CS 381 Introduction to computer networks
Lecture 2 1/29/2015 Introduction Overview: What is the Internet? What is a protocol?
Network edge Hosts Access networks Physical media Network core Packet/circuit switching Internet structure Performance Loss Delay Throughput Protocol layers Service models History Introduction Whats the Internet: nuts and bolts view
smartphone PC server wireless laptop millions of connected computing devices: hosts = end systems running network apps mobile network global ISP regional ISP home network institutional communication links fiber, copper, radio, satellite transmission rate: bandwidth wired links wireless Packet switches: forward packets (chunks of data) routers and switches router Introduction 3 Internet Appliances Introduction Whats a protocol? a human protocol and a computer network protocol:
Hi TCP connection request Hi TCP connection response Got the time? Get 2:00 time Q: other human protocols? Introduction A closer look at network structure:
Network edge: applications and hosts Access networks Connects end system to 1st router physical media: wired, wireless communication links Network core: interconnected routers network of networks Introduction The network edge: End systems (hosts): Reasons for this?
All Internet applications are implemented at the end systems. HTTP, FTP, SSH,SCP, DNS, SMTP Reasons for this? Introduction Access networks and physical media
Links connecting an end system to the first router (edge router)on the path to the Internet core. Edge router connects end system to Internet. Question: How to connect end systems to edge router? In other words, how can you connect your smartphone orlaptop to the first router on campus? Introduction Access networks and physical media
Question: How to connect end systems to edge router? Most common ways: residential access networks Cable modems, DSL, Dial-Up modem NAT router with Wi-Fi, Ethernet institutional access networks (school, company) mobile access networks Introduction 1-9 9 Access networks and physical media
Two important characteristics of access networks bandwidth (bits per second) of access network Residential (Outgoing): 2Mbps 50Mbps (and higher) Residential (Local): 11Mbps 1.2Gbps Institutional (Outgoing): 100s Mbps multiple Gbps Institutional (Local): 10Mbps 10Gbps Mobile: Kbps - ~40Mbps shared or dedicated Introduction 1-10 10 Dial-up Modem Uses existing telephone infrastructure
network Internet home dial-up modem ISP modem home PC centraloffice Uses existing telephone infrastructure Computer software makes phone connection to ISP Handshake: determines link speed, IP address Home modem converts digital output to analog and sends it across phone line. Modem:Modulate/Demodulate The ISP modem converts from analog back to digital and pushes data to edge router. Dial-up Modem Problems: Extremely slow with max speed of 56 kbps
telephone network Internet home dial-up modem ISP modem home PC centraloffice Problems: Extremely slow with max speed of 56 kbps ~42.5 hours to download 1GB worth of data ~4KHz bandwidth compared to 500MHz using CAT6a cable Have to choose: Computer or telephone. Circuit switched, non-shared access to ISP Digital Subscriber Line (DSL)
Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data Internet home phone telephone network splitter home PC DSL modem central office Also uses existing telephone infrastructure Advantages over Dial-up: Increased upload and download throughput Can use computer and telephone at the same time Digital Subscriber Line (DSL)
Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data Internet home phone telephone network splitter home PC DSL modem central office Telephone line carries both digital and telephone signals Encoded at different frequencies. Phone line at 0 - 4KHz Upstream data at KHz(128 kbps - 1 mbps) Downstream data at 50KHz - 1MHz(1 - 2 megabits per second) New technologies emerging for DSL: up to 1Gbps (~2016) to/from central office
Home Network wireless devices to/from central office often combined in single box wireless access point (54 Mbps 1.2 Gbps) router, firewall, NAT cable or DSL modem wired Ethernet (100 Mbps 1 Gbps) Introduction Ethernet Internet access
Typically used in companies,universities, etc. 10 Mbps, 100Mbps, 1Gbps,10Gbps Ethernet Multiple switches per building Serves rooms with Ethernet ports and Wi- Fi access points Fiber connection betweenswitches 100 Mbps 1 Gbps server Ethernet switch Institutional router To Institutions ISP Ethernet Internet access
Few routers on campus Why? Campus network can be thoughtof as a large LAN (Local AreaNetwork) Similar to your network at home, butwith thousands of end systems Greater complexity, but basic topologyis exactly the same Large number of switches allow localcommunication (layer 2 routing) Only communication off campusrequires the use of routers (layer 3routing) 100 Mbps 1 Gbps server Ethernet switch Institutional router To Institutions ISP Wireless access networks
