- Problem with
DCF protocol when there is
a hidden node:
- The DCF protocol
(using SIFS and
DIFS
mechanism) will have problems if
there is a
hidden node problem
- The hidden node will
cause collision at the
receiver that the
sender
cannot detect:
When C transmits to B, the hidden node A can interfere
- The hidden node will
cause collision at the
receiver that the
sender
cannot detect:
- The DCF protocol
(using SIFS and
DIFS
mechanism) will have problems if
there is a
hidden node problem
- If a node
detects that some node
frequently
fails
to reply
with an ACK frame:
- The node may (optionally) use the Virtual Carrier Sense mechanism to transmit data frames
- Control frames used in the
Virtual Carrier Sensing protocol:
- Request to send (RTS) frame
- Clear to send (CTS) frame
(The RTS and CTS frames are short frames --- like an ACK frame)
- Usage:
- A node that wants to
send a data frame will
transmit:
RTS frame to its receiver
- The receiver will
send back a
(high priority)
CTS frame
- The sender will then send a (high priority) data frame
(There are other side effect that will be explained soon)
Graphically:
- A node that wants to
send a data frame will
transmit:
- The IFS for
the RTS frame:
DIFSNote:
- The node
actually wants to
transmit a
data frame
- Instead of
sending a
data frame
(which can be
lost due to
collision with a
hidden node):
- The node transmits the RTS frame to solve the possible hidden node problem !!!
- The node
actually wants to
transmit a
data frame
- The Request To Send (RTS) frame:
Explanation:
- The Request To Send
(RTS) frame contains:
- Source ID
- Destination ID
- a (time) duration value
- Effect:
- Desitination Node:
- The destination node must respond with a (high priority) Clear to Send frame (see below)
- The other nodes
that received a
RTS frame:
- The other node
must
refrain from transmitting
for duration sec
(given in RTS frame)
(This is implemented using the Network Allocation Vector - see below)
- The other node
must
refrain from transmitting
for duration sec
(given in RTS frame)
- Desitination Node:
- The Request To Send
(RTS) frame contains:
- The IFS for
the RTS frame:
SIFS !!! (High priority, like an ACK frame)Note:
- The CTS frame will
follow the
RTS frame
immediately
- Because SIFS is the shortest IFS time
- The CTS frame will
follow the
RTS frame
immediately
- Reason to use
CTS:
- The destination node uses the CTS frame to silent all hidden nodes in its neighborhood !!!
- The Clear To Send (CTS) frame:
Explanation:
- The Clear To Send
(CTS) frame also contains:
- Source ID
- Destination ID
- a (time) duration value
- Effect:
- The destination node that
receives the
CTS frame
must transmitting
for the duration time
in CTS frame
- All node that receives the CTS frame must refrain from transmitting for the duration time in CTS frame
- The destination node that
receives the
CTS frame
must transmitting
for the duration time
in CTS frame
- The Clear To Send
(CTS) frame also contains:
- Consider the
following scenario:
- Suppose node A wants to
send a data frame to
node B
Node C is a hidden node that can cause a collision !!!
- Suppose node A wants to
send a data frame to
node B
- Solving the
hidden node problem:
- Node A first
send a RTS(t) frame to
node B:
Result:
- Node B will respond with a CTS frame
- Node E will
refrain from transmission for
t1 seconds
- Node E will not collide with the subsequent CTS frame !!!
Graphically:
- Node B
transmits
a CTS(t) frame
back to
node A:
Result:
- Node A will now transmit a data frame (after sensing for SIFT duration)
- Node C will
refrain from transmission for
t2 seconds
- The hidden node C will not interfere !!!
Graphically:
- Node A
now sends its data frame
to
node B:
Notice that:
- The hidden node (C) will not interfere with node A's transmission !!!
Graphically:
- Finaly: Node B
sends the ACK frame
back to
node A:
Without any node interfering !!!
Graphically:
- Node A first
send a RTS(t) frame to
node B:
- Network Allocation Vector:
- Network Allocation Vector =
a data structure (= variable)
in the 802.11 protocol that
records
reservation periods
- Reserved period = a period (time interval) where a node cannot transmit any frame (except ACK frames)
- Network Allocation Vector =
a data structure (= variable)
in the 802.11 protocol that
records
reservation periods
- How to implement
Virtual Carrier Sensing using
the NAV (variable):
- When a node receives a
RTS(t) or
CTS(t) frame, a
node:
- Mark off the reservation period in its NAV.
- When a node receives a
RTS(t) or
CTS(t) frame, a
node:
- Example:
How to set the values of t1 in RTS and t2 in CTS:
t1 = (SIFS + CTS) + (SIFS + DATA) + (SIFS + ACK) // Sender knows DATA length !!! t2 = t1 - (SIFS + CTS) // Receiver uses t1 to compute t2