Genişbandlı şebekelerde hizmet adaptasyon protokolleri
Başlık çevirisi mevcut değil.
- Tez No: 83108
- Danışmanlar: PROF. DR. GÜNSEL DURUSOY
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 161
Özet
ÖZET Günümüzde, şebekelerin çoğu telefon, TV yayım, paketlenmiş veri iletimi gibi belirli hizmetler için kurulmuştur. Sınırlı da olsa, tek bir evrensel şebekeye ilk adım, sesin ve verinin aynı ortam üzerinden aktarıldığı N-ISDN ile atılmıştır. N-ISDN, daha yeni yaygınlaşırken mühendisler, çok hızlı gelişen bağlaşma ve optik fiber iletim teknolojilerinden faydalanarak telekomünikasyonda bir dönüm noktası olan B-ISDN*e geçişi düşünmektedirler. Tam bu noktada, ATM tekniği devreye girmekte ve yüksek bit hrzh hizmetlerin kullanıcıya sunulmasının yam sıra farklı tipteki tüm hizmetleri tek bir şebeke üzerinden sağlayabilen mimarilerin önünü açmaktadır. Birçok kuruluş, geleceğe yönelik ve geçmişle uyumlu bir çözüm arayışı içindedir. Yakm zamana kadar çok-hedefli kitle iletişim hizmetleriyle ilgili pek fazla uygulama yapılmamasına karşm, B-ISDN olanaklarıyla çok hızlı gelişebilecek bir potansiyel olduğu görülmektedir. Bu sebeple, araştırmalar ve çalışmalar, yeni işaretleşme protokollerinin geliştirilmesi ve üst seviye fonksiyonlarının uyumlaştırılması üzerinde yoğunlaşmaktadır. Başlatılan pilot projelerden elde edilen bilgiler sonucunda, akış kontrol mekanizması, ATM katman yönetimi, kullanıcı-şebeke arayüz fiziksel parametreleri ve AAL hazırlama altkatmam ile ilgili tavsiyeler oluşturulmakta; ATM şebeke performansı, hizmet tanımlan, işaretleşme protokolleri ve birleştirilmiş ATM/STM şebekelerinin çalışma şartlan belirlenmektedir. Bu çalışmada, devre bağlaşmak analog haberleşmeden paket bağlaşmak sayısal haberleşmeye geçiş özetlenmekte ve kullanıcı gereksinimlerine bağlı olarak başlayan çalışmalar neticesinde ortaya çıkan, devre ve paket bağlaşmak çözümlerin yararlı yönlerini birleştiren ve çok esnek bir yaprya sahip olan ATM sisteminin kapsamı, protokol referans modeli, temel fonksiyonları ve çeşitli hizmetler için adaptasyon protokolleri standartlar çerçevesinde incelenmiştir. Bölüm 2'de, haberleşme şebekelerinin gelişimi ve hizmete-bağk şebekeleşme yapısının getirdiği sorunlar vurgulanmış; B-ISDNnin tanımı, sunduğu hizmetler ve bu kadar hızlı gelişmesinin arkasındaki etkenler ele alınmıştır. Bölüm 3te, B-ISDN için transfer modu olarak seçilen ATMnin tanımı, ilkeleri ve şebekeleşme gerekleri ortaya konmuştur. Bölüm 4te, ATM tekniğinin mimarisi, özellikleri, önemli fonksiyonları, katman yapısı ve hizmet sınıflarına göre protokol yapılan ayrıntılı olarak incelenmiştir. Bölüm 5te, B-ISDN işletme ve bakım prensipleri, fonksiyonları, uygulanma biçimi ve şebeke yönetim sistemi açıklanmıştır. Bölüm ffda, B-ISDNde işaretleşme kavramı, fonksiyonlan ve gelişim sürecinin işaretleşme protokollerine etkisi belirtilmiştir. Bölüm 7de ise, B-ISDNde şebekeye erişim ve çeşitli hizmetler için gerekli üst seviye protokol yapıları, dönüşümleri ve parametreleri tanımlanmış; ATM-FR adaptasyonu sırasında AAL ve ATM katmanları üzerindeki veri akışı görsefleştirihrriştir. Telekomünikasyon hizmetlerinde büyük bir atılım olan ATM teknolojisine kolay bir geçiş yapılabilmesi için müşteri, erişim ve bağlaşma şebekelerindeki dönüşüm etkilerinin çok iyi hesaplanması ve planlanması gerekmektedir. Müşteri profiline bakıldığında, evlerde daha çok TV yaymı ve telefon hizmetleri; iş yerlerinde ise LAN ve internet erişimi temel haberleşme olanaklarıdır. Ama yakm bir gelecekte isteğe ıxbağlı video, evden ahş-veriş, görüntülü telefon gibi çok-ortamh hizmetler; birbirlerinden uzak mesafelerdeki yerel alan şebekelerinin bağlanması; güvenli, hwh ve ucuz veri üetirninin sağlanması için genişbandlı şebekelere ihtiyaç duyulacağının ilk işaretleri görülmektedir. Bu yeni hizmetlerin düşük maliyetle ve sağlıklı biçimde karşılanabilmesi için iletim ortamı olarak fiberin çok uygun ve hatta zorunlu olduğu açıktır. Optik fiberin bit hata olasılığının ve zayıflama seviyesinin çok düşük olması, hücre temelli iletim ile çok sayıda aboneye tüm bandgenişliğmi en verimli şekilde paylaştıran ATM şebekesinin başarısını ve avantajlarım ön plana çıkartacaktır. ATMnin şu anda kullanılan şebekeleri destekleyebSmesi ise uyumlaştırma protokolleri ve işaretleşme standartlarının hızlı bir şekilde uygulamaya geçirilmesine bağlıdır. ATMnin ilk kullanıma gireceği alanlar ticaret ve bilim sektörleri olacaktır. X.25 paket bağlaşmasının yerini alan çerçeve aktarma ve 802.6 DQDB-MAN hizmetlerinin önümüzdeki yıllarda ATM şebekelerine geçmesi beklenmektedir.
