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Transcript
Direct Link Networks • Encoding and Framing • Error Detection/Correction • Reliable Transmission will be covered later (with TCP) • Media Access Control • (Wired) LAN Technologies • Readings – Chapter 2 except Section 2.5, which is delayed to the discusson on Transport layer. Csci 183/183W/232 Direct Link networks 1 Encoding • Signals propagate over a physical medium – modulate electromagnetic waves (on amplitude, frequency, and phase): the process of encoding source data onto a carrier signal with frequency f_c – e.g., vary voltage (amplitude) • Encode binary data onto signals – e.g., 0 as low signal and 1 as high signal – known as Non-Return to zero (NRZ) Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Csci 183/183W/232 Direct Link networks 2 Problems: Consecutive 1s or 0s • Low signal (0) may be interpreted as no signal • High signal (1) leads to baseline wander – Receiver keeps an average of the signal it has seen so far and uses this average to distinguish between low and high signals • Unable to recover clock – Clocks at the sender and receiver use signal transition to synchronize each other Csci 183/183W/232 Direct Link networks 3 Alternative Encodings • Non-return to Zero Inverted (NRZI) – make a transition from current signal to encode a one; stay at current signal to encode a zero – solves the problem of consecutive ones • Manchester – transmit XOR of the NRZ encoded data and the clock – only 50% efficient. Csci 183/183W/232 Direct Link networks 4 Encodings (cont) • 4B/5B – every 4 bits of data encoded in a 5-bit code – 5-bit codes selected to have no more than one leading 0 and no more than two trailing 0s – thus, never get more than three consecutive 0s – resulting 5-bit codes are transmitted using NRZI – achieves 80% efficiency Csci 183/183W/232 Direct Link networks 5 Encodings (cont) Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Clock Manchester NRZI Csci 183/183W/232 Direct Link networks 6 Framing • Break sequence of bits into a frame – why? determine where a frame starts and ends • Typically implemented by network adaptor Node A Adaptor Bits Adaptor Node B Frames Csci 183/183W/232 Direct Link networks 7 Approaches • Sentinel-based – delineate frame with special pattern: 01111110 – e.g., HDLC, SDLC, PPP 8 Beginning sequence 16 Header 16 Body 8 CRC Ending sequence – problem: special pattern appears in the payload – solution: bit stuffing • sender: insert 0 after five consecutive 1s • receiver: delete 0 that follows five consecutive 1s • What if the receiver sees a 1? Csci 183/183W/232 Direct Link networks 8 Approaches (cont) • Couter-based 8 8 8 14 42 SYN SYN Class – include payload length in header – e.g., DDCMP Count Header 16 Body CRC – problem: count field corrupted – solution: catch when CRC fails – How many frames are affected when counter or sentinel is corrupted? Csci 183/183W/232 Direct Link networks 9 Approaches (cont) • Clock-based – each frame is 125us long – e.g., SONET: Synchronous Optical Network Overhead Special bit pattern Payload 9 row s 90 columns Csci 183/183W/232 Direct Link networks 10 Handling Errors • Data can be corrupted during transmission – Bit values changed • Frame includes additional information – Set by sender – Checked by receiver • Error-detection vs error-correction – Both need redundant information – Detection: error exists or not. – Correction: repair if there was an error • Statistical guarantee Csci 183/183W/232 Direct Link networks 11 Error Detecting and Correcting Codes • How many check/redundancy bits? – To detect single-bit error • 1 bit: even/odd parity – To correct a single-bit error in m-bit message • Need a minimum of r bits such that • (m + r + 1) 2r • Example: 3-bit message needs 3-bit redundancy • Correction or detection+retransmission? Csci 183/183W/232 Direct Link networks 12 Error Detection Techniques • Checksum – – – – Treat data as sequence of integers Compute and send