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 1-1 Introduction 1-2 The network edge: The network edge: client/server model ❍ client host requests, receives service from always- end systems (hosts): on server ❍ run application programs ❍ e.g. Web browser/server; email client/server ❍ e.g. Web, email ❍ at “edge of network” why such a popular model? Introduction 1-3 Introduction 1-4 The network edge: Internet Services Models Connection-oriented service Connectionless service peer-peer model: ❍ minimal (or no) use of dedicated servers Applications ❍ e.g. Gnutella, KaZaA ❍ FTP, Internet Phone, Web, Internet radio, ❍ SETI@home? email Introduction 1-5 Introduction 1-6 1 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 time ❍ same as before! TCP - Transmission Control Protocol UDP - User Datagram Protocol [RFC 768]: ❍ Internet’s connection-oriented service ❍ ❍ reliable, in-order byte-stream data transfer connectionless • loss: acknowledgements and retransmissions ❍ unreliable data transfer ❍ flow control: • sender won’t overwhelm receiver ❍ no flow control ❍ congestion control: ❍ • senders “slow down sending rate” when network congested no congestion control What’s it good for? Introduction 1-7 Introduction 1-8 A Comparison The Network Core 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? App’s using UDP: ❍ circuit switching: dedicated circuit per streaming media, teleconferencing, DNS, call: telephone net Internet telephony ❍ packet-switching: data sent thru net in discrete “chunks” Introduction 1-9 Introduction 1-10 Network Core: Circuit Switching Circuit Switching: FDM and TDM Example: FDM End-end resources 4 users reserved for “call” link bandwidth, switch frequency capacity dedicated resources: no sharing time circuit-like (guaranteed) TDM performance call setup required frequency must divide link bw into pieces... time Introduction 1-11 Introduction 1-12 2 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 resources used as needed B queue of packets resource contention: waiting for output aggregate resource demand can exceed amount available link ❍ what happens if bandwidth is not available? congestion: packets queue, wait for link use D E store and forward: packets move one hop at a time ❍ Node receives complete packet before forwarding Sequence of A & B packets does not have fixed pattern statistical multiplexing. Introduction 1-13 Introduction 1-14 Packet switching versus circuit switching Packet switching versus circuit switching Packet switching allows more users to use network! Is packet switching a “slam dunk winner?” 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 N users Excessive congestion: packet delay and loss 1 Mbps link ❍ protocols needed for reliable data transfer, circuit-switching: congestion control ❍ 10 users Circuit Switching = Guaranteed behavior packet switching: ❍ good for which apps? ❍ with 35 users, probability > 10 active less than .0004 Introduction 1-15 Introduction 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 ❍ R = 1.5 Mbps routes may change during session link or R bps ❍ analogy: driving, asking directions delay = 15 sec Entire packet must virtual circuit network: arrive at router ❍ each packet carries tag (virtual circuit ID), tag determines next 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 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 FDM TDM Keep in mind: 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 1-19 Introduction 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 1-21 Introduction 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 Company access: local area networks Wireless access networks company/univ local area shared wireless access network (LAN) connects network connects end system end system to edge router to router router Ethernet: ❍ via base station aka “access point” base ❍ shared or dedicated station wireless LANs: link connects end ❍ 802.11b (WiFi): 11 Mbps system and router wider-area wireless access ❍ 10 Mbs, 100Mbps, ❍ provided by telco operator mobile Gigabit Ethernet ❍ 3G ~ 384 kbps hosts LANs: chapter 5 • Will it happen?? ❍ WAP/GPRS in Europe Introduction 1-25 Introduction 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 router/firewall/NAT wires physical link: what lies ❍ Category 3: traditional Ethernet between transmitter & phone wires, 10 Mbps wireless access receiver Ethernet ❍ Category 5: point wireless guided media: 100Mbps Ethernet to/from laptops cable router/ ❍ signals propagate in solid cable modem firewall media: copper, fiber, coax headend wireless unguided media: access Ethernet ❍ signals propagate freely, point e.g., radio Introduction 1-27 Introduction 1-28 Physical Media: coax, fiber Physical media: radio Fiber optic cable: Coaxial cable: signal carried in Radio link types: glass fiber carrying light electromagnetic spectrum terrestrial microwave two concentric copper pulses, each pulse a bit conductors no physical “wire” ❍ e.g. up to 45 Mbps channels high-speed operation: bidirectional bidirectional LAN (e.g., Wifi) ❍ 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 ❍ ❍ multiple channel on cable up to 50Mbps channel (or multiple smaller channels) ❍ HFC ❍ 270 msec end-end delay ❍ geosynchronous versus low altitude Introduction 1-29 Introduction 1-30 5 Internet structure: network of networks Tier-1 ISP: e.g., Sprint roughly hierarchical Sprint US backbone network at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage ❍ treat each other as equals Tier-1 providers also interconnect Tier-1 Tier 1 ISP at public network providers NAP access points interconnect (NAPs) (peer) privately Tier 1 ISP Tier 1 ISP Introduction 1-31 Introduction 1-32 Internet structure: network of networks Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs “Tier-3” ISPs and local ISPs ❍ Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs ❍ last hop (“access”) network (closest to end systems) local local ISP Tier 3 local local Tier-2 ISPs ISP ISP ISP ISP Local and tier- Tier-2 ISP pays Tier-2 ISP Tier-2 ISP also peer Tier-2 ISP Tier-2 ISP tier-1 ISP for privately with 3 ISPs are Tier 1 ISP Tier 1 ISP connectivity to each other, customers of NAP NAP rest of Internet interconnect higher tier tier-2 ISP is at NAP ISPs customer of connecting Tier 1 ISP Tier 1 ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP tier-1 provider them to rest local Tier-2 ISP Tier-2 ISP of Internet Tier-2 ISP Tier-2 ISP ISP local local local ISP Introduction 1-33 ISP ISP Introduction 1-34 Internet structure: network of networks