|
FAST Protocol
< Previous Next >
Re: Help with pmap
Shashank Bhatt / Faculties India
18 Feb 2010 10:48AM ET Hello Daniel,
pmap will not have a fixed length of 1 or 2 bytes, it's length depends on the number of fields in message that needs pmap bit to be present. We need to determine from stop bit everytime.
You can refer 1.1 spec doc: Section 10.5 Presence Map:
"A presence map is represented as a stop bit encoded entity. Logically a presence map has an infinite suffix of zeroes. This makes it possible to truncate a presence map that ends in a sequence where the bits are all zero. The length of the remaining part must be a multiple of seven."
Suppose, in one message, total 7 fields need pmap, then pmap length will be one byte (1 stop bit and 7 pmap bit). e.g. 10011011
If in second message, 10 fields need pmap, then pmap length will be 2 bytes. e.g. 00011011 10010000
Now in third message, 21 fields need pmap, and last 7 fields are having pmap bit as 0 (from encoder side), then PMAP in wire will be 01100111 10011011. For last 7 fields, pmap bit will be taken implicitly as 0 in decoder side.
Regards
Shashank
> I'm trying to implement a FAST decoder for CME. I'm wondering if the
> pmap always has a fixed length of 1 or 2 byte or ｄｏ Ｉ need to determine
> from stop bit everytime？ thx!
Re: Help with pmap
Shashank Bhatt / Faculties India 18 Feb 2010 10:48AM ET
|
