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4.3 Etherent

4.3 Etherent. IEEE 802 Group. 4.3 Etherent. 4.3 Etherent. Classical Ethernet. IEEE 802.3: 1-persistent CSMA/CD. Switched Ethernet. Fast Ethernet (100Mbps), Gigabit Ethernet, 10G Ethernet. 4.3.1 Classical Ethernet Physical Layer. MIT->Harvard->Hawaii->Xerox PARC

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4.3 Etherent

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  1. 4.3 Etherent IEEE 802 Group

  2. 4.3 Etherent

  3. 4.3 Etherent Classical Ethernet IEEE 802.3: 1-persistent CSMA/CD Switched Ethernet Fast Ethernet (100Mbps), Gigabit Ethernet, 10G Ethernet

  4. 4.3.1 Classical Ethernet Physical Layer MIT->Harvard->Hawaii->Xerox PARC (Palo Alto Research Center)->Ethernet ->3COM

  5. 4.3.1 Classical Ethernet Physical Layer The Xerox Ethernet was so successful that DEC, Intel, and Xerox drew up a standard in 1978 for a 10-Mbps Ethernet, called DIX standard. DIX became IEEE 802.3 in 1983.

  6. 4.3.1 Classical Ethernet Physical Layer

  7. 4.3.1 Classical Ethernet Physical Layer Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented

  8. 4.3.1 Classical Ethernet Physical Layer To allow larger networks, multiple cables can be connected by repeaters. A repeater is a physical layer device. It receives, amplifies, and retransmits signals in both directions. As far as the software is concerned, a series of cable segments connected by repeaters is no different than a single cable.

  9. 10BASE5 10BASE2 1BASE5 10BROAD36 10BASE-T Ethernet Cheaper net StarLAN Broadband Twisted-pair coaxial cable 50ohm-10mm coaxial cable 50ohms-5mm twisted-pair unshielded coaxial cable 75ohms 2 simplex TP unshielded medium 10Mbps Manch 10Mbps Manch 1Mbps Manch 10Mbps DPSK 10Mbps Manch signals maximum segment 500m 185m 500m 1800m 100m maximum distance 2.5km 0.925km 2.5km 3.6km 1km nodes per segment 2 100 30 activity on receiver and transmitter collision detection 2 active hub inputs transmission =reception excess current Notes slot time=512 bits; gap time=96 bits; jam=32 to 48 bits

  10. 4.3.1 Classical Ethernet Physical Layer Manchester Encoding

  11. 4.3.2 Classical Ethernet MAC Sublayer Protocol Frame formats. (a) Ethernet (DIX). (b) IEEE 802.3. >1500 is type, otherwise interprets as length

  12. 4.3.2 Classical Ethernet MAC Sublayer Protocol The first 3 bytes are OUI (Organizationally Unique Identifier) (Manufacturer) 802.3 frame format single address 0 multicast (all 1's for broadcast) 1 group address 0 local address No significance outside one of 246 unique address 1 global address

  13. 4.3.2 Classical Ethernet MAC Sublayer Protocol 802.3 frame format Minimum frame length: 64 bytes (6+6+2+46+4)

  14. 4.3.2 Classical Ethernet MAC Sublayer Protocol For a 10 Mbps LAN with a maximum length of 2500 meters (with 4 repeaters), the round-trip time is 50 msec in the worst case. (10M)x(50 msec) =500 bits~512 bits=64 bytes Checksum= 32-bit CRC= x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

  15. 4.3.2 Classical Ethernet MAC Sublayer Protocol 802.3 frame format As the network speed goes up, the minimum frame length must go up or the maximum cable length must come down proportionally. For a 2500-meter LAN operating at 1 Gbps, the minimum frame size would have to be 6400 bytes. Alternatively, the minimum frame size could be 64 bytes and the maximum distance between any two stations 250 meters.

  16. 4.3.2 Classical Ethernet MAC Sublayer Protocol Ethernet Frame Structure v2 (or DIX Ethernet, for DEC, Intel, Xerox) 7 1 6 6 2 4 Data preamble SFD DA SA type CRC 60 to 1514 bytes synchronize the receiver Cyclic Redundancy Check Type>0x0600=1536 0800: IPv4 datagram 0806: ARP request/reply 8035: RARP request/reply 86DD: IPv6 start frame delimiter

  17. 4.3.2 Classical Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm If a frame has collided n successive times, where n<16, then the node chooses a random number K with equal probability from the set {0,1,2,3,...,2m-1} where m=min{10,n}. The node then waits for bit times. (slot time=512 bit time) after first collision after second collision after third collision select one to start transmission

  18. 4.3.2 Classical Ethernet MAC Sublayer Protocol Acknowledgements As far as CSMA/CD is concerned, an acknowledgement would be just another frame and would have to fight for channel time just like a data frame. (What is the problem?) A simple modification would allow speedy confirmation of frame receipt. All that would be needed is to reserve the first contention slot following successful transmission for the destination station.

