BROADBAND WIRELESS

 BROADBAND WIRELESS

We have been indoors too long. Let us go outdoors, where there is quite a bit of interesting networking over the so-called ‘‘last mile.’’ With the deregulation of the telephone systems in many countries, competitors to the entrenched telephone companies are now often allowed to offer local voice and high-speed Internet service. There is certainly plenty of demand. The problem is that running fiber or coax to millions of homes and businesses is prohibitively expensive. What is a competitor to do?

The answer is broadband wireless. Erecting a big antenna on a hill just outside of town is much easier and cheaper than digging many trenches and stringing cables. Thus, companies have begun to experiment with providing multimegabit wireless communication services for voice, Internet, movies on demand, etc.

To stimulate the market, IEEE formed a group to standardize a broadband wireless metropolitan area network. The next number available in the 802 numbering space was 802.16, so the standard got this number. Informally the technology is called WiMAX (Worldwide Interoperability for Microwave Access). We will use the terms 802.16 and WiMAX interchangeably.

The first 802.16 standard was approved in December 2001. Early versions provided a wireless local loop between fixed points with a line of sight to each other. This design soon changed to make WiMAX a more competitive alternative to cable and DSL for Internet access. By January 2003, 802.16 had been revised to support non-line-of-sight links by using OFDM technology at frequencies between 2 GHz and 10 GHz. This change made deployment much easier, though stations were still fixed locations. The rise of 3G cellular networks posed a threat by promising high data rates and mobility. In response, 802.16 was enhanced again to allow mobility at vehicular speeds by December 2005. Mobile broadband Internet access is the target of the current standard, IEEE 802.16-2009.

Like the other 802 standards, 802.16 was heavily influenced by the OSI model, including the (sub)layers, terminology, service primitives, and more. Unfortunately, also like OSI, it is fairly complicated. In fact, the WiMAX Forum was created to define interoperable subsets of the standard for commercial offerings. In the following sections, we will give a brief description of some of the highlights of the common forms of 802.16 air interface, but this treatment is far from complete and leaves out many details. For additional information about WiMAX and broadband wireless in general, see Andrews et al. (2007).

 Comparison of 802.16 with 802.11 and 3G

At this point you may be thinking: why devise a new standard? Why not just use 802.11 or 3G? In fact, WiMAX combines aspects of both 802.11 and 3G, making it more like a 4G technology.

Like 802.11, WiMAX is all about wirelessly connecting devices to the Internet at megabit/sec speeds, instead of using cable or DSL. The devices may be mobile, or at least portable. WiMAX did not start by adding low-rate data on the side of voice-like cellular networks; 802.16 was designed to carry IP packets over the air and to connect to an IP-based wired network with a minimum of fuss. The packets may carry peer-to-peer traffic, VoIP calls, or streaming media to support a range of applications. Also like 802.11, it is based on OFDM technology to ensure good performance in spite of wireless signal degradations such as multipath fading, and on MIMO technology to achieve high levels of throughput.

However, WiMAX is more like 3G (and thus unlike 802.11) in several key respects. The key technical problem is to achieve high capacity by the efficient use of spectrum, so that a large number of subscribers in a coverage area can all get high throughput. The typical distances are at least 10 times larger than for an 802.11 network. Consequently, WiMAX base stations are more powerful than 802.11 Access Points (APs). To handle weaker signals over larger distances, the base station uses more power and better antennas, and it performs more processing to handle errors. To maximize throughput, transmissions are carefully scheduled by the base station for each particular subscriber; spectrum use is not left to chance with CSMA/CA, which may waste capacity with collisions.

Licensed spectrum is the expected case for WiMAX, typically around 2.5 GHz in the U.S. The whole system is substantially more optimized than 802.11. This complexity is worth it, considering the large amount of money involved for licensed spectrum. Unlike 802.11, the result is a managed and reliable service with good support for quality of service.

With all of these features, 802.16 most closely resembles the 4G cellular networks that are now being standardized under the name LTE (Long Term Evolution). While 3G cellular networks are based on CDMA and support voice and data, 4G cellular networks will be based on OFDM with MIMO, and they will target data, with voice as just one application. It looks like WiMAX and 4G are on a collision course in terms of technology and applications. Perhaps this convergence is unsurprising, given that the Internet is the killer application and OFDM and MIMO are the best-known technologies for efficiently using the spectrum.

