Showing posts with label Wireless Networks. Show all posts
Showing posts with label Wireless Networks. Show all posts
  • Cell Phone Technologies Cell phone technology seems an obvious approach to mobile computer communication, and indeed data services based on cellular standards are commercially available. One drawback is the cost to users, due in part to cellular’s use of licensed spectrum (which has historically been sold off to cellular phone operators for astronomical sums). The frequency bands that are used for cellular telephones (and now for cellular data) vary around the world. In Europe, e.g., the main bands for cellular phones are at 900 and 1800 MHz. In North America, 850-and 1900-MHz bands are used. This global variation in spectrum usage creates problems for users who want to travel from one part of the world to another, and has created a market for phones that can operate at multiple frequencies (e.g., a tri-band phone can operate at three of the four frequency bands mentioned above). That problem, however, pales in comparison to the proliferation of incompatible standards that have plagued the cellular communication business. Only recently have some signs of convergence on a small set of standards appeared. And finally, there is the problem that most cellular technology was designed for voice communication, and is only now starting to support moderately high-bandwidth data communication.

    Like 802.11 and WiMAX, cellular technology relies on the use of base stations that are part of a wired network. The geographic area served by a base station’s antenna is called a cell. A base station could serve a single cell or use multiple directional antennas to serve multiple cells. Cells do not have crisp boundaries, and they overlap. Where they overlap, a mobile phone could potentially communicate with multiple base stations. This is somewhat similar to the 802.11 picture. At any time, however, the phone is in communication with, and under the control of, just one base station. As the phone begins to leave a cell, it moves into an area of overlap with one or more other cells. The current base station senses the weakening signal from the phone, and gives control of the phone to whichever base station is receiving the strongest signal from it. If the phone is involved in a call at the time, the call must be transferred to the new base station in what is called a handoff .


    As we noted above, there is not one unique standard for cellular, but rather a collection of competing technologies that support data traffi c in different ways and deliver different speeds. These technologies are loosely categorized by “ generation. ” The first generation (1G) was analog, and thus of limited interest from a data communications perspective. Most of the cell phone technology currently deployed is considered second generation (2G) or “ 2.5G ” (not quite worthy of being called 3G, but more advanced than 2G). The 2G and later technologies are digital. The most widely deployed 2G technology is referred to as GSM — the Global System for Mobile Communications — which is used in more than 200 countries. North America, however, is a late adopter of GSM, which helped prolong the proliferation of competing standards.

    Most 2G technologies use one of two approaches to sharing a limited amount of spectrum between simultaneous calls. One way is a combination of frequency-division multiplexing (FDM) and time-division multiplexing (TDM). The spectrum available is divided into disjoint frequency bands, and each band is subdivided into time slots. A given call is allocated every n th slot in one of the bands. The other approach is code division multiple access (CDMA). CDMA does not divide the channel in either time or frequency, but rather uses different chipping codes to distinguish the transmissions of different cell phone users.

    The 2G and later cell phone technologies use compression algorithms tailored to human speech to compress voice data to about 8 Kbps without losing quality. Since 2G technologies focus on voice communication, they provide connections with just enough bandwidth for that compressed speech — not enough for a decent data link. One of the first cellular data standards to gain widespread adoption is the General Packet Radio Service (GPRS), which is part of the GSM set of standards and is often referred to as a 2.5G technology.

    GSM networks make use of a multiplexing technique called time-division multiple access (TDMA). (Confusingly, there is also a particular cellular standard that is sometimes called TDMA, but is known formally as IS-136.) You can think of TDMA as being like TDM — traditionally used for telephone services — with the additional feature that the timeslots can be dynamically allocated to users or devices that need them (and deallocated from devices that no longer need them). The number of timeslots that are available for GPRS at a given frequency depends on how many cellular voice calls are currently in progress, since voice calls also consume timeslots. As a result, GPRS data rates tend to be lower in busy cells. In practice, users often get between 30 and 70 Kbps — coincidentally, just about the same as a user of a dial-up modem on a landline. Nevertheless, GPRS has proven quite useful and popular in some parts of the world as a way to communicate wirelessly when faster connection methods (such as 802.11) are not available. Other 2.5G data standards have also become available and some manage to be quite a bit higher in bandwidth than GPRS.

