From geraldo.cc.utexas.edu!cs.utexas.edu!koriel!decwrl!nic.hookup.net!swrinde!sdd.hp.com!vixen.cso.uiuc.edu!howland.reston.ans.net!agate!boulder!cnsnews!spot.Colorado.EDU!barr Tue Dec 14 16:38:19 CST 1993 Article: 4728 of comp.dcom.lans.ethernet Newsgroups: comp.dcom.lans.ethernet Path: geraldo.cc.utexas.edu!cs.utexas.edu!koriel!decwrl!nic.hookup.net!swrinde!sdd.hp.com!vixen.cso.uiuc.edu!howland.reston.ans.net!agate!boulder!cnsnews!spot.Colorado.EDU!barr From: barr@spot.Colorado.EDU (BARR DOUG) Subject: FAQ, Part 1 Message-ID: Sender: usenet@cnsnews.Colorado.EDU (Net News Administrator) Nntp-Posting-Host: spot.colorado.edu Organization: University of Colorado, Boulder Date: Tue, 14 Dec 1993 17:42:07 GMT Lines: 431 ftp site: dorm.rutgers.edu, pub/novell/DOCS Last-modified: 1993/04/19 Version: 2.11 Ethernet Network Questions and Answers Summarized from UseNet group comp.dcom.lans.ethernet Version 2.11 of 19 April 1992 Sections: 01: Introduction, contributors, how to contribute to the FAQ and network etiquette. 02: General information about Ethernet and standards. 03: Ethernet Cabling Information. 04: Ethernet Devices and Components. 05: Errors and Related Terms. 06: Testing and Troubleshooting. 07: Additional Information. 01.01Q: What is this document? A: This is the official FAQ (Frequently Asked Questions) listing for UseNet newsgroup comp.dcom.lans.ethernet. It is intended to be a reference to the most commonly asked questions and basic informa- tion about Ethernet. 01.02Q: How is this document made available? A: This FAQ is posted monthly to newsgroup comp.dcom.lans.ethernet, comp.answers and news.answers on UseNet. You can also retrieve this file via anonymous FTP from dorm.rutgers.edu (128.6.18.15) in path pub/novell/DOCS as file Ethernet.FAQ (this is a UNIX machine, note proper character case). 01.03Q: Who maintains this list? A: This list is currently maintained by Mark Medici. My preferred mail address is medici@gandalf.rutgers.edu, and I would greatly appreciate it if you could use a Subject: line that starts "Ethernet FAQ". 01.04Q: Where does all this information come from? A: The questions and answers are mostly summarized from the UseNet newsgroup comp.dcom.lans.ethernet, many of which are derived from various IEEE, ISO and EIA/TIA documents. Specifically, the following persons have contributed to this FAQ. Their knowledge and experiences are gratefully acknowledged. Doug Barr John Breeden TP Brisco Peter Desnoyers Daniel Huber Bob Jaques Paul Joslin Dave Kapalko Rich Lawrence Nick Hennenfent Ray Hunter Mark Medici Dave Mitton Rich Seifert Charles Spurgeon (Note: If you have contributed something to this FAQ but your name is not listed above, please take no offense. There was some loss of information in this file a while back. Just mail the current FAQ maintainer your preferred named and mail address, and which section(s) you contributed). 01.05Q: How can I submit new contributions or corrections to the FAQ? A: New contributions, suggestions and corrections should be mailed to the current FAQ maintainer, who is listed in Q&A 01.03 above. However, you should note that if you are submitting a correction you must provide both the old and suggested new text -- messages to the effect of "this is wrong, fix it" will be ignored. 01.06Q: Are there any restrictions on the distribution of this FAQ? A: You may freely distribute this document for non-commercial purposes as long as the contents remain unchanged (including credits) and you do not gain any direct profits from the distribution. 01.07Q: Are there any guidelines regarding postings on this newsgroup? A: The standard UseNet guidelines apply to this newsgroup. Explaining these guidelines in detail requires a FAQ of its own. If you are not familiar with standard network etiquette, you should review the documents posted regularly in the newsgroup news.announce.newuser. 01.08Q: Are the vendors and/or models of equipment listed in this FAQ the only or best suited for the application described? A: Not necessarily. This document does not attempt to rate equipment from different manufacturers, and does not endorse nor specifically support any one vendor's product over another. Any references to a specific vendor or product is implicitly used as an example of all like devices. 