The 802.16 Architecture and Protocol Stack

The 802.16 architecture is shown in Fig. 4-30. Base stations connect directly to the provider’s backbone network, which is in turn connected to the Internet. The base stations communicate with stations over the wireless air interface. Two kinds of stations exist. Subscriber stations remain in a fixed location, for example, broadband Internet access for homes. Mobile stations can receive service while they are moving, for example, a car equipped with WiMAX.

The 802.16 protocol stack that is used across the air interface is shown in Fig. 4-31. The general structure is similar to that of the other 802 networks, but with more sublayers. The bottom layer deals with transmission, and here we have shown only the popular offerings of 802.16, fixed and mobile WiMAX. There is a different physical layer for each offering. Both layers operate in licensed spectrum below 11 GHz and use OFDM, but in different ways.

Above the physical layer, the data link layer consists of three sublayers. The bottom one deals with privacy and security, which is far more crucial for public outdoor networks than for private indoor networks. It manages encryption, decryption, and key management.

Next comes the MAC common sublayer part. This part is where the main protocols, such as channel management, are located. The model here is that the base station completely controls the system. It can schedule the downlink (i.e., base to subscriber) channels very efficiently and plays a major role in managing

the uplink (i.e., subscriber to base) channels as well. An unusual feature of this MAC sublayer is that, unlike those of the other 802 protocols, it is completely connection oriented, in order to provide quality of service guarantees for telephony and multimedia communication.

The service-specific convergence sublayer takes the place of the logical link sublayer in the other 802 protocols. Its function is to provide an interface to the network layer. Different convergence layers are defined to integrate seamlessly with different upper layers. The important choice is IP, though the standard defines mappings for protocols such as Ethernet and ATM too. Since IP is connectionless and the 802.16 MAC sublayer is connection-oriented, this layer must map between addresses and connections.

The 802.16 Physical Layer

Most WiMAX deployments use licensed spectrum around either 3.5 GHz or 2.5 GHz. As with 3G, finding available spectrum is a key problem. To help, the 802.16 standard is designed for flexibility. It allows operation from 2 GHz to 11 GHz. Channels of different sizes are supported, for example, 3.5 MHz for fixed WiMAX and from 1.25 MHz to 20 MHz for mobile WiMAX.

Transmissions are sent over these channels with OFDM, the technique we described in Sec. 2.5.3. Compared to 802.11, the 802.16 OFDM design is optimized to make the most out of licensed spectrum and wide area transmissions. The channel is divided into more subcarriers with a longer symbol duration to tolerate larger wireless signal degradations; WiMAX parameters are around 20 times larger than comparable 802.11 parameters. For example, in mobile WiMAX there are 512 subcarriers for a 5-MHz channel and the time to send a symbol on each subcarrier is roughly 100 μsec.

Symbols on each subcarrier are sent with QPSK, QAM-16, or QAM-64, modulation schemes we described in Sec. 2.5.3. When the mobile or subscriber station is near the base station and the received signal has a high signal-to-noise ratio (SNR), QAM-64 can be used to send 6 bits per symbol. To reach distant stations with a low SNR, QPSK can be used to deliver 2 bits per symbol. The data is first coded for error correction with the convolutional coding (or better schemes) that we described in Sec. 3.2.1. This coding is common on noisy channels to tolerate some bit errors without needing to send retransmissions. In fact, the modulation and coding methods should sound familiar by now as they are used for many networks we have studied, including 802.11 cable, and DSL. The net result is that a base station can support up to 12.6 Mbps of downlink traffic and 6.2 Mbps of uplink traffic per 5-MHz channel and pair of antennas.

One thing the designers of 802.16 did not like was a certain aspect of the way GSM and DAMPS work. Both of those systems use equal frequency bands for upstream and downstream traffic. That is, they implicitly assume there is as much upstream traffic as downstream traffic. For voice, traffic is symmetric for the most part, but for Internet access (and certainly Web surfing) there is often more downstream traffic than upstream traffic. The ratio is often 2:1, 3:1, or more:1.

So, the designers chose a flexible scheme for dividing the channel between stations, called OFDMA (Orthogonal Frequency Division Multiple Access). With OFDMA, different sets of subcarriers can be assigned to different stations, so that more than one station can send or receive at once. If this were 802.11, all subcarriers would be used by one station to send at any given moment. The added flexibility in how bandwidth is assigned can increase performance because a given subcarrier might be faded at one receiver due to multipath effects but clear at another. Subcarriers can be assigned to the stations that can use them best.

