The throughput for 802.11g is dependent upon three factors, the transmit power of your base dBm and reception in dBi and distance between the two wireless devices. 802.11g is older wifi technology, but it's still practical. In theory, the total bandwidth is 54Mbps, which means the throughput should be around half that much (27Mbps at Line of Sight), since all wireless signals are transmitted at half-duplex. If you don't have Line of Sight, then expect the throughput to be much lower. Materials between your wireless nodes do in fact impede throughput and degrade signal strength, which can cause the node(s) to send recurring re-transmission of packets.

Yes i am using TCP, the data rate is 54 Mbps. Please Viengvilay Xayavongsa i do not understand you mean the value is correct or not ? also please tell me is there specific  transmit power or reception power when the use of 802.11g? 

The value itself isn't static, therefore and meaning, the value will fluctuate according to signal impedance so there is no "correct" value. In other words, it depends on your environmental conditions.

You must understand that bandwidth and throughput aren't the same thing. 54 Mbps is the bandwidth, but that's not throughput. There is no static throughput, the bandwidth is just an indicator of the total "send and receive throughput" in respect to availability and limitations of the hardware's capacity.

What you're experiencing is variable throughput, keyword variable, which means it can change according to the signal-to-noise ratio (SNR) signal strength. The SNR is dependent upon the transmit and receive antennas and their transmit powers. The reception (dBi) is labeled as decibels in respect to isotropic radiation and transmit power is labeled (dBm) which stands for decibels in respect to miliwatts.

In simple terms, either remove obstacles from between your wireless devices and put them in line of sight to increase throughput or increase the transmit power as well as reception sensitivity of the receive antenna to achieve optimal throughput value. Usually the minimum transmit power of your wifi router and receive device transmits at 75 mw. I have a wifi router that has DD-WRT firmware instead of its factory one and have increased this default transmit power (on the router side) to 251 mw.

I would say either find a way to increase the transmit power manually using after-market firmware that's compatible with the device or buy a new router that transmits a wireless signal at a higher default power to get better wireless speeds. I've personally achieved an average throughput of 21 to 25Mbps on an 802.11g wireless signal.

The theoretical maximum is around 40 % and I tried the same in NetSim and got 21 Mbps. Perhaps you have some error in the link which is causing it to drop a bit more.