Direct Link Lan


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  • Direct Link Lan

    1. 1. Direct Link Networks <ul><li>Encoding and Framing </li></ul><ul><li>Error Detection/Correction </li></ul><ul><li>Reliable Transmission will be covered later (with TCP) </li></ul><ul><li>Media Access Control </li></ul><ul><li>(Wired) LAN Technologies </li></ul><ul><li>Readings </li></ul><ul><ul><li>Chapter 2 except Section 2.5, which is delayed to the discusson on Transport layer. </li></ul></ul>
    2. 2. Encoding <ul><li>Signals propagate over a physical medium </li></ul><ul><ul><li>modulate electromagnetic waves </li></ul></ul><ul><ul><li>e.g., vary voltage </li></ul></ul><ul><li>Encode binary data onto signals </li></ul><ul><ul><li>e.g., 0 as low signal and 1 as high signal </li></ul></ul><ul><ul><li>known as Non-Return to zero (NRZ) </li></ul></ul>Bits NRZ 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
    3. 3. Problems: Consecutive 1s or 0s <ul><li>Low signal (0) may be interpreted as no signal </li></ul><ul><li>High signal (1) leads to baseline wander </li></ul><ul><li>Unable to recover clock </li></ul><ul><ul><li>Clocks at the sender and receiver use signal transition to synchronize each other </li></ul></ul>
    4. 4. Alternative Encodings <ul><li>Non-return to Zero Inverted (NRZI) </li></ul><ul><ul><li>make a transition from current signal to encode a one; stay at current signal to encode a zero </li></ul></ul><ul><ul><li>solves the problem of consecutive ones </li></ul></ul><ul><li>Manchester </li></ul><ul><ul><li>transmit XOR of the NRZ encoded data and the clock </li></ul></ul><ul><ul><li>only 50% efficient. </li></ul></ul>
    5. 5. Encodings (cont) <ul><li>4B/5B </li></ul><ul><ul><li>every 4 bits of data encoded in a 5-bit code </li></ul></ul><ul><ul><li>5-bit codes selected to have no more than one leading 0 and no more than two trailing 0s </li></ul></ul><ul><ul><li>thus, never get more than three consecutive 0s </li></ul></ul><ul><ul><li>resulting 5-bit codes are transmitted using NRZI </li></ul></ul><ul><ul><li>achieves 80% efficiency </li></ul></ul>
    6. 6. Encodings (cont) Bits NRZ Clock Manchester NRZI 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
    7. 7. Framing <ul><li>Break sequence of bits into a frame </li></ul><ul><li>Typically implemented by network adaptor </li></ul>Frames Bits Adaptor Adaptor Node B Node A
    8. 8. Approaches <ul><li>Sentinel-based </li></ul><ul><ul><li>delineate frame with special pattern: 01111110 </li></ul></ul><ul><ul><li>e.g., HDLC, SDLC, PPP </li></ul></ul><ul><ul><li>problem: special pattern appears in the payload </li></ul></ul><ul><ul><li>solution: bit stuffing </li></ul></ul><ul><ul><ul><li>sender: insert 0 after five consecutive 1s </li></ul></ul></ul><ul><ul><ul><li>receiver: delete 0 that follows five consecutive 1s </li></ul></ul></ul>Header Body 8 16 16 8 CRC Beginning sequence Ending sequence
    9. 9. Approaches (cont) <ul><li>Couter-based </li></ul><ul><ul><li>include payload length in header </li></ul></ul><ul><ul><li>e.g., DDCMP </li></ul></ul><ul><ul><li>problem: count field corrupted </li></ul></ul><ul><ul><li>solution: catch when CRC fails </li></ul></ul>
    10. 10. Approaches (cont) <ul><li>Clock-based </li></ul><ul><ul><li>each frame is 125us long </li></ul></ul><ul><ul><li>e.g., SONET: Synchronous Optical Network </li></ul></ul>
    11. 11. Handling Errors <ul><li>Data can be corrupted during transmission </li></ul><ul><ul><li>Bit values changed </li></ul></ul><ul><li>Frame includes additional information </li></ul><ul><ul><li>Set by sender </li></ul></ul><ul><ul><li>Checked by receiver </li></ul></ul><ul><li>Error-detection vs error-correction </li></ul><ul><ul><li>Both need redundant information </li></ul></ul><ul><ul><li>Detection: error exists or not. </li></ul></ul><ul><ul><li>Correction: repair if there was an error </li></ul></ul><ul><li>Statistical guarantee </li></ul>
    12. 12. Error Detecting and Correcting Codes <ul><li>How many check/redundancy bits? </li></ul><ul><ul><li>To detect single-bit error </li></ul></ul><ul><ul><ul><li>1 bit: even/odd parity </li></ul></ul></ul><ul><ul><li>To correct a single-bit error in m-bit message </li></ul></ul><ul><ul><ul><li>Need a minimum of r bits such that </li></ul></ul></ul><ul><ul><ul><li>(m + r + 1)  2 r </li></ul></ul></ul><ul><ul><ul><li>Example: 3-bit message needs 3-bit redundancy </li></ul></ul></ul><ul><li>Correction or detection+retransmission? </li></ul>
