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Data-link Layer

Data-link Layer. Computer Networks. Where are we?. The Data Link Interface. The Local Area Network. Popular (most data links are LANs) High Throughput Low Cost Short Distances Often shared medium access Most new installations usually "switched". Shared Medium Access.

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Data-link Layer

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  1. Data-link Layer Computer Networks

  2. Where are we?

  3. The Data Link Interface

  4. The Local Area Network • Popular (most data links are LANs) • High Throughput • Low Cost • Short Distances • Often shared medium access • Most new installations usually "switched"

  5. Shared Medium Access • A Shared Medium Used by All • Only One Station Transmits at a Time • Stations "Take Turns” • MAC Protocol defines fairness policy

  6. Topology Review

  7. Data Link Bit Encoding

  8. Example Bus: Ethernet • Most Popular LAN • IEEE Standardized as 802.3 • Several Generations • Same frame format (mostly) • Changing data rates • Different physical layer requirements The book: Gigabit Ethernet, Rich Seifert

  9. Ethernet Transmission • Only one station transmits at a time • Signal propagates entire cable length • All stations receive all transmissions • CSMA/CD medium access control scheme

  10. CSMA/CD • Carrier Sense (CS) • Wait until medium is idle • Begin to transmit frame • Multiple Access (MA) • Multiple stations attached to shared media • Each station uses the same access algorithm • Simultaneous Transmission is Possible

  11. CSMA/CD [continued] • Simultaneous Transmission: • Interfere with each other • Known as a collision • CSMA with Collision Detect (CD) • Listen to media during transmission • Detect whether another station’s signal interferes • Back off from interference and try again

  12. Transmission Logic 1. If media is idle, transmit. 2. Else, continue to listen to the media and when it is available, transmit. 3. Listen to media while transmitting. 4. If collision is detected while transmitting, send jam and back-off 5. Go to step 1 until max-try counter is reached.

  13. Exponential Back-off Algorithm • Let 1 Slot Time = 512 bit times • Upon 1st collision, randomly choose among {0,1} slot delay • Upon 2nd collision, randomly choose among {0,1,2,3} slot delay • Up to a maximum of 16 transmission attempts with a range of delay from {0 to 1024} bit times 0 <= r < 2k-1 • Where r is the random number generated, where k = MIN(n,10) and where n is the n-th retransmission attempt

  14. The Collision Domain • Minimum Length Frame Must Be >= Maximum RTT of the Ethernet segment • Minimum Frame is 512 bits • Requires 46 bytes of data whether the upper layer has them or not • Distances decrease as speed increases • Full-duplex mode eliminates the collision domain

  15. An Aside - Collisions • They are NOT bad, unless they’re late • Collision statistics are mostly meaningless • Monitor utilization • Distance Matters • Becoming irrelevant with switching • The name "Collision” is misleading

  16. Ethernet Addressing • Standardized by IEEE • Each station assigned a unique 48-bit address • First 24-bits are the OUI • Second 24-bits are vendor assigned • Usually set when NIC is manufactured • Canonical address format

  17. Ethernet Address Recognition • Each Frame Contains a Destination Address • All Stations Receive All Transmissions • Station Discards Any Frame Not Destined for It • Important: interface hardware, not software, checks address

  18. Possible Destinations • 1. Single destination (unicast) • 2. All stations on the Ethernet (broadcast) • 3. Subset of stations on the Ethernet (multicast) • MAC address is used to distinguish between the destinations

  19. Ethernet Destination Addresses

  20. Promiscuous Mode • Designed for testing/debugging • Allows interface to accept all frames • Available on most Ethernet hardware

  21. IEEE 802.3 Frame Format • Sender fills in: • Sender’s source address • Recipient’s destination address • Type of data in the frame type field • Cyclic Redundancy in FCS field

  22. Demultiplexing on Frame Type Field • Network Interface Hardware • Receives a copy of each transmitted frame • Examines address and either accepts or discards • Passes accepted frame to system software • Network device software • Examines frame type • Passes frame to correct software module

  23. Ethernet Wiring - 10BASE5 • Thick Ethernet (Thicknet) • Heavy coaxial cable

  24. Ethernet Wiring - 10BASE2 • Thin Ethernet (Thinnet) • Smaller coaxial cable

  25. Ethernet Wiring - 10BASE-T • Uses a hub • Twisted-pair wiring

  26. Ethernet Office Wiring

  27. High-speed Ethernet • Fast Ethernet • Operates at 100 Mb/s • Standardized in IEEE 802.3 as 100BASE-T and 100BASE-F standards • 10/100 Devices available • Gigabit Ethernet • Operates at 1 Gb/s • Mostly fiber systems using switches • Even higher speeds coming!

