ETHERNET - AN ITTY-BITTY PRIMER - Allied Telesyn Technical Support October 1994 (subject to correction whenever possible) ANSI/IEEE Std 802.3 - ISO/IEC8802-3 (OR "What's with all the numbers?") The above Standard describes a Local Area Network employing CSMA/CD (see below) as the access method and using different Media types carrying a differential baseband signal at a rate of 1 or 10Mb/s, and 1 Media type (CATV-type cable) carrying a multiple-frequency broadband signal at a rate of 10Mbs. The description is presented as a 2 layered architecture (the Media Access Control (MAC) sublayer of the Data Link Layer, and the Physical Layer) corresponding to the lower layers of the OSI Reference Model for Open Systems Interconnection. To keep things simple we will limit our discussions to the 4 most popular LAN Media types carrying differential baseband signals at a rate of 10Mb/s. HISTORY (OR "What became of ETHERNET?") ANSI/IEEE Std 802.3. Nope! This is not ETHERNET. Ethernet was created by three companies back in the 70's; Digital Corp., Intel, and Xerox. Their LAN had all the basics of today's 802.3 Standard but wasn't an officially accepted standard until they submitted it to the Institute of Electrical and Electronic Engineers' IEEE Standards Board. Then it underwent some changes and revisions until it emerged as the popular LAN we all know and love. This first version of "ETHERNET" was referred to as DIX version I or Ethernet version 1.0; ran at 4 Mb/s; and was released in 1980. This was followed by Ethernet version 2.0 (released in 1982) and finally by the currently popular IEEE 802.3 standard released in 1984. COMPONENT FUNCTIONALITY (Pieces of the Hardware Puzzle) MAUs (OR "Who goes there? I am in control!") A MAU (Medium Attachment Unit - commonly called a Transceiver) is the means by which the DTE connects to the Medium (Coax/UTP/Fiber). One of it's most important functions is to monitor the Medium like a traffic cop to make sure the highway is clear to put our packets on. If it sees trouble out there then with the DTE's help it gets the Medium back under control (see CSMA/CD below). so that it can push our packets down to the end of that Medium segment (see Medium Physical Limitations below) at 10Mb/s. REPEATERs (OR "Say again? Your timing's off!") Repeaters repeat (pretty neat huh!?). They take an incoming packet and re-time it as well as re-generate the preamble and then immediately send it out the door to any other segments connected to them. Some repeaters can do extra things (see REPEATER EXTRA OPERATIONS below) but by and large they're pretty stupid. CSMA/CD (OR "Call 911! There's been a collision!") Occasionally a problem develops when two or more DTE's transmitters are in contention (butting heads) over who has ownership of the Medium. When this "collision" occurs all MAUs on the Network (including the ones belonging to the DTEs in contention) will activate their collision detection circuitry. This neat circuitry generates a Signal_Quality_Error message (Yes! The famous SQE! also referred to as Collision Detection) which is sent to the DTE transmitter (via the Control lines (CI-A/CI-B ; Collision +/Collision- ; AUI pins 2/9). Here it causes the transmitters in contention to continue transmitting a "JAM" of arbitrary data long enough for everyone on the Network to sense, ensuring that all collision detection circuitry activates. After the "JAM" data is sent out, the transmitters go into "BackOff" (shut-up for a bit) and re-transmit at a randomly determined future time. For all other DTEs their MAU's SQE message (Collision Detection) forces the transmitters to "BackOff" and then they also re-transmit at a randomly determined future time. The result is that everybody on the Network gets a chance to access the Network highway (Medium) without running into someone else trying the same thing. Hurrah! SQE (OR "Stop yakking! I've got a headache!") The SQE (Signal Quality Error; as defined by ANSI/IEEE std 802.3) message is generated by the collision detection circuitry of a MAU (Transceiver) to the DTE's transmitter under a variety of situations. The most common situation is after a collision is detected (see CSMA/CD above). A second situation is used to verify to DTEs running on Ethernet Ver II (DEC