Mạng và viễn thông P25

Chia sẻ: Hug Go Go | Ngày: | Loại File: PDF | Số trang:12

0
49
lượt xem
7
download

Mạng và viễn thông P25

Mô tả tài liệu
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

Broadband, Multimedia Networks and the B-ZSDN The emergence of ‘multimedia’ computers and software which use all sorts of different audio, data, image and video signals simultaneously has heralded a new generation of computers and computer ‘applications’ and spurred the need to develop and deploy a new universal technology for telecommunications

Chủ đề:
Lưu

Nội dung Text: Mạng và viễn thông P25

  1. Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) PART 4 MULTIMEDIA NETWORKS
  2. Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) Broadband, Multimedia Networks and the B-ZSDN The emergence of ‘multimedia’ computers and software which use all sorts of different audio, data, image and video signals simultaneously has heralded a new generation of computers and computer ‘applications’ and spurred the need to develop and deploy a new universal technology for telecommunications. The broadband-ISDN (B-ISDN) will be this new universal technology. It will be a powerfulnetwork, capable of carryingmanydifferent types of services and communication bitrates. 25.1 MULTIMEDIA APPLICATIONS: THE DRIVER FOR BROADBAND NETWORKS A multimedia application is one in which different types of communication signal are transmitted simultaneously. Thus, for example, multimedia computers are able simul- taneously to present spreadsheets, work processing and presentation foil ‘windows’ on the screen while simultaneously also showing avideo, projecting a ‘video telephone call’ and playing a soundtrack (Figure 25.1). Multimedia applications greatly increase the possibilities of themoderncomputingandentertainmentindustries,and they are finding their way into all walks of modern life: e Teleworking 0 Telemedicine e Tele-education 0 Teleshopping 439
  3. 440 BROADBAND, MULTIMEDIA AND NETWORKS THE B-ISDN Figure 25.1 A modern multimedia computer workstation (Courtesy of Siemens A G ) The simplest type of teleworking is that allowing a company executive to log his laptop PC into the company computer network and electronic mail facility no matter what his location. Thus working at home and during time away on a business trip becomes possible. Other forms of teleworking include applications designed to bring together groups of remotelylocatedindividualsinioa workgroup(workgroupapplications). A telemedicine application might allow a group of specialist surgeons (in different locations) to carry out an emergency operation together, whde sharing sight a video of signal broadcast from the operating theatre, as well asfullreference to a patient’s medical records and historic X-rays. A distant surgeon could indicate directly on the video exactly where an incision should be made, or maybe even conduct some of the surgery himself. Tele-education offers the prospect of sharing the best possible educational facilities, information resources and equipment between a large number of dispersed locations. Thus books, video lectures and demonstrations can be available to students, even in their own homes. Multimedia applications are also flourishing in the home environment. Here, cable television companies alreadywidely offer ‘cable’ connections bringing simultaneous live television, public telephone service and video-on-demand services, as well as teleshop- ping (catalogue shopping from the television or computer screen).