02.01Q: What is Ethernet? A: Ethernet is a type of network cabling and signalling specifications (OSI Model layers 1 [physical] and 2 [data link]) originally developed by Xerox in the late 1970. In 1980, Digital Equipment Corp. (DEC), Intel and Xerox (the origin of the term DIX, as in DEC/Intel/Xerox) began joint promotion of this baseband, CSMA/CD computer communications network over coaxial cabling, and published the "Blue Book Standard" for Ethernet Version 1. This standard was later enhanced, and in 1985 Ethernet II was released. The IEEE's (Institute of Electrical and Electronics Engineers') Project 802 then (after considerable debate) used Ethernet Version 2 as the basis for the 802.3 CSMA/CD network standard. The IEEE 802.3 standard is generally interchangeable with Ethernet II, with the greatest difference being the construction of the network packet header. A complete description of all Ethernet specifications is far out- side the scope of this document. If this area interests you, you are encouraged to obtain (hopefully legally) copies of the IEEE 802.3 documents, and perhaps the ISO 8802-3 documents as well. 02.03Q: What is an 802.3 network? A: That's IEEE-ish for Ethernet, but with a few small differences. The physical layer specifications are identical (though DIX Ethernet never specified standards for UTP and Fiber-Optic media) and the MAC sublayer are somewhat different. See "What is Ether- net for more info. 02.02Q: What is CSMA/CD? A: CSMA/CD is the media access control mechanism used by Ethernet and 802.3 networks; in other words, it determines how a packet of data is placed on the wire. CSMA/CD stands for "Carrier Sense Multiple Access, with Collision Detection". Before an Ethernet device puts a packet "on the wire", it listens to find if another device is already transmitting. Once the device finds the wire is clear, it starts sending the packet while also listening to hear if another device started sending at the same time (which is called a collision). Refer to the Q&A on collisions for more info about this phenomena. 02.03Q: What is a baseband network? A: A baseband network is one that provides a single channel for communciations accross the physical medium (e.g., cable), so only one device can transmit at a time. Devices on a baseband network, such as Ethernet, are permitted to use all the available bandwidth for transmission, and the signals they transmit do not need to be multiplexed onto a carrier frequency. An analogy is a single phone line such as you usually have to your house: Only one person can talk at a time--if more than one person wants to talk everyone has to take turns. 02.04Q: Ok, so what is a broadband network? A: Simplisticly, it is the opposite of a baseband network. With broadband, the physical cabling is virtually divided into several different channels, each with its own unique carrier frequency, using a technique called "frequency division modulation". These different frequencies are multiplexed onto the network cabling in such a way to allow multiple simultaneous "conversations" to take place. The effect is similar to having several virtual networks traversing a single piece of wire. Network devices "tuned" to one frequency can't hear the "signal" on other frequencies, and visa-versa. Cable-TV is an example of a broadband network: multiple conversations (channels) are transmitted simultaneously over a single cable; you pick which one you want to listen to by selecting one of the frequencies being broadcast. 02.05Q: What is an OSI Model? A: The Open Systems Interconnect (OSI) reference model is the ISO (International Standards Organization) structure for the "ideal" network architecture. This Model outlines seven areas, or layers, for the network. These layers are (from highest to lowest): 7.) Applications: Where the user applications software lies. Such issues as file access and transfer, virtual terminal emulation, interprocess communication and the like are handled here. 6.) Presentation: Differences in data representation are dealt with at this level. For example, UNIX-style line endings (CR only) might be converted to MS-DOS style (CRLF), or EBCIDIC to ASCII character sets. 5.) Session: Communications between applications across a net- work is controlled at the session layer. Testing for out-of-sequence packets and handling two-way communication are handled here. 4.) Transport: Makes sure the lower three layers are doing their job correctly, and provides a transparent, logical data stream between the end user and the network service s/he is using. This is the lower layer that provides local user services. 3.) Network: This layer makes certain that a packet sent from one device to another actually gets there in a reasonable period of time. Routing and flow control are performed here. This is the lowest layer of the OSI model that can remain ignorant of the physical network. 2.) Data Link: This layer deals with getting data packets on and off the wire, error detection and correction and retransmission. This layer is generally broken into two sub-layers: The LLC (Logical Link Control) on the upper half, which does the error checking, and the MAC (Medium Access Control) on the lower half, which deals with getting the data on and off the wire. 1.) Physical: The nuts and bolts layer. Here is where the cable, connector and signaling specifications are defined. 