Page 1Peter A. Steenkiste, SCS, CMU1Lecture 8Home – Virtual ConnectionsPeter 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, CMU2Outlinel Connectivity to the home.» Modems» ISDN» xDSL» Cable modemlCircuit switching.l ATM overview.l IP over ATM and SONET.Page 2Peter A. Steenkiste, SCS, CMU3Modemsl Modem offers a bit stream.» Aggressive signal processing has dramatically increased the available throughput - beats the Nyquist limit!l SLIP: Serial Line IP.» Protocol to sent IP packets with minimum framing» Lacks authentication, error detection, non-IP support, ..l PPP: Point-to-Point Packets.» Better framing, error control, and testing support» Can negotiate choice of higher layer protocols, IP address» Can support unreliable and reliable transmissionHomePCModemBankTelephoneNetworkPeter A. Steenkiste, SCS, CMU4Integrated Services Digital Network (ISDN)l ISDN integrates voice and data services.l Provides a set of bit pipes that can be used for voice, data, signaling.» Implemented by using time multiplexingl Basic rate ISDN offers to 64Kbs data bit pipes and one 16 Kbs signaling channel.HomePCISDNExchangeTelephoneNetworkPublicBranchExchangePhoneLANPage 3Peter A. Steenkiste, SCS, CMU5Digital Subscriber Linel Squeeze more bandwidth out of the telephone line using advanced signal processing.l Asymmetric digital subscriber line (ADSL).» More “download” bandwidth, e.g. video on demand or web surfing» Example: T1 incoming path, 64 Kbs outgoing pathl (Symmetric) digital subscriber line (DSL).» Same bandwidth both ways, e.g. 768 KbsHomePCADSLNetworkUnitTelephoneNetworkADSLSubscriptionUnitPhonePeter A. Steenkiste, SCS, CMU6Cable Modeml Use cable infrastructure for data service.» Inherently has more bandwidthl The last mile is a shared infrastructure that was designed for broadcasting.» Meaning: the bandwidth is shared by users» Example: 27 Mbs shared incoming path; 768 Kbs common outgoing pathFiberInfrastructurePage 4Peter A. Steenkiste, SCS, CMU7Comparisonl Modems use “worst case” technology.» Has to fit within any voice channel so encoding suboptimallISDN can be more aggressive.» But it is old technology by nowlDSL is highly optimized for the transmission medium.» But there are some constraints on distancelCablemodem uses a transmission medium that has inherently a higher bandwidth, but the network architecture will limit throughput.» Designed for broadcasting, not for point-point connectionslGo to web for example products/services.Peter A. Steenkiste, SCS, CMU8Packet Switchingl Source sends information as self-contained packets that have an address.» Source may have to break up single message in multiplelEach packet travels independently to the destination host.» Routers and switches use the address in the packet to determine how to forward the packetsl Destination recreates the message.l Analogy: a letter in surface mail.Page 5Peter A. Steenkiste, SCS, CMU9Circuit Switchingl Source first establishes a connection (circuit) to the destination.» Each router or switch along the way may reserve some bandwidth for the data flowl Source sends the data over the circuit.» No need to include the destination address with the data since the routers know the pathl The connection is torn down.l Example: telephone network.Peter A. Steenkiste, SCS, CMU10Circuit Switching Discussionl Traditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth.» Physical connection - clearly no need to include addresses with the datal Advantages, relative to packet switching:» Implies guaranteed bandwidth, predictable performance » Simple switch design: only remembers connection information, no destination address look uplDisadvantages:» Inefficient for bursty traffic (wastes bandwidth)» Delay associated with establishing a circuitlCan we get the advantages without (all) the disadvantages?Page 6Peter A. Steenkiste, SCS, CMU11Virtual Circuitsl Each wire carries many “virtual” circuits. » Forwarding based on virtual circuit (VC) identifier» A path through the network is determined for each VC when the VC is established» Use statistical multiplexing for efficiencyl Can support wide range of quality of service.» No guarantees: best effort service» Weak guarantees: delay < 300 msec, …» Strong guarantees: e.g. equivalent of physical circuitPeter A. Steenkiste, SCS, CMU12Packet Switching andVirtual Circuits: Similaritiesl “Store and forward” communication based on an address.» Address is either the destination address or a VC identifierl Must have buffer space to temporarily store packets.» E.g. multiple packets for some destination arrive simultaneouslyl Multiplexing on a link is similar to time sharing.» No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern» Reservations: some flows are guaranteed to get a certain number of “slots”AB ACBDPage 7Peter A. Steenkiste, SCS, CMU13Virtual Circuits Versus Packet Switching l Circuit switching:» Uses short connection identifiers to forward packets» Switches know about the connections so they can more easily implement features such as quality of service» Virtual circuits form basis for traffic engineering: VC identifies long-lived stream of data that can be scheduledlPacket switching:» Use full destination addresses for forwarding packets» Can send data right away: no need to establish a connection first» Switches are stateless: easier to recover from failures» Adding QoS is hard» Traffic engineering is hard: too many packets!Peter A. Steenkiste, SCS, CMU14ATM Historyl Telephone companies supported voice telephony: 4 kHz analog, 64 kbps digital.l They already provided lines for data networking.» ISDN: 64 + 64 + 16 kbps» T1 (1.544 Mbps)» T3 (44.736 Mbps)lThey wanted to become the primary service provider for data networking services.» file transfer: bursty, many Mbps peak» database access: bursty, low latency» Multimedia: synchronized » Video: 6 MHz analog, 1.2-200 Mbps digitallHow?Page 8Peter A. Steenkiste, SCS, CMU15One BISDN: STMl Synchronous Transfer Model Provide multirate frame structure:iH4 + jH3 + kH2 + lH1 + mH0 + nB + Dl Problems» complex channel assignment/subdivision» poor support for bursty
View Full Document