1.4.4 Throughput in Computer Networks

 

• The instantaneous throughput at any instant of time is the rate (in bits/sec) at which Host B is receiving the file.

• If the file consists of F bits and the transfer takes T seconds for Host B to receive all F bits
•  the average throughput of the file transfer  ==  F/T bits/sec.     

• Rs < Rc   :  the bits pumped by the server will “flow” right through the router and arrive at the client at a

                     rate of Rs bps, giving a throughput of Rs bps

  Rs > Rc  : the router will not be able to forward bits as quickly as it receives them. In this case, bits will only leave                      the router at rate Rc, giving an end-to-end throughput of Rc.

   

• two-link network, the throughput is min{Rc, Rs}, that is, it is the transmission rate of the bottleneck link.

 

 

 

• the constraining factor for throughput in today’s Internet is typically the access network.
• Suppose Rs=2 Mbps, Rc=1 Mbps, R=5 Mbps, and the common link divides its transmission rate equally among the 10 downloads.
• Then the bottleneck for each download is no longer in the access network, but is now instead the shared link in the core, which only provides each download with 500 kbps of throughput.
• Thus the end-to-end throughput for each download is now reduced to 500 kbps.
• Figure 1.19 and Figure 1.20(a) show that throughput depends on the transmission rates of the links over which the data flows.

• Figure 1.20(b) shows that more generally the throughput depends not only on the transmission rates of the links along the path, but also on the intervening traffic.

 

 

1.5 Protocol Layers and Their Service Models

 

1.5.1 Layered Architecture

 

• Each layer provides its service by (1) performing certain actions within that layer (for example, at the gate layer, loading and unloading people from an airplane) and by (2) using the services of the layer directly below it (for example, in the gate layer, using the runway-to-runway passenger transfer service of the takeoff/landing layer).]
• if the gate functions were changed (for instance, to have people board and disembark by height), the remainder of the airline system would remain unchanged since the gate layer still provides the same function (loading and unloading people); it simply implements that function in a different manner after the change.
• For large and complex systems that are constantly being updated, the ability to change the implementation of a service without affecting other components of the system is another important advantage of layering.