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Networking Basics CCNA 1 Chapter 6

Networking Basics CCNA 1 Chapter 6. Ethernet Fundamentals. The History of the Internet Created by Robert Metcalfe and others at Xerox in early-to-mid 1970s They later teamed with Digital Equipment Corporation (DEC) and Intel; published a set of proprietary standards

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Networking Basics CCNA 1 Chapter 6

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  1. Networking Basics CCNA 1Chapter 6

  2. Ethernet Fundamentals The History of the Internet • Created by Robert Metcalfe and others at Xerox in early-to-mid 1970s • They later teamed with Digital Equipment Corporation (DEC) and Intel; published a set of proprietary standards • These standards became known as the DIX (DEC, Intel, Xerox) standards • Version 2 of the standards, DIXv2, was published in 1980 and became 10BASE5

  3. Ethernet Fundamentals The History of the Internet • IEEE took over the development of these standards in the early 1980s • The first committee (committee 802) was given the task of developing IEEE standards for LANs • The 802.3 subcommittee worked on Ethernet standards • The 802.2 subcommittee worked on several types of LANS, including Ethernet and Token Ring; both 802.2 and 802.3 relate to Ethernet

  4. Ethernet Fundamentals The History of the Internet • Ethernet standards include: • 10BASE5 (1980) • 10BASE2 (1985) • 10BASE-T (1990) • Fast Ethernet – 100 Mbps (1995) • Gigabit Ethernet – 1 Gbps (1998) • 10 Gigabit Ethernet – 10 Gbps (2002)

  5. Ethernet Fundamentals The History of the Internet • Each new type of Ethernet supports the same basic Ethernet frame • By using common framing, Ethernet has remained simple and successful • Reasons for Ethernet’s continued success: • Relatively simple • Adding a new type of Ethernet is easy, because other types are already understood

  6. Ethernet Fundamentals The History of the Internet • Reasons for Ethernet’s continued success (continued): • Ethernet is reliable, with well-tested components and protocols • Ethernet is inexpensive; new types generally experience rapid price reductions within a few years of introduction

  7. Ethernet Fundamentals The Names of Different IEEE Ethernet Types • Ethernet standards differ in two main respects: • Speed • Type of cabling supported

  8. Ethernet Fundamentals The Names of Different IEEE Ethernet Types • Commonly used Ethernet names list the basic differentiating features: • Speed – the speed is listed in Mbps before the word “BASE” • Baseband transmission – all current Ethernet standards use baseband transmission • Cabling – the text after the word “BASE” indicates the type of cabling; e.g. “T” means “twisted-pair”

  9. Ethernet Fundamentals The Names of Different IEEE Ethernet Types • Commonly used Ethernet names list the basic differentiating features: • Example: 10BASE-T • 10 Mbps • Baseband transmission • Twisted-pair cabling

  10. Ethernet Fundamentals The Names of Different IEEE Ethernet Types • Baseband versus broadband • Baseband means that a single frequency is used to encode bits • Some earlier (now obsolete) Ethernet technologies used broadband to send signals • 10BROAD36 • Broadband is a range of frequencies

  11. Ethernet Fundamentals IEEE Ethernet Standards and the OSI Model • IEEE 802.3 Standards • Committee began in 1980 • Goal was to standardize 10BASE5 Ethernet • Standard matched the DIXv2 standard in most ways • One big difference was in the way 802.3 does framing

  12. Ethernet Fundamentals IEEE Ethernet Standards and the OSI Model • IEEE 802.3 Standards concern several details: • Physical transmission details (cables, connectors, encoding, speeds) • Media access issues (carrier sense multiple access with collision detection – CSMA/CD) • Errors during transmission • MAC address (location and format) • Ability of NICs to synchronize to the incoming signal by using a preamble and Start Frame Delimiter (SFD)

  13. Ethernet Fundamentals The IEEE 802.2 Logical Link Control Standard Original IEEE 802 Subcommittees

  14. Ethernet Fundamentals The IEEE 802.2 Logical Link Control Standard • 802.1 and 802.2 subcommittees had responsibility for features than could be used by all three types of LANs • Original focus of 802.2 subcommittee was to standardize any feature that spanned all three types of LANs • This subcommittee determined all three LAN types needed to somehow control the LAN, using a set of logic and rules • The standard was named Logical Link Control (LLC)

