1. Introduction1-1 Chapter 1 Computer Networks and the Internet Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.  This slide show has been modified by Merrie Bergmann 1/26/03 Thanks and enjoy! JFK/KWR All material copyright 1996-2002 J.F Kurose and K.W. Ross, All Rights Reserved

2. Introduction1-2 A closer look at network structure:  network edge: applications and hosts ----------------------------  network core: m routers m network of networks  access networks, physical media: communication links

3. Introduction1-3 The Network Core  mesh of interconnected routers  the fundamental question: how is data transferred through net? m circuit switching: dedicated circuit per call: telephone net ------------------------- m packet-switching: data sent thru net in discrete “chunks”

4. Introduction1-4 Network Core: Packet Switching each end-end data stream divided into packets  Packets from different users share network resources  each packet uses full link bandwidth  resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation

5. Introduction1-5 Packet Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern: statistical multiplexing. (Compare: In TDM each host gets same slot in revolving TDM frame.) A B C 10 Mbs Ethernet 1.5 Mbs D E statistical multiplexing queue of packets waiting for output link

6. Introduction1-6 Packet switching versus circuit switching  1 Mbps link  each user: m Constant date rate of 100 Kbps when “active” m active 10% of time  circuit-switching: m 10 users (1 Mbps / 100 Kbps)  packet switching: m with 35 users, probability > 10 active less than.0004 Packet switching allows more users to use network! Example: N users 1 Mbps link

7. Introduction1-7 Network Core: Packet Switching each end-end data stream divided into packets  Packets from different users share network resources  each packet uses full link bandwidth  resources used as needed Delays and loss:  store and forward: packets move one hop at a time m transmit over link m wait turn at next link  congestion: packets queue, wait for link use  If router output buffer is full, packets can get lost

8. Introduction1-8 Packet-switching: store-and-forward  Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps  Entire packet must arrive at router before it can be transmitted on next link: store and forward  delay = #links x L/R Example:  L = 7.5 Mbits  R = 1.5 Mbps  #links = 3  L/R = 5  delay = 15 sec R R R L

9. Introduction1-9 Network Core: Packet Switching each end-end data stream divided into packets  Packets from different users share network resources  each packet uses full link bandwidth  resources used as needed Delays and loss:  store and forward: packets move one hop at a time m transmit over link m wait turn at next link  congestion: packets queue, wait for link use  If router output buffer is full, packets can get lost

10. Introduction1-10 Packet switching versus circuit switching  Great for bursty data m resource sharing m simpler, no call setup  Excessive congestion: packet delay and loss m protocols needed for reliable data transfer, congestion control  Unsolved problem (chapter 6): How to provide circuit-like behavior? m bandwidth guarantees needed for audio/video apps  Qu: Is packet-switching appropriate for telephone networks? Is packet switching a “slam dunk winner?”

11. Introduction1-11 Packet Switching: Message Segmenting We saw the store-and-forward delay for one 7.5 Mb packet on a 1.5 Mbps link. Now break it into 5000 packets.  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

12. Introduction1-12 Advantages of message segmentation over message switching  Pipelining reduces time delay  Errors in transmission: need to discard and retransmit a much smaller chunk of data  However, more packet overhead: control information must be repeated in 5,000 packets rather than appear once, in a single message  Demonstration applet: http://www.aw.com/kurose-ross

13. Introduction1-13 Packet-switched networks: packet forwarding  Goal: move packets through routers from source to destination m we’ll study several path selection (i.e. routing)algorithms (chapter 4)  virtual circuit network (e.g. ATM ): m each packet carries tag (virtual circuit ID), tag determines next hop m fixed path determined at call setup time, remains fixed thru call m routers maintain state information for ongoing connections  datagram network (e.g. the Internet): m destination address in packet determines next hop m routes may change during session

14. Introduction1-14 Incoming Interface Incoming VC #Outgoing interface Outgoing VC # 112222 138319 Virtual Circuit Networks Portion of VC-number translation table:

15. Introduction1-15 Packet-switched networks: forwarding  Goal: move packets through routers from source to destination m we’ll study several path selection (i.e. routing)algorithms (chapter 4)  virtual circuit network (e.g. ATM ): m each packet carries tag (virtual circuit ID), tag determines next hop m fixed path determined at call setup time, remains fixed thru call m routers maintain state information for ongoing connections  datagram network (e.g. the Internet): m destination address in packet determines next hop m routes may change during session

16. Introduction1-16 Datagram Networks  Destination address has a heirarchical structure, like postal addresses  Packet switches (routers) examine a portion of the address to determine which next switch to sent the packet to  Like the postal system, or you and I travelling to visit a friend in a distant city  No connection-state information needed in the switches  Demo: http://www.traceroute.org/http://www.traceroute.org/

17. Introduction1-17 Network Taxonomy Telecommunication networks Circuit-switched networks FDM TDM Packet-switched networks Networks with VCs Datagram Networks Datagram network is not either connection-oriented or connectionless. Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.

