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Aug 30, 2023 •453 likes •543 views

A closer look at network structure: network edge: applications and hosts network core: Network Overview routers network of networks access networks, physical media: communication links Introduction Introduction 1-1 1-2

  1. A closer look at network structure:  network edge: applications and hosts  network core: Network Overview ❍ routers ❍ network of networks  access networks, physical media: communication links Introduction Introduction 1-1 1-2 The network edge: The network edge:  client/server model ❍ client host requests, receives service from always-  end systems (hosts): on server ❍ e.g. Web browser/server; ❍ run application programs email client/server ❍ e.g. Web, email ❍ at “edge of network”  why such a popular model? Introduction Introduction 1-3 1-4 The network edge: Internet Services Models  Connection-oriented service  Connectionless service  peer-peer model: ❍ minimal (or no) use of  Applications dedicated servers ❍ e.g. Gnutella, KaZaA ❍ FTP, Internet Phone, Web, Internet radio, ❍ SETI@home? email Introduction 1-5 Introduction 1-6 1

  2. Connection-oriented service Connectionless service Goal: data transfer between end systems  Goal: data transfer between end systems  handshaking: setup (prepare for) data transfer ahead of ❍ same as before! time  UDP - User Datagram Protocol [RFC 768]:  TCP - Transmission Control Protocol ❍ Internet’s connection-oriented service ❍ connectionless ❍ reliable, in-order byte-stream data transfer • loss: acknowledgements and retransmissions ❍ unreliable data transfer ❍ flow control: ❍ no flow control • sender won’t overwhelm receiver ❍ congestion control: ❍ no congestion control • senders “slow down sending rate” when network congested  What’s it good for? Introduction Introduction 1-7 1-8 The Network Core A Comparison App’s using TCP:  mesh of interconnected routers  HTTP (Web), FTP (file transfer), Telnet  the fundamental (remote login), SMTP (email) question: how is data transferred through net? ❍ circuit switching: App’s using UDP: dedicated circuit per  streaming media, teleconferencing, DNS, call: telephone net Internet telephony ❍ packet-switching: data sent thru net in discrete “chunks” Introduction Introduction 1-9 1-10 Circuit Switching: FDM and TDM Network Core: Circuit Switching Example: End-end resources FDM 4 users reserved for “call”  link bandwidth, switch frequency capacity  dedicated resources: no time sharing TDM  circuit-like (guaranteed) performance  call setup required frequency  must divide link bw into pieces... time Introduction 1-11 Introduction 1-12 2

  3. Network Core: Packet Switching Packet Switching: Statistical Multiplexing 10 Mb/s C each end-end data stream divided into packets A Ethernet statistical multiplexing  user A, B packets share network resources  each packet uses full link bandwidth 1.5 Mb/s B  resources used as needed queue of packets resource contention: waiting for output  aggregate resource demand can exceed amount available link ❍ what happens if bandwidth is not available? D  congestion: packets queue, wait for link use E  store and forward: packets move one hop at a time Sequence of A & B packets does not have fixed ❍ Node receives complete packet before forwarding pattern  statistical multiplexing . Introduction Introduction 1-13 1-14 Packet switching versus circuit switching Packet switching versus circuit switching Is packet switching a “slam dunk winner?” Packet switching allows more users to use network!  1 Mb/s link  Great for bursty data  each user: ❍ resource sharing ❍ 100 kb/s when “active” ❍ simpler, no call setup ❍ active 10% of time  Excessive congestion: packet delay and loss N users ❍ protocols needed for reliable data transfer, 1 Mbps link  circuit-switching: congestion control ❍ 10 users  Circuit Switching = Guaranteed behavior  packet switching: ❍ good for which apps? ❍ with 35 users, probability > 10 active Introduction Introduction less than .0004 1-15 1-16 Packet-switching: store-and-forward Packet-switched networks: forwarding L  Goal: move packets through routers from source to destination R R R ❍ we’ll study several path selection (i.e. routing) algorithms (chapter 4)  Takes L/R seconds to Example:  datagram network: transmit (push out)  L = 7.5 Mbits ❍ destination address in packet determines next hop packet of L bits on to ❍ routes may change during session  R = 1.5 Mbps link or R bps ❍ analogy: driving, asking directions  delay = 15 sec  virtual circuit network:  Entire packet must ❍ each packet carries tag (virtual circuit ID), tag determines next arrive at router hop before it can be ❍ fixed path determined at call setup time , remains fixed thru call transmitted on next ❍ routers maintain per-call state link: store and forward  delay = 3L/R Introduction 1-17 Introduction 1-18 3