  19. 4.3.3 Ethernet Performance Performance Assume k stations are always ready to transmit and a constant retransmission probability in each slot. (A rigorous analysis of the binary exponential backoff algorithm is complicated.) If each station transmits during a contention slot with probability p, the probability A that some station acquires the channel in that slot is

  20. 4.3.3 Ethernet Performance Performance The probability that the contention interval has exactly j slots in it is A(1-A)j-1, so the mean number of slots per contention is given by Since each slot has a duration 2t, the mean contention interval, w, is 2t/A. Assuming optimal p, the mean number of contention slots is never more than e, so w is at most 2te5.4t.

  21. 4.3.3 Ethernet Performance Performance If the mean frame takes P sec to transmit, when many stations have frames to send, channel efficiency= Here we see where the maximum cable distance between any two stations enters into the performance figures. The longer the cable, the longer the contention interval. By allowing no more than 2.5km of cable and four repeaters between any two transceivers, the round-trip time can be bounded to 51.2 msec, which at 10Mbps corresponds to 512 bits or 64 bytes, the minimum frame size.

  22. 4.3.3 Ethernet Performance Performance Let P=F/B (frame_length/bandwidth) and t=L/C (cable_length/signal_propagation_speed). For the optimal case of e contention slots per frame, channel efficiency= Increasing network bandwidth or distance (the BL product) reduces efficiency for a given frame size. Unfortunately, much research on network hardware is aimed precisely at increasing this product. People want high bandwidth over long distances, which suggests that 802.3 may not be the best system for these applications.

  23. 4.3.3 Ethernet Performance

  24. 4.3.3 Ethernet Performance Many theoretical analysis assume the input traffic is Poisson. It now appears that network traffic is rarely Poisson, but self-similar. What this means is that averaging over long periods of time does not smooth out the traffic. The average number of packets in each minute of an hour has as much variance as the average number of packets in each second of s minute. The consequence of this discovery is that most models of network traffic do not apply to the real world and should be taken with a grain of salt.

  25. 4.3.4 Switched Ethernet Not necessarily this kind of wiring Just like a single cable Ethernet (a) Hub. (b) Switch. Must know which station is in which port

  26. 4.3.4 Switched Ethernet An Ethernet switch.

  27. 4.3.5 Fast Ethernet Fast Ethernet The three primary reasons that the 803 committee decided to go with a souped-up 802.3 LAN (instead of a totally new one) were: 1. The need to be backward compatible with thousands of existing LANs. 2. The fear that a new protocol might have unforeseen problems. 3. The desire to get the job done before the technology changed.

  28. 4.3.5 Fast Ethernet Fast Ethernet The basic idea behind fast Ethernet was simple: keep all the old packet formats, interfaces, and procedural rules, but just reduce the bit time form 100 nsec to 10 nsec. Technically, it would have been possible to copy 10Base5 or 10Base2 and still detect collisions on time by just reducing the maximum cable length by a factor of ten. However, the advantages of 10BaseT wiring were so overwhelming that fast Ethernet is based entirely on this design. Thus all fast Ethernet systems use hubs and switches.

  29. 4.3.5 Fast Ethernet Fast Ethernet The category 3 UTP scheme, called 100Base-T4, uses a signaling speed of 25 MHz, only 25 percent faster than standard 802.3’s 20 MHz. To achieve the necessary bandwidth, 100BaseT4 requires four twisted pairs.

  30. 4.3.5 Fast Ethernet Fast Ethernet Of the four twisted pairs, one is always to the hub, one is always from the hub, and the other two are switchable to the current transmission direction. To get the necessary bandwidth, Manchester encoding is not used, but with modern clocks and such short distances, it is no longer needed.

  31. 4.3.5 Fast Ethernet Fast Ethernet Ternary signals are sent, so that during a single clock period the wire can contain a 0, a 1, or a 2. With three twisted pairs going in the forward direction and ternary signaling, any one of the 27 possible symbols can be transmitted, making it possible to send 4 bits with some redundancy. Transmitting 4 bits in each of the 25 million clock cycles per second gives the necessary 100 Mbps. In addition, there is always a 33.3 Mbps (100/3) reverse channel using the remaining twisted pair.

  32. 4.3.5 Fast Ethernet Fast Ethernet For category 5 wiring, the design, 100Base-TX, is simpler because the wires can handle clock rates up to 125 MHz and beyond. Only two twisted pairs per station are used, one to the hub and one from it. Rather than just use straight binary coding, a scheme called 4B5B is used at 125 MHz. Every group of 5 clock periods is used to send 4 bits in order to give some redundancy, provide enough transitions to allow easy clock synchronization, create unique patterns for frame delimiting, and be compatible with FDDI in the physical layer.

