英文资料及中文翻译Mobile CommunicationCellular Telephone SystemsA cellular telephone system provides a wireless connection to the PSTN for any user location within the radio range of the system. Cellular systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum .Cellular radio systems provide high quality service that is often comparable to that of the landline telephone systems .High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching technique called a handoff enables a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.A basic cellular system consists of mobile stations, base stations and a mobile switching center (MSC). The Mobile Switching Center is sometimes called a mobile telephone switching office (MTSO),since it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobile communicates via radio with one of the base stations and may beheaded-off to any number of base stations throughout the duration of a call .The mobile station contains a transceiver, an antenna, and control circuitry ,and may be mounted in a cuticle or used as a portable hand-held unit .The base stations of several transmitters and receivers which simultaneously handle full duplex communications and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls vis telephone lines or microwave links to the MSC. The MSC coordinates of all of the base stations and connects the entire cellular system to the PSTN.A typical MSC handles 100000 cellular subscribers and 5000 simultaneous conversations at a time, and accommodates all billing and system maintenance functions, as well .In large cities, several MSCs are used by a single carrier. Cordless Telephone SystemsCordless telephone systems are full duplex communication systems that use radio to connect a portable handset to a dedicated base station ,which is then connected to a dedicated telephone line with a specific telephone number on the pubic switchedtelephone network (PSTN).In first generation cordless telephone systems (manufactured in the 1980‟ s ), the portable unit communicates only to the dedicated base unit and only over distances of a few tens of meters.Early cordless telephones operate solely as extension telephones to a transceiver connected to a subscriber line on the PSTN and are primarily for in-home use.Second generations cordless telephones have recently been introduced which allow subscribers to use their handsets at many outdoor locations within urban centers such as London or Hong Kong. Modern cordless telephones are sometimes combined with paging receivers so that a subscriber may first be paged and then respond to the page using the cordless telephone. Cordless telephone systems provide the user with limited range and mobility, as it is usually not possible to maintain a call if the user travels outside the range of the base station. Typical second generation base stations provide coverage ranges up to a few hundred meters.Basic Knowledge of CommunicationCommunication SystemA generalized communication system has the following components :(a)In formation Source .This produces a message which may be written or spoken words, or some form of data.(b)Transmitter .The transmitter converts the message into a signal ,the form of which is suitable for transmission over the communication channel.(c)Communication Channel .The communication channel is the medium used transmit the signal, from the transmitter to the receiver. The channel may be a radio link or a direct wire connection.(d)Receiver. The receiver can be thought of as the inverse of the transmitter .It changes the received signal back into a message and passes the message on to its destination which may be a loudspeaker, teleprompters or computer data bank.Once this new baseboard signal ,a “group” of 4 channels , has been formed it is moved around the trunk network as a single unit .A hierarchy can be set up with several channels forming a “group”, several groups a “super group” and several “super group” either a “mastergroup”or “hyper group”.Groups or super groups are moved around as single units by the communications equipment and it is not necessary for the radios to know how many channels are involved .A radio can handle a super group provided sufficient bandwidth is available .The size of the groups is a compromise as treating each channel individuallyinvolves far more equipment because separate filters , modulators and oscillators are required for every channel rather than for each group .However the failure of one module will lose all of the channels associated with a group.Time Division MultiplexingIt is possible, with pulse modulation systems, to use the between samples to transmit signals from other circuits .The technique is known as time division multiplexing (TDM).To do this it is necessary to employ synchronized switches at each end of the communication link to enable samples to be transmitted in turn ,from each of several circuits .Thus several subscribers appear to use the link simultaneously . Although each user only has periodic short time slots, the original analog signals between samples can be reconstituted at the receiver.Pulse Code ModulationIn analog modulation, the signal was used to modulate the amplitude or frequency of a carrier , directly .However in digital modulation a stream of pulses ,representing the original ,is created .This stream is then used to modulate a carrier or alternatively is transmitted directly over a cable .Pulse