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Section 3 The OSI Data Link Layer

Section 3 The OSI Data Link Layer. CSIS 479R Fall 1999 “Network +” George D. Hickman, CNI, CNE. Objectives. Identify the basic purpose of the OSI Data Link layer Identify the characteristics of the two logical topologies Identify the characteristics of the three media access methods

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Section 3 The OSI Data Link Layer

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  1. Section 3 The OSI Data Link Layer CSIS 479R Fall 1999 “Network +” George D. Hickman, CNI, CNE

  2. Objectives • Identify the basic purpose of the OSI Data Link layer • Identify the characteristics of the two logical topologies • Identify the characteristics of the three media access methods • Describe how addresses are defined and managed at the Data Link layer

  3. Objectives (con’t) • Describe the transmission synchronization techniques used at the data link layer • Describe the connection services implemented at the Data Link layer • Describe the IEEE 802.x standards • Describe the 802.3 standard and Ethernet

  4. Objectives (con’t) • Describe the 802.3u Fast Ethernet standard • Describe the 802.5 and Token Ring standards • Describe the Fiber Distributed Data Interface (FDDI) standard • Describe commonly used wide area networking protocols

  5. Data Link Layer • Media Access Control • Sublayer controls how transmitters share single media • Logical Link Control • Sublayer establishes and maintains device to device link • Organize Physical layer’s bits into frames • Detect and sometimes correct errors • Control Data Flow

  6. Data Link Layer • Identify Computers on the network • Data Link layer header contains: • Source and destination addresses • Frame length information • Indication of upper layer protocols involved

  7. Data Link Devices • Network Connectivity Devices • Bridges • Switches • NICs

  8. Data Link--MAC • Logical Topology Process • Bus and Ring methods • Media Access Process • Contention, Token Passing, & Polling methods • Addressing Process • Physical device method

  9. Data Link -- LLC • Transmission Synchronization Process • Asynchronous, Synchronous, Isochronous methods • Connection Services Process • LLC-level flow control • Error control

  10. Logical Topology • The actual signal path • As opposed to the Physical Topology, which is the physical layout of wires • Logical and Physical paths do not have to be the same • Token Ring Example • Physical star • Logical Ring

  11. Media Access Control • Contention • Devices transmit when ever they want • Causes collisions • Carrier Sense • Newer contention scheme • Listens to media, transmits if no signal detected • CSMA • Carrier Sense, Multiple Access

  12. Carrier Sense • CSMA • Collision detection and retransmission is the responsibility of a “higher layer” protocol • Waiting and overhead of going up and down OSI models make CSMA less effective • CSMA/CD • Adds collision detection at or below the DL layer by sensing cable before and after transmitting • After collisions, wait random time and retransmit • Good for bursty traffic

  13. Token Passing • The Token (a small frame) is passed to give media access control. • Only devices with the token may transmit • Devices know where they get token from and where they pass it to • IEEE 802.5 Token Ring Standard • Token passing access control, physical or logical ring topology • FDDI • Good for time sensitive (voice, video) traffic or heavily populated networks

  14. Polling Systems • One device (the controller, primary, or master) is media access administrator • Queries other devices (secondaries) in a predefined order to see if they need to transmit • Ideal for networking time-sensitive devices like automation equipment

  15. Addresses–Defined and Managed • Data Link layer is concerned with the physical device address or MAC address • Most DL layer implementations place the source and destination addresses in the frame header • The frame is sent to every device on network, which reads header and reads or ignores the data as needed • Bridges use these addresses to let frames “through” or not • Switches use these addresses to know which port to send data frames to

  16. Transmission Synchronization Techniques—DL layer • The physical layer synchronization was bits • The Data Link layer synchronization is the coordination of frame transmission

  17. Asynchronous • Each device has own clock, not synchronized with the other • Start and Stop bits used • Good for random interval transmissions • Parity bit can be added to detect some errors • Even parity • Parity bit is set to give an even number of 1 bits/byte • Odd parity • Parity bit is set to give an odd number of 1 bits/byte • May not detect multiple bit errors

