Last Lecture. Nuts-and-bolts description of the Internet. ! The topology. ! The communication links. ! The core. ! The edge
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1 Last Lecture Nuts-and-bolts description of the Internet! The topology! The core! The edge! The communication links SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 1
2 This Lecture! How to send data from end to end: two switching methods! Circuit switching! Packet switching! Packet loss and delay in a packet switched network SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 2
3 How is data transferred through a network? Two switching methods: 1. Circuit Switching: dedicated physical circuit is established, maintained, and terminated over a communication session (e.g. ISDN) 2. Packet Switching: data are transferred in packets (chunks of data of a fixed size), possibly go through different paths to reach the destination (e.g. ATM, X.25, Frame Relay, Internet) SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 3
4 1. Circuit Switching Three step process o Source establishes connection to destination " Find path " Reserve resources o Data exchanged (no need for destination address) o Connection torn down " Resources released SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 4
5 Sharing a Link: Multiplexing! To combine multiple signals (analog or digital) for transmission over a single line or medium.! Multiplexing technologies:! Frequency Division Multiplexing (FDM) : each signal is assigned a different frequency range (e.g. FM radio).! Time Division Multiplexing (TDM) : each signal is assigned a fixed time slot in a fixed rotation.! Statistical Time Division Multiplexing (STDM): time slots are assigned to signals dynamically to make better use of bandwidth.! Wavelength Division Multiplexing (WDM) : each signal is assigned a particular wavelength; used in optical fiber. SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 5
6 Circuit Switching: FDMA and TDMA FDMA Example: 4 users frequency TDMA time frequency time SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 6
7 2. Packet Switching SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 7
8 Packet Switching: Statistical Multiplexing Packets SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 8
9 Packet Switching vs. Circuit Switching: In Theory! Packet Switching! CS wastes bandwidth when data is sporadic! PS is statistically more efficient and less costly! CS takes time to establish the circuit! PS is simpler to implement! Side Question: what about packet sizes? Small or Large?! Circuit Switching! PS is not suitable for real time application! A sudden surge of traffic could overflow router s buffers! PS could deliver packets in wrong order! CS is transparent (carrier does not need to know packet format) SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 12
10 Common View of the Telco Network (CS) brain (smart) brick (dumb) lock (you can t get in) SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 13
11 Common View of the IP Network (PS) SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 14
12 PS vs CS in Practice Common assumptions about the Internet (That you found in many textbooks and research papers)! IP dominates global communications! Packet switching is more efficient than circuit switching! Packet switching is robust! IP (and PS) is simpler! Quality of Service (QoS) can be realized over IP SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 15
13 IP Dominates Global Communications? NO! [US-census 2002] Revenues: Satellite Telecom (5.7B), ISPs (18.7B), Radio/TV broadcast (48.5B), Cable Distribution (77.7B), Cellular & other wireless Telecom (96.5B), Wired telecom-carriers (237.6B).! [Nielsen/NetRatings survey 2004 & others] Percentage of US households having access: Internet (75%), Cable/Pay TV (78%), TV (98%)! [RHK Industry Reports 2002] Public Telecom Infrastructure Expenditures: Core routers (1.7B), Edge routers (2.4B), SONET/SDH/WDM (28.0B), Telecom Multi-Service Switches (4.5B) SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 16
14 PS is more efficient than CS? Yes, but! More efficient means better utilized (both in transmission lines and switching equipments)! True for networks with scarce bandwidths! However, does it really matter today?! Average utilization levels! ATT switched voice (33%), Internet backbones (15%)! Private lines networks (3-5%), LANs (1%)! Various Reasons! Internet traffic is asymmetric and bursty, links are symmetric! Operators tend to over-provision because PS networks behave very badly once congested (oscillation, routing loops, black holes, disconnections, etc)! Over-provision to ensure low delay (satisfy customers), it s more economical to add capacity in large increments SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 17
15 PS is more robust than CS? Not necessarily! Downtime per year:! Internet: 471min [Labovitz et al. 2000]! Phone networks: 5min [Kuhn 1997]! Recover time! Internet: median 3min, frequently > 15min (due to slow BGP convergence time)! SONET/SDH rings: < 50ms (via pre-computed backup paths)! Routing in the Internet! Routing info affected by user traffic, suffering from congestion (in-band routing)! Routing computation complex # overload processors! Probability of mis-configuring a router is high, one router s error affect the whole network SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 18
