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Introduction to Computer Networks

Introduction to Computer Networks. September 9-11, 2003. Assignments. Due – Homework 0 I should have email from everyone Lab 1 Finish reading chapter 1 Read chapter 2 – 2.1-2.2 and 2.6-2.8 for next week. Network Structure. Network edge applications and hosts Network core routers

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Introduction to Computer Networks

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  1. Introduction to Computer Networks September 9-11, 2003

  2. Assignments • Due – Homework 0 • I should have email from everyone • Lab 1 • Finish reading chapter 1 • Read chapter 2 – 2.1-2.2 and 2.6-2.8 for next week

  3. Network Structure • Network edge • applications and hosts • Network core • routers • network of networks • Access networks, physical media • communication links

  4. End systems (hosts): run application programs e.g. Web, email at “edge of network” The Network Edge

  5. Client/server model client host requests, receives service from always-on server examples? Why such a popular model? The Network Edge

  6. Peer-to-Peer model minimal (or no) use of dedicated servers Gnutella, KaZaA SETI@home? The Network Edge

  7. Internet Service Models • Connection-oriented • Connectionless • Applications • FTP (SCP), Internet Phone, Web, Internet Radio, Email

  8. Connection-oriented Service • Hosts send control messages to establish connection (state) • Handshaking • Transmission Control Protocol (TCP) • reliable data transfer • flow control • congestion-control not all CO protocols provide these features • Why wouldn’t I want to use TCP?

  9. Connectionless Service • Just start sending • User Datagram Protocol (UDP) • no handshaking • no reliability • What’s it good for?

  10. How is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” The Network Core

  11. Circuit Switching • End-end resources reserved for “call” • link bandwidth • switch capacity • No sharing • Guaranteed performance • Call setup required

  12. Circuit Switching • Link bw divided into pieces • TDM vs FDM

  13. Example: 4 users FDMA frequency time TDMA frequency time FDMA and TDMA

  14. Packet Switching • Routers process packets • Resources used (requested) as needed • each pkt uses full bw • Store-and-forward • What if bandwidth not available?

  15. Queue = queuing delays No silent periods D E Statistical Multiplexing 10 Mbs Ethernet C A statistical multiplexing 1.5 Mbs B queue of packets waiting for output link

  16. Packet Switching v. Circuit Switching • 1 Mbit link • Each user • 100 kbps when “active” • active 10% of time • Circuit-switching • 10 users • Packet switching • with 35 users, probability > 10 active less than .0004 N users 1 Mbps link

  17. Circuit Switching Guaranteed behavior good for which apps? Packet Switching Good for bursty data simpler resource sharing More efficient and less costly to impl. Packet Switching v. Circuit Switching

  18. Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward delay = 3L/R Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec Store-and-Forward L R R R

  19. Message Segmentation

  20. Pipelining • Each packet 1,500 bits • 1 msec to transmit packet on one link • pipelining: each link works in parallel • Delay reduced from 15 sec to 5.002 sec http://wps.aw.com/aw_kurose_network_2/0,7240,227091-,00.html

  21. Forwarding • Move packets through routers from source to destination • Datagram network • destination address in packet determines next hop • routes may change during session • analogy: driving, asking directions • Virtual circuit network • fixed path determined at call setup time, remains fixed thru call • routers maintain per-call state

  22. Packet-switched networks Circuit-switched networks FDM TDM Datagram Networks Networks with VCs Network Taxonomy Telecommunication networks • Datagram network is not either connection-oriented • or connectionless. • Internet provides both connection-oriented (TCP) and • connectionless services (UDP) to apps.