arithmetic sum Detects some multiple bit errors not all Internet Checksum • Cyclic Redundancy Check – Mathematical function of data – More complex to compute – Handles more errors Csci 183/183W/232 Direct Link networks 13 Cyclic Redundancy Check • Add k bits of redundant data to an n-bit message – want k << n – e.g., k = 32 and n = 12,000 (1500 bytes) • Represent n-bit message as n-1 degree polynomial – e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1 • Let k be the degree of some divisor polynomial – e.g., C(x) = x3 + x2 + 1 Csci 183/183W/232 Direct Link networks 14 CRC (cont) • Transmit polynomial P(x) that is evenly divisible by C(x) – shift left k bits, i.e., M(x)xk – subtract remainder of M(x)xk / C(x) from M(x)xk • Receiver polynomial P(x) + E(x) – E(x) = 0 implies no errors • Divide (P(x) + E(x)) by C(x); remainder zero if: – E(x) was zero (no error), or – E(x) is exactly divisible by C(x) Csci 183/183W/232 Direct Link networks 15 CRC Example Generator 11111001 1101 10011010000 1101 Message 1001 1101 1000 1101 1011 1101 1100 1101 1000 1101 101 Csci 183/183W/232 Direct Link networks Remainder 16 Choice of C(x) • Want to ensure C(x) doesn’t divide E(x) • We can detect – All single-bit errors if • C(x) has at least 2 terms – All double-bit errors if • C(x) doesn’t divide xj + 1 – X15 + x14 + 1 doesn’t divide xj + 1 for any j below 32768 – Any odd number of errors if • C(x) contains the factor x+1 – Any “burst” error of length less than or equal to k bits – Most burst errors of length greater than k bits Csci 183/183W/232 Direct Link networks 17 Error Detection Summary • To detect data corruption – Sender adds additional information to packet – Receiver checks • Techniques – Parity bit – Checksum – Cyclic Redundancy Check Csci 183/183W/232 Direct Link networks 18 Hamming Error Correcting Code • Achieve the theoretical lower limit of the number of redundant bits • Correcting single-bit errors • Method: – Put information bits at positions that are not equal to powers of two – Compute the check bits at positions that are power of two • A check bit at position 2^j, where j=0, 1, …, checks an information bit at position i if and only if the binary representation of i contains a 1 at position j. • The value of an “even” check bit equals the xor of all checked information bits • At the receiver side, all checked bits and the check bit are xored and a zero indicates error free; otherwise, the sum of the positions of the check bits with a non-zero value indicates the bit that is in error • Example – Information bits: 1001000 – Coded bits: 00110010000 Csci 183/183W/232 Direct Link networks 19 Error Recovery • Reliable delivery over unreliable channel – How to recover from corrupted/lost packets • Error detection and retransmission – With acknowledgements and timeouts – Also called Automatic Repeat Request (ARQ) – Retransmission incurs round trip delay • Error correcting codes – Also called Forward Error Correction (FEC) – No sender retransmission required Csci 183/183W/232 Direct Link networks 20 Summary • Encoding • Framing • Error correction/detection codes – Parity/Checksum/CRC Csci 183/183W/232 Direct Link networks 21 Shared Media and Local Area Networks • Shared Media Access Problem – Media access control (MAC) and LAN – MAC addresses and network adaptors (NICs) – MAC protocols: random access vs. controlled access • Ethernet • Token Ring and FDDI (read yourself) • 802.11 Wireless LAN (WiFi and WiMAX) (next lecture) Csci 183/183W/232 Direct Link networks 22 Multiple Access Links and LANs Two types of “links”: • point-to-point, e.g., – PPP for dial-up access, or over optical fibers • broadcast (shared wire or medium), e.g. – traditional Ethernet – 802.11 wireless LAN Csci 183/183W/232 Direct Link networks 23 Typical LAN Structure • Transmission Medium • Network Interface Card (NIC) • Unique MAC “physical” address Ethernet Processor RAM ROM RAM Csci 183/183W/232 Direct Link networks 24 Adaptors Communicating