    13. 13. Error Detection Techniques <ul><li>Checksum </li></ul><ul><ul><li>Treat data as sequence of integers </li></ul></ul><ul><ul><li>Compute and send arithmetic sum </li></ul></ul><ul><ul><li>Detects some multiple bit errors not all </li></ul></ul><ul><li>Cyclic Redundancy Check </li></ul><ul><ul><li>Mathematical function of data </li></ul></ul><ul><ul><li>More complex to compute </li></ul></ul><ul><ul><li>Handles more errors </li></ul></ul>
    14. 14. Cyclic Redundancy Check <ul><li>Add k bits of redundant data to an n -bit message </li></ul><ul><ul><li>want k << n </li></ul></ul><ul><ul><li>e.g., k = 32 and n = 12,000 (1500 bytes) </li></ul></ul><ul><li>Represent n -bit message as n -1 degree polynomial </li></ul><ul><ul><li>e.g., MSG=10011010 as M ( x ) = x 7 + x 4 + x 3 + x 1 </li></ul></ul><ul><li>Let k be the degree of some divisor polynomial </li></ul><ul><ul><li>e.g., C ( x ) = x 3 + x 2 + 1 </li></ul></ul>
    15. 15. CRC (cont) <ul><li>Transmit polynomial P ( x ) that is evenly divisible by C ( x ) </li></ul><ul><ul><li>shift left k bits, i.e., M ( x ) x k </li></ul></ul><ul><ul><li>subtract remainder of M ( x ) x k / C ( x ) from M ( x ) x k </li></ul></ul><ul><li>Receiver polynomial P ( x ) + E ( x ) </li></ul><ul><ul><li>E ( x ) = 0 implies no errors </li></ul></ul><ul><li>Divide ( P ( x ) + E ( x )) by C ( x ); remainder zero if: </li></ul><ul><ul><li>E ( x ) was zero (no error), or </li></ul></ul><ul><ul><li>E ( x ) is exactly divisible by C ( x ) </li></ul></ul>
    16. 16. CRC Example Generator 1101 11111001 10011010 000 Message 1101 1001 1101 1000 1101 1011 1101 1100 1101 1000 1101 101 Remainder
    17. 17. Choice of C(x) <ul><li>Want to ensure C(x) doesn’t divide E(x) </li></ul><ul><li>We can detect </li></ul><ul><ul><li>All single-bit errors if </li></ul></ul><ul><ul><ul><li>C(x) has at least 2 terms </li></ul></ul></ul><ul><ul><li>All double-bit errors if </li></ul></ul><ul><ul><ul><li>C(x) doesn’t divide x j + 1 </li></ul></ul></ul><ul><ul><ul><ul><li>X 15 + x 14 + 1 doesn’t divide x j + 1 for any j below 32768 </li></ul></ul></ul></ul><ul><ul><li>Any odd number of errors if </li></ul></ul><ul><ul><ul><li>C(x) contains the factor x+1 </li></ul></ul></ul><ul><ul><li>Any “burst” error of length less than or equal to k bits </li></ul></ul><ul><ul><li>Most burst errors of length greater than k bits </li></ul></ul>
    18. 18. Error Detection Summary <ul><li>To detect data corruption </li></ul><ul><ul><li>Sender adds additional information to packet </li></ul></ul><ul><ul><li>Receiver checks </li></ul></ul><ul><li>Techniques </li></ul><ul><ul><li>Parity bit </li></ul></ul><ul><ul><li>Checksum </li></ul></ul><ul><ul><li>Cyclic Redundancy Check </li></ul></ul>
    19. 19. Error Recovery <ul><li>Reliable delivery over unreliable channel </li></ul><ul><ul><li>How to recover from corrupted/lost packets </li></ul></ul><ul><li>Error detection and retransmission </li></ul><ul><ul><li>With acknowledgements and timeouts </li></ul></ul><ul><ul><li>Also called Automatic Repeat Request (ARQ) </li></ul></ul><ul><ul><li>Retransmission incurs round trip delay </li></ul></ul><ul><li>Error correcting codes </li></ul><ul><ul><li>Also called Forward Error Correction (FEC) </li></ul></ul><ul><ul><li>No sender retransmission required </li></ul></ul>
    20. 20. Summary <ul><li>Encoding </li></ul><ul><li>Framing </li></ul><ul><li>Error correction/detection codes </li></ul><ul><ul><li>Parity/Checksum/CRC </li></ul></ul>
    21. 21. Shared Media and Local Area Networks <ul><li>Shared Media Access Problem </li></ul><ul><ul><li>Media access control (MAC) and LAN </li></ul></ul><ul><ul><li>MAC addresses and network adaptors (NICs) </li></ul></ul><ul><ul><li>MAC protocols: random access vs. controlled access </li></ul></ul><ul><li>Ethernet </li></ul><ul><li>Token Ring and FDDI </li></ul><ul><li>802.11 Wireless LAN </li></ul><ul><li>Readings </li></ul><ul><ul><li>Sections 2.6-2.7 </li></ul></ul>
    22. 22. Multiple Access Links and LANs <ul><li>Two types of “links”: </li></ul><ul><li>point-to-point, e.g., </li></ul><ul><ul><li>PPP for dial-up access, or over optical fibers </li></ul></ul><ul><li>broadcast (shared wire or medium), e.g. </li></ul><ul><ul><li>traditional Ethernet </li></ul></ul><ul><ul><li>802.11 wireless LAN </li></ul></ul>
    23. 23. Typical LAN Structure ROM Ethernet Processor <ul><li>Transmission Medium </li></ul><ul><li>Network Interface Card (NIC) </li></ul><ul><li>Unique MAC “physical” address </li></ul>RAM RAM
