Chapter15 - Chapter 15: N etw or k s D ata communication...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chapter 15: N etw or k s D ata communication netw or k s ar e set up in tw o basic configur ations. T he mesh configur ation uses dir ect, point-to-point connections betw een each pair of communicating devices. While this appr oach guar antees a dir ect connection betw een any pair of devices, the amount of cabling and I /O har dw ar e r equir ed at each device mak es it impr actical in most situations. T he alter native is to establish a br oadcast netw or k in w hich netw or k lines ar e shar ed. Simple configur ations associated w ith the br oadcast appr oach include the r ing and bus topologies, at left and r ight, r espectively. Chapt er 15 N et w or k s Page 1 L ocal Ar ea N etw or k s L AN s ar e pr ivately ow ned netw or k s containing per haps dozens of devices. Example: Ether net Bus Ca bl e Computer Computer Computer Computer Computer Computer Ether net uses a bus configur ation and a pr otocol for accessing the bus called CSM A/CD . CS - Car r ier Sense: Each machine constantly listens to the tr affic that's passing by on the bus. M A - M ultiple Access: Ever y machine has equal access to the communication medium (i.e., the bus). CD - Collision D etection: Each machine is capable of detecting w hether its tr ansmitted message "collided" w ith that of another machine, thus cor r upting the message and for cing a r etr ansmission. Chapt er 15 N et w or k s Page 2 Ether net Car r ier Sense: I ncoming As messages pass by, each machine examines the fir st T r affic few bits of the message, the "addr ess" of the message, and deter mines w hether or not the message is intended for itself. Bus Ca bl e messa ge Computer Computer Computer Computer Computer messa ge Computer When the desti na ti on ma chi ne sees i ts a ddr ess i n the messa ge's pr efi x, i t copi es the pa ssi ng messa ge a s the or i gi na l pr oceeds down the bus. Chapt er 15 N et w or k s Page 3 Ether net Car r ier Sense: Outgoing When a machine w ants to tr ansmit a message, it w aits a T r amount system-dependentaffic of time, using its car r ier sense to see if any tr affic appear s on the bus. I f not, it tr ansmits its message and "hopes" that no collision occur s. Bus Ca bl e Computer Computer Computer Computer Computer messa ge Computer When the tr a nsmi tti ng ma chi ne detects no pa ssi ng tr a ffi c for a cer ta i n ti me i nter va l , i t pl a ces i ts messa ge on the bus. Chapt er 15 N et w or k s Page 4 Ether net Car r ier Sense: Colliding After a machiner affic a message, it continues to T tr ansmits if ther e's any differ ence use its car r ier sense to see betw een its tr ansmitted message and the message on the bus. I f so, it inter pr ets the pr oblem as a collision. Bus Ca bl e Computer Computer Computer Computer Computer Computer When ea ch tr a nsmi tti ng ma chi ne detects tha t i ts messa ge ha s been cor r upted, i t wa i ts a r a ndom i nter va l of ti me, a nd then r etr a nsmi ts. Chapt er 15 N et w or k s Page 5 A L AN Alter native: T ok en Ring Collisions tend to exacer bate tr affic pr oblems on congested Ether net L AN s, so an alter native to the CSM A/CD appr oach is desir able. messa ge Ri ng Computer Computer Computer Computer Computer T OKEN Computer Computer T OKEN Computer Computer Computer Computer messa ge messa ge Computer Computer T ok en Ring uses a r ing topology, w ith a specially for matted "tok en" message per petually tr aver sing the r ing. When a machine w ishes to tr ansmit, it mer ely w aits for the tok en to ar r ive, r emoves it, and tr ansmits its message. When its message r etur ns to the tr ansmitting machine, it's r emoved fr om the r ing, and the machine places the tok en back on the r ing. Chapt er 15 N et w or k s Page 6 Wide Ar ea N etw or k s WAN s ar e lar ge collections of smaller netw or k s, w ith special inter connection devices k now n as "r outer s" to mak e adjacent sub-netw or k s compatible. Router