  4. VIDEO COMMUNICATION 441 25.2 VIDEO COMMUNICATION The advance in video coding schemes and videocommunications has been one of the main contributing factors in the rapid emergence multimedia applications. A number of of standards have laid the basis for standardized coding of video information, provid- ing for different qualitiesand bitrates. At the low bitrate end of the scale, ITU-T recom- mendation H.261 provides a standard algorithm for a videoconference codec (coder/ decoder), suitable for coding relatively static images for transmission across low bitrate digital lines. Transmission at rates even as low as 64 kbit/s are possible, giving accept- able quality for a ‘picture telephone’ conversation or a videoconference between seated (and therefore relatively slow moving) participants in two or more different meeting room locations locations. The H.261 algorithm works in a similar manner to ADPCM (Chapter 38). In effect the whole areaof a video screenis assumed to be made up of a matrix of dots, each of a given colour (typically oneof 256 shades). Thus each dot (orpixel, picture element) may berepresented by an 8-bitdigitalvalue. Theentirepicturecan be represented by sending all the digital values corresponding to the complete picture dot matrix (or bit map like the fax image discussed inChapter 4). As a large number of dots make up the picture, it takes a little while at the start to establish the initial video frame. However, once the firstframehas been established,it is sufficient to sendtheinformation correspondingtothedifferencebetween ‘freeze frames’ to recreateallsubsequent frames. Provided that the movement of the image is relatively restricted, sending only the differences in the picture enables a much lower transmission bitrate to be used. At 384 kbit/s, fairly high quality video images can be reproduced using the H.261 algorithm, and even public television signals are easily recognizable. The quality, how- ever, does not match that of the original signal. For this reason, a number of other coding techniques are also used, where very high quality video images are to be stored and transmitted in conjunction with modern computer storage devices and telecom- munications networks. These are the standards of the Motion Pictures Experts Group ( M P E G ) , an industrycommon-interestgroup of majorcompanies,cooperating to promote standards in this area. 25.3 THE EMERGENCE OF THEB-ISDN The demands of new graphical computer software, the emergence of videotelephony and cable television, and the political pressure for the development of the Information Highway haveallcombinedintheirvarious ways tostimulatethedevelopment of switched broadband networks. Theresult has been the development broadband ISDN of (broadband integrated services digital network, or B-ISDN). We saw in Chapter 10 how the integrated services digital network ( I S D N ) was one step in the development chain a universal integrated network, and how it may replace of the conventional telephone and data networks in the nextfew years. However, narrow- band ISDN (N-ISDN) its current form falls a long short of the ultimate needs a in way of multi-service network, becauseof the relatively restricted bit rate available on individual channels and because of the lack of service flexibility arising from the fact that it evolved
  5. 442 BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN Television @ e % ?k!i\al audio Multiservice Figure 25.2 The concept of the Broadband Integrated Services Digital Network (B-ISDN) out of telephone network principles. The maximum rate currently possible on ISDN is n X 64 kbit/s, up to 2 Mbit/s. Such rates are not enough forvery high speed file transfer between mainframe computers, or for interactive switching of broad bandwidth signals such as high definition television (HDTV) and video. Increasing the unit bit rate (64 kbit/s) of ISDN would increase bit rates and make broadband and multimedia services possible, but it would be at the expense of gross inefficiency in network resources, particularly when carrying bursty data signals. So B-ISDN is not merely an extension of N-ISDN, but instead also capitalizes on thelatest data switching techniques. Current world technical standards for B-ISDN describe the general functions which are to be performed by a B-ISDN, and define the network interfaces tobe used, but the technological details are not yet fully defined. 25.4 THE SERVICES TO BE OFFERED BY B-ISDN The services offered by B-ISDN networks split into two categories: interactive services and distribution services (Figure 25.3 shows). Interactive services are normal communications between just two parties. There are three subtypes: messaging, conversational and retrieval services. An example of aconversational serviceis a telephone conversation or a point-to-point dataconnection, wherethetwoend-points of thecommunicationarein real-time connected with one another and thus converse. A messageservice is a telecommunication service similar to the postal service. A message is submitted to the network (asa letter is posted). Sometime later, the message is delivered to the given address.Message services are usually not guaranteed. The network is not able to check whether the recipient address is valid, and may return no confirmation to the sender of receipt. Thus ‘no reply’ may result either because the recipient never got the message or because he chose not to respond. A retrieval service is one in which acaller accesses a central server, database or storage archive, requesting the delivery of certain specified information. He might, for example, receive data about holiday or request news video clips, etc.
  6. THE EMERGENCE OF THE ATM SWITCHING TECHNIQUE AS THE HEART OF ATM 443 B-ISDN services 1 1 I interactive services II distribution services I I[ I I conversational service; I1 I individual individual I messaging services II retrieval services I presentation control presentation control Figure 25.3 TheservicetypesofferedbyB-ISDN Distribution services are services in which the information from a single source is distributed to many recipients at the same time. Distribution services are subdivided intothose with or withoutindividual user presentationcontrol. An example of a distribution service without individual user presentation control is the broadcasting of national television. All therecipients receive thesamesignal at the same time. An example of a distribution service with individual user presentation control is video-on- demand. Here only those users who wish to pay and receive a given film do so. 25.5 THE EMERGENCE OF THE ATM SWITCHING TECHNIQUE AS THE HEART OF ATM Much oftheinitialdevelopmentwork which led toB-ISDNstarted in thedata networking field, as engineers tried to develop fast packet switching techniques suitable for the carriage of emerging video and ‘image’ applications. The switching technique Table 25.1 Potential applications of the various B-ISDN service categories type Service Category Potential applications Interactive services Conversational services Voice telephony Messaging services Internet electronic mail Retrieval services On-line database service Distribution control services Broadcast Without user TV With user control Pay-on-view TV or video-on-demand
  7. 444 BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN developed as the basis for B-ISDN is an adaption of the packet switching technique used in datacommunications networks. It is called ATM (asynchronous transfer mode). A common failing of telecommunication switching techniques previous to ATM was their restriction to low bit rate data conveyance or fixed bandwidth circuit switching. Thus the major difficulty which had tobe overcome in developing ATM was the need to optimize B-ISDN to carry simultaneously both low and very high bit rate services, and to meet simultaneously the exacting demands and rapid signal conveyance necessary for speech and equivalent signals. In classical data packet switching networks the emphasis is placed on assured and error-free delivery of data. This rules out such networks for the carriage of live speech and video signals due to the prolonged anduneven delays experienced during propaga- tion and delivery of the user’s information. Figure 25.4 illustrates the degradation ofapacketizedspeechchannel,sharinga packet network with other speech and data users. The network subject to congestion, is so that occasional packets are held up in the buffer. The result is a rather disjointed signal, with ‘staccato’, ‘snowy’ and ‘clipping’ effects. Anotherlimitation of pre-ATMdatanetworks, whenused to carrythebitrates typically used for voice and video signals, was their relative inefficiency. When sending single packetsof a small numberof bits (eight for a single speech sample), the network is kept heavily loaded just carrying andprocessing packet headers! Furthermore the error detection and correction procedures which ensure that each packet value is received correctly are largely irrelevant for speech video signals, because theear or eye tends and to ‘fill-in’ any slight inconsistencies in the signal anyway. Despite these problems, statistical multiplexing (as used in data networks) is con- sidered to be the ideal basis for broadband networks, as large numbers of connections can be accommodated of different and varying bandwidths. A large number of packets per second give a high bandwidth for one user, whereas a smaller number of packets per second yield lower bit rates for others. Delay buffer Undelayed (normally spaced) [normally spaced) Figure 25.4 Degradation of speech on normal packet networks