    24. 24. Adaptors Communicating <ul><li>link layer implemented in “adaptor” (aka NIC), with “transceiver” in it </li></ul><ul><ul><li>Ethernet card, dial-up modem, 802.11 wireless card </li></ul></ul><ul><li>sending side: </li></ul><ul><ul><li>encapsulates packet in frame </li></ul></ul><ul><ul><li>adds error checking bits, flow control, reliable data transmission, etc. </li></ul></ul><ul><li>receiving side </li></ul><ul><ul><li>looks for errors, flow control, reliable data transmission, etc </li></ul></ul><ul><ul><li>extracts packet, passes to receiving node </li></ul></ul><ul><li>adapter is semi-autonomous </li></ul><ul><li>data link & physical layers are closely coupled! </li></ul>sending node rcving node network packet adapter adapter link layer protocol frame frame
    25. 25. MAC (Physical) Addresses <ul><li>Addressing needed in shared media </li></ul><ul><ul><li>MAC (media access control) or physical addresses </li></ul></ul><ul><ul><li>To identify source and destination interfaces and get frames delivered from one interface to another physically-connected interface (i.e., on same physical local area network!) </li></ul></ul><ul><li>48 bit MAC address (for most LANs) </li></ul><ul><ul><li>fixed for each adaptor, burned in the adapter ROM </li></ul></ul><ul><ul><li>MAC address allocation administered by IEEE </li></ul></ul><ul><ul><ul><li>1 st bit: 0 unicast , 1 multicast . </li></ul></ul></ul><ul><ul><ul><li>all 1’s : broadcast </li></ul></ul></ul><ul><li>MAC flat address -> portability </li></ul><ul><ul><li>can move LAN card from one LAN to another </li></ul></ul>
    26. 26. MAC (Physical, or LAN) Addresses <ul><li>MAC addressing operations on a LAN: </li></ul><ul><li>each adaptor on the LAN “sees” all frames </li></ul><ul><li>accept a frame only if dest. (unicast) MAC address matches its own MAC address </li></ul><ul><li>accept all broadcast (MAC= all 1’s) frames </li></ul><ul><li>accept all frames if set in “ promiscuous ” mode </li></ul><ul><li>can configure to accept certain multicast addresses (first bit = 1) </li></ul>
    27. 27. MAC Sub-layer Data link layer 802.3 CSMA-CD 802.5 Token Ring 802.2 Logical link control Physical layer MAC LLC 802.11 Wireless LAN Network layer Network layer Physical layer OSI IEEE 802 Various physical layers Other LANs
    28. 28. Broadcast Links: Multiple Access <ul><li>Single shared communication channel </li></ul><ul><li>Only one can send successfully at a time </li></ul><ul><li>Two or more simultaneous transmissions </li></ul><ul><ul><li>interference! </li></ul></ul><ul><li>How to share a broadcast channel </li></ul><ul><ul><li>media access control uses same shared media </li></ul></ul><ul><li>Humans use multi-access protocols all the time </li></ul>
    29. 29. Random Access <ul><li>Stations contend for channels </li></ul><ul><li>Overlapping transmissions ( collisions ) can occur </li></ul><ul><ul><li>Carrier sensing? </li></ul></ul><ul><ul><li>Collision detection? </li></ul></ul><ul><li>Protocols </li></ul><ul><ul><li>Aloha </li></ul></ul><ul><ul><li>Slotted Aloha </li></ul></ul><ul><ul><li>Carrier Sense Multiple Access: Ethernet </li></ul></ul>
    30. 30. Controlled Access <ul><li>Stations reserve or are allocated channel </li></ul><ul><ul><li>No collisions </li></ul></ul><ul><ul><li>Allocation: static or dynamic </li></ul></ul><ul><li>Protocols </li></ul><ul><ul><li>Static channel allocation </li></ul></ul><ul><ul><ul><li>Time division multiple access </li></ul></ul></ul><ul><ul><li>Demand adaptive channel allocation </li></ul></ul><ul><ul><ul><li>Reservation protocols </li></ul></ul></ul><ul><ul><ul><li>Token passing (token bus, token ring) </li></ul></ul></ul>
    31. 31. Taxonomy of MAC Protocols WiFi (802.11)
    32. 32. Pure (unslotted) Aloha <ul><li>Simpler, no synchronization </li></ul><ul><li>Just send: no waiting for beginning of slot </li></ul>
    33. 33. Slotted Aloha <ul><li>Time is divided into equal size slots </li></ul><ul><li>Nodes transmit at the beginning of a slot </li></ul><ul><li>If collision, retransmit later </li></ul>Success (S), Collision (C), Empty (E) slots
    34. 34. Performance of Aloha Protocols S = throughput = “goodput” (success rate) G = offered load = Np 0.5 1.0 1.5 2.0 0.1 0.2 0.3 0.4 Pure Aloha Slotted Aloha
    35. 35. Carrier Sense Multiple Access <ul><li>Aloha is inefficient (and rude) </li></ul><ul><ul><li>Doesn’t listen before talking </li></ul></ul><ul><li>CSMA: Listen before transmit </li></ul><ul><ul><li>If channel idle, transmit entire packet </li></ul></ul><ul><ul><li>If busy, defer transmission </li></ul></ul><ul><ul><ul><li>How long should we wait? </li></ul></ul></ul><ul><ul><li>Human analogy: don’t interrupt others </li></ul></ul><ul><li>Can carrier sense avoid collisions completely? </li></ul>
