VSAT Networks - computeristoy

158 NETWORKING ASPECTS
where TS−ALOHA is the average delay (s) for S-ALOHA, Tp the
satellite link propagation delay (about 0.25 s), τ the packet duration
(s): τ = L/RTDMA, where RTDMA is the carrier transmission rate (bs −1 ),
G the offered traffic load (packet per time slot) and k the maximum
retransmission interval (time slots).
Figure 4.34 displays packet delay curves for a constant length
packet traffic, with packet length L = 1000 bits, and packet generation
rate λ = 0.1 s −1 as a function of the number of VSATs in
the network. Three multiple access protocols are compared: FDMA
with delay given by (4.27), TDMA with delay given by (4.29) and
S-ALOHA with delay given by (4.30). For FDMA, it is assumed
that a VSAT transmits one carrier only (K = 1). For S-ALOHA,
three values of the maximum retransmission interval have been
selected: k = 10, 50 and 100 slots. The network capacity is constant
and equal to R = 100 kbs −1 .Thus,RFDMA = 100 kbs −1 /KN =
100 kbs −1 /N, and RTDMA = R = 100 kbs −1 . The TDMA frame duration
TF varies with the number of VSATs: TF = (number of VSATs) ×
L/RTDMA = number of VSATs × 10 −3 seconds.
Clearly, S-ALOHA delivers a shorter delay than FDMA and
TDMA. This demonstrates its greater ability to convey bursty traffic.
Packet delay (s)
10
1
0.1
WWW.DANVIENTHONG.COM
l = 0.1 s −1
Network capacity
100 kbs −1
FDMA
TDMA
S-ALOHA
k = 100
k = 50
k = 10
0 50 100 150 200 250 300 350 400
Number of VSATs
Figure 4.34 Packet delay for a constant length packet traffic, with packet
length L = 1000 bits, and packet generation rate λ = 0.1 s −1 per VSAT as
a function of the number of VSATs in the network. The network capacity
is R = 100 kbs −1 With S-ALOHA, k represents the maximum retransmission
interval in time slots