Computer Networks and the Internet PDF

Introduction to Computer Networks and the Internet Intro to Internet, protocol, network edge (hosts, access net, physical media), packet/circuit switching, network core, Internet structure Chapter 1 sections 1.1, 1.2, 1.3...

Introduction to Computer Networks and the Internet Intro to Internet, protocol, network edge (hosts, access net, physical media), packet/circuit switching, network core, Internet structure Chapter 1 sections 1.1, 1.2, 1.3 Introduction 1-1 What’s the Internet: “nuts and bolts” view PC ▪ billions of connected mobile network server computing devices: wireless laptop hosts = end systems global ISP smartphone running network apps home ▪ communication links network regional ISP wireless fiber, copper, radio, links satellite wired links transmission rate: bandwidth ▪ packet switches: forward router packets (chunks of data) institutional routers and switches network Introduction 1-2  “Fun” Internet-connected devices Tweet-a-watt: monitor energy use bikes Pacemaker & Monitor Amazon Echo Web-enabled toaster + IP picture frame weather forecaster Internet refrigerator Slingbox: remote cars control cable TV Security Camera AR devices sensorized, scooters bed mattress Gaming devices Others? Internet phones Fitbit What’s the Internet? mobile network nuts and bolts view global ISP ▪ Internet: “network of networks” Interconnected ISPs home network ▪ protocols control sending, receiving of regional ISP messages e.g., TCP, IP, HTTP, Skype, 802.11 ▪ Internet standards RFC: Request for comments IETF: Internet Engineering Task Force institutional network a service view ▪ infrastructure that provides services to applications: Web, Video Streaming, VoIP, email, games, e-commerce, social nets, … ▪ provides programming interface to apps hooks that allow sending and receiving app programs to “connect” to Internet provides service options, analogous to postal service Introduction 1-4 What’s a protocol? ▪ protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt human protocols: network protocols: ▪ “what’s the time?” ▪ machines rather than humans ▪ “I have a question” ▪ all communication activity in ▪ introductions Internet governed by protocols … specific messages sent … specific actions taken when messages received, or other events Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 time Q: other human protocols? Introduction 1-5  A closer look at network structure: ▪ network edge: mobile network hosts: clients and servers servers often in data centers global ISP ▪ access networks, physical media: wired, wireless communication links home network Q: How to connect end systems to edge regional ISP routers? ▪ residential access networks ▪ institutional access networks (school, company) ▪ mobile access networks ▪ network core: interconnected routers institutional network of networks network ▪ keep in mind: bandwidth (bits per second) of access network? ▪ shared or dedicated? Introduction 1-6 Important Aspects of Computer Networks Hardware: can be classified by: Transmission Technology: Broadcast Links: Channel shared among all machines e.g., Satellite, Multi-access Ethernet Multicast Links: Same as broadcast with access limitation to a subset of machines Point-to-Point (Unicast) Links: Connections between pairs of machines. Scale: Software Introduction 1-7  Access network: digital subscriber line (DSL) Dual-Tone Multi-Frequency (DTMF) Network Interfact Device (NID) Analog Signals (< 4 kHz) xDSL Transmission Unit (xTU) central office telephone Digital Signals (> 4 kHz) network Plain Old Telephone Services (POTS) Local Loop 0 < f < 4 kHz (PCM Signalling) DSL splitter modem DSLAM ISP voice, data transmitted at different frequencies over DSL access dedicated line to central office multiplexer ▪ use existing telephone line to CO DSLAM (support up to 64 homes) data over DSL phone line goes to Internet voice over DSL phone line goes to telephone net Asymmetric Access ▪ Medium-speed upstream channel (4 < f < 50 kHz), Tx rate (typically < 1 Mbps) ▪ High-speed downstream channel (50 kHz < f < 1 MHz), Tx rate (typically < 10 Mbps) Frequency Division Multiplexing (FDM) Introduction 1-8  Frequency V.S. Time Division Multiplexing ▪ Frequency Division Multiplexing Each user is assigned a separate frequency. Frequencies are spaced out in order to avoid interference. ▪ Time Division Multiplexing Time is subdivided into slots of fixed size. Each user is allocated a specific time slot. Information only transmitted during allocated slot over the same frequency. Access network: DSL – Cont’d. Introduction 1-10  Access network: DSL – Cont’d. Plot of BW V.S. distance over Cat-3 UTP for DSL BUSINESS MEETS TECHNOLOGY Objectives: Work over Cat-3 twister pair local loops. Not affect existing telephones / fax machines. Be faster than 56 kbps. Always on with just monthly charge (no charge per minute). Possible Solution: Build several mini end offices in the neighborhoods (expensive) Introduction 1-11 ANY ALTERNATIVE?  