    36. 36. Persistent and Non-persistent CSMA <ul><li>Non-persistent </li></ul><ul><ul><li>If idle, transmit </li></ul></ul><ul><ul><li>If busy, wait random amount of time </li></ul></ul><ul><li>p-persistent </li></ul><ul><ul><li>If idle, transmit with probability p </li></ul></ul><ul><ul><li>If busy, wait till it becomes idle </li></ul></ul><ul><ul><li>If collision, wait random amount of time </li></ul></ul>
    37. 37. CSMA/CD <ul><li>CSMA with collision detection (CD) </li></ul><ul><li>Listen while talking </li></ul><ul><li>Stop transmitting when collision detected </li></ul><ul><ul><li>Compare transmitted and received signals </li></ul></ul><ul><li>Human analogy </li></ul><ul><ul><li>Polite conversationalist </li></ul></ul><ul><li>Worst case time to detect a collision? </li></ul>
    38. 38. Collisions A B A B A B A B
    39. 39. Worst Case Collision Detection Time
    40. 40. CSMA/CD: Illustration
    41. 41. Ethernet Overview <ul><li>History </li></ul><ul><ul><li>developed by Xerox PARC in mid-1970s </li></ul></ul><ul><ul><li>roots in Aloha packet-radio network </li></ul></ul><ul><ul><li>standardized by Xerox, DEC, and Intel in 1978 </li></ul></ul><ul><ul><li>similar to IEEE 802.3 standard </li></ul></ul><ul><li>CSMA/CD </li></ul><ul><ul><li>carrier sense </li></ul></ul><ul><ul><li>multiple access </li></ul></ul><ul><ul><li>collision detection </li></ul></ul><ul><li>Frame Format </li></ul>Dest addr 64 48 32 CRC Preamble Src addr Type Body 16 48
    42. 42. Ethernet <ul><li>“ Dominant” LAN technology: </li></ul><ul><li>cheap $20 for 100Mbs! </li></ul><ul><li>first widely used LAN technology </li></ul><ul><li>Simpler, cheaper than token ring LANs and ATM </li></ul><ul><li>Kept up with speed race: 10, 100, 1000 Mbps </li></ul>Metcalfe’s Ethernet sketch
    43. 43. Ethernet Frame Format <ul><li>Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame </li></ul>DIX frame format IEEE 802.3 format <ul><li>Ethernet has a maximum frame size: data portion <=1500 bytes </li></ul><ul><li>It imposes a minimum frame size: 64 bytes (excluding preamble) </li></ul><ul><li>If data portion <46 bytes, pad with “junk” to make it 46 bytes </li></ul><ul><li>Q: Why minimum frame size in Ethernet? </li></ul>Dest addr 8 bytes 6 4 CRC Preamble Src addr Type Data 2 6 0-1500 Dest addr 8 bytes 6 4 CRC Preamble Src addr Length Data 2 6 0-1500
    44. 44. Fields in Ethernet Frame Format <ul><li>Preamble: </li></ul><ul><ul><li>7 bytes with pattern 10101010 followed by one byte with pattern 10101011 (SoF: start-of-frame) </li></ul></ul><ul><ul><li>used to synchronize receiver, sender clock rates, and identify beginning of a frame </li></ul></ul><ul><li>Addresses: 6 bytes </li></ul><ul><ul><li>if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol (specified by TYPE field) </li></ul></ul><ul><ul><li>otherwise, adapter discards frame </li></ul></ul><ul><li>Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk </li></ul><ul><ul><li>802.3: Length gives data size; “protocol type” included in data </li></ul></ul><ul><li>CRC: checked at receiver, if error is detected, the frame is simply dropped </li></ul>
    45. 45. IEEE 802.3 MAC: Ethernet <ul><li>MAC Protocol: </li></ul><ul><li>CSMA/CD </li></ul><ul><li>Slot Time is the critical system parameter </li></ul><ul><ul><li>upper bound on time to detect collision </li></ul></ul><ul><ul><li>upper bound on time to acquire channel </li></ul></ul><ul><ul><li>upper bound on length of frame segment generated by collision </li></ul></ul><ul><ul><li>quantum for retransmission scheduling </li></ul></ul><ul><ul><li>max{round-trip propagation, MAC jam time} </li></ul></ul><ul><li>Truncated binary exponential backoff </li></ul><ul><ul><li>for retransmission n: 0 < r < 2 k , where k=min(n,10) </li></ul></ul><ul><ul><li>give up after 16 retransmissions </li></ul></ul>
    46. 46. IEEE 802.3 Parameters <ul><li>1 bit time = time to transmit one bit </li></ul><ul><ul><li>10 Mbps  1 bit time = 0.1  s </li></ul></ul><ul><li>Maximum network diameter  2.5km </li></ul><ul><ul><li>Maximum 4 repeaters </li></ul></ul><ul><li>“ Collision Domain” </li></ul><ul><ul><li>Distance within which collision can occur and be detected </li></ul></ul><ul><ul><li>IEEE 802.3 specifies: </li></ul></ul><ul><ul><ul><li>worst case collision detection time: 51.2  s </li></ul></ul></ul><ul><li>Slot time </li></ul><ul><ul><li>51.2  s = 512 bits = 64 bytes </li></ul></ul><ul><li>Why minimum frame size? </li></ul><ul><ul><li>51.2  s => minimum # of bits can be transited at 10Mpbs is 512 bits => 64 bytes is required for collision detection </li></ul></ul>
