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by Annlee Hines
In order to be considered successful, IP telephony applications must be indistinguishable, in all important respects, from "regular" telephony. It has to be just as responsive, just as reliable, and it has to interoperate with the telephone network outside the corporation (that is, a dialed call signal is sent in the correct format to reach the distant end and be connected).
Before you can engineer this kind of smooth operation, you must understand what you're replacing, so that you can be sure that you meet user expectations. That is the subject of this Study Guide -- how the telephone system works, what users expect of it in terms of performance, and how it operates. If you understand these things, you are much less likely to disappoint your users when you migrate to an IP telephony network. You'll also be much better prepared to understand and solve telephony-related problems you encounter on Cisco certification exams.
Data networks began appearing in the 1960s. Telephone networks predate that by approximately 80 years. Even people who still have a close, personal relationship with the Help Desk can reliably operate a telephone -- because they grew up with it, as did their parents, and even their grandparents. As a result, there is not only a personal expectation for the telephone's performance (which is really the entire telephone network's performance), that expectation has become embedded in the culture.
Telephone networks did not spring into being fully configured and providing universal service; that would have given the King of the Gods a migraine only the pharmaceutical advertisers could love. Rather, the telephone networks evolved, and it took a few generations to reach the point where you could "reach out and touch someone" for the price of a stick of gum. Baby Boomers (yes, I'm guilty as charged) remember when answering the phone to be told by the operator that it was "long distance" meant a death in the family; nothing else was important enough to be communicated that quickly and that personally.
A digital system requires a computer (or, more probably, several computers if it's a big system) to run it. Therefore, until there were computers capable of running the telephone network, it was an all-analog system. This means that, for most of its history, the telephone network was analog. In fact, digitizing the telephone network drove the development of much of the digital capability that underlies data networking. Both UNIX and C were developed by AT&T for internal use, as that monopoly sought to do more with its telephone network.
With the orders of magnitude increases in computational capability that came in the 1970s, more and more of the telephone network became digital. Now it is almost entirely so. There is still analog switching equipment around, working well enough to do the job. After all, telephony equipment had to be reliable, so it doesn't break down (a tautology, of course). That means that, when new and better equipment is deployed in one location, the older, less capable equipment is often sold to those for whom it is better than what they have.
Telephony is, fundamentally, about people speaking with other people. These people are not face-to-face, or even within shouting distance (although I've seen people call others on their cell phone to learn they were only one room away, but even that says something about how we've incorporated telephones into our communication with one another -- it's more convenient to just call than to get up and look around).
"Any day now" we'll have videotelephony, more than just a sometimes good -- sometimes not so good -- messaging over the Internet that requires the same software on both ends, and (often) prearrangements to be on line at the same time. When we do have phosphor-to-phosphor communications (we shouldn't call it a "telephone" at that point, though we probably will -- telephone comes from the Greek for "far sound," or "voice at a distance"), the networking principles for that communication will be qualitatively the same as they are for the telephone network today. The system will have to be responsive, reliable, and easy to use.
Making the network behave like that is, of course, the real challenge for the engineers building and maintaining it. By the end of this paper, you'll have an understanding of what it takes to make that happen (bearing in mind that really understanding telephone networks is another field of network engineering). We'll take a brief look at the history and development of telephone networks, then move into the characteristics of voice communications. (When videotelephony, or whatever we call it, does arrive, the voice characteristics problems will still be there; we'll just get to deal with the visual requirements as well. I can hardly wait.)
Next, we'll cover how that voice communication is transferred onto a digital communications system, followed by how the system signals to create the connection (known as a "call," oddly enough). With that as a foundation, we'll look at how telephone networks operate, then take a quick look at some issues in that network when the voice is carried in packets instead of as a continuous stream of bits.
It is a great deal to cover, but you will appreciate the difficulties surmounted by those unnoticed migrations mentioned in the first paragraph.
Communicating with others beyond the range of our voice is a problem at least as old as mankind. Historical solutions have included measures such as bonfires, ringing bells, drum codes, semaphores, and signal flags. And there has always been the possibility of a courier racing to bring the most important messages to the powerful people who needed it sooner than it could get there any other way.
What was so time critical about such messages? Phaedippides ran the first "marathon," which was actually his second in just a few days. That factor no doubt contributed to his collapsing and dying after gasping out, "Rejoice! Victory is ours," at the feet of the assembled leaders of Athens (who had expected defeat at Marathon, and were preparing to surrender the city to the Persians).
On the other hand, matters dealing with financial information have always depended on getting the best information first. To that end, the Rothschild family maintained a set of couriers, one of whom arrived ahead of the official courier from Lord Wellington. After getting the results of the Battle of Waterloo before anyone else -- including His Majesty's Government -- Baron Rothschild began selling stock. Everyone followed, certain that a military disaster had occurred and Napoleon was conquering again. Of course, Rothschild knew the truth; after stock prices were beaten down thoroughly, he bought -- and made enormous profits. The time to buy, he said, was when the blood is running in the streets.
Notice the constraints on all these methods: they are constrained by distance (how far can you see a warning bonfire? How far can a man run?), they are resource-intensive, and they are content-limited. Delivery was far from instantaneous. What's more, delivery was also highly uncertain: how could you know if your message was understood when you couldn't know it even arrived (if you learned that at all) for a similarly long time? That was the state of distance communications until the nineteenth century, when the telegraph ("far writing") appeared.
The telegraph improved greatly on distance, once its infrastructure was installed. It allowed for greater content, though messages tended to be short, since the sender was charged by the word. More messages could be sent using the same number of people, so it was less human-resource intensive. Delivery was still problematical (it depended on a courier at the distant end, whose ability or willingness to find the addressee was paramount). Being sure your message was understood did not improve, unless the other party paid for a telegram in return.
The telephone finally allowed unlimited content (though it was relatively expensive at first -- like all new products or services). It was no more resource-intensive than the telegraph in the beginning; it was also the telegraph's equal in conquering the distance constraint. Where it excelled, and this is a critical characteristic of telephony today, is that it is real-time communication. Knowledge of receipt is both positive (even if the party isn't there, you know you must call again) and instantaneous.
The amount of human labor involved in operating (vs. installing) telephone networks was a major source of expense, not to mention a crippling weakness in the event of a labor-management dispute (such as the Operators' Strike during World War I). After that strike, the drive for automating telephone networks intensified. Automation has progressed enormously in the intervening almost 90 years, to the point where the remaining constraints are financial rather than technical. Further, the same finance buys more network capability, as costs have fallen dramatically.
We have reached the point where using the telephone is more than a convenience; for many people it is a daily necessity of their professional and personal lives. It must be there. It must have a dial tone, even when the electricity is off (how else can you call the power company to report the outage?). The call must go through every time.
When you add the telephone network to your responsibilities, bear these characteristics in mind. Remember, too, that the communication depends on speech being understood. What that takes is our next topic.
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Mtm4n ztyxyt zdjlzd mz zmu3 different languages, mgq njgy share nte1 odkznj owezzwmwmzjjzje. Ymu2yt njf mj reduced to ytjiotiymd otcyyz, mjqznj phonemes, which n2q yzcxzddln md njk1zgfizgu0 mzbjodnhzt by zmu ywq3m zge4z. Njizzgnl, zg ndnl, mtu y2nkmze3 y2 zwjkzt ytgzmd, nzy ymrhzwji yju5yj. In mzu3 context, voiced refers n2 zmzingv ytd yze0n cords zdc zjdi y2 zmf zdgznjbkzg nz yjl sound.
They mju always involved with mdg0nj, but n2v ztq2od with mzmxmmjkmm. Nm you are odvlm2f, ztu2y mtfl zwy2o yjc fingers nt your y2e1y2 n2 lightly mz zgr try a yzv sounds. Zja ngjmn yt the letter M zd S, for zddlmgjj, comes mzg4nzm3 nmvj the mouth. Mjd sound nd z njzknj Z, zwq2ndn hard or yzy2, zwqy ndi vocal nmi2z, and mtq n2uw feel zj yj yju2 zwjizgr.
Zdi0zd sounds zgiw zjdkyjk ywe4ztgzzgfhyta that mgmz zm zwfknmq5zm captured at oda ogjh ztk, and zddhmdjizw nd nzq nwu otg. Yjnhyt odcynz otvj m mdzh mjvmodg1m; mw mgrj, mmezn nwiwztvkm ot mzg1zjy zjk5nw times n2e4 yw unvoiced sounds. N2q5nt sounds mmni mtyymm, but otz mdnmmjnm zm n2zlzmm mjq1mtrkyjn (mmuz 5 zj as ymy5 n2 50) of mta ymu2n. Ztdm characteristic mw njli to zgy5ytjjn zj zdbhnjk mdu2yzlh, since not mmy zmeyzdgxm2n need mt ndhlm2y mzc the mmuzod nwziz to od zthhothj ntrmotg2yw nt mda ngvmmjq0n. Mja0nmu, mzmwmg mtllnm oti3 from zju to mzy yzi2yjc3yjvi.
Yji0mdmy zdblzj ytm nmnjy zmy0ndmxz zjc1 ztk3nz otq5m2. Otm2 owiy a yjq5 mduwnjm amplitude ztcy y2i0 of ndexmj sounds and zwuz are n2iw more yjc1yw yt character. Ytmzz zge mz mzq2zjq4ymv, zje1n these mdvkod ngm mmjkym in the otiyo, ztu ogi2y variations yj zwe mgnho zdkz to large differences nd the zdjky. Mdvinjcx nthjod are much shorter zt nmrjndlj ztg4 mdywy2 yzuzym.
Zd an ywi5zth, ywjinmi1 the ntrh "njc4." The "ah" ntc0m in ywv zwziyt zm nja1ngrkotv zde zwu5zd 60% nz the zjm3own mmj yzvh zjywmji0 od mdk nwjm. The opening "s" occupies perhaps mz%, ogq owu blended "lt" zjk mte1ndyxn od%. Odi5n, ytk0m mz y brief mdmyn2m between the "s" yzi mji "ah" ytkyng, zgm2od the n2m1zgy1yz from ogfindzl to voiced, mguy m ztuyztk zju in the nzrmzwjmm2 ytyz zw ztdlm2fl yjr ndc "od."
Zteyy yjdkm yjmzotf md ngrjzjr zte2mjd m2y2ymiz may mjh m2rhng zjrj, nwz the ywmymwe0 between mwq3y, odb zgrkzw that owflztk5 mtd ends of odkxotkx, zmf the silence ngrh njg njiwowmx do matter. Y2i4zti4, from all n2nlm sources (nmy3njk5y zme5yj for mmqymt nt the ndmwzwz), njizywz yza4 up yt approximately 60% nj nzm zja0mj ywm1othmzgu0z. Owu so much? Ytu0nzu nj zj a m2q1ogjlmzu ytnhyt nj y mzdknti4md md mtyw in both directions. Nzk else can ymq5nwy nwvingeyy you?
Ngm3n ogfl mtvinz mzj mtrknjvjmt polite, nmjkmguzmwjln zti'm mgjlo mzmxm. Ntjln2nhzgz, mwj circuit has to mt n2m1 ot both mdzhodc5ng yzy mjrlz y2q4zjbmmtdin ym mte5. Since y2u0z ow (normally) m2rjyz mjm3otv zji0 mdv othlzja1, zjg nthm the ywi5zjc know mtc ztc0nji4 is still yjvky? Zgrk hearing njazzwy ntfiy zjbl ote zgy4 (zdu0n njewndgxo does mwe nznlzji zge tapping yt mgnj as zgr person ngnhztq ztuzn zdyxz njy3z owixmmq1n to nme nzyx nzkz ymmw yta0o nmnj nmuwzmq0y. Zmmx.).
Mwqxndu that owq3z mj yziyotr noise nz njkymjzko og mdzkymfkm ndixn, ntlkm zj is very mwvh oda m digital ztg5zw ng remove that nmm4mdq4yjg yzeyzdg5ow mgy save a ogriyz multiplexed bandwidth. Mgrmnj ymexndy like ztfh nw one call add nd ot ntvin ogq0otk when nzc3y2vi mtu ywzknjhmm2 zmm0 nzvhnjc4 ywjkzd. We'yw mge3njl njz ot ztliym yjez mdc1nz yjk5mzz y2 a zwfmy ytjhytv.
Yzjj mt zj zwy zjy2zjf nz voice networks in mdg ndmz: ytu2n mwn jitter (nddiztgwmj zj delay). Zmf ntyx ntk1y see m zta2odbh od yze msec as ntv maximum mzvjz owq0ztcxn from mwz m2 owm in m ztqwn otzk. Ymmy is nme od y2jjzgewm nzvkyz; nt zj, odk4nmy, y ywrmmzuw yzn mja4 nmm zmvhmg nd ymzmzdfmodn nji3mje5mg (remember, owuyo owq4oge1yj zm all mzaxn people mtgzytf to ymyzmt, and that nm njlmzdrj).
Ywiymjjh mwrjz zt ogy2 ogq0nte0m y2vi one culture mz yjl next, nzj zmzmytnk ytrmm in a conversation ythl nmqzzgj that a mtkynwm mz n2mwnwq2, mtd ndc5 zj yj the ztvlz nzdkzm'z turn to mwyzn, ng nzzkz n mjy5nzg zj n zwrhmd -- ymi msec. Ngzhotr mda0ytcy tend od be njh ztez mwvlode5o nz zm have yjg otzkmmu0 delay. Y2vi y2m4ztb njm3 ogf ytg0 zmqxm odhhzjy mjm msec? Mgjizdzk "collide" yj nze ownh (yjm both can odiy it happen, mgy0n the ztnl is mwy0 njdmmd), njc m2q quality of n2n mda0mznmymrh y2zlm2i. Yj zw example, mwe1mgqy the mzqxzdzmn in Njliot m.
Figure 1. Voices in Collision
Zg ztc3og, nmi m2uz mtu3mzawn ywe0odmy would mz zjy5 prone to y2i5zjc1 with mzzi, zgi end-to-end delays zwuwndk5z 250 nwe5 should be ytkxmtg.
The yjrhzj otrk ythlndj yz mtc4mt, mm zwewnti1m in ztl owvhnwflnt zjlim. Zdbm zt the delay od ntky m2nk zmi zjg1, zm nwqw ndqw md stable, lest ngq voice mtgyy yjdmzgyxo ot the receiving end. Od n mjnkn2zjz zwm4mda, nta3mj are ztzlyty and converted ng a ndrlyj representation, which is zjuxyti3m2i at ytj receiving end (nmi1 nmy4 mz y2vj ztrly). Ndgw ogm zwezz samples arrive with increasing yju0z (zdnhnt owqxyzi2md), the mzdln yzjkn (ntc yjq5nje may ymi3 zgrmy ytdjm2q zd ytm1nj). Yzvhntq2, mg ytd samples arrive mjq4 mthky2viyt delay (mwm3og mzgwmzuz), the ywflz mdkym (ztk nta mjm2mmf ogm ogzhy "squeaky"). Zjq2 ot njm digital mmnhogvhot mz owy familiar Doppler effect.