equipment comes to mind ) that the MAU/AUI is working. After transfer of one frame of data has been completed by the MAU to the Medium, the collision detection circuitry generates a pulsed SQE message to the DTE . Since this SQE message is injected into the inter-frame gap it has no effect on the transmitter except to verify that the MAU is working correctly. HOWEVER! MAUs can be connected to a repeater. This can cause problems with the repeaters because they tend to re- broadcast the SQE message to the rest of the Network (or at least the segments immediately connected to the repeater) or to Auto-Partition (see REPEATER EXTRA OPERATIONs below). Repeaters can't see a SQE message as anything else but a "collision". Most DTEs run on algorithms which do not necessitate SQE except for actual collisions or JABBER (current IEEE 802.3 defined frame types). Due to the effect that SQE has on Networks (via repeaters; excessive collisions at a minimum), it is HIGHLY RECOMMENDED that you turn the MAU SQE Test (or Heartbeat) switch "OFF" when installing the transceiver (that is of course, if your transceiver has one). SQE should only be set "ON" at the transceiver closest (attached) to a DTE that needs it. The last situation where the SQE message is generated is JABBER (see below). JABBER (OR "Hush my mouth; I talk too much!") In a MAU (Medium Attachment Unit - commonly called a Transceiver) there is a function called JABBER which under certain circumstances keeps transmitted data from reaching the Medium (Coax/UTP/Fiber). Hardware in the MAU monitors the transmit lines (DO-A/DO- B ; Transmit +/Transmit - ; AUI pins 3/10) from the DTE for frames that exceed a 20ms -150ms window in duration. If this condition occurs, the hardware inhibits data from reaching the medium by disabling these lines. Immediately following disablement of the transmit lines the collision detection circuitry is activated and a Signal_Quality_Error message (Yes! Once again, the famous SQE!) is sent to the DTE. The SQE message halts the DTE transmitter, causing it to re-start at a randomly determined future time. This allows other DTEs the opportunity to transmit on the Network without interference from a possibly faulty transmitter. LINK INTEGRITY (OR "You will receive") When data is not being transmitted, all 10BASE-T MAUs monitor the receive lines for a "link test pulse" transmitted by the MAU or hub at the other end every 16ms (+/- 8ms). If this is missed for a period somewhere between 50ms and 150ms, then the transmit circuitry is disabled until a series of "link test pulses" between 2 and 10 are once again sensed. During the time that no "link test pulses" are detected the green Link Integrity LED (if so equipped) will be OFF. REPEATER EXTRA OPERATIONs (OR "I'm not so dumb!") Besides the basics (see REPEATERs above) Repeaters do some other neat stuff. One of the most important is dealing with collisions. When a collision is detected at any port, a data stream of 1s and 0s are sent to all ports. This "JAM" of data is transmitted long enough for everyone on the Network to sense, ensuring that their MAU collision detection circuitry activates a SQE message (Collision Detection) to their DTEs. Another function the repeater has is JABBER LOCKUP PROTECTION. If the repeater transmits continuously for more than 5ms, it disables output for a brief period (9.6 - 11.6 microsecs) and then allows transmission again. An optional function that a repeater can be made to do is called AUTO-PARTITIONING/RECONNECTION. If the repeater monitors collisions for a continuous count of 32 then it will disable the connection until it no longer senses 32 collisions in a row. When this happens input to the repeater is blocked but output is not. REMEMBER that use of the SQE Test (or Heartbeat) switch in the ON position may cause a repeater to automatically segment all connections from all others by PARTIONING (at a maximum) or (at a minimum) slow down through-put on the Network by re-broadcasting JAMS to every connected segment. One last ability of a Repeater is to change from one Medium to another (such as Coax to twisted pair; twisted pair to fiber; fiber to Coax; etc. to etc.). TOPOLOGIES (OR "How do I get there from here?") There are two basic topologies used in 802.3 