shared wireless access network connects end system to router via base station aka access point wide-area wireless access provided by telco (cellular) 10s km between 1 and 10 Mbps 3G, 4G:LTE wireless LANs: within building (~100 ft) 802.11b/g/n/ac (WiFi) to Internet to Internet Introduction Host: sends packets of data
host sending function: takes application message breaks into smaller chunks, known aspackets, of length L bits transmits packet into access networkat transmission rate R link transmission rate link capacity link bandwidth two packets, L bits each 2 1 R: link transmission rate host L (bits) R (bits/sec) packet transmission delay time needed to transmit L-bit packet into link = = Physical media bit: propagates between transmitter/receiver pairs
physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper,fiber, coax twisted pair (TP) two insulated copper wires Category 5: 100 Mbps, 1Gpbs Ethernet Category 6: 10Gbps Introduction Physical media: coax, fiber
Coaxial cable: Center copper conductor surrounded by insulation bidirectional broadband: multiple channels on cable Fiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 10s - 100s Gpbs transmission rate) low error rate: repeaters spaced far apart immune to electromagnetic noise Introduction Physical media: radio Unguided media: Propagation environment effects:
signals propagate freely, e.g., radio signal carried in electromagnetic spectrum no physical wire Bidirectional Propagation environment effects: reflection obstruction by objects Interference radio link types: Terrestrialmicrowave Up to 45 Mbps channels LAN (e.g., WiFi) 11Mbps 1.3 Gbps Wide-area (e.g., cellular) 3G/4G cellular: ~ few Mbps Satellite Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude Introduction Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge
end systems, access networks 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switchednetworks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction The Network Core Mesh of interconnected routers
The fundamental question: how is data transferred through network? Compare telephone network andInternet Telephone network employs circuit switching resources necessary to make call are reserved forduration of communication Introduction Network Core: Circuit Switching
End-end resources reserved for call link bandwidth,switch capacity Finite capacity, all circuits are busy dedicated resources: no sharing circuit-like (guaranteed) performance Always true? call setup required Call request time: time to obtain dial tone Selection time: user dialing numbers, transmitting tones ofdifferent frequency Post selection time: time needed to process dialednumbers until connection to destination device Some differences in traditional telephone service andcellular telephone service Introduction Network Core: Circuit Switching
Network resources (e.g., bandwidth) divided intopieces Pieces allocated to calls for duration of call When you are not talking, no one else can utilize your piece of thenetwork How can bandwidth of a link be divided into pieces? Introduction Network Core: Circuit Switching
Two techniques for dividing link bandwidth intopieces frequency division multiplexing (FDM) time division multiplexing (TDM) Introduction 1-27 27 Network Core: Circuit Switching
Frequency division multiplexing the frequency spectrum isdivided among the connections across the link Recall that with DSL telephone link is divided into three frequencybands Telephone use Data upload Data download Link dedicates a frequency band for each connection for duration ofcommunication The width of the frequency band allocated to a particular connectionis called ????? Bandwidth! Introduction 1-28 28 Network Core: Circuit Switching
Time division multiplexing Time is divided into frames of fixed duration Example: 4 users, each user has access to the link for time per frame Each frame is divided into slots of fixed duration User has full bandwidth access to the link when active Each connection gets one time slot per frame User is idle for N-1/N time, where N = number of connections per frame Introduction 1-29 29 Circuit Switching: FDM and TDM
4 users Example: Assume frequency domain divided into 4 circuits FDM frequency time Example:Total bandwidth is 40Mhz Each user is allocated of the total bandwidth, 10Mhz each Resources are dedicated for the duration of the connection DSL works this way Instead of multiple users: 3 channels telephone, data upload, data download Two simple multiple access control techniques. Each mobiles share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction 1-30 30 Circuit Switching: FDM and TDM