sending node network packet link layer protocol frame frame adapter • link layer implemented in “adaptor” (aka NIC), with “transceiver” in it adapter • receiving side – Ethernet card, dial-up modem, 802.11 wireless card • sending side: – encapsulates packet in frame • – adds error checking bits, flow control, reliable data transmission, etc. Csci 183/183W/232 rcving node – looks for errors, flow control, reliable data transmission, etc – extracts packet, passes to receiving node data link & physical layers are closely coupled! Direct Link networks 25 MAC (Physical) Addresses • Addressing needed in shared media – MAC (media access control) or physical addresses – 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!) • 48 bit MAC address (for most LANs) – fixed for each adaptor, burned in the adapter ROM – MAC address allocation administered by IEEE • 1st bit: 0 unicast, 1 multicast. • all 1’s : broadcast • MAC flat address -> portability – can move LAN card from one LAN to another Csci 183/183W/232 Direct Link networks 26 MAC (Physical, or LAN) Addresses MAC addressing operations on a LAN: • • each adaptor on the LAN “sees” all frames accept a frame only if dest. (unicast) MAC address matches its own MAC address • accept all broadcast (MAC= all 1’s) frames • accept all frames if set in “promiscuous” mode • can configure to accept certain multicast addresses (first bit = 1) Csci 183/183W/232 Direct Link networks 27 MAC Sub-layer OSI IEEE 802 Network layer Network layer 802.2 Logical link control LLC Data link layer MAC 802.11 802.3 802.5 CSMA-CD Token Ring Wireless LAN Physical layer Other LANs Various physical layers Csci 183/183W/232 Direct Link networks Physical layer 28 Broadcast Links: Multiple Access Single shared communication channel • Only one can send successfully at a time • Two or more simultaneous transmissions – interference! • How to share a broadcast channel – media access control uses same shared media • Humans use multi-access protocols all the time Csci 183/183W/232 Direct Link networks 29 Random Access • Stations contend for channels • Overlapping transmissions (collisions) can occur – Carrier sensing? – Collision detection? • Protocols – Aloha – Slotted Aloha – Carrier Sense Multiple Access: Ethernet Csci 183/183W/232 Direct Link networks 30 Controlled Access • Stations reserve or are allocated channel – No collisions – Allocation: static or dynamic • Protocols – Static channel allocation • Time division multiple access – Demand adaptive channel allocation • Reservation protocols • Token passing (token bus, token ring) Csci 183/183W/232 Direct Link networks 31 Taxonomy of MAC Protocols WiFi (802.11) Csci 183/183W/232 Direct Link networks 32 Pure (unslotted) Aloha • Simpler, no synchronization • Just send: no waiting for beginning of slot Csci 183/183W/232 Direct Link networks 33 Slotted Aloha • Time is divided into equal size slots • Nodes transmit at the beginning of a slot • If collision, retransmit later Success (S), Collision (C), Empty (E) slots Csci 183/183W/232 Direct Link networks 34 Performance of Aloha Protocols 0.4 0.3 Slotted Aloha 0.2 0.1 Pure Aloha 0.5 Csci 183/183W/232 1.0 1.5 2.0 G = offered load = Np Direct Link networks 35 Carrier Sense Multiple Access • Aloha is inefficient (and rude) – Doesn’t listen before talking • CSMA: Listen before transmit – If channel idle, transmit entire packet – If busy, defer transmission • How long should we wait? – Human analogy: don’t interrupt others • Can carrier sense avoid collisions completely? Csci 183/183W/232 Direct Link networks 36 Persistent and Non-persistent CSMA • Non-persistent – – – • If idle, transmit If busy, wait random amount of time If collision, wait random amount of time p-persistent – – – If idle, transmit with probability p If busy, wait till it becomes idle If collision, wait random amount of time Csci 183/183W/232 Direct Link networks 37 CSMA/CD CSMA with collision detection (CD) • Listen while talking • Stop transmitting