    The concept of a third generation (3G) was established before there was any implementation of 3G technologies, with the aim of shaping a single international standard that would provide much higher data bandwidth. Unfortunately, at the time of writing, several mutually incompatible 3G standards are emerging. Thus, the possibility that cellular standards will continue to diverge seems quite realistic. Interestingly, all the 3G standards are based on variants of CDMA. For example, the Universal Mobile Telecommunications System (UMTS) is based on wideband CDMA (W-CDMA). UMTS appears poised to be the successor to GSM, and in fact is sometimes referred to as 3GSM (i.e., the third-generation version of GSM). UMTS is intended to support data transfer rates of up to 1.92 Mbps, although real network conditions will probably result in lower rates in practice. Nevertheless, it should represent a significant performance improvement over GPRS. And like GSM, it should have quite widespread (if not actually universal) adoption around the world.

    There are a number of commercial UMTS networks in operation at the time of writing with many more announced or planned. And to make it quite clear that work in this field is far from complete, we note that 3.5G and 4G standards are also in the works.

    Finally , it should be noted that there is a class of mobile phones that are not cellular phones but satellite phones, or satphones . Satphones use communication satellites as base stations, communicating on frequency bands that have been reserved internationally for satellite use. Consequently, service is available even where there are no cellular base stations. Satphones are rarely used where cellular is available, since service is typically much more expensive. Satphones are also larger and heavier than modern cell phones because of the need to transmit and receive over much longer distances, to reach satellites rather than cellphone towers. Satellite communication is more extensively used in television and radio broadcasting, taking advantage of the fact that the signal is broadcast, not point-to-point. High-bandwidth data communication via satellite are commercially available, but its relatively high price (for both equipment and service) limits its use to regions where no alternative is available.

    Source of Information : Elsevier Wireless Networking Complete 2010

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  • Wireless Networks - WiMAX (802.16) WiMAX , which stands for Worldwide Interoperability for Microwave Access, was designed by the WiMAX Forum and standardized as IEEE 802.16. It was originally conceived as a last-mile technology. In WiMAX’s case that “ mile ” is typically 1 – 6 miles, with a maximum of about 30 miles, leading to WiMAX being classifi ed as a metropolitan area network (MAN). In keeping with a last-mile role, WiMAX does not incorporate mobility at the time of this writing, although efforts to add mobility are nearing completion as IEEE 802.16e. Also in keeping with the last-mile niche, WiMAX’s client systems, called subscriber stations , are assumed to be not end-user computing devices, but rather systems that multiplex all the communication of the computing devices being used in a particular building. WiMAX provides up to 70 Mbps to a single subscriber station.

    In order to adapt to different frequency bands and different conditions, WiMAX defi nes several physical layer protocols. The original WiMAX physical layer protocol is designed to use frequencies in the 10- to 66-GHz range. In this range waves travel in straight lines, so communication is limited to line-of-sight (LOS). A WiMAX base station uses multiple antennas pointed in different directions; the area covered by one antenna’s signal is a sector . To extend WiMAX to near-LOS and non-LOS situations, several physical layer protocols were added that use the frequencies below 11 GHz (in the 10- to 11-GHz range, WiMAX can use either the original physical layer or one of the newer ones). Since this range includes both licensed and license-exempt frequencies, each of these physical layer protocols defines a variant better adapted to the additional interference and the regulatory constraints of the license-exempt frequencies.

    The physical layer protocols provide two ways to divide the bandwidth between upstream
    (i.e., from subscribers to base station) and downstream traffic: time-division duplexing (TDD) and frequency-division duplexing (FDD). TDD is simply STDM of the two streams; they take turns using the same frequency, and the proportion of upstream to downstream time can be varied dynamically, adaptively , by the base station. FDD is simply FDM of the two streams: one frequency is used for upstream and another for downstream. In license-exempt bands, the protocols use only TDD.

    Both channels, upstream and downstream, must be shared not just among the many subscriber stations in a given sector, but also among the many WiMAX connections that each subscriber can have with the base station. WiMAX — unlike 802.11 and Ethernet — is connection oriented. One reason for this is to be able to offer a variety of QoS guarantees regarding properties such as latency and jitter, with the aim of supporting high-quality telephony and high-volume multimedia in addition to bursty data traffic. This is conceptually similar to some of the wired last mile technologies (such as DSL) with which WiMAX is intended to compete.

