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C H A P T E R
4
F E A T U R E S
If after the initial 600ms, the call setup has not completed (this can be the case with DAMA networks) but
the call timeout has not been reached, the caller will hear a "call pending" tone pattern (fast beeps, fast
repeat). This will give way either to the ringing tone pattern if successful, or to the "unavailable" tone
pattern if the call fails or times out.
4.5
Dynamic Bandwidth Allocation
The versatility of the V100 multiplexer is a direct result of its packetised data transport architecture.
Voice/FAX, MPEG, ISDN BRI, synchronous and asynchronous data are processed in a prioritised manner
which reflects their individual demands for bandwidth.
The primary contention for bandwidth comes from Voice/FAX channels, which demand bandwidth when a
call is active and from synchronous data channels, which either demand permanent bandwidth as in the
case of a transparent channel, or which may demand bandwidth on a sporadic basis according to the
traffic they carry. Dynamic Bandwidth Allocation(DBA) on the V100 data ports uses a sophisticated
internal rate-change protocol to resolve the problem, which allows bandwidth to vary dynamically (on DBA
ports) in both transmit and receive directions independently.
Data channels may be set to operate at any speed up to 2Mbps. In DBA mode, the maximum clock
frequency of a synchronous tributary channel is configured by the user and the V100 then varies the
actual clock rate applied to the channel according to traffic demand. DBA mode uses the channel's RXC
and TXC phase-locked loops to generate an internal clock which is output to the connected device.
Following any successful new connection, which is routed through the same aggregate as the DBA sync
channel, the V100 calculates the highest permissible clock rate that is consistent with a total capacity of
87.5% on the chosen route and smoothly varies the output clock to the connected device. This allows a
LAN Router with a relatively low loading factor to operate without penalty while the V100 simultaneously
supports a voice/FAX call. As the voice traffic increases, the V100 is able to successively reduce the clock
speed to the Router so as to permit high-priority calls to be made at the cost of minor and temporary
reduction in Router throughput, which will probably go unnoticed by the LAN users. The clock rate is
successively restored as soon as competing connections are closed again.
By convention, the TX clock at the local end sets the originating DBA maximum rate and the RX clock at
the far end sets the terminating DBA maximum rate. They should be set equal and must both be set. The
V100 also supports asymmetrical Dynamic Bandwidth Allocation to cater for those cases where network
topology produces different bandwidth demands in each direction on an aggregate. In this case, the above
rules are still obeyed, but the local TX/remote RX rate and the local RX/remote TX rate are set to different
maximum values. In most cases, DBA will be used to drive bandwidth-agile devices such as routers and
therefore will be used in V.11 single-clock mode. Here the DBA RX and TX rates are set the same
throughout the connection, with the RX clock sources set to TXC. As with all other cases where the phase-
locked loops are used, the clock reference(s) must be sourced from the correct internal global clock bus
(GRX or GTX).
All data channels configured with "DBA" clock source are assigned a pool of dynamic bandwidth, which is
distributed between them in proportion to their configured bit rate. The V100 smoothly adjusts the
V100 Versatile Multiplexer Technical Manual Version 2.2
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