02.06Q: What does an ethernet packet look like? A. See the information below, as described in the National databook. The ethernet packet preamble is normally generated by the chipset. Software is responsible for the destiantion address, source address, type, and data. The chips normally will append the frame check sequence. +------------+ | | Preamble - | 62 bits | A series of alternating 1's and 0's used by the | | ethernet receiver to acquire bit synchronization. | | This is generated by the chip. +------------+ | | Start Of Frame Delimiter - | 2 bits | Two consecutive 1 bits used to acquire byte | | alignment. This is generated by the chip. +------------+ +------------+ | | Destination Ethernet Address - | 6 bytes | Address of the intended receiver. | | The broadcast address is all 1's. +------------+ | | Source Ethernet Address - | 6 bytes | The unique ethernet address of the sending | | station. +------------+ | | Length or Type field - | 2 bytes | For IEEE 802.3 this is the number of bytes of | | data. For Ethernet I&II this is the type of | | packet. Types codes are > 1500 to allow both to | | coexist. The type code for IP packets is 0x800. +------------+ | 46 bytes | Data - | to | Short packets must be padded to 46 bytes. | 1500 bytes | +------------+ +------------+ | | Frame Check Sequence - | 4 bytes | The FCS is a 32 bit CRC calculated using | | the AUTODIN II polynomial. | | This field is normally generated by the chip. +------------+ The shortest packet is: 6 + 6 + 2 + 46 = 60 bytes The longest packet is: 6 + 6 + 2 + 1500 = 1514 bytes 02.07Q: What is the difference between an Ethernet frame and a IEEE802.3 frame? Why are there two types? Why is there a difference? A: Ethernet was invented at Xerox Palo Alto Research Center and later became an international standard. IEEE handled making it a standard; and their specifications are slightly different from the original Xerox ones. Hence, two different types. 802.3 uses the 802.2 LLC to distinguish among multiple clients, and has a "LENGTH" field where Ethernet has a 2-byte "TYPE" field to distinguish among multiple client protocols. TCP/IP and DECnet (and others) use Ethernet_II framing, which is that which Xerox/PARC originated. 02.08Q: What is SNAP A: An extension that allows vendors to create their own ethernet sub-types. Sub-Network Access Protocol, an extention to the original 802.2 data link level format. (SNAP is described in IEEE 802-1990) The 802.2 data link format replaced the Ethernet Protocol Type concept with two 8 bit fields; Source SAP, and Destination SAP. Unfortunately that causes problems with migration of protocols, and the lack of SAP space that is available. So one SAP as allocated for this scheme which greatly expands the available protocol space. When using the SNAP SAP the first 5 bytes of data are used as a protocol ID. The first 3 bytes should be a value allocated to you as a vendor id, the same as you get for Source address values. The is called the OUI (Organizationally Unique ID) The second 2 bytes is a protocol type. Note that this is 802.2 and applies across all 802 LAN media types. For translation bridging, there is a convention, if you set the OUI to zero, you are representing a mapped Ethernet frame. So that a bridge will translate such a frame back into Ethernet format, and not into an 802.3 frame format. 802.2 SNAP frame: +-------+------+------+------+-------+------+------+ | MAC | DSAP | SSAP | UI | OUI | Type | data | | Header| 0xAA | 0xAA | 0x03 | 3bytes|2bytes| | +-------+------+------+------+-------+------+------+ This will appear the same on all 802 compliant LAN media. On 802.3, there will be a Length field between the SA and the DSAP but not on 802.5 or FDDI. 02.09Q: Where can I find out which Protocols use which Ethernet type numbers? A: Look at IETF RFC-1340 - Assigned Numbers RFC. 02.10Q: What is a MAC address? A: It is the unique hexadecimal serial number assigned to each Ether- net network device to identify it on the network. With Ethernet devices (as with most other network types), this address is permanently set at the time of manufacturer, though it can usually be changed through software (though this is generally a Very Bad Thing to do). 02.11Q: Why has the MAC address to be unique? A: Each card has a unique MAC address, so that it will be able to exclusively grab packets off the wire meant for it. If MAC addresses are not unique, there is no way to distinguish between two stations. Devices on the network watch network traffic and look for their own MAC address in each packet to determine whether they should decode it or not. Special circumstances exist for broadcasting to every device. 02.12Q: Is there a special numbering scheme for MAC addresses? A: The MAC addresses are exactly 6 bytes in length, and are usually written in hexadecimal as 12:34:56:78:90:AB (the colons may be omitted, but generally make the address more readable). Each manufacturer of Ethernet devices applies for a certain range of MAC addresses they can use. The first three bytes of the address determine the manufacturer. RFC-1340 (available via FTP) lists some of the manufacturer-assigned MAC addresses. A more up-to-date listing of vendor MAC address assignments is available on ftp.lcs.mit.edu in pub/map/Ethernet-codes. 