Subject: [Roll] What is "topological distance"? And "rank"? And "fixed point number"?

From: Alexandru Petrescu <[email-masked]>
Date: Fri, 09 Apr 2010 19:49:33 +0200

I am not sure in which order to mention this, but that's how I read it.

Alex

What is "topological distance"?

This pair of words appears precisely once in the draft, as help
explaining another term.

However, "topological distance" as such is not an easy term to
understand.  It could mean "IP hop count" if we knew there existed IP
hops between links but here we don't.

It could also mean "distance in meters".

It could also mean some energy-weighted distance.

Depending on these meanings the original term ("Rank") could have
various meanings ("Rank").

For example:
> The DODAG parent of a node will have a lower rank than the node
> itself

What does "lower" mean?  A smaller figure like in 2 is lower than 3? 
But in my company a "rank 1" means top level above everybody.

> Rank may be thought of as a fixed point number

What's a fixed point number?  A fixed point number between 0 and 1?  OR
a fixed point number between 1 and infinity?  Or did you mean integer
and not fixed point?

> When an objective function computes rank, the objective function
> operates on the entire (i.e. 16-bit) rank quantity.

What's the representation of the 16-bit fixed point number?  On which 
bit is the point?  What's the range of values?  How could it be represented?

Or you meant an "integer"?

Alex

---

From: Robert Cragie <[email-masked]>
Date: Fri, 09 Apr 2010 19:23:09 +0100

Hi Alex,

I agree that ranking does seem perverse, with 'higher' rank meaning 
further down the tree (an Australian tree, whose root is at the top ;-). 
In terms of superiority, you would think the root was most superior node 
but, well, I guess as long as it is consistent it doesn't really matter 
- there aren't actually any strict rules on the use of cardinal numbers 
in ordinality, despite the clear lexical relationship (e.g. third, 
fourth). On that note, the term 'deeper' is still used in a couple of 
places and should be changed to 'higher rank'.

I think rank is clearer - it is a fixed point number with an integer and 
fractional part. The integer part is used for the ordinal position and 
the integer and fractional part are used in rank arithmetic. Where the 
point sits is dependent on the objective function. Or at least that's 
how I read it. There are a couple of oddities, such as in OF0, 
MinHopRankIncrease is set to 256 but this cannot strictly be carried in 
the DODAG configuration suboption as there is only 1 byte for it.

Robert

Robert Cragie (Pacific Gas & Electric)

Gridmerge Ltd.
89 Greenfield Crescent,
Wakefield, WF4 4WA, UK
+44 (0) 1924 910888
http://www.gridmerge.com <http://www.gridmerge.com/>

On 09/04/2010 6:49 PM, Alexandru Petrescu wrote:
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---

From: Alexandru Petrescu <[email-masked]>
Date: Fri, 09 Apr 2010 23:47:35 +0200

Le 09/04/2010 20:23, Robert Cragie a écrit :
> further down the tree (an Australian tree, whose root is at the top
> ;-). In terms of superiority, you would think the root was most
> superior node but, well, I guess as long as it is consistent it
> doesn't really matter - there aren't actually any strict rules on the
> use of cardinal numbers in ordinality, despite the clear lexical
> relationship (e.g. third, fourth). On that note, the term 'deeper' is
> still used in a couple of places and should be changed to 'higher
> rank'.

To me it is unnecessary complication.  BEtter use standard things 
wherever available.

> I think rank is clearer - it is a fixed point number with an integer
> and fractional part.

How is a fixed point on 16bit represented in practice please?  NEver
programmed it before... I don't personally know I may be fully ignorant.

In C is it like an "fint16_t" (like the standard uint_t, or fpfloat or 
similar?)

How can one compare these meaningfully if the spec doesn't say?

What does "/" mean in:
>              DAGRank(rank) = floor(rank/MinHopRankIncrease)

Is it an integer divide?  A real divide?

What does "floor" mean?  It's not standard C.  Is it C++?  Does one 
_have_ to use C++ to implement this?  (knowing C compilers generate 
faster less memory code...)

In C I am afraid never seen a fixed point on 16bit.

Alex

> The integer part is used for the ordinal position and the integer and
> fractional part are used in rank arithmetic. Where the point sits is
> dependent on the objective function. Or at least that's how I read
> it. There are a couple of oddities, such as in OF0,
> MinHopRankIncrease is set to 256 but this cannot strictly be carried
> in the DODAG configuration suboption as there is only 1 byte for it.
>
> Robert
>
> Gridmerge Ltd. 89 Greenfield Crescent, Wakefield, WF4 4WA, UK +44 (0)
> 1924 910888 http://www.gridmerge.com <http://www.gridmerge.com/>
>
>
>> understand. It could mean "IP hop count" if we knew there existed
>> IP hops between links but here we don't.
>>
>> What's a fixed point number? A fixed point number between 0 and 1?
>> OR a fixed point number between 1 and infinity? Or did you mean
>> integer and not fixed point?
>>
>> What's the representation of the 16-bit fixed point number? On
>> which bit is the point? What's the range of values? How could it be
>>  represented?
>>
>> _______________________________________________ Roll mailing list
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