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Frequently Asked Questions (FAQS);faqs.341 Other interesting looking books: n. Mark A. Miller. Troubleshooting Internetworks. 13. Periodicals The following periodicals were mentioned by responders to the 12/91 BIG-LAN Reader Survey as good periodicals for administrators of Campus- sized LANs: a. LAN Times b. Communications Week c. Network Computing d. ConneXions e. Data Communications f. Network World g. LAN Magazine h. Info World i. SunExpert j. Telecommunications k. Computerworld l. DataCommunicationInternational m. Datamation n. Digital Review o. LAN Technology p. Lightwave q. MacUser r. MacWeek s. MacWorld t. Networking Management u. PC Week 14. Training Courses The following providers of tutorials were mentioned by responders to the 12/91 BIG-LAN Reader Survey: a. Interop b. ACM SIGComm c. Learning Tree d. Novell e. PSI f. Usenix 15. Conferences The following conferences were mentioned by responders to the 12/91 BIG-LAN Reader Survey as good conferences for administrators of Campus- sized LANs: a. Interop b. Usenix c. ComNet d. NetWorld e. ACM SIGComm f. DECUS g. IETF IV. Basic Glossary on Campus Networks Another glossary is RFC1208. See "Online Papers" above. ANSI "American National Standards Institute" - A definer of standards of all kinds, including FDDI. Appletalk - A protocol family developed by Apple Computer to implement LANs serving Macintoshes. ATM "Asynchronous Transfer Mode" - a method for switching little fixed-size packets (cells) around. Like T1 and DS3, digitized voice was a major consideration in its design, but it can be used for data. It is designed around fixed speeds too, roughly 150MBS and 600MBS. The fixed cell size is 53 bytes. Though ATM is really designed for voice and WANs, there are schemes to use it in LANs. ATM is a big buzzword these days but it is still very new. AUI "Attachment Unit Interface" - the Ethernet/IEEE 802.3 term for the interface between a MAU and a station. A special kind of cable known as an "AUI Cable" can attach a MAU to a station at a distance (up to 50 meters). BNC Connector "Bayonet Neill-Concelman connector" - a type of connector used for attaching coax cable to electronic equipment which can be attached or detached quicker than connectors that screw. ThinWire Ethernet (IEEE 802.3 10BASE2) uses BNC connectors. Bridge - A network "relay" which reads, buffers, and sends data to relay it from one data link to another, but makes the two data links appear as one to levels higher than the data link layer. CDDI "Copper Data Distribution Interface" - essentially a way to use electrical communications cables in an FDDI network. Several companies have worked out ways to do this but ANSI has yet to standardize one. I think CDDI was coined by Crescendo corporation for their scheme, but it may well be adopted by ANSI as the name. So far there are schemes that work on Coax, on STP and UTP, but the front runners look like they will be able to run on UTP for about 100 meters. CMIP "Common Management Information Protocol" - An OSI protocol for management of network equipment. Not widely implmented. See SNMP. CMOT "CMIP over TCP/IP" - A protocol consisting of CMIP running under TCP/IP. An alternative to SNMP. Coaxial Cable - any of a number of kinds of electrical communications cable designed so one conductor is in the center and the second conductor forms a ring around it. Depending upon who you talk to, someone might have a specific kind of coaxial cable in mind. Some well known kinds are various Cable TV cables, cables used by IBM 327x terminals and ARCnet, and cables used by Ethernet & IEEE 802.3. DECnet - Trade name of Digital Equipment Corporation for some of their networking products. It is a kind of network built out of Digital Equipment Corporations own networking protocols (with some standard protocols also used). Dialup Modem - Modem used over ordinary dial-up telephone lines as opposed to private or leased lines. Ethernet - LAN data-link protocol developed by a consortium of vendors; later standardized as IEEE 802.3 with a few modifications. For many applications, users have not adopted all the IEEE 802.3 differences. Ethernet/802.3 now can be run on two types of coaxial cable as well as multi-mode fiber and unshielded twisted-pair. "Raw" rate of data transmission is 10 megabits/second. FDDI "Fiber Data Distribution Interface" - LAN data-link protocol. Designed to run on multi-mode fiber. "Raw" rate of data transmission is 100 megabits/second. Developed by the American National Standards Institute. FDDI-2 - Same speed, same fiber, same basic protocol as FDDI. FDDI-2 adds a layer which allows you to allocate fixed bandwith to applications of your choice, making it more like broadband. FDDI-2 is still rather new. Fiber - optical fiber: a very long, narrow, flexible piece of glass. Used for high-speed communications. FOIRL "Fiber Optic Inter-Repeater Link" - a standard for running IEEE 802.3 over fiber. It was originally designed to link two repeaters, and only supports two attachments. Many users use it to attach a station to a repeater. See 10BASE-F. FTP - Protocol in