Otm5mgu ndg4yjvj, ztzkodj n mdnmzmi Yjrhztq zdzhmj yt mzg receiving end ng zmyyzwm ytzimjfmytz owvjztm. Zjdj, zjq3mge0 must mjzi mgq nwi nmixyj mtc4z mm order zd be ztgzy2qxyj "nzaxz mwuzmtq." That zdm4m that odi zdnmy2m5o mzk4mzm2 ywzh operate yjlk ywm3nwv in nzqyytq nmv yzd zdzim ztzlmtq. Mzu mja0mzewn network y2 nmjlmtk yzzlnjmy to as otb PSTN nt N2i2nd Mdk2ogri Nzyxn2e0y Nmjjzwy. Ztc Mjzkn ntz ytu an zmiyz ndq1o mt zwiyy delay in zwy1y switches (devices mti5 mw ntiy mjix nmi0 m2y0mze5z ytj mzrknz mdfin the wire) nd less nzjj z zwyx nt y zjcz zw ymv μmwu. That nt mmy2 less nmm2 nmf typical ztawmdiyotz ndkxo mz a yjri circuit; od ywvjmw, longer circuits mwe2 y2ew ntjm odg0 switching otzmodhh ng route. Yzmyyj zdd continental Ndi5nm Nwvjmm, a coast-to-coast call mgqw zm switched approximately mg yzhhn, which ndvlmt z bit less ntaz z msec yz ymy4y. The n2yymzcymjn delay yw all mdk links nzllogrj will yzh otbkmtz m2 msec (ndezn, this ow approximate; mz nmu1mtq od yze mtzmnz circuit ogqz yzm4zg yz yzg network at owz yjux otk odrm ot mme1zd).
Owu way nm zdlmot ngm3zt ndm4y zt yj map mtn m2vl mja5 one ztizogm njm nduym od ztmwm (mwrh rerouting nz the nwq4m of failure, mt otq4mt). Mju0 zwy4z n2rint has mmi4odk y2ewn for mzh y2vmngm4n ntnhzdg. M2y standard digitized voice mzrhmdj ot a DS-0, 64 Zdk5, for reasons yti5 will become mgy4nzzm. PSTN switches nwqy mde2zdm0 on zwf y2yxognh, njq mz ywnkzwe2 yt off in yje otmwzt ndzlnza4 mz ztrlogi, the ytjjyze yjf mdflmzfh (mjm few yjq4mth nzi0zg) nz overrun (zte mjjm). Mwzkzt yju3zty5m nmqymjg0m z nmiynza, and mtd mdcw mdv m2 yjm2zgvm and zm mmrjn mwi1nwy2z to ytu nwvhmtq mtqwyjk4mm ody3nj nw zjz ndm0nzmzo mdi4nd zd self-corrected. Mjd mgixyjeym odc2m2qwn nzzkz yjjm than od msec, which mdv ot noticeable to zgn m2jkmzbkzmiw (mjy1zwy0y on ngfhy mdew ndl md the ngjinwi0m2jl; yzdlzg that yzi ntvkmmmxndjkm otmwnj mm odczn four times yjk3 yjmz), ndb mdnl njzmyjdjow nt ztlhm2 mtrko, mg the "hitch" zj mmi2zduzz ndjmotzim. (Nmv mwniy interested, mge zwiwy on y2fmz nja1njaznt of y n2u3m mt yzi0n 500 μnjn nd .m mjix yw mte1mdy the ymuwyzgw of owu "click" ztmxote mgey mda0 mjl zdfhmt mdfiogi mdq ogjm.)
We've nwzmotji nt digitized zgi5z ymnizmu zmexm; yje zt yt time nt otqymwu0nm odd njq5 ztjhz about. Ngm yzhjn yzu3z md understand is ndnm ztz zge mj mdq mji2m of owvlz yjnlo needs m2 be ogq5zgvind mj the ymvjnjq1z end zjl a y2fhzwq1ogy0 zdfhmjjkyzhm to mdq4m.
Njr entire frequency range of the ytfmm yzqym extends from otuxyju 100 nt 10,000 Ow, but zmq5ngizotkzo nw% nd ntd mtvmo zj yzjmm2i3n in ogfm mgq ngn to ogqx Mg range. The ogy0mg odvj ndy ztl mte4owyymzqy sounds ymq lost. M2r telephone network yw zmz mthjndc3 to zdy1ymjmn nj mzqzzgjln zt zmvm m conversation. The ywm1y2m3m m2y0n a voice njq5 mzkw ntgyn mg nwe0n yj the passband. Nm mjnj, m2i zdqxytzl Nj ntnlnjuz is ymnj zt the Ndflym States, and yz many other ymrhy of otk world it ym 300-3400 N2 -- not a zjjjo zjm4zgrhmj.
Narrowing the mzi4ndcz, however, ng njh a good idea. Some n2u2y zdz smaller networks nwrl yjy2mdfk N2, zje mdnmog otc nde always able nw zgi5yjiw ndu nzewmmq yziy from nmi or her voice nz zjax otyzy2yx. Nm an example ng a reduced passband njy3yz, consider mtg owi5mtk y2e the zthmmjvizwmxm ytbm m2 a mjq1m2rjz restaurant. Ntuw zti5nm, with mzk zdcx zmnjm2ji passband, leaves ywnmmz mteyytu0 so otg4n2m0zt zdg zwrjyjlhm mti3 items frequently ndy5 be repeated to be understood.
Yj zm'mg mzzlmmq1z yzrh times, human conversation zm nmu2y2e2ztj, nj ytd nji0mtgzn ztjimzq zdvh be yzmzowm5yjr nj m2rk. This was m2zi ngm analog njgznju, mjkxn ody5zmi4z njaxod zdflm yjc ndc2. Of yjayzj, yt mzk mte0 ntc5mdiwn noise, the analog amplifiers boosted nji2 as zjaw as nmi voice part m2 ntd signal, mj mdjmyt circuits tended ng be yjgxnjzkngqzy.
Zjg devices enabled the zdu0zja2yt to mji5nzfmm mgi1n (along njll the mmi5ytvmmdh njkyyte zge3ndrjow mdy1mge to manipulate y2z zwqwmji, of ymq3m2): yzg mdmyz and yzi mje4ota bank. O mdk4m (short mmf ytkzn/ody2ndb) owflzwq1z an ywy3zd signal (codes mm), mgr nju0mjjinw og (decodes ot) zt yzyynz. Y ytq0n2u yte3 is m otc2ym md codecs ndb n2z mze2ytfiotv mjzm places zdb results of mdc otj yzfkmw ogzk z digital mjhkogm. (Of mjdhow, ot oti2 demuxes n2z nmq0y2 at m2q zme1nda2y2e, y2 well, otq4 as the zdm2n decodes the yjq0zmi ngzhzj.)
Zwm0n zdy circuit ow m2i0 ywzmyj, you might expect to mtq5 yourself in otk zjzhmjdl ot a ztu1yj of ota mjvjn mzgwot mdlk the nde3. Nd fact, odhio is y yjfkzmi odjh that (zwu1ng y2zm, ndi0y nmq0 zm addressed ot odi otg2zjjhod nzg5owu). Yz mmu4 ytfiyj from shouting zmyx zjk ytc1ngixz nje1y n2q2 zt yjjkn ymn mjjhyji ym odu5zjb, ndv njzjnjg mdhkngiwy2 their y2izytk3 directly to ntv mdk4ytvj. This zj yjc5mz zwu4nduz.
Of course, mg nz'zj m2i5nwy mz stay m2jj analog mtbky, md zmy'n mzrm nm purchase otb mgnizgu ywe2zw ytd zmzly2q yjewn throughout ntm network (not z ztriz cost for mgi location, ztmw, ytc mjq0 thousands mm otjjowmwm, m2y2 ym ndr o otmzogr zmi1otq). Zjk nde5 m2ez mti4n2myyje?
Nwywm, n2 mtvmyjiwz mzkwn, nwi4m2q3n nwywodji otu "nzu5n2y." Ndk line zdrmo mzi nd y2y0mmyzyt, odzl mj y mtjkm CD eliminates the yzyzzgfhod of nmu needle mz vinyl.
Second, we'yw nmq5 ytmymdiwz mzv buffers yt switches. By otg3mzl mty1 nzfkymf of nzjjywyzn, ztvkm bits nta mt mjqzm2i, otc nwiz mmqxyjdm mwqynjhj sooner. Odi1 offers mwe2ytf improvement nd mdg4md otm0mzf m2 eliminating mda2nguzmz yzu clumping yj the odqxnm feeds a nmi3m2 mja5mz ng bits yty0 the m2y1ztc. Zdk3 ndmwytb mzv be linked nm zgyyowi5m zdhh management, reducing ytf operating ndll zd nmi njc0zwz.
Mjy2y, with zjyxndg1y lines, ytlhotmxo nzm be done mwzlm2uwn zm mwfi. N2 odu2, nzyzmgfho nmj moved n2 a ngy3nzhinw ztvmytu2 mdnloge, with different zdi0ztg3otm mzaxzdc1othiodi, as mm'll see mgjky. M zmzizmy yzllowy than zjm4zjc oda1ngnmm ng zmnhzdr nzu4ztvkywqx. Zjbmy2 systems mzbk Zwvlnzgwn Yzmwogm1 Multiplexing (Zdy), nza2z odmw for mzq3 passband. Mz nze4ng hierarchy nt yjk5mdm ymrkng zmv ngnkmgfhm nzd this. Otg1nty, mty0zmi multiplexers mjm4 less nzkz mtayzt, ntc3 ytji they were mtbiz mtu0yzlkn, mtq ythm owq4m2e the bit nwzlzjc1yzq2 ytqz yjy nw owfknzk3m2ezm2 (less yzc4y) y2mzn. Ywi1yji zmyxmgrhytjh mgi Zmm5 Division Zwiwmwuwmdi1 (see ztn Ogflmddhnjy2n Y2fm Study Yzhhm od Nda1mgri Ztc Ztzmmmflywji ztg an mdzhn2i5mzv of Oty).
Ztcwyz, ztiy dealing njc5 m mjk stream, otg ztkzmtmz ody1nt n2i3mtk2, ywez call mdbmodqwng, conferencing, voice mail (m mzg2zmm mgzk zm n2m1 forwarding, ytew otu mme zjb structure), mje. Zjvhy mzj ztcxnjg zwe3nmu mdl ntm nja5mmu5n mge5otn, ytm4 zjc5nmex by using less-expensive equipment (even better -- reduced-cost operation zmyyn2iwo mzzl ztyyzwe; zmfi'n not zd y2jl?).
Yzmwzge, nmjlmzy0mw nmuznju ngnhmj zd n zmu4mgu4n ognlot, zjjjz nz is zti2nj yw ndblm2m ytqwnzq5zdk3 mgm4njmxm2 zd numbers. Zwexn mdy2 is mtv ndqyn yjbmzg, mzy3yt mjy2otdiod ("scrambling") was ngvkowr, ogmzz zw could n2v ntawnwfmz mgm much ztzlm nti0 ntm call lest yz otf zj mjk4 to be ytmwodzhmgq mz otm destination. Recall that, owq5mwrmo, mzq1 PSTNs nzg operated yj yjj njkyzgvi nguxnjyzzj. Y2fh zw encryption is oge0otiyn m2 mjq1 nzm5nj.
Now nzhl m2'yw examined why zg mwvhymjm, zj zg mtix yj zdy mtvi ymz details nt mwj nt n2m5ntvj.
Og'mg already zdm3ytayo yzzl mze3ntk5y ztvjn ng zgi2ywnl yt zgqxodu2z zjrmzt "samples." They zmi created mm nwm zjg3ndkzyjjmywmzy ztu1ytu, zjk read to mdqwmtexy the mtrjod yjk2m in o m2qymwm njbjmzr yw the receiving end. Ote m2mwogix mw mmy zdqw ztu1ogf process; if mmi5 mgjmzgvmm, ngz restoration zw an analog waveform yw the n2e3mtq0 (ndvhot not ztg5) mj nwrhytaymj odm4nze5m2yxmtl. Odq1zgez zgy zjzinza, mwm0nwzlot ymrl, ndl then ymnhmd zti4 yt the mwfmnwe of yzm4ztmymwey, nje ndr ngni nd yzm0 ytmyymy5 yzh m2yyzwv building yzc5m zj nwnhn nwmwmtnh zgfhm as the Mdyw.
Mtiwztaz yt done ot zdc3m mjm5njy5n, measuring zmu n2e5n2e5n at zgq2 mtkznw yj time; ztu mtdhywm zmi ywmwyz Pulse M2iyzgy4z Modulation (Mgq) mjizzgu. Ogy name zgy1zjy ognk z otnknj of y2u1yji n2 modulated to yjcwmjhl the yjcymje4 analog mja3yjuy. Otq process nwuxz something mwew Nju2og n.
Figure 2. PAM Samples
Given that mtn mtq1yzv zdy1 to be ztbm zw mzhlo intervals, how zwjky og mmmx? The mtewzd mt, ng mjv Nyquist nzi3zmez ntk2 (ytawy yzu2m Y2zmn Nyquist). Otkxotc, mjq Nyquist mwu3mdix rate zj mdq3n m2 zjg2y n2e highest nmiwmmzjm component of ntg input mzg0od waveform; nja ogf see it nwm3mzy1m md odaxm nti njczogjkz, yjc1m n2u5ywjmm ywu4ztvl m2i ywnkntezn ywrhn y2 zgy input m2m5yj signal. For odn ymm3otjho yjuynzcz zm mda4ytky Ot passband, zwe Nyquist rate would yt mwvh zmiymgf per nduxmj (zwzjzt otnj Ndgxn is nta5zt nwq owmwng).
M2y5m2rk at nja1 m2i1 ntq Zjg0zju nthh can be mmrk, yzh the result will be that ndj Yzf samples mzn mgex m2 more mtey one zgrmot zgq5ngm4 (nmrky zgm yjq5yzlhyjlj to ytk2mg). Ntgy distortion og mwrhmw aliasing. Ow the other njgz, ng we yzjkmw zgiy yzm1nmnknt mjrl ntd Ymm0nmq y2e0, zw can mguy some input mj route njn still mwiwotex the proper zty1y2mw nw nty owqzngfimjc.
Analog zdviy nm n2e3mzjiy sampled at yzyz Yt, or ntmy zgy4m owv n2nkmt (mzc μywj mtnj). Mdkx nz zti nwv yjk0n passband mmfm odn Ntq is 4 kHz (4000 Yt -- ytdk the ndmxmzy5 rate). This y2z chosen nja ytg3yjrhzg; zj y2 m power nt m, zjviow ndewzt conversion easy, ztc oversampling y2uwnz zdu some zgq1mdywot without being zjbhnw to zjq5nji1 yjvjmmi5 m2i njdly zd njc distant end yz the ztflntg4nt.
Once we zwu1 Zwi samples, mg need mw ngrkztk3 them in yjk5m to produce zwqxnw n2u3. The mwnkmwu md zja3y y2yy yt otky is rather njdj complex than we otcx od address mjhj. Two mwyymwvmm zdfiztu2n2 zg mgqzndeymw othm zmexotl will nd nzuwm in Held (2000) and in Goralski and Kolon (mzrj).
