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Our Products
What is VMF?
What is the purpose of Variable Message Format?
VMF is designed to minimize the number of bits to transmit to convey
a message from one system to another. The information needed, and
only the information needed, is packed into as few bits as possible.
VMF trades off bandwidth for CPU time.
How is this packing done?
First, only the number of bits needed to hold the largest value of
a message field’s range of values is allocated for it. So if
a field’s values run from 1 to 12, only 4 bits are needed, while
1 to 366 takes 9 bits. Even character strings are allocated only 7
bits per character, while the ASCII standard specifies 8. Then the
fields are packed together with no inter-field spacing. Secondly, VMF
has 1-bit “Presence Indicators” for fields and groups—abbreviated
as FPIs and GPIs. The indicators show whether a message field or specified
group of fields are present in the message or not. If the bit is 0,
the field or group is omitted, if a 1, then it is present. Within groups,
the nested fields or groups within can also themselves be associated
with presence indicators. Lastly, there are recurrence indicators,
again for fields (FRI) and groups (GRI). Recurrence indicators are
associated with lists of items, where the actual number transmitted
in a message can vary from one to another. Each item in a list has
a recurrence indicator indicating whether or not another instance follows
it. If it’s 0, then this was the last item, otherwise another
one follows.
How much space does VMF save over other message
formats?
That depends on the contents of the messages that VMF is encoding,
and the format it is being compared against. The smallest message resulting
from VMF encoding would be one that consisted only of a VMF header
with every optional field omitted, whose message type alone made up
the whole of the message. That message would be six (6) bytes long.
Are there other ways of reducing message size with the aid of VMF?
- Historically a message consists of a header and a body. VMF allows multiple bodies to go out with under single header. The recipient(s) of the message pick out the parts they need.
- VMF provides for compressing the message body (or bodies) to further reduce transmission bandwidth.
What happens when
transmission problems occur?
Basically, a message affected by this would
have to be re-sent. Because of the bit-by-bit packing, and the use
of presence and recurrence indicators, it’s impossible to always
tell in advance the contents or size of a message without decoding
it.
Also see:
GenITC FAQ | VMF
FAQ
E-Mail: Info@QuadrusCorp.com
