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Unit – 4 -Wired LANs: Ethernet

Unit – 4 -Wired LANs: Ethernet. Overview. IEEE standards Standard Ethernet Changes in the standards Fast Ethernet Gigabit Ethernet Wireless LAN IEEE 802.11. IEEE STANDARDS.

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Unit – 4 -Wired LANs: Ethernet

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  1. Unit – 4 -Wired LANs: Ethernet

  2. Overview • IEEE standards • Standard Ethernet • Changes in the standards • Fast Ethernet • Gigabit Ethernet • Wireless LAN IEEE 802.11

  3. IEEE STANDARDS In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols. ISO approved 802 under the designation ISO8802 and is adopted by ANSI in 1987. Data Link LayerPhysical Layer

  4. IEEE Standards Relationship of 802 with OSI model • Data link layer is subdivided into logical link control (LLC) and media access control (MAC) • IEEE also created several physical layer standards for different LAN protocols.

  5. Data Link Layer • Logical Link Control (LLC) • Handles framing, flow control and error control. • In IEEE 802 project flow control, error control and part of the framing duties are collected into one sublayer called LLC • Framing • LLC defines a protocol data unit PDU that is somewhat similar to that of HDLC. • The header contains control field like the one in HDLC; this field is used for flow and error control. • Destination service access point DSAP and the source service access point SSAP

  6. HDLC Frames • Frames contain 6 fields: Flag, address, control, Information, Frame check sequence, and end Flag

  7. IEEE Standards HDLC frame compared with LLC and MAC frames

  8. Data Link Layer • Need for LLC • Provides flow and error control for the upper layer protocols • Media access control: • IEEE project 802 has created a sublayer called media access control and defines the specific access methods for each LAN. Ex: CSMA/CD access method for Ethernet LANs and token passing method for Token Ring and Token Bus LANs.

  9. STANDARD ETHERNET The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has gone through four generations. Standard Ethernet (10 Mbps) Fast Ethernet (100 Mbps) Gigabit Ethernet (1 Gbps) Ten-Giga Ethernet (10 Gbps) MAC SublayerPhysical Layer

  10. Ethernet Ethernet evolution through four generations

  11. MAC Sublayer

  12. 802.3 Frame Format • Preamble: • Contains 7 bytes of alternating 0s and 1s. Alerts the receiver and enables the receiver for synchronizing its input timing • 56 bit pattern allows the receiver to miss some of bits at the beginning of the frame. • The preamble is actually added at the physical layer and is not part of the frame.

  13. 802.3 Frame Format • Start frame delimiter (SFD): • 2nd field 1 byte: 10101011, signals the beginning of the frame. • Warns the stations that this is the last chance for synchronization. • Last 2 bits 11 alerts the receiver that the next field is the destination address.

  14. 802.3 Frame Format • Destination address (DA): • DA field is 6 bytes and contains the physical address of the destination to receive the packet • Source address (SA): • SA field is 6 bytes and contains the physical address of the sender of the packet

  15. 802.3 Frame Format • Length or type: • Number of bytes in the data field • Data: • Data encapsulated from the upper layer protocol. Min 46 and maximum of 1500 bytes • CRC: • Error detection information, CRC-32

  16. 802.3 Frame Length • The minimum length required for the correct operation of the CSMA/CD is 64 bytes. • 64-(6+6+2+4)=46 bytes is of data • The maximum length of a frame is 1500 bytes • Memory, buffer size, avoids the blocking of other stations

  17. Frame length: Minimum: 64 bytes (512 bits) Maximum: 1518 bytes (12,144 bits)

  18. Addressing • Each station on an Ethernet network has its own network interface card (NIC) and has 6 byte physical address Unicast and multicast addresses • Source address is always unicast addres, destination address however can be unicast, multicast or broadcast • If the LSB of the first byte in the destination address is 0 the address is unicast otherwise it is multicast • The broadcast destination address is a special case of the multicast address in which all bits are 1s.

  19. Access Method: CSMA/CD • Standard Ethernet uses 1-persistent CSMA/CD • Slot time: • Round trip time required for a frame to travel from one end of a maximum length network to the other plus the time needed to send the jam sequence is called the slot time. • slot time =round trip time + time required to send the jam sequence • Slot time in Ethernet is defined as the time required for a station to send 512 bits. 10-Mbps Ethernet it is 51.2 micro second.

  20. Access Method: CSMA/CD • Slot time and Maximum Network Length: • MaxLength=Propagation Speed*(SlotTime/2) • =(2*108)*(51.2*10-6/2)=5120m

  21. Physical layer

  22. Physical Layer • The standard Ethernet defines several physical layer implementations; four of the most common are:

  23. Physical Layer Encoding in a Standard Ethernet implementation

  24. 10Base5: Thick Ethernet • First implementation thick Ethernet or Thicknet, name is due to size of the cable and used for bus topology with an external transceiver connected via tap to thick coaxial cable. • Transceiver is responsible for transmitting receiving and detecting collisions, maximum length of the cable is 500 meters.

  25. 10Base2: Thin Ethernet • Used for bus topology but the cable is much thinner and more flexible. Transceiver is normally part of the network interface card (NIC) which is inside the station. • Collisions occur in the thin coaxial cable and cable is inexpensive than thick coaxial cable. Maximum length of the cable is 185 meter (close to 200).

  26. 10Base-T: Twisted pair Ethernet • Uses star topology • Stations are connected to a hub via two pairs of twisted cable. • Two pairs of twisted cable create two paths (one Tx and one for Rx) • Collisions occur in the hub • Maximum length of the cable is 100 meter.