Router Router Router Router Router Chapt er 15 N et w or k s Page 7 Sw itching D ata communications is char acter ized by tw o sw itching technologies. T echnology #1: Cir cuit Sw itching Once the ci r cui t i s esta bl i shed, i t i s ma i nta i ned unti l one of the endsta ti ons ter mi na tes the connecti on. Chapt er 15 N et w or k s Page 8 Pr oblems w ith Cir cuit Sw itching Cir cuit sw itching has one gr eat advantage: once established, the cir cuit is dedicated, i.e., a communication line is completely open until for mally ter minated. H ow ever , this appr oach has a number of ser ious pr oblems: M any netw or k ed applications don't r equir e a dedicated cir cuit, so r eser ving a communication line until an endstation for mally ter minates it can r epr esent a ser ious w aste of r esour ces. T he r oute or iginally selected for the cir cuit may be optimal to begin w ith, but may pr ove to be suboptimal as the communication continues. An entir e, end-to-end path must be found and r eser ved befor e any communication is allow ed betw een the tw o endstations; this is definitely not conducive to many moder n applications (e.g., Web sur fing, videoconfer encing). T r ansmission er r or s ar e pr opagated all the w ay to the destination, r equir ing r etr ansmission acr oss the entir e netw or k . Chapt er 15 N et w or k s Page 9 An Alter native to Cir cuit T echnology #2: Pack et Sw itching Sw itching T he sour ce's messa ge i s br oken i nto ma na gea bl e "pa ckets" tha t a r e tr a nsmi tted to the desti na ti on i ndi vi dua l l y, not necessa r i l y a l ong the sa me pa th. Chapt er 15 N et w or k s Page 10 Pack et sw itching r T he Pritching's emedies of Pack et os and Cons cir cuit sw pr incipal pr oblems: Sw itching L ines ar en't dedicated, so their utilization is higher . M essages ar e "pack etized", so line-shar ing is r easonably fair . Routing may be dynamic, i.e., an alter nate r oute may be chosen w hen tr affic patter ns change. ow entir r oute does itching H T heever ,epack et sw not have does have its ow n set of to be chosen pr oblems: pr ior to sending any data. be pr ogr ammed Sw itches mustpr opagated end- to mak e sophisticated r outing Er r or s ar en't decisions. to-end. Sw itches must manage memor y for queued pack ets that aw ait for w ar ding. Pack ets must be pr efixed w ith contr ol header s, incr easing over head. Endstations must deal w ith missing pack ets and out-of-or der Chapt er 15 N et w or k s Page 11 M ultiplexing T o mor e efficiently utilize a physical medium, multiple higher -level connections might "shar e" the medium simultaneously. F r equency-D ivision M ultiplexing T he spectr um of fr equencies tr ansmittable via the physical medium is divided into sever al channels (e.g., cable T V). T ime-D ivision M ultiplexing Each tr ansmitter is allocated a per iodic time inter val in w hich to tr ansmit. Chapt er 15 N et w or k s Page 12 Code D ivision M ultiplexing Fr equency D i vi si on M ul ti pl exi ng Ever yone gets to ta l k a t the sa me ti me, but onl y a cr oss thei r na r r ow cha nnel s. (Commonl y used wi th copper ca bl es) T i me D i vi si on M ul ti pl exi ng Ever yone gets to ta l k usi ng the enti r e ba ndwi dth, but they ha ve to ta ke tur ns ta l ki ng. (Commonl y user wi th fi ber opti cs) Code D i vi si on M ul ti pl exi ng Ever yone gets to ta l k si mul ta neousl y, usi ng the enti r e ba ndwi dth! T hey do thi s by codi ng thei r tr a nsmi ssi ons i n a uni que fa shi on (a s i f ever y pa i r wer e spea ki ng a di ffer ent l a ngua ge, a nd ea ch other l a ngua ge mer el y sounds l i ke ba ckgr ound noi se). (Commonl y used wi th wi r el ess communi ca ti ons) Chapt er 15 N et w or k s Page 13 M odems D igital data must be modulated into analog signals if it's going to be tr ansmitted acr oss an analog medium. After tr ansmission, it must be demodulated back into its or iginal digital for m. Cables betw een the w or k station and the modem and betw een the modem and the