Özet (Çeviri)
SUMMARY Virtually everyone uses the phone nowadays as an efficient communication tool. It provides us with the capability of exchanging information between different places around the world; it is quick because of its real-time operation and it is easy to use. These assets, namely global availability, user-friendliness and short response times have made telephony the communication evergreen. Engineers developing integrated broadband network concepts and products are dreaming that their ideas will be successfull as plain old telephony was. Of course, not all experts are yet convinced that this will actually happen, nor are potential customers, who often do not even know what powerfull communication tools the future will offer them. At present, most networks are dedicated to specific purposes like telephony, TV distribution, circuit-switched or packetized data transfer. Some applications, such as facsimile, make use of the widespread telephone network. Using pre-existing networks for new applications may lead to characteristic shortcomings, however, as such networks are not usually tailored to the needs of services that were unknown when the networks were implemented. So data transfer over the telephone network is confined by a lack of bandwidth, flexibility and quality of analogue voice transmission equipment. Telephone networks were engineered for a constant bandwidth service, therefore using them for variable bit rate data traffic reqires costly adaptation. Since in general the public telephone network was not able effectively to support non-voice services to the extent that was reqired by the customer, other dedicated networks arose, such as public data networks or private data networks connecting a big company's plants or several research institutes. An example of a large data network is the Internet. A first step towards a single universal network, is the introduction of narrowband ISDN (N-ISDN) in which voice and data are transported over a single medium. The N-ISDN concept laid down in the 1984 recommendations has since been further elaborated; its evolution is documented in the 1988 CCTTT Blue Books, later ITU-T recommendations. N-ISDN cannot transport TV signals so it can be considered as being rather limited due to its limited bandwidth capabilities. N-ISDN is being implemented in this decade. Their benefits for the user and network provider include:. common user-network interface for access to variety of services,. enhanced (out-of-band) signalling capabilities,. service integration,. provision of new and improved services. Telephony is one of its main applications; in fact, N-ISDN is based on the digitized telephone network. N-ISDN is an all-digital network; all source information is transmitted and switched as digital signals end to end. Thus a common signal transfer mechanism can be employed in the network to serve several applications that are quite different in nature, such as voice and data transmission. N-ISDN seems to be a naturel development of plain telephony with good prospects of success as well. xiThe highest bit rate N-ISDN can offer to the user is 1.5 or 2 Mbit/s, that is the HI channel bit rates. Connection of local area networks (LANs) or transmission of moving images with good resolution may require considerably higher bit rates. Consequently, the conception and realization of a broadband ISDN was desirable. Integrated broadband networks are being conceived as an extention of 64 kbit/s based N-ISDN. Broadband integrated services digital network (B-ISDN) will add a tremendous new feature: considerably higher bit rates will be available for fast data transmission and these bit rates may be demanded per connection with great flexibility in choosing the value actually needed. This study will address the following items:. What broadband capabilities are and where they are needed.. The main ingredients of B-ISDN: asynchronous transfer mode (ATM) and optical transmission.. ATM networking: virtuel paths and viruel channels.. User-network access configurations, interfaces and protocols.. How to evolve towards B-ISDN. In principle, B-ISDN should be suitable for both business and residential customers. Thus as well as data communication, the provision of TV programme distribution and other entertainment facilities has to be considered. B-ISDN will