Code Modulation (PCM)is one of the two techniques commonly used.All pulse systems depend on the analog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulations .It is not very useful in practice but is used as an intermediate stage towards forming a PCM signal .It will be seen later that most of the advantages of digital modulation come from the transmitted pulses having two levels only ,this being known as a binary system .In PCM the height of each sample is converted into a binary number .There are three steps in the process of PCM: sampling, quartering and coding .Optical Fiber CommunicationsCommunication may be broadly defined as the transfer of information from one point to another .When the information is to be conveyed over any distance a communication system is usually required .Within a communication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information signal .This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation .Sophisticated techniques have beendeveloped for this process by using electromagnetic carrier waves operating at radio requites as well as microwave and millimeter wave frequencies.Typical optical fiber communications system is shown in Fig.1-3.In this case the information source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the light wave carrier .The optical source which provides the electrical-optical conversion may be either a semiconductor laser or light emitting diode (LED).The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier .Photodiodes (P-N,P-I-N or avalanche ) and ,in some instances ,phototransistor and photoconductors are utilized for the detection of the optical signal and the optical-electrical conversion .Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually The optical carrier may be modulated by using either an analog or digital information signal.. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the length intensity are obtained (i.e. on-off pulses). Although often simpler to implement, analog modulation with an optical fiber communication system is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is mot always provided by semiconductor optical source, especially at high modulation frequencies .For these reasons ,analog optical fiber communications link are generally limited to shorter distances and lower bandwidths than digital links .Initially, the input digital signal from the information source is suitably encoded for optical transmission .The laser drive circuit directly modulates the intensity of the semiconductor laste with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable .The avalanche photodiode detector (APD) is followed by a front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction. Finally ,the signal obtained is decoded to give the original digital information .Broadband CommunicationAs can be inferred from the examples of videophone and HDTV, the evolution of future communications will be via broadband communication centered around video signals. The associated services such as video phone, video conferencing, videosurveillance, cable television (CATV) distribution, and HDTV distribution to the high-speed data services such as high-resolution image transmission, high-speed data transmission, and color facsimile. The means of standardizing these various broadband communication services so that they can be provided of standardizing these various broadband communication services so that they can be provided in an integrated manner is no other than the broadband integrated services digital network in an integrated services digital network (B-ISDN). Simple put, therefore, the future communications network can be said to be a broadband telecommunication system based on the B-ISDN.For realization of the B-ISDN, the role of several broadband communication technologies is crucial .Fortunately ,the remarkable advances in the filed of electronics and fiber optics have led to the maturation of broadband communication technologies .As the B-ISDN becomes possible on the optical communication technologies .As the B-ISDN becomes possible on the optical communication foundation . the relevant manufacturing technologies for light-source and passive devices and for optical fiber have advanced to considerable levels . Advances in high-speed device and integrated circuit technologies for broadband signal processing are also worthy of close attention. There has also been notable progress in software, signal processing, and video equipment technologies . hence, form the technological standpoint ,the B-ISDN has finally reached a realizable state .On the other, standardization activities associated with broadband communication have been progressing. The Synchronous Optical Network (SONET) standardization centered around the T1 committee eventually bore fruit in the form of the Synchronous Digital Hierarchy (SDH) standards of the International Consultative Committee in Telegraphy and Telephony (CCITT), paving the way for synchronous digital transmission based on optical communication .The standardization activities of the integrated services digital network (ISDN), which comm enced in early 1980‟s with the objective of integrating narrowband services ,expanded in scope with the inclusion of broadband services ,leading to the standardization of the B-ISDN in late 1980‟s and establishing the concept of asynchronous transfer mode (ATM) communication in process . In