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Ethernet Basics Explained
- 2. SchutzvermerknachDIN34beachten The diagram .. was drawn by Dr. Robert M. Metcalfe in 1976 to present Ethernet to the National Computer Conference in June of that year. Ethernet
- 3. SchutzvermerknachDIN34beachten Topics • History, Standards, Terminology • Transmission media • Topologies • Protocol • Access methods, Collision management
- 4. SchutzvermerknachDIN34beachten What is Ethernet ? Ethernet is a certain type of a local area network (LAN) which was developed in 1972 in the renowned PARC- research facility of Xerox in Palo Alto by Robert Metcalfe. In the meantime the companies Intel, DEC and Xerox have specified a common standard that has been established in the IEEE-standard 802.3.
- 5. SchutzvermerknachDIN34beachten History • 1969 student Robert Metcalfe (founder of 3Com in 1979) develops a Host Interface Controller for DARPA (Defense Advanced Research Projects Agency) in the company DEC. • 1970 the ALOHA-Net (multiple access protocol) is developed and tested at the university of Hawaii • 1972 the idea is picked up by the XEROX Palo Alto Research Center (Metcalfe works there by then). The project goal is: experimental Ethernet
- 6. SchutzvermerknachDIN34beachten History • 1976 the results of the project are published. The companies DEC, Intel and Xerox join in the company DIX and complete Ethernet to the market entry stage. • 1980 Ethernet version 1.0 is passed. • 1981 IEEE starts standardization efforts. The Ethernet specification is accepted without major modifications. • 1982 Publication of Ethernet version 2.0 • 1985 worldwide recognition of the Ethernet standard as ISO/DIS 8802/3
- 7. SchutzvermerknachDIN34beachten History • 1986 Publication of the 10Base2- and 10BroadT standards • 1987 Standardization of the 10BaseT spezification • 1991 Publication of the 10BaseF standard • 1994 more than 10.000 suppliers support the Ethernet globally • 1995 Standardization of the 100 Mbit/s Ethernet • 1997 Standardization efforts for the Gigabit Ethernet and presentation of first products prior to the completion of the standard
- 9. SchutzvermerknachDIN34beachten Access method: CSMA/CD Station is ready to send check “Ether” Sending of data and checking the “Ether” Waiting according to back-off algorithm Medium occupied Discovered collision medium available send jam signal No collision New attempt
- 10. SchutzvermerknachDIN34beachten Back-Off Algorithm • If a collision has occurred, the stations try to send again after a certain period of time. • After the first collision there a two different back-off times available, from which one is chosen at random. Transmission probability is 50% • After the second consecutive collision there are four different back-off times available, from which one is chosen at random. • The transmission probability now is 75%
- 11. SchutzvermerknachDIN34beachten Truncated Binary Exponential Back-Off-Algorithm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ... ...... ......... ............... ............ ............... ............... ............... ............... ............... ............... ............... Nuber of back-off times to be selected at random 2 4 8 16 32 64 128 256 512 1024 1024 1024 1024 1024 1024 1024 50% 75% 87,5% 93,75% 96,88% 98,44 99,22% 99,61% 99,80% 99,90% 99,90% 99,90% 99,90% 99,90% 99,90% 99,90%
- 12. SchutzvermerknachDIN34beachten Which waiting times are used ? • The 0...1024 fold of the double max. signal travel time between the most remote stations + Offset is used • With 10 Mbit/s Ethernet that means: • The waiting time is also called collision window, the offset (9.6µs) is called gap. • Only after the time of the collision window has passed, you can be certain that there will be no more collision. Station 1 Station 2 25.6µs 25.6µs 51.2µs
- 13. SchutzvermerknachDIN34beachten Example • After the first collision the stations willing to send select a random waiting time of either 9.6µs or 9.6µs plus 51.2µs (duration of the collision window). Condition: Only two stations are involved, no new stations enter the scene in the collision management phase. Waiting time(A) waiting time(B) transmission 9.6µs 9.6µs NO 9.6µs 9.6µs+51.2µs YES 9.6µs+51.2µs 9.6µs YES 9.6µs+51.2µs 9.6µs+51.2µs NO
- 14. SchutzvermerknachDIN34beachten Delay depending on the network load delay 20 % 40 %30 %10 % 50 % 60 % 70 % 80 % Network load low high High throughput Beginning problems Overload
- 15. SchutzvermerknachDIN34beachten Ethernet address • Also called "MAC address" • Globally unique ID for each device • Burnt into ROM, cannot be modified • Six Bytes in which manufacturer, device model and serial number are coded • Readable with many auxiliary tools e.g. WINIPCFG
- 16. SchutzvermerknachDIN34beachten Ethernet frame Preamble SFD DA SA LEN FCSPadData 7 1 6 6 2 >=46 4 Preamble DA SA Type FCSPadData 8 6 6 2 >=46 4 Ethernet II DIX Frame: IEEE 802.3 Frame:
- 17. SchutzvermerknachDIN34beachten Ethernet frame • Preamble Trailer consisting of the bit sequence “0101010101...” serving the bit synchronization of the receiver. • SFD (Start Frame Delimiter) Start character consisting of the bit pattern “10101011” showing the recipient that the actual information will follow now. • DA (Destination Address) Evaluated by the recipient‘s address filter; only data frames destined for this recipient will be passed on to the communication software. • SA (Source Address) Sender‘s address • LEN (Length) Indicates the length of the subsequent data field in Bytes according to IEEE 802.3.