    47. 47. Ethernet MAC Protocol: Basic Ideas <ul><li>1-persistent CSMA/CD </li></ul><ul><li>Carrier sense: station listens to channel first </li></ul><ul><ul><li>Listen before talking </li></ul></ul><ul><li>If idle, station may initiate transmission </li></ul><ul><ul><li>Talk if quiet </li></ul></ul><ul><li>Collision detection: continuously monitor channel </li></ul><ul><ul><li>Listen while talking </li></ul></ul><ul><li>If collision, stop transmission </li></ul><ul><ul><li>One talker at a time </li></ul></ul><ul><li>Exponential binary back-off algorithm </li></ul>
    48. 48. Ethernet CSMA/CD Illustration
    49. 49. Ethernet CSMA/CD Alg. Flow Chart
    50. 50. Ethernet CSMA/CD Algorithm <ul><li>1. Adaptor gets datagram from and creates frame </li></ul><ul><li>2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits </li></ul><ul><li>3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! Signal to network layer “transmit OK” </li></ul><ul><li>4. If adapter detects another transmission while transmitting, aborts and sends jam signal </li></ul><ul><li>5. After aborting, adapter enters exponential backoff : after the mth collision, adapter chooses a K at random from {0,1,2,…,2 m -1}. Adapter waits K*512 bit times and returns to Step 2 </li></ul><ul><li>6. Quit after 16 attempts, signal to network layer “transmit error” </li></ul>
    51. 51. Ethernet’s CSMA/CD (more) <ul><li>Jam Signal: make sure all other transmitters are aware of collision; 48 bits; </li></ul><ul><li>Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec </li></ul><ul><li>Exponential Backoff: </li></ul><ul><li>Goal : adapt retransmission attempts to estimated current load </li></ul><ul><ul><li>heavy load: random wait will be longer </li></ul></ul><ul><li>first collision: choose K from {0,1}; delay is K x 512 bit transmission times </li></ul><ul><li>after second collision: choose K from {0,1,2,3}… </li></ul><ul><li>after ten collisions, choose K from {0,1,2,3,4,…,1023} </li></ul>
    52. 52. CSMA/CD Efficiency <ul><li>Relevant parameters </li></ul><ul><ul><li>cable length, signal speed, frame size, bandwidth </li></ul></ul><ul><li>t prop = max prop between 2 nodes in LAN </li></ul><ul><li>t trans = time to transmit max-size frame </li></ul><ul><li>Efficiency goes to 1 as t prop goes to 0 </li></ul><ul><li>Goes to 1 as t trans goes to infinity </li></ul><ul><li>Much better than ALOHA, but still decentralized, simple, and cheap </li></ul>
    53. 53. IEEE 802.3 Physical Layer IEEE 802.3 10 Mbps medium alternatives Thick Coax: Stiff, hard to work with T connectors flaky Hubs & Switches! (a) transceivers (b) Point-to-point link Star Bus Bus Topology 2 km 100 m 2 00 m 5 00 m Max. Segment Length Optical f iber T wisted pair Thin coax Thick coax Medium 10base FX 10base T 10base 2 10base 5
    54. 54. Ethernet Technologies: 10Base2 <ul><li>10: 10Mbps; 2: under 200 meters max cable length </li></ul><ul><li>thin coaxial cable in a bus topology </li></ul><ul><li>repeaters used to connect up to multiple segments </li></ul><ul><li>repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! </li></ul><ul><li>has become a legacy technology </li></ul>
    55. 55. 10BaseT <ul><li>10 Mbps rate </li></ul><ul><li>T stands for Twisted Pair </li></ul><ul><li>Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub </li></ul><ul><li>Hubs are essentially physical-layer repeaters: </li></ul><ul><ul><li>bits coming in one link go out all other links </li></ul></ul><ul><ul><li>no frame buffering </li></ul></ul><ul><ul><li>no CSMA/CD at hub: adapters detect collisions </li></ul></ul><ul><ul><li>provides net management functionality </li></ul></ul>hub nodes
    56. 56. Ethernet Hubs & Switches Twisted Pair Cheap Easy to work with Reliable Star-topology CSMA-CD Twisted Pair Cheap Bridging increases scalability Separate collision domains Full duplex operation (a)       Single collision domain (b)     High-Speed backplane or interconnection fabric