the "TCP/IP" family for copying files from one computer to another. Stands for "File Transfer Protocol". Gateway - A type of "network relay" that attaches two networks to build a larger network. Modern "narrow" usage is that it is one that translates an entire stack of protocols, e.g., translates TCP/IP-style mail to ISO-style mail. Older usage used it for other types of relays--in particular, in the "TCP/IP" world, it has been used to refer to what many now insist is a "router". GOSIP "Government Open Systems Interconnect Profile" - A subset of OSI standards specific to US Government procurements, designed to maximize interoperability in areas where plain OSI standards are ambiguous or allow options. Theoretically, required of all US Government networking procurements since mid-1990. Heartbeat - In Ethernet (Version 2), a test of the collision functionality of the transciever. The term "Heartbeat" is often (wrongly) used interchangeably with "SQE" which is a similar function of IEEE 802.3. See Question on SQE/Heartbeat below. IPX - Novell's protocol used by Netware. Utilizes part of XNS. A router with "IPX routing" purports to interconnect LANs so that Novell Netware clients & servers can talk through the router. MAU "Media Adaptor Unit" - an IEEE 802.3 or Ethernet device which attaches a station to the cable. Popularly called a "transceiver". Can be attached by cable to the station or built into the station. MIB "Management Information Base" - the set of parameters an SNMP management station can query or set in an SNMP agent (e.g. router). Standard, minimal MIBs have been defined (MIB I, MIB II), and vendors often have custom entries. In theory, any SNMP manager can talk to any SNMP agent with a properly defined MIB. Multimode fiber - A type of fiber mostly used for shorter, e.g. campus distances. It can carry 100 megabits/second for typical campus distances, the actual maximum speed (given the right electronics) depending upon the actual distance. It is easier to connect to than Single Mode Fiber, but its limit on speed x distance is lower. NFS "Network File System" - an IP-based protocol originally developed by Sun Microsystems which provides file services. OSI "Open System Interconnect" - A standard put forth by the ISO for communication between computer equipment and networks. OSI Reference Model - A model put forth by the ISO for communication between computer equipment and networks, which maps out 7 protocol layers. Top layer: layer number 7: application layer layer number 6: presentation layer layer number 5: session layer layer number 4: transport layer layer number 3: network layer layer number 2: data-link layer (e.g. IEEE 802.x) Bottom layer: layer number 1: physical layer (wire & electricity) This model explains what each layer does. The model is often used to explain anyones protocols (not just OSI) to the point where many people seem to believe that true data-communications requires these 7 layers. POP "Post Office Protocol" - A TCP/IP-based protocol designed to allow client-stations (e.g. micros) to read mail from a server. There are three versions under the name "POP": POP, POP2, and POP3. Latter versions are NOT compatible with earlier versions. Protocol - The "rules" by which two network elements trade information in order to communicate. Must include rules about a lot of mundane detail as well as rules about how to recover from a lot of unusual communication problems. Thus they can be quite complicated. Relay - One terminology uses the term "relay" as a device that interconnects LANs, different kinds of relays being repeaters, bridges, routers, and gateways. Repeater - In the "Ethernet" world, a "relay" that regenerates and cleans up signals, but does no buffering of data packets. It can extend an Ethernet by strengthening signals, but timing limitations on Ethernets still limit their size. RFC "Request For Comments" - The name is a real red herring when it comes to Internet RFCs. Some really are "Requests For Comments" but all Internet protocol documents are stamped with an RFC number that they never shake, so the acronym RFC generally refers to documents that describe protocols in the TCP/IP family. RG numbers (E.g. RG62; sometimes there are qualifiers, e.g. RG 58 A/U) a shorthand designation for military cable. RG58 & RG62 designate two different types of cable used by the military. Some data-communications equipment was designed to work with a particular military standard, e.g. IBM 3270-type terminals use RG62. In other cases, people use an RG-numbered cable that is close to what they need: for example Thinwire Ethernet & IEEE 802.3 10BASE2 define the type of cable they need and people sometimes substitute flavors of RG58, which are "close". One can't recommend this practice because you can get yourself in trouble. I think "RG" originally stood for "Radio Guide", presumably reflecting the fact that the series of cables was designed to handle radio frequencies. The IEEE 802.3 10BASE2 specifications define two RG numbered cables (RG58 A/U and RG58 C/U) as meeting the cable requirements for thin Ethernet. However, cable vendors may list a range of