  18. Synchronous • Devices responsible for a framing clock • Can be separate channel • Or use SYN or SYNC characters for “start” • A CRC value can be put near end for error checking • Both devices must use same algorithm to compute CRC

  19. Isochronous • A clock signal is sent out to all network devices to create time slots • Other devices may fill “slots” with data • Clock signal is not provided with every frame (like asynchronous) or at start of a string of data (like synchronous)

  20. DL layer implementations of Connection Services • Unacknowledged connectionless services • Send and receive frames with no flow, error, or packet sequence control • Connection-oriented services • Flow, error, and packet sequence control provided by use of acknowledgments • Acknowledged connectionless services • Flow and error control provided by acknowledgements between point to point transmissions

  21. LLC level Flow Control • Guaranteed Rate Flow control • A rate is agreed upon before transmission and is maintained as long as the transmission lasts • Window flow control • Static • An acceptable window or buffer size is determined. That number of frames is maximum sent without an acknowledgement • Dynamic • Window or buffer size can be adjusted. A choke packet is sent by receiver when the buffer exceeds a specified level, slowing down the sender. Transmission is slowly increased until another choke packet is sent.

  22. Error Control • How lost or scrambled frames are handled • Sending device receives NAK (or nothing) • Checksums do not match • Packet size is off (too small) • Can be caused by Noise, Interference, or Distortion • Can be caused by a buffer overflow

  23. IEEE 802.x standards • 1980 IEEE defined LAN standards for Physical and Data Link layers • 802.1 • Allows 802 compliant device to speak with another 802 device on another LAN or WAN • 802.2 • Defines LLC sublayer of Data Link layer

  24. IEEE 802.x standards • 802.3 • Physical layer specifications • Baseband/broadband • Media type • Topologies • Data rate • Three part naming convention • Speed (megabits per second) • BASE or BROAD • Special designator or effective distance • 10BASE2

  25. IEEE 802.x standards • 802.4 • Factory and Industrial automation needs • Physical bus topology • Token Passing media access • Baseband or Broadband media • 75 Ohm CATV-type cable or optical fiber • 802.5 • Based on IBM Token Ring • Token Passing media access • 1, 4, or 16 Mbps • No specific transmission media or physical topology mandated (IBM Token Ring mandates both.)

  26. IEEE 802.x standards • 802.6 • Distributed Queue Dual Bus (DQDB) • 802.7 • Standards for broadband communications • 802.8 • Fiber Optic standards • 802.9 • Isochronous Ethernet (voice/data)

  27. IEEE 802.x standards • 802.10 • Used with encryption key information • 802.11 • Used with wireless LAN implementations • 802.12 • 100 Mbps physical star, contention based (100VG-AnyLAN)

  28. Ethernet • Combination of 802.2 and 802.3 • Designed as simple, low access-overhead LAN architecture • See diagram on page 3-36 • Can use Thick or thin Co-axe • BUS Topology • Can use Twisted Pair, or fiber optic cable, using either switches or hubs • STAR Topology

  29. 10BASE5 Thick Coaxial cabling • NICs use external transceiver • 50 Ohm terminator both ends, 1 grounded • 500 Meter maximum segment length • 100 devices per segment maximum (incl repeater) • 3 populated segments maximum • 2.5 M between taps / 5 M (max) tap to node

  30. 10BASE2 Thin Coaxial • NICs use internal transciever • 50 Ohm terminator both ends, 1 grounded • 185 M maximum segment length • 30 devices per segment max (incl repeaters) • 3 populated segments maximum • .5 M minimum between T connectors

  31. 10BASE-T Twisted Pair • 100 M maximum segment length • 1,024 maximum workstations (theoretical) • 4 repeaters maximum between communicating devices

  32. 5-4-3 Rule • Coaxial • 5 cable segments maximum • 4 repeaters maximum • 3 segments populated • UTP • 5 cable segments maximum • 4 hubs maximum

  33. IEEE 802.3u (Fast Ethernet) • Physical and Logical Topologies • Physical hierarchical star / Logical Star • Media Independent Interface (MII) • 100BASE-TX, 2 pair Cat 5 UTP 100M/segment • 100BASE-T4, 4 pair Cat 3+ UTP 100M/segment • 100BASE-FX, 2 strand FO, 412-10,000M/segment • Auto Negotiation (AUTONEG) • 10 or 100 Mbps auto negotiated • Media Access Control (MAC) • CSMA/CD

