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83C152 HARDWARE DESCRIPTION
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Figure 3.2. GHG cienerator
3.2.3 INTERFRAME SPACE
The interframespace
is the amountof time that trans-
missionis delayedafter the linkis sensedas beingidle
and is usedto separatetransmittedframes. In alternate
backoffmock the interframespacemay alsobe includ-
ed in the determma“ tion of whenretrans
missionsmay
actually begin. The C152allowspro
grammableinter-
frarmespaces of even numbersof bit times from 2 to
256. The hardware enforces the interfkasnespace in
SDLCmodeas wellas in CSMA/CDmode.
Theperiodofthe interframespaceis det
erminedby the
contents of Ill% IFS is an SFR that is programmable
fromOto 254.The interframespaceis measuredin bit
times. The value in IFS multipliedby the bit time
equalsthe interframespaceunlessIFS equalsO.If IFS
doesequal O,then the istterframespace will equal 256
bit times. Gne of the considerationswhereloadingthe
IFS is that onlyevennumbers(L3Bmust be O)can be
usedbecauseonlythe 7 mostsignificantbits are loaded
into Ii% The LSBis controlledbythe GSCand deter-
mineswhichhalf of the IFS iscurrentlybeingused.In
somemodes,the istterfratnespacetimer is m-triggered
ifactivityis &tected duringthe fmt half of the period.
TheGSCdetermm
“ eswhichhalfofthe interfratnespw
is currently being used by
examiningthe LSB.A one
indicates the first half and zero indicates the
second
half of the IFS.
After reset IFS is O,whichdelaysthe first transmission
for both SDLCand CSMA/CDby256bit times(atk
reaet,a bit time equals8 oscillatorclock periods).
In mostamlicatiorm the rxriod ofthe interframespace
will be
e@lto or ”greakr than the amount of iime
needed to turn-around the receivedframe. The tum-
armsndperiodis the amount of time that is neededby
user softwareto completethe handlingof a received
frame and be prepared to receivethe next frame. An
interframespacesmallerthan the requiredturn-around
periodcouldbe used,but wouldallowsomeframesto
be missed.
Whena GSCtransmitter has a newmessageto aend,it
willfmt sensethe link.If activityis detected,trrmamis-
sionwillbe deferredto allowthe framein progressto
complete.Whenlinkactivityceases,the stationcontin-
ua deferringfor one interframespaceperiod.
As mentionedearlier,the interframespaceis useddur-
ingthe collisionresolutionperiodas wellasduringnor-
mal tr
ansmission.The backoffmethodselectedaffects
how the deferenceperiod is handled during normal
transmission.If normal backoffmode is selected,the
intcrframespacetimer is reset if activityoccursduring
approximatelythe tint half of the interframespace. If
alternate backoffor deterministic backoffis selected,
the timer is not
reset.In all cases whenthe interfmme
spacetimer expires,transmissionmaybegin,regardless
if there is activity on the link or not. Although the
C152resetsthe interframespacetimer inactivityia de-
tectedduringthe fmt one-halfof the interffamespace,
this is not necessarilytrue of all CSMA/CD systems.
(IEEE 802.3recommendsthat the interframespacebe
resetifactivityis detectedduringthe firsttwo-thirdsor
lessof the interframespace.)
7-22
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