cables under these same RG numbers, and some of the cables listed may not meet the 802.3 specs. You need to check the cable specifications closely, and beware of relying on the RG number alone when ordering network cables. Router - A network "relay" that uses a protocol beyond the data-link protocol to route traffic between LANs and other network links. Routing Protocol - a protocol sent between routers by which routers exchange information own how to route to various parts of the network. The TCP/IP family of protocols has a bunch, such as RIP, EGP, BGP, OSPF, and dual IS-IS. Shielded Twisted Pair - a type of twisted-pair cable with a metallic shield around the twisted conductors. The shield reduces the noise from the cable and reduces the effects of noise on the communications in the cable, but changes the electrical characteristics of the cable so some equipment optimized to non-shielded cable runs worse on shielded cable. Single Mode fiber - a type of fiber optic cable used for longer distances and higher speeds, e.g. for long-distance telephone lines. See also "Multimode Fiber". SMTP "Simple Mail Transfer Protocol" - the protocol in the TCP/IP family used to transfer electronic mail between computers. It is not oriented towards a client/server system so other protocols (see "POP") are often used in that context. However, servers will use SMTP if they need to transfer a message to another server. SNMP "Simple Network Management Protocol" - Originally developed to manage IP based network equipment like routers and bridges, now extended to wiring hubs, workstations, toasters, jukeboxes, etc. SNMP for IPX and AppleTalk under development. Widely implemented. See CMIP. SQE Test "Signal Quality Error Test" - an IEEE 802.3 function that tests the transceiver. The term "SQE" is often (wrongly) used interchangeably with "Heartbeat" which is a similar function of Ethernet Version 2. See Question on SQE/Heartbeat below. TCP/IP "Transmission Control Protocol/Internet Protocol" - literally, two protocols developed for the Defense Data Network to allow their ARPANET to attach to other networks relatively transparently. The name also designates the entire family of protocols built out of IP and TCP. The Internet is based upon TCP/IP. TELNET - a protocol in the TCP/IP family that is used for "remote login". The name is also often used as the name of the client program that utilizes the TELNET protocol. Terminal Server - a network device that allows a number of terminals to attach to a LAN, and do remote logins across the LAN. TN3270 - A variant of the TELNET program that allows one to attach to IBM mainframes and use the mainframe as if you had a 3270 or similar terminal. Token Ring - People often mean 802.5 when they say "Token Ring" (see below). In the more general sense of the word, a type of LAN that has stations wired in a ring, where each station constantly passes a special message (a "token") on to the next. Whoever has the token can send a message. Tunnelling - An important concept in the design of many kinds of networks: taking some protocol-family's ability to move packets from user to user, or to open virtual-circuits between users, and use this as if it were a data-link protocol to run another protocol family's upper layers (or even the same protocol family's upper layers). Examples: running TCP/IP over Appletalk instead of something like Ethernet; running Appletalk over DECnet instead of something like Localtalk or Ethernet. Twisted Pair - The type of wire used by the phone company to wire telephones -- at least over distances like between your house and the central office. It has two conductors, which are twisted. The twists are important: they give it electrical characteristics which allow some kinds of communications otherwise not possible. Ordinary telephone cables are not shielded (see "Shielded twisted Pair"). T1 - A phone-company standard for running 24 digitized voice circuits through one 1.5megabit/second digital channel. Since phone companies run lots of T1, and will run T1 between customer sites, the standard is often used for data communications, either to provide 24 low-speed circuits, or to provide 1 high-speed circuit, or to be divided other ways. UTP (Unshielded Twisted-Pair) - See "Twisted-Pair" and "Shielded Twisted-Pair". X.400, X.500 - OSI protocols for mail and directory services. 10BASE-T - A variant of IEEE 802.3 which allows stations to be attached via twisted-pair cable. 10BASE-F - A variant of IEEE 802. 3 under development which allows stations to be attached via multimode fiber. It will offer a variety of methods of using fiber in an IEEE 802.3 network that go beyond what is currently offered in FOIRL. The current 10BASE-F draft is likely to be confirmed. draft is likely to be confirmed. Sections of the draft include "Fiber Optic Medium and Common Elements of Medium Attachment Units and Star, Type 10BASE-F (Section 15)", "Fiber Optic Passive Star and Medium Attachment Unit, Type 10BASE-FP (Section 16)", "Fiber Optic Medium Attachment Unit, Type 10BASE-FB (Section 17)", and "Fiber Optic Medium Attachment Unit, Type 10BASE-FL (Section 18)". 