This practical guide to IBM’s System Network Architecture (SNA) discusses the basic principles of SNA, namely how IBM machines and compatibles can be interconnected over short as well as long distances. The SNA architecture and protocols are discussed in sufficient detail that the book would be useful to both a beginning SNA user (as a textbook) and an advanced SNA programmer (as a practical handbook). Although one can probably find most of the SNA information in numerous IBM documents, Kapoor has done an excellent job in assimilating this information from numerous sources and then extracting and presenting the useful information in a unified, logical, and easy-to-comprehend manner. For the book’s intended audience, a prerequisite is an undergraduate introductory course on computer networking or familiarity with equivalent material.

The book is well organized, well written, and well edited. It consists of 14 chapters in five parts. It also contains five appendices and a bibliography of SNA documents.

Part 1, which contains the first two chapters, sets the stage for the rest of the book. It provides background information on IBM products, the need for teleprocessing, and the evolution of SNA. Kapoor discusses the layered architecture of SNA, the data structures SNA employs, and how SNA layers compare with OSI layers. The author continues with the logical structure of SNA, that is, how the upper layers can be considered to form the network addressable units (NAUs) and how the lower layers form the path control (end-to-end) network. Fundamentals of NAU types, subareas, SNA sessions, and networks spanning multiple domains are outlined. These concepts are elaborated on in the remainder of the book.

Two chapters--one primarily on data link control and another on path control--make up Part 2, “Transport Architecture in SNA.” This part covers the path control network, the three lower layers in the SNA architecture. Little is said about the physical layer. The fundamentals of layer 2, synchronous data link control (SDLC), why it was chosen, and specific information on some of its operational parameters in SNA are discussed in one chapter. The other chapter is devoted to path control (layer 3). Items discussed include SNA’s routing philosophy; related terminology and concepts, such as boundary network node, intermediate network node, virtual route, and explicit route; and the supporting data structures required.

Part 3, which consists of three chapters, is on “Upper Layers, PU/LU Types, and Control Sequences.” Two major protocols are discussed--chaining (congestion control via the use of message sizing, fragmentation, reassembly, and sequence numbers) and session pacing (better known as admission control in the networking literature). A chapter is devoted to the specifics of physical units (PUs), which are NAUs for performing network management and control; logical units (LUs), which serve the user as ports into the SNA network; their various types; and how they interact. An additional chapter is devoted to SNA commands and flow sequences for starting a single-domain system and for activating its various resources (links, routes, PUs, and LUs) and sessions.

Part 4, “Implementation, Multiple Systems Networking (MSN), and Management,” contains three chapters, the first two of which are devoted to SNA implementation details and to SNA networking across multi-domain configurations. Particularly interesting is the chapter on network management and how IBM’s NetView product performs this function.

An additional four chapters form Part 5, “APPC, Gateways, and Trends.” A chapter on SNA gateways discusses, among other items, SNA-to-IBM Token Ring Gateway, SNA-to-X.25 interface, and internetworking between SNA and other international networking standards. Support for distributing processing in SNA is discussed in another chapter, which covers LU version 6.2, which is also known as Advanced Program-to-program Communications (APPC), and PU version 2.1, which can support host-independent sessions and is also known as Low Entry Networking (LEN). Another chapter discusses APPC transaction programs that communicate with the APPC through an Application Program Interface (API); details of the API are also given. Future trends, namely what capabilities SNA is expected to offer and how it is expected to be affected by evolving networking concepts and standards, are discussed in the book’s final chapter.

Five appendices are provided, one each on “SNA Headers and Trailers,” “SNA Commands,” “Network Management,” “APPC and LEN Formats,” and “Acronyms.” The appendix on acronyms is particularly helpful because the SNA world uses so many of them.

Although my impressions of the book are positive, it could have been improved in certain areas. For those who know about other network architectures but are new to SNA, it would have been beneficial to relate SNA terminology and concepts to their equivalents that are generally accepted in the networking literature (for example, admission control versus session pacing, or network management terminology while discussing the various elements of NetView). Some basic concepts are undefined, such as “permissions” (p. 54), “load balancing” (p. 113), and “expert system” (p. 281), and the reader is expected to know about them. A few acronyms are undefined. The illustrations are generally good, but information is not easy to extract from some of the computer outputs. Finally, I hope that the author does not turn readers off by being a little too critical of SNA.

Overall, Kapoor seems to have done an excellent job in assembling the material for this book, based on his vast experience as an SNA developer and teacher. I recommend this book highly for beginning as well as advanced SNA users.