LAN Standards
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DATA PRO Data Networking 2770 1 Standards LAN Standards In this report: Datapro Summary IEEE 802.3 No single technological trend other than the development and proliferation of the PC has had (CSMNCO) .......................... 3 the impact upon our work habits and styles as have local area networks. It was not that long ago when we viewed PCs as sophisticated replacements for typewriters. Networking PCs IEEE 802.4 was something ever distant-the cost of networking was prohibitive. But those costs have (Token Bus) .......................... 6 plummeted. An Ethernet connection, which in 1982 cost approximately $2,000, is often less than $150 today. The development of LAN standards has fueled PC networking even more IEEE 802.5 than the technology itself. (Token-Ring) .....•..•............... 6 IEEE 802.2 (Logical Link Control) ................................ 9 Introduction (A Little are implementations of Arcnet that operate at IEEE 802.6 (Metropolitan 20M bps, an order of magnitude greater than the Area Network) ...................... 9 History) original2.5M bps LAN. Efforts to have the Arc During the mid-to-Iate 1970s, a small company net implementation adopted as a "de jure" stan ANSI Fiber Distributed in Texas developed a capability to provide ac dard have finally borne fruit in the form of Data Interface (FOOl) ••...•... 10 cess to shared direct access storage device-disks "ANSI standard" status. (DASDs) from microprocessor workstations lo Unfortunately, however, the patient has be Synchronous Optical cated within a reasonable distance from the come almost comatose. In the past year, only Network (Sonet) .....•.....•..... 10 DASD. The company was Datapoint Corp. and 300,000 Arcnet adapters have been sold-com the technology was Arcnet-Attached Resource pared to 8 million Ethernet and 2 million token Computer Network. At about the same time, Xe ring adapters. The "multivendor breeding" of rox Corp. was developing its experimental Eth Ethernet, along with its highly flexible cabling ernet. These were the first LANs to be offered as options, has led to a stronger, more widely ac commercial products. ceptable product. Arcnet, on the other hand, with In the 1980s, with the near demise of its pedigree status, seems to have passed its Datapoint, and Xerox's alliance with Digital peak. Equipment Corp. and Intel, the IEEE had little With the ever increasing popularity of LANs, choice when it adopted an "Ethernet-like" ap new requirements have developed calling for proach for its local network standard. Other or higher-speed networks of greater geographic ganizations, such as General Motors and IBM, range. In response to these requirements, ven had their own ideas about what the ideal LAN dors have developed alternatives to the standard should look like. The result was a family of LAN implementations, and standards bodies such as standards to be known as the IEEE Project 802 ANSI have produced Fiber Distributed Data in LAN standards. terface (FOOl) while the IEEE, despite several Despite the 802 "steamroller," Arcnet did false starts, has finally adopted its IEEE 802.6 not quite disappear. Licensees continued to make Metropolitan Area Network (MAN). Arcnet interfaces, but now focused on the PC as The bottom line is that while there are cur the workstation. Today, Arcnet continues to en rently numerous standard implementations, joy a useful life as a mature and highly func there are also proprietary implementations from tional proprietary implementation. In fact, there leading vendors, which have become "de facto" standards, and may suit one's needs very well. As long as technology continues to improve, en trepreneurs will continue to come up with "bet -By Michael L. Rothberg ter ways." Some of these "better ways" will President Applied Network Solutions, Inc. C 1994 McGrllW-Hili. Incorporated. Reproduction ProhibHed. FEBRUARY 1994 Datapro Information Services Group. Delran NJ 08075 USA 2 2770 LAN Standards Data Networking Standards Figure 1. IEEE Project 802 Working Groups I OSI .802.1 Internetworking Layers The IEEE 802 standards address the two lower layers (Physical, Data Link) of the OSI Reference Data Model. 802.2 Logical Unk Control Link Address Management 802.3 802.4 802.5 802.6 802.9 Architecture CSMAlCD Token-Bus Token-Ring MAN ISDN Data Link MAC MAC MAC MAC MAC PHY PHY PHY PHY PHY Physical fade quickly after initial flurries of excitement, while others will The LLC sublayer provides a mechanism accommodating pave the way for new and better standards. those functions of wide area network Data Link protocols that The standards organizations naturally have a responsibility to pertain to LAN management. Unlike the wide area Data Link assume. that improvements are compatible and interoperable with protocols such as High-level Data Link Control (HOLC), which earlier standard versions. Standards are not static, and the net address specific nodes, LLC protocol data units only contain