  4. Network Taxonomy Access networks and physical media Telecommunication Q: How to connect end systems networks to edge router?  residential access nets Circuit-switched Packet-switched  institutional access networks networks networks (school, company)  mobile access networks Networks Datagram Keep in mind: FDM TDM with VCs Networks  bandwidth (bits per second) of access network? • Datagram network is not either connection-oriented  shared or dedicated? or connectionless. • Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps. Introduction Introduction 1-19 1-20 Residential access: point to point access Residential access: cable modems  Dialup via modem  HFC: hybrid fiber coax ❍ up to 56Kbps direct access ❍ asymmetric: up to 30Mbps downstream, 2 to router (often less) Mbps upstream ❍ Can’t surf and phone at same  network of cable and fiber attaches homes to time: can’t be “always on” ISP router  ADSL: asymmetric digital subscriber line ❍ homes share access to router ❍ up to 1 Mbps upstream (today typically < 256 kbps)  deployment: available via cable TV companies ❍ 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 Introduction Introduction 1-21 1-22 Cable Network Architecture: Overview Cable Network Architecture: Overview Typically 500 to 5,000 homes cable headend cable headend home home cable distribution cable distribution network (simplified) network (simplified) Introduction 1-23 Introduction 1-24 4

  5. Company access: local area networks Wireless access networks  shared wireless access  company/univ local area network connects end system network (LAN) connects to router router end system to edge router ❍ via base station aka “access  Ethernet: base point” ❍ shared or dedicated station  wireless LANs: link connects end ❍ 802.11b (WiFi): 11 Mbps system and router  wider-area wireless access ❍ 10 Mbs, 100Mbps, mobile ❍ provided by telco operator Gigabit Ethernet hosts ❍ 3G ~ 384 kbps  LANs: chapter 5 • Will it happen?? ❍ WAP/GPRS in Europe Introduction Introduction 1-25 1-26 Home networks Physical Media Typical home network components: Twisted Pair (TP)  Bit: propagates between  ADSL or cable modem  two insulated copper transmitter/rcvr pairs wires  router/firewall/NAT  physical link: what lies ❍ Category 3: traditional  Ethernet between transmitter & phone wires, 10 Mbps  wireless access receiver Ethernet point ❍ Category 5:  guided media: wireless 100Mbps Ethernet to/from laptops ❍ signals propagate in solid cable router/ cable media: copper, fiber, coax modem firewall headend wireless  unguided media: access Ethernet ❍ signals propagate freely, point e.g., radio Introduction Introduction 1-27 1-28 Physical Media: coax, fiber Physical media: radio Fiber optic cable: Radio link types: Coaxial cable:  signal carried in  glass fiber carrying light electromagnetic spectrum  terrestrial microwave  two concentric copper pulses, each pulse a bit  no physical “wire” ❍ e.g. up to 45 Mbps channels conductors  high-speed operation:  LAN (e.g., Wifi)  bidirectional  bidirectional ❍ high-speed point-to-point ❍ 2Mbps, 11Mbps  propagation environment  baseband: transmission (e.g., 5 Gps) effects:  wide-area (e.g., cellular) ❍ single channel on cable  low error rate: repeaters ❍ reflection ❍ e.g. 3G: hundreds of kbps ❍ legacy Ethernet spaced far apart ; immune to ❍ obstruction by objects  satellite  broadband: electromagnetic noise ❍ interference ❍ up to 50Mbps channel (or ❍ multiple channel on cable multiple smaller channels) ❍ HFC ❍ 270 msec end-end delay ❍ geosynchronous versus low altitude Introduction 1-29 Introduction 1-30 5

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