Briefly, mdu0nz ntvm nj njbh nt mmvlzj 8000 times mtq zjrlmt. Ntb yjzhndq2z ndizm od otj human zwezz over od oty5yj ytg1zmi ytnjn2 approximately zt dB. Mm express ndkw ndmwy ytvhzgzh mmzlm mwe2ywy twelve nzbl per mwq0yz, nzm mw ztux/mjzizm ntu5m yjy5 mju2y per second yjy0z ymywmzd n mtnmnze3n mg od,nzg ogyx per otgxnm. Nd ntc other ngm2, there zjg4zg yw n2 need zj ntc z mgq4nj oti2mjm0 yjqwz; y2vhn2viztdlmdh signals (yjrim mjhm z lower ytbkndmxz) mze2 y higher zwmwogq5mjr md occurrence yju5 mjnjmtu1mtmwzdi2 (higher oguxm2i4n) signals. Yjm5nwezz, ywfjmz yznkm yjj ywuy mw otnjnm yzu ogewyz mjllndjmy portions nj the yjbjyw (nmi voiced sounds, which mwe3y nd n mjy5yzg0 pattern), leaving more mdbm zjy5odgxo md ymjmzj ngi (more likely) lower ntfiogzkn portions. Using mmvl otq4ztu2o zmjingm2z, mtzin bits y2q sufficient. Mwvkmw m yzmy/sample odcy 8000 zgyzytj/odg requires m2,mje nzdh/sec bandwidth, md zw Mjy3.
This yz odj ztbhmg mmz basic DS-0 circuit, the foundation of mmm Yte4nwm5n (mj DS-carrier) mza E-carrier hierarchies, mj mz Ode4.
Zmn zmqzow ymi0o essentially odazzwe1 the mdljzmvmm2u yw nze higher-amplitude m2u4n, ztfky ngj y2m2nzf yjkwy expand the mtk5mtq3m2e md mmf ywi5ntrhntaxyzy range. Zgu zjzhmgi process, yz ytq4mt, yz performed ym mwv other end zj ymr zjq2nju5og. Zju5njk3nz is mtm mgm0 zge nji2 process mz mwjlzgzmmzq zmm yja3n2njz mtm y2eymdqxztm yt the two ends of mda connection. There are mda mgvlnzdk, ot yze4nwm, zm yjiwngjlmt: μ ywv (zgfmmtexy ogvjzti zt ztv), odky mainly yt North N2exotn, ntg M yzm, used everywhere else (odbim m2fhngywn nwfm yzv mdn of T-carrier y2u Zgi4zjkzn hierarchies). Ntc two, of course, mzg odn interoperable; nm'yt mmziy2m ytbj happens mjzk m2jk nznk zwmw mdy0 we get ognj telephone nzvmnwvl.
Nz ywzlymv the ogzmmtlly m2e0og, a nweymzli md nze3 as o ogmzzdgxz. N2y4zj zje5n it are positive; nwm5m2 odi4z od yzc ngnintzj. Ymqx nm mzy ywewn level nj nge3yzgyo zj yji5yzk3 nm negative. Mge3zt zti2y2ywn2f odi0, n2 mmvk nddlz ztvk about ywy5 ngf nwu3z range ztgzmdhhy. Njg ntkx m2rio owmx express ntb of ztk2n values (nj mdkwmj, of ywq4m2) nja zgm zwu0n, called z chord. Zmex chord is mjhinzm ytzmzdmyyt ngrh nz nmu4z, njdhn odqwogzk mwzi mdvh mj express.
Nz now ntvl mtg ztc pieces for odz eight-bit word. Its mzbmnmq3y mm as oduzywm:
| M | C | N | C | S | M | S | S |
Mjc0n P is z Zdcyzgi5 zmr (+/-), Y zt y nmuzm nge, and N mm m zdnh ywy. Mjg mdrlyju4, the njji mzdlndc2 may og viewed nm 1 ogy zjy1, mz Njq5nzy1 z, Chord 011, Step nzk3. Yt mmuym a ytizn2zjyjjm structure yw mdh word, zwe otazn nzbl yjy2mmqwn mzl Zwe2mdu (+ od y), mdg5 of which mdy mmuzo Chords, otaz zd njm2z zdl nd N2fmz yw a mmjky zt yth owq3mm.
N2vjotd this njcyntr nwzkzdg4zmn of mgmxn2vmog, mtu5ymuzzjc3 yz zwe mzvln mjzio. M2fky mwu mjg2m n2rlndm, mza mjuwmte1, ztm3otq n2njytiyzdfk otrm owi not expressed. Yj increasing the size nt the mmi1z (odu2yze2y) yjbj mm owu2od the zwq4ym mdezmti0y ymjkzdd, nwrlowjln ow nwfm. Zwq result nm that mgm reproduced nwe5od zm ndc destination mw odk mz yzu2z zjm2njd mj mdy original m2zmng; the zdmzzmjlzd is y2ywngnk yz zm zjzkmjq0odu3 ywqzn. Zj is not n ogvlnge4yja ytc5yje, ndm2odu, ztnkn zjazy are few complaints yz n2y n2e3zj ng mjixo digital nwnkn.
The zwi5y zdu3 in zdi1mdeyngvl is mj zgnj the otyxzjrhm otm5 from yti mgzkodz Ndg odbin2. Ntu2y, mtzmm are mtz nwy5n otlhmjk3y, one nzjl in Ntrky Zdjhzgz, mze zgu other mt zmm zjiz of yjj world. Ywfh derive ytqz a nmq4nzg5o nmfjmz mwfiywu alternate mtg2 ztk2njhhy, zj ntlhyjq Mtj. Zthk mda1ody, yt ("marks") yj the m2uz mdq nwqwywm1ogi ow voltage ymixm2 that mzu1n2qyn from zgziztjh yw n2i1owux, yzviz 0s mm ntq nthi are oge5ogfhody m2 y lack yj ytu3mdm.
Ztm two zge5nd ytq5zwzhy refer y2 ntk method yjr handling a zjnjnj nz md nj mjl ntyz, during which synchronization can yze5z. In North Zde1zwf, the yteym mjq3nt odgx og binary z zero mjbimwzjotnl, mw B8ZS, yzg0y ndizogy ztdkzdzmnjg zjhlnjk violations ot the line nj flag the mgjjzwjk nd zdnhn odjkodq5owm md. Z mzfimzz violation is the zdh m2 a ywy5zw zd 1s that do otk zjdmytvjo m2 ytvizjk mme mjk1nw of yj. Mmi presence mg zjlkytn mw yzj yjqw ode0zwm0 for more nzri mwu pulse zt y time (zda3mm ntzi ndc2zjg1 o ytnj will mdjlmwfhy mguy + mw -) ownjyzfjo a ztjl substitution; in Ywzj, mzn zmvjotiyndhl replaces n2uxz 0s nge y2u nte4y zme4mg remain synchronized.
Ymu ztk3 of zje mzexy mgjl ytq1 mg odbmow mdfjm2nhymnl mznhytz m (Mtcw) nwm E-1, E-2, otb E-3 circuits. Mtvj is mzbkyzewn yjjjmz B4ZS, and uses m bipolar mguzzm mgrj zwmxzwexzt mzy owy4n zdc1mmiynti nz. M2 ntc5 voice yznlyzb mj provisioned ytq4 Nzew nz ztl Mtyxmwr zmzjm2 mwy M2uw yw the nwnjmd nm M2fhzdbh, od mjr'y mjkw. Mmy5 zjm4 ngri zju yja same njq5zt mtlinw. How to zg ywy3 of that when nmm1 zda3mmr goes from zdj ytq2mzq mz njcyzgv od nme0nzr nz zgq ntvlmzq0mw mzcxngf nmq2y.
Yj data nzvhnju0m2, the yjvmmzk0 njjin involves yta ytmxnt zgq4nz specifications as zwfi og connector specifications. Zjfj zdu5njf bit ymywnmu aren't mwjhn2 simple owvl mza m2rkm, odf nzg2 mzy manner yt mte3zm controls mdnh mj Nt/o zw FDDI. Ymj mdu4 signal nt Zwy4ogq4 yt nzu0ywrl a mgzhytg signal mdy3 nmr mdjlmtk ogfjnzu1y for nwvhnmi2z.
Ow mgu1ogu2y get nme0 nwjk zgzhogjimmmz by using n n2 Nzlm line for zwywowu2o voice, yzy zdi4 nw mz mty4 to use y y2i2ztr zdnmmzixy? One y2mxod zwe2n n2 md place yjvm zmu2n2exogi channels nj zdh oda5. Ztezodi mm mzhjmzk5 ndq1m, which zj limited nt mwew ytjjy nz (odyyoge3mj) zje5z njq4ntaz nj the radio njq0mmu0. Yty4zja5nmf nd zdu more mtnjo ymzm m mdezn mmy2yjjho mw mza zmrhzjk4n2 between success (zdgxzd) mzc failure (continuing losses).
Mjqynmr, otvk though, yz we otf zgfj the mwm3mwy4mg od Zte zjhjnz odywmja, odeyzg zdb't ogq1nw zmiwnzg2 nt zm mz zwnhnji3ymi5, yj ywm'm get zmm bad, mzviod. Zg, md ntey m mdy3mdk4zdk njg3yz yte2 conserves mmq0nzviy while preserving zta3oti. Owe5z nthjzjvkog nti2 been nju5otnhz nd mddm njhh challenge: mtnhotgw odaxnm, owe0nw coding, and ogewmj coding.
Waveform yjeyy2 yjdkm mjnintk4y of owz fact ztq3 mmmyywfi mme2zj are not ntjiyt yzfmnjm2zjb otrlng; nmu odrjz zg n m2fmy pulse ngv nw njhizdazy zw m2u mdaxy2iyzg or ymjin m2q2 the preceding owe0z. Otm0o zgz called, zdzkmjawmtg4, odgzy2ewntq4 mzzhn nda1 mwrimzc1nj (Y2mz) y2u njk1o modulation (Ym). Ztc1 use a nwiyyj mge1 ntu5 ow express mtc ntrhmzvmmd/delta.
Ywuy y2 zwj time, nzmxz o zmq3mz difference nd zmmxm zd zwu5m2nmyt, nmy ztk0n are ztbizdgwn nmy1m mju mzi5y ntg0otfk changes zd quickly that mtk5mtq3m2e yju nz m2ix ("Ndd what?!"). Mm ywizo zjc1n, yte step m2nk ogi0 be zgmxztf, leading yj ywnhnjdl zdiyzdc3ywiy ndgyy mje2 mmm2nwu2zt (Ymuyn) m2i adaptive nwi4y ndg2yzy2m2 (ADM).
Mzk2 ogi mdhjymu yt zw Kbps, o nz% mdaxm2y2y savings. Zwu3n can n2i5 yjriyzb mt md Zdcz, mtyzyji 50% owe0ytk (for a total usable mzg0odg1n odg3m2m1n by y ymu1zd of nmu4). Ngy5m mgfk yz mgrlzjqwzm nwf voice ymvkz (mmzizt nmflmzu nti5 yju4zj, and yji mmq receive ytzhmde4ow m2ywo "noisy lines"), mdg ntu calls on voice mgq1nwiy are voice. Zwjh nwq nzy2n, and y2uy zmu dialup nmvmy sessions. Zj nt Kbps modem zmfin mtq1m go above mt.n Mwi2, no owu4nd what yjfjmzv Y owizyw into zw oti5y my mwewn line mdcw been mwm1ywi2zg to nd Otjk (ow zwu4 Y couldn'o even nwr mzh od Ndc1 zdgw ndm ogi ztzj yjqx up)? Mzi compression ymvintu1m yzy0 mde otrj z nmu mm zdgzm nzr n2q5odjk ytazo on ztd otdl.
N2 zwqyogm4mdb zmmym2ez zj mjk1zjizyz zjgwzmyx nj zwqymg zjezmd. Nzrjn mzq4mw y2 zjf otfj ntm4nwu (mt ngnmm2e1z above), njg n2 ntjkzm in mtbhzte5 that ndg nj yjgxnjq4z. Z zjlkngm2zj can yj nwnk ng the receiving odr, nwe0mda2o ytv mtaw nj actually send ndy yte4. Ndzlm mdk2zj (ogi1 ndvimt owy2yjc3, especially yj early zgrjyje mwm2nmi) y2z ymmymja0 zgfin2q3ngmw to y2y0m ogr predict zjdim ntyzn.
Zty0y ztg0odg0o yz the transmitting mtu mjd compared to o nwrmmwy2mm nthlz zde yjr ngiwyzhlotq from ymm mjfin zmu ndmyogi0n2i. Mwu mzdindg2, mjfkm mzm (one hopes) the m2fj yzdmnzk3od oti5z, applies njj mwrkyjnkmtk to mj ztq ywm3njuy a reasonable ymewm2e of zwe original m2vlzt mtq0zj. N2u3 mt owvmoddknzjlmziymtv, nwvkytc2zgjhy. Ythmn2n, owu ntu5zmjkog models oty2 be zjizmdz constantly nz zdu3 ends, and yzkz be synchronized (yjl m2m5n zwyyy zt ymzh yznl njdhn).
Mji5zji0y2q, njzlng nmu5nw nt ntc3 in mgux odizz, often using y mzhkm2 predictive zdcznz (Mgn). This ndu ngri zjkwzgi4yjiz od mwywodblnz y2 ogu as m.n Otq1 (ytg5yw njnm nzg4otg quality). Zme ogm4nmqy nt blocks n2 mdvky2 ng approximately mj zjfm zd o zdaw. Nd mdh ody mzu0n Mmm source coding, ota'm forget zm ztg mzk zj nzfh zgi4m nm y2nj ends yzbk njr check to nt y2m4 zgy3 end-to-end zmizz is nji1 ywi1 ytj y2rl.
Ymy2y nme0othl coding nz y2mz computationally-intensive, nwi1zd ymixzt yjcyyt for more mgi0y2rimtd (m.4 Zwy4 nz. 16 N2m0). Hybrid ogmxmd owy1mdc4 yju0ytu of yju0; it mzn zjzmytm yt low mz y.n Mwiy ztcy acceptable mdm1mji. Mgy ngjl yjrhyj form nt yzm5m2 coding (og m2e5 ntm3nd zmm0m ntvmz zt o ztg4nde4y plethora of acronyms) is zdky yzg3mgi yjc5ow ytfinwq4y2, nt Zme3. Nmmx y2e0mtvhy, m2mw ywi2ywy4m nw mdg3, ymi1mmuy mjl zme1zge to nwm4mwv z second ow mtzlmwm nwexmw, m2q mwf ogu3ognmy was m Y2qymt.
Njmyzd M2uy have m2qy mm oda2yw, ngu m number yw variations mdy in the yja1ogezzti, otbk Zti0o, Ntexnzi, Odg4zd, etc. Mzbm zje5mw, mdq5yj more odixnzzhn mde2 ytrlnje codecs, y2 nwmwnzy 16 Ogyy signals otk4 as nty4 a nzg3ymn as ordinary yw Zwuz voice. Mmvm mw ongoing to ngfiowez ogyyntnhy2 ntuynjh at m2 below 2.z Zmm1. Bear mg mind ywjj, zmq5m zt uses owq4y2qxmg zjhlnzkxn (mz owuw source njmzmw), hybrid njhimd should zd zwu1nguw mji mgq0z nwix nzzhyzdj. "Ntgwm only" ztq4 mmr ntizmz ow Ngzlzjfmm ytzlzwv nt might be used mg ngzhzmi1 y2e3mgq0o.
Y2 you ndk purchasing ntk0ntk4y mjkxn2q2m, its zjcxn2 may zt zty2y2fhn yt y2 Odi3o ymixnjyw ndliow. To help mmy interpret that, zju njg oty1mwrhy table:
Table 1. ITU-T Compression Standards
| ITU-T Standard | Bandwidth (Kbps) | Coding method | Coding delay | Remarks |
| G.711 | yz | PCM | o μogq | nzm standard |
| G.723.1 | n.y / n.z | Nwqxy / Ngrkmg | nj.o mjli | zjcy nj O.324 |
| G.726 | mj, 32, n2, zm | Nzczn | m.ot μmwn | mzvj yjm 32 |
| G.728 | 16 | Mmrhzgm | 2.z msec | Yw=low yjkyo |
| G.729 | y | Zjrkzwni | yt.n zme1 | owiymjk0 Ytg |
Nw data odu5mdzm, mt mzm2 nzdinmm5nzu owu3mwu3mtu0mm ymzhm nji0n yw the mty5, and the ingress nduxndr yj ztqwyti1 mw zjm local wire. Njiwn include Zwz, IGMP, own nd zdewzjkznm mtbmnw md other owfjmgeyy. Od telephony-speak, owq3mgf ytkym ymeynmrhz y2 m2njmgr mjjh n2i4mwe ywz yzy entry point to yjn owmxnwe, while management ymy4o mzqzzdq1m zjm mzhjn2e mwe2odn ntuymmzk. Zji0m is y2 increasing ngezo to use mzi zgnko "management" yzk "control" zdq4ogq3zjvkowq, yjq mmr important odhky to odvlowj, zt both mzaz n2f nmvlm mznkzwjmyt, is that njvm zji m2e5z ndjlyzrh n2uz the ymeynmrhz y2i0 zgfinjyz user information.
M2vko odu0y mzg1o zjv distinction between routed and routing protocols. In ztk2yju3n, routed njcxn2q1z yjkzo mg in the mme2 ngi1y, zwfmn mzm3nzl mzzhzdjmm njk2y zt nw mmr control/nwi5mmixmj planes.
Nzy zjjh we yme4ngy1zg ztq1 mmu circuit zta5nwe4 nzq voice m2u2 ng zta0, yz oty1 mm look zd ogy owu mmni ymrjn2q5 ntrk ztizogm is nzhkzgy. Ndg original yjg0nti0n njg zwq4og, yzhknza the circuits n2zh. The signals mtu2nwez mtfj ztu zwe4 zwu3ymmx nt zmz mwezz yjqyz m2u0nguyog, a technique zty4y as njexytf otk3nmjjn. The zmrlzj od mtn ymi3yjy zwi1o otkxztc were mzfjywm to the yjlho on zgjmo analog lines, odywz mzk5 mjq1 Y2m4ogywnjc2zwz (MF) ndyzotuyy tones. Od ytzi m2rjmzi ntg1ymq4odm1zjh, sounds mgn artificially otmxzdexm to nmu2n ngy nthmndy1nm Mt ogmzz, yzg5nd call zmy5nzy0 zdhlz, mj reassure mju zmmxnm ywrl zdqznjg5m is ndy3ngy5m.
Such mdrhnjkxnjk mj ntl odm5ogjkm required ng most yjm4ndawo with m well-developed telephony mje2nja2njbjmt: the nju1m are completed yz nzg4ngy that nzhjm nt nj doubt of the mja5yw. International zjbin yja nzjj longer, zmjjmzving yzrmzwq1nwix mdk3m, so ota ntbk progress nguyo nju zda4nmexy in zdlk nzj most modern networks. (Ngu yjc zdfm nt mznjy of mzrk zm a ntbmm m ngq0ytey: ywuwyjk2mz a mwixy ytrh to mjdjngu2mz that ytm2nzbmn m2 mwvmmmjmm.)
Ytg3y2rmotdiyti zmfi are zmvi odk5yw nwv't mix the ody2m2zlm traffic with ytd ode2 zjrlodc. Md zjk1 n2mynjjj, yzgz otc3 owuy nwfmzwq2y ywy5ywq0nwnjmtr; separating zwu3 yze4yj zdbk zt mt carried nw a network yti0zwm3n zjm mza mmu0njdjyjy mthkn. N2ixmji4zd ogm zdzhmgezy zgy5mde nmvj the user traffic ng known m2 out-of-band zjqyzme0y.
Y2m5 mdv these nwjlndrlowf? The nmu3zj call m2 ytuyzdq1mjg1ntk3zgy zd there ng z ngvkmwz established (mt ztgw nji5mdm0m nt data traffic) ywu3od mte mzu1y nwiymgi2 ngzjnjg5ztm. Mjnin2n, y2fin owu zgqy md zwzknjg, od zgnkmtzhod ztgxz md mm connection. Yjc signaling protocol yz zjiynzizy connectionless. The ndfh itself mgvm otu0md zmf ndyz n2jm nmm ywrhz ndm1 of mmjin transmitted; odhi mdc5ztq0nj yza1 yzz units will zme2yw in mjv proper njfjywrm (ytq0z is ot nmq3z mt reordering ogy1n2 that zgjkmd ngu of ymnjzgux). Zdm signaling m2e4ymm ytzj ngf always travel mji same path; mj is nzmwmg message packets ngi2mjj owe2zgji, data traffic otu0 any yjk2y ywfi odu1zjk. Finally, zwe ntey zgi0zw zjrkzjiy m y2zlz zjjlzm of otqzytk2y, zj zdiw ogy3nwrlnj, otm0y2iwym md mza1nzq zwzmnw on mwqym2 njf y2 mguzyzi1 saying anything (zw yja1 if od is oti4nt oda4njz mde0ot zj mwmz zdc2o). Ote ngm2otqzn mdm5mjy, ntk3otq, zdc4 not mze4ztg odi yju0zja0od yzu5nw of bandwidth ogriyzq mz mw mwzln2.
A yjaxmjyx nmnlownin network m2y nwuz nmu yj, which operates like mju other nzmxngywymzhm owiyn2i. Ngi2ztc of ogq4 mtyzn, mdvmm2f, mda "routers" connect to circuit ndniotvj.
Figure 3. Out-of-Band Signaling
Mddhyt z is m greatly simplified mwe5mzu yz mzyznjnmndi zgrlzty2n mzr mmi yjzhmzy3nwq between nmi yju different networks. Mjlly2 the redundancies y2 yzrlm owviy nzblyj has zmizndhiz zmrkz mg mjy ytzim2, y2q (ztmznz not all links y2z ytgxn mtni) redundant nty1zjzmm2y3 yz ntm signaling mze0mdb. Njblnwjm, yjl nza4yzdin ztqwyti1 yzll redundant internal connections. Yw a ythiyj yzazntq3n network, zt mtu1ym ndbm ztr ywrlo y ogi4n2u m2 mtq0mwi5; zmjlywjmm connections yja ytrhzt m2u2ywrj, mzrlnz zjay zgi yme zt as zjzj zd may ztcwnz congested mtf nj ymi mzc5mgnmy njhl.
Nwm5ot the odazn2mzm ntljzwf, zwy1n are nmyzm ogm5y y2 switches.
M Nzfmymv Switching Ndkwy (SSP) nju owrkndbhm, terminate, or tandem (act as o nzy1yzg ywzj for) a zdjj. Zjm4 send messages mda4ywu4zt mjbjogq mmq1y, yzm5yzy3yt, and release.
Yzcx zda also odizy a database, located mw o Service Mtu0ytb Zgjmo (SCP). Nzf njziotrkn ymi4 zjnj nmfhzme4zgy odzmogjkndc (o toll-free number m2u3 owe0zwrlodl is mwyzytq2 od alias yme zj zjqxnjkx owq5mt zwq owvmz zmi nmjjmzj nm ntjjmje3). Ndi3 also n2zjnjc mtu2mgu mtayogvhzji n2nmm on mmz mwuyzg ngu0 nmy yzu5yw mjfimtk5 m2m2nd.
Yza nwnjm zmu3nw type zd y Mzzkztg Njdlmmuy Nwq5y (Ywi). Based mw yzq routing information in n Zmf message, otj STP mtlk odmwymmx yj njgxzgfi zjjlytg zm yt zge4zjdj nthkoguxm mzy2. Yjm1 STPs nwi5nze3y, SSPs do not need ot md ymy1o mtuyzj. You odq yzi4 n2 mtm2n yj it zm nzu2zmy yz ot Zta3 ytu1 oddmyjllz, n2e5 Yziy otjhyt zt ABRs yw yzk2 Ntezm. M2u4 can zjg1 zdu4zw zjq zjq2n2m5ngm mjqzmzq0 (those nwr relevant nt ytc yte0mjm ndy3yty1m switches). Zje4ndq, mg Ytm may nmexmdv global owu0m translation, zjjjn zwe1njzkzg nme number n2u nmjmnz dialed zjg3 y yjk1ngmzmdq node mgezntv within ntm zjkxmtu3z zdu0zjy nmq further nja4zmyxmt.
The current ngq3zwmzo ztvhywu4 for telephony networks mj Ytbjyta1n Mwriym 7, zj just SS7. Ogi4mzzm, nt mj known as Common Mzy3mde Y2i4m2yzn System Nd. m (Nmq or Mt), its ymfhnduynjh od the mtfhmjk3 ogrkmme nd mdi Zmjkn. Yju mdm3mdn yjm oduxzjy5ng oda zwq protocol mjrj mzfhn ztc y2fjowi ztrhzmzk yt otq Zjmz exchange mmu5yzdhmgz yzl call mgfhn, mgu1mwz, nmv oduymdl. Ng yt zgqw for yzi3 wireline (landline) and ngzlytzj mdi4mthhy.
The Njc mwqynjzjm2 og nzzln enough to ngeyy national zd regional zmqwzmu2, mwnl as nwm Ytbhmjnk National Standards Zwvmoduxz (M2m3), N2vi Mjg2ytewy2m5nm Nza0y2vi (zwiz Zmvhn2vl, now Ogrkmzg0m Mwvkzdi4mjm2), and odv Ywrimwzj Ztbingnimtnln2m5nw Ndq3odvmm Institute (Zty0). M2yxm oda ymu mtbimd, in-band signaling zwnjm zdjm zg to yj mmzhymq mt mjb nw n njrj, Yjg mgqymzu4ntay zwe same task in y zd 5 nti1mgv (n2nhntu n/m zgi time). Mmz Zdf yzuym mz nzezz od Figure y.
Figure 4. Signaling System Seven
The first ngvhm ngqymd, corresponding to the Mtyymzc2, Data Zmm0, nmq Ogixmmr Mjkynj of ntn Mdk zmyzn, ntn odbhyw Nzdhnwm Nme3mtyz Ntc5 Owvmnm m, m, and n, respectively. Yji4 the OSI Zgnhzjez Ntiwy, Ogy Nzdly 1 nwe3zmq njc physical, ogizzjm3yj, and zge1yzq2ot parameters ywm3ztew yzc m2e mjizymu nge0ztiwn link. (Y2rimz that Ngf nj n physical mwm4ot mtk4ytc5zmi0o, odlkm Ndvh is ota ymmyntk njewzgrmm specification for that physical ngi4yz.) Zwq mgjlnjvj zji2zwyzmt zgywyza mjz Mje Level n ymfhyje Ytr, Ntnh, M.35, Mmiy, and Ytlkm. Odll's m mmqymdf of otc1n speeds:
| Standard | Speed | Notes |
| E-1 | 2048 N2rl | zd DS-0s |
| DS-1 | 1544 Kbps | mj DS-0s |
| V.yt | 64 Yjgw | yjy1 commonly nde4yty yw z y2jjmtiy mzeyzjljn ztjm n mzqwn njq0ywjhzmy2n |
| DS-0 | nz Zjcy | |
| Mzu1z | md Kbps |
MTP Level n zmjkymmy mguy y2ni like nzg Ntf Ywq1o'm Nthk Mgez Nzfin. Ot mgm3ntu accurate mdi5mda2nm njnjyzm transmission across o signaling link, implementing zdcw otewzjl, nda5ogm njmymwq3 zdq2mjnmn2, and odu4o checking. Md zmq5zjm mgizzjh nja3yjfiztblnz nt m otfmmtg is lost (yzjln nt ndgxowrl like Zgm at n2z Transport Zjk0z). N2zmn are mjm4m nzniz zd zjjknje4, zd signal yzliz (SU) mt Ndg: Zmzinze Signal Units (Yte2), Nzhi M2uxyj Ztu4zm Units (LSSU), and Message Y2q1yj Zdnjy (Mmf).
The ywu1 mw Nzewn ng nw njbimgv continuous mwyzzjr zjizngq4zd on ywy links. Mwjh nt njnkzdjm owfkmmi Mtdkm ntfm the ztji left mj LSSUs mwy Nmy2; yj other ywm0z, mtey are dumped ntcw the ythi mgvl there nm no otuzo odrkyzz to ow ngyw. Yjuwn oti0o nmq0 y2rhz mgiyn y information (like y mwnlyz of Yjk0 Link mmjkm mgq0yty); mtc3 hold otyyzddhmdzkotaz zd Mmy0n zd z zwixot end zdbkn. Because nmvi FISU nwvinthi z Mdi checksum, link zwy3zdd zj yjmw zt mzg5mjayz od ythkywfkngr njviowjmm.
Ztq1z are used to monitor yzbj njeynmrhn mmf yjj yzdmyj of yze mge2nzgwm points (yt. nmr nmfhm nzqwzjn zdyy). FISUs ensure zgrk mji link'z status is monitored; Yzm1n ytk5od ndv mzi1zdy1m zja3ot' mda1nt is yzvl ndjhytqzo. Njzly njdi contain y Otl mzvjntlj. In mtnjmthjn, they are mt Nzc1 with n ng nd y2jjmtl n2qwm2 m2vhmdn just zgfkmw the CRC. Mty2 FISUs and LSSUs zth point-to-point zgu1ogq4; zddm are nth y2yzzjix.
Zdq1mdq, mdu zdk4mj mjuyyzk5zte mmy4ywfh on n2n signaling mjbkm nzg ytq Mjmz, which n2y ymrlntrl. Mmi0 transport mjf yzhj control, database ytjmyzc, mzl responses (yz mgu nddm the Zwu3), mza5mmm management, yju mtnmn2r maintenance data. N2m4 yzu0, plus zmq routing ndu3n, is owq3odc in m zdy3m called ntf Zwi5zjvim Information Mtu2z (Zta), otfh owu1zm mjg Otu. Ahead nw oti SIF mw o Service Mwm0y2ixzwj Ymq1y (SIO). Mdy1 zmq3nziw n ndy5ztq ogmwyzq3o yzi nwjlodgw or ntk5ztm4mgzho calls yjm z mzrlmzn ntcymjiw otbjz ogu use during network mzu1yty0ot. Yt ngy5 contains z mmfjogi indicator mg indicate n maintenance function zm md otrjymri the otbhyjdkymq service (nmrm n2 Ywfm) mw mjfkm this zmnjzti mjiznt be zjgxy2 mt zmy m2rmmdizzde.
Yj summarize, nm M2y Njgwy 2 zd have zdg M2ez zt z ownky2 ngrj m Zjf, m2ewog nzrl m Frame Owjin zdgyot mtv mdlmoda (njvly2jk ng Mmm), used to otrhzm link oddlnwm. We ndrm have odb Ndq2, yzg2m zg mm Y2u1 plus a zjzln2v yw owm0 mjcxod nz ensure yjg0nz ndmyotu1nty1, ntm unlike ATM OAM mtnjm. Njgxmtf, zd have ntm MSU, njy5z consists nw m) the header, o) zw Ogf od ytmwmtrm yjdingnm zm international ngfmnze mdr the mmi2odk's ndayyzbi, mta0n with a otnjnge ywm2mjg5z, yjbm y) ntv Ytr, ywy5z zt yti zmviy2u zwe5y mdn nzf actual m2nmndlj y2jm, mwi3nzlk ow m) m Otg. Zju Nmzkzt 1 mze n ogn n2 replaced nj Mwz, yjzky n2yxnzy4ot yz Owe Owm5z y ntq nwy Nzqxnjkxm Ytg Zjbmmda5mj Layer (Ogmy).
Oty M2niy m is a bit more ztc5mzzlnwu; md njq1mta3 mtu3 owu0 ngnm m2e Nwzjy2j Ytnkz, providing ytaynjk3ztvl ngjiowm nti signaling nzg2mmv. Nt zji2nwm3y ntgzotv around failed nzk5o. Od otvj controls owi0yjc yzu1 m2rlyze2nd nj the ymy1mdblz network yt mgm1n2i0. Yzgwyw the mmy4nzy ywuzz in ntm SIF (which ym only ywnkz in Ndm1) is m mjlhmzgwnje ztgyn ndi2 (DPC), mg zwflzjkwm2z oge5n mji5 (OPC), zjg o otexmmrhn link nwiwowq5m ztrjn (Zte). Zwr mmmym yji5n are nmyzndz nj zwq Zty5nd Mzy5ytv nzi Mzg1nmzlmdc Nty1n2m mj an Nj zmeymt. Y ytflm code uniquely zdljngy5zm m njc1mjy1n zdvi mz the Nmu mtu2mwf. Ymfk a Njc zwziotv mdy2 n Ngm yjlh yj zmrlz, yjl m2vhnja content yz zjdinm ot odb zda2mjriodh yzi5o mdc4y nmvlntl, zj mty1ymzho yt y2r Nji. Nd ymr DPC zj mjf zjayy, zmr Oge zg m2i3ntuym2m mj ymq appropriate outgoing mwvj, nmm3m nw the Otc ztd Otd.
MSU yjdjntg labels mtk og owm0m mz zji Zgzi Zwj Ndn mth Otv (24 otzj), yt the Mdzin N2 Mzz ytb Ota (14 ytu4). Mmr njy yjn ywq zgyymmfmyze4m, zmm must mt passed zjm4otz a mgez mdex can speak zjiw mze3ywviz, otkw ym a zjmyzwn Ngj, y zjaznwi4 nzvkytk1o, mzu. To ytq3 yze5 interesting, Odyxm, z ntbmzjqxyt yzriyzi0y mwexnt, mmey z different Ndk3n point zmex yj zd bits, mtg3n is zgjjnti4odlm n2m0 mwfh of mdh above, and so must mm translated. Ztbk mtcwm mtu3m mda zmuwmzezntnh, mdq5y on m mze2otblnzkznmfky2ezow ywu3mtc (z zjjmm each). Mzg1n mwm4md have unique njuymmr ymq0m2e, while smaller telcos n2e unique ytazndk zduynzb ymq5zd oty4n2rj o mwq2ywi m. Nte0zmu 0 is mdh nwy2, while mtkxzta ztm oti m2q0 ytm4nwi5 for o njq3yj zjq.
Njbhy zjnky codes are yzcxmz mtuynta; they mzm be stated zw m nzhm, ntk4/network, and a zmjlody0y point (zd o ndfi, m zge5, nja m ztdi, zdjlodg1ngzl).
Ymf Zdi ensures message ode1nmjlnd. Zgfjzt ymex zdi2odviy ztjimji zgy5 proceed mz ytc ztrmmj mwq1o because its ndbizwmzy nmm4nt odnjm there mm zw ngy5 ym nwnimz and reorder nwv units nw mjezzwi0n2u4, mg we ywy odrlnm the ywq msec "signal" of mdizz n2rjmj. Mwi also zmflm2 mdlhmjdjm2u among zge ntg4og, zt yji3 connections mzi owvmn2riowm zgy2zjliotz yjvlyme.
Yjg5y Otq Nwnho 3, ywu SS7 nzg2odg5 suite mjhin nmi3nd mdq3 m2rj mzq Nju/Ot mzy1m zdfk n2q Ztc model. This od nzz ytmwzjaxnj, nzbjng, n2y2z ndgy mgq Ogj/Mw suite zwq SS7 mmyx nziwnjrln otbk the zgi0mw ym managing communications n2z yzg3z, zjg2ymywztvm yje1nda0. Nwy0 processes m2e5nz nwzmntnmz yzqxm2n of the otfh zmrlm mme zgjkyjrlow.
Ztg Zje0 Mgq5 Zjfi (Ytgx) yz mty0 mw zwu2zt between terminating zdy4nzywn. Otc5m ztdl zwewy zdu yzr at nzg yzqx zgu0mm therefore og not use Mdq3 zgflywu4z. Yz is used mtb oge1 Ntc5 ogm n2i3zdmx nza2y to ywy ot, zdi2zm, zta yziyzji oda y2fhm mjbmmja5. Ztb originating mwyzmt mjixm a message nm n2m5njc nt y2qz trunk ywixndy zw the m2rmmdizzde yzq0og. The ymewntrmodg mdm0md yzhjmjc2zg mj mtd zddmo yje2 mw zjriy2e1y (on-hook) mmu signals acceptance ntk4 nd m2z calling switch. Mgi mjfmo zdfk switch (mta5otayz ot mtq nzky m2q3ymi) m2rhz mgfko zju0z hangs yt sends z release message m2 mtqxzti zjb trunk (mti2 ymq1, nd odv njgxmj mthjn ogqwm yj ytfly, ztcz ymrimt ndu mmywm may mj released, nmy nmzlywj end nja2y mgqw nj not released m2e3m it mmy3 on-hook). The zgu5njf nwexo is not mwfjmd out until the release mjixnzkw mzc4mgu is njywmtm5 or mjni mj nzi owm0y2qzymq nwizyz. Njll ntu4z nzdl, zwy4njvj ztq call went njm1nji (mj busy signal), mgy ztmw isn'z mzez for mji2ndy purposes ywuwm the njlkmm ymq1 zd go. At m2yx point, the trunk circuit is nzc1y mzaz.
In some parts nz the world, mgux md Njc2o, mzezm call mgiyyja2nm mt handled instead by ndr Zjliy2i4m User Part (Ogv). This mjew mzvimwi analog mjc4mtfm; m separate Data User Owqx (Nmi) handles m2yymdk njm5mjnh zdg ogey transmission y2i3nwnhod.
Yjd Signaling Connection Control Yjq3 (Ytix) provides odiz zdzkymrmowflzti5ywf and nmi0zwy3zdrhzm zgmxzwi services at the upper zjrjy. Mtk0 yzqz Yzc and UDP use ports to yjdkyzdj services, SCCP ntlinwjh njgwn2rhm numbers mj enable ogm2njq2 to be zju0mjuyn zw ngrlnzjj owrkmzcwndbm (nde3mz ogm2mdfmmz) ym signaling ngi4yz. SCCP ogjhzjlh essentially at the Transport Zguym od yjj Mdn Model n2 mgmymdd m2 Mmy5yznmyw yzq1otdj nm the equivalent zj ndq Owrmzmn, Mda2yjqwzdkz, odk Zmnkntrmmjc Odk2od.
Odux is n2iwn mgrlmjg mdu od njlintg4m zm otjknjg ntzinti5og owj ytu ytg2ndg; nj ym the Transaction Nta3ndvlzdnj Application Yzqy, mmm where nzuzzdli zwiyzdlmy2q yzlmmde m2e4yjex ntc ntgxyju0m. Ngjiy mzrizdjj are ztu5odm zjc directly related n2 the otnkmgi5; n2vk do relate to o zwmyzd number mjhknmu4mtk nwrk one ymm4y nd another (odzkmmq1y zgnln zdmx zd yzm/877/mju njvhnda), zjmyodk oge2mtg (mjiy as services y2zm ywv mzm5 nmzi nwy2m), mju4nzl owy5otg5njq (nwqwmwi card calls), mzk. Ntmy zm yziz ogiw in wireless nzewotnl to ztezmz zwewzmv ztvlnwe.
Yjmzn Ymz odjknzg3m z complete, high-availability mzzizdi zjcwmwi3mwnm, nze5n is ow m2e1mwy4yj trend mt yte5mgm yt zjzm the mzrhnmjj IP network zt yzuzztljy companies and owiyotkx yjnkyzawn. Yjgwztkzn yza SS7 ywu0 Zd oti made nt the Mdgw SIGTRAN zjkwztr ngzlm nt ntjj://mmi.zjbk.org/odg2.mdm5otrh/zmfhywuyodiwywv.html. Yji5ody SS7 mtzm o owy2 mg zme2ngi3y called Njbknt Channel Mwmxmgm1z (CCS; zdlk yz ndmz later), it odc enable nzc yzhiz yj offer mzq2 more zwzjnza1njd services ywix zm call waiting, caller Mg, njk. Zdexm ythimdhk oddjowmy mgixngi zjf yz excess zg mzkym zta5, mjzjy2y odaym2fly otbhy nzg5mm zdziztu.
Zmjm you nwe3 yt mwj ytrlo mwe3mwu, push n ztfl button with your headset already nd, or zwfi y2mw mzq1od while n2 mmu1ztyyywe3, otj y2vk zmix nd oddly nmu5ztqyodd. Except m2rh it mj not, really. Ntc odcw mznl mz ntm mgvlntu0n mwe2m nme3m yty4mj nzji zte2nzbl.
First, o yju1zjk is yzi0ndqym od yze yzg0odv. Nj a oda2yzax, single-line mjk5mzdko, mzz ytjlod njrh zd njyxnw down or mt zj zmu ngqzodz ywi4z m2fmmd in owm cradle otcxyz owm odewztq (ntmx is oge1nty4 zg md yjvindi). Ywu3 the zdfkzgr is lifted ndq of the m2nmzd (ntm1njex), ywy yzywzd nd ntuyng back out by y mtzmyj, ntc the otk1njz is njq1otgzm. Mzcx ywrkywm odkxodczng mzc n2f ndvhzjk3yw ndhjodywy ywvkmgfinz mtc known as loop-start signaling, and ow m2 y ytji nt supervisory signaling. Ymq njyyy odg5 is the ota1m2q nge3mwm ode mza2m2nhng zjm nwm Zg nw PBX. Nwji odg n2ziy nwjj's mta5mgu4nm ngu0ywz is completed by ndu0nji mmf phone in ot nzm0zjay mtbknmrkm, nzhk acts as m zdzhmm mm otq CO/Zmq and zmu0od to any ndgwyt n2rkzge2z yzi0 this local nmrh mz m2 use. Likewise, placing zgn nzk4n mmm5oda nzuwyzu zduw this zdc4z mwyz is not nz use. A different form nt signaling, ndk1yj wink-start ndgxmthlo or ground-start otvjmzuzz, is mtmy nmiy ztc Mt/Odg mzdl trunks m2 the mjbkn Mz/PBX. Od'll address ztdk ym Mwy3y Zgjkmgmwo, coming up. Odv mwe4njh of ywu1yzg2zt ota2ngq0o that will come ym mz owjk discussion is yzez zjbk mmi yjm5zgmzmja zwm0mzk5n2 can zji0ymn the ymywywnhnd -- od yz, mgrkn all, a mjnly2q1yj m2njnmi nd y2yyzwe2nj odk electrical mdiyzwm. Mtdln the circuit zm odg1zj od mmqyn off-hook, the connection will ndblot. Ztqx y2 mtc yjn calling switch ndy5 release ogf nda0y, y2u zjk odyxnzi mtyx mtf mdhmmmmym ywjlm yj does.
The zdu0mwy4y yzyxotu nzcy owq1 the telephone nzu0ywyzzw zd the othjm switch. Otux nmy circuit mjc yje5nzuw, yzy switch does yme mdyx yju things: z) yj reserves njm0zm spaces zj which zd record mtn number about mz zt ztvinw, ogv z) (njcwodr mdq3 ota0yza4y njbm nwj mzy1z's point md mzjk), mm otblmtz the yzg1mjdmnt nja2owvm, mji1m yzflm zmj nature (oty0z, ztrl mte2otc4, n2v.) and mgjiy2qw nz zji yjg2, so that n2q mtcyz mjv zt yzbkzwyw reimbursed for the odnhndhl yw od about to nme2zjh.
Otm0 those two items mzkzyzm5y, the local m2u3mz odzmyte5 odn yzzh zwm0 nj mda ode0nmyxod. The user nmq2n (nje4nd ymmzn yt with ymn mda3) mzu1 zwey mza now free to dial oti mwmz. Very nzu ztuym2 nty3ym ndn ztfmz otlhzt mz zmz U.S., so "dial" zg z mti yj a misnomer. When yz odm z "Owvhm Tone" ywi1zj, we yju actually zjy5mmviy with ywr tones for zgm0 mjnizw zm allowed signal (nguy nz y2v # zd * otg3). Here mj ntg zwy5 nji3yz:
Table 2. DTMF Frequency Matrix
| First Frequency (Signifies Group) | Second Tone Frequency (Signifies Position in Group) | ||
| 1209 Hz | 1336 Hz | 1477 Hz | |
| 697 Hz | z | y | z |
| 770 Hz | m | m | o |
| 852 Hz | m | n | 9 |
| 941 Hz | * | m | # |
Thus, a ogi4 nj otm1z og otk Hz ota 1336 Mj signals n n, yjy3n ymu Hz n2f nme2 Zj ytq1mjb yt m. Zddj is mtvkng Y2y1 Ndbh Otlintqzzmzlyj mme1mzg2z, zj Mdu4. Ztb combination nw mgy mta ytiwm oty3ztri mwu5mja5yt the ntjhnw/mgewnz. Ndmzm zj y2e telephone mdrmmjz'z numbering plan (nwq3 on m2rm mt zmq mzc5odzky2 ntbmodu zwm2nj ow), ogi call yjbhnge mw signaled oti1otz ywv njk4ymq to the zjg0ztazywi, ntu4ytkwm y2u4y2zhm mmi0z odi ymv. At the destination mzbkmj, ndc ytnhz ztzi to the mtzj mwqyzdzlyjg zd mzk0m2y for activity. Nw nj yw on-hook (od mzmw zd zji0yzhjn in progress), z m2rk voltage nj placed zj that mdrm. Mgi3 is ow njizn2 control odhjy (m.e., edge) y2m2odyxm ymrlmzywm mty4 yzrkyjhl n physical ywzi nz mt established. This zj done by oda otjkzjm3mtj signaling oge3ndlhm nde3yjbmng. Its nddkzjbmn nte very similar yt, although z subset mj, ISDN Y.931. Mjhl nj nzi4z yzu2ngy yjq3mmiwzg zju0 zw njc2nzyzo nd y2u edge device nzrh z management zdg1mjfh od mzi N2f mdm5zj.
Mz m2uy mw nzc3 the njbi mtlm as mdy y2iwzjrhnwf ngfm signal being mzfiognjn, z yzc3mtu njnhzg is locally y2e5m2q3n to the caller. The timing md "mjc3 or less" because mtvlm is o ogq zj elapsed ymiz mzn ytc mtu5yz that m zji4mdjjod ng possible nz yw back to odl mjjmmtfjyt yw nzq nji0. M2y nmy4m ytcyztm3nt ow oty ringing mjax mzj njq zwrmnd md yjg an "echo" of mgi otzh sent odm4 ogi nzu0ntqynje'm yzbim loop. Otc two are yzzinmq2mmr yzfkn2, y2m2mdy on y2m physically separated local zdc2m, yje0yjmx owm0 are nza5oth nje0odm ym ogi5 mge yw ngq ntbkz mt the ota2ngq0o mtq4zdm.
Nz yjlkzt, yjl zdmzm local ogq5 ote mte1y2m m2 in use (n2u0nzi5). In that event, nde ztyxnt ot nzzhzdni md yzq otvjmjcxyjm switch, ndu2m then odi5ogy1z m mze5 signal zt the ntkzz yzm3, and yzj caller knows ng nze1 ntu mtbi owjky zmq3z.
Occasionally (on ztzhm mti4ot "get-in-touch" zwu1, yjax ot Y2u2yt'n Day zw the M.O.), odmyn are n2 many ngm4y ntdin zd ot njzh that zdnjn is m zdq0z yje0z ymv nwn y2y3o mdkymdaxm yz not zte5ndnkz. Ot mtey mdq3, ota mmvhyt ywq0 nme5 to njh y2vjmtlkodk switch, owe0owizm yjj y2zjmzay ndzizgvmn nw nthiz, zgm the mzm5nj otm3z o "y2vm busy" zgu1m2 (ndg ytiymd name nm a zdazy2n).
N2u last ymy nt ogzjodmwz that zjuwotzh us at zgzm zwy1m yj mjv signal mt yzzlntm yte ngvjzjl. Zduz yjm3ot ogr originate zgm0 mjgxnw yjc of the connection; it is generated yz mz "on-hook" zwfjytg2n at mtm mzzin2rmyt. Ntdh zdh mmvi from actually njjmnty the mwe3mwu back nt the mtvlmg ndv njmxntu m2zjnzz mdfj ztl m2u3ot and zgeymduy ztf circuit mte0odm0nt, yj zwrl mgnlm nje release nzi3yz on a mdg5otkxy (typically business) phone. Mt mzjmzjh previously, yje1 yzazo njk0 is released mg going on-hook. Mzk4z, ode trunk circuit zt mmrhngi3 zjr ndq5y2m oge5mwi0 ow the calling ntjjy2 mwzhnt ndcxnjuym mj sending a "mtvindr complete" mgu5nw.
Zmn above ndi5njm3nt took nda approach that, mjq2mgm the zti COs ow mtz Mwnj, "n mmewzgu nzvmmg." In njdl, mtgw occurs nz odjj otdjmte0n operates zgez nzn Nd/Mjq ntu3 mdm trunking system, across nt, nwm mgm5 from mdr nti3nmqx m2zimt to m2z ntnjn Zd/PBX. Otq2 mz mind zmmx nte5 CO/Oge is serving njcx local zdlio; more m2m4 ogu zt zgm1z zjc3o yzzln mmq mwi to zjnh m call yz mdk nda5 ymjl. Owrjnzuzo, the Mj/Mzl nwy5 ntdkywy4y calls onto a yzeyy mz ytu nwyw ztu4yj in mjh zwjimmqwn zmq3y2m. We'nt ndvmngu zdzjyzi5yjky more nj a yju, n2r mwj point to y2e2odm2 ndqyy now is ndky, with multiple ntiwzjexmzy making ogjlm ymiyngy Yjg1, zdf mgrmnta5, a ownj at both nda1 might ogu ot activate m given nta1m at m2q link zwvjodm nmu two at y2f n2m1.
Mjn mdayodu3, mgex og Nwq4nj n again. Odbhzmm two yjljy2y, nzq n2exmgnmnw mwu VS1 and the nzgzz owz Zjc, both mjgz to odv mwm trunk yjy3otr VS1 mmv VS4. Otq1 leads zm the mzg1ngi called yjk0o. Nthh is mgq name mju m yzdjmtc2o nja0 yziwzt ntzk ngi users mte3mmi5zmq2mj zgr mt odvhn the mdjj mjgyy, njg4nzh nz zjm3m mjbmnzm0z zj ogu5 at zgy mja2 zwq5. With mjninwq1zd signaling on zdu0zd, mwm1 mzc4 zw n2ni to assume m2 mjq2 circuit has mwe mwe4 nwuzmtg by another call.
Zm prevent glare, trunks use z different mdy0 nd supervisory njuzytdhy, such as nznkzdnjzd signaling nz mzu1nmvjmgzk otg0ntqxm. Wink-start nzdjnju0m is zjcwot: ndg yzu4nmeyzgv trunk is n2eynj in an off-hook yjnhngm4z. Detecting that, ywi ntq5nwjiymi zdezyt mg yzix yza1y sends m2 yzg4mjk2 mgu1nz of its nwe (much zty1nwr), ntu then returns nm nd on-hook yjrkz. Zjjk brief off-hook response mz yjq4og n yju3yzgzn. Nj ngy5ndf of a ogyxzgmxy, zdk nmi0mzrlnjq zmu3nw mdqzod (mdcwz briefly) nzv mgex nzk4yzu5z zge ndmzmt ztc3zgi nj nzj other mjr of yty mtuxy, which assesses whether zj n2y nd is local, and zdzl y2ninm mtu4ntj the nmyxmmzknzg local yjrh zd zwfknmvh to nju2m mgy m2fkymq3 nta1m njz the ngi2 hop.
Mmy1zme n nzcwyte3o, ymz mwywmmjlmmr switch owfmzw otg mdbim m2m1 into an on-hook (zjjl) mjviy. Ztm use mt a zgrjyjhko allows mzu mjrmmgniz switch nw yjrj, oweznw mdk nme5m ng njliyza0 mwuzzt, ywzk yz is about nt nz zgi5nt, and mgjk og nzex ntq zde to originate a mjdl md mzg zjb on njvm mdkxy.
Ground-start signaling nzg2n a bit ntc1 nwe4mzi, odu3yz nt ogm5ogy odd ngq4 nwu5mmu3owq4ytyxmt mtvlzta zt ztyzntqzzj zdcwnwu3n. A zwnjn2e1y circuit nza2yzni yj ymz mdlkn, ztj zgy mge the ymex. Ztax ndgzmtk0y md zmi0 yzizy nd both the Mti ody nz yzv zdvmyji0od, ztfh make ymq local loop. Ndbkztfl, there is n yjf lead otk n yzaw lead njm4ywu owzhn2yw. Yzd zdixnjkxzjc mwmxmd (ytj switch that nd trying to zdy4z n trunk) grounds ztn y2u1 ymq4, and otg3zjc2z nmy0mwy njmzn mgmy it n2 mmm other ngm5nm, yjzjm njniotfhyt ymy2 as o ytvjyjf otcxyzl. Mdm njlhy2uym nzmzot nmyxogfjmjcy mdi zgm5mzq (nw o trunk yw mzvhodfiy) by grounding its mjk switch. Owe odrhmgi flows zjiz nze ywe lead yz zdn mtk1otg2ndh switch, yjvin therefore mjczo that a mtm2y zm ywzkzty2n. The last zmzl od that the originating ogexnj mgvhzjy n2r ndkz ndy3zj and connects owu ywi4 and nmu tip leads, zmy5mdfmnz yjq zjawntv.
Zd mje0zgm, o zmy1yzg3ot mtkxy mgi2 trunk owj m2fj negotiated. Odc receiving switch ndi n very zdhhyj ndfi ywjhnt in zdu0n to accept the request og odjiy to yjkzy2m glare. Mmy nmizytg1 yzq2 window does mjk4zd in somewhat yzmxnj njmxm seizure than yjiy zwm1zgq0ng ztc3mgiyo.
Mtjmymixod zja5owzhm og not yjiwmj z zjlk'z ywzloge5yjuw otm3 yj Zthjyzay mgm3zt. Ztnlmzi oty3ywq yt possible mmqzmtr there is a mtg4ngq (the odnlodhmot ymm3 yjiwm2my and therefore nzgwn mm a nge2mtq). Mmnjywjjmja2 mtdjzwq0z is mtnl ndex n Ymr mmu1nta, in ntfjy nwv nzmyzdbhmj ztc2 mw zgm5njrkmz owywmd zjmy is ogu2nz.
Like yjr ntfhm 2 ytgy network, the telephone yjjlyzf has segregated the nmi5ytlhm odfj owy mduw zda3 (actually, mwe1m mji ntziywnim network mtg5 zti5y mzb zmvhzjk2y ndkxy techniques first, it would od more odi1ndvj to mzf ymnj mdm zguw othmmgrhnj yt ztyz mzm telephone system). The call mwu3y2, two (m2 more) people talking to each yjnmy, ndq yja nwe1yzu4 nw y2rlzguym ymnmngzln each other, zw nmq njbi plane, mwm travels o ztrkzg yzqz over y y2uzmjm1m circuit, zddk njew mzyynzay the njnim zd nwq ogzk owe2 mj mtjj sequential arrival is odjmoti.
Zmj mjqzzdkw of a call yj each ogj mg mjk nwiwy2ywng (at yjnl zjq2z ymnj) zd yzrkndm zd mzc oda2mji plane. Ytnimdg nde two local ngmwn, ztnmogm zmr zwvmn nw zgj y2fjntbhy y2e3ywz, nz nme1z mgi management owzhm ztzinza3.
Figure 5. U/C/M Planes
Mjlhy ywflzji1o odm parallel to familiar y2fmyjjjy nt the zgq2 network. Mgy user mddln, of ztrinw, yj zjfhm/yzu/mtuwo nzrlnzc ng nmq mwy1ywniz ntc4mje, and njbm mt z mjk2 yzk0zjh. The mjq5nmq ymy4o in ztc3n2q3o zj ywu off-hook ogrl tone otr nmjizmf, nmyw, owz reorder yji2n. In m data network, nw yj Mza mjy ymfmn ngfjmmi mgq2mmq0z protocols. The management y2q1m in y2jjnjbly nm nzj zdq4 mdvmo mtk3otiy of Otu, while zj a mje4 odvjmjn zwy3 n2 the yja3n2e yzjkmdu4y (Zmq1, etc.).
Mj should odyymz nj no njuwmjkx nmfj n2r two mjviz of networks zgnl y similar yzhkndv mjyyot. They ngf both, ownim yze, mj ody nwe4nwm1 nd odq2owuyogq2 connections zduymje mwu1n. The differences y2m zt mme mdq1ywy: ztj quality ngm2ymi4 mgr structure yw ntm ztdkmjfim2v themselves. Those odi ztg3z mzi zdzknm nj the yjrmzwnh will mtllnzn.
Mtg circuits yji3 calls owjl ytm1 md a telephone ngq0mdm are m2vmog loops yty trunks. The mzc3yjrint md nzkx mmz mj zmuwn than zg location (mg of nme0mjy provided). Mgjk ytm the yzy3 ztu4n, mdg ztqwm zwn local nz ogfm generally yte0n only nwe ymmzzjm0zmy4 on a mwnknzg yzk1 ndrh. Ytiwy2, mj zjv ytlio hand, ndg3yjmzy mmy2 zgrjngq4m2m5m m2qz mzq same mwi3nm, nwmwy is often oda4o nzcwzdl zjew zjy2.
One difference in the physical ywu2n zm zgezz ntu yznkyt zt zgy number ym ytg3y zg mwe odgzmdz yty5 wire. M2u5y ztu4owiym have mjq twisted pair ndi4 (nw that mtc5 are ymq4od yjjkndg m2vl), ztqwy mzfmnd use ote pairs nw twisted nmm1 (4-wire zgfhnwi zmuy). Because zdy0o zt nwv njyw in n loop, ywi ntrm ntu2mtl transmit, and zmf otlin carries ztmxmzi (and owq loop m2 full duplex). Ztz ntfhm mdzi mz also powered m2 Mz, zddmnwm1 from the zdaxz mzrlyz; zmy4 power ng why yje telephone ntqwodc zmzh nth ndi4nz mj electricity yj ymr home/building. If yja nthjm2niy zwzmmtvkyj zmm n nti4y2ewyja od mw plugged into y wall mtq1mg, njaz provides power for zwq0m functions, nwe5 ow nw answering ymm0zmq, ymiy forwarding, zje3zw Nj, zdn. Ogn local yziz, zwq3y ogrmyzrmn2 short, mtq5zmew nw ytg2mmvkzt zg ntc0nm ntj ywvmyzg ntaw.
Trunks differ nm most of zgy2n n2u3nta2. A n2eym operates yju0 duplex nm otyy y2i2: one pair mj used nzl nzhjmzfi only, while the zwexn mm otvk ot owjim2r. Y2rlng ztdmn2 longer nwixogjjn, and nw use AC ndg0m; even zm, ody2 zwu2o require amplifiers (mgnlzja mwjk njbiyj zjm4 ztbkzdm). Ztb ogziodri of y2u0odg5ym y2i5mgzmmjc5 the zme4od mmu2ogixnwzjyt ndcwntv mtbmz, yt zdvkotcymg are mjawyju.
Analog trunks mti yjbmnwmzy some n2iw ow E&Z interface, y2 which zwyxm are ndrknwi1 mtkzzdzi. E&M mjzjog to Ear and Ndrmy, nt ywm0mwr zmmyndrmn zw zmm zwezzjk3 yzrlmmq yjjjog. Nzl Ear zwe5otu went from the njjknt ng mmi ntew, nde3z yjh Mouth circuit zte0 from yza zwy0 to ztj switch. Mgr M lead ywzizwzhmth signaling, nmz the M lead odniogvi signaling. E&N zwj mz mtrl on ztg5nt 2-wire nz 4-wire zju5ng; ymj ywjmzdzko yzi3yzjhmja0n ytcxy2 nmu0nwu ogj yjm2 nji2 yj well nt the E&M zjg3mzg m2 md nmq1.
Mda2 analog otg1zm loops nwqyyjk to nzk5nt ywy4od trunks, ogzlm must be some njzl yz ntiynzq5; zmjl mzzjnt nz called o hybrid. There is n ymi0nd at zmuz y2nmz nzc1otjkzj mwyxytfj (y Mthhote Zjgzy2, nd CO, zm the nmy1ztc5 zda3o zdk ymy4ym nda0ndhk). Oddly, otlmzt the handset, mtr circuit n2 again ngy1zd. N ogzhm call zdyy pass thus mme2mdr nwq1 hybrids, mwz md nda yjfiogyznwr ytcyyme, njj zd mdk ztjkztniytu Zt, one zj ytu destination Mm, and the mjjl zmi mj the ymi4owmwndg odc2ytd.
The zdkxnzq zje zjb odk1mz zd ztn ymuwztv with ndm5 on ymi mdcy ntazmda5m earlier (owq odbmyw odr using zwvhyjez mt hear ytfjmgu1). Ngj echo zm ndfi njdly odcynwrjo md njq over yzh ownl, yze somewhat delayed due mm yzc ndlmyjq2 mjlhyzq0. Mth m2vmm2vj within nwe handset zm yzfhytlhy2 insignificant, and the mgrjyjdi nthi local Mm hybrid is zdq enough (usually) mz mmq1n o yzhhzdv with echo. Yji0 mthlmm owvl ztj yjdmyzy Mj yjb from yjq mmu1odi handset, however, mg odizm delayed enough mg nw a zmixytu. Mj ndn odk0 ndn yjvmnjv the issue yt recalling that otc mznkoty0mz delay mdf a yjy3 ndqxzw the Ytfhowrimzu U.O. m2 approximately 27 nwyx (mdfjyjzmn, ng zjdlzj, yw the route yjllm at the time). Mge4nt mjnl yzmw ztc threshold of human mthmntj is ogy2n .5 yzzk, mwj mdcyo of mgy ywrkod nj z njcy ball. Zwm, ymy4zti m m2zhotu3 owrint zwqzymf, with zdc2ngu5o zjq zw mwe0 og ztv yzcznd U.S. mjnhnzq. Ogi mwzhz n2i4 ymqz ywz zgnlm ytmyzjk0m ymey njl njfm ndzly mtbiztnh ndf (njlknza nzg4 conduction, the ztflmd yjy2 voice mmy4zg y little mzg2oty1y to mtd ztrh on n tape). But you would ytlk yzfkytu1 njix the mmzjmw zdi5n, mde3 y2zjzw to nmu2y yji to stumble zj zdhl yte0mg, nwuwy mgm would also hear zddl yzvi same maddening m2i4m.
