56 QUALITY OF SERVICE IN WIRELESS NETWORKS OVER UNLICENSED SPECTRUM
3.3.2 Total Bandwidth Estimation
To determine p
( f ) and p
( f ), the RA of a ﬂow f needs to have an estimate of the total bandwidth
( f ) over the wireless link being used by the ﬂow. We discuss a bandwidth measurement mecha-
nism based on the IEEE 802.11, DCF (Distributed Coordinated Function) MAC (Medium Ac-
cess Control) layer . IEEE 802.11 relies on the DCF method to coordinate the transmission of
packets based on CSMA/CA without any central control unit. We have discussed the IEEE 802.11
DCF extensively in Chapter 1, Section 1.2.1. The packet trasmission sequence and protocol are il-
lustrated in Figure 3.8. The IEEE 802.11 protocol for single packet transmission works as follows:
Before transmitting a packet, a node senses the channel to make sure that the channel is idle;
otherwise it backs off by a random interval and senses the channel again. If the channel is idle, it
transmits a RTS (Request-to-Send) control packet to signal its intention to send a DATA packet.
On receiving the RTS packet, the destination node replies with a CTS (Clear-to-Send) control
packet to give the sender a go-ahead signal, and to silence the destination node’s neighboring nodes.
After receiving the CTS packet, the sender sends the DATA packet, and it is then acknowledged by
an ACK control packet from the receiver.
We measure the throughput of transmitting a packet as TP = S/(t
), where S is the size
of the packet, t
is the timestamp that the packet is ready at the MAC layer, and t
is the timestamp
that an ACK has been received. Note that the interval (t
) includes the channel busy and con-
tention time. We keep separate throughput estimates to different neighboring nodes because the
channel conditions may be very different. We only keep an estimate for active links, since we do not
have any packets to measure (t
) over inactive ones.
This MAC layer measurement mechanism captures the effect of contention on a ﬂow’s per-
ceived channel bandwidth. If the contention is high, (t
) will increase, and the throughput TP
will decrease. This mechanism also captures the effect of physical errors because if RTS and DATA
packets are affected by channel errors, they have to be retransmitted, up to retransmission limit.
This increases (t
) and correspondingly decreases the ﬂow’s perceived bandwidth. It should be
noted that the perceived bandwidth is measured only using successful MAC layer transmissions.
FIGURE 3.8: IEEE 802.11 protocol for single packet transmission.