    57. 57. Evolution of Ethernet <ul><li>From early 80’s 10Base Ethernet to 90’s 100Base (Fast) Ethernet </li></ul><ul><li>to today’s Gigabit Ethernet to 10 Gigabit Ethernet, …… </li></ul><ul><li>IEEE 802.3 Original Parameters </li></ul><ul><li>transmission Rate: 10 Mbps </li></ul><ul><li>Min Frame: 512 bits = 64 bytes </li></ul><ul><li>slot time: 512 bits/10 Mbps = 51.2  sec </li></ul><ul><ul><li>51.2  sec x 2x10 5 km/sec =10.24 km, 1 way </li></ul></ul><ul><ul><li>5.12 km round trip distance </li></ul></ul><ul><li>max Length: 2500 meters + 4 repeaters </li></ul><ul><li>For compatibility, desire to maintain same frame format! </li></ul><ul><ul><li>Each x10 increase in bit rate, must be accompanied by x10 decrease in distance ?! </li></ul></ul>
    58. 58. 100Base T (Fast) Ethernet: Issues <ul><li>1 bit time = time to transmit one bit </li></ul><ul><ul><li>100 Mbps  1 bit time = 0.01  s </li></ul></ul><ul><li>If we keep the same “collision domain”, i.e., </li></ul><ul><li>worst case collision detection time kept at 51.2  s </li></ul><ul><ul><li>Q: What will be the minimum frame size? </li></ul></ul><ul><ul><li>51.2  s => minimum # of bits can be transited at 100Mpbs is 5120 bits => 640 bytes is required for collision detection </li></ul></ul><ul><ul><li>This requires change of frame format and protocol! </li></ul></ul><ul><li>Or we can keep the same minimum frame size, but reduce “collision domain” or network diameter! </li></ul><ul><ul><li>slot time from 51.2  s to 5.12  s! </li></ul></ul><ul><ul><li>maximum network diameter  100 m! </li></ul></ul>
    59. 59. Fast (100Mbps) Ethernet IEEE 802.3 100 Mbps Ethernet medium alternatives <ul><li>To preserve compatibility with 10 Mbps Ethernet: </li></ul><ul><li>Same frame format, same interfaces, same protocols </li></ul><ul><li>Hub topology only with twisted pair & fiber </li></ul><ul><li>Bus topology & coaxial cable abandoned </li></ul><ul><li>Category 3 twisted pair (ordinary telephone grade) requires 4 pairs </li></ul><ul><li>Category 5 twisted pair requires 2 pairs (most popular) </li></ul><ul><li>Most prevalent LAN today </li></ul>Star Star Star Topology 2 km 100 m 100 m Max. Segment Length Optical fiber multimode Two strands Twisted pair category 5 UTP two pairs Twisted pair category 3 UTP 4 pairs Medium 100baseFX 100baseT 100baseT4
    60. 60. Gigabit Ethernet Gigabit Ethernet Physical Layer Specification (IEEE 802.3 1 Gigabit Ethernet medium alternatives) Star Star Star Star Topology 100 m 25 m 5 km 550 m Max. Segment Length Twisted pair category 5 UTP Shielded copper cable Optical fiber single mode Two strands Optical fiber multimode Two strands Medium 1000baseT 1000baseCX 1000baseLX 1000baseSX
    61. 61. Gigabit Ethernet <ul><li>use standard Ethernet frame format </li></ul><ul><li>allows for point-to-point links and shared broadcast channels </li></ul><ul><li>in shared (half-duplex hub) mode, CSMA/CD is used </li></ul><ul><ul><li>- slot time increases to 512 bytes </li></ul></ul><ul><ul><li>- small frames need to be extended to 512 B </li></ul></ul><ul><ul><li>- carrier extension: </li></ul></ul><ul><ul><li>frame bursting: allow stations to transmit burst of short frames </li></ul></ul><ul><li>Commonly used today: Gigabit switches! </li></ul><ul><ul><li>Full-Duplex at 1 Gbps for point-to-point links </li></ul></ul><ul><ul><li>Frame structure preserved but CSMA-CD essentially abandoned </li></ul></ul><ul><ul><li>Extensive deployment in backbone of enterprise data networks and in server farms </li></ul></ul>
    62. 62. Carrier Extension <ul><li>For 10BaseT : 2.5 km max; slot time = 64 bytes </li></ul><ul><li>For 1000BaseT: 200 m max; slot time = 512 bytes </li></ul><ul><li>Carrier Extension : continue transmitting control characters [R] to fill collision interval. </li></ul><ul><li>This permits minimum 64-byte frame to be handled. </li></ul><ul><li>Control characters discarded at destination. </li></ul><ul><li>For small frames net throughput is only slightly better than Fast Ethernet . </li></ul>RRRRRRRRRRRRR Frame Carrier Extension 512 bytes
    63. 63. Frame Bursting <ul><li>Source sends out burst of frames without relinquishing control of the network. </li></ul><ul><li>Uses Ethernet Interframe gap filled with extension bits (96 bits) </li></ul><ul><li>Maximum frame burst is 8192 bytes </li></ul><ul><li>Three times more throughput for small frames. </li></ul>512 bytes Extension Frame Frame Frame Frame Frame burst