support services with both constant and variable bit rates, data, voice, still and moving picture transmission and multimedia applications which may combine data, voice and picture service components. Another salient feature of B-ISDN is the cost-effective provision of high speed data links with flexible bit rate allocation for the interconnection of customer networks. The residential B-ISDN user may appreciate the combined offer of text, graphics, sound, still images and films giving information about such things as holiday resorts, shops or cultural events, as well as interactive video services and video on demand. Of all solutions proposed today it would seem that the technology that could best answer these demands is ATM which is considered the ground on which B-ISDN is to be built. The words“transfer mode”are used by ITU-T to describe a technique which is used in a telecommunication network, covering aspects related to transmission, multiplexing and switching. In today's telecommunications, the enormous range of speeds required is not the only problem. The biggest problem is that transmissions occur at statistically random intervals. Voice is time-sensitive so its transmission must be synchronous and the bandwidth required for a voice conversation in digital communication is relatively small and constant. Data is statistical in nature and therefore, ideally the data communication channel should be as flexible as possible to allow big bursts to take place without the obligation of the user to purchase committed bandwidth to handle the peak. Also the information is extremely error-sensitive, so extreme caution must be taken in transmission. Another type of traffic for a modem network is video and its transmission is synchronous. The amounth of bandwidth varies from a few kbit/s to several Mbit/s. Error control should be very tight. So what is to be done if we want to combine data, voice and video on the same link? The solution is rather simple, to use fixed and relatively short packets. This way the delays produced by each packet are going to be short and probably fixed; so if voice and video traffic can be assured priority handling, they can be mixed with data without diminishing any reception quality. xnThis is where ATM fits. ATM is a transmission technology that uses fixed- size packets called cells. The cell size is small, just 53 bytes, which has led some experts to think of ATM cells as just bigger bits. But the cells include a basic packet. header, and have a variety of transmission modes (unicast, multicast, best-effort delivery, guaranteed delivery) that allow cells to act like small packets. The truth seems to be that ATM should be thought of as big bits and small packets, and which description is more accurate depends on the context. For instance, at high bandwidths (a few 100 Mbit/s or faster) ATM cells look very much like bits. Each cell takes a neglible amount of time to send, and is too small for a sending or receiving computer to handle efficiently. But at low bandwidths (64 kbit/s or slower) cells start to look big. A cell takes over 6 ms to transmit, a long time in a world where processors perform an instruction every few nanosconds. If one assumes that cells for a given conversation on an ATM network will be nicely spread out, then one can model ATM performance in a fashion similar to the way we currently model telephone network performance. However, if one assumes that related cells clump together then the telephony models generally fall apart. Data trafic appears to follow fractal models rather than the more comfortable Poisson models. ATM lies in this middle ground because it is an attempt to combine the best features of telephone and data networks into a coherent link-layer protocol. What makes ATM so exciting to many people is that it has apparently partially succeededin merging two approaches. One of the things that makes ATM so challenging is that, to understand it, one needs an appreciation of both telephone and data networking. The ATM standards available for implementation have matured in the past four years. Though still not complete, a considerable number of new issues have been tackled and at least partly resolved. This covers mainly operation and maintenance of ATM-based networks, signalling scenarios and procedures, resource management, ATM adaptation layer type 5. The full set of possible B-ISDN services can not be offered from the