telephone jack D ial-in pool of 96 modems: Six ter minal ser ver s, each connecting 16 modems to a L AN Chapt er 15 N et w or k s Page 14 M odulation and cos( t) D emodulation c m 1(t) m 2(t) X X cos(ct) m 1(t) + si n(ct) m 2(t) M odulation of the digital signals m 1 and m 2 is effected at left via pr oducts w ith tr ig functions. U pon ar r ival, the message is demodulated below via tr ig multiplications, tr ig identities, and highfr equency filter ing. si n(ct) 2cos(ct) X cos(ct) m 1(t) + si n(ct) m 2(t) X m 1(t)+cos(2ct) m 1(t) +si n(2ct) m 2(t) m 2(t)+cos(2ct) m 2(t) +si n(2ct) m 1(t) FI LT ER OU T T H E H I GH FREQU EN CI E S m 1(t) m 2(t) 2si n(ct) Chapt er 15 N et w or k s Page 15 M odem I nter faces T he EI A-232D connector connects a modem to a computer . I t uses a 25-pin connector , w ith each pin having a differ ent meaning. T he meaning of each pin is activated by applying a voltage acr oss it. F or example... Pin 20: D ata T er minal Ready (i.e., "H ey! T he computer 's r eady to send something to the modem!") Chapt er 15 N et w or k s Page 16 Nnetw ort k Pr otocolorLs, they ar e often ayer s I an effor to simplify netw k or ganized as layer ed hier ar chies of pr otocols, w ith har dw ar e-intensive pr otocols on the bottom and user applications at the top. L a yer 5 L a yer 5 L a yer 4 L a yer 4 L a yer 4 L a yer 4 L a yer 3 La yer 3 La La yer 3 L a yer 3 L a yer 2 La yer 2 La yer 2 La yer 2 L a yer 2 La yer 1 Sour ce H ost La yer 1 La yer 1 L a yer 1 L a yer 1 Physi cal M edi um I nter medi a te Low-Level Router Br i dge I nter medi a te D esti na ti on Router H ost Chapt er 15 N et w or k s Page 17 Communicating Via communicate L ayer ed Consecutive netw or k nodes only dir ectly at thePr otocols e) layer ; to low est (har dw ar communicate at higher layer s, netw or k ing softw ar e inser ts cer tain r elevant data as header s and tr ailer s to the message coming fr om the sour ce. La yer 5 messa ge La yer 4 hdr 4 messa ge La yer 4 hdr messa ge 4 La yer 3 hdr 3hdr 4 msgB hdr 3hdr 4 msgA La yer 3 hdr 3 hdr 4 msgA hdr 3 hdr 4 msgB La yer 2 hdr 2 hdr 3 hdr 4 msgBtr l 2 hdr 2 3 hdr hdr 4 msgAtr l 2 La yer 2 hdr 2 hdr 3hdr 4 msgAtr l 2 hdr 2 hdr 3hdr 4 msgBtr l 2 La yer 1 La yer 1 Chapt er 15 N et w or k s Page 18 hdr 1hdr 2hdr 3 hdr 4msgB tr l 2 hdr 1hdr 2hdr 3 hdr 4msgA tr l 2 L ayer ed Pr otocol M odels Sever al models have been developed to implement pr otocol hier ar chies for netw or k s. Refer ence M odel #1: Open Systems I nter connection (OSI ) Chapt er 15 N et w or k s Page 19 Refer ence M odel #2: T r ansmission Contr ol Pr otocol/I nter net Pr otocol (T CP/I P) Chapt er 15 N et w or k s Page 20 T henter netProtocol w as designed for thr ee I P Pr otocol T he I pr imar y pur poses: 1. D efine the data for mat to be used by messages tr avelling thr ough T CP/I P netw or k s. 2. Route the data thr ough the I nter net by selecting appr opr iate paths. 3. Pr ocess pack ets, gener ate er r or messages, and discar d pack ets in such a w ay to ensur e "unr eliable" pack et deliver y. Chapt er 15 N et w or k s Page 21 T he I P H eader Version HdrLen Time To Live Service Type Flags Source IP Address Destination IP Address Options & Padding (if any) Protocol Total Length Fragment Offset Header Checksum Identification Ver sion: T he ver sion of I P used to cr eate the pack et, used by nodes to pr ocess it dr L r ectly. length of the header in 32-bit w or ds (because the Options field H cor en: T he hasvicefixed size).bits to r epr esent the r elative pr ior ity and delay sensitivity Ser no T ype: Six of the L ength: T he length of the entir e pack et in bytes (16-bit field means a T otal pack et. 65,535-byte max). I dentification: All fr agments of the same pack et have the same I D number . F lags: D on't-F r agment flag and M or e-F r agments flag. F r agment Offset: Offset fr om star t of pack et (in bytes) of cur r ent fr agment. T ime T o L ive: L ength of time (in seconds) the pack et may stay in the I nter net. Pr otocol: Global I