Access network: DSL – Cont’d. More Efficient Alternative Solution Discrete Multi-Tone (DMT). Available local loop spectrum (i.e. 1.1 MHz) divided into 256 independent channels: Each channel is 4.3125 kHz wide. Channel Usage 0 POTS 1 Unused 2 Unused 3 Unused 4 Unused 5 Unused 6 to 250 1 channel for upstream control 1 channel for downstream control. 25 channels upstream. 218 channels downstream. Introduction 1-12 Access network: DSL – Cont’d. DMT Channel Allocation Introduction 1-13  Metropolitan Access network: cable network Area Network cable headend … broadcasts TV channels cable splitter modem distribution network of coaxial cables and amplifiers C O V V V V V V N I D I D I D I D I D I D D A D A T R FDM E E E E E E T T O O O O O O O A A L 1 2 3 4 5 6 7 8 9 Channels Hybrid Fiber Coax (HFC): Fiber optics connect cable headend to neighborhood-level junctions. Traditional coaxial cable is used to read individual houses. Requires special cable modems. Introduction 1-14  Access network: cable network cable headend … cable modem termination system cable splitter turns analog signal modem CMTS sent from cable modems into digital data, TV transmitted at different format frequencies over shared cable ISP distribution network ▪ HFC: hybrid fiber coax Cable modems divide the HFC network into two asymmetric channels up to 30Mbps downstream transmission rate, 2 Mbps upstream transmission rate ▪ network of cable, fiber attaches homes to ISP router homes share access network to cable headend unlike DSL, which has dedicated access to central office Introduction 1-15  Local Access network: home network Area Network wireless devices to/from headend or central office often combined in single box cable or DSL modem wireless access router, firewall, NAT point (54 Mbps) wired Ethernet (1 Gbps) Introduction 1-16 Enterprise access networks (Ethernet) Local Area Network institutional link to ISP (Internet) institutional router Ethernet institutional mail, switch web servers ▪ typically used in companies, universities, etc. ▪ 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates ▪ today, end systems typically connect into Ethernet switch Introduction 1-17 Wireless access networks ▪ shared wireless access network connects end system to router via base station aka “access point” wireless LANs: wide-area wireless access ▪ within building (100 ft.) ▪ provided by telco (cellular) ▪ 802.11b/g/n/ac/ax (WiFi): 11 operator, 10’s km M/54 M/450 M/6.9 G/9.6 G ▪ between 1 and 10 Mbps (bps) transmission rate ▪ 3G, 4G: LTE, 5G and Beyond to Internet to Internet Introduction 1-18 Physical media ▪ bit: atomic entity propagates between Tx/Rx pairs ▪ physical link: what lies Twisted Pair (TP) between transmitter & ▪ two insulated copper wires receiver Category 5: 100 Mbps, 1 Gbps ▪ guided media: Ethernet Category 6: 10Gbps signals propagate in solid media: copper, fiber, coax Achievable rates depend on: Wire’s thickness. ▪ unguided media: Tx/Rx distance. signals propagate freely, ▪ Unshielded TP (UTP): e.g., radio Used for indoor LANs and residential access nets. Introduction 1-19 Physical media: Coaxial Cable ▪ Common in cable TV systems. ▪ Two concentric copper conductors with special shielding and insulation. ▪ Bidirectional ▪ Can achieve high bit rates. ▪ Can be used as guided shared medium: ▪ Multiple end systems directly connect to cable. ▪ Each end system receives what is sent by other end systems. ▪ Broadband: Multiple channels on cable HFC Introduction 1-20 Physical media: Fiber Optic Cables Introduction 1-21 Physical media: radio radio link types: ▪ signal carried in ▪ terrestrial microwave electromagnetic spectrum e.g. up to 45 Mbps channels ▪ no physical “wire” ▪ LAN (e.g., WiFi) ▪ bidirectional 54 Mbps ▪ propagation environment ▪ wide-area (e.g., cellular) effects: 4G cellular: ~ 10 Mbps reflection ▪ satellite obstruction by objects Kbps to 45Mbps channel (or interference multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude Introduction 1-22 STARLINK Satellite Meshes Introduction 1-23 Introduction 1-24  The network core ▪ mesh of interconnected routers ▪ packet-switching: hosts break application-layer messages into packets forward packets from one router to the next, across links on path from source to destination ▪ Each packet contains destination Address (dst) ▪ Each packet treated independently each packet transmitted at full link capacity Introduction 1-25  Host: sends packets of data host sending function: ▪ takes application message two packets, L bits each ▪ breaks into smaller chunks, known as packets, of length L bits 2 1 ▪ transmits packet into access network at transmission rate R R: link transmission rate host link transmission rate, aka link capacity, aka link bandwidth packet time needed to L (bits) transmission = transmit L-bit = delay packet into link R (bits/sec) Introduction 1-26 Packet-switching: store-and-forward L bits per packet 3 2 1 source destination R bps R bps ▪ takes