In this column we will take a survey of wireless networking technology. Since 1997 wireless technology has grown in the IEEE802.11 standards realm. One of the best ideas of the 802.11 standards is the backward compatibility that’s baked in – although at slower data rates as you move down in standard revisions. New wireless routers typically come compatible with 802.11 a and b/g/n/ac. Let’s take a closer look into the different revisions to the 802.11 standard.
As mentioned, the standard allows switching between a variety of data rates dynamically. Poor RF conditions can cause the wireless nodes to step down data rates through the standard revisions to maintain the connection and step back up when conditions improve.
In wired Ethernet terms, network traffic collisions are detected and data is retransmitted when the air is free. Wireless connections don’t have that luxury due to transmitting nodes that are hidden to the sender. Instead wireless technology uses a collision avoidance concept. This utilizes a request to send (RTS) packet containing just the sending and intended receiving addresses and intended transaction duration information. The intended receiver replies with a clear to send packet (CTS). Other nodes sensing these packets will hold off transmitting for at least the transaction duration.
Table 1 shows the original standard with the several amendments and corrections. The standard revisions use the name of the main document – 802.11 plus the rev level. For example: 802.11b, 802.11g and so on. In other words, the rev level designates the physical medium specification.
The IEEE standard first appeared in 1997 as 802.11. It was released with both a frequency hopping (FHSS) method and a distributed sequence (DSSS) method – both as spread spectrum. An interesting side note – Hedy Lamarr the famous actress of the 1940s co-developed a radio guidance system for torpedoes at the beginning of World War II. It used spread spectrum and frequency hopping techniques to avoid the radio jamming of the Axis powers. The principles of that work lives on in today’s Wi-Fi technology. Hedy and her co-inventor, composer George Antheil, are in the National Inventors Hall of Fame. (see table 1)
802.11a defined in 1999 overcame some of the original 802.11 specs by moving to the 5GHz range from the 2.4GHz band. Higher data rates (up to 54MBs) are achieved by using Orthogonal Frequency-Division Multiplexing (OFDM). However, with its error correction code the actual data throughput is closer to 20MBs.
OFDM uses many subcarriers that are closely spaced in an orthogonal (or 90 degree apart) fashion. Each subcarrier carries part of the data stream at a much slower data rate. Combined, the subcarriers add up to the faster data rate. OFDM techniques also deal much better with signal fading due to multi-path. Multi-path occurs when reflected RF signals recombine causing interference, fading and phase shifting.
802.11b was the first generally accepted protocol and was in use earlier than 802.11a causing many to think that it was released before 802.11a. Rev B uses the original 2.4GHz range. While the 11MBs data rate beats the original 802.11 rate of 2MBs. Note that the 2.4GHz range can get interference from household appliances such as microwave ovens and cordless phones. Rev B uses Complimentary Code Keying (CCK) at a smaller chip rate than the original spec. The original used 11 chips per bit – in other words each digital 1 or 0 of data was represented by a series of 1 or 0 “chips” to help avoid RF interference. Rev B reduces the number of chips to 8 allowing for more data bandwidth but with higher RF interference probability. When network gear indicates that it works with Rev A or Rev B they actually run them side by side with the different frequency ranges used.
802.11g took advantage of newer encoding techniques to increase data bandwidth up to 54MBs. One additional idea was Packet Binary Convolutional Coding.
Packet Binary Convolutional Coding
A convolutional code is error-correcting by using parity symbols. With PBCC it’s within the data packet with a sliding function of a Boolean polynomial working on a data stream. The sliding function is the “convolution” over the data, which gives rise to the term “convolutional coding.” However newer techniques have taken over. Rev G was to be backward compatible with Rev B but reduced data rates in the process.
802.11n adds multiple-input multiple-output antennas (MIMO) to the scene. Rev N works on both main Wi-Fi frequencies – the 2.4GHz and the 5GHz bands. However, technically, support for the 5GHz bands are optional. It operates at a maximum net data rate from 54Mbit/s to 600Mbit/s. MIMO multiplies the transmit and receive antennas to utilize intentially generated multipath RF broadcasts to multiply link capacity.
802.11ac has a bandwidth that’s rated up to 1300 MBps on the 5 GHz band plus up to 450 MBps on the 2.4 GHz band.
IEEE 802.11ac was released in 2013 building on the 802.11g protocol. The changes included wider channels from 40 MHz to 80 and 160 MHz in the 5 GHz band. Rev AC added up to 8 spatial streams and a higher-order modulation up to 256 quadrature amplitude modulation (QAM). This all adds up a 1300 MBps data rate.
A new technology is also used, multiuser MIMO (MU-MIMO). Whereas Rev N is like a wired Ethernet hub that can only transfer a single frame at a time to all its ports, MU-MIMO can send multiple frames to multiple nodes simultaneously using the same frequency spectrum. In wired Ethernet terms think of this as behaving like a wireless switch.
Other revs in the standard family not listed in Table 1 (c–f, h, j etc.) are changes used to add to the breadth of the standard, which can include corrections to a previous rev. Next month, Tech Savvy looks into wireless security.
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