LAN Manager Saves The User From NetBIOS Inconsistencies

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Local Area Networks: LAN Manager

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Last week we did a quick review on the innovative aspects the Lan Manager Microsoft. It is now time to go into detail and examine its special architectural aspects, comparing them with those of its predecessor: Microsoft Networks for Ms- Dos. The task is difficult to deal with, and there we avail ourselves with the statements made by Darryl Rubin, the main designer of this new network software. Lan Manager, being a Microsoft product, was founded essentially to simplify the work of the software designer, providing the openness to multiple users that Os 2, alone, cannot offer. The Pc LAN Program of IBM, 3 + 3Com and all the various products derived from Microsoft Networks are essentially directed to allow for the sharing of files and printers, internally in a network of independent workstations. The basic qualifications of MS- Net do not allow one to write network applications easily, as Darryl Rubin admits. To develop a multi-user program under MS- Net, the programmer has to create from scratch a whole series of advanced functions that make up for the poor MS- Dos functionality in file management , printing and linking of sessions amongst different users. IBM tried, at the moment when announcing the first Pc Network, to improve this situation with an edition containing a special interface for the network known universally by the name of NetBios. Its function is to act as a buffer between the operating system of the workstation or server and the inferior network protocols (those that manage data traffic along the cable).Taking as a reference the model, Open system interconnect, which subdivides the management functions of a network into seven levels , NetBios is in the fifth level, that is the sessions level. This means that this portion of the software is responsible for establishing a logical connection between two users keeping the connection live until the exchange of information is complete. Therefore in a normal Ms- Net workstation, the messages coming from the application are intercepted by a special software entity called Redirector (which is the essential core of Ms- Net), which then determines whether the instructions are addressed to the local workstation, or to the network. In the first case the request of the application is transferred to Ms- Dos premises, in the second case, the Redirector will address itself to the network, using the service level immediately below it: the NetBios records. On the server, a similar procedure is repeated, with the only difference that above the Redirector is a special application, called the server, which works to the benefit of remote users sharing the resources of the station that is serving them. Because the software server is an application in all respects and since MS-DOS is an operating system that deals with only a single application, it becomes clear at this point, why in a Ms -Net network it is necessary to have to be dedicated the server (Lan Manager, thanks to Os 2, has surpassed this limitation ) . The fact is, however, that IBM was not content to produce a layer dedicated to session management, but has incorporated, within NetBios, many management functions, with the NetBios Extended User Interface (NetBEUI), not only of the session but also the two lower levels: transport and network. NetBios has become a de facto standard; therefore in the design of Lan Manager it has been necessary to provide a function that replicates the characteristics (at the session level), as well as a software module that acts as a Redirector. In this way, Lan Manager is compatible with all applications available today in the context of the network while also solving an important problem: the inconsistency of NetBios. With the advent of IBM PC Networks and NetBios, almost all vendors will have rushed into the compatibility race, producing their own versions of NetBios adapted to their corresponding networks. Unfortunately, almost no one has reproduced all the functions provided by IBM, with the majority merely providing the minimum interface functions in relation to the Redirector (level of elementary sessions). Consequently, anyone who develops applications for IBM LAN will find themselves in trouble when having to move their software onto other networks. The inefficiency of some Ms -Net networks was, therefore, due just to the incompleteness and mediocrity of its NetBios module. The Lan Manager architecture is designed to solve this problem in two ways: by reducing the functions of NetBios only to the sessions level (minimum common denominator among all the NetBios commercially available) and building new standard protocols for the underlying zones (transport, network, and data link). In this way, any OEM that wants to produce a NetBIOS driver for their respective network can manage to do so by writing 2 Kbytes of code, instead of 50-80 Kbytes. Furthermore, applications developed for a specific environment, will also work in other environments because Lan Manager has implemented and made many of their own functions that NetBios by IBM had and which the others were lacking. In the near future we will continue the examination of Lan Manager architecture ,scrutinizing the lower layers of NetBIOS , amongst which is included Media Access Control (MAC ), which is mainly responsible for the quantum leap in performance of the new Microsoft Networks.

Roberto Mazzoni