© Copy Right: Rai University 11.676. 5 19 N T W O M A N A T LESSON 6: NETWORK TOPOLOGY Lesson Objectives •I ntrodu ction to Topology •Major Types of Topologies: Physical & logical topology Phys i calTopo logiesi • Point-to-Point Topolog y ii. • Multipoint Topologyiii. • Sta r Topology iv. • Ring Topology v. • Bus Topologyvi. • Hybrid Topology Introduction to Topology Th e w or dt opo l og y is commonly used to discuss the properties of va rious typ es of ne tworks. Topology is the branc h of ma thema tics that examines the cha racteristics of ge ometric shape s. Networks have sha pes, and the sha pe a network t ake s has much to do with the way it functions. Tw o d iffe rent d e fin it io ns o f t o p o lo g y areas fo llo w s: physical an d l og i c al . Y ou enc ounter the distinction betwe en phys i ca l and logica l freque ntly in networking. The disti nction is ne ces sa ry beca use the way something looks can be quite different from the way it functions. A m otion picture m a y look like a na tural moving sce ne , but it ac tuallyconsistsof ma nysna pshotsfl as hedonthescre ens oqui ckly that your ey e interprets the individual pictures a s m oving . T he logica l, m oving pi cture is m uch differe nt from the phys ical rea lity. So it is with compute r networks. Y our eye ma y te ll you one thing as you exa mine the cabli ng, but the networ k ma y operate very differe ntly a t the invisible level of elec trica l si gn a l s in the wire. Major Types of Topologies NOTE: The following rule of thumb distinguishes physical from logical: I f you c a n se e it and touc h it, it is phy sica l. I f you ca nnot se e it or touch it, it is logica l. The p hys i cal topology of a network is the layout or actual appe aranceof the cabling sche meused on a network. Thelog icaltopo logy of a network descri bes how the da ta flows through the ph ys ical topology . Y ou soon discover that sim il a r- looking networks can have quite different logical topologies. A network’s l ogical topology i s closely relate d to the m echa ni sm use d t o ma nag e the way stations acces s the network. Becaus e a given ca bl e in a base band ne twork servi ces only one station at a time , it is nece ssa ry to have procedures in place that ma nag e network acc es s so that a l l sta tions ha ve access w i thout confl icting with ea ch ot her. Thes e procedures a re ca l l edacc essco nt rol me thods . 1. Physical Topologies All physical topologies are variations of two fundamental me thods of connec ting de vi ce s: point-to-point and multipoint . After exa mining the se two funda me ntal topologies, se vera l variati ons a re e xam i ned. 1. Point-to -Point Topo log y Point-to-point (PTP) topology conne cts two node s directly toge ther. Thefollowingexa mp lesarep urepo i nt-to-pointlinks: • Tw o com p u t ersc o mmun ic a t in g v ia mo d ems • A mainframe terminal communicating with a front-end processor • A workstation comm unicatinga l onga pa rallelca bleto aprinter. I nap oint-to-point link,twode vices m onopolizea com m unica tion me dium. Becaus e the me dium is not shared, a m echa nism is not nee de d to ide ntify the c ompu ters. The refore, a s im ple, two-dev ice point-to-point ne twork has no nee d for addre ss i ng . • Point-to-point links can be simpl e x , half-duplex , or full-duplex . When de vi ces mu st eng ag e in bi- di rectional commu ni cation on a half-duplex link, some turnaround me cha nism s m ust be i npla cetoswitchtheroles of these ndingandrec eivingdevices . 2. Multipoint Topolog y Multipoint topologies link three or more devices together through a single communication medium. Multipoint topol ogies w ork m uch like a party-li ne telephone s ervice whe re seve ral subscr i bers a re connected to the same tel ephone line. Becaus e mu l tipoint topologi es sha re a comm onchannel , each de vi ce nee ds a wa y to i dentify itself and the de vi ce to which it wants to se nd informa tion. The m ethod use d to identify se nde rs and rece ivers is ca lledaddre ss i ng . Thefoll ow i ngfourtype sofphysic altopologi esa refreq uen tly used in computer ne tworki ng: Star Bu s Ring Hybrid