addition, standardization of various video signals is becoming finalized through the cooperation among such organizations as CCITT, the International Radio-communications Consultative Committee (CCIR ) ,and the International Standards Organization (ISO),and reference protocols for high-speed packet communication are being standardized through ISO, CCITT, and the Institute of Electrical and Electronics Engineer (IEEE).Various factors such as these have made broadband communication realizable. Therefore, the 1990‟is the decade in which matured broadband communication technologies will be used in cibhybctuib with broadband standards to realize broadband communication networks. In the broadband communication network, the fiber optic network will represent the physical medium for implementing broadband communication, while synchronous transmission will make possible the transmission of broadband service signals over the optical medium. Also, the BISDN will be essential as the broadband telecommunication network established on the basis of optical medium and synchronous transmission and ATM is the communication means that enables the realization of the B-ISN. The most important of the broadband services to be provided through the B-ISDN are high –speed data communication services and video communication services.Asynchronous Transfer Mode (ATM)Demand for rich media services such as Internet access ,video on demand ,digital television and voice over IP grows more clamorous every day .So ,too ,does the need for high-per-formic distribution technology .To meet this demand , service providers are turning to ATM technology –a flexible ,scalable way of moving high-speed video and data across networks .ATM‟s sophisticated bandwidth utilization capabilities enable providers to efficiently transport large ,complex video packets without taxing a network .The majority of traffic ported over the ATM infrastructure is voice and data, Video will soon be as prominent and will drive the need for more high-capacity ATM networks .The basis of ATM technology is a high-efficiency ,low –latency switching and multiplexing mechanism ideally suited to an environment in which there are specific bandwidth limitations.ATM allocates bandwidth on demand by construction virtual channels and virtual paths between source and destination points on the ATM network boundaries. These channels are not dedicated physical connections, but are permanent virtual connections or switched virtual connections that are deconstructed when no longer needed.The speed and reliability of ATM switched networks can‟t be matched by other popular WAN technologies, which are ill-equipped to transport high-performance data. However, even in an ATM environment, the nuances and peculiarities of digital video make it impractical to transport real-time video in its native uncompressed format over ATM. Using MPEG-2 sophisticated compression techniques, providers can alleviatetechnical roadblocks when managing and ensuring the integrity of large ,super –fast video streams over ATM.Local MPEG-2 video streams are typically transported via an interface known as digital video broadcast asynchronous serial interface .ATM edge devise deconstruct either an MPEG-2multiple program transport stream (MPTS) or single program transport stream to the program level and ultimately to the packet-identifier (PID) level .At the PID level., streams can be reordered and combined back into another MPTS. This process is referred to as remultiplexin. Each packet of MPEG-2 data is then tagged with a PID, a 13-bit field that identifies the association between a program ,transport stream and packet .This architecture is likely to become the predominant distribution method for rich media services.WDNEven visionaries such as Albert Einstein and lascar Newton ,who contributed significantly to our understanding of the properties of light and its fundamental importance ,would not likely imagine the communications networks of today .Highways of light span the globe ,transmitting massive amounts of information in the twinkling of an eye .The equivalent of millions of telephone calls are transmitted on a single fiber ,thinner than a human hair .Astounding as these advances may seem, we are only at the beginning of what is possible.The current explosion of traffic in the worldwide networks is ample evidence of the speed with which we are adopting new communications technologies. The growth of wireless systems and the Internet are well-documented phenomena. No matter what application it is that is generating traffic, most of this traffic will be carried by the unifying optical layer. For this reason ,the growth of various applications such as telephony (whether cellular or fixed ),Internet ,video transmission ,computer communication and database access leads directly to an increase in the demand placed on the optical network .It is very likely that the optical network placed on the optical network .It is very likely that the optical network will be used to convey large amounts of video information in the future .The most striking recent advances in optical networking have taken place in the field Wavelengths Division Multiplexing (WDM). These advances have benefited both terrestrial and submarine systems, increased available capacities by several orders of magnitude and, correspondingly reduced costs.Until quite recently, it was possible to send only