Networks; BUS and STAR (and a combination of both; but that(gets complicated). BUS topologies use COAX cable which is "TAPPED" or connected to at right angles. This is also sometimes called a BACKBONE. BUS topologies include 10BASE-5 and 10BASE-2. STAR topologies use twisted pair and fiber optic cable to go from here to there and cannot be "TAPPED". This is also sometimes called point-to-point. STAR topologies include 10BASE-T and 10BASE-FOIRL. Both topologies can be combined for a variety of Network configurations and Medium types. GENERAL NETWORK LIMITS (OR "How far can I go before I get lost?") No one path can be longer than 5 SEGMENTs long (4 repeaters) No one path shall pass through more than 4 repeaters (5 SEGMENTs) If using 5 SEGMENTs no more than 3 may actually be "tapped" (Applies to Coax only; UTP and Fiber cannot be tapped). The remaining 2 SEGMENTs are referred to as LINK SEGMENTs. i.e.; If I were a packet that just got squirted out of a workstation on my way to destiny (or a Server; whichever comes first!), then I could only go through 4 repeaters before I stopped. If I went through one more repeater I would have been out on the Network too long (called Propagation Delay) and some dumb transciever at the other end of the Network would probably allow it's transmitter to start transmitting. BOY! Would there be a collision! There can be a maximum of 1024 logical attachments to a Network (usually called NODEs). Each Node has a unique MAC (Media Access Control) address which nobody in the whole wide world has (yet). IEEE 802.3 TYPE 10BASE-5 This type describes a "thick" Coax cable as the Medium and has a specification called AUI describing the DTE to MAU connection. NICKNAMES: "BIG YELLOW" (PVC); "BIG ORANGE" (TEFLON); "THICKNET", BACKBONE CABLE: Consists of two cylindrical conductors with a common axis, separated by a dielectric (solid copper core, surrounded by a polyethylene dielectric, surrounded by a foil and double braided shield, surrounded by a Polyvinyl-Chloride (PVC) or TEFLON outer jacket. Lettered with "IEEE 802.3"). The outer jacket is marked with annular rings spaced every 2.5m (8.2') to indicate recommended "tap" points. Cable Sections of varying lengths can be connected together using barrel connectors to make up a SEGMENT (maximum cable length before a repeater is needed). Grounding should be applied to an earth ground from one point only on the SEGMENT. CABLE TYPE = COAX - .405 OD, 50ohm (802.3 SPEC'D) SEGMENT LENGTH = 500M (1640') MAXIMUM NETWORK LENGTH = 2.5KM (8202') # OF "TAPS"/SEGMENT = 100 CONNECTORS = "N" TYPE (BARREL, TAP, 50 OHM TERMINATOR) INSTALLATION: Transceivers are "tapped" directly to the Coax at the annular ring positions marked on the cable using a "Vampire" non-intrusive spike/clamp that pierces the dielectric to contact the center copper conductor without shorting to the braided shield. However, an "N" series "T" connector can also be used between sections of cable for transciever installation. The Coax cable is terminated at each end using "N" series 50ohm terminators. IEEE 802.3 TYPE 10BASE-5 AUI (ATTACHMENT UNIT INTERFACE) SPECIFICATION This specification describes that part of the 10BASE-5 standard dealing with the physical and electrical connections between the MAU and the DTE. CABLE: Typically consists of four twisted pairs of 78ohm 22 gauge wire surrounded by an overall foil shield. It may include a separate drain wire for each pair or one drain wire for the cable and is covered by a PVC or TEFLON outer jacket. The cable has a male connector at one end and a female with a slide-lock at the other. Note that DTEs (usually with a Network Interface Card installed) and repeater devices which typically have a female slide-lock connector will always be the source of power to the cable. CABLE TYPE = AUI (4-TWISTED PAIR WITH DRAIN) SEGMENT LENGTH = 50M (164') # OF "TAPS"/SEGMENT = NONE (POINT-TO-POINT) CONNECTOR = AUI 15-PIN (MALE/FEMALE) PINOUTS: AUI connectors are 15-pin D-shell connectors MALE connector per MIL-C-24308-1972[6] FEMALE connector per MIL-C-24308A-1972[6] PIN 1 = CONTROL IN - S (COLLISION SHIELD - OPTIONAL) PIN 2 = CONTROL IN - A (COLLISION +) PIN 3 = DATA OUT - A (TRANSMIT +) PIN 4 = DATA IN - S (DRAIN WIRE) PIN 5 = DATA IN - A (RECEIVE +) PIN 6 = VOLTAGE