Lokal alan şebekelerinin köprüler kullanılarak birbirine bağlanması
Interconnection of local area networks via bridges
- Tez No: 19272
- Danışmanlar: PROF.DR. GÜNSEL DURUSOY
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 122
Özet
ÖZET Bu çalışmada Lokal Alan Şebekelerinin ISO (International Standardization Organization) terminolojisinde MAC altkatmanı aktarıcıları olarak adlandırılan köprüler (bridges) kullanılarak birbirine bağlanması incelenmiştir. Bir işletmede çeşitli nedenlerle birden fazla sayıda LAN kurulmak istenebilir. Bunun sebepleri arasında, şebekenin kapsadığı alanın genişletilmesi, şebekeye bağlanabilen istasyon sayısının artırılması, işletmenin büyümesi, gövenliğin artırılması sayılabilir. Birden fazla lokal alan şebekesine sahip bu tür işletmelerde farklı şebekelere bağlı kullacıların birbirleriyle haberleşmelerini sağlayacak yöntemlere gereksinim vardır. Bu tür şebekeleri birbirine bağlamak için köprü olarak adlandırılan aktarıcılar kullanılabilir. Köprüler basit köprü, öğrenen köprü ve kaynaktan yöneltme köprüsü olarak üçe ayrılabilir. Köprülerin standartlaştırma çalışmaları IEEE 802.1 komitesi tarafından yapılmıştır. Bu çalışma sonucunda saydam köprüleme ve kaynaktan yöneltme köprüleme yöntemleri standart yöntemler olarak kabul edilmiştir.
Özet (Çeviri)
SUMMARY INTERCONNECTION OF LOCAL AREA NETWORKS VIA BRIDGES About ten years ago, advanced computer and workstation users began to realize that a local communication system was required to intercconnect diverse data processing equipment to enable better sharing of the resources available in an establishment. This idea led to the development of whwt has become known as local area networks (LAN's), systems that typically employ a shared medium and a unique protocol to access this medium. Initially, there was much enthusiasm among engineers to solve the local networking problem with a single LAN technology, but for various technical and commercial reasons, it soon turned out that a plurality of LAN media and access protocols was unavoidable and even desirable. This created the situation we are facad today: the existance of multiple diverse LAN's within one organization, each of servicing a specific operation or functional group. This situation is one of the reasons why interconnection of LAN's has become an important issue. Multiple LAN's within one organization need to be interconnected so that all users can communicate with all others if necessary. Local Area network products available today do not conform to a single standart. They exist on a variety of media, such as twisted pair, coaxiel cable, and optical fibre; use two signalling schemes (baseband and broadband); and work with a variety of media access techniques, such as carrier sense with collision detection (CSMA/CD), token passing, and so on. There is wide support among all segments of the computer industry for the Institute of Electrical and Electronics Engineers (IEEE) 802 standarts, and it is expected that within the next few years most LAN products will conform to one of these standarts. Currently, IEEE 802 standarts support the following schemes: VI i- CSMA/CD on baseband cable - CSMA/CD on broadband cable - Token-passing bus on broadband cable - Token-passing ring on baseband cable Although they do not offer a single scheme, the IEEE 802 standards have produced the number of options for new products. A number of LAN's may coexist in a single organization for several reasons: - Number of stations - Security - Area of coverage - Media access schemes - Organizational growth - Maintenance A class of devices known as bridges can be used to interconnect local area networks. A bridge (also referred to as Data Link relay) is a device which interconnects LAN's and allows stations connected to different LAN's to communicate as if both stations were on the same LAN. The collection of LAN's and bridges is referred to as an extended LAN. Any locol network (or a collection of local networks) that carries traffic between two other local networks operates as a backbone with respect to these networks. Bridges differ from devices such as amplifiers and repeaters in that they are intelligent filtering devices which store-and-forward frames. Bridges, therefore, can interconnect LAN's. Repeaters, on the other hand, are used to interconnect cable segments within a LAN. Bridges connecting LAN's have useful properties such as: - Traffic Filtering: Bridges isolate LAN's from traffic which does not need to traverse that LAN. Because of this filtering the load on a given LAN can be reduced, thus improving the delays experienced by all users on the extended LAN. vi 11- Increased Physical