Kurumların veri iletişim ağı alt yapılarının yeni gereksinimlere göre geliştirilmeleri

Abstract

Günümüzde bilişim dünyası hızlı bir değişim göstermektedir. Eskiden, hiyerarşik yapıya sahip olan ve merkezcil veri işlemeye, sadece klasik veri ve ses trafiğine, tahmin edilebilir veri akışına dayalı olan iletişim sistemleri, günümüzde yerini masa üstü veri işlemeye, klasik veri, LAN verisi, ses ve video trafiğine, dağıtılmış veri iletimine, patlamalı veri akışına olanak sağlayan iletişim sistemlerine bırakmış durumdadır. Bunun sonucu olarak, iletişim ağlarından istenilen servisler değişmiş ve eski iletişim ağları beklentileri karşılayamaz duruma gelmiştir, işte bu noktada, kurumların sadece veri hizmeti veren eski tip yapıya sahip iletişim ağlarının, çoklu ortam olanağını sağlaması, çeşitli karakterlerde trafiklerin geçmesine imkan tanıması ve yüksek bit hızlarını desteklemesi zorunluluğu doğmuştur; dolayısı ile eski ağların yeniden yapılanması ve yeni kurulacak ağların ise bu kriterlere göre oluşturulması gerekliliği ortaya çıkmıştır. Bu tezde, belli bir senaryoya göre yeni bir iletişim ağı kurulması üzerinde durulmuştur. Bunun için önce, mevcut teknolojiler incelenmiş, avantaj ve dezavantajları tartışılmıştır. Daha sonra bu incelemeler ışığında, eski yapıya sahip bir iletişim ağının, yeni beklentilere ve trafiklere olanak sağlayacak şekilde oluşturulması ele alınmıştır.

The communication technology has been changing rapidly. The communication Systems that based on hierarchical architecture, central computing, legacy data traffic, voice traffic, has become to be based on distributed computing, LAN traffic, bursty traffic, voice and video traffic, due to the new requirements. As a result of this, The expected services from the networks have changed and the old networks could not satisfy the expectations. At this point of view, the old network of the organizations that only used to serve data was not capable of serving high speed, multimedia, different traffics. Thus, the need for reorganization of the old networks and having these properties for the new designed networks have raised. This thesis also explains a new network design in terms of a scenario. Discussing the advantages and the disadvantages with the current technologies has been the firs step of the study. After that, With the help of these discussions, the old network has been redesign, in terms of the new expectations and the traffics. The examined technologies are given below briefly. Frame Relay; Frame Relay is a packet transport technology suited to the telecommunication networks of the first half of the 1990s. Frame Relay provides a packet-switching data communications capability that is used across the interface between user devices (for example, routers, bridges, host machines) and network equipment (for example, switching nodes). User devices are often referred to as data terminal equipment (DTE), while network equipment that interfaces to DTE is often referred to as data circuit- terminating equipment (DCE). The network providing the Frame Relay interface can be either a carrier- provided public network or a network of privately owned equipment serving a single enterprise. As an interface to a network, Frame Relay is the same type of protocol as X.25. However, Frame Relay differs significantly from X.25 in its functionality and format. In particular, Frame Relay is a more streamlined protocol, facilitating higher performance and greater efficiency. As an interface between user and network equipment, Frame Relay provides a means for statistically multiplexing many logical data conversations (referred to as virtual circuits) over a single physical transmission link. This contrasts with systems that use only time-division-multiplexing (TDM) techniques for supporting multiple data streams. FrameRelay's statistical multiplexing provides more flexible and efficient use of available bandwidth. It can be used without TDM techniques or on top of channels provided by TDM systems. Another important characteristic of Frame Relay is that it exploits the recent advances in wide-area network (WAN) transmission technology. Earlier WAN protocols such as X.25 were developed when analog transmission systems and copper media were predominant. These links are much less reliable than the fiber media/digital transmission links available today. Over links such as these, link-layer protocols can forego time-consuming error correction algorithms, leaving these to be performed at higher protocol layers. Greater performance and efficiency is therefore possible without sacrificing data integrity. Frame Relay is designed with this approach in mind. It includes a cyclic redundancy check (CRC) algorithm for detecting corrupted bits (so the data can be discarded), but it does not include any protocol mechanisms for correcting bad data (for example, by retransmitting it at this level of protocol). Another difference between Frame Relay and X.25 is the absence of explicit, per-virtual-circuit flow control in Frame Relay. Now that many upper-layer protocols are effectively executing their own flow control algorithms, the need for this functionality at the link layer has diminished. Frame Relay, therefore, does not include explicit flow controlprocedures that duplicate those in higher layers. Instead, very simple congestion notification mechanisms are provided to allow a network to inform a user device that the network resources are close to a congested state. This notification can alert higher-layer protocols that flow control may be needed. Current Frame Relay standards address permanent virtual circuits (PVCs) that are administratively configured and managed in a Frame Relay network. Another type, switched virtual circuits (SVCs), has also been proposed. The Integrated Services Digital Network (ISDN) signaling protocol is proposed as the means by which DTE and DCE will communicate to establish, terminate, and manage SVCs dynamically. Asynchronous Transfer Mode; Asynchronous Transfer Mode (ATM) was selected by the CCITT as the basis for broadband ISDN ( B-ISDN) in 1988. Asynchronous Transfer Mode (ATM) technology is based on the efforts of the International Telecommunication Union Telecommunication XI Standardization Sector (ITU-T) Study Group XVIII to develop Broadband Integrated Services Digital Network (BISDN) for the high-speed transfer of- voice, video, and data through public networks. Through the efforts of the ATM Forum (jointly founded in 1991 by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint), ATM is capable of transferring voice, video, and data through private networks and across public networks. ATM continues to evolve today as the various standards groups finalize specifications that allow interoperability among the equipment produced by vendors in the public and private networking industries. There are seven main objectives for B-ISDN that can be achieved using ATM transport. Each of seven is described briefly. * Provide Low - and High - Bandwidth Services; In order to form the basis of B-ISDN, ATM must be capable of carrying high - bandwidth services, such as high definition TV, and also more conventional low-bandwidth services, such as voice. * Provide High - Bandwidth Transport; ATM should be capable of providing transport on high - bandwidth networks, linking central offices in the public network environment. In private networks, high - bandwidth is also required within a corporate site and between sites. * Provide a Single Network for all Services; The major attraction of ATM from the viewpoint of a public network operator is not the high bandwidth itself. It is the ability to integrate a variety of different services onto a single network using a single mode of transmission and switching. The benefit of integration is cost savings in terms of : ** reduced equipment costs, ** reduced operating costs, ** reduced maintenance costs. Xll Essentially, the aim is to gradually introduce a single ATM network as an alternative to the multiple networks that currently exist for voice, X.25 data, frame relay. * Provide LAN/WAN Integration; Prior to introduction of ATM, the protocols used to provide data transport on LANs were quite differnt frame those used over WANs. Bridges and routers are necessary to convert between one protocol and another. With the deployment of ATM in both local and wide area networks, the need for protocol conversion is reduced, resulting in significant ipmrovements in performance, particularly in terms of reduced end to end delay. * Free Users of Bandwith Granularity; Prior to introduction of B-ISDN, network operators allocated bandwith to users in discrete blocks, usually multiples of 64 Kbps ( e.g., 64 Kbps, 384 Kbps, 786 Kbps, 1.5 Mbps, 45 Mbps). The aim of ATM is to allow users much more flexibility to select any bandwith they choose to suite their application. The majority of the installed base of user equipment has ports with bandwiths designed to mach the telecommunication system to which they interface. The majority of ports are therefore at 64 Kbps, 768 Kbps, and so on. The aim of ATM is to allow more choice for designers of user equipment, so that the bandwiths can be more closely matched to the requirements of the user applicaiton. * Allow Dynamically Changing Bandwith; Another aim of ATM is to allow a user's bandwith to vary during the course of a call. This may be due to the natural variations in the traffic being sent (e.g., the burstiness of LAN interconnect traffic). Alternatively, it may be due to the user wishing to change the bandwith requirements during the course of a call (e.g., during a voice call, the user wishes to send a high resolution image to illustrate a point during a discussion). It should be noted that all forms of communication in their raw form require dynamically varying bandwith. Voices contains pauses between words, fax and still image contain areas with more detail than others, and motion video has a higher information rate when the scene involves more movement. Tehese variations are coded into constant bit rates in order to fit the requirements of fixed bandwith telecommunication channels. One of the benefits of ATM is that it removes the necessity for fixed-bit-rate coding and buffering of voice, fax, image, and video. As a result of the study, the new network will be a network such that ATM network will be built in the backbone and Frame Relay, SNA and TCP/IP networks will be built at periphery. By this structure, it is expected to get a Xlll better result in terms of performance, multiprotocol support, congestion control and bandwidth management. SNA network and TCP/IP network will be built on top of the Frame Relay network. The protocols supported over the Wide Area Network will only be SNA and TCP/IP. The network architecture is subdivided into entities, or building blocks, characterized by the network services they have to provide to the users population. The network services are, for example, the capability to support data only, voice and/or video. The network architecture building blocks essentially differ with respect to the connectivity and bandwidth functional objectives. The various requirements can be met by four categories of building blocks leading to the definition of four node types: the service location node, the hub location node, the backbone access location node, the backbone transport node The Backbone Transport Node The WAN backbone transport node provide the reliable and transparent high-speed bit transmission over the wide area network and support high speed serial upstream links. Those nodes is linked by high-speed lines to form the highway for all types of traffic, providing reliability through meshing and back-up features, and providing high-speed line sharing over long distances. The Backbone Access Node The backbone access node is where a connection end gets the required bandwidth over the transmission network (the backbone) to reach its other end, for example a compressed or uncompressed voice channel, a video channel or a data circuit. The purpose of this type of node is to isolate the connectivity functions from the transport functions to keep maximum flexibility. Such isolation is either required because the market does not always offer all the required functions in the same platform or cost justified to provide, for example, a high number of connections which have to be concentrated before the transport. The Hub Location Node The hub location node provide the network services to the end-user equipment and concentrates the traffic from subordinate locations, the XIV spoke locations. This is the first level of connection for end-users equipment. It is likely that more than one equipment is necessary to provide all the services required from the hub location nodes. Typically, a combination of a router and a multiplexer (or an ATM-based cell switch) would fulfill all the functions. The Spoke Location Node The spoke location node provide the network services to the end-user equipment in locations where a hub location node is not justified. The spoke location node is equivalent to the hub location node except that it will provide less connectivity (less connection ports, less bandwidth support) and less power because a spoke location is at a lower level in terms of business volume, hence does not need the same capacity