Subject: [manet] Questions about hidden terminal and hops in ad hoc networks.

From: "Changchun Xu" <[email-masked]>
Date: Thu, 20 May 2004 20:46:29 +0800

Hi,every one!

     The questions proposed by me may be quite simple, sorry for I'm a 
beginner.
1. when we say that node i and j are hidden terminals with each other, does 
the definition of
hidden terminals also be expressed as "node i and j are 2 hops apart with 
each other"? I think these
two expressions are same.

2. The effective wireless communication range is R, the distance between 
node i and j is 1.8R, but there is no middlenode serves as bridge between 
them, then we can't say that node i and j are 2 hops apart?

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From: "John Mullen" <[email-masked]>
Date: Thu, 20 May 2004 11:29:41 -0600

Hi,

The hidden terminal problem can be defined in terms of the receiver.  If
a node can receive transmissions from two different transmitters that
are unaware of each other, you have a hidden terminal problem.  There
are two classic situations.  

In the first, there are three nodes, A, B, C, arranged so that links AB
and BC are usable, but AC is not.  Then, A and C are unaware of each
other and could both send packets intended for B, which would cause a
data collision.

In the second situation, there is some sort of barrier between A and C,
such as a building, but both can still reach B.  Now, A and C could be
closer than 1.8R and still be unaware of each other.

Bearing in mind that carrier and hello messages travel further than
transmitted packets, the second scenario is actually more common in the
field than the first.  This also means "hidden terminal" and "two-hop"
are not the same.  For example, if nodes in the first scenario are 0.7R
apart, they are within two hops of each other, but would probably detect
each other's presence, hence not be hidden.

Generally, Node i and Node j are said to be two hops apart if there is
an intermediate node that they can relay through, but direct comm is not
possible.  In the situation you describe in #2, the two nodes would be
disconnected.  

Hope this helps,

John Mullen

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Changchun Xu
Sent: Thursday, May 20, 2004 5:46 AM
To: [email-masked]
Subject: [manet] Questions about hidden terminal and hops in ad hoc
networks.

Hi,every one!

     The questions proposed by me may be quite simple, sorry for I'm a 
beginner.
1. when we say that node i and j are hidden terminals with each other,
does 
the definition of
hidden terminals also be expressed as "node i and j are 2 hops apart
with 
each other"? I think these
two expressions are same.

node i and j is 1.8R, but there is no middlenode serves as bridge
between 
them, then we can't say that node i and j are 2 hops apart?