Figure 1. Channel Throughput when
only one node is transmiting
Figure 2. Channel Throughput when
two nodes are transmitting simultaneously
Figure 3. Compare the channel
throughput of 1-node Tx and 2=node Tx
Figure 4. Channel throughput when
1-node encounter bad-link-quality
Figure 5. Channel throughput when
1-node encounter bad-link-quality, but MAC scheduling enabled
Figure 6. Channel throughput
Comparison: Master Node
Figure 7. Channel throughput
Comparision: Slave Node 1 ( with good link quality)
Figure 8. Channel throughput
Comparision: Slave Node 2 ( with bad link quality)
Figure 9. Channel throughput
Comparision: Slave Node 2 ( bad link quality vs. good limk
quality of the other node link)
Throughput Analysis:
802.11b provides 11Mbps in physical link channle capacity. But, It's
only in a sense of time-instant.
However, In practice, it cannnot achieve such a rate in a general
sense.
Many factors compromise the data rate:
1. Channel Co-ordination Functions.
the rules to be used to co-ordiante channel access
wastes a size of channel capacity, such as slot times, contention
wondows.
2. MAC header overhead
Each packet has an overhead of 34 bytes
3. RTS/CTS
From Figure 1,
When packet rate is more than 680 packet/sec, the Rx bitrate is
keep in a stable value as near4Mbps
This is the case when only one node is acccesing channel, thus its real
meaning is that
"The (CPU) cannot put more data in that link" . Two possible reason:
1. CPU( Hardware device)'s cabability is stressed!
2. the physical channel is already saturated with bits?
The real reason is 2. Actually, the link could achieve ~4.3Mbps.
From Figure 2,
When link is in good condition, two nodes divides the channel capacity,
each has an upper limit of Tx rate as ~2.1Mbps.
In the experiment, Tx node2 is a 486 CPU, whie Tx node is a PII
machine. The different CPU power maybe accounts for
the slight difference of the channel throughput when packet rate is
more than 340 packet/s.
As we adding all the overhaeds of the layer header:
UDP header : 8 bytes
IP header : 20 bytes
MAC header: 34 bytes
The corrected total "datarate" over this wireless link is 4.3
*(750+20+8+34)/750 = 4.66 Mbps
From Figure 3:
