Hubs, Bridges, and Swit ches * Hubs (mor e) � Used f or ext ending LANs in t erms of geographical � Hub Limit at ions: coverage, number of nodes, administ rat ion o Single collision domain result s in no increase in max t hroughput ; t he mult i -t ier t hroughput same as t he single capabilit ies, et c. segment t hroughput � Dif f er in regards t o: o collision domain isolat ion o I ndividual LAN rest rict ions pose limit s on t he number of o layer at which t hey operat e nodes in t he same collision domain (t hus, per Hub); and on t he t ot al allowed geographical coverage � Dif f erent t han rout ers o Cannot connect dif f erent Et hernet t ypes (e.g., 10BaseT o plug and play and 100baseT) o don’t provide opt imal rout ing of I P packet s * Kurose and Ross, “Computer Networking” Hubs Br idges � P hysical Layer devices: essent ially repeat ers � Link Layer devices : t hey operat e on Et hernet operat ing at bit levels: repeat received bit s on one f rames, examining t he f rame header and int erf ace t o all ot her int erf aces select ively f orwarding a f rame base on it s dest inat ion � Hubs can be arranged in a hierarchy (or multi- tier design ), wit h a backbone hub at it s t op � Bridge isolates collision domains since it buf f ers f rames � When a f rame is t o be f orwarded on a segment , t he bridge uses CSMA/ CD t o access t he segment and t ransmit Hubs (mor e) Br idges (mor e) � Each connect ed LAN is ref erred t o as a LAN � Bridge advant ages: segment o I solat es collision domains result ing in higher t ot al max � Hubs do not isolate collision domains: a node may t hroughput , and does not limit t he number of nodes nor collide wit h any node residing at any segment in geographical coverage t he LAN o Can connect dif f erent t ype Et hernet since it is a st ore and f orward device � Hub Advant ages: o Simple, inexpensive device o Transparent : no need f or any change t o host s LAN o Mult i -t ier provides gracef ul degradat ion: port ions of t he adapt ers LAN cont inue t o operat e if one of t he hubs malf unct ion o Ext ends maximum dist ance bet ween node pairs (100m per Hub)
Bridge Filt ering Connect ing dif f erent LANs � Bridges learn which host s can be reached t hrough which int erf aces and maint ain f ilt ering t ables � A f ilt ering t able ent ry: (Node LAN Address, Bridge I nt erf ace, Time St amp) � Filt ering procedure: if dest inat ion is on LAN on which f rame was received t hen drop t he f rame else { lookup f ilt ering t able if ent ry f ound f or dest inat ion t hen f orward t he f rame on int erf ace indicat ed; else f lood; / * f orward on all but t he int erf ace on which t he f rame arrived*/ } Backbone Bridge Bridge Learning � When a f rame is received, t he bridge “learns” f rom t he source address and updat es it s f ilt ering t able (Node LAN Address, Bridge I nt erf ace, Time St amp) � St ale ent ries in t he Filt ering Table are dropped (TTL can be 60 minut es) I nt erconnect ion Wit hout Backbone Bridges Spanning Tree � For increased reliabilit y, it is desirable t o have redundant , alt ernat e pat hs f rom a source t o a dest inat ion � Wit h mult iple simult aneous pat hs however, cycles result on which bridges may mult iply and f orward a f rame f orever � Solut ion is organizing t he set of bridges in a spanning t ree by disabling a subset of t he int erf aces in t he bridges: Disabled � Not recommended f or t wo reasons: - Single point of f ailure at Comput er Science hub - All t raf f ic bet ween EE and SE must pat h over CS segment
The Spanning Tree Bridges result ing � To prevent mult iple f looding bridges need t o t r ee communicat e t o produce a spanning t ree The Spanning Tree Prot ocol (P erlman, 1992) Bridges vs. Rout ers � Bot h are st ore-and-f orward devices, but Rout ers are Net work Layer devices (examine net work layer headers) and Bridges are Link Layer devices � Choose a root bridge � Rout ers maint ain rout ing t ables and implement rout ing � Const ruct a t ree of short est pat hs (Dij kst ra’s alg.) algorit hms, bridges maint ain f ilt ering t ables and implement f ilt ering, learning and spanning t ree algorit hms � Block bridges t hat are not on t he spanning t ree � I f a bridge or LAN f ails, re-comput e t he spanning t ree Spanning Rout ers vs. Bridges Tr ee example � Bridges + and - + Bridge operat ion is simpler requiring less processing bandwidt h - Topologies are rest rict ed wit h bridges: a spanning t ree must be built t o avoid cycles - Bridges do not of f er prot ect ion f rom broadcast st orms (endless broadcast ing by a host will be f orwarded by a bridge)