  34. IEEE 802.5 Token Ring • Specifications for Physical Layer and MAC sublayer of Data Link • Physical Star Logical Ring Topology • 802.5 does not specify cabling type • Cable lengths differ with media used (page 3-46) • 3 cable segments per series • 33 MSAUs maximum • 802.5 specifies 250 nodes maximum • IBM STP specifies 260 nodes • IBM UTP specifies 72 nodes • All network devices must run at same speed (4 or 16 Mbps) unless connected by a bridge

  35. 802.5 Token Ring MAC • Token Passing • Special packet allowing device to transmit • Device receives packet, transmits a frame. When frame returns to sender, sender puts a new token out on ring. • Early token release-creates/releases new token immediately after sending data frame • Active monitor performs maintenance on ring

  36. Beaconing • Allows some automatic error recovery • Upon ring break, stations send beacon frames until they receive a beacon frame from an “upstream neighbor” • Soon only one station (after break) is beaconing • MSAU attempts to reconfigure ring around the break

  37. FDDI Standard • Fiber Distributed Data Interface • Physical Layer and MAC sublayer+SMT • Assumes 802.2 (LLC sublayer specification) • Fills need for secure high bandwidth • Backbone implementation (connects LANs) • Computer room networks (mainframes, minis) • High Data Rate LANS (CAD/video needs)

  38. FDDI and 802.5 • Both use Token passing MAC • Both physical star logical ring • Both (can) use fiber-optic media • FDDI has higher maximum data rate

  39. FDDI Network Design • 2 counter rotating rings • Primary caries data, secondary management • Secondary becomes primary if P. media fails • 1000 workstations max, 200 km total cable • 500 / 100 incase of media failure • Multi-mode fiber optic cable/62.5 micrometer • Repeater required every 2 km or less

  40. FDDI Network Design (con’t) • Class A Workstations • Connect to both rings. Higher fault tolerance • Class B Workstations • Connects to primary ring. Can’t reconfigure • FDDI ALWAYS releases new token at end of transmitted frames, so there may be multiple frames on network at once. • See figure 3-27 on page 3-53

  41. WAN Protocols • Dial up connections • SLIP (Serial Line Internet Protocol) • Physical layer protocol • Not a strict standard, may not work • PPP (Point-to-Point Protocol) • Physical and Data Link layer protocol • Allows Dynamic IP addressing • Support multiple protocols on same link • Password login • Error control

  42. WAN Protocols (con’t) • PPTP (Point-to-Point Tunneling Protocol • PPP extension • Encapsulates other protocols for IP transmission • Corporations use Internet to connect their LANs • Can read multi-protocol packets • X.25 • Attaching computer to a packet-switched network • Physical, Data Link, Network Layers • SprintNet, Tymnet, GTE

  43. WAN Protocols (con’t) • Frame Relay • Designed for high speed bursts on digital network • No error checking while transmitting, so faster • Error checking at receiving point • Ethernet, X.25, Token Ring common • 56 kpbs to 1.544 Mbps common (56K, T-1, T-3) • Physical and Data Link layers • Purchased by CIR (Committed Information Rates), the minimum guaranteed capacity of virtual circuit

  44. WAN Protocols (con’t) • ISDN B-ISDN Integrated Services Digital Network • Channel A 4 KHz analog channel • Channel B 64 Kbps digital channel • Channel C 8 or 16 Kbps digital • Interconnects X.25, PPP, frame relay • Physical, Data Link, Network Layers • Popular with SOHO

  45. WAN Protocols (con’t) • ATM Asynchronous Transfer Mode • B-ISDN and Cell Relay (Cell is 53 byte block) • Considered a LAN and WAN protocol • Primarily Data Link and Network Layer • Designed to be independent of Physical Layer for faster processing speeds • 155 Mbps and 622 Mbps specified. 10Gbps expected • FDDI or others specify the Physical layer

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