data is aligned to FSYNC again.
Fractional bit offsets are handy for adjusting the sampling point on a Rx highway. With a 1/4-bit resolution possible,
setup and hold time requirements on the Rx TDM highways for the TSI should be easily met. On transmit high-
ways, fractional bit offsets can be used to shift the outgoing highway data slightly, so the destination device’s setup
and hold times can be met with adequate margins. Note that the time slot, bit, and fractional bit offsets are relative
to the highway data rate and imply different durations on different speed highways. For example, a 1/4-bit offset on
a 2.048 Mbits/s highway means 122 ns, on a 4.096 Mbits/s highway, it is 61 ns, and on an 8.192 Mbits/s highway,
it implies a 30.5 ns offset.
FSYNC
Rx HIGHWAY
PHYSICAL FRAME, N
PHYSICAL FRAME, N + 1
Tx HIGHWAY
Rx OFFSET
VIRTUAL Rx FRAME, N
VIRTUAL Rx FRAME, N + 1
Tx OFFSET
VIRTUAL Tx FRAME, N
VIRTUAL Tx FRAME, N + 1
Figure 10. Virtual and Physical Frames
5-7464(F)r.2
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Lucent Technologies Inc.