Sensors 2017, 17, 1637 3 of 14
that there are at least two concurrent transmission at the same time slot,
T
suc
is the average time
period associated with one successful transmission and
T
col
is the average time period associated
with collisions.
As shown in Equation (1), the channel business ratio accurately reflects the contention and
collision at the MAC layer. When the channel business ratio
R
b
increases, the channel utilization
and network throughput will become higher. However, the channel business ratio is too complex to
be deployed on the TCP protocol. Specifically, the key problem lies in the time interval involved in
calculating
R
b
being difficult to set. The ideal situation appears only when the time interval is exactly
equal to the transmission cycle of a frame. However, the transmission cycle of a frame will change
with many factors, such as frame length, flow concurrency and channel status.
Compared with the above solutions, this paper proposes a congestion notification scheme RECN,
which uses the frame transmission efficiency as the congestion metric. With only a very small
modification at the MAC layer, frame transmission efficiency characterizes the channel congestion
status more accurately and can be deployed on the TCP protocol using the standard ECN signal
method, improving the efficiency and fairness of the resource distribution in wireless ad hoc networks.
3. Definition and Theory Analysis
RECN is the cross-layer congestion notification scheme, based on the frame transmission efficiency
R
FTE
and using the standard ECN mechanism. RECN works at the MAC layer and adjusts the sending
rate of the transport protocol automatically and transparently. In this scheme, the frame transmission
efficiency is the key point, being put as the new congestion metric. Thus, in this section, we give the
definition and analysis of the frame transmission efficiency to demonstrate the relationship between
the frame transmission efficiency and the link layer congestion.
3.1. Definition
IEEE (Institute of Electrical and Electronics Engineers) 802.11 is the standard MAC protocol of
wireless networks. It includes RTS/CTS (Request To Send/Clear To Send) and basic access mechanisms
for data transmission in DCF mode [23].
In the RTS/CTS scheme, the RTS frame is sent to the reserve channel when the TCP source
wants to transmit the data packet. If the receiver successfully receives the RTS frame, it returns a CTS
(Clear To Send) frame. After that, the data and the ACK (Acknowledgment) frame will be transmitted
respectively. Only the RTS frame may have a collision in this mechanism. Since the RTS frame has
no data and is a short frame, it avoids long data frame collision and the hidden terminal problem;
while in the basic access scheme, the data will be sent directly once nodes access the channel instead of
sending RTS.
In the DCF (Distributed Coordination Function) mode, each node uses the CSMA/CA (Carrier Sense
Multiple Access with Collision Avoidance) mechanism to compete for wireless channel. Firstly, the sender
will detect the channel. If the idle time lasts for the DIFS (Distributed Inter-frame Spacing) time interval
and a random backoff time, RTS or the data frame will be sent. If no CTS or ACK is received within a
specified period, this frame is considered lost. Then, the transmitter will double the size of the contention
window and choose a new backoff timer.
The model of MAC layer service delay is shown in Figures 1 and 2. No matter which way it
works, we define the frame transmission efficiency R
FTE
as the ratio of successful transmission delay
to the frame service delay in the MAC layer, that is
R
FTE
=
T
SL
T
FSD
(2)
where
T
SL
is the delay of data successful transmission, which refers to the interval from the start of
sending data to the time ACK is received.
T
FSD
is the total frame service delay from listening on the
channel for transmitting this frame to this frame being transmitted successfully.