As well as having asymmetric traffic, stations usually alternate between sending and receiving. This method is called TDD (Time Division Duplex). The alternative method, in which a station sends and receives at the same time (on different subcarrier frequencies), is called FDD (Frequency Division Duplex). WiMAX allows both methods, but TDD is preferred because it is easier to implement and more flexible.

Fig. 4-32 shows an example of the frame structure that is repeated over time. It starts with a preamble to synchronize all stations, followed by downlink transmissions from the base station. First, the base station sends maps that tell all stations how the downlink and uplink subcarriers are assigned over the frame. The base station controls the maps, so it can allocate different amounts of bandwidth to stations from frame to frame depending on the needs of each station.

Next, the base station sends bursts of traffic to different subscriber and mobile stations on the subcarriers at the times given in the map. The downlink transmissions end with a guard time for stations to switch from receiving to transmitting. Finally, the subscriber and mobile stations send their bursts of traffic to the base station in the uplink positions that were reserved for them in the map. One of these uplink bursts is reserved for ranging, which is the process by which new stations adjust their timing and request initial bandwidth to connect to the base station. Since no connection is set up at this stage, new stations just transmit and hope there is no collision.

The 802.16 MAC Sublayer Protocol

The data link layer is divided into three sublayers, as we saw in Fig. 4-31. Since we will not study cryptography until Chap. 8, it is difficult to explain now how the security sublayer works. Suffice it to say that encryption is used to keep secret all data transmitted. Only the frame payloads are encrypted; the headers are not. This property means that a snooper can see who is talking to whom but cannot tell what they are saying to each other.

If you already know something about cryptography, what follows is a oneparagraph explanation of the security sublayer. If you know nothing about cryptography, you are not likely to find the next paragraph terribly enlightening (but you might consider rereading it after finishing Chap. 8).

When a subscriber connects to a base station, they perform mutual authentication with RSA public-key cryptography using X.509 certificates. The payloads themselves are encrypted using a symmetric-key system, either AES (Rijndael) or DES with cipher block chaining. Integrity checking uses SHA-1. Now that was not so bad, was it?

Let us now look at the MAC common sublayer part. The MAC sublayer is connection-oriented and point-to-multipoint, which means that one base station communicates with multiple subscriber stations. Much of this design is borrowed from cable modems, in which one cable headend controls the transmissions of multiple cable modems at the customer premises.

The downlink direction is fairly straightforward. The base station controls the physical-layer bursts that are used to send information to the different subscriber stations. The MAC sublayer simply packs its frames into this structure. To reduce overhead, there are several different options. For example, MAC frames may be sent individually, or packed back-to-back into a group.

The uplink channel is more complicated since there are competing subscribers that need access to it. Its allocation is tied closely to the quality of service issue. Four classes of service are defined, as follows:

1. Constant bit rate service.

2. Real-time variable bit rate service.

3. Non-real-time variable bit rate service.

4. Best-effort service.

All service in 802.16 is connection-oriented. Each connection gets one of these service classes, determined when the connection is set up. This design is different from that of 802.11 or Ethernet, which are connectionless in the MAC sublayer.

Constant bit rate service is intended for transmitting uncompressed voice. This service needs to send a predetermined amount of data at predetermined time intervals. It is accommodated by dedicating certain bursts to each connection of this type. Once the bandwidth has been allocated, the bursts are available automatically, without the need to ask for each one.

Real-time variable bit rate service is for compressed multimedia and other soft real-time applications in which the amount of bandwidth needed at each instant may vary. It is accommodated by the base station polling the subscriber at a fixed interval to ask how much bandwidth is needed this time.

Non-real-time variable bit rate service is for heavy transmissions that are not real time, such as large file transfers. For this service, the base station polls the subscriber often, but not at rigidly prescribed time intervals. Connections with this service can also use best-effort service, described next, to request bandwidth.

Best-effort service is for everything else. No polling is done and the subscriber must contend for bandwidth with other best-effort subscribers. Requests for bandwidth are sent in bursts marked in the uplink map as available for contention. If a request is successful, its success will be noted in the next downlink map. If it is not successful, the unsuccessful subscriber have to try again later. To minimize collisions, the Ethernet binary exponential backoff algorithm is used.

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