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in Section 24.4 that gets around this current limitation of TCP.
The value 1,544,000 bits/sec for a Tl phone line is the raw bit rate. The data rate is actually 1,536,000
bits/sec, since 1 bit in 193 is used for framing. The raw bit rate of a T3 phone line is actually 44,736,000
bits/sec, and the data rate can reach 44,210,000 bits/sec. For our discussion we'll use 1.544 Mbits/sec and 45
Mbits/sec. Either the bandwidth or the delay can affect the capacity of the pipe between the sender and
receiver. In Figure 20.11 we show graphically how a doubling of the RTT-doubles the
capacity of the pipe. Figure 20.11 Doubling the RTT doubles the capacity of the pipe.
In the lower illustration of Figure 20.11, with the longer RTT, the pipe can hold eight
segments, instead of four.
Similarly, Figure 20.12 shows that doubling the bandwidth also doubles the capacity of the
pipe. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (17 of 24) [12/09/2001 14.47.22] Chapter 20. TCP Bulk Data Flow Figure 20.12 Doubling the bandwidth doubles the capacity of the pipe.
In the lower illustration of Figure 20.12, we assume that the network speed has doubled,
allowing us to send four segments in half the time as in the top picture. Again, the capacity
of the pipe has doubled. (We assume that the segments in the top half of this figure have the
same area, that is the same number of bits, as the segments in the bottom half.)
Congestion can occur when data arrives on a big pipe (a fast LAN) and gets sent out a
smaller pipe (a slower WAN). Congestion can also occur when multiple input streams arrive
at a router whose output capacity is less than the sum of the inputs.
Figure 20.13 shows a typical scenario with a big pipe feeding a smaller pipe. We say this is
typical because most hosts are connected to LANs, with an attached router that is connected
to a slower WAN. (Again, we are assuming the areas of all the data segments (9-20) in the
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