  28. Ethernet - Final Notes • Data Link Layer Usually Implemented with Physical Layer • Link Beat • Interframe Gap Time • Capture Effect • Modern Ethernet is a star-shaped bus • news://comp.dcom.lans.ethernet • IETF increasing maximum frame size?

  29. Example Ring: Token Ring • Popular in IBM environments • IEEE Standardized as 802.5 • Operates at 4Mb/s, 16Mb/s • Quickly Being Abandoned • 802.5 working group moved to "hibernation" status in July 2000 • Still worth learning about!

  30. Token Ring Transmission • Station waits for token before sending • Signal travels the entire ring • Sender receives its own transmission

  31. Token Passing Paradigm • Frames travel in a unidirectional fashion around the ring • Stations must wait for token to transmit • Stations can reserve the token • Token will circle indefinitely until a station wants to transmit

  32. MAC Frames • Ring management and control frames • Beacon, Ring purge, claim token, report error • Ring Poll every 7 seconds • Active monitor present • Standby monitor present • NAUN notification process

  33. Active and Standby Monitor • Only 1 Active Monitor per ring • AM is the master clock for the ring • AM inserts 24-bit delay to transmissions • AM ensures tokens/frames are present • AM removes circulating frames • SMs are ready to take over if AM fails

  34. Monitor Contention • Ring elects a new Active Monitor • Initiated when: • Loss of signal is detected • Active monitor not detected • Time-outs of token timer, NAUN, etc. • Highest MAC address wins • Everyone else is Standby Monitor

  35. Token Ring Insertion Process • Phase 0 - Media Lobe Check • Phase 1 - Physical Insertion • Phase 2 - Address Verification • Phase 3 - Participation in Ring Poll • Phase 4 - Request Initialization

  36. The Token Frame When no station is transmitting, the token frame travels continuously around the ring.

  37. Token Ring Addressing • Standardized by IEEE • Each station assigned a unique 48-bit address • First 24-bits are the OUI • Second 24-bits are vendor assigned • Usually set when NIC is manufactured • Non-canonical address format

  38. Token Ring Address Recognition • Each Frame Contains a Destination Address • All Stations Receive and Repeat All Transmissions • Stations Copy Any Frame Destined for It • Important: interface hardware, not software, checks address

  39. Token Ring Destination Addresses

  40. Token Ring Frame Format • Sender fills in: • Sender’s source address • Recipient’s destination address • Cyclic Redundancy in FCS field • Other stations may change: • Frame Status

  41. High-speed Token Ring • HSTR • Operates at 100 Mb/s • 1 Gb/s was being worked on • Standardized in IEEE 802.5 • Some 4/16/100 devices

  42. Why Token Ring Lost • IBM was the only systems manufacturer that promoted it • Cost • Complexity • Support throughout the industry • Only one vendor left to develop product!

  43. Token Ring - Final Notes • Jitter • Early Token Release • Backup Path • Token Transmission Timer • Needs LLC - we haven’t talked about it yet • news://comp.dcom.lans.token-ring

  44. Example Ring: FDDI • Uses Optical Fiber cabling • High reliability (dual rings) • Immune to interference • Standardized by ANSI • Transmission rate of 100 Mb/s • Similar to token ring

  45. FDDI Dual Ring Operation

  46. Logical Link Control • Standardized by IEEE 802.2 • Often used for MACs that don’t use type field

  47. LLC with SNAP

  48. What else? • ATM • Wireless (802.11) • Fiber Channel • HIPPI • Token Bus (802.4) • IEEE 802 standards may become free!

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