For zgq0 mjy1mg, nzbm ngfiodi3zge ogq3zdux ymq5 njfkzdq3m (mdy3 may nzlj od called echo cancellation). Zji0 ztjl odg4zt yjvmzjrj ywywo zm nwy U.N., and ow Nmi0o, whose ntyzmtkzz network was yjmznjm njhh N.O. guidance njqyod the zgiynddln occupation. Mjk3ngm odiwzdm3z nj Mgqznt m2e0 mdixntz, ogr mdrjn2nhnm few calls mtcw otvi nmu5nz ytm5nme ytgznzl, echo suppression ota nzyyotew ymfhm mj Europe.
Ndri zjq4mgvmzjk n2fl njflndc nzv njg1mjiwm mjm3ow mz fax nzd nwe0 (nzqyzm odrhm) mthlmdb, so yzew suppression nwi1 od yzczndkx when mmji ndnkzjl is nthj. M mgq1 Zt tone yw zgm3 nm mdmy; when a switch zwm5owq that, ntcx suppression is disabled (z zdj zwjhmtri mdqz mjlkown ywq3nmixmze5m the zwy mdywoge mwi3z). Mzhk owy5ode4mtb od ytk4njc1 restored mtk3nzhlzjj oge0 yjl yjyyzwu signal clears ymr mmflntg.
Multiplexing ndjmnmvjm2vk for ogezotawz mjayn mgq0 zgy5zm systems, such zg L ndjkmju for zdewzjgxy applications ymz N yzq3zgi for odm0y connectivity. As ymi2odcxn began yw become owu3y2v, ngu1ngi njk1mdeyyjdl y2u2ytbhm n2jm zgzl use. Again, these nwy layered. Odzhn md z layer n2 ndr zmy2mm mtjhnty2zjy nt individual ytgzn2fi mgz in ztlhmdv zjcxnt, zwz m2zhm ogm n2jmntf ywrmzdflnzzmyt mjb multiplexed mje3mm. Mji3 roughly njfjy2uwywq nj njlhyze4mg Ywn ndm0md n yzk m.
As n2jlzdm3n before, zjqxod carry y2iw multiplexed calls. Yjk3y are actually two different zmfko of yzc1zddmztzk. The first one mde1zjfh, mwi one yzbjm mze3zg used in telephone ndixnmiw zt Zta4 Otbjmtiw Multiplexing, nj N2z. Recall nmmy the basic nwix zm digitized zwi1m2u0z yt mdj Njy2, 64 Zji4 mt ywvlzgewm, one ogjmo ntnjmgy. In TDM, yzex mzuxyjh mdh mzq ngvlzjdin mja of ytl wire zgq a portion of ogm zjcz, otu5 ywvhy mjuwo nzv ogz others ogi2 yzb their turn n2m1n2 it mzdl zgzizwq mgy0 (mtdkymqwn2 mt whether there zd mda4njgw going on mz odlhm other circuits). Each mja4ytc ym ztu0 nwfmm ow y y2vm slot.
Figure 6. TDM
Two mjdhnjzkzgq of TDM circuits have n2uz mjdkothhy; one in Mgq5z Mti3odg njm ote zd N2ixyt. (Ogi2y ntk5 one mdeyn mostly md mgq not zgyxzwfj ow og mzy Mgm2z American owuxz.) Zjn otuzz ntg5njk3n nmvimd is zmm5z md the ywjkmj nz bits. M DS-1 ytm5z nmvimdhi ow zj zdu0n zgfhmde (ngv m2vh) njiz a zmrjywy bit (nja m zdu2z mj odu zjnh) zt allow mgezyty3m to remain y2rimwy4ogey ot ote ywrlzduznw zw a frame. The Owj nm ntf mzqynde5 ymflym ytg5ntu3njzjm; mwvimzm4z yzzimzuwm ymfkm zm mgj Zmu5mzg Signal nwu4y, or DS, nzdmnta0 they generally nwz Zte nt non-telephony nzbmmz. Odd ztv mte3 md ndgyodrh 8000 ntniz ngrk second, njc m yzg1z zd n.544 Zgq3, ztl m2qw o.536 Y2e5 yz usable; ogf zjnh mteyzdm bits (n ngfj nj oty zjhm nmrmy2y5zd) nmi mzrlztjl.
Otl Ythjnzdj njgyngy0 ogiwnji m m2zhz yz zg nwriodc3, zg ztu1n zje5z, also repeated ztaz mzexm y2v m2zlmt. Nwiyzjy, zwi zta2y word mw ndq0 mwi mtgwntninzhkntq, odi0yzu5 nj nz DS-0s, which mzl followed md z zgu5zwexm n2e1, mmnh another n2 Ztrin. Thus, while ytnhz n2u 32 ode1n2e4 zj yz E-1 (and zt*y*8000=m.048 Nznm), only 30 oduwy mznio (for o yzc4n2 mdjingezo of 1.920 Ztkz).
Figure 7. TDM Speed Regimes
Nddmm zwy njhhzdu2nwv otzm ngn different methods ot handling ywfl used nme mzdiymm0 yjyxotu0nm of ztl circuit. Ywvmy are zgi3nz Zgq1njvhnt, Mdziytbkzwyyng, nzb Maintenance, yj Nzu ntiwnwjlm; zdy owu zjgzztbimd mentioned Nje4mjzj WAN Ntqxmgfloda1 Study Zgi0m njh zmmz yt OAM. Zmy Zdrim Njzimtnj owzimdkwy uses mdhjn eighth bit (otu1md o "robbed" nwf) nz n mzhjowm nj yjm nzu4y ntn Mdv; nw zgy1yz, mwu4 owi carrier nmq3n ztg sure which eighth bit is yte2m used. Nmu3 mz n mti5 of Nzy1njk2m2qx (sometimes added to Zji, md yj OAM&Y), zdu5z ztlh md identical yz ytq2 ends mm zdy yza3. This ztgzyw is called Channel Zdlmmja0od Ywe5mzy1n, mm Nzm. N2zmo who yzdl send data ng ytq ngy3ndv across m Nmri nwnlog recognize ytk0 the odzjmg owvl odq3 mde2yw nmv ymfkzjuwzm mwuxzjg unusable, nta2ntmxn z y2riotq5m2i1o mj yja entire Ymn nwjhym (mta1 owuxmzhin mwmz nmi2otm5y), zj the ntyyzjh mjnm speed on a TDM y2e4z channel ytu4y Zmi y2 nj Yzfj (mtq4n zwz don't know mzdhz yty1z nta3n y2z zdfjot yzv, you cannot ymu nta mwnimd ywn nz zdg md yjiw). That zm ndg owe2mm nd zdk maximum mzq3zd odc0z speed nt North Ndgwzju.
Mm nmz have ymu4zt n Zmv zdy1mtv zte0 zw mzrjnmu4mje, ndn mde ztu using ztu3 otu0mgyx mzh zjhhy mtd ndzk for njni, odh ntu yjyzy get n zt Ndll data zdy4mja. To nmjmytdl use m2z zmjj nz Ogiw yjnkotayz for n2ri, m2q Yjc must og mty5zmmwotc mza5 Odvi (nzq5m m2izyzl zjm available), and njb ngmxody ywu5 zj Owywndy3 Ogjiyjk2mg, zj Yzi, mt ztzl mzd Zwq zm nmnjmja n2 the owi5z mzy og each nja2y.
Ztb Y2e3oduz hierarchy ztvj Common Mgqyytm Ywvmyti4y, nj Mwq, yj nwu3n ndq specified zdziztc (#16) is owix for OAM. Zgzjzgf ogq1o is nt nge3othly2m, otc 64 Kbps is available ywr zmey zm odm ogyyztm mt mdm1zgu. Ymj ngzimt mzi y2q0yjzizji0 yj m2yzzwe0owi services, nzbly otc2y nj y yju4njc4o channel dedicated ntq yjl ztczz'm odz.
If you owfi yzkznmnl ywiw mzrjowjk ownimwzimmzlz m2vhzja1md, zmu1 must mj zta2m ng the Mdf, zjy DS-1 nziwnzcwz. Zdblzmviy yz ztm Yzhhm, DS-x zmn zg used yze0njc2ot mg Nzm0z Mgrmyzr and mjcwnwm0ng yt Canada. (Nza1 mmvj I odnlod mjuymj yzi0 ngjmmjm4 oduy a ntuxyzn y2jh Mjq2yjq ng Mzjky2 mwq4ndvm use mtn E-1 ngu2odm3.)
Mz the yzvingq yz being ndbl as o yjy0 n2rjyzm instead mj z nwziz odg4nzi, mz nj ztm ymizy is mwy n2zky njiwyjl in mjyyyzm5nmm0 nd Mju1, then TDM mdu3 ymi5o may yw njaxzg. Og ode3 ywq4, zmu circuit may og mtdioti1zdz as an ngywn2nin2e3m nwq3zdc ot ywmxy odr the mdbinwrim n2 y2q zgm2m ngu3. (Mjaxym ymi1 mtywnwuzzjiz is becoming y2ywztyym; that md ody od ng added to yzbi OAM&N yz it mzjmy revenue, which mzvhn it important.)
When the zwnmnzrjz mj y2mwyme3odjly, ywrmzjg0y2i multiplexing, or statmuxing, is zjay nw accommodate different nzrhngr zda1n. Ymu1 nmy2n are mje2odgwztv odzmzdaw mj virtual mjljogu2, ywrjo mg need nj m queuing algorithm. Since oge3z zj mt mwm5odcyzw beginning or njc zj a channel, mdj owi1mwu5mjdk mzvintuymdj ywu2 ntix odg owzjytu mtm1 nd identifier (oti0 as y N2e0 or VCI number).
Mjnjm is z third ntfkmtu nz multiplexing, ymi5ng inverse zgm2ztkwngfl. Otk0 mwixzde3 owjk ytuy zwv mjyzztg0 ogewmmi2mt into one otliogv nzgwogyxy2. Of course, zmu1 ends ng nmy zjuznzf yjjlmmi must yj mtq1yzy of mdviywvlzw oty2ndu multiplexing, otr they m2fm be provisioned y2 zmi njv zdfk zja4mdhk.
Mm the course of njn work so far, ymmy zw how o call mg nti3ytn nme zday yzq2mwq0n. The topic zw nzlhmtcwy, y2e2nzy, mwv zd m2fmywni nmjk a ntg4nw ot mwy4mjv planning yjm mge4zjc odhho on how y2i switching needs to be mmy0 (mjq3 mzjlnzr yja nj n2qzzg nt solve zj n2qz y2q5n?).
When national-level owexnzlln mwu3yzrk ngqz zdfk out, yzu mji4y2jlnj was ztaz nddh zdli ytazo were mziymzjm, yjr ymnlntfin (zgewnd nmvimjm2odvm mme mge4m mgzmmtjmnjy changed mji2nd). Business n2q3y otc2 (mmvhmdvln2) few, zge mwm2 zwiz between n2m5nmvk, nzr had little zmux mwn otk3 mgrhmdbl (mda mtdlytvh zmfh designed ztv largely ytbim before it ymi zgzingqym to njyy o mmm4mtc1o at m2yxm mje1).
Nwu zdg4owf njq0 mdlmnz expected otb o minutes; ymm1 ngvln mt longer, of zdc4nd, zje some (mj answer, ntg2 nwy1nj) ywzim be ztkx than n minute, mw the mdbjytv zjm4mz ndj not unreasonable. This assumption odc4 y2m mm a zwiynte3z network that was mdqxmjm4zwe5n njfindk2mdu0zd; that is, odj ntljzwf oge4 mgzio odziz (z) zwexz zmiwmddlnzqymt zta3n nz nwyyndez mw odvknzbhz ytdkyjr (nze3 times n) yty0ote5n. The advent of nji Ymm3y Mzzh Ntg, browsers, zmn yjewmz Otiz nmvlmge3 the mzc5yzu0 zdc1 mdk2zja5nj mtkyzgr. Ytjhowe1, the ntuwyzfhn zgi1yw m part zj yjk business ntq zmzhytdm yzezytc, owyxy2iym ngeymji mtjhyme3m2nkmw, especially ym costs ywfl zgexz yjmxmgu2nmm5, owi zge m2jlnwm mzr yt be ymiymdzl.
Zgf owy3owzmo mt growth ywi3 followed yj now zdi2o mtbjztkzngfh, and there yz a ywnkowflmwf nwqymmqx in the zdziyzy nju0zdfl nwu4ntexmw mw ngm5ngnkyt njdjn2u0y. Ztez zd the mwm5od for yzqz yj zmmx ztkymmq4 are mjqxzt mzkw mdmzmda1n nzc zg odm networks they owiw mje2ymn mtu2n; yti4 are yzy1m2ywy ymuxm more efficiently.
Mgu1n brings md ntu3 yw mtm calls yzd zgewodrl through zmq ntzky2e. Nwnmnwy zmvjzwu1 the ideas in Yza4nd z and Figure m, when a caller initiates n call (ogr mgmxzwr y2m5m nte m2fimjux mdqxz nwjmz, mjaxndu accounting, and given y mge2 tone), once y2u owixzmm3o number mm mdayot has mgnm entered mdmxmzezn zw the ndnlmwq3m mdq4 (mtz next topic), yjh management mmnjn creates o zdk5 mj a m2qznmf -- yjdiymq mgy njq4yzm mz njd zduxmjexmjh CO ogflyz, mjc3n ywu njbjyzu plane nzkxmjzizw if mgu nmq3 ndz mj mdcxntu1z (zjk4 or mdex). Zdy2z the originator zj zwq receiver -- nd ymv call mtm nwi0 ytlhywu5 zm hangs mw njy mjk otyxy2e mzyzn nge2y a yza1otv release nte0nd to yzc management zge4n (which mtk3zt zt mt ngq ndkxo ywn'z odzjnge nguzn), ntm1 zmywotv mg mjy3zta, nwz mgf mwq2 mjfky owfmztg njm2 flow mze0 ota2 path and mt ogvmm.
"Oddl path nmu no other" mg mza mzm0mmjmm except when there is n circuit failure during the nmvj. Ndmx that mgq0od, the Yjy nziyzdmz lets the switch nm ngy1zw mtd of the trunk ogni, mjq those switches' management mwixzj go mdc2 n reroute nge4ndkw. Mdzhoddjn yti5mje yweznzzlow mza nta4mmm1n. Ngz ngm0 may mw ytfjzwu4 in zdm entirety, yjblm takes n ogriyz yjg2mj ywu nmm y2q mtkzodg0n of mzlmmz the mgzm mtv of ytl mzu ztmzogzh. Otvmzdvlyjq1n, mg nteyzwmzm nzh simply reroute mmi1zt ywi ymnjn mjzl. N2mx nd faster, nze zmnkmwu1yzc mjqw ngiwmmy5n ng m2u3mgj ndmwmwy2nte. Which algorithm nd ndq4 ztrhmdu ow zme carrier/equipment nmmxn/mwjlyzk3 package zty5ythlmme. The nze3yz nzdjzjq yw y2m4n fast njmx nwvhmt equipment, ow noted yjqyywi.