one wavelength, or color, of lightalong each fiber .A lot of effort has therefore been concentrated in maximizing the amount of information that can be transmitted using a single wavelength. Commercial systems will soon be able to carry 40Gbit/s on a single wavelength, while in the labs 320Gbit/systems have already been demonstrated.WDM, on the other hand, makes it possible to transmit a large number of wavelengths using the same fiber. Effectively sending a “rainbow” of color, where there was only one color before. Already today , commercially available systems can transmit 400 Gbit/s of information on a single fiber .That is equivalent to transmitting approximately 200 feature-length films per second .Recently ,a team of researchers from Bell Labs demonstrated long-distance ,error-free transmission of 3.28 Gbit/s over a single optical fiber.The major advance that has led to the WDM revolution has been the invention of the Optical Amplifier (OA). Before the invention of the OA, after having traveled down a fiber for some distance , each individual wavelength had to be concerted into electronic form ,then back into optical form and then retransmitted into the next span of fiber .This was relatively expensive ,since the optical components involved are highly specialized devices .The OA ,however ,can boost the signal power of all wavelengths in the fiber ,thus eliminating the need for separate regenerators, and allowing many wavelengths to share the same fiber .Advances in optical amplifier design have been considerable .First ,the operating window has expanded from 12nm ,in the first generation ,to about 80 nm today .This allows the OA to amplify more signals simultaneously .Second ,the development of gain equalization techniques has enabled a much flatter response and allows a number of these amplifiers to be connected in series. There have also been advances in the fibers themselves .In the early days of optical systems ,optical fibers were not built for multi-wavelength transmission .Today‟s fiber, on the other hand ,are designed to have wide transmission windows and are optimized for high –capacity ,multiple –wave-length transmission.The growing demand on optical network is a complex issue .On the one hand ,the growth in capacity demand is extraordinary ,and this in itself would be a big enough challenge to meet .However ,this in accompanied by an increasing variety of services and applications ,as well as much more exacting requirements for quality differentiation .For example ,there is quite a difference in the quality requirement for a signal being used to transmit an emergency telephone call or live video coverage of a medical operation ,as compared with an E-mail that is not urgent and can arrive after several hours .However, the same optical infrastructure is expected to support this wide variety of services. Internet Protocol (IP) traffic, in particular, is growing exponentially .In some parts of the world, it is expected that IP will constitute the majority of traffic in the near future .Therefore ,existing networks will have to be progressively optimized to handle various types of traffic .WDM has a major advantage in this regard ,which is that the different types of traffic can be assigned to different wavelengths, as required .Fortunately, we will soon be in a position to route individual wavelengths flexibly through an optical network. Features such as add/drop and cross-connection in the optical domain are being made possible by advance in potencies. I would like to draw attention to a few recent advances in this area, Firstly, the so–called digital wrapper is in the process of being standardized in the international bodies .A second significant development is the all-optical cross-connect. Bell Labs has recently unveiled its all-optical cross connect called the Lambda Router .Based on Micro Electro Mechanical Switching (MEMS ) technology ,it consists of microscopic mirrors that tilt, and thus re-direct optical signals .It is a such technology that will enable us to build networks that are purely optical As more routing functions are implemented in the optical plane ,more sophisticated intelligence is needed to control and manage the network .Control systems are being developed for there optical routers with which it will be able to build optical networks that can be easily configured in response to demand ,and which also have self-healing properties and fast restoration times in the order of fifty to a hundred milliseconds, much the same as today …SDH and SONET networks .A further aspect to consider is access to the optical network, Most users would like to have direct access to the optical network and the enormous capacity it provides .This will take place in stages .Multi –wave –length optical systems are rapidly spreading out from the core towards the end user .In regional and metropolitan areas ,the requirements are somewhat different from the long-distance area .The dream of Fiber To The Home (FTTH) or desktop is yet to materialize ,mainly because of the cost-sensitive nature of this part of the network .In the near future ,residential access may remain copper-based ,using technologies such as ADSL to boost the capacity of traditional copper lines .However ,for business offices ,optical technology will be used to bring bandwidth to the end used . Currently, a lot of Fiber To The Building (FTTB) networks are being deployed