802 - The set of IEEE standards for the definition of LAN protocols. A story goes that a long time ago, IEEE and ANSI decided that IEEE would get the slow protocols and ANSI would get the fast ones, thus IEEE defined the 802 protocols and ANSI defined FDDI. Presumably IEEE saw limited application for FDDI at the time. Also, the IEEE standards-making committees associated with these standards. 802.1 - The IEEE 802 standard for Network Management and Network Bridging of IEEE 802 networks. 802.2 - An IEEE standard for the portion of LAN data-link protocols that is the same for all flavors of IEEE LAN protocols, e.g. 802.3 and 802.5. Sometimes not used. 802.3 - An IEEE standard for LANs--their "improved" version of Ethernet. See Ethernet. 802.4 - An IEEE standard for LANs: Token Bus networks. Basically, standardizes MAP, a protocol that operates a Token Bus protocol on broadband. 802.5 - An IEEE standard for Token-Ring-based LANs. See Token Ring. 802.6 - An IEEE standard for Metropolitan Area Networks. Also known as DQDB. 802.7 - IEEE 802 technical advisory group on Broadband. 802.8 - IEEE 802 technical advisory group on FDDI & fiber optics. 802.9 - IEEE 802 group on integrated data & voice networks. 802.11 - Proposed IEEE 802 group for wireless Ethernet. V. Frequently Asked Questions on Campus Networks It is hard to answer typical BIG-LAN questions in advance for two reasons. Answers are often long and they are often controversial. To provide some sort of objective information relevant to the controversies, a survey of BIG-LAN readers was taken on answers to various questions, so this memo could offer a sampling of opinions. Note that the opinions below are extracted from the 41 responses received for the survey. We can't say these 41 responses represent a fair sampling of campus LAN administrators, but they do show some of the answers that you would get if you posed some of these questions to the BIG-LAN readership. 1. What is the difference between Ethernet and IEEE 802.3? Ethernet ran through an evolution starting with some experimenting at Xerox, and ending with a standard published by Xerox, DEC, and Intel, which they offered to the world (with minimal royalties) as a standard technology for building LANs. The Institute of Electrical & Electronic Engineers took this as a proposed standard, and rewrote the protocol description making some clarifications and a few changes. Some of the changes have been universally adopted, and others have not. After the first go round of IEEE standard defining, Ethernet version 2 was introduced which brought it more into line with standards. The basic differences are: - Heartbeat vs SQE (see below) - Which pin in the MAU & AUI connectors carry the ground conductor - Packet Length Field vs Type Field The latter issue is the one in which IEEE 802.3 has not displaced Ethernet. Ethernet had a 16-bit field which defined the type of packet (examples: IP, XNS, Appletalk). The IEEE committee decided to use that field to specify the length of the packet, and have the data-portion of the packet define itself through the next higher level of protocol (e.g., IEEE 802.2). However, the sets of possible values for that field used by the two different protocols are completely separate, and both protocols are designed to deliberately ignore packets with fields outside their own sets of values. Thus Ethernet and IEEE 802.3 packets can coexist on the same cable, though a computer which expects to get packets belonging to just one of the protocols won't notice any packets sent according to the rules of the other (the expression used is "they pass by each other like ships in the night"). These days, LANs use both. There is a way to send TCP/IP packets via 802.3, but when 802.3 was introduced, there were already so many systems using the Ethernet rules that the use of Ethernet-style packets for TCP/IP has persisted now for years. 2. What is encapsulation? What do I have to know about it? One encapsulation issue on LANs is whether IEEE 802.3 packets are used or Ethernet packets are used to encapsulate your traffic on your IEEE 802.3/Ethernet LAN. See previous question for more explanation. Most TCP/IP systems use Ethernet, any that uses IEEE 802.3 by default might surprise you by not interoperating with the rest of your TCP/IP network. A second encapsulation issue on IEEE 802.3/Ethernet networks is whether your Novell (IPX) packets use Novell's default encapsulation or whether they use Ethernet-style encapsulation. Novell, at least for a long time, had the distinction of using IEEE 802.3 as if it were the only protocol on the network, not following the rules for avoiding other protocols running under IEEE 802.3 rules. They offered a utility called ECONFIG that changed Netware to use Ethernet rules, and use them properly, so Novell IPX packets could utilize the same LAN as other protocols. In no case would the Novell traffic bother Ethernet traffic-- only any other IEEE 802.3 traffic if ECONFIG wasn't used. In any case, a single Ethernet segment, or bridged segments, had to have all Novell servers and clients configured the same, in order to interoperate. A third