Rout ers vs. Bridges Et hernet Swit ches (more) � Rout ers + and - Dedicated + Arbit rary t opologies can be support ed, cycling is limit ed by TTL count ers (and good rout ing prots) + P rovide f irewall prot ect ion against broadcast Shared st orms - Require I P address conf igurat ion (not plug and play) - Require higher processing bandwidt h � Bridges do well in small (f ew hundred host s) while rout ers are required in large net works (t housands of host s) Et her net Swit ches I EEE 802.11 Wireless LAN � A swit ch is a device t hat incorporat es bridge � Wireless LANs are becoming popular f or mobile f unct ions as well as point-to-point ‘dedicat ed I nt ernet access connect ions’ � Applicat ions: nomadic I nt ernet access, port able � A host at t ached t o a swit ch via a dedicat ed point- comput ing, ad hoc net working (mult ihopping) to-point connect ion; will always sense t he medium � I EEE 802.11 st andards def ines MAC prot ocol; as idle; no collisions ever! unlicensed f requency spect rum bands: 900Mhz, � Et hernet Swit ches provide a combinat ions of 2.4Ghz shared/ dedicat ed, 10/ 100/ 1000 Mbps connect ions � Basic Service Sets + Access P oints => Distribution System � Like a bridged LAN (f lat MAC address) Et her net Ad Hoc Net wor ks � Some E-net swit ches support cut-t hrough � I EEE 802.11 st at ions can dynamically f orm a group swit ching: f rame f orwarded immediat ely t o wit hout AP dest inat ion wit hout await ing f or assembly of t he � Ad Hoc Net work: no pre-exist ing inf rast ruct ure ent ire f rame in t he swit ch buf f er; slight reduct ion � Applicat ions: “lapt op” meet ing in conf erence room, in lat ency car, airport ; int erconnect ion of “personal” devices (see bluet oot h.com); bat t elf ield; pervasive � Et hernet swit ches vary in size, wit h t he largest comput ing (smart spaces) ones incorporat ing a high bandwidt h � I ETF MANET int erconnect ion net work (Mobile Ad hoc Net works) working group
I EEE 802.11 MAC Pr ot ocol Point t o Point prot ocol (PPP) CSMA (carrier-sense mult iple access) P rot ocol: - sense channel idle f or DI SF sec (Dist ribut ed I nt er Frame Space) � P oint t o point , wired dat a link easier t o manage - t ransmit f rame (no Collision Det ect ion) t han broadcast link: no Media Access Cont rol - receiver ret urns ACK af t er SI FS (Short I nt er � Several Dat a Link P rot ocols: P P P , HDLC, SDLC, Frame Space) Alt ernat ing Bit prot ocol, et c -if channel sensed busy � P P P (P oint t o P oint P rot ocol) is very popular: used t hen binary backof f in dial up connect ion bet ween resident ial Host and I SP ; on SONET/ SDH connect ions, et c � P P P is ext remely simple (t he simplest in t he Dat a NAV : Net work Allocat ion Link prot ocol f amily) and very st reamlined Vect or (min t ime of def erral) Hidden Terminal ef f ect PPP Requirement s � CSMA inef f icient in presence of hidden t erminals � Pkt f raming: encapsulat ion of packet s � Hidden t erminals: A and B cannot hear each ot her � bit t ransparency: must carry any bit pat t ern in t he because of obst acles or signal at t enuat ion; so, dat a f ield t heir packet s collide at B � error det ect ion (no correct ion) � Solut ion? CSMA/ CA � mult iple net work layer prot ocols � CA = Collision Avoidance � connect ion liveness � Net work Layer Address negot iat ion: Host s/ nodes across t he link must learn/ conf igure each ot her’s net work address Collision Avoidance: RTS-CTS exchange Not Provided by PPP • CTS “f reezes” st at ions wit hin range of receiver (but possibly hidden f rom t ransmit t er); t his prevent s collisions by hidden st at ion during dat a � error correct ion/ recovery • RTS and CTS are very short : collisions during dat a phase � f low cont rol are t hus very unlikely (t he end result is similar t o Collision Det ect ion) � sequencing � mult ipoint links (e.g., polling) •Not e: I EEE 802.11 allows CSMA, CSMA/ CA and “ polling ” f rom AP
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