Introduction to Networks Introduction to Networks 1 Modulat on and - PowerPoint PPT Presentation

Introduction to Networks Introduction to Networks 1

Modulat on and Demodulat on Modulation and Demodulation  Common examples: radio, l s: di television channels for channels for analog signals o Bandwidth in hertz  Can also be used for digital signals (encoding binary cos( cos( 2 2 ) ) π π + + θ θ d t ) data) A A f f t t 0 1/24/2017 Intro to networks (Simon S. Lam) 1-2 2

Shannon’s Theorem Shannon s Theorem C = B log 2 (1 + S/N) where where C max capacity in bits/sec max capac ty n b ts/sec B bandwidth in hertz S/N signal to noise ratio 1/24/2017 Intro to networks (Simon S. Lam) 1-3 3

FDM vs. TDM FDM vs. TDM Duration of frame (or superframe) is 125 µsec in digital telephone networks t k 1/24/2017 Intro to networks (Simon S. Lam) 1-4 4

TDM in Telephone Networks TDM n Telephone Networks  Sampling rate for p g  Why 125 µ sec for y µ voice = 8000 frame duration? samples/sec or one  Sampling Theorem: voice sample every 125 voice sample every 125 An analog signal can be An analog signal can be µ sec reconstructed from  Digital voice channel samples taken at a (uncompressed), rate equal to twice the 8 bits x 8000/sec = signal bandwidth 64 Kbps 64 Kbps  Bandwidth for voice  Bandwidth for voice signals is 4 Khz; for hi fidelity music, 22.05 Kh Khz per channel h l 1/24/2017 Intro to networks (Simon S. Lam) 1-5 5

Other Mult plex ng Techn ques Other Multiplexing Techniques  Space division  Wavelength division multiplex multiplex multiplex multiplex o Same frequency used in  Light pulses sent at different cables different wavelengths in optical fiber in optical fiber o Same frequency used in Same frequency used in different (nonadjacent)  Code division multiplex cells A A A d e.g., CDMA for cell phones r A G G F B F F B B A A A E C A E C D D A 1/24/2017 Intro to networks (Simon S. Lam) 1-6 6

The Network Core The Network Core  mesh of interconnected routers routers  the fundamental question: how is data transferred through net? o circuit switching: dedicated circuit per dedicated circuit per call: telephone net o packet-switching: data sent thru net in discrete “chunks” 1/24/2017 Intro to networks (Simon S. Lam) 1-7 7

Network Core: Circuit Switching End-to-end resources End to end resources reserved for each “call”  E.g., link bandwidth o FDM, TDM  end-to-end circuit-like d t d i it lik  (guaranteed) performance  call setup required call setup requ red o resource piece idle if not used by the call (no sharing) o state information each step o state information each step along the way 1/24/2017 Intro to networks (Simon S. Lam) 1-8 8

Packet Switching: Statistical Multiplexing 100 Mb/s C A Ethernet statistical multiplexing 1.5 Mb/s B queue of packets q p waiting for output link D E  Sequence of A & B packets does not have fixed pattern bandwidth shared on demand  statistical multiplexing b d idth h d d d  t ti ti l lti l i  queueing delay, packet loss 1/24/2017 Intro to networks (Simon S. Lam) 1-9 9

Network Core: Packet Switching each end-end data stream resource contention : divided into packets divided into packets  aggregate resource t   packets of different users demand can exceed share network resources amount available  each packet uses full link congestion: packets bandwidth queue, wait for link use  store and forward: t d f d packets move one hop at a time Bandwidth division into “pieces” p Dedicated allocation  Each node receives the complete packet before Resource reservation forwarding it g 1/24/2017 Intro to networks (Simon S. Lam) 1-10 10

Circuit vs. Message vs. Packet Switching violates store- and-forward? 1/24/2017 Intro to networks (Simon S. Lam) 1-11 11

