e., placed in HOL position) will be
dropped (note the difference with P{LOSS}, which instead, represents the probability that a
randomly chosen frame, among the contending ones, is lost). Assuming independent frame
dropping, the average number of dropped frames between two successful deliveries is thus
given by the ratio pR+1/(1-pR+1). A dropped frame will be forced to cross all the backoff
stages, from stage 0 to stage R, and in each stage i it will spend, in average, (1+ ??
i ) slots.
Hence the average delay for a successfully transmitted frame is given by the average interdeparture
time between successful frames, namely, the first term in Eq. (48), minus the
time spent by dropped frames. Hence, we might have directly written Eq. (48) from this
intuitive reasoning, with no need to provide any formal derivation at all!
{ } ( )
( )
( ) ??‘
??‘
=
+
+
=
+
+
+
??’
??’ =
+ ??’
??’
??’
=
??’
=
R
i
i R
R
R
i
i R
R
p
p ] slot [ E
] P [ E / S
N
] P [ E / S
) p ( N
p
p N
] P [ E / S
LOSS P N D
0
1
1
0
1
1
1
1
1 1
1
1
??
?? ?„
4.3.4 Non-Ideal Channel Conditions
The performance analyses described in the previous sections were based on the assumption
of ideal channel conditions.
Pages:
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260