4 users Example: TDM TDM frequency time Frame Frame Example:Total bandwidth is 40Mhz Each user is allocated all of the total bandwidth, 40Mhz for of the time of a frame Resources are dedicated for the duration of the connection Bluetooth works this way Instead of multiple users: 40 channels Data divided into packets, each packet transmitted on one of the 40 channels Two simple multiple access control techniques. Each mobiles share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction Time Division Multiplexing
frequency Frame Frame time Assume transmission rate of link is 4000 bits per second Frame = 1 second, link is divided among four communications (I.e., link is supporting 4 circuits: 0, 1, 2, 3). Each circuit gets a 1/4 second timeslot per second. During timeslot gets full transmission rate: 1/4 second * 4000 bps = 1000 bps. Transmission rate for each circuit is 1000 bps How long to transmit a 5000 bit file? 5 seconds (Note: The example does not consider setup time) Two simple multiple access control techniques. Each mobiles share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction 1-32 32 Time Division Multiplexing
frequency Frame Frame time Assume transmission rate of link is 6000 bits per second Another Example: Frame = 2 seconds, 4 circuits Each circuit gets second timeslot per frame How long does it take to transmit a bit file? rate link: 6kbps, 12kbps throughput per frame second * 6000 bps = 3kbps 3kb transmitted per frame 5 frames needed to transmit 13kb. total time: ~9 seconds, excluding setup time Two simple multiple access control techniques. Each mobiles share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction 1-33 33 Frequency Division Multiplexing
4 users frequency time Assume bandwidth of the link is 4000 bps and each communication (circuit) receives equal bandwidth. Each circuit gets of the 4000 bps throughput for the duration of the communication. How long for a given circuit to transmit a 5000 bit file? * 4000 bps = 1000 bps 5 seconds, excluding setup time Two simple multiple access control techniques. Each mobiles share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction 1-34 34 Frequency Division Multiplexing
4 users frequency time Assume bandwidth of the link is 6000 bps and each communication (circuit) receives equal bandwidth. Another Example: Each circuit gets of the 6000 bps throughput for the duration of the communication. How long for a given circuit to transmit a bit file? * 6000 bps = 1500 bps ~9 seconds, excluding setup time Two simple multiple access control techniques. Each mobiles share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction 1-35 35 One more example How long does it take one connection to send a file of640,000 bits from host A to host B over a circuit-switchednetwork? Assuming: All links are Mbps Frame rate is 1 second Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit Introduction Numerical example How long does it take one connection to send a file of640,000 bits from host A to host B over a circuit-switchednetwork? Capacity of link is Mbps With 24 slots, each connection gets bandwidth of 1.536/24 =64Kbps So each connection has bandwidth of 64Kbps (640,000)/64 = 10 seconds to transmit file msec toestablish end-to-end connection = 10.5 seconds. Introduction 1-37 37 Network Core: Packet Switching
Internet is a packet switching rather than circuitswitching network. Reservations not accepted No reserving of communication links, no guarantee of given bandwidth In fact, No guarantees at all! How can we demonstrate this? Ping command Introduction Network Core: Packet Switching
Internet is a best-effort network: It will allocate whatever resources are available at the timethey are requested. Hopefully all data will make it from sender to receiver: Might take a very long time Might not arrive in the same order it was sent Might not arrive at all The application is not informed if any of these problems happen (ordont). Introduction Packet Switched Networks
Distributed applications communicate by sending messages to eachother. Can contain any kind of data: video, audio, jpeg, mp3,, Sender divides long messages into smaller chunks called packets. Each layer of the OSI model will attach a header with information to the packet Packet generation happens on client devices.Network core components do little to change packetheader information. Packets get shuttled between packet switches (routers, link-layerswitches). Network Layer protocols: communication between source and destination client devices Link Layer Protocols: single hop communication between clients, switches, and routers Packet switches have input links and output links Routing vs. forwarding Store-and-forward