very beginning of ATM networking for several reasons. First, most of the envisaged services are not completely defined. Second, an implementable subset of network services has to be identified that will attract customers. These services must include existing applications. However, new ATM-specific features should be visible for marketing reasons. The basic service of an ATM network is the transport and routing of ATM cells. This ATM bearer service is also named cell relaying. The network does not need to know anything about the end-to-end application which is running on an ATM connection. Users can employ this ATM service to exchange data, voice, pictures or a combination across the network. This will stimulate users to experiment with new applications such as multimedia via ATM. The practical exprience gained from such trials will assist in eventually defining appropriate service characteristics. Early ATM implementations serve as backbone networks mainly for data communications. Therefore existing or upcoming data services, such as X.25, frame relay and switched multi-megabit data service (SMDS) have to be supported. This support, in terms of adaptation/interworking equipment and interworlring protocols, will be specified in more detail in chapter 7. SMDS was introduced by Bellcore as a high-speed, connectionless packet-type data service at bit rates up to 45 Mbit/s and subsequently 155 Mbit/s. SMDS uses the ITU-T-defined address shceme of Recommendation E.164 to support global addressing. SMDS will first be run on top xuiof a distributed queue dual bus (DQDB) metropolitan area network, but later on can also use an ATM network. The European version of SMDS is called connectionless broadband data service (CBDS). Frame relay is an enhanced packet-type service. Higher throughputs and less delay are achieved by reducing error control and forgoing end-to-end flow control in contrast to X.25. That's why the frames are passed in a relay fashion, very fast, from switch to switch, with only three questions asked:. Is the routing information in the frame intact?. Is the Data Link Connection Identifier (DLCI) known?. Is the node congested, and if so, is the frame eligible for discard? Should the answer to any of these questions be conducive to discarding the frame, that action is taken and no notification about it takes place. Frame relay frames have very little overhead (seven bytes). However, because frame length vary, their transit through the switch ports suffers variable delays. Therefore mixing data, voice and video is not recommended. ATM acts just like frame relay and does not protect data from errors. ATM relies on user equipment error control and therefore works well on digital lines with low bit error rates. The cells are transmitted over a permanent virtual connections (P VC) established/released by network management, whereas at a later stage switched virtual connections (SVC) established/released via signalling procedures will follow. While the cells carry the information some will get lost due to noise or equipment failure, others due to congestion. Therefore, various types of traffic generators with their different requirements have to carefully prepare or adapt their messages for travel over the ATM network. This is done in each case by a piece of software or firmware called ATM adaptation layer (AAL). The AAL has two stages:. A service-dependent sublayer called convergence sublayer (CS),. A service-independent segmentation and reassembly sublayer (SAR). The CS assures the necessary error control and sequencing as well as the sizing of information. The SAR chops the CS message into the 48-byte payload packets and attaches them to the five-byte header. There are four types of adaptation layer services; AAL type 1 prepares voice traffic, AAL type 2 is intended for video traffic, AAL type 3/4 prepares connectionless data communication like SMDS and AAL type 5 is used for connection-oriented services such as frame relay. The higher layers of the user plane comprise all the service-specific protocols which are necessary for end-to-end communication. The higher layer protocols should be independent of the protocols used by the underlying layers. The need for fast and effective interconnection of LANs was the main incentive for the development of frame relay (FR). It allows statistical multiplexing of several connections, thus taking into account the bursty nature of typical