C H A P T E R 4 F E A T U R E S
V100 Versatile Multiplexer Technical Manual Version 2.2
Page 159 of 231
4.10.2
Compression
The data compression feature uses a derivative of the Lempel-Ziv algorithm with a code size of 12 bits.
The algorithm runs above the error-correction layer, which is always enabled when data compression is
selected.
The 100mS data buffers are compressed and fed into the error-corrector, which builds up a table of 12-bit
codes according to the data coming in. If the code table ever fills up completely (i.e. all 2
12
Code entries
are used) then the table is reset and begins to accumulate all over again. Similarly, the compression is
turned off whenever the throughput reaches 100%, in which case there is no gain achievable and the data
is passed transparently.
The compressor also stops operating when the channel goes idle for more than 0.5 seconds and drops
into transparent mode. This is to optimise the transfer of single characters or sequences of data less than
four characters – the compression will only start again when a block of at least four characters are present
in a buffer. If for any reason, the compressor turns off, perhaps during a sequence of incompressible data,
then it will retry after 5 seconds. This allows it to pick up again to optimise transmission automatically.
4.10.3
General characteristics
Error-correction or compression on channels below 4800bps may start to impose excessive delays. The
usefulness of this feature should be carefully assessed before configuration.
The operation of the error-correction facility imposes a processing burden upon the host. Testing has
indicated that a safe performance limit of up to four channels per chassis running at 57600bps should be
observed to prevent any adverse effects on the normal functioning of the unit. Performance degradation
may be indicated by a slowing of the supervisor update speed.
4.11
TDM Data Channels
The V100 TDM Framer card (VI68713) is a multi-purpose adaptor card, which is fitted internally into the
Special Options slot of the Rev.C V100 chassis. Into this card, one of three interface modules may be
inserted: (i) a TDM Serial Interface (VI68714), (ii) a basic-rate ISDN S0/T0 OEM card (VI68711-SE or –
SU), (iii) a basic-rate ISDN U0 OEM card (VI68711-U). With the TDM Serial Interface fitted, the card
functions as an aggregate serial interface and framer allowing the V100 to split off TDM bandwidth for
other applications. In the second and third applications, the card converts the OEM ISDN modules into a
footprint compatible with the existing Rev.C V100 motherboard connectors to allow the modules to plug
into an existing Rev.C motherboard. On later motherboards (Rev.D+), the framer card is fully integrated
allowing the motherboard to support all modules directly.
4.11.1
Functionality Restrictions
In the case of the ISDN S0/T0 and U0 applications, no other V100 resources are affected. In the case of
the TDM framer application, the internal architecture imposes restrictions on the choice of operation. In
this case, the High-Speed Channel (HSC) may not be used and the HSC menu items on the SYSTEM
SETTINGS page are replaced by two fields for the control of the TDM port interface and Transmit Clock.