    64. 64. 10 Gigabit Ethernet IEEE 802.3 10 Gbps Ethernet medium alternatives <ul><li>Frame structure preserved </li></ul><ul><li>CSMA-CD protocol officially abandoned </li></ul><ul><li>LAN PHY for local network applications </li></ul><ul><li>WAN PHY for wide area interconnection using SONET OC-192c </li></ul><ul><li>Extensive deployment in metro networks anticipated </li></ul>300 m – 10 km 40 km 10 km 300 m Max. Segment Length Two optical fibers multimode/single-mode with four wavelengths at 1310 nm band 8B10B code Two optical fibers Single-mode at 1550 nm SONET compatibility Two optical fibers Single-mode at 1310 nm 64B66B Two optical fibers Multimode at 850 nm 64B66B code Medium 10GbaseLX4 10GbaseEW 10GBaseLR 10GbaseSR
    65. 65. Typical Ethernet Deployment Server 100 Mbps links 10 Mbps links Server Server Server 100 Mbps links 10 Mbps links Server 100 Mbps links 10 Mbps links Server Gigabit Ethernet links Gigabit Ethernet links Server farm Department A Department B Department C Hub Hub Hub Ethernet switch Ethernet switch Ethernet switch Switch/router Switch/router
    66. 66. Ethernet Summary <ul><li>1-persistent CSMA/CD </li></ul><ul><li>51.2  s to seize the channel </li></ul><ul><li>Collision not possible after 51.2  s </li></ul><ul><li>Minimum frame size of 64 bytes </li></ul><ul><li>Binary exponential backoff </li></ul><ul><li>Works better under light load </li></ul><ul><li>Delivery time non-deterministic </li></ul>
    67. 67. Token based Multiple Access <ul><li>Grant access to one station at a time </li></ul><ul><ul><li>Have one token circulating among all stations </li></ul></ul><ul><li>To transmit </li></ul><ul><ul><li>Station must grab token (remove from media) </li></ul></ul><ul><ul><li>Transmit packet while holding token </li></ul></ul><ul><ul><li>Release token </li></ul></ul>
    68. 68. Ring Topology
    69. 69. Token Ring (IEEE 802.5) <ul><li>Station </li></ul><ul><ul><li>Wait for token to arrive </li></ul></ul><ul><ul><li>Hold the token and start data transmission </li></ul></ul><ul><ul><ul><li>Maximum token holding time  max packet size </li></ul></ul></ul><ul><ul><li>Strip the data frame off the ring </li></ul></ul><ul><ul><ul><li>After it has gone around the ring </li></ul></ul></ul><ul><ul><li>When done, release the token to next station </li></ul></ul><ul><li>When no station has data to send </li></ul><ul><ul><li>Token circulates continuously </li></ul></ul><ul><ul><li>Ring must have sufficient delay to contain the token </li></ul></ul>
    70. 70. Token Ring Performance <ul><li>Efficiency </li></ul>where
    71. 71. Token Release T oken Frame T oken Frame Release after Transmission Release after Reception
    72. 72. Tokens and Data Frames Body Checksum Src addr Variable 48 Dest addr 48 32 End delimiter 8 Frame status 8 Frame control 8 Access control 8 Start delimiter 8
    73. 73. Token Ring Frame Fields <ul><li>Access Control </li></ul><ul><ul><li>Token bit: 0  token 1  data </li></ul></ul><ul><ul><li>Monitor bit: used for monitoring ring </li></ul></ul><ul><ul><li>Priority and reservation bits: multiple priorities </li></ul></ul><ul><li>Frame Status </li></ul><ul><ul><li>Set by destination, read by sender </li></ul></ul><ul><li>Frame control </li></ul><ul><ul><li>Various control frames for ring maintenance </li></ul></ul>
    74. 74. Priority and Reservation <ul><li>Token carries priority bits </li></ul><ul><ul><li>Only stations with frames of equal or higher priority can grab the token </li></ul></ul><ul><li>A station can make reservation </li></ul><ul><ul><li>When a data frame goes by </li></ul></ul><ul><ul><li>If a higher priority has not been reserved </li></ul></ul><ul><li>A station raising the priority is responsible for lowering it again </li></ul>
    75. 75. Ring Maintenance <ul><li>Each ring has a monitor station </li></ul><ul><li>How to select a monitor? </li></ul><ul><ul><li>Election/self-promotion: CLAIM_TOKEN </li></ul></ul><ul><li>Responsibilities </li></ul><ul><ul><li>Insert additional delay </li></ul></ul><ul><ul><ul><li>To accommodate the token </li></ul></ul></ul><ul><ul><li>Check for lost token </li></ul></ul><ul><ul><ul><li>Regenerate token </li></ul></ul></ul><ul><ul><li>Watch for orphan frames </li></ul></ul><ul><ul><ul><li>Drain them off the ring </li></ul></ul></ul><ul><ul><li>Watch for garbled frames </li></ul></ul><ul><ul><ul><li>Clean up the ring and regenerate token </li></ul></ul></ul>
    76. 76. Fault Scenarios <ul><li>What to do if ring breaks? </li></ul><ul><ul><li>Everyone participates in detecting ring breaks </li></ul></ul><ul><ul><li>Send beacon frames </li></ul></ul><ul><ul><li>Figure out which stations are down </li></ul></ul><ul><ul><li>By-pass them if possible </li></ul></ul><ul><li>What happens if monitor dies? </li></ul><ul><ul><li>Everyone gets a chance to become the new king </li></ul></ul><ul><li>What if monitor goes berserk? </li></ul>