LAN traffic. However, frames with bit errors and invalid routing information are discarded at FR network elements. The endpoints of a connection are responsible for detecting lost frames and have to initiate retransmission when required. This is to be done by higher layer protocols. FR is a packet-based technology and therefore similar to ATM. The main difference between the technologies is the variable length of the frames in FR as compared with the fixed-sized cells in ATM. The high degree of commonality between FR and ATM facilitates interworking. Three scenarios have been identified for the support of FR by B-ISDN in ITU-T Recommendation 1.555:. Frame relay service function (FRSF) inside B-ISDN, xiv. FRSF outside B-ISDN,. Frame relay protocol support via ATM bearer capability. For the adaptation of frame relay, AAL type 5 is used operating in message mode without the corrupted data delivery option. For the emulation of FR by and for interworking with B-ISDN, the frame relaying service-specific convergence sublayer (FR-SSCS) has been defined. This sublayer preserves the FR-SSCS-SDU sequence integrity. Interworking functions perform the Q.922 core operations and remove the flags and frame check sequence (FCS) from the original FR frame. The mapping of other indicators such as discard eligibility (DE) to cell loss priority (CLP) and forward/backward explicit congestion notification (FECN/BECN) to congestion indication in the ATM cell header is explained in ITU-T Recommendation 1.555. ATM represents a major step forward with respect to the telecommunication services offered, compared with the existing services. However, since most of the today's installed terminals and telecommunication networks are not ATM-based, a careful introduction plan must be worked out, to guarantee a smooth transition from the current situation to the final target solution, in which all terminals have a direct ATM interface and where all services are transported via the ATM network. To reach this final solution, ATM will be introduced in several places of the network, like the customer's network, the access network and the switching network. The introduction phases of these 3 solutions can occur at a different moment in time for each specific part of the network. One can for instance start with the introduction of ATM in the private customer's network without an ATM access or switching network. In this case only local ATM high speed switching functionality is offered. One can imagine the use of ATM only in the access network, where only the existing services of the access network are multiplexed on a single ATM access network and where the different services are further in the network directed to the proper network- So terminals and the switching network remain unmodified. Or, one can also imagine a public ATM switching system without private ATM systems, offering wide area broadband interconnectivity for all existing services. Residential customers today are mainly interested in plain old telephone service and TV distribution. Since some years, there is also a growing interest in services like video on demand, home shopping, games. In order to prepare the access network for these new services and to offer the existing services at a lower cost, several operators have plans to start deployment of fiber in the loop (FTTL). On the other hand, business customers today are frequently using a PABX for voice; routers and LANs for data. They are interested to get the different types of information across the wide area network. It is also expected that they will install in the future an ATM-LAN and will thus need a B-ISDN public interface. Therefore, a local access architecture can be deployed which is fully future safe and is not more expensive than the existing services. FTTL system is well suited for both provision of today's narrowband services and for use as B-ISDN local loop to offer future B-ISDN services like SMDS, frame relay, ATM, video on demand. This access architecture are a cell based ATM transport, sharing the total bandwidth over multiple subscribers in the access network. It is however clear that, if the introduction phase of these 3 individual network elements can optimally be aligned in time, the interworking problems can be reduced, thereby reducing the cost and enhancing the chances for an early success of the broadband ATM solution. xv
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