L/R seconds to transmit one-hop numerical example: (push out) L-bit packet into link at R bps ▪ L = 7.5 Mbits ▪ R = 1.5 Mbps ▪ store and forward: entire packet must arrive at router before it ▪ one-hop transmission delay = 5 sec can be transmitted on next link ▪ end-end delay = 2L/R (assuming zero propagation delay) more on delay shortly … Introduction 1-27  Packet Switching: queueing delay, loss Sources of delays and losses: Introduction 1-28 Packet Switching: queueing delays behaviour Introduction 1-29  Two key network-core functions routing: determines source- destination route taken by packets ▪ routing algorithms & protocols: forwarding: move packets from ▪ automatically build and maintain router’s input to appropriate the forwarding tables router output routing algorithm local forwarding table header value output link 0100 3 1 0101 2 0111 2 3 2 1001 1 destination address in arriving packet’s header Introduction 1-30 Alternative core: circuit switching end-end resources allocated to, reserved for “call” between source & dest: ▪ in diagram, each link has four circuits. call gets 2nd circuit in top link and 1st circuit in right link. ▪ dedicated resources: no sharing circuit-like (guaranteed) performance ▪ circuit segment idle if not used by call (no sharing) ▪ commonly used in traditional telephone networks Introduction 1-31 Circuit Switching: Example Introduction 1-32 Packet Switching V.S. Circuit Switching packet switching allows more users to use network! example: ▪ 1 Mb/s link ▪ each user: N users 100 kb/s when “active” active 10% of time 1 Mbps link ▪ circuit-switching: 10 users ▪ packet switching: Q: how did we get value 0.0004? with 35 users, probability > 10 active at same time is less Q: what happens if > 35 users ? than.0004 * * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Introduction 1-33 Packet switching versus circuit switching is packet switching a “slam dunk winner?” ▪ great for bursty data resource sharing simpler, no call setup ▪ excessive congestion possible: 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 1-34 Internet structure: network of networks ▪ End systems connect to Internet via access ISPs (Internet Service Providers) residential, company and university ISPs ▪ Access ISPs in turn must be interconnected. so that any two hosts can send packets to each other ▪ Resulting network of networks is very complex evolution was driven by economics and national policies ▪ Let’s take a stepwise approach to describe current Internet structure Introduction 1-35 Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? access access net net access net access access net net access access net net access access net net access net access net access net access net access access net access net net Introduction 1-36 Internet structure: network of networks Option: connect each access ISP to every other access ISP? access access net net access net access access net net access access net net connecting each access ISP access to each other directly doesn’t access net scale: O(N2) connections. net access net access net access net access net access access net access net net Introduction 1-37 Internet structure: network of networks Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement. access access net net access net access access net net access access net net global access net ISP access net access net access net access net access net access access net access net net Introduction 1-38 Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. access access net net access net access access net net access access net net ISP A access net ISP B access net access net ISP C access net access net access net access access net access net net Introduction 1-39 Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. which must be interconnected access access Internet exchange point net net access net access access net net access IXP access net net ISP A access net IXP ISP B access net access net ISP C access net access peering link net access net access access net access net net Introduction 1-40 Internet structure: network of networks … and regional networks may arise to connect access nets to ISPs access access net net access net access access net net access IXP access net net ISP A access net IXP ISP B access net access net ISP C access net access net regional net access net access access net access net net Introduction 1-41 Internet structure: network of networks … and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users access access net net access net access access net net access IXP access net net ISP A Content provider network access net IXP ISP B access net access net ISP C access net access net regional net access net access access net access net net Introduction 1-42 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 “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage content provider network (e.g., Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs Introduction 1-43