  8. THE EMERGENCE OF THE ATM SWITCHING TECHNIQUE AS THE HEART OF ATM 445 In 1988, a statistical multiplexing technique called ATM was provisionally accepted by CCITTasthe switching basis forfuture B-ISDNs, and anumber of detailed CCITT(nowITU-T) recommendations were issued to define the basic technique. These provide for 0 simultaneous carriage of mixed telecommunication service 0 aconnectionestablishment contract, in which the end-user device may specify during connection set-up the required connection bitrate and connection quality parameters, including maximum permissible delay and delay variation; this is then to be guaranteed by the network for the duration of the call 0 network overload control, preventing new connections being established, and allocatingavailablecapacity to higher priority or delay sensitive (e.g. voice and video) connections first 0 a limit to the delay experienced by video, voice and other delay-sensitive signals 0 relatively high efficiency inthe use of bandwidthforall types of applications, achieved by removing error correctionmethodsin cases (e.g. voice and video applications) where these are superfluous The ‘packets’ conveyed by ATM are called cells. Like the packets of X.25, each cell of ATM consists of an information field (in which the user data is held and conveyed) and a header which ‘tells’ the network where to deliver thepacket, and providesa sequence number so that cells can be re-assembled in the correct order at the receiving end).But,unlikethepackets of X.25, the cells in ATMare all of a fixed length (48 byte payload plus a 5 byte header). The fixed length gives the scope for overcoming the limitations of earlier packet networks. The relatively short length of the cell (as compared with a normal data packet or frame) means that user data messages must be broken up into a large number of indi- vidual cells for transmission. This is inefficient, but on thepositive side the smaller cells allow a transmission priority scheme to be established. High priority cells (e.g. deriving from a voice or video connection) can nowbe inserted between the cells which make up a single data frame,rather than having to wait untilthecompleteframehas been transmitted. They thus experience significantly lower delay than they would have done in the case of a classical packet-switched data network. Unfortunately, from a voice and video perspective the 48 byte information content of an ATM cell is very large compared to the single byte samples normally transmitted. The inefficiency associated with processing cell headers militates against sending very small(e.g.one byte) cells, corresponding to individual speech samples.Instead we must make do with conveying a number of consecutive speech samples in a single cell. A packet size of 48 bytes allows 48 separate samples to be sent simultaneously. This corresponds to6 milliseconds worth of conversation (at 64 kbit/s). Thusa 6 ms propaga- tion delay is inflicted on the signal as we wait for the cell to fill up before we can send it. Nonetheless, this is asmall(andmaybeunnoticed) price to pay for high network efficiency and the scope of B-ISDN.
  9. 446 BROADBAND, MULTIMEDIA NETWORKS THE AND B-ISDN CEQ used and paid for on-demand CEQ = Customer Equipment U Figure 25.5 The basic switching capabilities of B-ISDN 25.6 CONNECTIONTYPESSUPPORTED BY B-ISDN Over and above the switching of straightforward point-to-point connections, B-ISDN (through ATM)distinguishes itself from its predecessing networks only in its ability not to switch high bitrate connections, but also in its ability to set up broadcast (i.e. mufti- cast) distributions of high bandwidth signals (e.g. video or television programmes). The broadcastcapability,meanwhile, will enablecableTVcompaniestodevelopeco- nomically efficient metropolitan fibre cable networks for the distribution of pay-TV or video-on-demand (Figure 25.5). Figure 25.6 illustrates the main typesof connections and services which B-ISDN and ATM networks will offer. The diagram also shows how ATM relates to B-ISDN. 25.7 USER DEVICE CONNECTION B-ISDN TO The user-network interface ( U N I ) illustrated in Figure25.6 is the standardized interface for connecting users end devices to a B-ISDN network for the purpose of communication. Various reference points (defined by ITU-T recommendation 1.413) are slightly different variants of the UN1 interface, according to how the device is connected to the network These are similar to the narrowband ISDN reference points, as Figure 25.7 shows. The T B interface is the basic UN1 interface by which user equipments are connected to a B-ISDN at the B-NT1. The B-NTl (broadband ZSDN network termination type l ) provides line terminating functions for the public network operator (power feeding to line, management test functions, etc.). Where the user device is connected directly to the