02.13Q: What is a preamble ? A: A seven octet field of alternating one and zero binary bits sent prior to each frame to allow the PLS circuitry to reach its steady state synchronization with received frame timing. (802.3 standard, page 24,42). 02.14Q: What is a Start Frame Delimiter (SFD)? A: A binary sequence of '10101011' immediately following the preamble and indicating the beginning of a frame. (802.3 standard, page 24). 02.15Q: What means CRC? A: Cyclical Redundancy Check - A method of detecting errors in a message by performing a mathematical calculation on the bits in the message and then sending the results of the calculation along with the message. The receiving work-station performs the same calculation on the message data as it receives it and then checks the results against those transmitted at the end of the message. If the results don't match, the receiving end asks the sending end to send again. 02.13Q: What is a broadcast address? A: The unique address that identifies a packet as appropriate to all receiveing stations. In 802.3 any address in which the second byte is an odd number. (1,3,...F). 02.14Q: What exactly means 10Base5, 10BaseT, 10Base2, 10Broad36, etc. A: These are the IEEE names for the different physical types of Ethernet. The "10" stands for signalling speed: 10MHz. "Base" means Baseband, "broad" means broadband. Initially, the last section as intended to indicate the maximum length of an unrepeated cable segment in hundreds of meters. This convention was modified with the introduction of 10BaseT, where the T means twisted pair, and 10BaseF where the F means fiber (see the following Q&A for specifics). This actually comes from the IEEE committee number for that media. In actual practice: 10Base2 Is 10MHz Ethernet running over thin, 50 Ohm baseband coaxial cable. 10Base2 is also commonly referred to as thin-Ethernet or Cheapernet. 10Base5 Is 10MHz Ethernet running over standard (thick) 50 Ohm baseband coaxial cabling. 10BaseF Is 10MHz Ethernet running over fiber-optic cabling. 10BaseT Is 10MHz Ethernet running over unshielded, twisted- pair cabling. 10Broad36 Is 10MHz Ethernet running through a broadband cable. 02.15Q: What means FOIRL? A: Fiber Optic Inter Repeater Link. A "IEEE 802 standard" worked out between many vendors some time ago for carrying Ethernet signals across long distances via fiber optic cable. It has since been adapted to other applications besides connecting segments via repeaters (you can get FOIRL cards for PCs). It has been superseded by the larger 10BaseF standard. 02.16Q: What is LattisNet? A: LattisNet is a pre-10BaseT quasi-standard for running Ethernet over twisted-pair cabling. It was developed by Synoptics, and several other vendors made compatible equipment for a while. LattisNet is not compatible with 10BaseT, but you can have LattisNet hubs and 10BaseT hubs in the same hub chassis or connected to the same network backbone. The primary difference is that 10BaseT synchron- izes the signals at the sending end, while LattisNet syncrhonizes at the receiving end. 02.17Q: What is StarLAN-10? A: StarLAN-10 is AT&T's variety of Ethernet over twisted-pair cabling. Older StarLAN-10 is not 100% 10BaseT compliant, as it does not provide link integrity to the AUI. However, many 10BaseT interfaces can be configured to work with StarLAN-10 hubs, alongside StarLAN-10 NICs. Beware, though, that the original StarLAN-10 is NOT in any way compatible with 10BaseT, and worse, there seems to be no way to tell other than trying it to see what happens. The current StarLAN products supported by AT&T/NCR are fully 802.3 compliant. This includes the SmartHUB model E, SmartHUB model B, SmartHUB XE, and the other fiber and wire SmartHUB models. 03.01Q: What is coax? A: Coaxial cable (coax) is a metallic electrical cable used for RF (radio frequency) and certain data communications transmission. The cable is constructed with a single solid or stranded center conductor that is surrounded by the dielectric layer, an insulating material of constant thickness and high resistance. A conducting layer of aluminum foil, metallic braid or a combination of the two encompass the dielectric and act as both a shield against interference (to or from the center conductor) and as the return ground for the cable. Finally, an overall insulating layer forms the outer jacket of the cable. Coaxial cable is generally superior in high-frequency applications such as networking. However, for shorter distances (up to 100 meters), UTP or STP cable is generally just as reliable when using differential modulation techniques (such as with 10BaseT). There are three types of RG-58 cable, as far as I can tell. There are probably other subtle differences, but for 10BASE2, impedance and velocity of propagation are the important ones. The table below summarizes: Cable Impedance Velocity ---------- ---------- -------------- RG-58A/U 50 ohms .66 or .78 RG-58C/U 50 ohms .66 RG-58/U 53.5 ohms .66 or .695 End of part I. .