  23. Assignments • Due – Homework 1 • Read chapter 2 – 2.1-2.2 and 2.6-2.8 for next week

  24. Access Networks and Physical Media • Last hop connection to edge router • residential access • institutional access networks (school, company) • mobile access networks

  25. Residential Access: Point to Point Access • Dialup via modem • up to 56Kbps direct access to router (often less) • Can’t surf and phone at same time: can’t be “always on” • ADSL: asymmetric digital subscriber line • up to 1 Mbps upstream (today typically < 256 kbps) • up to 8 Mbps downstream (today typically < 1 Mbps) Why asymmetric? Why faster than modem? • FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone

  26. Residential Access: Cable Modems • HFC: hybrid fiber coax • asymmetric: up to 10Mbps upstream, 1 Mbps downstream • Network of cable and fiber attaches homes to ISP router • shared access to router among home • issues: congestion, dimensioning

  27. Cable Network Architecture Typically 500 to 5,000 homes cable headend home cable distribution network (simplified)

  28. Cable Network Architecture cable headend home cable distribution network (simplified)

  29. Company Access: Local Area Networks • Company/Univ local area network (LAN) Ethernet: • shared link connects end system and router • 10 Mbs, 100Mbps, Gigabit Ethernet

  30. router base station mobile hosts Wireless Access Networks • Shared wireless net connects end system to router • via base station aka “access point” • Wireless LANs: • 802.11b (WiFi): 11 Mbps • 3G – Wide area • telecom providers

  31. Home Networks • Typical home network components • ADSL or cable modem • router/firewall/NAT • Ethernet • wireless access point wireless laptops to/from cable headend cable modem router/ firewall wireless access point Ethernet (switched)

  32. Physical Media • Bit • propagates between transmitter/rcvr pairs • Physical link • what lies between transmitter & receiver • Guided media • signals propagate in solid media: copper, fiber, coax • Unguided media • signals propagate freely, e.g., radio

  33. Twisted Pair (TP) • Two insulated copper wires • Category 3: traditional phone wires, 10 Mbps Ethernet • Category 5 TP: 100Mbps Ethernet

  34. Coax • Two concentric copper conductors • High bit rates • Formerly used for Ethernet

  35. Fiber • Glass fiber carrying light pulses • each pulse a bit • High-speed operation • 5 Gps • Low error rate • repeaters spaced far apart;immune to electromagnetic noise • Expensive – lots in backbone but not in LANs

  36. No physical “wire” ubiquitous Environment effects reflection obstruction by objects interference Radio link types: terrestrial microwave up to 45 Mbps channels LAN (e.g., WaveLAN) 2Mbps, 11Mbps Wide-area (e.g., cellular) 3G: hundreds of kbps Satellite up to 50Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus LEOS Radio

  37. NAP Tier-1 providers also interconnect at public network access points (NAPs) Tier-1 providers interconnect (peer) privately Tier 1 ISPs • Internet is roughly hierarchical • At center: “tier-1” ISPs • UUNet, BBN/Genuity, Sprint, AT&T, national/international coverage • treat each other as equals Tier 1 ISP Tier 1 ISP Tier 1 ISP

  38. Sprint US Backbone Network

  39. “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 pays tier-1 ISP for connectivity to rest of Internet • tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier 2 ISPs Tier 1 ISP Tier 1 ISP Tier 1 ISP

  40. “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 Tier 3 ISPs Tier 1 ISP Tier 1 ISP Tier 1 ISP

  41. 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 Path of a Packet Tier 1 ISP Tier 1 ISP Tier 1 ISP

  42. Delay A B

  43. Processing check bit errors determine output link transmission A propagation B nodal processing queueing Delay • Queuing • time waiting at output link for transmission • depends on congestion level of router

  44. Transmission delay R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R Propagation delay d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s transmission A propagation B nodal processing queueing Delay

  45. 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

  46. Packet Loss • What happens when the queue is full? • Does that mean that my message never gets to the other side?

  47. Diagnostic tool http://www.traceroute.org/ 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. Traceroute 3 probes 3 probes 3 probes

  48. Layering • How can we organize the structure of the network? • Why is this important?

  49. 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

  50. A Layered Approach Counter-to-counter delivery of person+bags baggage-claim-to-baggage-claim delivery people transfer: loading gate to arrival gate runway-to-runway delivery of plane airplane routing from source to destination

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