Page 1Peter A. Steenkiste, SCS, CMU1Lecture 6Datalink – Framing, SwitchingPeter SteenkisteSchool of Computer ScienceDepartment of Electrical and Computer EngineeringCarnegie Mellon University15-441 Networking, Spring 2004http://www.cs.cmu.edu/~prs/15-441Peter A. Steenkiste, SCS, CMU2From Signals to PacketsAnalog Signal“Digital” SignalBit Stream0 0 1 0 1 1 1 0 0 0 1Packets0100010101011100101010101011101110000001111010101110101010101101011010111001Header/BodyHeader/BodyHeader/BodyReceiverSenderPacketTransmissionPage 2Peter A. Steenkiste, SCS, CMU3Datalink Functionsz Framing: encapsulating a network layer datagram into a bit stream.» Add header, mark and detect frame boundaries, …zMedia 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?z Error control: error detection and correction to deal with bit errors.» May also include other reliability support, e.g. retransmissionzFlow control: avoid that the sender outruns the receiver.Peter A. Steenkiste, SCS, CMU4Datalink Lecturesz Framing and error coding.z Datalink architectures.z Switch-based networks.» Packet forwarding» Flow and error controlzTaking turn protocols.z Contention-based networks: basic Ethernet.z Ethernet bridging and switching.z Connectivity to the home.z Circuit-based communicationPage 3Peter A. Steenkiste, SCS, CMU5Framingz A link layer function, defining which bits have which function.z Minimal functionality: mark the beginning and end of packets (or frames).z 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)Peter A. Steenkiste, SCS, CMU6Character and Bit Stuffingz 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.z 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.Page 4Peter A. Steenkiste, SCS, CMU7Example: Ethernet Framingz Preamble is 7 bytes of 10101010 (5 MHz square wave) followed by one byte of 10101011z Allows receivers to recognize start of transmission after idle channelpreamblepreambledatagramdatagramlengthlengthmore stuffmore stuffPeter A. Steenkiste, SCS, CMU8SONETz SONET is the Synchronous Optical Network standard for data transport over optical fiber.z One of the design goals was to be backwards compatible with many older telco standards.z Beside minimal framing functionality, it provides many other functions:» operation, administration and maintenance (OAM) communications» synchronization» multiplexing of lower rate signals» multiplexing for higher ratesPage 5Peter A. Steenkiste, SCS, CMU9Standardization Historyz Process was started by divestiture in 1984.» Multiple telephone companies building their own infrastructurez SONET concepts originally developed by Bellcore.z First standardized by ANSI T1X1 group for the US.z Later picked up by CCITT and developed its own version.z SONET/SDH standards approved in 1988.Peter A. Steenkiste, SCS, CMU10A Word about Data Ratesz Bandwidth of telephone channel is under 4KHz, so when digitizing:8000 samples/sec * 8 bits = 64Kbits/secondz 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/secondPage 6Peter A. Steenkiste, SCS, CMU11Synchronous Data Transferz Sender and receiver are always synchronized.» Frame boundaries are recognized based on the clock» No need to continuously look for special bit sequencesz 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 rowsPeter A. Steenkiste, SCS, CMU12SONET Framingz Base channel is STS-1 (Synchronous Transport System).» Takes 125 µsec and corresponds to 51.84 Mbps» 1 byte/frame corresponds to a 64 Kbs channel (voice)» Also called OC-1 = optical carrierz Standard ways of supporting slower and faster channels.» Support both old standards and future (higher) data ratesz 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 rowsPage 7Peter A. Steenkiste, SCS, CMU13How Do We Support Lower Rates?z 1 Byte in every consecutive frame corresponds to a 64 Kbit/second channel.» 1 voice call.z Higher bandwidth channels hold more bytes per frame.» Multiples of 64 Kbit/secondz 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 µsec 125 µsec125 µsecPeter A. Steenkiste, SCS, CMU14How Do We SupportHigher Rates?z Send multiple frames in a 125 µsec time slot.z The properties of a channel using a single byte/ST-1 frame are maintained!» Constant 64 Kbit/second rate» Nice spacing of the byte samplesz Rates typically go up by a factor of 4.z Two ways of doing interleaving.» Frame interleaving» Column interleaving– concatenated version, i.e. OC-3c125 µsec 125 µsec125 µsecPage 8Peter A. Steenkiste, SCS, CMU15The 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 Gbs129,024129,024STS-768STS-76839.8 Gbs39.8 Gbs516,096516,096DS0 (POTS)DS0 (POTS)64 Kbs64 Kbs11DS1DS11.544 Mbs1.544 Mbs2424DS3DS344.736 Mbs44.736 Mbs672672Peter A. Steenkiste, SCS, CMU16Using SONET in NetworksmuxmuxmuxmuxmuxmuxDS1OC-3cOC-12cOC-48Add-drop capability allows soft configuration of networks,usually managed manually.Page 9Peter A. Steenkiste, SCS, CMU17Self-Healing SONET RingsmuxmuxmuxmuxmuxmuxDS1OC-3cOC-12cOC-48muxmuxPeter A.
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