18. Introduction1-18 A closer look at network structure:  network edge: applications and hosts  network core: m routers m network of networks ------------------------  access networks, physical media: communication links

19. Introduction1-19 Access networks Q: How are end systems connected to edge router?  residential access nets  institutional access networks (school, company)  mobile access networks

20. Introduction1-20 Residential access: point to point access  Dialup via modem m up to 56Kbps direct access to router (often less) m Can’t surf and phone at same time: can’t be “always on”  ADSL: asymmetric digital subscriber line m up to 1 Mbps upstream (today typically < 256 kbps) m up to 8 Mbps downstream (today typically < 1.5 Mbps) m FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone  Both use twisted-pair copper

21. Introduction1-21 Residential access: cable modems  HFC: hybrid fiber coax (coax = coaxial cable) m network of cable and fiber attaches homes to ISP router m Asymmetric: more bandwidth and therefore faster downstream than upstream m shared access to router among home m issue: congestion downstream, collisions upstream  deployment: available via cable companies, e.g., MediaOne

22. Introduction1-22

23. Introduction1-23 DSL and HFC  Both are always on, as compared to dial-up modem connections  Do not tie up the telephone line  DSL and HFC both have higher transmission rates than dial-up connections DSL v. HFC  DSL is point-to-point between home and ISP: dedicated bandwidth  HFC: with “reasonable dimensions” gives higher bandwidth

24. Introduction1-24 Company access: local area networks  company/univ local area network (LAN) connects end system to edge router  Ethernet (most prevalent LAN technology): m shared or dedicated link connects end system and router m 10 Mbs, 100Mbps, 1 or 10 Gbps Ethernet m Twisted copper or coax  LANs and Ethernet: chapter 5

25. Introduction1-25 Wireless (mobile) access networks  shared wireless access network connects end system to router m via base station aka “access point”  wireless LANs  wider-area wireless access m Managed by telecommunications providers base station mobile hosts router

26. Introduction1-26 Physical Media  physical link: what lies between transmitter & receiver  guided media: m signals propagate in solid media: copper, fiber, coax  unguided media: m signals propagate freely, e.g., radio signals Twisted Pair (TP)  two insulated copper wires m Category 3 – voice- grade twisted pair: traditional phone wires, 10 Mbps Ethernet m Category 5 TP – more twists and Teflon™ insulation: 100Mbps Ethernet m Used by almost all new Ethernet installations

27. Introduction1-27 Physical Media: coax, fiber Coaxial cable:  two concentric copper conductors  Bidirectional  1 –km cables: up to 2 Gbps, but slower for longer lengths unless amplifiers are used  baseband: m single channel on cable  broadband: m multiple channels on cable  Can be used as shared medium Fiber optic cable :  glass fiber carrying light pulses, each pulse a bit  high-speed operation: m high-speed point-to-point transmission (e.g., 5 Gbps and more)  low error rate: repeaters spaced far apart; immune to electromagnetic noise  Hard to tap  High cost hinders use for short-distance communication

28. Introduction1-28 Physical media: radio links  signal carried in electromagnetic spectrum  no physical “wire”  bidirectional  propagation environment effects: m obstruction by objects m interference Radio link types:  Terrestrial  Satellite m Bandwidths in the Gbps range

29. Introduction1-29 Internet structure: the internet is a network of networks; loosely heirarchical

30. Introduction1-30 Internet structure: network of networks  at center: “tier-1” ISPs or Internet backbone networks (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage m treat each other as equals m Extremely fast (e.g. 622+ Mbps links) Tier 1 ISP Tier-1 providers interconnect (peer) privately NAP Tier-1 providers also interconnect at public network access points (NAPs)

31. Introduction1-31 Tier-1 ISP: e.g., Sprint Sprint US backbone network

32. Introduction1-32 Internet structure: network of networks  “Tier-2” ISPs: smaller (often regional) ISPs m Connect to one or a few tier-1 ISPs, possibly other tier-2 ISPs Tier 1 ISP NAP Tier-2 ISP Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet  tier-2 ISP is customer of tier-1 provider Tier-2 ISPs also peer privately with each other, interconnect at NAP

33. Introduction1-33 Internet structure: network of networks  “Tier-3” ISPs and local ISPs m last hop (“access”) network (closest to end systems) Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet

34. Introduction1-34 ISP terminology  Peers: two ISPs that are directly connected with each other  Point of Presence (POP): point within an ISP network at which it connects to other ISPs  NAP (network access point): connects ISPs but is owned by a third party

35. Introduction1-35 Internet structure: network of networks  a packet passes through many networks! "Choosing an ISP" Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP

36. Introduction1-36 Seniors, take note! “… anyone of us can become an access ISP as soon as we have an internet connection. All we need to do is purchase the necessary equipment (for example, router and modem pool) to allow other users to connect to us.” (Kurose-Ross, p. 41)