ser work architect must realize that there will always be a better so vice access points or internal memory addresses of software enti lution tomorrow. Of course, if one continues to wait for the better ties. Physical node addresses and error-detection are handled by solution, no solution will ever be implemented. the MAC sublayer. It is also essential to realize that there are a variety of compet There are four basic access methods defined, as well as a sub ing forces in the standards development world-each with its set of higher Data Link Layer functions. In addition, there are own agenda (hidden or unhidden). The result is often a less than several working groups whose activities are focused on specific perfect compromise. It is often said that the two things you really technologies which are applicable across a broad range of the do not. want to watch being made are sausages and computer access methods. network standards. The Carrier Sense Multiple Access with Collision Detection (CSMAlCD) method was the first to be developed by the IEEE, and was modeled after the DigitaVInteVXerox (DIX) Ethernet. Overview of IEEE 802 Standards While many people refer to the 802.3 specification as The IEEE 802 standards essentially address only the two lower "Ethernet," the two are not really compatible. There are a num layers of the Open Systems (OSI) Reference Model (see Figure ber of differences in the physical and medium access implemen 1). tations, but these have usually been resolved through "interface The Physical Layer corresponds to the OSI Physical Layer, agreements" and special-purpose bridges. Since it is more diffi while the OSI Data Link Layer is divided into two sublayers: cult to change tradition than to accept it, we will continue to use medium access control (MAC) and logical link control (LLC). the terms Ethernet and IEEE 802.3 interchangeably. Table 1 de The MAC sublayer addresses the specific procedural issues asso fines the differences between Ethernet and IEEE 802.3 imple ciated with distributed arbitration of channel access. mentations. Table 1. Ethernet/IEEE 802.3 Differences Feature Ethernet Version 1 Ethernet Version 2 IEEE 802.3 Specification 1980 Blue Book 1982 Blue Book 1983, 1985 Transceiver Cable 3 pairsAWG 22 4 pairs AWG 20 4 pairsAWG 20 Grounding at Host Inner/outer shield common at Same as Version 1 Inner shield to backshell; outer to backshell & pin 1 pin 4 Electrical Signal o V to negative +&-signal +&-signal Signal Quality Error (SQE) None; no heartbeat Yes; heartbeat Yes; heartbeat Repeater Specification None None Multiple collision protection Jabber Control None Yes Yes Type/Length Field Type (>1500) Type (>1500) Length «1500) CoaXial Cable 50n Double shield Same Same FEBRUARY 1994 @ 1994 McGraw-HiII, Incorporated. Reproduction Prohibited. Datapro Information 8e<vices Group. Delran NJ 08075 USA Data Networking LAN Standards 2770 3 Standards Figure 2. 802.2 Logical Link Control - Type 1 Connectionless IEEE StandlJrds Variations Type 2 Connection Oriented Type 3 ACK'ed Connectionless There are variations within each of the IEEE 802 standards. 802.6 802.3 802.4 802.S Metropolitan CSMAlCD Token-Bus Token-Ring Area Network Base- Broad- Broad- Carrier band band band Band Baseband DaDB 1SSM bps Single Cable 1M bps 1M bps Dual 5Mbps 4Mbps Cable 10M bps 16M b s "Ethernef' 10M bps 5Mbps "Cheapernef' 10M bps 10M bps Starlan 1M bps Twisted Pair 10BASE-T Figure 3. The 802.4 specifications were developed primarily in re lOBASE5 Termination Hardware sponse to requirements for the deterministic perfonnance of to ken passing, coupled with the facility of bus-oriented cabling. The employment of broadband technology provided the addi tional benefits of increased bandwidth, geographic coverage, and numbers of tenninations. The 802.5 token-ring specification was developed under the Bus Controller "guidance" oflBM and reflected the emerging "blue" perspec tive on local area networking. While the initial versions of the Board network provided less capacity than Ethernet, the expected im provements associated with detenninistic perfonnance and prior ity mechanisms yielded other benefits. Over time, however, we have seen a wide variety of imple mentations emerge---each reflecting specific vendors' "better ideas. " Some of these have been standardized, while others will likely become standards in the near future (see Figure 2). The chainnan of the 802.3 working group, Don Loughry, once spoke at an Interface Conference session with this author, and remarked that the "nice thing about standards is that you have so many to choose from." While this is certainly humorous, and may also evoke some concern for the viability of so many varia tions, it does in fact insure that there will be a standard implemen tation to meet most user requirement scenarios. One more important background note-many of the 802 specifications have been adopted as ISO/IEC 8802-n series stan dards: where "n" is the IEEE 802 extension. Thus, IEEE 802.3 becomes ISO/IEC 8802-3.