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conve:rsmon generate, create none none with none projectsprint data matrix .net Communications Oracle's Java Chap. 5 echo suppressors on th none none e.cbannel are all reversed. Though the telephone companies have undertaken a program to upgrade their equipment in order to eJjminate echo suppressors, many remain and probably will remain for the foreseeable future.

This is primarily a problem for dialed lines, as d.edic:ated tines are conditioned in many ways which preclude the need for echo suppressors..

The tumaround delays r equired by channel settling time and by echo suppressors may be established by timers in the terDrlDal or host equipment or may be compensated for in the modem itself. The modem provides two signals for this purpose: 1. A Request To Send (RTS) sigDal to the modem, requesting permission to send data.

2. A Oear To Send (crs) sigoal from the modem, indicating that data may DOW be sent..

In actual fact, the 0D none for none ly logic that links the sigoal to the RTS sigoal is a timer in the modem. The time-out is set by the user to the minimum safe time determined for channel tumarouDd. Typical timer values range from tens of nriUjseconds to hundreds of.

nrilJjseconds. Oearly. channel tumarouDd delays can have a sigDificant impact on communication channel perf"0l1D3IlCe.

A lOO-msec. tamaround penalty for every 2OO-byte block over a 24OO-bitIsec. c:bam:ael requiring (8)(200)12400 667 msec.

per block is not to be taken ligbtly. This becomes even worse for a polled channel, wherein a typical poll sequence might be 3 eharacters and a poll response 1 character (10 msec. and 3.

3 DJsec., IeSpectively, at 2400 bitslsec.).

Adding 100 JDsec. to each of these completely distorts the pmfomJance picture..

FulI-Dup1ex CIIann_. A full..duplex channel is conc:epcua1ly simple.

Both sides of the conversation can transmit simu1taDeous1y. Full-duplex c:bannels can be derived in several ways from physical channels. For iDs1aDc:e.

two separate cImmeIs may be provided. one to be used for each direction. This is usually the tedmiqueused for higher speed CQI1JID1IDicaDODS.

For lower data rates. a single physical channel may be divided iDro two logical channels using FlequeDcy Division Multiplexing (FDM). as described earlier.

The lower half of the frequeacy spectmm supported by 1be channel may be used to send data in one diIection, and the upper half of 1be.frequeacy specuum may be used to send data in the opposite ctiIection. This teclmique is used by.

low speed (300 bit per second) modems. One significant advantage of this technique is that a fuD-duplex cmmec:tion can be established over a dialed line. Though fuD-duplex channels me c:onceptualIy simple.

the message tmDsfer pr0tocols RqDired to take maximum advantage of the full-duplex capability can be quite a bit more complex dian those used with half-duplex chaUDels. These protocols are discussed in detail in the next section. .

. Chap. 5 Protocols PROTOCOLS The previous discussio ns can be perceived as descnbing the process of data transmission. The facilities and teclmiques described allow sequences of bytes to be delivered between. users. Let us now disc none for none uss data communications. In order to have a meaniDgful communication of data between a sender and a receiver, the data must be identifiable and transferred reliably.

The procedure for accomplishing this is called a protocol.. A protocol is an agIee meDt between the sender and receiver of data as to exactly how that data will be transferred. Protocols provide tIuee primary functions:. 1. Message identificot none for none ion-They identify the bounds of messages carrying the transaction and response data. 2.

Do.tIl protection-They protect data agaiDst euor, ensuring its reliable delivery..

3. Chtmnel alloco.tion -They provide the mec:lwrism for allocating the channel in an O1derly mauner to the various competing users of the cbarmel.

. Protocols typically ha none for none ve tIuee distinct parts:. 1. The establishment p rocedure, which serves to establish a virtual CODDeCtion . between two users (or many users in the case of a bmadcast).

. 2. The message transfer procedure, which describes the form of message ttaDsfer.

3. The termino.tion procedure, which bJeaks the virtual c:cmnection .

.The establishment and none for none tenxrination plocedutes saDsfy the ehaUDe! allocatiOD functiOD. The message-traDsfeprocedure is the message-ideDtifcaticm fImction.

DaIa potectiOD spans and sigDUicandy complicates all pmcedm:es. Tbere me many standan:Iized protocols, and their SIUdy alcme would fill volumes. Our COIlCeED is to 1IIidea:staud1be basics ofpmtocols fmma paformaDce Viewpoint so tbat, given a protocol specific:atioD, we can evaluaIe i1s pedomumc:e wiI:biD a given ccm""PIWc:atiODs eavimnment Therefole, we wiJl SIIIdy some classes of prorocols and zeJate tbem geoen1ly to some of 1be IIlOIe popular protocoJs in USe today.

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