  15. Ethernet Fundamentals Comparing Ethernet Standards to the OSI Model

  16. Ethernet Fundamentals Comparing Ethernet Standards to the OSI Model • Each new type of Ethernet standard defines some physical layer standards and data link layer details • E.G., all Ethernet standards define the details of physical transmission over some medium, so they match the OSI physical layer

  17. Ethernet Fundamentals Comparing Ethernet Standards to the OSI Model • The IEEE 802.3 standard defines many physical details, as well as the lower half of the data link layer - Media Access Control (MAC)

  18. Ethernet Fundamentals Comparing Ethernet Standards to the OSI Model • MAC protocol performs several functions the physical layer (Layer 1) standards cannot • Layer 1 standards cannot communicate with upper-layer protocols, but the MAC protocol can • Layer 1 standards cannot identify other computers, but MAC addressing can • Layer 1 standards define how to send bits but cannot interpret their meaning, but MAC can by defining framing • Layer 1 standards cannot manage the process of which device can send at what time, so MAC protocol defines CSMA/CD

  19. Ethernet Fundamentals Ethernet Framing • Framing refers to two things: • The process of encapsulating data inside a header and possible a trailer • The meaning given to the bits inside those headers and trailers • Framing defines the meaning of transmitted bits • Mailing a letter • Write the letter • Put in envelope • Write address on envelope in a particular place • Put the stamp in a particular place

  20. Ethernet Fundamentals Ethernet Framing • Framing is similar to mailing a letter • The NIC adds a header and trailer around the Layer 3 protocol data unit (PDU) to encapsulate the data • The header and trailer must have properly formatted information to be delivered correctly

  21. Ethernet Fundamentals Encapsulating Packets Inside Ethernet Frames • The process of adding a header and trailer around the Layer 3 PDU is called “encapsulation” • Sometimes called “framing” • The term “frame” refers to the resulting bits that include the Ethernet header and trailer

  22. Ethernet Fundamentals Ethernet Encapsulation and the IEEE 802.3 Frame

  23. Ethernet Fundamentals Encapsulating Packets Inside Ethernet Frames • The encapsulation process: • The IP software gives the IP packet to the Ethernet software • The Ethernet software encapsulates the IP packet between and Ethernet header and trailer • The Ethernet NIC physically transmits the bits that comprise the frame over an Ethernet LAN

  24. Ethernet Fundamentals Three Styles of Ethernet Framing

  25. Ethernet Fundamentals The Fields in the IEEE 802.3 Frame • Header and trailer contain a defined number of bytes • Different sets of bytes have different meanings • E.G., the destination field contains the six-byte MAC address

  26. Ethernet Fundamentals IEEE 802.3 Header

  27. Ethernet Fundamentals IEEE 802.3 Header

  28. Ethernet Fundamentals The Fields in the IEEE 802.3 Header • Destination MAC address – allows a switch to forward the frame, allows a PC to determine if the frame is meant for it • Source MAC address – identifies the sending NIC or interface, helps a switch build a switching table

  29. Ethernet Fundamentals DIX Framing and IEEE Framing • IEEE 802.3 framing for Ethernet is slightly different than the DIXv2 framing specification • DIXv2 did not use the term SFD, called the first eight bytes the Preamble • DIXv2 Type field identifies the contents in the frame • IEEE 802.3 was planned to be backwardly compatible with DIXv2 • IEEE 802.3 can tell if the frame is DIXv2 or IEEE 802.3 by looking at the value of the Length/Type field

  30. Ethernet Fundamentals The Format of MAC Addresses • Media Access Control address characteristics: • 6-byte hexadecimal (hex) numbers • Each NIC has a burned-in (permanent) MAC address • First 3 bytes are called an Organizationally Unique Identifier (OUI) and refer to the manufacturer • Second 3 bytes are unique numbers to the manufacturer (a serial number)

  31. Ethernet Fundamentals Structure of a MAC Address

  32. Ethernet Fundamentals Sending Frames over Different Types of Ethernet • Each type of Ethernet uses the exact same framing • An Ethernet frame can be sent by a device on one type of Ethernet and can go over links using different types of Ethernet with no problems • By using the same type of framing, the IEEE has enabled companies to slowly migrate to newer types of Ethernet