Extent: The extended LAN cover a larger extent than an individual LAN. - Increased Maximum Number of Stations: Since the bridge contends for access to the LAN as a single station, one bridge may represent many nodes on another LAN or extended LAN. - Use of Different Physical Layers: Bridges allow a variety of physical media to co-exist in the same extended LAN. - Interconnection of Dissimilar LAN's: It is possible to build a bridge in which its LAN's are dissimilar (with some constraints}. About the interconnection of LAN's each of the nine combinations of 802.x to 802. y has its own unique set of problems. Each of the LANs uses a different frame format. There is no valid techninal reason for this incompatibility. It is just that none of the corporations supporting the three standarts (Xerox, GM and IBM} wanted to change theirs. As a result any copying between different LANs require reformatting, requires a new chechsum calculation and introduces the possibility of undetected errors due to bad bits in the bridge's memory. None of this would have been necessary if the three committees had been able to agree on a single format. A second, and far more serious problem, is that interconnected LANs do not necessarily run at the same data rate. Each of the standarts allow a variety of speeds. A third problem is that, all three 802 LANs have a different maximum frame length. For 802.3 it depends on the parameters of the configuration, but for the standart 10-Mbps systen it is 1518 bytes. For 802.4 it is fixed at 8191 bytes. For 802.5 there is no upper limit, except that a station may not transmit longer than the token holding time. With the default value of 10 msec, the maximum frame length is 5000 bytes. Bridges can be classified in three groups: simple bridge, learning bridge, source routing bridge. IXIn a simple bridge, the address table is static, based on a priori knowledge of station addresses. Filtering performed for purposes of access control also relies on static tables. Simple bridges have the prşmary advantage of simplicity and speed. Their singular advantage is the lack of flezibility - simply moving a station from from ona network to another may require changing the configurations of all bridges in the extended network. A learning bridge, on the other hand, modifies its address table dynamically as each packet is received. Station locations relative to the location of the bridge are also learned so that traffic is not forwarded onto a link unnecessarily. Should a station address be unknown, the bridge will flood all links other than the one from which the packet was received. Learning bridges rely on a loop free topology, i.e., a graph with no cycles. This type of bridge is self-configuring, requiring a priori knowledge only of its unique MAC address and a relative priority in the set of bridges bound by the extended local network. A bridge protocol is employed to compute the topology, known as a spanning tree, and to propagate parameters, e.g,. path cost, timer values. IBM has specified a bridging technology for routing frames through a multiple-ring local network. The route is determined by the source station foreach frame sent through one or more bridges to the destination station. The routing information is contained within each frame. This information is used by each bridge to determine the next path the frame should follow. Source routing is acquired by the originating station by employing a protocol that searches through the network for the destination station. An originating station sends to the address of the destination. If the destination is not on the local ring, the originator sends an all-rings broadcast TEST or XID frame. Bridges creste copies of the test frame in search of the destination. Each copy is updated with the source address of each bridge that received it. As the test frame makes it way through the extended ring, many such copies may exist. As each copy is received by the destination station, it is sent back to the source station by the same route it took originally. multiple routes are examined and a preferred one is selected by the source station.IEEE 802.1 committee began its work on MAC bridges in 1984. This committee has produced two draft MAC bridge specifications. The first, which forms Part D of Draft IEEE Standard 802.1, defines a bridge that can be used to interconnect LAN segmants based on any of the IEEE 802 MACs. The second, which is in the form of an additional chapter to be added to IEEE Standard 802.5, defines a bridge that is designed specifically to interconnect token-ring LAN segments. Xi
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