Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge end - PowerPoint PPT Presentation

Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge � end systems, access networks, links 1.3 Network core � circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-1 Introduction 1-1

Connecting two nodes � Direct connection � simplest � no flexibility � "permanent" � S witched connection � Network of intermediate paths j oined by switches � PS TNs do this � PS TNs are very early examples Introduction 1-2

The Network Core � mesh of interconnected routers � the fundamental question: how is data transferred through net? � circuit switching: dedicated circuit per call: telephone net � packet-switching: data sent thru net in discrete “chunks” Introduction 1-3 Introduction 1-3

Network Core: Circuit Switching End-end resources reserved for “call” � link bandwidth, switch capacity � dedicated resources: no sharing � circuit-like (guaranteed) performance � call setup required Introduction 1-4 Introduction 1-4

Network Core: Circuit Switching network resources � dividing link bandwidth (e.g., bandwidth) into “pieces” divided into “pieces” � frequency division � pieces allocated to calls � time division � resource piece idle if not used by owning call (no sharing) ‏ Introduction 1-5 Introduction 1-5

Circuit S witching � End-to-end path established for duration of communications session � Paths traverse links from switch to switch � Inter-switch links each carry multiple circuits – multiplexing � Links transmit a range or band of (analog) signal frequencies � FDM –Frequency-Division Multiplexing - constantly divides total bandwidth between channels � TDM – Time-Division Multiplexing –allocates entire bandwidth to each channel cyclically Introduction 1-6

Circuit Switching: FDM and TDM Example: FDM 4 users frequency time TDM frequency time Introduction 1-7 Introduction 1-7

FDM � "Plain Old Telephone S ervice" (POTS ) requires 3KHz bandwidthfor voice, carried in 4KHz channel � A link capable of 100 MHz total bandwidth carries? � Broadcast television channels (used to) require 6MHz � see <www.csgnetwork.com/ tvfreqtable.html> � How much bandwidth does a tv cable need, to deliver Bruce S pringsteen's 57 channels? • <www.kovideo.net/ lyrics/ b/ Bruce-S pringsteen/ 57-Channels-And- Nothin-On.html> Introduction 1-8

TDM � Link is divided between a (typically fixed) number of timeslots � Timeslot duration is sufficient to carry a certain amount of information � Channels are assigned to timeslots � A single timeslot from each channel forms a frame � S ONET (S ynchronous Optical Networking) –frame duration is 125 microseconds, divided into nine timeslots Introduction 1-9

Numerical example � How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? � All links are 1.536 Mbps � Each link uses TDM with 24 slots/sec � 500 msec to establish end-to-end circuit Let’s work it out! Introduction 1-10 Introduction 1-10

Network Core: Packet Switching each end-end data stream resource contention: divided into packets � aggregate resource � user A, B packets share demand can exceed network resources amount available � each packet uses full link � congestion: packets bandwidth queue, wait for link use � resources used as needed � store and forward: packets move one hop at a time Bandwidth division into “pieces” � Node receives complete packet before forwarding Dedicated allocation Resource reservation Introduction 1-11 Introduction 1-11

Packet Switching: Statistical Multiplexing 100 Mb/s C A Ethernet statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand � statistical multiplexing . TDM: each host gets same slot in revolving TDM frame. Introduction 1-12 Introduction 1-12

Packet-switching: store-and-forward L R R R � takes L/R seconds to Example: transmit (push out) � L = 7.5 Mbits packet of L bits on to � R = 1.5 Mbps link at R bps � transmission delay = 15 � store and forward: sec entire packet must arrive at router before it can be transmitted on next link � delay = 3L/R (assuming more on delay shortly … zero propagation delay) ‏ Introduction 1-13 Introduction 1-13

Packet switching versus circuit switching Packet switching allows more users to use network! � 1 Mb/s link � each user: � 100 kb/s when “active” � active 10% of time N users � circuit-switching: 1 Mbps link � 10 users � packet switching: � with 35 users, Q: how did we get value 0.0004? probability > 10 active at same time is less than .0004 Introduction 1-14 Introduction 1-14

Packet switching versus circuit switching Is packet switching a “slam dunk winner?” � great for bursty data � resource sharing � simpler, no call setup � excessive congestion: packet delay and loss � protocols needed for reliable data transfer, congestion control � Q: How to provide circuit-like behavior? � bandwidth guarantees needed for audio/video apps � still an unsolved problem (chapter 7) ‏ Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Introduction 1-15 Introduction 1-15

Internet structure: network of networks � roughly hierarchical � at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage � treat each other as equals Tier-1 Tier 1 ISP providers interconnect (peer) privately Tier 1 ISP Tier 1 ISP Introduction 1-16 Introduction 1-16

Tier-1 ISP: e.g., Sprint POP: point-of-presence to/from backbone peering … … . … … … to/from customers Introduction Introduction 1-17 1-17

Internet structure: network of networks � “Tier-2” ISPs: smaller (often regional) ISPs � Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier-2 ISPs also peer Tier-2 ISP Tier-2 ISP pays Tier-2 ISP privately with tier-1 ISP for Tier 1 ISP each other. connectivity to rest of Internet � tier-2 ISP is c ustomer of Tier 1 ISP Tier 1 ISP tier-1 provider Tier-2 ISP Tier-2 ISP Tier-2 ISP Introduction 1-18 Introduction 1-18

Internet structure: network of networks � “Tier-3” ISPs and local ISPs � last hop (“access”) network (closest to end systems) ‏ local local Tier 3 ISP local local ISP ISP ISP ISP Local and tier- Tier-2 ISP Tier-2 ISP 3 ISPs are Tier 1 ISP customers of higher tier ISPs connecting them to rest Tier 1 ISP Tier 1 ISP Tier-2 ISP of Internet local Tier-2 ISP Tier-2 ISP ISP local local local ISP ISP ISP Introduction 1-19 Introduction 1-19

Internet structure: network of networks � a packet passes through many networks! local local Tier 3 ISP local local ISP ISP ISP ISP Tier-2 ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local Tier-2 ISP Tier-2 ISP ISP local local local ISP ISP ISP Introduction 1-20 Introduction 1-20