  33. Ethernet Fundamentals Sample Campus LAN with Different Types of Ethernet

  34. Ethernet Fundamentals LLC, SNAP and Determining the Type of Protocol • Additional Background on IEEE 802.2 Logical Link Control (LLC) • “Link” in LLC refers to the concept of how two devices communicate over a transmission medium – the two devices must be “linked” • Two base features of the IEEE 802.2 header • Identify type of data inside frame’s data field • Control the transmission between two devices on a LAN, specifically to perform error recover when frames are lost

  35. Ethernet Fundamentals LLC, SNAP and Determining the Type of Protocol • Additional Background on IEEE 802.2 Logical Link Control (LLC) (continued) • Two base features of the IEEE 802.2 header (continued) • The error recovery function is not used today, but the first function is • The de-encapsulation process needs to know the type of data in the data field; however, many networks use only TCP/IP as a Layer 3 protocol

  36. Ethernet Fundamentals LLC, SNAP and Determining the Type of Protocol • The 802.3 subcommittee defined a field to identify the type of data, the Destination Service Access Point (DSAP) • This field holds a number that identifies the contents of the data

  37. Ethernet Fundamentals IEEE 802.3 and 802.2 Headers

  38. Ethernet Fundamentals The IEEE SNAP Header • The DSAP field (1 byte long) did not allow enough protocol values – ran out before one could be assigned to IP! • Original solution was to use an optional header called the Subnetwork Access Protocol (SNAP) • Header includes an OUI field (mostly unused) and a 2-byte protocol Type field

  39. Ethernet Fundamentals The DIX and IEEE Type Field • Three Type fields • 1970s – original DIX frame format (2-byte Type field) • mid 1980s – IEEE standard for framing, with SNAP header (802.2 and 802.3), and 1988 (SNAP header part) • 1997 – IEEE revised standard for framing with Length/Type field

  40. Ethernet Fundamentals Ethernet Framing with Variations on the Type Field

  41. Ethernet Operation Rules Governing When a Device Can Transmit: CSMA/CD • CSMA/CD algorithm defines rules that NICs must follow • In the original Ethernet physical bus topology, only one device can send at a time • If multiple devices try to send at the same time, their electrical signals overlap and are added together • The resulting signal does not represent the original signal of either device • Such an occurrence is called a “collision”

  42. Ethernet Operation Rules Governing When a Device Can Transmit: CSMA/CD - the CSMA/CD Algorithm • The original MAC standard suggests: • Wait until the LAN is unused, then send the frame • Listen to detect if the frame collided with another frame • If no collision occurred, the frame must have made it across the LAN • If a collision did occur, wait and then try to send the original frame again

  43. Ethernet Operation Rules Governing When a Device Can Transmit: CSMA/CD - the CSMA/CD Algorithm • The CSMA/CD algorithm in detail: • Wait until the LAN is silent • Send a frame • Transmitting device listens for collisions while transmitting • If no collisions occur, process is complete • All devices whose transmitted frames collided send a jamming signal (32 bits of 1s and 0s) • All devices whose frames collided set semi-random collision back-off timers. • Once the timer expires, a device can begin the process again

  44. Ethernet Operation Origins of the Name CSMA/CD

  45. Ethernet Operation CSMA/CD and Collisions on 10BASE-T LANs with Hubs • Hubs operate at Layer 1, simply repeating all received signals out all other ports

  46. Ethernet Operation CSMA/CD and Collisions on 10BASE-T LANs with Hubs 10BASE-T NICs use a loopback circuit on the NIC to be able to recognize collisions on a hub. When the two signals combine, it is recognized as a collision.

  47. Ethernet Operation Full Duplex, Half Duplex, and Collision Domains • CSMA/CD causes throughput over a LAN to degrade as the LAN gets busier • Collisions cause devices to wait to retransmit, further slowing the network • Originally, Ethernet LAN performance was restricted because Ethernet NICs used half duplex logic – they could send or transmit (duplex), but not do both simultaneously • To improve LAN performance, standards evolved where collisions cannot occur: full duplex (receive and transmit simultaneously)

  48. Ethernet Operation Preventing Collisions with Switch Buffering • Switches prevent collisions by buffering frames • If several PCs send frames to the same address at the same time, the switch holds the frames in memory - a process called buffering • The switch then forwards the frames one at a time

  49. Ethernet Operation Switch Buffering Example

  50. Ethernet Operation Collision Domains with a Switch

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