when collision detected – Compare transmitted and received signals – Save time and bandwidth – Improvement over persistent and nonpersistent protocols • Human analogy – Polite conversationalist • Worst case time to detect a collision? Csci 183/183W/232 Direct Link networks 38 Collisions Csci 183/183W/232 A B A B A B A B Direct Link networks 39 Worst Case Collision Detection Time Csci 183/183W/232 Direct Link networks 40 CSMA/CD: Illustration Csci 183/183W/232 Direct Link networks 41 Ethernet Overview • History – developed by Xerox PARC in mid-1970s – roots in Aloha packet-radio network – standardized by Xerox, DEC, and Intel in 1978 – similar to IEEE 802.3 standard • CSMA/CD – carrier sense – multiple access – collision detection • Frame Format 64 48 Preamble Dest addr Csci 183/183W/232 48 Src addr 16 Type Direct Link networks 32 Body CRC 42 Ethernet “Dominant” LAN technology: • cheap $20 for 100Mbs! • first widely used LAN technology • Simpler, cheaper than token ring LANs and ATM • Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch Csci 183/183W/232 Direct Link networks 43 Ethernet Frame Format Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame DIX frame format 8 bytes Preamble 6 Dest addr 6 Src addr 2 Type 0-1500 Data 4 CRC IEEE 802.3 format 8 bytes Preamble 6 Dest addr 6 Src addr 2 0-1500 Length Data 4 CRC • Ethernet has a maximum frame size: data portion <=1500 bytes • It imposes a minimum frame size: 64 bytes (excluding preamble) If data portion <46 bytes, pad with “junk” to make it 46 bytes Q: Why minimum frame size in Ethernet? Csci 183/183W/232 Direct Link networks 44 Fields in Ethernet Frame Format • Preamble: – 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 (SoF: start-of-frame) – used to synchronize receiver, sender clock rates, and identify beginning of a frame • Addresses: 6 bytes – 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) – otherwise, adapter discards frame • Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk – 802.3: Length gives data size; “protocol type” included in data • CRC: checked at receiver, if error is detected, the frame is simply dropped Csci 183/183W/232 Direct Link networks 45 IEEE 802.3 MAC: Ethernet MAC Protocol: • CSMA/CD • Truncated binary exponential backoff – for retransmission n: 0 < r < 2k * ASlotTime, where k=min(n,10) – give up after 16 retransmissions • Slot Time is the critical system parameter – upper bound on time to detect collision – upper bound on time to acquire channel – upper bound on length of frame segment generated by collision – quantum for retransmission scheduling – max{round-trip propagation, MAC jam time} Csci 183/183W/232 Direct Link networks 46 IEEE 802.3 Parameters • 1 bit time = time to transmit one bit – 10 Mbps 1 bit time = 0.1 s • Maximum network diameter 2.5km – Maximum 4 repeaters • “Collision Domain” – Distance within which collision can occur and be detected – IEEE 802.3 specifies: worst case collision detection time: 51.2 s • Slot time – 51.2 s = 512 bits = 64 bytes • Why minimum frame size? – 51.2 s => minimum # of bits can be transited at 10Mpbs is 512 bits => 64 bytes is required for collision detection Csci 183/183W/232 Direct Link networks 47 Ethernet MAC Protocol: Basic Ideas 1-persistent CSMA/CD • Carrier sense: station listens to channel first – Listen before talking • If idle, station may initiate transmission – Talk if quiet • Collision detection: continuously monitor channel – Listen while talking • If collision, stop transmission – One talker at a time • Exponential binary back-off algorithm Csci 183/183W/232 Direct Link networks 48 Ethernet CSMA/CD Alg. Flow Chart Csci 183/183W/232 Direct Link networks 49 Ethernet CSMA/CD Algorithm 1. Adaptor gets datagram and creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! Signal to network layer “transmit OK” Csci 183/183W/232 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2 6. Quit after 16 attempts, signal to network layer “transmit