  27. 10Base-F: Fiber Ethernet • Uses star topology • Stations are connected to a hub via two pairs of twisted cable. • Two pairs of twisted cable create two fiber optic cables. • Collisions occur in the hub • Maximum length of the cable is 2000 meter.

  28. Summary of Standard Ethernet implementations

  29. CHANGES IN THE STANDARD The 10-Mbps Standard Ethernet has gone through several changes before moving to the higher data rates. Bridged EthernetSwitched EthernetFull-Duplex Ethernet

  30. Bridged Ethernet Sharing bandwidth • Bridges raise the bandwidth and they separate the collision domains. • In an unbridged network the total bandwidth (10 Mbps) is shared among all the stations. • If only one station has frames to send it benefits from the total bandwidth. If more than one station use the network then the capacity is shared. • Figure shows the two stations occupy the slots in alternated period, in average the BW for each station is of 5 Mbps.

  31. Bridged Ethernet A network with and without a bridge • Bridge divides the network into two or more networks. • With bridge 10-Mbps capacity is now shared by 6 stations i.e 10/6 instead of 10/12 Mbps assuming that the traffic is not going through the bridge.

  32. Bridged Ethernet • Bridge divides the network into two or more networks. • Collision domains is much smaller and the probability of collision is reduced.

  33. Switched Ethernet • Instead of two to four networks N networks where N is the number of stations on the LAN • A two Layer switch allows faster handling of the packets

  34. Full-Duplex Ethernet • 10Base5 and 10Base2 is half duplex system, a station can either send or receive. • Full duplex mode increases the capacity of each mode

  35. FAST ETHERNET Fast Ethernet was designed to compete with LAN protocols such as Fiber Distributed Data Interface (FDDI) or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. MAC SublayerPhysical Layer

  36. MAC Sublayer The goals of Fast Ethernet can be as follows: Upgrade the data rate to 100 Mbps Make it compatible with standard Ethernet Keep the same 48 bit address Keep the same frame format Keep the same minimum and maximum frame lengths

  37. MAC Sublayer • Changing Ethernet from 10 Mbps to 100 Mbps was to keep the MAC sublayer untouched. • Bus topology was dropped and continued with star topology • CSMA/CD for half duplex • For full duplex no need for CSMA/CD. • Auto negotiation: • It allows a two devices to negotiate the mode or data rate of operation. • It was designed for the following purposes: • To allow incompatible devices to connect to one another (different speed 10 Mbps can communicate with 100 Mbps) • To allow one device to have multiple capabilities • To allow a station to check a hub’s capabilities

  38. Physical Layer • Physical layer in Fast Ethernet is more complicated than the one in standard Ethernet. • Topology: • Fast Ethernet is designed to connect two or more stations together. • If only two stations they can be connected point-to-point. • If more than two stations need to be connected then star topology with hub or switch at the center.

  39. Fast Ethernet - Implementation • Fast Ethernet implementation at the physical layer can be categorized as either two-wire or four wire. • Two wire may be either category 5 UTP(10 Base-Tx) or fiber-optic cable (100 Base-Fx) • Four wire implementation is designed only for category 3 UTP (100Base-T4)

  40. Fast Ethernet - Implementation

  41. Fast Ethernet - Implementation

  42. Encoding • Manchester encoding needs 200-Mbaud Bandwidth for a data rate of 100 Mbps which is unsuitable for a medium such as twisted pair cable. • 100Base-Tx: uses two pairs of twisted-pair cable, MLT-3 encoding and is not a self synchronous. 4B/5B block coding is used to provide bit synchronization preventing occurrence of a long sequence of 0s and 1s.

  43. Multiline Transmission: MLT-3 • Uses three levels(+V, 0 and -V) and three transition rules to move between the levels. • If the next bit 0, no transition. If the next bit is 1 and the current level is not 0, the next level is 0. If the next bit 1 and the current level is 0, the next level is the positive of the last nonzero level. • The signal rate for MLT-3 is one-fourth the bit rate • MLT-3 when we need to send 100Mbps on a copper wire that cannot support more than 32MHz

  44. 4B/5B Mapping Codes

  45. Encoding • 100Base-FX: Two pairs of fiber-optic cables, NRZ-I encoder and 4B/5B block coding. • 100Base-T4: Buildings already have wired for voice grade twisted pair (Category 3 UTP). Four pairs of UTP for transmitting 100 Mbps. One pair switches between sending and receiving. Three pairs can carry only 75 Mbps (Maximum 25 Mbps/Pair). 8B/6T encoding scheme is used to carry 100 Mbps. (75*8/6=100)

  46. Encoding

  47. Encoding Summary of Fast Ethernet implementations

  48. GIGABIT ETHERNET The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z. MAC SublayerPhysical Layer Ten-Gigabit Ethernet

  49. MAC Sublayer The goals of Fast Ethernet can be as follows: Upgrade the data rate to 1 Gbps Make it compatible with standard Ethernet or Fast Ethernet Keep the same 48 bit address Keep the same frame format Keep the same minimum and maximum frame lengths To Support Autonegotiation

  50. MAC Sublayer • Changing Ethernet from 100 Mbps to 1 Gbps was to keep the MAC sublayer untouched. • Half Duplex Mode: • Switch is replaced by a hub, collision may occur, CSMA/CD • Central switch is connected is connected with star topology Full Duplex Mode: • In full duplex mode, there is a central switch which is connected to all computers or other switches, no need for CSMA/CD. • 512*1000-6=0.512 micro seconds, maximum cable length is 25m • Frame Bursting: • To transmit short frame, each frame carries redundant data, multiple frames are sent and these multiple frames look like one frame, padding is added between the frames.

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