Chapter 1 Computer Networks and the Internet
Internet traffic
What s the Internet? (hardware) PC server wireless laptop cellular handheld wired links millions of connected computing devices: hosts = end systems access points running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth Mobile network Global ISP Home network Regional ISP Institutional network router routers: forward packets (chunks of data) Introduction 1-3
Internet appliances
Your work Find me very interesting an Internet appliance Present me with picture and description If your friends buy it, you get a point You can invent a new one also whether it is not for real
What s the Internet? (software) communication infrastructure enables distributed applications: Web, VoIP, email, games, e-commerce, file sharing communication services provided to apps: reliable data delivery from source to destination best effort (unreliable) data delivery
Too many types of computing devices! How to make networks which allow different devices to communicate? Analogous to human languages Need speak the same language
Network protocol Hi Hi Got the time? 2:00 TCP connection request TCP connection response <file> time Q: Other human protocols?
Who make network protocols? Internet Engineering Task Force (IETF) Internet standards Request for Comments (RFCs)
A closer look at network structure: Network edge: applications and hosts Access networks, physical media: wired, wireless communication links Network core: interconnected routers network of networks Introduction 1-10
The network edge: End systems (hosts): run application programs e.g. Web, email at edge of network Client/server model peer-peer client host requests, receives service from always-on server client/server e.g. Web browser/server; email client/server Peer-to-Peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent Introduction 1-11
Access networks Three categories: Residential access Company access Wireless access Help me identify them Mobile network Global ISP Home network Regional ISP Institutional network Introduction 1-12
Physical media Guided media: signals propagate in solid media: copper, fiber, coax Unguided media: signals propagate freely, e.g., radio
Twisted-pair copper wire (crosstalk reduced) Guided media Unshielded twisted pair (UTP) Shield twisted pair (STP)
Coaxial cable Guided media
Fiber optics Guided media
Radio links Unguided media Microwave WiFi 3G Satellite
Network core (review slide 10) 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
Circuit switching (telephone system) 19
Packet switching (store-and-forward)
Packet-switching: store-and-forward L R R R takes L/R seconds to transmit (push out) packet of L bits on to link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link delay = 3L/R (assuming zero propagation delay) Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay = 15 sec Introduction 1-21
Packet switching versus Circuit switching Packet switching allows more users to use network! 1 Mb/s link each user: 100 kb/s when active circuit-switching: 10 users packet switching: >> 10 users N users 1 Mbps link Introduction 1-22
Packet switching versus Circuit switching Packet Shared resources No call setup Long delay when traffic is high Circuit Reserved resources Call setup needed Delay is known s N users supported Low cost < N users supported High cost
How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers 1-24
Four sources of packet delay 1. nodal processing: check bit errors determine output link 2. Queuing: time waiting at output link for transmission depends on congestion level of router A transmission propagation B nodal processing queueing Introduction 1-25
Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s A transmission Note: s and R are very different quantities! propagation B nodal processing queueing Introduction 1-26
Caravan analogy 100 km 100 km ten-car caravan toll booth cars propagate at 100 km/hr toll booth takes 12 sec to service car (transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? toll booth Time to push entire caravan through toll booth onto highway = 12*10 = 120 sec Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes Introduction 1-27
Caravan analogy (more) 100 km 100 km ten-car caravan toll booth Cars now propagate at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth? Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! toll booth See Ethernet applet at AWL Web site Introduction 1-28
Nodal delay d nodal d proc d queue d trans d prop d proc = processing delay typically a few microsecs or less d queue = queuing delay depends on congestion d trans = transmission delay = L/R, significant for low-speed links d prop = propagation delay a few microsecs to hundreds of msecs Introduction 1-29
Queueing delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more work arriving than can be serviced, average delay infinite! Introduction 1-30
Experiment delay Traceroute (XP) Tracert (Linux) Provide delay measurement from source to router along end-end Internet path towards destination.
Packet loss See applet examples queue (aka buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not at all A buffer (waiting area) packet being transmitted B packet arriving to full buffer is lost 1-32
Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time server server, sends with bits (fluid) file of into F bits pipe to send to client pipe link that capacity can carry fluid R s bits/sec at rate R s bits/sec) pipe link that capacity can carry Rfluid c bits/sec at rate R c bits/sec) 1-33
Throughput (more) R s < R c What is average end-end throughput? R s bits/sec R c bits/sec R s > R c What is average end-end throughput? R s bits/sec R c bits/sec bottleneck link link on end-end path that constrains end-end throughput 1-34
Throughput: Internet scenario per-connection end-end throughput: min(r c,r s,r/10) R s R s R R s in practice: R c or R s is often bottleneck R c R c R c 10 connections (fairly) share backbone bottleneck link R bits/sec 1-35
Layered architecture Layered architecture is our day life Why layer? Things are too complex Computer networking is too complex
Sending a mail Encapsulation Decapsulation
In-class assignment Please spend a few minutes to find other example of a layered system Show me to get a point
ISO issues OSI (7-layer architecture) The International Standards Organization (ISO) Open Systems Interconnection (OSI) model
Seven layers of the OSI model
The interaction between layers in the OSI model
An exchange using the OSI model
Physical layer The physical layer is responsible for movements of individual bits from one hop (node) to the next
Data link layer The data link layer is responsible for moving frames from one hop (node) to the next
Hop by hop delivery
Network layer The network layer is responsible for the delivery of individual packets from the source host to the destination host
Source to destination delivery
Transport layer The transport layer is responsible for the delivery of a message from one process to another
Reliable process-to-process delivery of a message
Session layer The session layer is responsible for dialog control and synchronization
Presentation layer The presentation layer is responsible for translation, compression, and encryption
Presentation layer The application layer is responsible for providing services to the user
Summery
Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits on the wire application transport network link physical Introduction 1-54
Internet protocol stack and OSI
Homework Assignment Chapter 1 problems: P2 P4 P5 P6 P9 P12 P13
Discussion Questions Make a group of two persons and choose one question from the list below D1 D7 D11