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Transcript
Norman White Stern School of Business Csntier.ppt Background Data communications TCP/IP Physical networks Database management systems Data Communications Networking … Some Basics SOURCE SINK MEDIUM SOURCE telephone cable satellite MEDIUM SINK Telephone Satellite Cellular Wifi Cable Fiber Optic Blue Tooth etc. Analog Represent information as a waveform Music, video,voice Digital Represent Information as a string of bits Digitization Convert from analog to digital by sampling the waveform at fixed intervals ISDN phone line 64000 bits per second 8000 samples per second (each sample an 8 bit (0-255)) Number Converts Digital signal to analog so it can be sent over phone lines Reconverts analog to digital on other end 1 1 1 0 0 01110000 0 0 0 Need rules to communicate between systems Communications protocols are the rules for a particular method of communication Who says what, when How are errors handled How much information in a message How is message routed from sender to receiver Asynchronous - A single character is a message Errors sometimes detected, but no recovery Horizontal parity used to detect single character errors Synchronous - A group of characters constitute a message Synchronous protocols usually recover from errors Bisynchronous Point to point conversation Not used much any more Each message is acknowledged as received Multilateral protocols Messages routed from one machine to another Message includes address of destination Needs to be routed by intermediate nodes Application Layer – Applications and processes that use network Host-host Transport Level – end to end data delivery Internet layer – defines the datagram and handles data routing Network Access layer – routines for accessing physical networks Protocol of the Internet Originally developed as a protocol that would withstand wide network outages (Arpanet) Now it has become the standard protocol for almost all data communications Consists of two parts, IP and TCP Handles communications between 2 processes anywhere on internet Guarantees correct data is received by receiver Uses IP for delivery and routing of packets Fixed size blocks of information that are sent using TCP Packet includes information like Address of host to send this to Address of host it is coming from Destination Port Number Sending Port Number MAC address of sender/receiver (hardware address) Sequence info Data Handle datagrams Defines Internet addressing scheme Routes datagrams to remote hosts Performs fragmentation and reassembly of datagrams Every host has an address in the form of www.zzz.yyy.zzz (the “IP” address) Each subcomponent refers to a particular breakdown of all of the machines on the internet. 128.122.197.133 = sales.stern.nyu.edu 128 = edu domain 122 = nyu network 197 = subnet at stern 133 = address of sales computer The TCP/IP protocols include the concept of “name lookup”, where names like sales.stern.nyu.edu are translated into their numeric addresses (128.122.197.133) Hence one has to specify the locations of the “name servers” you wish to use. These are servers that are given a name (sales.stern.nyu.edu) and return its address. Gateways (also called IP routers) are the computers used to connect your network to the internet TCP/IP uses “ports” to connect services between machines The destination machine has particular ports offering different types of service 23 = telnet , 80 = WWW etc. Client machine uses random port when it connects to a service (i.e. port 80 for http requests) Combination of destination port and originating port = socket (i.e. a destination port that is in use) telnet - 23 Ssh - 22 ftp - 21 NFS - Network File Service - 2049 Talk - 517 Sendmail - 25 http - 80 World Wide Web (usually port 80) Hyper media interface to internet Connects clients and servers using HTTP Hyper Text Transfer Protocol Subset of the internet Problem – How do we fully connect thousands of computers together? (How many wires?) 2 computers – 1 3 computers – 3 4 computers - 6 5 computers – 10 N computers – (N*(N-1)/2) 1000 computers – about .5 million connections Solution Don’t connect every computer to every other computer, instead “route” information from one computer to another Protocol Rules to follow telling each computer what to send where, when Packet switched protocols include routing of packets from source to destination Protocols also define rules to follow if there are errors, I.e. how to recover, retransmit etc. Low level protocols route packets (of bits) around the network. Packets include address High level protocols know what the packets contain TCP/IP client server protocols Client sends requests over network to a “server” program running on another computer WWW C/S application A program on one computer (the Client) can request services from a program on another computer (the Server) Client issues requests to a server someplace on the network Requests are specially formatted messages which can be understood by almost any type of computer Requests are sent using a “standard” telecommunications protocol Server responds with a message in the “standard” format Client receives message and processes it In the WWW model, the client is the browser (Firefox, Safari, I.E.) running on your computer The server is the web server running somewhere on the internet Both the client and server must be connected through a TCP/IP communications network The situation gets more complex, if the initial (Tier 1) server in turn becomes a client and requests services from another server in order to process the web request. There are now 3 programs running on 3 different computers involved in the processing of the web request. User Web Browser Web Server Data Base Server Two tier Clients connect directly to servers Example PC clients, LAN DB server, Web server Three … N tier Clients connect to intermediate servers which in turn issue requests to other servers Examples PC to UNIX to IBM Mainframe User Web Browser Application Server Web Server Data Base Server JAVA – Sun developed language that runs code on the user’s machine (i.e. built in to the web browser) Off loads processing from server Basis for new JINI language for appliances FAT Client Term used to refer to putting all of application code on the user’s machine (on the hard drive) THIN Client – Distribute applications directly over the internet using JAVA. App is downloaded everytime your run it. CACHE Keep recently used html pages, images, applets etcs on user hard drive. Only download when they change TCO HTTP Hyper Text Transfer Protocol – Protocol of the WWW Applet Total Cost of Ownership – includes support A compiled Java program ready for distribution over WWW Digital Signature Numeric identifier indicating that a document hasn’t been compromised or that authenticates the creator. Data Base Servers Application Servers Web Servers Ad Servers (Double Click) Multimedia (audio/video servers) Calendar Servers Authentication (LDAP) servers Mail servers Proxy Servers CS computing isolates each layer from the other layers. Computers can be added as necessary to scale performance Processing can happen anywhere, so that bottlenecks can be eliminated Machines can be dedicated/optimized for different kinds of processing Can start with everything on one machine, but add machines as load grows Needs TCP/IP network protocol