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CS 372 – introduction to computer networks* Lecture 3: Wednesday June 23

Announcements: Assignment 1 is posted online and is due next Tuesday Quiz on next Tuesday Lab 1 is posted and is due next Monday No late lab and assignment will be accepted!. CS 372 – introduction to computer networks* Lecture 3: Wednesday June 23.

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CS 372 – introduction to computer networks* Lecture 3: Wednesday June 23

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  1. Announcements: Assignment 1 is posted online and is due next Tuesday Quiz on next Tuesday Lab 1 is posted and is due next Monday No late lab and assignment will be accepted! CS 372 – introduction to computer networks*Lecture 3: Wednesday June 23 * Based in part on slides by Bechir Hamdaoui, Paul D. Paulson, and Dina Katabi. Chapter 1, slide: Acknowledgement: slides drawn heavily from Kurose & Ross

  2. The network core: Packet switching • Data transmitted in small, independent pieces • Source divides outgoing messages into packets • Destination recovers original data • Each packet travels independently • Includes enough information for delivery • May follow different paths • Can be retransmitted if lost Chapter 1, slide:

  3. The network core:Functions of packet-switching networks • Packet construction • encode/package data at source • Packet transmission • send packet from source to destination • Packet interpretation • unpack/decode data from packet at destination • acknowledge receipt Chapter 1, slide:

  4. The network core: other functions • Route discovery • Traffic/congestion control • Retransmitting lost packets • Determining type of data • messages • service requests/responses • files • audio/video • etc. • etc. Chapter 1, slide:

  5. Packet switching: Reordering and different path Host C Host D Host A Node 1 Node 2 Node 3 Node 5 Host B Host E Node 7 Node 6 Node 4 Chapter 1, slide:

  6. Chapter 1: roadmap 1 What is the Internet? 2 Network edge 3 Network core 4 Network access and physical media 5 Internet structure and ISPs 6 Protocol layers, service models 7 Delay & loss in packet-switched networks Chapter 1, slide:

  7. Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile/wireless access networks Access networks and physical media Chapter 1, slide:

  8. why is it needed? to propagate bits between sender/receiver pairs what is it? a physical link that lies between sender & receiver two types of media: guided media: signals propagate in solid media unguided media: signals propagate freely, e.g., wireless radio Physical Media Chapter 1, slide:

  9. Dialup via modem regular twisted-pair copper phone lines up to 56Kbps direct access to router (often less) rate depends on thickness and distance may pick up interference (“noise”) can’t surf and phone at same time: can’t be “always on” Residential access: point to point access Chapter 1, slide:

  10. Residential access: point to point access • ADSL: asymmetric digital subscriber line • regular phone lines • transmission rates depend on length • point-to-point medium (dedicated) • up to 1 Mbps upstream (today typically < 256 kbps) • up to 8 Mbps downstream (today typically < 1 Mbps) • FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone Chapter 1, slide:

  11. two concentric copper conductors baseband: single channel on cable legacy Ethernet broadband: multiple channels on cable hybrid fiber-coax cable (HFC) Cable TV rate depends on thickness and distance less interference than twisted pair Guided Media: coaxial cable Chapter 1, slide:

  12. HFC: hybrid fiber coax asymmetric up to 30Mbps downstream up to 2 Mbps upstream network of cable and fiber attaches homes to ISP router Shared medium deployment: available via cable TV companies Residential access: cable modems Chapter 1, slide:

  13. Cable Network Architecture: Overview Typically 500 to 5,000 homes cable headend home cable distribution network (simplified) Chapter 1, slide:

  14. server(s) Cable Network Architecture: Overview cable headend home cable distribution network Chapter 1, slide:

  15. Cable Network Architecture: Overview cable headend home cable distribution network (simplified) Chapter 1, slide:

  16. C O N T R O L D A T A D A T A V I D E O V I D E O V I D E O V I D E O V I D E O V I D E O 5 6 7 8 9 1 2 3 4 Channels Cable Network Architecture: Overview FDM: cable headend home cable distribution network Chapter 1, slide:

  17. local area networks (LAN), more in chapter 5 connect end system to edge router E.g., universities, companies Example: Ethernet: shared or dedicated link connects end system to router 10 Mbs, 100Mbps, Gigabit Ethernet Company access: local area networks Chapter 1, slide:

  18. wireless access network connects end system to router via base station or “access point” Examples: wireless LANs: 802.11b/g (WiFi): 11 or 54 Mbps wider-area wireless access provided by telcomm operator 3G ~ 384 kbps GPRS in Europe/US router base station mobile hosts Wireless access networks Chapter 1, slide:

  19. Chapter 1: roadmap 1 What is the Internet? 2 Network edge 3 Network core 4 Network access and physical media 5 Internet structure and ISPs 6 Protocol layers, service models 7 Delay & loss in packet-switched networks Chapter 1, slide:

  20. roughly hierarchical: tier 1, tier 2, and tier 3 at center: “tier-1” ISPs e.g., MCI, Sprint, AT&T, Cable and Wireless, national/international coverage NAP Tier-1 providers also interconnect at public network access points (NAPs) Tier-1 providers interconnect (peer) privately Internet structure: network of networks Tier 1 ISP Tier 1 ISP Tier 1 ISP Chapter 1, slide:

  21. Seattle DS3 (45 Mbps) OC3 (155 Mbps) OC12 (622 Mbps) OC48 (2.4 Gbps) Tacoma New York Stockton Cheyenne Chicago Pennsauken Relay Wash. DC San Jose Roachdale Kansas City Anaheim Atlanta Fort Worth Orlando Tier-1 ISP: e.g., Sprint Sprint US backbone network Chapter 1, slide:

  22. “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs NAP Tier-2 ISPs also peer privately with each other, interconnect at NAP Tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Internet structure: network of networks Tier 1 ISP Tier 1 ISP Tier 1 ISP Chapter 1, slide:

  23. “Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems) Tier 3 ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP NAP Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Internet structure: network of networks Tier 1 ISP Tier 1 ISP Tier 1 ISP Chapter 1, slide:

  24. a packet passes through many networks! Tier 3 ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP NAP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Internet structure: network of networks Tier 1 ISP Tier 1 ISP Tier 1 ISP Chapter 1, slide:

  25. Chapter 1: roadmap 1 What is the Internet? 2 Network edge 3 Network core 4 Network access and physical media 5 Internet structure and ISPs 6 Protocol layers, service models 7 Delay & loss in packet-switched networks Chapter 1, slide:

  26. Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Protocol “Layers” Chapter 1, slide:

  27. ticket (complain) baggage (claim) gates (unload) runway landing airplane routing ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing airplane routing Organization of air travel • a series of steps Chapter 1, slide:

  28. ticket ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing ticket (complain) baggage (claim gates (unload) runway (land) airplane routing baggage gate airplane routing airplane routing takeoff/landing airplane routing departure airport intermediate air-traffic control centers arrival airport Layering of airline functionality Layers: each layer implements a service • via its own internal-layer actions • relying on services provided by layer below Chapter 1, slide:

  29. Why layering? Dealing with complex systems: • Easing assignment of tasks • identify relationship among pieces of complex systems • Easing maintenance, updating of system • change of implementation of layer’s service transparent to rest of system • e.g., change in gate procedure doesn’t affect rest of system Chapter 1, slide:

  30. application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” application transport network link physical Internet protocol stack Chapter 1, slide:

  31. network link physical link physical M M M Ht M Hn Hn Hn Hn Ht Ht Ht Ht M M M M Ht Ht Hn Hl Hl Hl Hn Hn Hn Ht Ht Ht M M M source Encapsulation message application transport network link physical segment datagram frame switch destination application transport network link physical router Chapter 1, slide:

  32. application transport network link physical ISO/OSI Model: late 70’s application presentation session transport network data link physical 5-layer Internet Protocol Stack 7-layer ISO/OSI model (OSI: open system interconnections) Chapter 1, slide:

  33. Chapter 1: roadmap 1 What is the Internet? 2 Network edge 3 Network core 4 Network access and physical media 5 Internet structure and ISPs 6 Protocol layers, service models 7 Delay & loss in packet-switched networks Chapter 1, slide:

  34. End-to-end delay (nodal delay) : Total time from initiating “send” (from source) to completed “receive” (at destination) Throughput : Rate (bits/sec) at which bits are actually being transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time Network performance metrics Chapter 1, slide:

  35. 1. nodal processing: check bit errors determine output link Sources of packet delay • 2. queueing • time waiting at output link for transmission • depends on congestion level of router A B nodal processing queueing Chapter 1, slide:

  36. 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) trans. delay = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s Sources of packet delay Note: s and R are very different quantities! transmission A propagation B nodal processing queueing Chapter 1, slide:

  37. packet being transmitted (delay) packets queueing (delay) packets get dropped (loss) if no free buffers How do loss and delay occur? A B Chapter 1, slide:

  38. Packet loss • queue (buffer) preceding link in buffer has finite capacity • packet arriving at a full queue is dropped (lost) • lost packet may be retransmitted by previous node, by source, or not at all buffer (waiting area) packet being transmitted A B packet arriving to full bufferis lost Chapter 1, slide:

  39. Cars run at 100 km/hr (speed of propagation) Booth takes 12 sec to service a car (transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? Time to “push” entire caravan through toll booth = 12*10 = 120 sec = 2 mns Time for last car to propagate from 1st to 2nd toll both: =100km/(100km/hr)= 1 hr A: 1 hr 2 minutes toll booth toll booth Caravan analogy 100 km 100 km ten-car caravan Chapter 1, slide:

  40. 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? 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! toll booth toll booth Caravan analogy (more) 100 km 100 km ten-car caravan Chapter 1, slide:

  41. Example Packet length = L bits trans. rate R = 1 Mbps Host A Host B distance = 1 km, speed= 2x108m/s • Question: • Which bit is being transmitted at the time the first bit arrives at Host B for • Answer: • First bit arrives after • 1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 = 6 µsec • After 6 µsec • 6 bits are already transmitted; so 7th bit is being transmitted Chapter 1, slide:

  42. 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 Chapter 1, slide:

  43. Every second: aL bits arrive to queue Every second: R bits leave the router Question: what happens if aL > R ? Answer: queue will fill up, and packets will get dropped!! queue Packet arrival rate = a packets/sec Link bandwidth = R bits/sec Packet length = L bits Queueing delay (revisited) aL/Ris called traffic intensity Chapter 1, slide:

  44. queue Packet arrival rate = a packets/sec Link bandwidth = R bits/sec Packet length = L bits Queueing delay (revisited) • La/R ~ 0: avg. queueing delay small • La/R -> 1: delays become large • La/R > 1: more “work” than can be serviced, average delay infinite! Chapter 1, slide:

  45. Exercise 1 Transmission vs. propagation L=100Bytes trans. rate R = ? Host A Host B distance = 2 km, speed= 2x108m/s • Question: • At what rate (bandwidth) R would the propagation delay equal the transmission delay? • Answer: • Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec • Transmission delay = 100x8 (bits)/R • Prop. Delay = trans. Delay => R=105x100x8 = 80 Mbps Chapter 1, slide:

  46. Exercise 2 Voice over IP L=48 Bytes trans. rate R = 1Mbps Host A a=64Kbps Host B delay_prop = 2msec • Host A • converts analog to digital at a=64Kbps • groups bits into L=48Byte packets • sends packet to Host B as soon it gathers a packet • Host B • As soon as it receives the whole pckt, it converts it to analog • Question: • How much time elapses from the 1st bit is created until the last bit arrives at Host B? Chapter 1, slide:

  47. Exercise 2 Voice over IP L=48 Bytes trans. rate R = 1Mbps Host A a=64Kbps Host B delay_prop = 2msec • Answer: • Time to gather 1st pkt: 48x8 (bits)/64x1000 (b/s) = 6 msec • Time to push 1st pkt to link: 48x8 (bits)/1x106 (b/s) = 0.384 msec • Time to propagate: 2 msec • Total delay = 6 + 0.384 + 2 = 8.384 msec Chapter 1, slide:

  48. Covered a “ton” of material! Internet overview what’s a protocol? network edge, core, access network packet-switching versus circuit-switching Internet/ISP structure performance: loss, delay layering and service models You now have: context, overview, “feel” of networking more depth, detail to follow! Introduction: Summary Chapter 1, slide:

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