Networks (Introduction to Medical Informatics) (http://www.cpmc.columbia.edu/edu/textbook) LAST REVIEWED: 8 October 1996 why many computers (lab, rad, ...) many small computers can do more than one big computer (even considering the cost of connecting them) local autonomy less vulnerable to failure network goals (diagram with printer): communication between users share resources and reduce costs -- eg printer reliability - through hardware redundancy share data definitions network=connect 2 or more devices capable of independent control baud rate = switching-values/second (bits/second, not bytes) LAN=kilometers, tightly coupled WAN=unlimited distance, but slower connections without a LAN: require N(N-1) connections usually cannot reuse connections with LAN: require N Look at a LAN from bottom up medium: physical link twisted pair=4 MB (25 seconds for CXR), cheapest, like telephone wire coaxial cable=100 MB (1 sec) fiber optics=1 GB (0.1 sec), fastest, no radio interference network interface (adapter) card (NIU) connects wire to computer baseband vs. broadband network topology (diagram) bus (ethernet) ring (token ring) passing information (diagram) ethernet uses contention access token ring uses token passing speed of light limits the size of a LAN (actually speed*size) different ways of passing the information virtual circuit -- like telephone call datagram -- send message, hope it got there broadcast -- tell many at once above that, see various protocols supply basic network resources and utilities these allow computer applications (MSW) to use the network make network resources look like local resources (printer, disk) encapsulate communications parameters (not re-enter printer address) common error handling examples TCP/IP Netware (Novell) -- ipx LAN Manager -- NETBIOS -- NETBEUI Appletalk SNA why so many protocols historical free market competition different strengths (netbios vs tcp/ip) International Standards Oranization (ISO) reference model (diagram): divide the complexity of network software and hardware into layers heterogeneous computers and even networks must connect create a framework within which protocols, media may coexist service isolation, reusability each layer communicates only with the layer immediately above and below it conceptually, each layer communicates with the corresponding layer on the other side (eg, user A thinks she is talking to user B, not to "layer 7"); like telephone: pick up receiver, feel like you are talking to a person lower levels in NIC, higher layers in computer network applications and utilities (diagram) telnet: use local machine as dumb terminal ftp: transfer files file sharing (nfs): make remote files look as if local printer support electronic mail client server: eg, Xwindows wide area networks may be simple modem over telephone lines (if SLIP, not merely telnet) up to 28KB (70 min for CXR) leased lines: 56 KB (30 min) ($300/month) to 1.5 MB (1 min) (T1) microwave versus LAN at 10 MB (10 sec) New Jersey's "fiber to the curbside" policy hope telecommunications can boost state economy hope to boost health, educations, social life but NYS added 13% "modem tax" to dial-up services internet now international wide area network small number of main hubs with fast connections others in geographic vicinity connect to them from original ARPANET now managed by several different entities regional - PSI (like NYNEX) global - NSFnet (like ATT) private networks gov't portion for non-commercial use TCP/IP's IP addresses 128.59.160.100: organization, ring, machine each number 0-255 cucis.cis.columbia.edu: machine, subdomain, organization, type can often use shorthand the two do not necessarily correspond internet resources can use telnet and ftp to distant machines anonymous ftp=files you can get to without a password archie=catalog of anonymous ftp resources on the internet automatically updates its directory gopher=directory plus search through and retrieve information (go for) (telnet columbianet) browse resources not know actual address not know detailed computer command WAIS (Wide Area Information Servers) protocol to retrieve information distributed in databases on internet private libraries follow Z39.50 ANSI standard offers bibliographic, full-text, meterological, astro, ... voluntary=> variable quality WAIS is the interface standard several different client interfaces available (xmosaic on cucis) World Wide Web (WWW) hypertext access to documents hierarchy of documents, able to move through hierarchy by clicking slow Ed Krol, "The Whole Internet: User's Guide and Catalog" CPMC network administration many computers many users who can use what application on what computers too big to define it on each computer tools for administer large list (20,000 users) bootp_give each machine an address (name) yellow pages_define users in one place for many computers kerberos_single server runs security for many machines netID_cross network types security if too strict: doctors cannot treat patients if too lax: confidential information is spilled problem is largely one of policy, not implementation authentication_ logon ID and password (who are you) authorization (who can look at AIDS info) encryption (transfer AIDS info) auditing (eg, check on pregnancy) network does not solve the N(N-1) problem at application level (next lecture) TB grant example ISO NETWORK REFERENCE MODEL: user A - - - - - - - - - - - - - user B layer 7 - - - - - - - - - - - - layer 7 ... ... layer 2 - - - - - - - - - - - - layer 2 layer 1 - - - - - - - - - - - - - layer 1 <<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>> 7. application layer anything not in first 6 eg, distributed systems, client-server, mail, rlogin, user network tools, terminal emulation, FTP, NFS devices: gateway (between disparate networks) 6. presentation layer transform data across architectures ASCII vs EBCDIC, byte order, compression, encryption 5. session layer tools for applications eg, synchronization, simultaneous connections 4. transport layer isolate higher layers from changes in networks connection-less - assume delivery, efficient connection-oriented - assume faulty, reliable 3. network layer efficient routing over sub-networks nodes have addresses 2. link layer putting bits together into "frame", "token" allowing one node to speak at a time devices: bridge (connect similar networks) 1. physical layer transmit raw bits over communications channel issues: baud rate, shielding cables: coaxial, twisted pair, fiber, microwave devices: converter (cable to cable), repeater, modem related reading: Simborg DW. Local area networks: why? what? what if? M.D. Computing 1984;1(4):10-20.