WIRELESS LANS - 1

WIRELESS LANS

Wireless LANs are increasingly popular, and homes, offices, cafes, libraries, airports, zoos, and other public places are being outfitted with them to connect computers, PDAs, and smart phones to the Internet. Wireless LANs can also be used to let two or more nearby computers communicate without using the Internet.

The main wireless LAN standard is 802.11. We gave some background information on it in Sec. 1.5.3. Now it is time to take a closer look at the technology. In the following sections, we will look at the protocol stack, physical-layer radio transmission techniques, the MAC sublayer protocol, the frame structure, and the services provided. For more information about 802.11, see Gast (2005). To get the truth from the mouth of the horse, consult the published standard, IEEE 802.11-2007 itself.

The 802.11 Architecture and Protocol Stack

802.11 networks can be used in two modes. The most popular mode is to connect clients, such as laptops and smart phones, to another network, such as a company intranet or the Internet. This mode is shown in Fig. 4-23(a). In infrastructure mode, each client is associated with an AP (Access Point) that is in turn connected to the other network. The client sends and receives its packets via the AP. Several access points may be connected together, typically by a wired network called a distribution system, to form an extended 802.11 network. In this case, clients can send frames to other clients via their APs.

The other mode, shown in Fig. 4-23(b), is an ad hoc network. This mode is a collection of computers that are associated so that they can directly send frames to each other. There is no access point. Since Internet access is the killer application for wireless, ad hoc networks are not very popular.

Now we will look at the protocols. All the 802 protocols, including 802.11 and Ethernet, have a certain commonality of structure. A partial view of the 802.11 protocol stack is given in Fig. 4-24. The stack is the same for clients and

APs. The physical layer corresponds fairly well to the OSI physical layer, but the data link layer in all the 802 protocols is split into two or more sublayers. In 802.11, the MAC (Medium Access Control) sublayer determines how the channel is allocated, that is, who gets to transmit next. Above it is the LLC (Logical Link Control) sublayer, whose job it is to hide the differences between the different 802 variants and make them indistinguishable as far as the network layer is concerned. This could have been a significant responsibility, but these days the LLC is a glue layer that identifies the protocol (e.g., IP) that is carried within an 802.11 frame.

Several transmission techniques have been added to the physical layer as 802.11 has evolved since it first appeared in 1997. Two of the initial techniques, infrared in the manner of television remote controls and frequency hopping in the 2.4-GHz band, are now defunct. The third initial technique, direct sequence spread spectrum at 1 or 2 Mbps in the 2.4-GHz band, was extended to run at rates up to 11 Mbps and quickly became a hit. It is now known as 802.11b.

To give wireless junkies a much-wanted speed boost, new transmission techniques based on the OFDM (Orthogonal Frequency Division Multiplexing) scheme we described in Sec. 2.5.3 were introduced in 1999 and 2003. The first is called 802.11a and uses a different frequency band, 5 GHz. The second stuck with 2.4 GHz and compatibility. It is called 802.11g. Both give rates up to 54 Mbps.

Most recently, transmission techniques that simultaneously use multiple antennas at the transmitter and receiver for a speed boost were finalized as 802.11n in Oct. 2009. With four antennas and wider channels, the 802.11 standard now defines rates up to a startling 600 Mbps.

We will now examine each of these transmission techniques briefly. We will only cover those that are in use, however, skipping the legacy 802.11 transmission methods. Technically, these belong to the physical layer and should have been examined in Chap. 2, but since they are so closely tied to LANs in general and the 802.11 LAN in particular, we treat them here instead.

The 802.11 Physical Layer

Each of the transmission techniques makes it possible to send a MAC frame over the air from one station to another. They differ, however, in the technology used and speeds achievable. A detailed discussion of these technologies is far beyond the scope of this book, but a few words on each one will relate the techniques to the material we covered in Sec. 2.5 and will provide interested readers with the key terms to search for elsewhere for more information.

All of the 802.11 techniques use short-range radios to transmit signals in either the 2.4-GHz or the 5-GHz ISM frequency bands, both described in Sec. 2.3.3. These bands have the advantage of being unlicensed and hence freely available to any transmitter willing to meet some restrictions, such as radiated power of at most 1 W (though 50 mW is more typical for wireless LAN radios). Unfortunately, this fact is also known to the manufacturers of garage door openers, cordless phones, microwave ovens, and countless other devices, all of which compete with laptops for the same spectrum. The 2.4-GHz band tends to be more crowded than the 5-GHz band, so 5 GHz can be better for some applications even though it has shorter range due to the higher frequency.

