This question concerns the delay-bandwidth product. We often use approximations

Question

This question concerns the delay-bandwidth product. We often use approximations when computing the delay-bandwidth product - so follow the guidance below to compute the required answer.

(Pay careful attention to units. Check your math with common sense and experience. If the total delay is significantly larger than the time it takes to retrieve a fast web page - then something is wrong with your computation. The delay-bandwidth product amount of memory (as a minimum) would be typically set aside for each connection. Given that a typical web page opens many connections - if your answer times 100 exceeds the RAM in your computer then there is something wrong with your computation.)

-----

Consider a path that is 2718 km in total length. The path is primarily fiber optic, so the propagation velocity we will use is 2.0*10^8 m/s.

There are 7 router hops. Assume that each router hop contributes 1.8 ms of queuing delay. The queuing delay is the same in both directions.

We will ignore the transmit delay.

We have a broadband network connection that is capable of 45 Mbit/s data rate.

What is the round-trip delay-bandwidth product in Mbits?

Convert your answer to Mbits

Explanation / Answer

In data communications, bandwidth-delay product refers to the product of a data link's capacity (in bits per second) and its end-to-end delay (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the maximum amount of data on the network circuit at any given time, i.e., data that has been transmitted but not yet acknowledged. Sometimes it is calculated as the data link's capacity multiplied by its round trip time.[1]

A network with a large bandwidth-delay product is commonly known as a long fat network (shortened to LFN and often pronounced "elephan".) As defined in RFC 1072, a network is considered an LFN if its bandwidth-delay product is significantly larger than 105 bits (12500 bytes).

Ultra-high speed LANs may fall into this category, where protocol tuning is critical for achieving peak throughput, on account of their extremely high bandwidth, even though their delay is not great.

An important example of a system where the bandwidth-delay product is large is that of GEO satellite connections, where end-to-end delivery time is very high and link throughput may also be high. The high end-to-end delivery time makes life difficult for stop-and-wait protocols and applications that assume rapid end-to-end response.

A high bandwidth-delay product is an important problem case in the design of protocols such as TCP in respect of performance tuning, because the protocol can only achieve optimum throughput if a sender sends a sufficiently large quantity of data before being required to stop and wait until a confirming message is received from the receiver, acknowledging successful receipt of that data. If the quantity of data sent is insufficient compared with the bandwidth-delay product, then the link is not being kept busy and the protocol is operating below peak efficiency for the link. Protocols that hope to succeed in this respect need carefully designed self-monitoring, self-tuning algorithms

First lets convert the TCP window size from bytes to bits. In this case we are using the standard 64KB TCP window size of a Windows machine.

64KB = 65536 Bytes. 65536 * 8 = 524288 bits

Next, lets take the TCP window in bits and divide it by the round trip latency of our link in seconds. So if our latency is 30 milliseconds we will use 0.030 in our calculation.

524288 bits / 0.030 seconds = 17476266 bits per second throughput = 17.4 Mbps maximum possible throughput

So, although I may have a 1GE link between these Data Centers I should not expect any more than 17Mbps when transferring a file between two servers, given the TCP window size and latency.

So in our example of a 1GE link between Chicago and New York with 30 milliseconds round trip latency we would work the numbers like this

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.