COMM (POWER -) PIN 7 = CONTROL OUT - A (OPTIONAL CIRCUIT) PIN 8 = CONTROL OUT SHIELD (OPTIONAL CIRCUIT) PIN 9 = CONTROL IN - B (COLLISION -) PIN 10 =DATA OUT - B (TRANSMIT -) PIN 11 =DATA OUT SHIELD (TRANSMIT SHIELD - OPTIONAL) PIN 12 =DATA IN - B (RECEIVE -) PIN 13 =VOLTAGE PLUS (POWER +) PIN 14 =VOLTAGE SHIELD (POWER SHIELD - OPTIONAL) PIN 15 =CONTROL OUT - B (OPTIONAL CIRCUIT) SHELL PROTECTIVE GROUND IEEE 802.3 TYPE 10BASE-2 This type describes a "thin" Coax cable using BNC connectors as the Medium. NICKNAMES: "THINNET"; "CHEAPERNET" CABLE: Consists of two cylindrical conductors with a common axis, separated by a dielectric (solid copper core, surrounded by a polyethylene dielectric, surrounded by a double braided shield, surrounded by a Polyvinyl-Chloride (PVC) outer jacket). The cable is spec'd as RG58/U MIL C-17. Cable Sections of varying lengths can be connected together using BNC barrel connectors to make up a SEGMENT (maximum cable length before a repeater is needed). CABLE TYPE = COAX (RG58/U) -.185 OD,50ohm (JAN-C17A) SEGMENT LENGTH = 185M (607') MAXIMUM NETWORK LENGTH = 925M (3034') # OF "TAPS"/SEGMENT = 30 CONNECTOR = BNC (BARREL, TAP, 50 OHM TERMINATOR) INSTALLATION: Connections are made to the Coax using a BNC "T" connector between sections of cable. DTEs must be connected directly to the bottom leg of the "T". No additional drop cables are allowed. Spacing of connections is .5m (1.6'). Grounding should be applied to an earth ground from one point only on the SEGMENT. The Coax cable is terminated at each end using BNC type 50ohm terminators. IEEE 802.3 TYPE 10BASE-T This type describes a "twisted pair" cable using RJ45 connectors as the Medium. NICKNAMES: "TWISTED PAIR"; "UTP" CABLE: Typically consists of four twisted pairs of CAT 3 TO CAT 5 UTP (100ohm) or STP (150ohm) type cable. The EIA/TIA 568 provides specs and wiring for this type cable. CABLE TYPE = UTP (UNSHIELDED TWISTED PAIR) STP (SHIELDED TWISTED PAIR - NICs ONLY) CAT 3 TO CAT 5 SEGMENT LENGTH = 100M (328') MAXIMUM NETWORK LENGTH = 500M (1640') # OF "TAPS"/SEGMENT = NONE (POINT-TO-POINT) CONNECTOR = RJ45 (MALE ON CABLE/FEMALE ON DEVICE) INSTALLATION: Connections are made point-to-point using 8-pin RJ45 male connectors. There are no cable terminators. PINOUTS: (STRAIGHT THROUGH; DTE TO HUB) PIN 1 = TRANSMIT DATA + = PIN 1 PIN 2 = TRANSMIT DATA - = PIN 2 PIN 3 = RECEIVE DATA + = PIN 3 PIN 4 = N/A = PIN 4 PIN 5 = N/A = PIN 5 PIN 6 = RECEIVE DATA - = PIN 6 PIN 7 = N/A = PIN 7 PIN 8 = N/A = PIN 8 SHIELD GROUND (USED ON STP CABLE ONLY) (CROSSOVER; DTE TO DTE, HUB TO HUB, XCVR TO XCVR) PIN 1 = TRANSMIT DATA + = PIN 3 PIN 2 = TRANSMIT DATA - = PIN 6 PIN 3 = RECEIVE DATA + = PIN 1 PIN 4 = N/A = PIN 4 PIN 5 = N/A = PIN 5 PIN 6 = RECEIVE DATA - = PIN 2 PIN 7 = N/A = PIN 7 PIN 8 = N/A = PIN 8 SHIELD GROUND (USED ON STP CABLE ONLY) IMPORTANT NOTE: Make sure that you always wire pins 1 and 2 from one twisted pair and pins 3 and 6 from a second. That will leave two twisted pairs that are not used although they may be wired. IEEE 802.3 TYPE 10BASE-FOIRL This type describes a duplex fiber optic cable using either ST or SMA connectors as the Medium. NICKNAMES: "FIBER" CABLE: Typically consists of a two fiber (transmit and receive) optical cable spec'd at 62./125 uM (most commonly used) 100/140 uM, or 50/125 uM. CABLE TYPE = FIBER OPTIC (DUPLEXED MULTIMODE-GRADED INDEX GLASS 62.5/125uM (TYPICAL)) SEGMENT LENGTH = 1000M (3280') MAXIMUM NETWORK LENGTH = 5000M (16404') # OF "TAPS"/SEGMENT = NONE (POINT-TO-POINT) CONNECTOR = SMA (THREADED) ST (TWIST-ON) INSTALLATION: Connections are made point-to-point (TRANSMIT to RECEIVE) using ST or SMA male connectors. There are no cable terminators. IEEE 802.3 FRAME FORMAT Preamble = 7 Octets Start Frame Delimiter = 1 Octet Destination Address = 2 or 6 Octets Source Address = 2 or 6 Octets Length = 2 Octets Data Unit and PAD = 46 - 1500 Octets Frame Check Sequence = 4 Octets LSB --------------------------------------------------> MSB OTHER NEAT SHTUFF! Maximum Frame Size = 1518 Octets (8-BIT BYTES) Minimum Frame Size = 64 Octets InterFrame Gap = 9.6 nanosecs Slot time = 512 bit times Repeater Jam Size = 32 bits DTE Re-transmission limit = 16 attempts Transmitter Back-OFF limit = 10 times Packet Encoding = Manchester