16 ! Number of lines of codes in IP (and PS) is simpler?! Typical Tel. Switches: 3 millions, extremely complex switch: 16M! Cisco s IOS: 8 millions [more susceptible to attacks]! Routers crash frequently, takes long time to reboot! Hardware! A line card of a router: OC192 POS has 30M gates + 1 CPU + 300MB packet buffers + 2MB forwarding table + 10MB other state memory! Current trend makes routers more complex (multicast, QoS, access control, security, VPN, etc) violation of E2E! A line card of a typical transport switch:! number of gates, no CPU, no forwarding table, one on-chip state memory! Density: highest transport switch capacity = 4 x highest router capacity, at 1/3 the price! WDM, DWDM push the difference further! IP s simplicity does not scale! SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 19
17 QoS can be realized over IP?! Belief: over-provisioning allows low e2e delay # guaranteeing QoS is possible! After > 10 years of research, IntServ and DiffServ are still not good enough.! Few financial incentive to provide QoS over IP! Watch out for VoIP, however.! On the other hand, current phone services are much better with very low price SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 20
18 Other measures! Scalability! CS scales more or less linearly! When data rates increase, routers can t keep up! Flexibility! IP is more flexible! Lead to high costs of end-systems! Need more sophisticated users [large organizations need a room of sys admin, just 1 phone operator] SUNY AT BUFFALO; CSE 489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 21
19 This Lecture! How to send data from end to end: two switching methods! Circuit switching! Packet switching! Packet loss and delay in a packet switched network SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 22
20 How do loss and delay occur? Packets queued in router buffers! packet arrival rate to link exceeds output link capacity! packets queued, wait for turn packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 23
21 ! 1. nodal processing:! check bit errors Four sources of packet delay! determine output link $ 2. queueing % time waiting at output link for transmission % depends on congestion level of router A transmission propagation B nodal processing queueing SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 24
22 Delay in packet-switched networks 3. Transmission delay:! R=link data-rate (bps)! L=packet length (bits)! time to send bits into link = L/R 4. Propagation delay:! d = length of physical link! s = propagation speed in medium (~2x10 8 m/sec)! propagation delay = d/s A transmission propagation Note: s and R are very different quantities! B nodal processing queueing SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 25
23 Caravan analogy 100 km 100 km ten-car caravan! cars propagate at 100 km/hr toll booth! toll booth takes 12 sec to service car (transmission time)! car~bit; caravan ~ packet! Q: How long until caravan is lined up before 2nd toll booth? toll booth! Time to push entire caravan through toll booth onto highway = 12*10 = 120 sec! Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/ hr)= 1 hr! A: 62 minutes SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 26
24 Caravan analogy (more) 100 km 100 km ten-car caravan toll booth! Cars now propagate at 1000 km/hr! Toll booth now takes 1 min to service a car! Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth? toll booth! Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.! 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!! See Ethernet applet at AWL Web site SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 27
25 Nodal delay! dproc = processing delay! typically a few microsecs or less! dqueue = queuing delay! depends on congestion! dtrans = transmission delay! = L/R, significant for low-speed links! dprop = propagation delay! a few microsecs to hundreds of msecs SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 28
26 Queueing delay (revisited)! R=link bandwidth (bps)! L=packet length (bits)! a=average packet arrival rate traffic intensity = La/R $ La/R ~ 0: average queueing delay small $ La/R -> 1: delays become large $ La/R > 1: more work arriving than can be serviced, average delay infinite! SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 29
27 Real Internet delays and routes! What do real Internet delay & loss look like?! Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i:! sends three packets that will reach router i on path towards destination! router i will return packets to sender! sender times interval between transmission and reply. 3 probes 3 probes 3 probes SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 30
28 Packet loss! queue (aka buffer) preceding link in buffer has finite capacity! packet arriving to full queue dropped (aka lost)! lost packet may be retransmitted by previous node, by source end system, or not at all A buffer (waiting area) packet being transmitted B packet arriving to full buffer is lost SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 31
29 Throughput! throughput: rate (bits/time unit) at which bits transferred between sender/receiver! instantaneous: rate at given point in time! average: rate over longer period of time server server, sends with bits (fluid) file of into F pipe bits to send to client pipe link that capacity R s bits/sec can carry fluid at rate R s bits/sec) pipe link capacity R c bits/sec that can carry fluid at rate R c bits/sec) SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 32
30 Throughput (more)! Rs < Rc What is average end-end throughput? R s bits/sec R c bits/sec $ R s > R c What is average end-end throughput? R s bits/sec R c bits/sec bottleneck link link on end-end path that constrains end-end throughput SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 33
31 Throughput: Internet scenario! per-connection endend throughput is R s R s R s R! in practice: R c or R s is often bottleneck R c R c R c 10 connections (fairly) share backbone bottleneck link R bits/sec SUNY AT BUFFALO; CSE489/589 MODERN NETWORKING CONCEPTS; Fall 2010; INSTRUCTOR: HUNG Q. NGO 34
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