    77. 78. Token Ring Summary <ul><li>Stations take turns to transmit </li></ul><ul><li>Only the station with the token can transmit </li></ul><ul><li>Sender receives its own transmission </li></ul><ul><ul><li>Drains its frame off the ring </li></ul></ul><ul><li>Releases token after transmission/reception </li></ul><ul><li>Deterministic delivery possible </li></ul><ul><li>High throughput under heavy load </li></ul>
    78. 79. Ethernet vs Token Ring <ul><li>Non-deterministic </li></ul><ul><li>No delays at low loads </li></ul><ul><li>Low throughput under heavy load </li></ul><ul><li>No priorities </li></ul><ul><li>No management overhead </li></ul><ul><li>Large minimum size </li></ul><ul><li>Deterministic </li></ul><ul><li>Substantial delays at low loads </li></ul><ul><li>High throughput under heavy load </li></ul><ul><li>Multiple priorities </li></ul><ul><li>Complex management </li></ul><ul><li>Small frames possible </li></ul>
    79. 81. FDDI <ul><li>Two counter-rotating rings </li></ul><ul><ul><li>Failure recovery </li></ul></ul><ul><li>Optical fiber </li></ul><ul><ul><li>High bandwidth </li></ul></ul><ul><ul><li>Difficult to tap without detection </li></ul></ul><ul><li>100 Mbps data rate </li></ul><ul><li>Up to 200 kms, 1000 stations </li></ul>
    80. 82. FDDI vs. 802.5 (Token Ring) <ul><li>Operationally are very similar </li></ul><ul><ul><li>In frame format and contents </li></ul></ul><ul><li>Some differences </li></ul><ul><ul><li>Special 4B/5B symbols in FC field </li></ul></ul><ul><ul><ul><li>To indicate token or type of frame </li></ul></ul></ul><ul><ul><li>Maximum frame size of 4,500 bytes </li></ul></ul><ul><ul><li>Release token after transmission </li></ul></ul><ul><ul><li>Enhanced quality of service </li></ul></ul><ul><ul><ul><li>Synchronous and asynchronous frames </li></ul></ul></ul>
    81. 83. Timed Token Algorithm <ul><li>Token Holding Time (THT) </li></ul><ul><ul><li>upper limit on how long a station can hold the token </li></ul></ul><ul><li>Token Rotation Time (TRT) </li></ul><ul><ul><li>how long it takes the token to traverse the ring. </li></ul></ul><ul><ul><li>TRT <= ActiveNodes x THT + RingLatency </li></ul></ul><ul><li>Target Token Rotation Time (TTRT) </li></ul><ul><ul><li>agreed-upon upper bound on TRT </li></ul></ul>
    82. 84. Timed Token Algorithm (cont) <ul><li>Each node measures TRT between successive tokens </li></ul><ul><ul><li>if measured-TRT > TTRT: token is late so don’t send </li></ul></ul><ul><ul><li>if measured-TRT < TTRT: token is early so OK to send </li></ul></ul><ul><li>Two classes of traffic </li></ul><ul><ul><li>synchronous: can always send </li></ul></ul><ul><ul><li>asynchronous: can send only if token is early </li></ul></ul>
    83. 85. FDDI Failure Recovery
    84. 86. Summary <ul><li>Local Area Networks </li></ul><ul><ul><li>Designed for short distance </li></ul></ul><ul><ul><li>Use shared media </li></ul></ul><ul><ul><li>Many technologies exist </li></ul></ul><ul><li>Media Access Control: key problem! </li></ul><ul><ul><li>Different environments/technologies-> different solutions! </li></ul></ul><ul><li>Topology refers to general shape </li></ul><ul><ul><li>Bus </li></ul></ul><ul><ul><li>Ring </li></ul></ul><ul><ul><li>Star </li></ul></ul>
    85. 87. Summary (continued) <ul><li>Address </li></ul><ul><ul><li>Unique number assigned to station </li></ul></ul><ul><ul><li>Put in frame header </li></ul></ul><ul><ul><li>Recognized by hardware </li></ul></ul><ul><li>Address forms </li></ul><ul><ul><li>Unicast </li></ul></ul><ul><ul><li>Broadcast </li></ul></ul><ul><ul><li>Multicast </li></ul></ul>
    86. 88. Summary (continued) <ul><li>Type information </li></ul><ul><ul><li>Describes data in frame </li></ul></ul><ul><ul><li>Set by sender </li></ul></ul><ul><ul><li>Examined by receiver </li></ul></ul><ul><li>Frame format </li></ul><ul><ul><li>Header contains address and type information </li></ul></ul><ul><ul><li>Payload contains data being sent </li></ul></ul>
    87. 89. Summary (continued) <ul><li>LAN technologies </li></ul><ul><ul><li>Ethernet (bus) </li></ul></ul><ul><ul><li>Token Ring </li></ul></ul><ul><ul><li>FDDI (ring) </li></ul></ul><ul><ul><li>Wireless 802.11 </li></ul></ul><ul><li>Wiring and topology </li></ul><ul><ul><li>Logical topology and Physical topology (wiring) </li></ul></ul><ul><ul><li>Hub allows </li></ul></ul><ul><ul><ul><li>Star-shaped bus </li></ul></ul></ul><ul><ul><ul><li>Star-shaped ring </li></ul></ul></ul>