  33. 4.3.5 Fast Ethernet Fast Ethernet Consequently, 100Base-TX is a full-duplex system; stations can transmit at 100 Mbps and receive at 100 Mbps at the same time. Often 100Base-TX and 100Base-T4 are collectively referred as 100Base-T. The last option, 100Base-FX, uses two strands of multimode fiber, one for each direction, so it, too, is full duplex with 100 Mbps in each direction. In addition, the distance between a station and the hub can be up to 2 km.

  34. 4.3.6 Gigabit Ethernet Gigabit Ethernet The ink was barely dry on the fast Ethernet standard when the 802 committee bagan working on a yet faster Ethernet. It was quickly dubbed gigabit Ethernet and was ratified by IEEE in 1999 under the name 802.3ab. An important design goal: remain backward compatibility

  35. 4.3.6 Gigabit Ethernet Gigabit Ethernet All configurations of gigabit Ethernet are point-to-point. Each individual Ethernet cable has exactly two devices on it, no more and no fewer.

  36. Chapter 4 The Medium Access Sublayer 4.3.6 Gigabit Ethernet Gigabit Ethernet Two different modes of operation: full duplex and half duplex The normal mode is full-duplex used when computers are connected to a switch. The sender does not have to sense the channel to see if anybody else is using it because contention is impossible. So CSMA/CD protocol is not used. So the maximum length of the cable is determined by signal strength issues rather than by the collision detection issue.

  37. 4.3.6 Gigabit Ethernet Gigabit Ethernet Half-duplex is used when the computers are connected to a hub. A hub does not buffer incoming frames. So collisions are possible and CSMA/CD is required. But now the transmission time for a 64-byte frame is 100 times faster. So the distance is 100 times less than Ethernet. That is, only 25 meters. The 802.3ab committee considered a radius of 25 meters to be unacceptable and added two features to the standard to increase the radius.

  38. 4.3.6 Gigabit Ethernet Gigabit Ethernet The first feature, called carrier extension, essentially tells the hardware to add its own padding to extend the frame to 512 bytes. Of course, using 512 bytes to transmit 64 bytes of data has a line efficiency of 9%. The second feature, called frame bursting, allows a sender to transmit a concatenated sequence of multiple frames in a single transmission. If the total length is less than 512 bytes, the hardware pads it again. Just for backward compatibility. Most will use switches.

  39. 4.3.6 Gigabit Ethernet Gigabit Ethernet Cabling Gigabit Ethernet uses new encoding rules on the fiber. Manchester encoding at 1Gbps would require 2G baud signal, too difficult and too wasteful.

  40. 4.3.6 Gigabit Ethernet Gigabit Ethernet • 8B/10B is used. Each 8-bit byte is encoded as 10 bits. • 256 out of 1024. Two rules are used: • No codeword may have more than four identical bits in a row. • No codeword may have more than six 0s or six 1s. In addition, many input bytes have two possible codewords assigned to them. When there is a choice, the encoder always chooses the one that tries to equalize the number of 0s and 1s transmitted so far.

  41. 4.3.6 Gigabit Ethernet Gigabit Ethernet 1000Base-T uses a different encoding scheme since clocking data onto copper wire in 1 nsec is too difficult. The solution uses four category 5 twisted pairs to allow four symbols to be transmitted in parallel. Each symbol is encoded using one of five voltage levels. This scheme allows a single symbol to encode 00, 01, 10, 11, or a special value for control purposes. The clock runs at 125MHz, allowing 1-Gbps operation.

  42. 4.3.6 Gigabit Ethernet Gigabit Ethernet Gigabit Ethernet supports flow control which consists of one end sending a special control frame to the other end telling it to pause for some period of time. For gigabit Ethernet, the time unit for pause is 512 nsec. The maximum is 33.6 msec.

  43. 4.3.7 10-Gigabit Ethernet 10 Gigabit Ethernet cabling

  44. 4.3.8 Retrospective on Ethernet Ethernet has been around for over 30 years and has no serious competitions. Few CPU architectures, operating systems, or programming languages have been king of the mountain for three decades going on strong. Clearly, Ethernet did something right. What?

  45. 4.3.8 Retrospective on Ethernet Simple and Flexible Simple translates into reliable, cheap, and easy to maintain. Ethernet interworks easily with TCP/IP, which has become dominant. (Both are connectionless) Lastly and perhaps the most importantly, Ethernet has been able to evolve in certain crucial ways

  46. 4.3.8 Retrospective on Ethernet • In anyway, Keep It Simple & Splendid Keep It Simple & Stupid Keep It Simple & Stable KISS

  47. 4.3.8 Retrospective on Ethernet • What I see in the Korea Customs: • Korea Immigration Smart Service

  48. 4.4 Wireless LANS

  49. 4.4.1 The 802.11 Architecture and Protocol Stack To Network AccessPoint Client 802.11 architecture – infrastructure mode

  50. 4.4.1 The 802.11 Architecture and Protocol Stack 802.11 architecture – ad-hoc mode

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