Data rates of HomePlug and 802.11 Wi-Fi

Engineers need to understand what manufacturers are representing with “retail box” figures

BY MARK E. HAZEN
Intellon
Ocala, FL
http://www.intellon.com

Users want to see the highest possible performance from their digital communications equipment, whether the technology is Wi-Fi, HomePlug powerline communications (PLC), or some other another technology. The data rate stated on the retail box is much higher than you will ever see registered on a PC. When large files are transferd over one of these network connections the actual data throughput rate seems very low in comparison. We need to understand what manufacturers are representing with these “retail box” figures.

Manufacturers label the retail box with the physical layer (PHY) bit rate—200 Mbits/s for HomePlug AV PLC and 54 Mbits/s for 802.11g Wi-Fi. However, each technology should be understood to see what is actually included in the maximum PHY rate number. To illustrate this, we will examine these two common network technologies.

Fig. 1. A breakdown of bit-rate usage for HomePlug AV and 802.11g shows considerable overhead.

Comparing these two technologies, you can see the importance of understanding the real bit rates. The HomePlug AV PLC standard is referred to as a 200-Mbits/s technology. This refers to the raw PHY rate and HomePlug AV is actually capable of almost 250 Mbits/s over the physical medium if the total bandwidth is used. In practice, certain portions of the approximately 2 to 30-MHz bandwidth will not be used in deference to the amateur radio bands. So, approximately 50 Mbits/s are forfeited.

Note that 802.11g is stated differently. The raw PHY rate is not used as the “identifying speed” in the Wi-Fi camp. Instead, the coded PHY rate, also known as link rate, is used. The coded PHY rate, for both HomePlug AV and 802.11g, excludes bits used for forward error correction, but does include the coding bits to wrap the Ethernet packets into the PLC packets, or wireless packets, for routing. The maximum coded PHY rate for HomePlug AV and 802.11g is ~150 and ~54 Mbits/s, respectively.

Throughput rates most often measured are those controlled by protocols in the medium access control layer.

The throughput rates that are most often measured are those controlled by protocols that exist in the medium access control (MAC) layer—these are: UDP, TCP, and Windows File Transfer.

User Datagram Protocol (UDP) is a protocol designed for streaming traffic that does not request an acknowledgment from the receiving end. The data either arrives successfully or it doesn’t. UDP is often used for streaming media, music, and video. Using UDP, the HomePlug AV maximum MAC throughput rate is roughly 92 Mbits/s and roughly 30 Mbits/s for 802.11g. Keep in mind that these figures represent the maximum theoretical UDP rate under conditions of no noise, interference, or signal attenuation.

Transmission Control Protocol (TCP) does require, unlike UDP, an acknowledgement from the receiving end for each packet sent. Naturally, this sending and listening sequence, that often requires retransmission of data, is more time consuming and less bit-efficient than UDP. As a result, the MAC throughput rate drops. For a HomePlug AV standard-based system, the maximum TCP MAC throughput rate is 65 Mbits/s, with some of the bits used for the protocol. The maximum TCP rate for 802.11g is approximately 24 Mbits/s.

The coded PHY rate excludes forward error correction bits

When using Windows-based PCs to transfer files over a network, and even downloads from the Internet, an additional protocol is added to the mix. The Windows File Transfer Protocol is based upon, and generically referred to as, the server message block (SMB) protocol. This protocol rides on top of the TCP protocol and adds a lot of extra bits to the transmission, thus reducing the MAC throughput rate even further. From Figure 1, you can see that the Windows File Transfer rate for HomePlug AV is roughly 35 Mbits/s and around 11 Mbits/s for 802.11g.

When noise, interference, and signal attenuation are added to the mix, each of the above PHY and MAC throughput rates decrease further. HomePlug AV technology from Intellon can optimize the throughput according to conditions over the physical medium. The true quality of a product is measured in how well the product performs under the adverse conditions of noise, interference, and attenuation. Product quality should rise above compliance and standards bodies’ testing and certification, which only indicates compliance, interoperability, and some minimum level of features/performance. That’s why product reviews can be useful in choosing a technology or devices within a technology.

So, the data rate that is stated on the retail box is a PHY rate that is based upon the performance of the related standard and technology under the best of conditions. The stated PHY rate may be the raw PHY rate or the coded PHY rate. The PHY rate includes physical medium coding bits and may include forward error correction bits. The throughput rates that are most often measured are associated with the MAC layer protocols: UDP, TCP and Windows File Transfer, each slower respectively because of necessary bit overhead as defined by each. Noise, interference and attenuation work to further reduce throughput rates in both the PHY and MAC layers. ■

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For more information on networking devices see http://electronicproducts-com-develop.go-vip.net/digital.asp.