  10. CONNECTION DEVICE USER TO B-ISDN 441 constant bit rate (CBR) service frame relay service X.25 service ATM -- network FDDI SMDS (DQDB) switched voice service sound retrieval service video on-demand Term Meaning CBRconstantrate bit (a point-to-pointconnectionacross an ATM network providing service similar to a private wire or leaseline DQDB queue bus technology SMDS) dual dual (the behind FDDI fibre distributed data interface(a campustechnologyinterconnecting for LAN routers) NNI network-node (an interfacenetwork ATM interface) SMDS switched multimegabit data service existing (an broadband networktype) UN1 user-network (an interfacenetwork ATM interface) Figure 25.6 ATMandB-ISDN:theconnectiontypesavailable private UN1 public UN1 private local interface public network interface SB TB UB B-TE1 B-NT2 B-NT1 ' to network I [m ' " + R B-TEZ Figure 25.7 B-ISDN reference configurations at the UN1 line the U , interface isused (e.g. the case where the optical version of the UN1 is employed). a Where an electrical interface is usedat the UNI, B-NT1 maybe required to perform electrical to optical conversion of the signal for connection to a fibre network connec- tion. Alternatively, the B-ISDN network operator may choose to install a device to
  11. 448 BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN provide for network monitoring or management functions. If so, this device is also classified as a B-NT1, and the user side interfaceto it is then the UNI, reference TB. The TB interface is equivalent to the Tinterface of narrowband ISDN. The B suffix is merely to denote its connection with B-ISDN. B-NT2 (broadband ZSDN network termination 2 ) is the notation used to denote a user-provided switching device which permits multiple end user devices to share the same network connection. Thus a B-NT2 has one TB connection, but may have multi- ple SB ports. The equivalent in a narrowband ISDN is an NT2. An example of an NT2 n a) centralized B-NT2 (hub-type) configuration I B-TE1 ............................................................................................... B-NT2 i MA I B-TE1 r l rri; B-TE2 B-TE2 c) ring type multipoint configuration 8-NT2 W I W sa l I I I MA I B-TE1 i MA I I - Y A -W ' - MA I W .................................... ........................... I RTE1 I Figure 25.8 Multipoint configurations of the B-ISDN user-network interface (UNI)
  12. EVOLUTION 449 device is an office PBX, an office-installed telephone exchange which switches a large number of individual extensions and a small number of main exchange lines. By using Ss an NT2, better use can be made of the main exchange lines. The interface will be very similar (if not identical) to the TB interface. B-TEI (broadband ISDN terminal equipment type 1 ) is the notation used to denote customer equipment (CEQ) which has been developed to operate over B-ISDN (i.e. ATM) networks. A B-TE1 is a B-ISDN-compatible end device. A B-TA is a broadband ISDN terminal adaptor. This is a device provided to convert signals from non-B-ISDN-compatible end devices ( B - T E 2 ) into a format suitable for carriage over a B-ISDN (ATM network). Thus the R reference is merely anotation point used to refer to other types of end user device interfaces (i.e. any interfacenot covered by B-ISDN or ATM specifications). ITU-T Recommendation 1.413 also defines a W reference point in conjunction with a medium adaptor ( M A ) .A number of medium adaptors perform the function of a B-NT2, allowing multiple end devices to be connected to a single UN1 network connection. This is W a multipoint application(Figure 25.8). The reference pointis to be found between MAs. The technical realization of the W reference point may be a non-standard (i.e. manu- facturer proprietary) interface. The various configurations that appear in Figure 25.8 reflect considerable scope and complexity for connection to B-ISDN. However, do not let yourself be put off by the mass of jargon before you notice that the possible configurations of the UN1 mirror the classical office wiring configurations of LANs as we discussed in Chapter 19. The bustopology(Figure 25.8b) is like that of aclassicalcoaxialcable ethernet LAN. The ring topology (Figure 2 5 . 8 ~ is like that of a token ring LAN or an FDDI ) network. 25.8 EVOLUTION BROADBAND-ISDN TO The evolution of existing networks and applications to B-ISDN will be complex and frustrating, as are complicated development projects.It will be many years before we all see a fully integrated variable bit rate, broadband and multi-service network. Leading up to this time we will see the emergence of specialized broadband networks, optimized for particular applications. However, as technology develops and demand grows, a gradual migration of services from existing networks is inevitable. Cynics might say ‘what demand?’, and ‘how can we possibly wish to convey so much information?’ But cynics have been confounded before. It is only a matter of time!
Đồng bộ tài khoản