    Sharing of the upstream and downstream channels is based on dividing them into equal-sized time slots. A WiMAX frame generally takes up multiple slots, with different frames taking different numbers of slots. The downstream channel (from base to subscribers) is relatively easy to subdivide into connections since only the base station sends on that channel. The base station simply sends addressed frames, one after the other. Each subscriber station in the sector receives all the frames, but ignores those not addressed to one of its connections.

    In the upstream direction, how a connection gets handled depends on its QoS parameters.
    Some connections get slots at a fixed rate, some get polled to determine how many slots they need currently, and some must request slots whenever they need them. Connections in this last category must contend to place their requests in a limited number of upstream slots set aside for contention. They use an exponential back off algorithm to minimize the chance of a collision, even on the first attempt.

    A European alternative to WiMAX is HIPERMAN, which stands for high-performance radio metropolitan area network and uses the 2- to 11-GHz range. South Korea’s WiBro (for wireless broadband) technology operates at 2.3 GHz, and is being brought into line with the emerging IEEE 802.16e standard for mobile WiMAX.

    Source of Information : Elsevier Wireless Networking Complete 2010

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  • Wireless Networks - Wi-Fi (802.11) This section takes a closer look at a specific technology centered around the emerging IEEE 802.11 standard, also known as Wi-Fi . Wi-Fi is technically a trademark, owned by a trade group called the Wi-Fi alliance, that certifies product compliance with 802.11. Like its Ethernet and token ring siblings, 802.11 is designed for use in a limited geographical area (homes, office buildings, campuses), and its primary challenge is to mediate access to a shared communication medium — in this case, signals propagating through space. 802.11 supports additional features (e.g., time-bounded services, power management, and security mechanisms), but we focus our discussion on its base functionality.


    Physical Properties
    802 .11 runs over six different physical layer protocols (so far). Five are based on spread spectrum radio and one on diffused infrared (and is of historical interest only at this point). The fastest runs at a maximum of 54 Mbps.

    The original 802.11 standard defined two radio-based physical layer standards, one using frequency hopping (over 79 1-MHz-wide frequency bandwidths) and the other using direct sequence (with an 11-bit chipping sequence). Both provide up to 2 Mbps. Then physical layer standard 802.11b was added. Using a variant of direct sequence, 802.11b provides up to 11 Mbps. These three standards run in the license-exempt 2.4 GHz frequency band of the electromagnetic spectrum. Then came 802.11a, which delivers up to 54 Mbps using a variant of frequency-division multiplexing (FDM) called orthogonal frequency-division multiplexing (OFDM) . 802.11a runs in the license-exempt 5-GHz band. On one hand, this band is less used, so there is less interference. On the other hand, there is more absorption of the signal and it is limited to almost the line of sight. The most recent standard is 802.11g, which is backward compatible with 802.11b (and returns to the 2.4-GHz band). 802.11g uses OFDM and delivers up to 54 Mbps. It is common for commercial products to support all three of 802.11a, 802.11b, and 802.11g, which not only ensures compatibility with any device that supports any one of the standards, but also makes it possible for two such products to choose the highest bandwidth option for a particular environment.


    Collision Avoidance
    At first glance, it might seem that a wireless protocol would follow the same algorithm as the Ethernet — wait until the link becomes idle before transmitting and back off should a collision occur — and to a first approximation, this is what 802.11 does. The additional complication for wireless is that, while a node on an Ethernet receives every other node’s transmissions, a node on an 802.11 network may be too far from certain other nodes to receive their transmissions (and vice versa).

    A related problem, called the exposed node problem, occurs under the circumstances, where each of the four nodes is able to send and receive signals that reach just the nodes to its immediate left and right. For example, B can exchange frames with A and C but it cannot reach D, while C can reach B and D but not A. Suppose B is sending signal to A. Node C is aware of this communication because it hears B’s transmission. It would be a mistake, however, for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission. For example, suppose C wants to transmit to node D. This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B. (It would interfere with A sending to B, but B is transmitting in our example.) 802.11 addresses these two problems with an algorithm called multiple access with collision avoidance (MACA).

