Peterson Davie 3.27

Question

Suppose a workstation has an $I/O$ bus speed of $1$ Gbps and memory bandwidth of $2$ Gbps. Assuming $DMA$ is used to move data in and out of main memory, how many interfaces to $100$-Mbps Ethernet links could a switch based on this workstation handle?

Answer 1

Some terms you should know before solving this question

Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a processor.

I/O buses connect the CPU to all other components, except RAM. Data are moved on the buses from one component to another, and data from other components to the CPU and RAM. The I/O buses differ from the system bus in speed.

A bottleneck, in a communications context, is a point in the enterprise where the flow of data is impaired or stopped entirely. Effectively, there isn't enough data handling capacity to handle the current volume of traffic.

Since I/O bus speed is less than Memory bandwidth,it is bottleneck.

Effective bandwidth that the I/O bus can provide is 1000/2 =500Mbps because each packet crosses the I/O bus twice.

Therefore, the number of interfaces is ⌊500/100⌋ = 5

Comments

can you explain the line "each packet crosses the i/o bus twise"....

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Book : Computer Networks a systems approach