- 18. SchutzvermerknachDIN34beachten Ethernet frame • Data and Pad The data field may contain 46 to 1500 user data bytes. Are there less than 46 bytes the Ethernet controller independently adds padding bytes, until the total amount (data + pad) is 46. This miminum length is crucial for the CSMA/CD procedure to work faultlessly. The data field can be used at will, it only has to contain complete bytes. • FCS (Frame Check Sequence) A check character. It is obtained by taking the rest of the division operation from the formula representing the wide-spread cyclic- redundancy-check procedure. This formula is applied to the bit sequence including the address field through to the padding field. In case of en error the whole frame is ignored, i.e. not passed on to the application program.
- 20. SchutzvermerknachDIN34beachten Naming of the cable types • Example: 10base5 – 10 Transmission rate in Mbytes/s – base Base or Broadband – 5Segment length in 100 meters • UTP unshielded twisted pair • STP shielded twisted pair • S/STP screened shielded twisted pair
- 23. SchutzvermerknachDIN34beachten Extension • The maximum extension depends on the medium and the transmission rate; here some examples: – 10base5 Segment: 500m Total: 2500m (with 4 repeaters) – 100baseTX UTP Hub-Station: 100m – 100baseFX Hub-Station: 400m 25km (with Mono mode fibre) – 1000baseSX Hub-Station: 550m
- 26. SchutzvermerknachDIN34beachten Switch = Multiport Bridge Bridge Switch
- 28. SchutzvermerknachDIN34beachten Characteristics of the switches • Cut-Trough Switch – noc cheking of the data frames • Store-and-Forward – checking of the data frames • Frames with same destination – kept in internal short term memory thus queueing them – discard them or create collision • Broadcast messages – go to all stations anyway (z.B. ARP) so switches are of no advantage here – there are specific approaches of different switch manufacturers to reduce broadcast data traffic
- 29. SchutzvermerknachDIN34beachten Typical office wiring Hub/Switch Hub/Switch PatchfieldPatchfield Network socket Patch cable „normal“ Cat 5 cable RJ 45
- 30. SchutzvermerknachDIN34beachten www • http://www.gigabit-ethernet.org • http://wwwhost.ots.utexas.edu/ethernet/ • http://www.3com.com/technology/tech_net/white_papers/index.html#ethernet • http://www.iaopennetworking.com/
Editor's Notes
- After the second collision all station choose a random waiting time of 9,6µs plus [0, 1, 2, or 3] times 51,2µs. With two stations having created the collision the probability for success will now be 75%. All stations memorize the number of consecutive collisions and double the number of waiting times to randomly choose from. After the third collision that means 9,6µs plus [0, 1, 2, 3, 4, 5, 6 or 7] times 51,2µs. After 15 consecutive collisions the algorithms cancels the transmission. Now it is the task of the superior communications software to decide further actions (e.g. renewed transmission).
- The algorithm descibed is called “Truncated Binary Exponential Backoff Algorithm, BEB”, short „Back-Off Algorithm“. The desciption of the algorithm makes clear, that in extreme cases there can be substantial delays in the transmission, it can even happen that the telegram is not transmitted after all.
- Repeater The repeater does not regard any protocol information, but simply amplifies the signal to allow longer distances for networks. It can also be used to switch between media, e.g. from copper to fibre optics. A repeater with several ports is called hub.
- Bridges A bridge connects two or more networks and exchanges data packages between them. Since a bridge operates on level 2 of the OSI model it is independent from higher level protocols (e.g. TCP/IP). Bridges are intelligent devices, which are monitoring the complete traffic. Depending on the telegrams MAC address in combination with internal address tables they decide if the telegram will be forwarded or ignored.
- Switch Switches, also named “Switching Hubs” or “Layer 2 Switches”, are improved brigdes. Switches are nothing but Bridges with several ports, why they are also called “Multiport-Bridge”. Each port of a switch can be regrded as a collision domain of its own. Collision between devices connected to a switch can so be excluded. As opposed to the hub, which reads, amplifies and then distributes the telegrams on all ports, the switch reads the telegrams, checks the destination address and then forwards the telegram only to the port the destination device is attached to.Defective telegrams will not be forwarded at all.
- The information, which device is located on which port, is gathered by the switch by analyzing the telegrams‘ source addresses. Thus it creates a table containing the relevant information about each station‘s location. Switches are capable of dealing with a bigger amount of data without experiencing the collisions-related delays that occur when using hubs. Hubs can be replaced by switches in order to enhance the performance of the network. To be able to maintain several connections at the same time a switch internally processes data much faster than they are arriving at the ports or being sent out. A switch that is labeled “Nonblocking” has an internal transmission rate corresponding to the sum of all maximum transmission speeds at the ports. For a 10/100MBit/s Switch (suitable for 10 as well as 100MBit/s) with 8 ports that means that – taking into account the full-duplex mode (will be explained later-on) - it has to offer an internal transmission speed of 1600MBit/s (100MBit/s * 8 Ports *2 [due to full-duplex]). This level of performance is normally not required and also too expensive. An internal speed of 800MBit/s is normally sufficient for such a switch, which will then be called “Blocking Switch”. In addition to the high internal apeed the switches offer a short-term memory to temporarily buffer telegrams arriving at the same time and determined for the same destination port.
- Nowadays most STP and UTP-wirings are realized with RJ45 connectors, also called Western connectors. The pinning of those connectors is standardized (compliant to IEC, DIN etc.), where the pairs are identical in each standard, but the colours can differ.