All of the transmission methods also define multiple rates. The idea is that different rates can be used depending on the current conditions. If the wireless signal is weak, a low rate can be used. If the signal is clear, the highest rate can be used. This adjustment is called rate adaptation. Since the rates vary by a factor of 10 or more, good rate adaptation is important for good performance. Of course, since it is not needed for interoperability, the standards do not say how rate adaptation should be done.

The first transmission method we shall look at is 802.11b. It is a spread-spectrum method that supports rates of 1, 2, 5.5, and 11 Mbps, though in practice the operating rate is nearly always 11 Mbps. It is similar to the CDMA system we examined in Sec. 2.5, except that there is only one spreading code that is shared by all users. Spreading is used to satisfy the FCC requirement that power be spread over the ISM band. The spreading sequence used by 201.11b is a Barker sequence. It has the property that its autocorrelation is low except when the sequences are aligned. This property allows a receiver to lock onto the start of a transmission. To send at a rate of 1 Mbps, the Barker sequence is used with BPSK modulation to send 1 bit per 11 chips. The chips are transmitted at a rate of 11 Mchips/sec. To send at 2 Mbps, it is used with QPSK modulation to send 2 bits per 11 chips. The higher rates are different. These rates use a technique called CCK (Complementary Code Keying) to construct codes instead of the Barker sequence. The 5.5-Mbps rate sends 4 bits in every 8-chip code, and the 11-Mbps rate sends 8 bits in every 8-chip code.

Next we come to 802.11a, which supports rates up to 54 Mbps in the 5-GHz ISM band. You might have expected that 802.11a to come before 802.11b, but that was not the case. Although the 802.11a group was set up first, the 802.11b standard was approved first and its product got to market well ahead of the 802.11a products, partly because of the difficulty of operating in the higher 5-GHz band.

The 802.11a method is based on OFDM (Orthogonal Frequency Division Multiplexing) because OFDM uses the spectrum efficiently and resists wireless signal degradations such as multipath. Bits are sent over 52 subcarriers in parallel, 48 carrying data and 4 used for synchronization. Each symbol lasts 4μs and sends 1, 2, 4, or 6 bits. The bits are coded for error correction with a binary convolutional code first, so only 1/2, 2/3, or 3/4 of the bits are not redundant. With different combinations, 802.11a can run at eight different rates, ranging from 6 to 54 Mbps. These rates are significantly faster than 802.11b rates, and there is less interference in the 5-GHz band. However, 802.11b has a range that is about seven times greater than that of 802.11a, which is more important in many situations.

Even with the greater range, the 802.11b people had no intention of letting this upstart win the speed championship. Fortunately, in May 2002, the FCC dropped its long-standing rule requiring all wireless communications equipment operating in the ISM bands in the U.S. to use spread spectrum, so it got to work on 802.11g, which was approved by IEEE in 2003. It copies the OFDM modulation methods of 802.11a but operates in the narrow 2.4-GHz ISM band along with 802.11b. It offers the same rates as 802.11a (6 to 54 Mbps) plus of course compatibility with any 802.11b devices that happen to be nearby. All of these different choices can be confusing for customers, so it is common for products to support 802.11a/b/g in a single NIC.

Not content to stop there, the IEEE committee began work on a high-throughput physical layer called 802.11n. It was ratified in 2009. The goal for 802.11n was throughput of at least 100 Mbps after all the wireless overheads were removed. This goal called for a raw speed increase of at least a factor of four. To make it happen, the committee doubled the channels from 20 MHz to 40 MHz and reduced framing overheads by allowing a group of frames to be sent together. More significantly, however, 802.11n uses up to four antennas to transmit up to four streams of information at the same time. The signals of the streams interfere at the receiver, but they can be separated using MIMO (Multiple Input Multiple Output) communications techniques. The use of multiple antennas gives a large speed boost, or better range and reliability instead. MIMO, like OFDM, is one of those clever communications ideas that is changing wireless designs and which we are all likely to hear a lot about in the future. For a brief introduction to multiple antennas in 802.11 see Halperin et al. (2010).

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