D # of the pr otocol used to cr eate the pack et (e.g., T CP). H eader Check sum: Er r or -check ing sum of all of the 16-bit values in the header . Sour ce I P Addr ess: 32-bit I P addr ess of the pack et's or iginal sour ce. D estination I P Addr ess: 32-bit I P addr ess of the pack et's final destination. Chapt er 15 Options & Padding: Options include: N o-oper ation-just-align; M ilitar yN et w or k s Page 22 secur ity-application; L oose-sour ce-r outing; Recor d-r oute; Str ict-sour cer outing; Recor d-inter net-timestamps. ITP Addr ee pr incipal classes of I P addr esses, esses her e ar e thr w ith all endstations on the same netw or k given a common pr efix: CL ASS A - for networ ks wi th at l east 216 (65536+) endstati ons: 0 Network ID Host ID CL ASS B -for networ ks wi th between 28 and 216 (256-65535) endstati ons: 10 Network ID Host ID CL ASS C - for networ ks wi th l ess than 28 (0-255) endstati ons: 110 Network ID Host ID Chapt er 15 N et w or k s Page 23 T he D omain N ame System (D N S) A hier ar chical system of domains and subdomains has been established to per mit stations to communicate w ith A station contacts its ser ver , w ho other stations by "name". k now s the location of the r equir ed domain ser ver , w ho k now s the location of the r equir ed subdomain ser ver , etc., until the r equir ed endstation is located, w her eupon its I P Chapt er 15 N et w or k s Page 24 Electr onic M ail U ser Sends M a i l U ser I nter fa ce U ser Rea ds M a i l Outgoi ng Ma i l Spool Ar ea M a i l boxes for I ncomi ng Mail Cl i ent T CP Connecti on (Ba ckgr ound T r a nsfer ) for Outgoi ng M a i l Ser ver (T o Accept Mail) T CP Connecti on for I ncomi ng M a i l T he back gr ound tr ansfer pr ocess sw eeps thr ough the spool ar ea per iodically (typically, tw ice an hour ). Whenever it finds an undeliver ed message or w henever a user deposits new outgoing mail, the back gr ound pr ocess attempts deliver y. Al i a s D a ta ba se U ser Sends Ma i l U ser I nter fa ce U ser Rea ds M a i l Al i a s Expa nsi on & For wa r di ng Ma i l boxes for I ncomi ng M a i l Outgoi ng M a i l Spool Ar ea Cl i ent T CP Connecti on (Ba ckgr ound T r a nsfer ) for Outgoi ng M a i l Ser ver (T o Accept Mail) T CP Connecti on for I ncomi ng M a i l Rather than r esor ting to the D omain N ame System for each outgoing message, and to accommodate locally used aliases for incoming messages, most systems pr ovide mail for w ar ding softw ar e w ith a mail alias Chapt er 15 N et w or k s Page 25 F ile T r ansfer Pr otocol (F T P) Client System contr ol pr ocess Ser ver System One a cti ve T CP connecti on befor e a nd a fter da ta tr a nsfer , just for contr ol . contr ol pr ocess Oper a ti ng System Oper a ti ng System T CP/I P I nter net Client System da ta tr a nsfer contr ol pr ocess Ser ver System T wo a cti ve T CP connecti ons dur i ng da ta tr a nsfer , one for contr ol a nd one for da ta . contr ol pr ocess da ta tr a nsfer Oper a ti ng System Oper a ti ng System T CP/I P I nter net Chapt er 15 N et w or k s Page 26 Foir ew allsity of a pr ivate T ensur e the secur netw or k , "fir ew all" pr ogr ams have been developed. A common appr oach is to filter incoming and outgoing pack ets based upon header infor mation, and to use an application gatew ay to inhibit application-specific tr affic. pa cket pa ck et pa ck et M y Secur e N etwor k pa ck et I ncomi ng pa ckets for ba d a ddr ess/por t combi na ti ons a r e r ejected (e.g., no outsi der ca n "fi nger " a n i nter na l si te). I ncomi ng or outgoi ng pa ckets a r e r ejected on the ba si s of si ze or pa yl oa d i nfo. Outgoi ng pa ckets for ba d a ddr ess/por t combi na ti ons a r e r ejected (e.g., no i nsi der ca n "http" a n exter na l si te). Chapt er 15 N et w or k s Page 27 ...
View Full Document

This note was uploaded on 08/26/2009 for the course CS 111 taught by Professor Klein,s during the Fall '08 term at Southern Illinois University Edwardsville.

Ask a homework question - tutors are online