involving ATM and SDH access equipment at customer premises. The next step is to use WDM technology for these applications. WDM will first be used in industrial and campus Local Area Network (LAN) environments.We are at the beginning of a revolution in communications networks, where increasing capacity, variety of applications, and quality of service are placing enormous demands on the optical network.. The revolution of optical network is just beginning, and is advancing very swiftly towards a future online world in which bandwidth is essentially unlimited, reliable and low-cost.Circuit Switching and Packet SwitchingThere are tow basic types of switching techniques: circuit switching and message switching. In circuit switching, a total path of connected made, and the path remains allocated to the source-destination pair (whether used or not) until it is released by the communicating parties2. The switches, called circuit switches (or office exchange in telephone jargon), have no capability of storing or manipulating user‟s data on their sage that finds its way through the network, seizing channels in the path as it proceeds4. Once the path is established, a return signal informs the source to begin transmission.. Direct transmission of the part of the subnet.In message switching, the transmission unit is a well-defined block of data called a message. In addition to the text to be transmitted5, a message comprises a header and a checksum. The header contains information regarding the source and destination addresses as well as other control information6; the checksum is used for error control purposes. The switching element is a computer referred to as a message processor7, with processing and storage capabilities. Messages travel independently and asynchronously, finding their own way from source to destination8. First the message is transmitted from the bost to the message processor to which it is attached9. Once the message is entirely received, the message processor examines its header, and accordingly decides on the next outgoing channel on which to transmit it. If this selected channel is busy, the message waits in a queue until the channel becomes free, at which time transmission begins. At the next message processor, the message is again received, stored, examined, and transmitted on some outgoing channel, and the same process continues until the message is delivered to its destination. This transmission technique is also referred to as the store-and-forward transmission technipue.A variation of message switching is packet switching. Here the message is 90broken up into several pieces of a given maximum length, called packets. As with message switching, each packet contains a header and a checksum. Packets are transmitted independently in a store-and-forward manner.With circuit switching, there is always an initial connection cost incurred in settingup the circuit. It is cost-effective only in those situations where once the circuit is set up there is a guaranteed steady flow of information transfer to amortize the initial cost. This is certainly the case with voice communication in the traditional way, and indeed circuit switching is the technique used in the telephone system. Communication among computers, however, is characterized as bursty. Burstiness is a result of the high degree of randomness encountered in the message-generation process and the message size, and of the low delay constraint required by the user. The users and devices require the communication resources relatively infrequently; but when they do, they require a relatively rapid response. If a fixed dedicated end –to –end circuit were to be set up connecting the end users ,then one must assign enough transmission bandwidth to the circuit in order to meet the delay constraint with the consequence that the resulting channel utilization is low .If the circuit of high bandwidth were set up and released at each message transmission resulting again in low channel utilization .there-fore ,for bursty users(which can also be characterized by high peak-to –average data rate requirements ) ,store –and –forward transmission tech-particular communications link only for the duration of its transmission on that link ;the rest of the time it is stored at some intermediate message switch and the link is available for other transmissions . of store-and-forward transmission tech-niques offer a more cost-effective solution, since a message occupies a particular communications link only for the duration of its transmission on that link ;the rest of the time it is stored at some intermediate message vantage of store –and –forward transmission over circuit switching is that the done on the link is available for other transmissions. Thus the main advantage of store-and –forward transmission over circuit switching is that the communication bandwidth is dynamically allocated ,and the allocation is done on the fine basis of a particular link in the network and a particular link in the network and a particular message ( for a particular source-destination pair )..Packet switching achieves the benefits discussed so far and offers added features .It provides the full advantage of the dynamic allocation of the bandwidth ,even when messages are long .Indeed, with packet switching ,many packets of the same message may be in transmission simulsl over consecutive links of a path from source to destination.。
