Introduction cont. Lecture goal: Overview: get context, overview, - PDF document

Introduction cont. Lecture goal: Overview: ¸ get context, overview, ¸ access net, physical media “feel” of networking ¸ performance: loss, delay ¸ more depth, detail later in course ¸ protocol layers, service models ¸ approach: ¸ backbones, NAPs, ISPs o descriptive o use Internet as example 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 1 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls ¸ resource piece idle if not used ¸ by owning call (no sharing) dividing link bandwidth into ¸ “pieces” frequency division o time division o 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 2 Network Core: Packet Switching each end-end data stream divided into resource contention: packets ¸ aggregate resource user A, B packets share network ¸ demand can exceed resources amount available each packet uses full link ¸ bandwidth ¸ congestion: packets resources used as needed , ¸ queue, wait for link use ¸ store and forward: packets move one hop at Bandwidth division into “pieces” a time Dedicated allocation o transmit over link Resource reservation o wait turn at next link 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 3 1

Network Core: Packet Switching 10 Mbs C A statistical multiplexing Ethernet 1.5 Mbs B queue of packets 45 Mbs waiting for output link D E Packet-switching versus circuit switching: human restaurant analogy 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 4 Network Core: Packet Switching Packet-switching: store and forward behavior Example ¸ 7.5 Mbit message ¸ 1.5 Mbps link transmission rate ¸ 5000 1.5 Kbit packets 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 5 Packet switching versus circuit switching Packet switching allows more users to use network! 1 Mbit link ¸ each user: ¸ 100Kbps when “active” o active 10% of time o N users circuit-switching: ¸ 1 Mbps link 10 users o packet switching: ¸ with 35 users, probability > 10 o active less that .004 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 6 2

Packet switching versus circuit switching Is packet switching a “slam dunk winner?” ¸ Great for bursty data o resource sharing o no call setup ¸ Excessive congestion: packet delay and loss o protocols needed for reliable data transfer, congestion control ¸ Q: How to provide circuit-like behavior? o bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 6) 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 7 Packet-switched networks: routing Goal: move packets among routers from source to destination ¸ we’ll study several path selection algorithms (chapter 4) o datagram network: ¸ destination address determines next hop o routes may change during session o analogy: driving, asking directions o virtual circuit network: ¸ each packet carries tag (virtual circuit ID), tag determines next hop o fixed path determined at call setup time , remains fixed thru call o routers maintain per-call state o 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 8 Access networks and physical media Q: How to connection end systems to edge router? ¸ residential access nets ¸ institutional access networks (school, company) ¸ mobile access networks Keep in mind: ¸ bandwidth (bits per second) of access network? ¸ shared or dedicated? 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 9 3

Residential access: point to point access ¸ Dialup via modem o up to 56Kbps direct access to router (conceptually) ¸ ISDN: intergrated services digital network: 128Kbps all- digital connect to router ¸ ADSL: asymmetric digital subscriber line o up to 1 Mbps home-to-router o up to 8 Mbps router-to-home o Ordinary PSTN 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 10 Residential access: cable modems ¸ HFC: hybrid fiber coax o asymmetric: up to 10Mbps upstream, 1 Mbps downstream ¸ network of cable and fiber attaches homes to ISP router o shared access to router among home o issues: congestion, dimensioning 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 11 Institutional access: local area networks ¸ company/univ local area network (LAN) connects end system to edge router ¸ Ethernet: o shared or dedicated cable connects end system and router o 10 Mbs, 100Mbps, Gigabit Ethernet ¸ LANs: chapter 5 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 12 4

Wireless access networks ¸ shared wireless access network connects end system to router router ¸ wireless LANs: base o radio spectrum replaces station wire o e.g., Lucent Wavelan 10 Mbps ¸ wider-area wireless access mobile o GPRS (General Packet hosts Radio Service) 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 13 Physical Media Twisted Pair (TP) ¸ physical link: ¸ two insulated copper transmitted data bit wires propagates across link o Category 3: traditional ¸ guided media: phone wires, 10 Mbps o signals propagate in ethernet solid media: copper, o Category 5 TP: fiber 100Mbps ethernet ¸ unguided media: o signals propagate freely e.g., radio 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 14 Physical Media: coax, fiber Coaxial cable: Fiber optic cable: ¸ wire (signal carrier) ¸ glass fiber carrying light within a wire (shield) pulses o baseband: single channel ¸ high-speed operation: on cable o 100Mbps Ethernet o broadband: multiple o high-speed point-to-point channel on cable transmission (e.g., 5 Gbps) ¸ bidirectional ¸ low error rate ¸ common use in 10Mbps Ethernet 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 15 5

Physical media: radio Radio link types: ¸ signal carried in electromagnetic ¸ microwave spectrum o e.g. up to 45 Mbps channels ¸ LAN (e.g., waveLAN) ¸ no physical “wire” o 2Mbps, 11Mbps ¸ bidirectional ¸ wide-area (e.g., cellular) ¸ propagation o e.g. GPRS, 10’s Kbps environment effects: ¸ satellite o reflection o up to 50Mbps channel (or o obstruction by objects multiple smaller channels) o interference o 270 msec end-end delay 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 16 Delay in packet-switched networks ¸ nodal processing: packets experience delay check bit errors on end-to-end path o determine output link o ¸ four sources of delay ¸ queuing at each hop time waiting at output link o for transmission depends on congestion o level of router transmission A propagation B nodal queuing processing 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 17 Delay in packet-switched networks Transmission delay: Propagation delay: ¸ R=link bandwidth (bps) ¸ d = length of physical link ¸ L=packet length (bits) ¸ s = propagation speed in medium (~2x10 8 m/sec) ¸ time to send bits into link = L/R ¸ propagation delay = d/s Note: s and R are very different quantities! transmission A propagation B nodal queueing processing 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 18 6

Queuing delay (revisited) ¸ R=link bandwidth (bps) ¸ L=packet length (bits) ¸ a=average packet arrival rate traffic intensity = La/R ¸ La/R ~ 0: average queuing delay small ¸ La/R -> 1: delays become large ¸ La/R > 1: more “work” arriving than can be serviced, average delay infinite! 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 19 Protocol “Layers” Networks are complex! ¸ many “pieces”: o hosts Question: o routers Is there any hope of organizing structure of network? o links of various media Or at least our discussion of o applications networks? o protocols o hardware, software 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 20 Organization of air travel ticket (purchase) ticket (complain) baggage (check) baggage (claim) gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing airplane routing ¸ a series of steps 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 21 7

Organization of air travel : a different view ticket (purchase) ticket (complain) baggage (claim) baggage (check) gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing airplane routing Layers: each layer implements a service o via its own internal-layer actions o relying on services provided by layer below 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 22 Layered air travel: services 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 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 23 Distributed implementation of layer functionality Departing airport ticket (purchase) ticket (complain) arriving airport baggage (check) baggage (claim) gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing intermediate air traffic sites airplane routing airplane routing airplane routing 9/9-02 Datorkommunikation & Internet, Anders broberg, Umu - Introduction 24 8