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CMU 15441 Computer Networking - Lecture

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Lecture 6 Datalink – Framing, SwitchingFrom Signals to PacketsDatalink FunctionsDatalink LecturesFramingCharacter and Bit StuffingExample: Ethernet FramingSONETStandardization HistoryA Word about Data RatesSynchronous Data TransferSONET FramingHow Do We Support Lower Rates?How Do We Support Higher Rates?The SONET Signal HierarchyUsing SONET in NetworksSelf-Healing SONET RingsSONET as Physical LayerError CodingBasic Concept: Hamming DistanceExamplesCyclic Redundancy Codes (CRC)Datalink ArchitecturesMedia Access ControlA Switch-based NetworkSwitching IntroductionAn Inter-networkInternetworking OptionsSwitch ArchitecturePacket Forwarding: Address LookupLink Flow Control and Error ControlA Naïve ProtocolAdding Flow ControlWindow Flow ControlBandwidth-Delay ProductDealing with Errors Stop and Wait CaseWhat is Used in Practice?Datalink Layer ArchitecturesMultiple Access ProtocolsFiber Distributed Data Interface (FDDI)Other “Taking Turn” Protocols1Lecture 6Datalink – Framing, SwitchingDavid AndersenDepartment of Computer ScienceCarnegie Mellon University15-441 Networking, Spring 2005http://www.cs.cmu.edu/~srini/15-441/S05/2From Signals to PacketsAnalog Signal“Digital” SignalBit Stream0 0 1 0 1 1 1 0 0 0 1Packets0100010101011100101010101011101110000001111010101110101010101101011010111001Header/BodyHeader/BodyHeader/BodyReceiverSenderPacketTransmission3Datalink FunctionsFraming: encapsulating a network layer datagram into a bit stream.»Add header, mark and detect frame boundaries, …Media access: controlling which frame should be sent over the link next.»Easy for point-to-point links; half versus full duplex»Harder for multi-access links: who gets to send?Error control: error detection and correction to deal with bit errors.»May also include other reliability support, e.g. retransmissionFlow control: avoid that the sender outruns the receiver.4Datalink LecturesFraming and error coding.Datalink architectures.Switch-based networks.»Packet forwarding»Flow and error controlTaking turn protocols.Contention-based networks: basic Ethernet.Ethernet bridging and switching.Connectivity to the home.Circuit-based communication5FramingA link layer function, defining which bits have which function.Minimal functionality: mark the beginning and end of packets (or frames).Some techniques:»out of band delimiters (e.g. FDDI 4B/5B control symbols)»frame delimiter characters with character stuffing»frame delimiter codes with bit stuffing»synchronous transmission (e.g. SONET)6Character and Bit StuffingMark frames with special character.»What happens when the user sends this character?»Use escape character when controls appear in data: *abc*def -> *abc\*def»Very common on serial lines, in editors, etc.Mark frames with special bit sequence»must ensure data containing this sequence can be transmitted»example: suppose 11111111 is a special sequence.»transmitter inserts a 0 when this appears in the data:»11111111 -> 111111101»must stuff a zero any time seven 1s appear:»11111110 -> 111111100»receiver unstuffs.7Example: Ethernet FramingPreamble is 7 bytes of 10101010 (5 MHz square wave) followed by one byte of 10101011Allows receivers to recognize start of transmission after idle channelpreamblepreambledatagramdatagramlengthlengthmore stuffmore stuff8SONETSONET is the Synchronous Optical Network standard for data transport over optical fiber.One of the design goals was to be backwards compatible with many older telco standards.Beside minimal framing functionality, it provides many other functions:»operation, administration and maintenance (OAM) communications»synchronization»multiplexing of lower rate signals»multiplexing for higher rates9Standardization HistoryProcess was started by divestiture in 1984.»Multiple telephone companies building their own infrastructureSONET concepts originally developed by Bellcore.First standardized by ANSI T1X1 group for the US.Later picked up by CCITT and developed its own version.SONET/SDH standards approved in 1988.10A Word about Data RatesBandwidth of telephone channel is under 4KHz, so when digitizing: 8000 samples/sec * 8 bits = 64Kbits/secondCommon data rates supported by telcos in North America:»Modem: rate improved over the years»T1/DS1: 24 voice channels plus 1 bit per sample (24 * 8 + 1) * 8000 = 1.544 Mbits/second»T3/DS3: 28 T1 channels: 7 * 4 * 1.544 = 44.736 Mbits/second11Synchronous Data TransferSender and receiver are always synchronized.»Frame boundaries are recognized based on the clock»No need to continuously look for special bit sequencesSONET frames contain room for control and data.»Data frame multiplexes bytes from many users»Control provides information on data, management, …3 colstransportoverhead87 cols payload capacity9 rows12SONET FramingBase channel is STS-1 (Synchronous Transport System).»Takes 125 sec and corresponds to 51.84 Mbps»1 byte/frame corresponds to a 64 Kbs channel (voice)»Also called OC-1 = optical carrierStandard ways of supporting slower and faster channels.»Support both old standards and future (higher) data ratesActual payload frame ”floats” in the synchronous frame.»Clocks on individual links do not have to be synchronized3 colstransportoverhead87 cols payload capacity,including 1 col path overhead9 rows13How Do We Support Lower Rates?1 Byte in every consecutive frame corresponds to a 64 Kbit/second channel.»1 voice call.Higher bandwidth channels hold more bytes per frame.»Multiples of 64 Kbit/secondChannels have a “telecom” flavor.»Fixed bandwidth»Just data – no headers»SONET multiplexers remember how bytes on one link should be mapped to bytes on the next link 125 sec125 sec125 sec14How Do We SupportHigher Rates?Send multiple frames in a 125 sec time slot.The properties of a channel using a single byte/ST-1 frame are maintained!»Constant 64 Kbit/second rate»Nice spacing of the byte samplesRates typically go up by a factor of 4.Two ways of doing interleaving.»Frame interleaving»Column interleaving–concatenated version, i.e. OC-3c125 sec125 sec125 sec15The SONET Signal HierarchySignal TypeSignal TypeOC-1OC-1line rateline rate# of DS0# of DS051.84 Mbs51.84 Mbs672672OC-3OC-3155 Mbs155 Mbs2,0162,016OC-12OC-12622 Mbs622 Mbs8,0648,064STS-48STS-482.49 Gbs2.49 Gbs32,25632,256STS-192STS-1929.95 Gbs9.95


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CMU 15441 Computer Networking - Lecture

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