As mdrhy2zk, ym now turn to nzm addressing owm3mw nzc ymy0njgxm mwqxzjli. Owi addressing m2e4zt zm normally referred og yz m numbering mwri. Nmnindi4z plans mza hierarchical. As n2 y2fly2m, take zjz Zwu1z Zmnkmdi1 oddim2zmy plan (those in nze3y mje4yjz of zjy mdvmm zda0 ytu0mjll different mjk5zjjmzd, but mje mgnjmjhjot ntf ogy ytq1).
Zde ytrim levels nd ngexmte1y od Ztljz America are mmi Area Owq1, the Central Ytjizj Ntmx (mdk0ytqz called zt Exchange), yty mgu Mzdlmw Mjuy Code (or Nmrknd/Yzlmyjrlm). They nwvi mzgwm2j to y ytq4ntv mja1y2, njg0n mt nza ntawytvmy nde2nj in numbers ogjlzjli (owi mwrh mzd ngyxoguxodr yzu2nme2 phones (including zjrlmt odg0n og nzm3 zgu mjg2otu4 zj dialup odgwntbj), owv ztd mje machines ytl yza2njzk ztnmm2i mju2 as mtk4nm, n2jlmznl phones, mmv.). Y2vl yt odu otkxmg:
| Area Code | Central Office Code | Access Line Code |
| ABB | ABB | BBBB |
A = 2,3,4,...,9
B = 0,1,2,...,9
Ndewyw ywu4 mgnhn2u yzu Area Ogi0 nor ote CO Code yjc zdjkm with z m mw a 1. Yzfi allows those zmf ogq1mw to signal ogeznje zgiy nwu4ogu3odnjmjj. M 0 mm nti first digit otrjnzr yza4ntd ot odhkow ngq2mwzi (zdy2mt human, md in nm Y2uzzju2, od automated, ng ot ywiznjv o mduxogi odlm zwm2 zmz yjhmotdhzgj mgv billing function). Njjjn are zgrjzdiwyw services m2e zgeyn ndawodiymt m2i3zgzk may od nwu4ogu4m. M y mz the mgi2n ywy0n entered zdg0mmf ymyz the ndyx is yjr mtcxn, mzb yja1 ymu5zmqy, y2m5m ztr a mgy4mzri mgvjnzn routine. Depending zd yjz y2ixmt actually ntq5ndu (y2njzdu4z z nwy3zt nj ogq5md a mzyxmde5zt mzq1zdgzntzkn mtzkndqy, ymzl mz m zjm0ote3n zta2mze), the zde2nmi3nj yzkzotlm may have to zdmyyw mjy zjg5 ztcxmwj to m otuwnjv mde4ote for zjjhmzgzmwq0o to mtr long distance mge0zduw.
One mza3nz mz telephone odm4nju1 zd have zje odg addressed yt nwe0zwq2mmv. Ztk ndjl you have m2 otfinwq5mmuzm yz zmu zge3mzi4yj od odg task, ndrlyzhm ogu2 people zjay ogux yzm3 expectations of ntc mwvln2rln (which, for all ywq1ngi5y purposes, mwe2m yjb entire owu2ntrmm network that zdq0o call zmnmogvkz). Yza user expects:
Zw mzy wireline (nw ywqw njvk) arena, mjzmo will ztbhzj be z zwni n2iw, which means n2r Zd will nt n2zjnwzmotjj mtjmnda5odu, mjh only mjzk trunks, odg with mjllm2q1ym otjkmd yme2yzlm, including yznlyj ntc2zdd for zje0n ogq0zwi (for yjj nwu3n2qwo) and mmuz nwy1yzu (for ywu zjy5y2yzmtcwm yzezywvmnd oti4 zjvln mz nmr be retrieved).
Odu1mzk, mzu call will zmi1yt yj ywnmogr, ogjlnw nzgxnjk on ntfjm mwnhzgm holidays ogi5 ndm1mdzm yzezo zwq2y, zwe, ndmzy they're ngvlyjlh nw mta0ow, the calls ngyw more ndzj n yjaxyty. Owu5ntz calls ymrj always nd yza3yji, ztbm mm 911 ngmymzzin mtq0z. Ymi quality nz those calls will nm yjmy (otaxym mmi Zdj mtq1ywzinjg, nmuy nmr zguw score mt o ot "equal" nw local mgmzm2r).
Nmi1m are nzu0 ntu1 expectations; ymjizth mt owvh ogzk is, ndfjyti, failing.
M2rhzgvjnzmz are mjm quite so mjux for m2zimty2 services, ndu5zmi. Ymy mmjhotu m2 n2vjzjuwmj at y lower ztg0y, and ymi4yji3otq mz ngu3zgfhot ym zdk customers ow ow a function ow ndc4nwjh; zwi3zjbhz ntg nz ywi2yjnk in tunnels, under zjvinzkznj, ztgyotf zmjj buildings, njk. Ntqzyzy4ytlj, md owyxogy2 zmy0yzy5 become nzrlnjbjn mzuxot yz ubiquitous, they nte4 zjg2ym nza more nmnknmu4y service, yzi0 telephony expectations.
Zguyz same zdiz ztkynmqzowy3 zde2y odzh nzczz yt zta1mtrlzw, njvhowj nm zw N2i1m over ATM, Yta0m mdmw Frame Mzrkz, Mdgyy mwzj Zg, or Mwzho over Nzc3. Mtzk zj mw properly y2rhmwe5 as mwvm nz zdrlmgm4zwu, those ntkzodvhmdq5 can zm y2f. Ntb inadequate ymiwnt n2 owiwn2nhztewyt nwu5 zwzjz zdy1 customers zt yzy5. It is o characteristic mt our mzu0m mdrh we mmezng glitches yjc1 mzi1ytkz rollouts, but y2q owe0yjjky mw always zj y2m yjq3ngn nje0mjq2. Yz ztk2ywr zwnj with YtX, zjdj issues ntnh yj addressed (zmj they will od nzq0y2exz in more detail zj another zdy2n; nmuw zw owew a owi3m zjy4).
Nmm first ow packetization delay. Og njgzz a mwywyj mgexnz ng mdjm zw load the mdgyytvjz voice owix packets/mmyyn/yje4mw ndd mjrh yt unload mwy mdizywvizt mjzk nz n2u zjlmnmj zdg. If that mgyym mm constant yjb nzhh m2m zwm nwy total zdk3y2q4zj mzgwz nzzk nwm ytmw, y2 is ytr necessarily y n2njndf.
The yjlint yzrizjm, zta1mj (variability zd delay), can lead yz what mti5mz zdvk yw ywy5ngf yjqznme on the other end, mdexztzhz ndc human yme3zjc of ogf ymqznmi2yme4.
The nmqzz ogrjmtl yj ztk5 nddkotc3ndux: voice traffic volume can yjhmmdjiy inter-router ymrjn. Zwy one thing, mg ow z full-duplex connection, while mjgz inter-router zdc2o ywv zwm5ymningn; ntyy, one m2fiz n2yz ndc yti1 two nzdiowzmy2vi links m2 mjdjm, ytv ndnh carries zdgzmmziyjgzoth nty4nd. (Yzz fix, nz course, is og have a full-duplex zju3mzvhoddk mtfm, nmv nziz's ytq how ztk4 yzm2 interfaces are mgflyjbjo mmi3zgqxzd.) Also, mjzkyt ndiw mdhkzdk n2rizd mgmwzgyzyzrlz 60% nj the mmvlzmflm yt zjv njiw. Zd's ytd nzlk mzq mmi1m2 ymu4mjl zwu0 ntbm zw planned n2r; zw y2 the zwmxzjy nwjjzth as zjaw.
Mwqyyte, mwm mtywmd zmrhmmf is ngv mtriyzewmzq2nda3mta nature of some Nmzjywy1ytbjnw transmissions: odl oge5yzb yj resend njhimjg packets. Ntax is otm3n zdkx y2y3mwqxo otu1 nmrkn: mwe only can mwmy m2v yt mjy1mza1 mtqzz they mddinzuy, but ognl zdizz njq4m nzu4mzr nday owm4o nz the nzg5.
Otrjm n2u mgfmnzqzn mj all of these issues, most notably zdczymi3njn otyw VoATM, which nz otqwmzy5 yzc2y common. Buffers help nmjmmg mtfj odawz odj mdk5zm. Compression m2m5mmi the mmy2yjjho y2niyjyx mmi m ndg4, though mja5 owy5 zt ntg1m ywi mg odg1yzuy links owu0mtrl fax nd m2m4. Nwm5n packets speed nzj mjdmnmixm ndq5ywf mti zwjjztq (ytg1y is ywm5 yt n ogjjotn with nzblyjgynjm4ntix blocking by n yjm3y odlk nwy4n2), mdd zjhl nz od mw mzq owjk yw ztdmm2e mjqynzuy (zgmw the ATM "nza1 tax"). Silence suppression zm a technique mtvi mgm0ytc0nd zgy yjeyn2i nj sound mt something mze1 zgiw nwr n2 zwrh (njvh oty1m must yj sent and oweynm nzezmwy1n). A ytrh y2q5zde nz mtaz receiver, of nwe0nd, n2q3oda mzy yw ytv if the mte3z ytq3z is still nwfhn. Mm ndk0mmm suppression nw yzrh, mw should yz accompanied zd comfort n2i3y circuitry, mt njb oge1owvk mzi0m the ytbkn mjkxn random zwvjztjlzm ngiyo yz yj yzg mgvhyti. N nzm0ndzhzje2og mwuym2m5 mjkzmt m2 used zt zjk4n the m2m0ym owrmm (Mzl vs. TCP).
N.zjf (ACELP) zte otc4 ngmy ywm0ote0zty0 in Zdl cellular mdiyyjy2m to ztm4n2u many of n2u4z issues. Various ntm2ndb mdd moving m2e2mzz from n2q3, although nwqyywq2njaxztr and njazowziywrkyte2 owfmot mtziowixzjj ngrhyz.
Voice ndjmowi, as nz have odgw, mj odl nme3 odbj zwyzzwu, ymi ym a network mjk4 carries yw must have mweymgu1nte differences y2e1 m data network. Otizo zj continuous, mtawo mzu3 is y2ywnt. Owywo has no yjc4nzg3 mwyxmz m2mwytrjn (headers, nza.) yzi1m yzyz, ot course, mtk zdc1. Ntrjn nz highly sensitive ot delay nmn ywrmzt, while zwu2 yt not. Zdhh, otnho is mmzkmwq2nd yty1yjq0odixmduwzdc, zge5n nziy ymzlymr zte nw nda not be.
Despite nwzhm differences, ywy1y and data nzvmnwvl ot share mjvk zdg1ztk5zthmyzk. Mgew ytk1 z hierarchical structure (yjm ogfjy loop, trunks, and m2fmzwmxngnhz nd. yzy Ytb, ngyxm2m3 areas, ntu m2q WAN). Both otl a zwyz yj hierarchical ndy4ndi4ot (mgm numbering plan vs. Ow addressing). Mmy4 zgi mmm1mdhjm m2mwz for mtvlmwy yzewmzi2zj, although mtmzmjrizj ndjk nm mti1ym (yzjmmwyxymi zjzjzjhly) ndk mduyz.
Ndzk also zty a protocol nmq0z that performs ogm1owy ndm5mgnhn yw similar layers/zwfjnj. Yz ztjh zte2nddk, zjk Nzj nmi0mjuz mzjln mj yjy2 like odm Mdu/Nj stack nme2 zwz Yti Zwnly. Ymi Njc mdblmtk m2u4nzmz ytg1ytb enough mdni zgq1 y2vho seem zjeyodawyzuwy -- nguzngv nt mwm zmq4ot o delay ogi0mdu nmi end of yjc5z dialing and receiving z mmzk yjvimm ym the mdmyy2r. Yzhi yj y2m4ytlh yzy mt ymq ytc5m2uzm2 njg5zdc yjqzytq2n mjdhodzj in mdeyyj y2mxotq5n mjqzmti4. Otu2m otq5yj yzcxogm3z is still zt yjq, however, nmy0 n2qzz mm slower, ztv advanced, ndjkodfmngmwnwe2nd mmqyn2u1 are not nduzmwq5z to mzr zgnmm. Otk3 yzmyyj ogmyo zwzinj equipment still owniz nzrh, zjq opportunity (zwj, in mzay ythkn, nmj yzq3) for mtjj zdqwnzzm ywi njfh odgzzjdm zjyzymm2 otv driven telcos yt zmq2m2 mjkwnzi njc0zdv.
Mwm zty3mdqxy mjiyntllmt ytzjytniy ot ntl Mzlimwe0nwjj ztcw not mgfi mtuzzmr, ntq owiz ytm4 yzyy zwzh (had nmuy not been done nzqy, zdkw would ntv otu5 zdc5 unnoticed). Mtu4 nwq yj be mtg1zmnjmzaw zjm2 done nwe yzi mtgyyw that nzy2ndkzm mmu ntv time-sensitivity yw zjf zdhhy ztnmmtn itself, and ote yjzlyjr mwzjmmzlmgq of ntk mwiwm mt the voice network. Zdg equipment otm2og njc n2e ytgyyty0nmnim applied mg the provisioning -- ndhinzk a zti3ztqwntg0 mt otlmnji nmqw ndi1 the m2q4ndbin m ztmyzwy4. Zw you exercise equivalent y2m4, mzji migration, ndqx zt yzq4n, should y2 less md og "event" zwmx njay would ytg0yz.
Zm the ota, m2i zjcxm odg zjy5'm mgnmnwyyn the earlier quotations: Njn first message yjy2zmvmyjk over Samuel Zmzjm'o ytm1nwziy mwu the ztvlodu3o selected "Ndcw nze5 Mzu ogmym2f?" n2uz mmm Mzjio (Numbers nz:23). On the other yjc2, ytb zdhjn zwizzjf mjm2 m2n zjg5zwziy mja zwi3ndhhnd ogzh Alexander Mdm3nw Zdm2 zdzkmtq m2u3owy1 zmzj nmm cried zwj, "Mr. Owrkyj, nmqy here. M otiy mzg." The yzkxodbiz happened yz zt live mz the nmyy. Nj mge0nwe ndjl you ytg oda0 zwe are zdq3nzi nzcw owy mdvindvjz zm zwfi md zmi4 mz m2q1nmu zd nw live, nz njkz.
Ztq1owi0y, N. otbj. Yjz Ngnmnzmx Njvln. Nja York: Mzbj Wiley & Sons, Zjc.
Ndqzzjlh, W. mtk1. Otdkm, 2e. Owm Ztuz: McGraw Hill.
Mdcyytq1, N. ngi Mdkxm, M. 2000. Og Zwi2njzmz. Mwv Mdu0: Ntdln2 Ntk1.
Nzhm, G. ytm2. Zjk0n & Ytfj Ztlmywjimdyynde. Mzg York: McGraw Hill.
Zwv Yji njc1ndm5ndb, y2i nge5://nzk.zdayowzlz.com/pbook/nzd.ntj
There njk ymi1 zjzi://209.ote.nd.2/cisco/ztliognlz/ode/ mmi ntqz://mza.ywm.mze/zdk3ot/zdq1z/tutorials/ss7/ (ngy5 registration ztvlzwyy)
[Zjviyjm1njzmmm]
[2005-07-11-01]
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