    The idea is for the sender and receiver to exchange control frames with each other before the sender actually transmits any data. This exchange informs all nearby nodes that a transmission is about to begin. Specifically, the sender transmits a request-to-send (RTS) frame to the receiver; the RTS frame includes a field that indicates how long the sender wants to hold the medium (i.e., it specifies the length of the data frame to be transmitted). The receiver then replies with a clear-to-send (CTS) frame; this frame echoes this length field back to the sender. Any node that sees the CTS frame knows that it is close to the receiver, and therefore cannot transmit for the period of time it takes to send a frame of the specified length. Any node that sees the RTS frame but not the CTS frame is not close enough to the receiver to interfere with it, and so is free to transmit.

    There are two more details to complete the picture. Firstly, the receiver sends an acknowledgment (ACK) to the sender after successfully receiving a frame. All nodes must wait for this ACK before trying to transmit. Secondly, should two or more nodes detect an idle link and try to transmit an RTS frame at the same time, their RTS frames will collide with each other. 802.11 does not support collision detection, but instead the senders realize the collision has happened when they do not receive the CTS frame after a period of time, in which case they each wait a random amount of time before trying again. The amount of time a given node delays is defined by the same exponential back off algorithm used on the Ethernet.

    Source of Information : Elsevier Wireless Networking Complete 2010

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  • Wireless Networks - Bluetooth (802.15.1) Bluetooth fills the niche of very short-range communication between mobile phones, PDAs, notebook computers, and other personal or peripheral devices. For example, Bluetooth can be used to connect a mobile phone to a headset or a notebook computer to a printer. Roughly speaking, Bluetooth is a more convenient alternative to connecting two devices with a wire. In such applications, it is not necessary to provide much range or bandwidth. This is fortunate for some of the target battery-powered devices, since it is important that they not consume much power.

    Bluetooth operates in the license-exempt band at 2.45 GHz. It has a range of only about 10 m. For this reason, and because the communicating devices typically belong to one individual or group, Bluetooth is sometimes categorized as a personal area network (PAN). Version 2.0 provides speeds up to 2.1 Mbps. Power consumption is low.

    Bluetooth is specified by an industry consortium called the Bluetooth Special Interest Group. It specifies an entire suite of protocols, going beyond the link layer to define application protocols, which it calls profiles, for a range of applications. For example, there is a profile for synchronizing a PDA with a personal computer. Another profile gives a mobile computer access to a wired LAN in the manner of 802.11, although this was not Bluetooth’s original goal. The IEEE 802.15.1 standard is based on Bluetooth but excludes the application protocols.

    The basic Bluetooth network configuration, called a piconet , consists of a master device and up to seven slave devices, as shown in Figure 2.3 . Any communication is between the master and a slave; the slaves do not communicate directly with each other. Because slaves have a simpler role, their Bluetooth hardware and software can be simpler and cheaper.

    Since Bluetooth operates in a license-exempt band, it is required to use a spread spectrum technique to deal with possible interference in the band. It uses frequency hopping with 79 channels (frequencies), using each for 625 μm at a time. This provides a natural time slot for Bluetooth to use for synchronous time-division multiplexing. A frame takes up 1, 3, or 5 consecutive time slots. Only the master can start to transmit in odd-numbered slots. A slave can start to transmit in an even-numbered slot, but only in response to a request from the master during the previous slot, thereby preventing any contention between the slave devices.

    A slave device can be parked: set to an inactive, low-power state. A parked device cannot communicate on the piconet; it can only be reactivated by the master. A piconet can have up to 255 parked devices in addition to its active slave devices. ZigBee is a newer technology that competes with Bluetooth to some extent. Devised by the ZigBee alliance and standardized as IEEE 802.15.4, it is designed for situations where the bandwidth requirements are low and power consumption must be very low to give very long
    battery life. It is also intended to be simpler and cheaper than Bluetooth, making it financially feasible to incorporate in cheaper devices such as a wall switch that wirelessly communicates with a ceiling-mounted fan.

    Source of Information : Elsevier Wireless Networking Complete 2010

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