Traffic Management for the Available Bit Rate (ABR) Service in ...

the performance is fair. Also, the sources use the entire ACR allocated to them. In
other words,the TCP sources are rate-limited and not window-limited. Note that given
su cient time, the ABR switch algorithm can control the rates of the VCs carrying
TCP tra c. We shall quantify this time and corresponding bu er requirements in
section 8.14 later in this chapter.
Window Size in bytes
two-tcp/option-g=1/optionb=295/optiont-g=6/sw_qsize-g=4096/tbe-g=1024/granularity=100/wnd_scale_factor=4/epd_thresh-g=2045/icr=10.0/air=1/xdf=0.5/tdf=0/headroom=1.0
t_threshold=900000.0/maxsrcrate=10.0/ontime=100000/offtime=100000/vbrrate=124.41/t0v=120/a=1.15/b=1.05/qlt=0.8/time_int=1000.0/sw_int=100/dist=1000 / Date:02/03/96
1.2e+06
1e+06
800000
600000
400000
200000
ICR: 10.00 10.00 10.00 10.00 / XRM: 32.00 32.00 32.00 32.00 / Graph: 1
Two TCP : Cwnds
Cwnd Size for S1
Cwnd Size for S2
0
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Time in milliseconds
(a) Congestion Window
Rates
two-tcp/option-g=1/optionb=295/optiont-g=6/sw_qsize-g=4096/tbe-g=1024/granularity=100/wnd_scale_factor=4/epd_thresh-g=2045/icr=10.0/air=1/xdf=0.5/tdf=0/headroom=1.0
t_threshold=900000.0/maxsrcrate=10.0/ontime=100000/offtime=100000/vbrrate=124.41/t0v=120/a=1.15/b=1.05/qlt=0.8/time_int=1000.0/sw_int=100/dist=1000 / Date:02/03/96
180
160
140
120
100
80
60
40
20
ICR: 10.00 10.00 10.00 10.00 / XRM: 32.00 32.00 32.00 32.00 / Graph: 1
Two TCP : ACRs
ACR for S1
ACR for S2
0
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Time in milliseconds
(b) ACR
Figure 8.6: Two TCP Source Con guration, Bu er=4096 cells, TBE=1024
8.10 E ect of Finite Bu ers
We now investigate the e ect of smaller bu ers, keeping the ABR capacity xed.
The bu er size is set to the product of TBE (512), the number of sources (2), and
a safety factor (2), i.e., 2048 = 512 2 2. The remaining con guration is the
same as in Section 8.9 i.e., n = 2, TBE = 512 and xed ABR capacity (no VBR
source). Since the bu ers are smaller, it is possible that they might over ow before
the ABR control loop is set up. We expect some cell loss and reduced throughput
due to timeout retransmission.
270