Hubs, Bridges, and Swit ches * Hubs (mor e) Used f or ext ending - - PDF document

Hubs, Bridges, and Swit ches *

Used f or ext ending LANs in t erms of geographical

coverage, number of nodes, administ rat ion capabilit ies, et c.

Dif f er in regards t o:

• collision domain isolat ion
• layer at which t hey operat e

Dif f erent t han rout ers

• plug and play
• don’t provide opt imal rout ing of I P

packet s

* Kurose and Ross, “Computer Networking”

Hubs

P

hysical Layer devices: essent ially repeat ers

• perat ing at bit levels: repeat received bit s on one

int erf ace t o all ot her int erf aces

Hubs can be arranged in a hierarchy (or multi- tier

design ), wit h a backbone hub at it s t op

Hubs (mor e)

Each connect ed LAN is ref erred t o as a LAN

segment

Hubs do not isolate collision domains: a node may

collide wit h any node residing at any segment in t he LAN

Hub Advant ages:

• Simple, inexpensive device
• Mult i -t ier provides gracef ul degradat ion: port ions of t he

LAN cont inue t o operat e if one of t he hubs malf unct ion

• Ext ends maximum dist ance bet ween node pairs (100m per

Hub)

Hubs (mor e)

Hub Limit at ions:

• 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 segment t hroughput

• I ndividual LAN rest rict ions pose limit s on t he number of

nodes in t he same collision domain (t hus, per Hub); and on t he t ot al allowed geographical coverage

• Cannot connect dif f erent Et hernet t ypes (e.g., 10BaseT

and 100baseT)

Br idges

Link Layer devices: t hey operat e on Et hernet

f rames, examining t he f rame header and select ively f orwarding a f rame base on it s dest inat ion

Bridge isolates collisiondomains 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

Br idges (mor e)

Bridge advant ages:

• I solat es collision domains result ing in higher t ot al max

t hroughput , and does not limit t he number of nodes nor geographical coverage

• Can connect dif f erent t ype Et hernet since it is a st ore

and f orward device

• Transparent : no need f or any change t o host s LAN

adapt ers

Connect ing dif f erent LANs Backbone Bridge I nt erconnect ion Wit hout Backbone

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

Bridge Filt ering

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 hendrop t he f rame else { lookup f ilt ering t able if ent ry f ound f or dest inat ion t henf 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*/ }

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)

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

Spanning Tree Bridges

To prevent mult iple f looding bridges need t o

communicat e t o produce a spanning t ree

The Spanning Tree Prot ocol (P

erlman, 1992)

Choose a root bridge Const ruct a t ree of short est pat hs (Dij kst ra’s alg.) 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 Tr ee example The result ing t r ee 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

Rout ers maint ain rout ing t ables and implement rout ing

algorit hms, bridges maint ain f ilt ering t ables and implement f ilt ering, learning and spanning t ree algorit hms

Rout ers vs. Bridges

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

Rout ers + and -

+ 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 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

• f host s)

Et her net Swit ches

A swit ch is a device t hat incorporat es bridge

f unct ions as well as point-to-point ‘dedicat ed connect ions’

A host at t ached t o a swit ch via a dedicat ed point-

to-point connect ion; will always sense t he medium as idle; no collisions ever!

Et hernet Swit ches provide a combinat ions of

shared/ dedicat ed, 10/ 100/ 1000 Mbps connect ions

Et her net

Some E-net swit ches support cut-t hrough

swit ching: f rame f orwarded immediat ely t o dest inat ion wit hout await ing f or assembly of t he ent ire f rame in t he swit ch buf f er; slight reduct ion in lat ency

Et hernet swit ches vary in size, wit h t he largest

• nes incorporat ing a high bandwidt h

int erconnect ion net work

Et hernet Swit ches (more)

Dedicated Shared

I EEE 802.11 Wireless LAN

Wireless LANs are becoming popular f or mobile

I nt ernet access

Applicat ions: nomadic I nt ernet access, port able

comput ing, ad hoc net working (mult ihopping)

I EEE 802.11 st andards def ines MAC prot ocol;

unlicensed f requency spect rum bands: 900Mhz, 2.4Ghz

Basic Service Sets +

Access P

• ints =>

Distribution System

Like a bridged LAN

(f lat MAC address)

Ad Hoc Net wor ks

I EEE 802.11 st at ions can dynamically f orm a group

wit hout AP

Ad Hoc Net work: no pre-exist ing inf rast ruct ure Applicat ions: “lapt op” meet ing in conf erence room,

car, airport ; int erconnect ion of “personal” devices (see bluet oot h.com); bat t elf ield; pervasive comput ing (smart spaces)

I ETF MANET

(Mobile Ad hoc Net works) working group

I EEE 802.11 MAC Pr ot ocol

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)

• t ransmit f rame (no Collision Det ect ion)
• receiver ret urns ACK af t er SI FS (Short I nt er

Frame Space)

• if channel sensed busy

t hen binary backof f NAV: Net work Allocat ion Vect or (min t ime of def erral)

Hidden Terminal ef f ect

CSMA inef f icient in presence of hidden t erminals Hidden t erminals: A and B cannot hear each ot her

because of obst acles or signal at t enuat ion; so, t heir packet s collide at B

Solut ion? CSMA/ CA CA = Collision Avoidance

Collision Avoidance: RTS-CTS exchange

• 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

• RTS and CTS are very short : collisions during dat a phase

are t hus very unlikely (t he end result is similar t o Collision Det ect ion)

• Not e: I EEE 802.11

allows CSMA, CSMA/ CA and “polling” f rom AP

Point t o Point prot ocol (PPP)

P

• int t o point , wired dat a link easier t o manage

t han broadcast link: no Media Access Cont rol

Several Dat a Link P

rot ocols: P P P , HDLC, SDLC, Alt ernat ing Bit prot ocol, et c

P

P P (P

• int t o P
• int P

rot ocol) is very popular: used 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 Link prot ocol f amily) and very st reamlined

PPP Requirement s

Pkt f raming: encapsulat ion of packet s bit t ransparency: must carry any bit pat t ern in t he

dat a f ield

error det ect ion (no correct ion) mult iple net work layer prot ocols 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

Not Provided by PPP

error correct ion/ recovery f low cont rol sequencing mult ipoint links (e.g., polling)

P P P Dat a Frame

Flag: delimit er (f raming) Address: does not hing (only one opt ion) Cont rol: does not hing; in t he f ut ure possible

mult iple cont rol f ields

P

rot ocol: upper layer t o which f rame must be delivered (eg, PPP-LCP , I P , I P CP , et c)

Byt e St uf f ing

For “dat a t ransparency”, t he dat a f ield must be

allowed t o include t he pat t ern < 01111110> ; ie, t his must not be int erpret ed as a f lag

t o alert t he receiver, t he t ransmit t er “st uf f s” an

ext ra < 01111110> byt e af t er each < 01111110> dat a byt e

t he receiver discards each 01111110 f ollowed by

anot her 01111110, and cont inues dat a recept ion

b5 b4 01111110 01111110 b2 b1

PPP Dat a Cont rol Prot ocol

P

P P

• LCP

est ablishes/ releases t he P P P connect ion; negot iat es opt ions

St art s in DEAD st at e Opt ions: max f rame lengt h; aut hent icat ion prot ocol Once P

P P link est ablished, I P CP (Cont rol P rot ocol) moves in (on t op of P P P ) t o conf igure I P net work addresses et c.

An hour long