error” Direct Link networks 50 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 32 bits; Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Csci 183/183W/232 Exponential Backoff: • Goal: adapt retransmission attempts to estimated current load – heavy load: random wait will be longer • first collision: choose K from {0,1}; delay is K x 512 bit transmission times • after second collision: choose K from {0,1,2,3}… • after ten collisions, choose K from {0,1,2,3,4,…,1023} Direct Link networks 51 CSMA/CD Efficiency Relevant parameters – cable length, signal speed, frame size, bandwidth • tprop = max prop between 2 nodes in LAN • ttrans = time to transmit max-size frame efficiency 1 1 5t prop / ttrans • Efficiency goes to 1 as tprop goes to 0 • Goes to 1 as ttrans goes to infinity • Much better than ALOHA, but still decentralized, simple, and cheap Csci 183/183W/232 Direct Link networks 52 IEEE 802.3 Physical Layer IEEE 802.3 10 Mbps medium alternatives Medium Max. Segment Length Topology 10base5 10base2 10baseT 10baseFX Thick coax Thin coax Twisted pair Optical fiber 500 m 200 m 100 m 2 km Bus Bus Star Point-topoint link (b) (a) Hubs & Switches! transceivers Thick Coax: Stiff, hard to work with Csci 183/183W/232 Direct Link networks T connectors flaky 53 Ethernet Technologies: 10Base2 • 10: 10Mbps; 2: under 200 meters max cable length • thin coaxial cable in a bus topology • repeaters used to connect up to multiple segments • repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! • has become a legacy technology Csci 183/183W/232 Direct Link networks 54 10BaseT • 10 Mbps rate • T stands for Twisted Pair • Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub nodes hub • Hubs are essentially physical-layer repeaters: – – – – bits coming in one link go out all other links no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality Csci 183/183W/232 Direct Link networks 55 Ethernet Hubs & Switches Single collision domain (a) High-Speed backplane or interconnection fabric (b) Twisted Pair Cheap Easy to work with Reliable Star-topology CSMA-CD Csci 183/183W/232 Twisted Pair Cheap Bridging increases scalability Separate collision domains Full duplex operation Direct Link networks 56 Evolution of Ethernet From early 80’s 10Base Ethernet to 90’s 100Base (Fast) Ethernet to today’s Gigabit Ethernet to 10 Gigabit Ethernet, …… IEEE 802.3 Original Parameters • • • transmission Rate: 10 Mbps Min Frame: 512 bits = 64 bytes slot time: 512 bits/10 Mbps = 51.2 sec • max Length: 2500 meters + 4 repeaters – 51.2 sec x 2x105 km/sec =10.24 km – 5.12 km round trip distance • For compatibility, desire to maintain same frame format! – Each x10 increase in bit rate, must be accompanied by x10 decrease in distance ?! Csci 183/183W/232 Direct Link networks 57 100Base T (Fast) Ethernet: Issues • 1 bit time = time to transmit one bit – 100 Mbps 1 bit time = 0.01 s • If we keep the same “collision domain”, i.e., worst case collision detection time kept at 51.2 s Q: What will be the minimum frame size? – 51.2 s => minimum # of bits can be transited at 100Mpbs is 5120 bits => 640 bytes is required for collision detection – This requires change of frame format and protocol! • Or we can keep the same minimum frame size, but reduce “collision domain” or network diameter! • slot time from 51.2 s to 5.12 s! • maximum network diameter 100 m! Csci 183/183W/232 Direct Link networks 58 Fast (100Mbps) Ethernet IEEE 802.3 100 Mbps Ethernet medium alternatives 100baseT4 100baseT 100baseFX Twisted pair category 3 UTP 4 pairs Twisted pair category 5 UTP two pairs Optical fiber multimode Two strands Max. Segment Length 100 m 100 m 2 km Topology Star Star Star Medium To preserve compatibility with 10 Mbps Ethernet: • Same frame format, same interfaces, same protocols • Hub topology only with twisted pair & fiber • Bus topology & coaxial cable abandoned • Category 3 twisted pair (ordinary telephone grade) requires 4 pairs • Category 5 twisted pair requires 2 pairs (most popular) • Most prevalent LAN today Csci 183/183W/232 Direct Link networks 59 Gigabit Ethernet Gigabit Ethernet Physical Layer Specification (IEEE 802.3 1 Gigabit Ethernet medium alternatives) Medium Max. Segment Length Topology Csci 183/183W/232 1000baseSX 1000baseLX 1000baseCX 1000baseT Optical fiber multimode Two strands Optical fiber single mode Two strands Shielded copper cable Twisted pair category 5 UTP 550 m 5 km 25 m 100 m Star Star Star Star Direct Link networks 60 Gigabit Ethernet • use standard Ethernet frame format • allows for point-to-point links and shared broadcast channels • in shared (half-duplex hub) mode, CSMA/CD is used - slot time increases to 512 bytes - small frames need to be extended to 512 B - carrier extension: – frame bursting: allow stations to transmit burst of short frames • Commonly used today: Gigabit switches! – Full-Duplex at 1 Gbps for point-to-point links – Frame structure preserved but CSMA-CD essentially abandoned – Extensive deployment in backbone of enterprise data networks and in server farms Csci 183/183W/232 Direct Link networks 61 Carrier Extension Frame RRRRRRRRRRRRR Carrier Extension 512 bytes • For 10BaseT : 2.5 km max; slot time = 64 bytes • For 1000BaseT: 200 m max; slot time = 512 bytes • Carrier Extension : continue transmitting control characters [R] to fill collision interval. • This permits minimum 64-byte frame to be handled. • Control characters discarded at destination. • For small frames net throughput is only slightly better than Fast Ethernet. Csci 183/183W/232 Direct Link networks 62 Frame Bursting Frame Extension Frame Frame Frame 512 bytes Frame burst • Source sends out burst of frames without relinquishing control of the network. • Uses Ethernet Interframe gap filled with extension bits (96 bits) • Maximum frame burst is 8192 bytes • Three times more throughput for small frames. Csci 183/183W/232 Direct Link networks 63 10 Gigabit Ethernet IEEE 802.3 10 Gbps Ethernet medium alternatives 10GbaseSR Medium 10GBaseLR 10GbaseEW 10GbaseLX4 Two optical fibers Multimode at 850 nm Two optical fibers Two optical fibers Single-mode at 1310 nm 64B66B code 64B66B Single-mode at 1550 nm SONET compatibility Two optical fibers multimode/singlemode with four wavelengths at 1310 nm band 8B10B code Max. Segment Length • • • • • 300 m 10 km 40 km 300 m – 10 km Frame structure preserved CSMA-CD protocol officially abandoned LAN PHY for local network applications WAN PHY for wide area interconnection using SONET OC-192c Extensive deployment in metro networks anticipated Csci 183/183W/232 Direct Link networks 64 Typical Ethernet Deployment Server farm Server Server Server Gigabit Ethernet links Switch/router Server Ethernet switch 100 Mbps links Hub 10 Mbps links Department A Csci 183/183W/232 Gigabit Ethernet links Ethernet switch 100 Mbps links Server Hub 10 Mbps links Department B Direct Link networks Switch/router Ethernet switch 100 Mbps links Server Hub 10 Mbps links Department C 65 Ethernet Summary • • • • • • • 1-persistent CSMA/CD 51.2 s to seize the channel Collision not possible after 51.2 s Minimum frame size of 64 bytes Binary exponential backoff Works better under light load Delivery time non-deterministic Csci 183/183W/232 Direct Link networks 66 Summary (continued) • Address – Unique number assigned to station – Put in frame header – Recognized by hardware • Address forms – Unicast – Broadcast – Multicast Csci 183/183W/232 Direct Link networks 67 Summary (continued) • Type information – Describes data in frame – Set by sender – Examined by receiver • Frame format – Header contains address and type information – Payload contains data being sent Csci 183/183W/232 Direct Link networks 68 Summary (continued) • LAN technologies – – – – Ethernet (bus) Token Ring FDDI (ring) Wireless 802.11 • Wiring and topology – Logical topology and Physical topology (wiring) – Hub allows • Star-shaped bus • Star-shaped ring Csci 183/183W/232 Direct Link networks 69