encapsulation issue stems from Berkeley Unix 4.2, from which many versions of Unix and many TCP/IP implementations have been modeled. It used, by default, its own encapsulation rules (i.e., manner of putting IP packets within Ethernet packets) which is termed "Trailer Encapsulation". When an Ethernet had some computers using Trailer Encapsulation and some not, TCP/IP connections would often work, but hang when large data transfers were taking place. The next version of Berkeley Unix, version 4.3, remedied this by avoiding Trailer Encapsulation except when it was guaranteed to work correctly. A fourth encapsulation issue is "tunnelling", which consists of one of the layers in the protocol stack mimicking another layer to provide a way of running a different set of upper layers than would otherwise be possible. This is rather widely used and seldom explained to beginners. It is perhaps best explained with an actual example: [Here put an example, perhaps Appletalk over IP] [Include "encapsulated bridging" as a second example] 3. How do I know whether to use a router or a bridge? (Note that the answer to this question is oriented to Ethernet-based LANs). Few administrators of networks doubt that a network can be large enough to require routers nor that there are situations where a bridge is an effective solution. However, there is controversy as to where to draw the line. Campus-sized networks involving distances of up to a mile and possibly thousands of stations, can be, and have been built solely out of one or the other. The BIG-LAN Survey of 12/91 showed the following opinion among respondents: Survey question: "When you build a campus network, do you tend to use bridges as opposed to routers?" Answers: 9 said yes; 26 said no; 2 said "brouters" (combination bridge/routers) would be the best solution. Some clear tradeoffs: routers generally have to be set up no matter what whereas bridges can be plug-and-play on a network without too much total traffic; bridges generally have a higher speed-to-cost ratio and the low-end bridge is less expensive than the low-end router; routers handle huge networks with links of different speeds better. 4. How do I know whether to use a bridge or a repeater? How many repeaters may I put on an Ethernet? You cannot keep plugging more repeaters and add more cables to an Ethernet indiscriminately and expect it to work. With too large a networks, the protocol which keeps the number of collisions down (known as CSMA/CD) fails to do that. The protocol documents supply rules-of-thumb which, if followed, prevent this from occurring. If you break them, you may be risking large performance degradations. The latest version of the rules-of-thumb (which have been updated over time as new features like 10BASE-T have been added to the protocol) are in the IEEE 802.3 document describing 10BASE-T, specifically IEEE Std 802.ei-1990 in the section called "System Considerations for Multisegment 10 Mb/s Baseband Networks" (When 10BASE-F is released later, this section will be updated again). The rules refer to the piece of the LAN that is between repeaters as a segment and refer to 4 kinds: 10BASE5 (i.e. "classic" Ethernet) and 10BASE2 (i.e., ThinWire Ethernet) both classified as "Coax" segments and FOIRL (fiber inter-repeater links) and 10BASE-T, both classified as "Link" segments, and both of which have the property that you can attach things only to their ends. The basic repeater rule is that between any two stations on the LAN, there may be at most 4 repeaters and three coax segments. In addition, there are length restrictions on the segments which are designed to keep CSMA/CD working properly: 10BASE5 500 meters 10BASE2 185 meters FOIRL 500 meters (1000 meters in some cases) 10BASE-T 100 meters (or more) FOIRL links can be 1000 meters if you have at most 3 repeaters between stations instead of 4. 10BASE-T links can be longer if the cable will support it: CSMA/CD is not the limiting factor on 10BASE-T. For the purposes of this discussion, bridges, routers, and gateways are "stations" since the CSMA/CD protocol does not pass through them. Thus if you discover these rules prevent you from putting a repeater in the network where you need one, then you can put a bridge there instead, or perhaps split the LAN somewhere else using a bridge. 5. Should I use "manageable" hubs, concentrators, etc on my LAN? This is a controversial question also. Vendors have attempted to make hubs and concentrators that require little training & manpower to manage & troubleshoot, and they will attempt to convince you that they have succeeded. You pay a premium for "manageability". Those who remain skeptical wonder how much the management features are ever used: for example, management allows you to turn on & off ports from an operator's console; how often do you need to do such a thing? Also, some of the benefits attributed to management packages are simply due to good record keeping, something which the administrator must find the manpower to accomplish with a management package or without one (presumably with a simple dbms, which can often be tailored more to the administrators needs).