Packet switching versus circuit switching g g  1 Mb/s link  each user:  each user: o 100 kb/s when “active” o active 10% of time (a “bursty” user) bursty user)  circuit-switching: N users N users o 10 users 10 1 Mbps link  packet switching: o with 35 users, probability > 10 active l at same time is less Q: how did we get value 0.0004? than .0004 1/24/2017 Intro to networks (Simon S. Lam) 1-12 12

Packet switching versus circuit switching Is packet switching a “slam dunk winner?”  great for bursty data g y o resource sharing o simpler, no call setup  excessive congestion -> packet delay and loss o protocols needed for reliable data transfer, congestion control congestion control  Q: How to provide circuit-like behavior? bandwidth guarantees needed for bandwidth guarantees needed for o interactive audio/video apps o providing virtual links to enterprise network p g p customers (under service contracts) 1/24/2017 Intro to networks (Simon S. Lam) 1-13 13

Network Taxonomy Telecommunication networks Circuit-switched Packet-switched networks networks k Datagram D Networks k FDM/WDM TDM with VCs* Networks Internet Internet A Any technology can be used in h l b d i won! link layer of Internet under IP VC examples: ATM networks, MPLS tunnels VC examples: ATM networks MPLS tunnels Intro to networks (Simon S. Lam) 1/24/2017 1-14 14

Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? access access net net access net access net access net access access net net access access net net access net access net access net access net net access access net net access net Intro to networks (Simon S. Lam) 1-15 1/24/2017 15

Internet structure: network of networks Option: connect each access ISP to every other access ISP? access access net net access net access net access net access access net net connecting each access ISP to each other directly doesn’t t h th di tl d ’t access access scale: O( N 2 ) connections. net net access net access net access net access net net access access net net access net Intro to networks (Simon S. Lam) 1-16 1/24/2017 16

Internet structure: network of networks 1970 1991: connect each access ISP to a global transit 1970-1991: connect each access ISP to a global transit ISP: 1. Financed by US government: ARPAnet, NSFnet access access net net access net access net access net access access net net global global ISP access access net net access net access net access net access net net access access net net access net Intro to networks (Simon S. Lam) 1-17 1/24/2017 17

Internet structure: network of networks Post 1991 -Transition to commercial ISPs - If one global ISP is viable business, there will be competitors …. access access net net access net access net access net access access net net ISP A access access ISP B net net ISP C access net access net access net access net net access access net net access net Intro to networks (Simon S. Lam) 1-18 1/24/2017 18

Internet structure: network of networks Two ISPs are connected in a “provider-customer” or “peer- Two ISPs are connected in a provider-customer or peer- peer” relationships according to peering agreements Internet exchange point Internet exchange point access access net (hundreds of ISPs) net access net access net access net IXP access access net net ISP A IXP access access ISP B net net ISP C access net access net private link access net access net net access access net net access net Intro to networks (Simon S. Lam) 1-19 1/24/2017 19

Internet structure: network of networks … and regional networks may arise to connect access nets to ISPs access access net net access net access net access net IXP access access net net ISP A IXP access access ISP B net net ISP C access net access net access regional net net access net net access access net net access net Intro to networks (Simon S. Lam) 1-20 1/24/2017 20

Internet structure: network of networks … and a content provider network (e.g., Akamai, Google, Microsoft) may run its own network to bring services, content close to end users access access net net access net access net access net IXP access access net net ISP A Content provider network p IXP access access ISP B net net ISP B access net access net access regional net net access net net access access net net access net Intro to networks (Simon S. Lam) 1-21 1/24/2017 21

Internet structure: network of networks Tier 1 ISP Tier 1 ISP Google IXP IXP IXP Regional ISP Regional ISP access access access access access access access access ISP ISP ISP ISP ISP ISP ISP ISP  at center: small # of well-connected large networks g o “ tier-1 ” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage o content provider networks (e g o content provider networks (e.g., Google): private network that Google): private network that connects its data centers to Internet, often bypassing tier-1 and regional ISPs 1-22 Intro to networks (Simon S. Lam) 1/24/2017 22