Network Tools, Data Compression 1. Networking in Practical Terms We first explored, in terms of a 1750 fax network, principles of round-robin scheduled, packet-switched networks with confirmation. We then sawhow those principles are essential elements of the design of the modern Internet. We saw how the Internet delivers data by preceding the bits of information with headers that describe, in order,(1) the address of the next router on the journey, (2) the address of the ultimate destination, (3) the sequence number,destination program, and request for confirmation, and (4) data-specific description. We now move from basic principles and system design to some practical how-to’s. In particular,welook at: a) Howthe Internet attaches names (likewww.xyz.com) to IP addresses b) Howtofind out network details/information c) Methods for compressing data 2. Getting to KnowYour Network We saw that the Internet is a vast collection of local area networks connected by routers. Each network has a net- work ID number,and each machine on that network is givenacomputer ID number.This combination of network- ID.computer-ID (the IP address) identifies every single computer on the Internet. Furthermore, every computer can send packets of data to every other computer,simply by specifying the IP address of the destination. Once you start to understand this model of data communication, a fewquestions arise: a. Howdonames likewww.tufts.edu work? Where is its IP address? b. What is my IP address? What is your IP address? c. What route does a packet taketoget from my machine to its destination? d. What is the MACaddress of my computer? e. What are the routers to get me out of this local network? f. Howcan I get a lot of data through a connection with limited capacity? 3. DNS -Names for IP Numbers Every computer on the Internet is assigned an IP Address.AnIPaddress is a four-part number of the form net- work#.computer#. Each part is an 8-bit number,which, in theory,allows for about four billion addresses. PROBLEM Howcan people remember IP addresses? PROCEDURE Assign names to numbers. Forexample, google.com is at IP address 74.125.47.147. Youcould just type 74.125.47.147 into your browser and connect to google.com. But it is easier to remember a name than a sequence of four 8-bit numbers. It is easier,for example, to remember www.tufts.edu than it is to remember 130.64.1.83. When you send email, you use a name for the machine to which you are sending email, but you could use the IP address, what everitis. PROBLEM Howdoes a computer convert www.tufts.edu to 130.64.1.83 PROCEDURE It calls directory assistance (aka a DNS server) DNS stands for domain name system.Scattered all overthe Internet are computers that do for computers what the telephone number 411 does for people: looks up numbers for names. When you type www.google.com into your browser,your browswer connects to a DNS server and asks for the IP number of www.google.com. The DNS server might knowthe IP address of the name you request, in which case it sends back the IP number.Or it might not. In which case, it says something like, "That’sa.com address. Ask the server in charge of the .com addresses. That server is at the following IP address." Then your computer makes a second connection, this time to aserver in charge of the .com addresses. That second server might knowwhere www.google.com is, or it might not. If it does not, it says "That’sagoogle.com address. Ask the server overatgoogle.com. That server knows the IP address of www.google.com." And your computer makes a third call, this time to the server at google.com. That server tells your computer the IP address for www.google.com. Dec 01, 2008 13:47 page 1 Network tools, Data Compression Each domain,likegoogle.com, just likeeach business, keeps its own directory of the computers within its organiza- tion. For example, the DNS servers at Tufts keep a list of the IP addresses assigned to computers within the Tufts network. The .edu server just has to knowthe address of the DNS server at Tufts. Note, the Internet would exist and would work without DNS. It would not be as convenient for humans. 4. Unix Commands to View Network Information Howcan we look up a name ourselves? Howcan we see what is going on in our network? Unix has commands like ls, who, ps, date to tell you what is happening on the local machine, and Unix has commands to tell you about the Internet connections. hostname hostname tells you the name of the computer you are using. The -i option asks for the IP address of the computer. ifconfig ifconfig tells you about the network card(s) in the machine you are using. It tells the MACaddress and the IP address. If there are multiple cards, ifconfig lists them all. dig dig asks DNS for the IP address for a name. traceroute traceroute shows you the path of a packet from one machine to another. Try /sbin/traceroute times.co.uk to see howyou read news from the Times of London or /sbin/traceroute www.lemonde.fr. ping ping tells you if a remote machine is alive and can tell you howlong it takes to get a packet there, and tells you howmanypackets get lost. route route tells you howthe local machine figures out which router to use to get to what addresses. arp arp tells you which IP addresses are assigned to local MACaddresses. 5. Sending Images - Fax vs Digital Images Afax machines scans a piece of paper and creates a black and white bitmap. The machine can scans at 98 lines per inch, or at 196 lines per inch. Faxalways scans at 203 bits per inch horizontally.Apage is 8.5" wide x 11" tall. ☞ Howmanypixels does one rowuse? ☞ Howmanypixels does a complete page use? ☞ Faxmachines transmit up to 14,400 bits/second. Howmanyseconds should it taketosend a complete page ? But fax machines do not takethat long usually.How dotheyreduce the time? 6. Compressing Data The Internet sends data as 1’sand 0’srepresented as pulses of electricity,flashes of light, bursts of radio wav es. And these 1’sand 0’sare ultimately stored on hard disks, RAM, flash drives, compact discs. The capacity of a wire, as measured in howmanypulses can be sent per second, and the capacity of a hard disk, as measured as howmany magnetic regions per square inch, are limited. Since the beginning of data storage and transmission, people have looked for ways to store more information in less space. What procedures have people devised to compress data? Why Compress Data? If some information can be represented in fewer bits, that information takes less time to transmit and takes less space to store. We knowhow touse bits to represent sound, text, and images. Howcan we compress songs, documents, and pictures? 6.1. Compressing Music MP3 is a format that compresses digital music. We hav e studied some of the techniques MP3 uses to store a piece of music using fewer bits than the full 16-bit, 2-channel, 44,100 samples per second CD standard. Some of the tech- niques, such as dropping quieter data that would be hard to hear along with louder sounds, is based on the way Dec 01, 2008 13:47 page 2 Network tools, Data Compression human hearing works. Some techniques, such as dropping the sample rate, changes the resolution but leavesit within acceptable ranges. 6.2. Compressing Text Adouble-spaced page of text at an average of 10 characters per inch contains about 1800 characters. Therefore a 10-page paper uses about 18,000 bytes, which is 144,000 bits. Howcan one store the same 18,000 characters worth of text in fewer bits? There are twomajor techniques, one is based on characters, the other is based on words and phrases. First, let us look at compression in the earliest days of sending bits using electricity -- the Morse Code. Samuel Morse designed his Morse Code to reduce Morse Code the number of dits and dahs sent overthe wire. The A._J.--- S ... 2 ..--- letter ’e’ is the most common letter in typical English B-... K -.- T -3...-- C-.-. L .-.. U ..- 4 ....- text. Morse assigned a single dit to the letter E and a D-.. M -- V ...- 5 ..... single dah to the letter T.Less common letters, like E. N-. W .-- 6 -.... Q, were assigned longer codes. Thus, data compres- F..-. O --- X -..- 7 --... G--. P .--. Y -.-- 8 ---.. sion has been built into electric communication sys- H.... Q --.- Z --.. 9 ----. tems since the beginning. What other techniques can I..R.-. 1 .---- 0 ----- we use to compress data? 7. Designing Your Own Compression System: Image Compression Consider,inthe spirit of the 1750 Fax System, an image transmitter from Earth to a colonyonMars. The transmis- sion station charges one dollar per bit. Youhav e a8x8 digital image you want to send to a pal of yours on a semester abroad on Mars. Drawapicture in the grid belowand we shall discuss systems to use to cut the cost down from $64 Apicture: Acompressed version: __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ 0-14 __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ 15-29 __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ 30-44 __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ 45-59 8.